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Renewable Sources of Energy

This book is a part of the course by Jaipur National University, Jaipur.This book contains the course content for Renewable Sources of Energy.

JNU, JaipurFirst Edition 2013

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JNU makes reasonable endeavours to ensure content is current and accurate. JNU reserves the right to alter the content whenever the need arises, and to vary it at any time without prior notice.

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Index

ContentI. ...................................................................... II

List of FiguresII. ....................................................... VII

List of TablesIII. ........................................................... X

AbbreviationsIV. ........................................................XI

Case StudyV. .............................................................. 195

BibliographyVI. ........................................................ 200

Self Assessment AnswersVII. ................................... 204

Book at a Glance

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Contents

Chapter I ....................................................................................................................................................... 1New and Renewable Energy Sources ......................................................................................................... 1Aim ................................................................................................................................................................ 1Objectives ...................................................................................................................................................... 1Learning outcome .......................................................................................................................................... 11.1 Need of Energy ........................................................................................................................................ 21.2 Types of Energy ....................................................................................................................................... 2 1.2.1 Potential Energy ....................................................................................................................... 2 1.2.2 Kinetic Energy ......................................................................................................................... 3 1.2.3 Electrical Energy ...................................................................................................................... 3 1.2.4 Magnetic Energy ...................................................................................................................... 4 1.2.5 Electromagnetic Energy ........................................................................................................... 4 1.2.6 Nuclear Energy ........................................................................................................................ 5 1.2.7 Heat Energy ............................................................................................................................. 5 1.2.8 Sound Energy ........................................................................................................................... 5 1.2.9 Mass Energy............................................................................................................................. 6 1.2.10 Chemical Energy .................................................................................................................... 61.3 Law of Conservation of Energy ............................................................................................................... 61.4 Conventional and Non-conventional Energy Sources ............................................................................. 8 1.4.1 Conventional Energy Sources .................................................................................................. 8 1.4.2 Non-conventional Energy Sources ........................................................................................ 101.5 Renewable Energy Sources .................................................................................................................... 14 1.5.1 Advantages of Renewable Energy ......................................................................................... 15 1.5.2 Obstacles to the Implementation of Renewable Energy Systems .......................................... 151.6 Energy and Environment ....................................................................................................................... 161.7 Pollution and Climate Change ............................................................................................................... 16 1.7.1 The Greenhouse Effect .......................................................................................................... 16 1.7.2 Most Rapid Change in Last 10,000 Years .............................................................................. 16 1.7.3 The Impact of Global Warming ............................................................................................. 17 1.7.4 Actions to Tone Down Climate Change ................................................................................. 19Summary ..................................................................................................................................................... 20References ................................................................................................................................................... 20Recommended Reading ............................................................................................................................. 21Self Assessment ........................................................................................................................................... 22

Chapter II ................................................................................................................................................... 24Solar Energy .............................................................................................................................................. 24Aim .............................................................................................................................................................. 24Objectives .................................................................................................................................................... 24Learning outcome ........................................................................................................................................ 242.1 Introduction ........................................................................................................................................... 252.2 Sun as Source of Energy ........................................................................................................................ 25 2.2.1 Characteristics of the Sun ...................................................................................................... 25 2.2.2 Electromagnetic Energy Spectrum ........................................................................................ 28 2.2.3 Solar Constant ........................................................................................................................ 29 2.2.4 Solar Energy Transfer ............................................................................................................ 302.3 Solar Energy on Surface of the Earth..................................................................................................... 30 2.3.1 Geometry of the Sun and Earth .............................................................................................. 31 2.3.2 Basics of Sun Earth Angles .................................................................................................... 31 2.3.3 Estimation of Solar Energy .................................................................................................... 332.4 Solar Energy Measuring Instruments ..................................................................................................... 35 2.4.1 Pyrheliometer ......................................................................................................................... 35 2.4.2 Pyranometer ........................................................................................................................... 36

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2.4.3 Sunshine Recorder ................................................................................................................. 37Summary ..................................................................................................................................................... 38References ................................................................................................................................................... 38Recommended Reading ............................................................................................................................. 39Self Assessment ........................................................................................................................................... 40

Chapter III .................................................................................................................................................. 42Solar Energy Systems: Thermal and Photovoltaic ................................................................................. 42Aim .............................................................................................................................................................. 42Objectives .................................................................................................................................................... 42Learning outcome ........................................................................................................................................ 423.1 Introduction ............................................................................................................................................ 433.2 Principle of Conversion of Solar Radiation into Heat ........................................................................... 433.3 Types of Collectors ................................................................................................................................ 43 3.3.1 Flat Plate Collector (Non-Concentrating Collectors) ............................................................ 44 3.3.2 Concentrating or Focusing Collectors ................................................................................... 45 3.3.3 Comparison of Flat Plate and Focusing Collectors ............................................................... 503.4 Solar Energy Storage ............................................................................................................................. 51 3.4.1 Types of Storages ................................................................................................................... 51 3.4.2 Solar Pond .............................................................................................................................. 533.5 Solar Thermal Devices ........................................................................................................................... 56 3.5.1 Solar Thermal Power Station ................................................................................................. 56 3.5.2 Solar Furnace ......................................................................................................................... 57 3.5.3 Solar Distillation .................................................................................................................... 59 3.5.4 Industrial Applications of Solar Energy (Industrial Process Heat) ........................................ 60 3.5.5 Solar Cooking ........................................................................................................................ 61 3.5.6 Solar Water Heating Systems ................................................................................................. 633.6 Energy Balance Equation ....................................................................................................................... 673.7 Economics of Solar Thermal Systems ................................................................................................... 683.8 Principle and Operation of Solar Cells .................................................................................................. 693.9 Solar Cell Electrical Characteristics ...................................................................................................... 703.10 Types of Solar Cells ............................................................................................................................. 73 3.10.1 Semiconductor - Semiconductor Junction Cells .................................................................. 73 3.10.2 Semiconductor- Metal junction Cells (Schottky Barrier cells) ............................................ 73 3.10.3 Semiconductor-liquid Junction Cells ................................................................................... 73 3.10.4 MIS and SIS Solar Cells ...................................................................................................... 743.11 Solar PV System .................................................................................................................................. 743.12 Applications of Solar Photo-voltaic System ........................................................................................ 763.13 Advantages and Limitations of Solar PV Systems............................................................................... 773.14 Economics of Solar PV Systems .......................................................................................................... 78Summary ..................................................................................................................................................... 80References ................................................................................................................................................... 81Recommended Reading ............................................................................................................................. 81Self Assessment ........................................................................................................................................... 82

Chapter IV .................................................................................................................................................. 84Wind Energy .............................................................................................................................................. 84Aim .............................................................................................................................................................. 84Objectives .................................................................................................................................................... 84Learning outcome ........................................................................................................................................ 844.1 History of Wind Energy ......................................................................................................................... 854.2 Origin and Classification of Wind ......................................................................................................... 85 4.2.1 Planetary Wind ....................................................................................................................... 85 4.2.2 Local Winds ........................................................................................................................... 864.3 Wind Energy and Assessment ................................................................................................................ 88

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4.3.1 Wind Energy Basics ............................................................................................................... 88 4.3.2 Wind Speed Measurement ..................................................................................................... 89 4.3.3 Wind Data Collection ............................................................................................................. 90 4.3.4 Wind Data Analysis ............................................................................................................... 914.4 Theory of Windmill ................................................................................................................................ 91 4.4.1 Wind Power Conversion ........................................................................................................ 91 4.4.2 Performance Characteristics of Windmill .............................................................................. 934.5 Wind Power Technology ........................................................................................................................ 95 4.5.1 Working of Windmill ............................................................................................................. 95 4.5.2 Vertical Axis Wind Machine .................................................................................................. 96 4.5.3 Horizontal Axis Wind Machine ............................................................................................. 99 4.5.4 Comparative Performance of Windmill ............................................................................... 100Summary ................................................................................................................................................... 102References ................................................................................................................................................. 102Recommended Reading ........................................................................................................................... 103Self Assessment ......................................................................................................................................... 104

Chapter V .................................................................................................................................................. 106Bio and Hydrogen Fuels ......................................................................................................................... 106Aim ............................................................................................................................................................ 106Objectives .................................................................................................................................................. 106Learning outcome ...................................................................................................................................... 1065.1 Fuels ..................................................................................................................................................... 107 5.1.1 Types of Fuels ...................................................................................................................... 107 5.1.2 Heating Value (Calorific Value) of Fuels ............................................................................. 1085.2 Fossil Fuels .......................................................................................................................................... 109 5.2.1 Limitations of Fossil Fuels .................................................................................................. 109 5.2.2 Coal, its Origin and Limitations .......................................................................................... 109 5.2.3 Petroleum Oil, its Origin and Limitations ............................................................................110 5.2.4 Natural Gas, its Origin and Limitations ................................................................................111 5.2.5 Pollution Due to Fuels ..........................................................................................................111 5.2.6 Alternative to Fossil Fuels: Fuel Substitution .......................................................................1115.3 Bio-fuels ................................................................................................................................................112 5.3.1 Types of Bio-fuels .................................................................................................................113 5.3.2 Applications of Bio-fuels ......................................................................................................115 5.3.3 Biofuel Technology Worldwide Development ......................................................................1165.4 Hydrogen Fuel ......................................................................................................................................116 5.4.1 Sources of Hydrogen ............................................................................................................116 5.4.2 Hydrogen Fuel Technology ...................................................................................................117 5.4.3 Methods of Production of Hydrogen ....................................................................................117 5.4.4 Hydrogen Storage ................................................................................................................ 123 5.4.5 Transport of Hydrogen ......................................................................................................... 1255.5 Applications of Hydrogen .................................................................................................................... 1265.6 Hydrogen Fuel Technology Worldwide Developments ....................................................................... 126Summary ................................................................................................................................................... 128References ................................................................................................................................................. 128Recommended Reading ........................................................................................................................... 129Self Assessment ......................................................................................................................................... 130

Chapter VI ................................................................................................................................................ 132Biogas ........................................................................................................................................................ 132Aim ............................................................................................................................................................ 132Objectives .................................................................................................................................................. 132Learning outcome ...................................................................................................................................... 1326.1 Introduction .......................................................................................................................................... 133

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6.2 Principle of Biogas Plant ..................................................................................................................... 133 6.2.1 Construction and Operation of Biogas Plant ....................................................................... 1346.3 Types of Biogas Plants ......................................................................................................................... 135 6.3.1 Floating Gasholder Biogas Plant: (KVIC Model) ............................................................... 135 6.3.2 Fixed Dome Biogas Plant .................................................................................................... 136 6.3.3 Greenhouse Multichamber Biogas Plant ............................................................................. 1376.4 Performance of Biogas Plant ............................................................................................................... 138 6.4.1 Factors Affecting Biodigestion or Gas Production .............................................................. 139 6.4.2 Various Types of Inputs of Biogas Plant .............................................................................. 1406.5 Applications of Biogas Plant ................................................................................................................ 1416.6 Future Prospects of Biogas .................................................................................................................. 142Summary ................................................................................................................................................... 144References ................................................................................................................................................. 144Recommended Reading ........................................................................................................................... 145Self Assessment ......................................................................................................................................... 146

Chapter VII .............................................................................................................................................. 148Biomass Energy Sources .......................................................................................................................... 148Aim ............................................................................................................................................................ 148Objectives .................................................................................................................................................. 148Learning outcome ...................................................................................................................................... 1487.1 Introduction .......................................................................................................................................... 1497.2 Biomass Sources .................................................................................................................................. 1497.3 Silvicultural Energy Farm .................................................................................................................... 149 7.3.1 Selection of Species ............................................................................................................. 150 7.3.2 Intensive Management ......................................................................................................... 150 7.3.3 Rotation and Spacing ........................................................................................................... 151 7.3.4 Species Recommended for Energy Plantation ..................................................................... 151 7.3.5 Cultivation Practices for Salvador ....................................................................................... 1527.4 Agricultural Energy Farm .................................................................................................................... 1527.5 Aquatic Biomass .................................................................................................................................. 1547.6 Biomass Energy Conversion ................................................................................................................ 155 7.6.1 Combustion .......................................................................................................................... 155 7.6.2 Dry Chemical Processes ...................................................................................................... 157 7.6.3 Aqueous Processes ............................................................................................................... 161Summary ................................................................................................................................................... 162References ................................................................................................................................................. 162Recommended Reading ........................................................................................................................... 163Self Assessment ......................................................................................................................................... 164

Chapter VIII ............................................................................................................................................. 166Ocean Energy .......................................................................................................................................... 166Aim ............................................................................................................................................................ 166Objectives .................................................................................................................................................. 166Learning outcome ...................................................................................................................................... 1668.1 Ocean as a Potential Source of Energy ................................................................................................ 1678.2 Ocean Temperature Difference and its Use as a Source of Energy ..................................................... 167 8.2.1 OTEC System (Method of Conversion of Ocean Energy into Electrical Energy) .............. 168 8.2.2 Open Cycle (Claude Cycle) OTEC System ......................................................................... 169 8.2.3 Close Cycle (Anderson Cycle) OTEC System .................................................................... 1708.3 Ocean Waves ........................................................................................................................................ 171 8.3.1 Energy and Power from the Waves ...................................................................................... 172 8.3.2 Estimation of the Energy and Power from Ocean Tides ...................................................... 1748.4 Tidal Energy ......................................................................................................................................... 175 8.4.1 Origin of the Tides ............................................................................................................... 175

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8.4.2 Tidal Energy System ............................................................................................................ 178 8.4.3 Multiple Basin System ......................................................................................................... 179Summary ................................................................................................................................................... 180References ................................................................................................................................................. 180Recommended Reading ........................................................................................................................... 180Self Assessment ......................................................................................................................................... 181

Chapter IX ................................................................................................................................................ 183Geothermal Energy ................................................................................................................................. 183Aim ............................................................................................................................................................ 183Objectives .................................................................................................................................................. 183Learning outcome ...................................................................................................................................... 1839.1 Introduction .......................................................................................................................................... 1849.2 Origin of Geothermal Energy .............................................................................................................. 1849.3 Types of Geothermal Energy Resources .............................................................................................. 185 9.3.1 Hydrothermal Systems ......................................................................................................... 186 9.3.2 Geopressured Systems ......................................................................................................... 187 9.3.3 Petrothermal Systems ......................................................................................................... 1889.4 Advantages and Disadvantages of Geothermal Energy ....................................................................... 1899.5 Environmental Problems: Impacts of Geothermal Development ........................................................ 190Summary ................................................................................................................................................... 192References ................................................................................................................................................. 192Recommended Reading ........................................................................................................................... 192Self Assessment ......................................................................................................................................... 193

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List of Figures

Fig. 1.1 Potential and kinetic energy.............................................................................................................. 3Fig. 1.2 Generation of electrical energy ......................................................................................................... 4Fig. 1.3 Nuclear fission and fusion ................................................................................................................ 5Fig. 1.4 Exothermic and endothermic reaction .............................................................................................. 6Fig. 1.5 Energy conversion chart ................................................................................................................... 7Fig. 1.6 Law of conservation of energy in view of mechanical energy ......................................................... 8Fig. 1.7 Total energy consumption spit up in (in %) ...................................................................................... 9Fig. 1.8 Record from an Antarctic ice core of temperature and carbon dioxide concentration ................... 17Fig. 1.9 Global carbon emissions from fossil fuel use ................................................................................. 18Fig. 2.1 Structure of the sun ......................................................................................................................... 25Fig. 2.2 Cross sectional view of the interior of the sun ............................................................................... 27Fig. 2.3 Electromagnetic energy spectrum................................................................................................... 28Fig. 2.4 The variation of wl with λ for solar spectrum ................................................................................. 29Fig. 2.5 Earth’s season pattern .................................................................................................................... 31Fig. 2.6 Geometry of the sun and earth ........................................................................................................ 31Fig. 2.7 Sun-earth angles ............................................................................................................................. 32Fig. 2.8 Schematic of Pyrheliometer ............................................................................................................ 36Fig. 2.9 Schematic of Pyranometer .............................................................................................................. 36Fig. 2.10 Schematic of sunshine recorder .................................................................................................... 37Fig. 3.1 Typical liquid-type flat plate collectors .......................................................................................... 44Fig. 3.2 Some common configurations of air heating collectors ................................................................. 45Fig. 3.3 Schematic of concentrating/ focusing collector .............................................................................. 46Fig. 3.4 (a) Cylindrical parabolic collector .................................................................................................. 47Fig. 3.4 (b) Basic geometry of paraboloid mirror ........................................................................................ 47Fig. 3.5 Cylindrical parabolic collector ....................................................................................................... 48Fig. 3.6 Receiver collector ........................................................................................................................... 49Fig. 3.7 Compound parabolic collector ........................................................................................................ 50Fig. 3.8 Schematic diagram of solar pond ................................................................................................... 53Fig. 3.9 Salt gradient solar pond .................................................................................................................. 54Fig. 3.10 Solar thermal power station .......................................................................................................... 56Fig. 3.11 Schematic of direct incidence type solar furnace ......................................................................... 58Fig. 3.12 Schematic of heliostat type solar furnace ..................................................................................... 58Fig. 3.13 Schematic diagram of solar still ................................................................................................... 59Fig. 3.14 Principle and types of solar cookers ............................................................................................. 61Fig. 3.15 Details of box type cooker ............................................................................................................ 62Fig. 3.16 Shallow trough of water ............................................................................................................... 64Fig. 3.17 Plastic water bags ......................................................................................................................... 64Fig. 3.18 Trough type ................................................................................................................................... 64Fig. 3.19 Japanese pipe solar water heaters ................................................................................................. 65Fig. 3.20 Schematic of a natural circulation solar water heater with auxiliary energy added to the

storage tank (Pressurised) ............................................................................................................. 65Fig. 3.21 Non-pressurised solar water heater ............................................................................................... 66Fig. 3.22 A typical solar water heater .......................................................................................................... 66Fig. 3.23 Schematic diagram of solar collector ........................................................................................... 67Fig. 3.24 Thermal circuits for flat plate collector ........................................................................................ 68Fig. 3.25 p-n junction with influence of local electric field ......................................................................... 70Fig. 3.26 Equivalent circuit of solar cell including series and shunt resistances ......................................... 70Fig. 3.27 Circuit diagram for photovoltaic with variable illumination ........................................................ 71Fig. 3.28 The current voltage characteristics obtainable on the same solar cell by three different methods 72Fig. 3.29 Terminal properties of p-n junction solar cell in the dark and when illuminated ......................... 72Fig. 3.30 Structure of MIS or SIS solar cell with thin interfacial layer ....................................................... 74Fig. 3.31 Current-field line commutated inverters ....................................................................................... 75Fig. 3.32 Basic photovoltaic system integrated with power grid ................................................................. 77

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Fig. 4.1 Planetary winds ............................................................................................................................... 86Fig. 4.2 Local winds ..................................................................................................................................... 87Fig. 4.3 Coastal Winds ................................................................................................................................. 87Fig. 4.4 Hilly and Mountain Winds ............................................................................................................. 88Fig. 4.5 Cup type anemometer ..................................................................................................................... 89Fig. 4.6 Variation in wind velocity and density of air with respect to height .............................................. 90Fig. 4.7 A typical month wise distribution of wind velocity ........................................................................ 90Fig. 4.8 (a) Velocity distribution curve (b) Power distribution curve .......................................................... 91Fig. 4.9 Modification of wind flow near the rotor ....................................................................................... 92Fig. 4.10 Variation of CP with ‘a’ ................................................................................................................. 94Fig. 4.11 Variation of CP with wind velocity ................................................................................................ 95Fig. 4.12 Important parts of wind machine .................................................................................................. 95Fig. 4.13 Vertical axis wind mills (a) Simple savonius wind mill (b) Forces acting on the cylinders of

wind mill (c) Multiple savonius type wind mill (d) Split type savonius machine (e) The distribution of wind and its direction at the split savonius wind machine ....................................................... 97

Fig. 4.14 Types of darrious vertical axis wind machine (a) Φ type and (b) Δ type ...................................... 97Fig. 4.15 H –Rotor vertical axis wind machine ........................................................................................... 98Fig. 4.16 Schematic diagram of H –Rotor vertical axis wind machine ....................................................... 98Fig. 4.17 Magnus type vertical axis wind machine ..................................................................................... 99Fig. 4.18 Horizontal axis wind mills (a) Mono blade (b) Twin blade windmill (c) Three blade

windmill and (d) Multi blade windmill ........................................................................................ 99Fig. 4.19 Performance characteristics CP versus Tip Speed Ratio, of different windmills ........................ 101Fig. 5.1 World energy consumption ........................................................................................................... 107Fig. 5.2 Biofuels cycle ................................................................................................................................113Fig. 5.3 Simple electrolytic cell ..................................................................................................................117Fig. 5.4 Tank type electrolyser ....................................................................................................................118Fig. 5.5 Filter-press electrolyser .................................................................................................................119Fig. 5.6 Photo-electrolysis of water ........................................................................................................... 122Fig. 5.7 Liquid storage of hydrogen ........................................................................................................... 123Fig. 5.8 Metal hydride for hydrogen .......................................................................................................... 124Fig. 6.1 Biogas plant .................................................................................................................................. 134Fig. 6.2 Floating gas holder biogas plant: (KVIC model) ......................................................................... 135Fig. 6.3 Fixed domed biogas plant (Janta design) ..................................................................................... 136Fig. 6.4 Fixed domed plant (Deenbandhu design) ..................................................................................... 137Fig. 6.5 Modified green house multi-chamber (GHMC) biogas plant ...................................................... 138Fig. 6.6 Effect of slurry temperature on the digestion period .................................................................... 139Fig. 6.7 Schematic of a single chamber solid oxide fuel cell .................................................................... 142Fig. 7.1 Biomass sources ........................................................................................................................... 149Fig. 7.2 Energy plantation farm ................................................................................................................. 150Fig. 7.3 Alternative methods of algae production ...................................................................................... 154Fig. 7.4 Biomass energy conversion processes and products .................................................................... 155Fig. 7.5 Vertical cyclone furnace ............................................................................................................... 157Fig. 7.6 The tech-air pyrolysis scheme ...................................................................................................... 158Fig. 7.7 Thermogravimetry of wood .......................................................................................................... 158Fig. 7.8 Arrhenius plot for the first order reaction in the isothermal degradation of cellulose in

air (-) and nitrogen (- - -) .............................................................................................................. 159Fig. 7.9 First order plot for the residual cellulose weight (normalised) versus time. Plots at 3100C

and 3250C for air and nitrogen are similar. ................................................................................... 159Fig. 7.10 The chemical formula for a) holocellulose and b) cellulose....................................................... 160Fig. 7.11 Schematic of up draught gasifier ................................................................................................ 160Fig. 8.1 Absorption of solar energy at different depth of ocean ................................................................ 167Fig. 8.2 (a) Variation of temperature with respect to depth of ocean ......................................................... 168Fig. 8.2 (b) Variation of density of water with respect to temperature ...................................................... 168Fig. 8.3 (a) Open cycle OTEC system ....................................................................................................... 169Fig. 8.3 (b) T-S diagram corresponding to Fig. 8.3 (a) .............................................................................. 170

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Fig. 8.4 Close cycle (Anderson closed cycle) OTEC system .................................................................... 170Fig. 8.5 Ocean waves ................................................................................................................................. 171Fig. 8.6 Conversion of ocean energy to electrical energy .......................................................................... 172Fig. 8.7 Two-dimensional progressive wave observed in an ocean ........................................................... 172Fig. 8.8 Energy and power from ocean tides ............................................................................................. 174Fig. 8.9 Principle of tidal power generation .............................................................................................. 175Fig. 8.10 Forces responsible for the phenomenon of tides ........................................................................ 176Fig. 8.11 Distribution of tractive forces ..................................................................................................... 176Fig. 8.12 Range of the tide ......................................................................................................................... 177Fig. 8.13 Relative high and low tides showing variation in the range during lunar month ....................... 177Fig. 8.14 One way single basin system ...................................................................................................... 178Fig. 8.15 Two way single basin .................................................................................................................. 178Fig. 8.16 Multiple basin system ................................................................................................................. 179Fig. 9.1 A typical geothermal field ............................................................................................................. 185Fig. 9.2 Vapour dominated power plant ..................................................................................................... 186Fig. 9.3 Liquid dominated single-flash steam system ................................................................................ 187Fig. 9.4 A schematic diagram of heat extraction from hot dry rock system .............................................. 189

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List of Tables

Table 1.1 Global consumption of various energy sources in 2004 (in %) ..................................................... 8Table 2.1 Composition of the Sun ................................................................................................................ 26Table 3.1 Configuration of PV array and their resulting voltage current production .................................. 75Table 3.2 Comparison of cost factor ............................................................................................................ 78Table 5.1 Calorific values of different fuels .............................................................................................. 109Table 5.2 Some of the biogas equivalents ...................................................................................................115Table 7.1 Biomass available from leucaena ............................................................................................... 152Table 7.2 Economics of leucaena plantation .............................................................................................. 153Table 7.3 Heats of combustion of solid energy sources (after Edward 1975) ........................................... 156Table 9.1 Impacts of large-scale geothermal development ........................................................................ 191

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Abbreviations

Å – AngstromARCI – International Advanced Research Centre0C – Degree Centigrade or Degree CelsiusC.G.S. – Centimetre-gram-second SystemC/N Ration – Carbon / Nitrogen RationCp – Power CoefficientCPC – Compound Parabolic CollectorCR – Concentration RatioCTARA – Centre for Technical Alternative for Rural AreaDNES – Department of Non-conventional Energy SourcesDP – Degree of PolarisationECPV – Electrochemical Photo-voltaicFeTi – Iron TitaniumGHMC – Green House MultichamberH2 – HydrogenHá – H-alphaHav – Average Horizontal Solar RadiationsHAWM – Horizontal Axis Wind MachineHDPE – High Density Poly EthyleneHDR – Hot Dry RockHe – HeliumHHE – Heli-hydroelectricHTT – Heat Treatment TemperatureI.C Engines – Internal Combustion EnginesIPH – Industrial Process HeatK.E. – Kinetic EnergyKVIC – Khadi and Village Industries CorporationLaNi5 – Lanthanum NickelLCZ – Lower Convective ZoneLH2 – Liquid HydrogenLNG – Liquefied Natural GasLPG – Liquefied Petroleum GasMCRC – Shri AMM Murugappa Chettiar Research CentreMg2Ni – Magnesium NickelMHD – Magneto HydrodynamicsMIS – Metal-insulator-semiconductorNCZ – Non-convective ZoneOPEC – Organisation of Petroleum Exporting CountriesOTEC – Ocean Thermal Energy ConversionP.E. – Potential EnergyPV – PhotovoltaicPVC – Poly Vinyl ChlorideRR – Total Radiationss – SlopeSCFC – Single Chamber Fuel CellSI – International System of UnitsSIS – Semiconductor-insulator SemiconductorSRTC – Savannah River Technology CentreSSF – SPIC Science FoundationTiO2 – Titanium Oxidetsr – Tip Speed Ratio


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UCZ – Upper Convective ZoneUH – University of HawaiiUSA – United States of AmericaUSC – University of South CarolinaVOME – Vegetable Oil Methyl Estersα – Altitude AngleαL – Absorption Coefficient of Solar Radiation for Landαw – Absorption Coefficient for Waterγs – Solar Azimuth Angleδ – Declination Angleθ – Incident Angleθz – Zenith Angleρ – Diffuse ReflectanceΦl – Latitude Angleω – Hour Angle

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Chapter I

New and Renewable Energy Sources

Aim

The aim of this chapter is to:

determine the need for energy and various types of energy•

explain the law of conservation of energy•

differentiate conventional and non-conventional sources of energy•

Objectives

The objectives of this chapter are to:

enlist various types of energy•

define the laws of conservation of energy•

explain renewable sources of energy•

Learning outcome

At the end of this chapter, you will be able to:

understand the need for energy to mankind•

define various types of energy•

describe energy and environment•


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1.1 Need of EnergyThe Industrial Revolution that began with the discovery of the steam engine brought about a many great changes. For the first time, man began to use a new source of energy, viz. coal. A little later, the internal combustion engine was invented and the other fossil fuels, oil and natural gas began to be used extensively. The fossil fuel era of using non-renewable sources had begun and energy was now available in a concentrated form. The invention of heat engines and the use of fossil fuels made energy portable and introduced the much needed flexibility in man’s movement. A new source of energy, nuclear energy, came on the scene after the, Second World War. The first large nuclear power station was commissioned about 40 years ago. Nuclear energy is already providing a small but significant amount of the energy requirements of many countries. Thus today, every country draws its energy needs, from a variety of sources.

Broadly, these sources can be categorised as commercial and non-commercial sources. The commercial sources include the fossil fuels (coal, oil and natural gas), hydroelectric power and nuclear power. The non-commercial sources include wood, animal wastes and agricultural wastes. In an industrialised country like the USA, most of the energy requirements are met from commercial sources, while in an industrially less developed country like India, the use of commercial and non-commercial sources is about equal.

In last few years, it has become noticeable that fossil fuel resources are fast depleting and that the fossil fuel era is gradually coming to an end. This is particularly true for oil and natural gas. It will be useful therefore to examine the rates of consumption of the different sources of energy and to give some indications of the reserves available.

1.2 Types of EnergyEnergy is defined as the capacity of a physical system to perform work. Energy exists in several forms such as heat, kinetic or mechanical energy, light, potential energy, electrical, or other forms. When one does the work there is change in energy of the system. If E1 is the energy before doing the work and E2 is the energy after doing the work, then the work done by the system is, W = E1-E2 = ΔE

Thus the change in energy ΔE is equal to the work done by the system. System will do variety of work and so will have the variety of forms of energy. There are different forms of energy given as

Potential Energy• Kinetic energy• Electrical energy• Magnetic energy• Radioactive energy• Nuclear energy• Heat energy• Sound energy• Mass energy• Chemical energy•

1.2.1 Potential Energy

The potential energy of the system is associated with the position of the system relative to earth’s surface. The • gravitational force due to earth is mg. Where m is the mass of the object and g is acceleration due to gravity. Force is the entity which sets the body at rest into motion or which brings body from motion to rest.•

Force = Mass x Acceleration Unit of force is Newton.

Work is performed when a body moves in the direction of the force, which is given as,• Work = Force x displacement

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Thus, if a mass m is lifted through height h against the gravity, then the work done (W) is stored as potential • energy. Hence

Potential Energy (P. E.) = m g h Where g is acceleration due to gravity = 9.8 ms-2 or 980 cms-2

S. I. Unit of energy is Joule (J) and in C.G.S. unit it is ergs. 1 Joule = 107 ergs

1.2.2 Kinetic Energy

The kinetic energy is associated with the motion of the body. If a body of mass m is moving along straight line • with velocity v, then kinetic energy E is,

E = The rotational kinetic energy of body moving with angular velocity, ω along the arc is•

Where, I is the moment of inertia of the body.

Fig. 1.1 Potential and kinetic energy

1.2.3 Electrical Energy

The most versatile form of energy is the electric energy. It is associated with the flow of electrons due to potential • difference between two electrodes. If electron of charge e is accelerated through a potential difference of V volts, then •

Electrical energy = eV e = 1.6 x 10-19 Coulomb.

The electrical energy can be transported very easily from one place to another and also converted into different • forms of energy. The units of electrical energy are:•

Watt. second = Joule. (1 Watt .1second = 1 Joule) �In practice we use unit as 1 kWh = 1000 Watt hour. �

Electrical generators produce electrical energy from mechanical energy.• Magneto hydrodynamic generators produce electrical energy from thermal energy. Fuel cells produce electrical • energy from chemical energy. The main advantages of electrical energy are:•

easily obtained from various primary energy sources �easily and quickly transmitted �


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easily distributed �easily measured, controlled, monitored, and accounted �easily converted to other forms for final energy consumption �safe, pollution free, reliable source of energy �

1.5 Volt Battery

1.5 Volt Battery

Switch Closed- Circuit Not Complete

Switch Open-Circuit Not Complete

Electrons do not flow through an open circuit Electrons do not flow through an open circuit

Fig. 1.2 Generation of electrical energy

1.2.4 Magnetic Energy

The magnetic field of a magnet can be used as a source of energy. When we apply magnetic field B to the electron • moving with velocity v then there occurs the deviation in the path of electron due to magnetic energy.If i is the current flowing through the coil, then magnetic flux f linked with the coil is directly proportional to • current flowing through it.

φ α i φ = Li Where, L is constant of proportionality, called as self-inductance of coil.

If there is change in flux linked with coil, then according to Faraday’s law of electromagnetic induction, e.m.f. • e is induced in the coil which is,

e = – e = –L

The energy E stored in coil (inductor) of self inductance L, and carrying current i is• E =

1.2.5 Electromagnetic Energy

The beam of light i.e., radiation has also energy. The light rays are electromagnetic waves and the energy E • associated with radiation of wavelength λ and frequency ν is

E = hv =

Where h is Planck’s constant = 6.63 x 10-34 JS and c is velocity of light = 3 x 108 m/s

For example, the electromagnetic energy associated with radiation of wavelength 4000Å is•

E = Joule

E = Joule

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1.2.6 Nuclear Energy

This form of energy is associated with the fusion and fission reactions. • In case of fission reaction, heavy element when bombarded with a neutron it breaks into two small elements. In • such cases the mass of basic ingredient is different than that of the total mass of the final products. Hence here is loss of mass Δm. This loss of mass in nuclear reaction is converted into tremendous amount of energy.

E = Δm c2

For example, when uranium 235 (U• 235) is bombarded by neutron gives rise to fission reaction producing tremendous amount of nuclear energy.Nuclear fusion is combining of two nuclei accompanied by release of heat• Nuclear fusion is likely to solve energy problem of the world during the 21• st century.

Fig. 1.3 Nuclear fission and fusion

1.2.7 Heat Energy

Heat is defined as a transfer (flow) of thermal energy across certain boundary (for example, from a hot body • to cold via the area of their contactIt is another form of energy depending upon the temperature of the substance.• Heat energy α Temperature T•

H = k T Where, k = 0.8626 x 10-4 eV / K, is Boltzman constant.

1.2.8 Sound Energy

Sound is a mechanical wave which propagates through any mechanical medium and hence it has energy. • Sound waves are longitudinal waves. The waves that produce a sense of sound on a human ear are called sound • waves. Only waves with frequencies lying in the range of 20Hz to 20 KHz is audible.• The intensity of sound ‘I’ is the energy transported by sound waves in unit time across a unit area of cross section • normal to the direction of propagation of wave. It is proportional to the square of the wave amplitude P. Thus,

I ∝ P2

I =

Where B is bulk modulus and ρ is the density of medium. The unit of intensity of sound is W/m2


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1.2.9 Mass Energy

The mass of a body is a measure of its energy content. It is known that anything that has mass is a source of • energyThe matter having a mass is source of energy. The energy associated with the mass m is given by well-known • Einstein’s mass-energy relation,

E = m c2

Where, c is velocity of light.

1.2.10 Chemical Energy

Chemical energy is the potential of a chemical substance to undergo a transformation through a chemical reaction • or to transform other chemical substances. Breaking or making of chemical bonds involves energy, which may be either absorbed or evolved from a chemical system.There are two types of chemical reactions called,•

Exothermic reaction �Endothermic reaction �

Exothermic reactions release heat. Endothermic reactions absorb heat. Heat evolved in the chemical reaction • is in the form of energy.Chemical energy is converted into thermal energy by chemical reactions and by combustion. Chemical energy • is converted into electrical energy in fuel cells, storage batteries etc. Chemical energy is an intermediate energy between primary energy source and final usable energy. Petroleum resources and natural gases are extracted from the natural reserves by means of production wells. Petrochemical sector deals with refining crude petroleum, natural gas and producing several petroleum products used as fuels and industrial raw materials.

Fig. 1.4 Exothermic and endothermic reaction

1.3 Law of Conservation of EnergyThe conservation of energy can be explained on the basis of thermodynamics. Thermodynamics is the branch • of science, which deals with the transfer, and transport of heat energy. The transfer of heat from one form to other or vice versa can be explained with first law of thermodynamics • and the transport of energy can be explained by second law of thermodynamics. Thus the law of conservation of energy is nothing but first law of thermodynamics.

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It states that, “energy can neither be created nor destroyed, the total energy of the universe is constant quantity, • and only one form of energy can be converted into other forms of energy.”The possible ways of conversion of different forms of energy is given in Fig.1.5. As shown in the figure there is • possible way that, electric, magnetic and electromagnetic (radiant) energies can be converted into heat, chemical or nuclear energy.

NuclearFossile

Sources, Biomass

BurnerHeat (e.g. H2O Vapor)

Depot, Tank,

Battery

Fuel

Cells

Elec

trolys

er

Geothermal

SolarSolar Cell

ElectricityElectric Motor

GeneratorMechanical

Energy

Wind, Water

ChemicalEnergy

(e.g. Hydrogen, Diesel,

Electrolyte)

Int. Comb. Engine

Turbi

ne

Steam Turbine

& Generator

Solar

Coll

ector

Heat Pump

Nuclear Fission

Refinery,

Reformer

User device (e.g. APU, FC vehicle, laptop)

Fig. 1.5 Energy conversion chart

The internal energy is associated with the internal changes within the solid. E.g. the vibrations of lattice called • phonons. Like that mass energy can be converted into nuclear energy and vice versa. The mass energy can be converted into kinetic energy,

E = m c2 or E =

The law of conservation of energy can be explained by considering the mechanical energy, which involves • potential energy (P. E.) and kinetic energy (K. E.). Consider a closed space with the boundary such that energy is not allowed to come in or go out from the closed • space. Let us consider at a moment t = t1 a small stone of mass m is held at height h within the closed space. At t = t1, Ein = 0 and Eout = 0, the energy of the system is potential energy V = m g h and total energy of the system is potential energy.At the next moment t = t• 2, the stone is allowed to fall down and it is just about to touch the border of the wall, at this moment also, Ein = 0 and Eout = 0. However it has kinetic energy and total energy of the system is kinetic energy. Thus from Fig. 1.6, since energy is not allowed to enter or to go out, the total energy of the system is conserved.


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Fig. 1.6 Law of conservation of energy in view of mechanical energy

The law of conservation of energy can also be explained by considering the electromagnetic source. For example, • the generation of radio waves or generation of microwave signals of mobile. The pointing vector S gives the amount of energy flowing out due to electromagnetic waves,

S =

Where, E and H are electric and magnetic field components. The physical significance of the Pointing vector S is the total normal outward electric field flux flowing through unit area in unit time.

1.4 Conventional and Non-conventional Energy SourcesThe variety of energy sources can be broadly categorised as •

conventional energy sources �non-conventional energy sources �

The conventional energy sources include the fossil fuels (coal, oil and natural gas), hydroelectric power and • nuclear power, while non-conventional energy sources include wood, animal waste and agricultural wastes.

1.4.1 Conventional Energy Sources

Major conventional sources of energy include:• fossil fuels i.e., solid fuels like coal, liquid and gaseous fuels including petroleum and its derivatives and �natural gas.water power or energy stored in water �energy of nuclear fusion �

The percentage use of various energy sources for the total energy consumption in the world is given in the table • below.

Coal 32.5%Oil 38.3%Gas 19.0%Uranium 0.13%Hydro 2.0%Wood 6.6%Dung 1.2%Waste 1.3%

Table 1.1 Global consumption of various energy sources in 2004 (in %)

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Fig. 1.7 Total energy consumption spit up in (in %)(Source: Energy Information Administration, Renewable Energy Consumption and Electricity

Preliminary Statistics 2008)

Fossil fuelsLooking at the percentage distribution one finds that world’s energy supply comes mainly from fossil fuels.•

CoalSince the advent of industrialisation, coal has been the most common source of energy. In last few decades, • the world switched over from coal to oil as a major source of energy because it is simpler and cleaner to obtain useful energy from oil.According to estimates, coal is abundant. It is enough for last 200 years. However, it is low in calorific value • and its shipping is expensive. Coal is pollutant and when burnt it produces CO• 2 and CO. Extensive use of coal as a source of energy is likely to disturb the ecological balance of CO2 since vegetation in the world would not be capable of absorbing such large proportions of carbon dioxide produced by burning large quantities of coal.

OilAlmost 40% of the energy needs of the world are fed by oil. The rising prices of oil have brought a considerable • strain on the economy of the world. With today’s rate of consumption and a resource amount of 250,000 million tones of oil, it would suffice for • about 100 years unless more oil is discovered. The question is whether an alternative to oil would then be available; the world must start thinking of a change • from a world economy dominated by oil.

GasGas is incompletely utilised at present and huge quantities are burnt off in the oil production process because of the non-availability of ready market. The reason may be high transportation cost of the gas. To transport gas is costlier than transporting oil. Large reserves are estimated to be located in inaccessible areas. Gaseous fuels can be classified as:

gases of fixed composition such as acetylene, ethylene, methane etc• composite industrial gases such as producer gas, coke oven gas, water gas, blast furnace gas etc•


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Water powerWater power is developed by allowing water to fall under the force of gravity. It is used most exclusively for electric power generation. In fact, the generation of water power on a large scale became possible around the beginning of the twentieth century only with development of the electrical power transmission. Prior to that, water power plants (hydroelectric plants) were usually of small capacities, less than 100 kW.

Potential energy of water is converted into mechanical energy by using prime moves known as hydraulic turbines. Water power is quite cheap where water is available in abundance. Hydroelectric power is one of the indirect ways in which solar energy is being used. Thus, the main factor in its favour is that it is the only renewable non-depleting source of the present commercial sources of energy. In addition it does not create any pollution problem. The development rate of hydropower is still low, due the following problems.

In a developing project, it will take about 6-10 years time for planning, investing and construction.• High capital investment is needed, and some parts of the investment have to derive from foreign sources.• The growing problems on relocation of villages involved, compensation of damage, selecting suitable resettlement • area and environmental impact.

Nuclear powerAccording to modern theories of atomic structure, matter consists of minute particles known as atoms. These • atoms represent enormous concentration of binding energy. Controlled fission of heavier unstable atoms such as U• 235, Th232 liberate large amount of heat energy. This enormous release of energy from a relatively small mass of nuclear fuels makes this source of energy of great importance. The energy released by the complete fission of one kg of U• 235, is equal to the heat energy obtained by burning 4500 tons of high-grade coal or 2200 tons of oil. The heat produced by nuclear fission of the atoms of fissionable material is utilised in special heat exchangers for • the production of the steam which is then used to drive turbo generators as in the conventional power plants. However, there are some limitations in the use of nuclear energy namely high capital cost of nuclear power plants, • limited availability of raw materials, difficulties associated with disposal of radioactive waste and shortage of well trained personnel to handle the nuclear power plants.

1.4.2 Non-conventional Energy SourcesWhile fossil fuels will be the main fuels for thermal power, there is fear that they will get exhausted eventually in coming years. Therefore other systems based on non-conventional and renewable sources are being tried by many countries. These are solar, wind, sea, geothermal and biomass.

Solar energySolar energy can be a major source for power. Its potential is 178 billion MW which is about 20,000 times the world’s demand. But so far it could not be developed on a large scale. Sun’s energy can be utilised as thermal and photovoltaic. The former is currently being used for steam and hot water production.

Energy comes to the earth from the sun. This energy keeps the temperature of the earth, above that in colder space, causes current in the atmosphere and in the ocean, causes the water cycle and generates photosynthesis in plants. The solar power where sun hits atmosphere is 107 watts, whereas the solar power on earth’s surface is 106 watts. The total worldwide power demand of all need of civilisation is 1013 watts. Therefore, the sun gives us 1000 times more power than we need. If we can use 5% of this energy, it will be 50 times what the world would require. The energy radiated by the sun on a bright sunny day is approximately 1 kW/m2. Attempts have been made to make use of this energy in raising steam, which may be used to generate electricity.

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The applications of solar energy which are enjoying most success today are:heating and cooling of residential building �solar water heating �solar drying of agricultural and animal products �solar distillation on a small community scale �salt production by evaporation of seawater or inland brines �solar cookers �solar engines for water pumping �food refrigeration �bioconversion and wind energy, which are indirect sources of solar energy �solar furnaces �solar electric power generation by �

solar ponds -steam generators heated by rotating reflectors (heliostat mirrors) -cylindrical parabolic reflectors -

Solar photovoltaic cells, which can be used for conversion of solar energy directly into electricity or for �water pumping in rural agricultural purposes.

Wind energyWind energy uses high wind velocity available in certain parts. Wind energy is used for pumping the water or power generation. About 1 million wind pumps are in operation in different countries. A minimum wind speed of 3 m/s is needed. Coastal, hilly and valley areas are suitable for this process. Potential in India is estimated between 20,000 and 25,000 MW. Coastal areas of Gujarat, Maharashtra and Tamil Nadu are considered as favourable. Energy of wind can be economically used for the generation of electrical energy.

Winds are caused from two main factors:Heating and cooling of the atmosphere, which generates convection currents. Heating is caused by the absorption • of solar energy on the earth’s surface and in the atmosphere.The rotation of the earth with respect to the atmosphere, and its motion around the sun.•

Many types of windmills have been designed and developed. However, only a few have been found to be practically suitable and useful. Some of these are:

multiblade type windmill• sail type windmill• propeller type windmill• savonius type windmill• darrieus type windmill•

The first three are the examples of horizontal axis windmills, while last two have a vertical axis. Some characteristics of wind energy are stated below:

It is a renewable source of energy.• Like all forms of solar energy, wind power systems are non-polluting, so it has no adverse influence on the • environment.Wind energy systems avoid fuel provision and transport.• On a small scale, upto a few kilowatt system, is less costly. On a large scale, costs are competitive with • conventional electricity and lower costs could be achieved by mass production.


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But with wind energy, following problems are associated.Wind energy available is dilute and fluctuating in nature. Because of dilute form, conversion machines have to • be necessarily large.Unlike water energy, wind energy need storage means because of its irregularity.• Wind energy systems are noisy in operation; a large unit can be heard many kilometers away.• Large areas are needed to install wind farms for electrical power generation.•

Energy from Biomass and Bio-gasBiomass is another renewable source of energy in the form of wood, agricultural residues, etc. the potential for application of bio-mass as an alternate source of energy in India is very great. We have plenty of agricultural and forest resources for production of bio-mass. Bio-mass is produced in nature through photosynthesis achieved by solar energy conversion. As the word clearly signifies, Bio-mass means organic matter. In the simplest form the reaction is the process of photosynthesis in the presence of solar radiation, can be represented as follows H2O + CO2

solar energy CH2O + O2

In the reaction, water and carbon dioxide are converted into organic material i.e. CH2O, which is the basic molecule of forming carbohydrates stable at low temperature, it breaks at high temperature, releasing an amount of heat equal to 112,000 cal/mol (469 kJ/mole). CH2O + O2 CO2 + H2O + 112 kcal/mol

The bio-mass is used directly by burning or is further processed to produce more convenient liquid and gaseous fuel.

Bio-mass resources fall into three categories:bio-mass in its traditional solid mass (wood and agricultural residue)• bio-mass in non-traditional form (converted into liquid fuels)• the first category is to burn the bio-mass directly and get the energy• in the second category, the bio-mass is converted into ethanol and methanol to be used as liquid fuels in • enginesthe third category is to ferment the bio-mass anaerobically to obtain gaseous fuel called bio-gas (bio-gas-55 to • 65% methane, 30 to 40% CO2 and rest impurities i.e. H2, H2S and some N2)

Bio-mass resources include the following:concentrated waste-municipal solids, sewage wood products, industrial waste• dispersed waste residue-crop residue, legging residue, disposed manure• harvested bio-mass, standby bio-mass, bio-mass energy plantation•

Bio-gas: The main source for production of bio-gas is wet cow dung or wet livestock (and even human) waste, to produce bio-gas. The bio-gas production is of particular significance for India because of its large cattle population. The total cattle population in country is about 250 million. Some of the other sources of bio-gas are:

sewage • crop residue• vegetable wastes• water hyacinth• poultry droppings• pig-manures• algae• ocean-kelp•

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In the rural sector, bio-gas finds great applications in cooking, lighting, mechanical power and generation of small electricity. The gas can also be used with advantage to improve sanitary conditions and also to check environmental pollution. Bio-gas can be used solely or with diesel in I.C. engines, for production of power.

Ocean thermal energy conversion• This is also an indirect method of utilising solar energy. A large amount of solar energy is collected and �stored in tropical oceans. The surface of water acts as the collector for solar heat, while the upper layer of the sea constitutes infinite heat storage reservoir. Thus the heat contained in the oceans, could be converted into electricity by utilising fact that the temperature �difference between warm surface waters of the tropical oceans and the colder water in the depths about 20-25oK.Utilisation of this energy, with its associated temperature difference and its conversion into work, forms the �basis of ocean thermal energy conversion (OTEC) systems. The surface water, which is at higher temperature, could be used to heat some low boiling organic fluid, the vapours of which would run a heat engine. The exit vapour would be condensed by pumping cold water from the deeper regions. The amount of energy available for ocean thermal power generation is enormous, and is replenished �continuously. All the systems for OTEC method work on a closed Rankine cycle and use low boiling organic fluids like ammonia, propane, R-12, R-22 etc.

Tidal energy• The tides in the sea are the result of the universal gravitational effect of heavenly bodies like sun and moon �on the earth. Due to fluidity of water mass, the effect of this force becomes apparent in the motion of water, which shows �periodic rise and fall in level which is in rhythms with daily cycle of rising and setting of sun and moon. This periodic rise and fall of the water level of sea is called tide. These tides can be used to produce electrical power, which is known as tidal power. �When water is above the mean sea level, it is called flood tide and when the level is below the mean sea �level, it is called ebb tide.

Geothermal energy• This is the energy, which lies embedded within the earth. According to various theories the earth has a �molten core. The fact that volcanic action takes place in many places on the surface of the earth supports these theories. The steam and hot water comes naturally to the surface of the earth in some locations of the earth. For large- �scale use bore holes are normally sunk with depth up to 1000m, releasing steam and water at temperatures upto 200 or 300oC and pressures upto 3000 kgN/m2. Two ways of electric power production from geothermal energy has been suggested. �In one of this heat energy is transferred to a working fluid, which operates the power cycle. This may be �particularly useful at places of fresh volcanic activity. Where the molten interior mass of earth vents to the surface through fissures and substantially high temperatures, such as between 450 to 550oC can be found. By embedding coil of pipes and sending water through them can be raised. In the other, the hot geothermal water and/or steam is used to operate the turbines directly. From the wellhead �the steam is transmitted by pipelines up to 1 m in diameter over distances upto about 3km to the power station. Water separators are usually required to separate moisture and solid particles from steam. At present only steam coming out of the ground is used to generate electricity, the hot water is discarded because it contains as much as 30% dissolved salts and minerals, and these cause serious rust damage to the turbine. The water however contains more than 1/3 of the available thermal energy.

Hydrogen energy• Hydrogen as an energy can play an important role as an alternative to conventional fuels. For that technical �problem of production, storage and transportation can be resolved satisfactorily and the cost could be brought down to acceptable limits.


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One of he most attractive features of hydrogen as an energy carrier is that it can be produced from water �which is abundantly available in nature. Hydrogen has the highest energy content per unit of mass than any chemical fuel and can be substituted �for hydrocarbons in a broad range of applications. Its burning process is non-polluting and it can be used in fuel cells to produce both electricity and useful heat.

Fuel cells• It may be defined as an electrochemical device for the continuous conversion of the free energy change in a chemical reaction to electrical energy. It is distinguished from a battery in that it operates with continuous replenishment of the fuel and the oxidant at active electrode area and does not require recharging. Main components of fuel cell are

a fuel electrode �an oxidant or electrode �an electrolyte. �

Some of the advantages of fuel cells are:It is a direct conversion process and does not involve a thermal process, so it has high operating �efficiency.The unit is lighter, smaller and needs less maintenance. �Fuel power plants may further cut generation costs by reducing transmission losses. �Little pollution, little noise, so that it can be readily acceptable in residential areas. �

The primary drawbacks of fuel cells are their low voltage, high initial costs and low service life.

Magneto hydro-dynamics generator• The principle of Magneto Hydrodynamics (MHD) power generation enables direct conversion of thermal �energy to electrical energy. MHD power generation works on the principle described by Faraday: When an electric conductor moves �across a magnetic field, a voltage is induced in it, which produces an electric current. In MHD generators, the solid conductors are replaced by a fluid, which is electrically conducting. The �working fluid may be either ionised gas or liquid metal. The hot, partially ionised and compressed gas is expanded in a duct, and forced through a strong magnetic �field; electrical potential is generated in the gas. Electrodes placed on the side of the duct pick up potential generated in the gas. In this manner, direct current is obtained which can be converted into AC with the aid of an inverter.

1.5 Renewable Energy SourcesRenewable energy sources include both ‘direct’ solar radiation intercepted by collectors and indirect solar energy • such as wind, hydropower, ocean energy and bio-mass resources that can be managed in sustainable manner. Geothermal fields tapped with present drilling technologies have a finite life but are sometimes considered • renewable for planning purposes. Traditional methods of using biomass and derivatives such as wood and charcoal are highly inefficient. If • broadly interpreted, the definition of renewable resources also includes the chemical energy stored in food and non-fuel plant products and even the energy in the air used to dry material or to cool and heat the interiors of the buildings.From an operative view point, the correct way to treat renewable energy is as a means to reduce the demand • for conventional energy forms. Thus, in performing economic and financial analysis, there is no real distinction between renewable energy technologies and those designed to improve the efficiency of conventional energy use.

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A further point is that cost-effective approaches to energy efficiency ranging from no or low cost measures. (E.g. • reducing excess air in the boilers, shutting down equipments when not needed) to systems requiring moderate capital investment. Improvements in the efficiency of the energy use can be teamed with a variety of energy supply technologies, • and this fact must be recognised when assessing the relative economics of renewable and conventional energy systems. Three independent primary sources provide energy to the earth: •

the sun �geothermal forces �planetary motion in the solar system �

In particular, direct solar radiation represents an enormous resource for a modern technological civilisation. • However, human capacity to harness these gigantic natural flows of energy to perform useful work depends • largely on the economic feasibility of the required conversion in comparison with fossil fuel options and the extent to which large scale applications affect food production, climate and ecology.

1.5.1 Advantages of Renewable EnergyEven though renewable options are not likely to supply a substantial amount of energy to developing countries over the short term, they do have following advantages.

Renewable energy is indigenous resource available in considerable quantities to all developing nations. It has • significant local, regional or national economic impact. The use of renewable energy could help to conserve foreign exchange and generate local employment if conservation technologies are designed, manufactured, assembled and installed locally.Several renewable options are financially and economically competitive for certain applications, such as in • remote locations, where the costs of transmitting electrical power or transporting conventional fuels are high, or in those well endowed with bio-mass, hydro or geothermal resources.Because conversion technology tends to be flexible and modular, it can usually be rapidly deployed. Other • advantages of modular over very large individual units include easy in adding new capacity, less risk in comparison with ‘lumpy’ investments, lower interest on borrowed capital because of shorter lead times and reduced transmission and distribution costs for dispersed rural locations.Rapid scientific and technological advantages are expected to expand the economic range of renewable energy • applications over the next 8-10 years, making it imperative for international decision makers and planners to keep abreast of these developments.

1.5.2 Obstacles to the Implementation of Renewable Energy SystemsExperience with renewable energy projects in the developing countries indicates that there are a number of barriers to the effective development and widespread diffusion of the systems. Among these are:

Inadequate documentation and evaluation of past experience, a paucity of validated field performance data and • a lack of clear priorities for future work.Weak or non-existent institutions and policies to finance and commercialise renewable energy systems. With • regard to energy planting, separate and completely uncoordinated organisations are often responsible for petroleum, electricity, coal, forestry, fuel wood, renewable resources and conservation.Technical and economic uncertainties in many renewable energy systems, high economic and financial costs • for some systems in comparison with conventional supply options and energy efficiency measures.Sceptical attitudes towards renewable energy systems on the part of the energy planners and a lack of qualified • personnel to design, manufacture, market, operate and maintain such systems.Inadequate donor coordination in renewable assistance activities, with little or no information exchange on • successful and unsuccessful projects.


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1.6 Energy and EnvironmentEnergy and environment are two sides of a coin. One thing to be noted is that, while man’s large-scale use of • commercial energy has lead to a better quality of life, it has also created many problems. The most serious of these is the harmful effect on the environment. The combustion of fossil fuels has caused serious air pollution problems. It has also resulted in the phenomenon • of the global warming which now a matter of great concern. Similarly, the release of large amount of heat from power plants has caused thermal pollution in lakes and rivers • leading to the destruction of many forms of plants and animal life. In the case of nuclear power plants, there is also concern over the possibility of radioactivity being released into • the atmospheres. The gravity of most of these environmental problems had not really been foreseen.Now, however, as a man embarks on the search for alternative sources of energy, it is clear that he would do • well to keep the environment clean.

1.7 Pollution and Climate ChangeMan extracts energy from the nature in the form of raw energy (primary energy sources). The primary energy • sources are processed and transformed to intermediate and finally useable energy forms. The energy conversion processes are accompanied with pollution problems. A major portion of energy is • transformed to electrical form by power plants. Coal fired power plants emit solid particles, SO• x, NOx, CO, CO2 and waste heat and chemicals, etc., into the environment. Pollution of the environment disturbs the ecological balance, which leads to global warming and climate change. • The world’s annual energy consumption rate is increasing at a rate of two to four percent. Nuclear power plants, thermal power plants, chemical conversion plants etc. are emitting solid, liquid and gaseous • pollutants in the environment. Gaseous pollution is causing green house effect and global warming.

1.7.1 The Greenhouse Effect

The earth absorbs the heat energy of sunshine mainly at the surface. To maintain a steady temperature, a balancing • amount of energy is then radiated upwards from the surface at longer, infrared, wavelengths. Some of the gases in the atmosphere, which are present naturally, particularly water vapour, carbon dioxide and • methane, absorb some of this infrared radiations so acting as ‘blankets’ over the surface. Close control is thereby kept on global temperature; with the earth’s surface nearly 30• 0C warmer providing an average climate for the earth, which is suitable for human life. Increases in the amount of gases such as carbon dioxide in the atmosphere are occurring because of emissions • from human activities such as the burning of fossil fuels (coal, oil and gas) or through deforestation. These increases are sufficient to lead on average to substantially increased warming. It is called the ‘greenhouse • effect’ because the glass in a greenhouse possesses similar properties to the atmosphere.

1.7.2 Most Rapid Change in Last 10,000 Years

The climate record over many thousands of years can be built up by analysing the composition of the ice, and • the air trapped in the ice, obtained from different depths from cores drilled from the Antarctic or Greenland ice caps. The earth’s climate is in a long-term warm phase that began when the last ice age ended about 20,000 years • ago; the last warm period was about 120,000 years ago. The main triggers for ice ages have been the small regular variations in the geometry of the Earth’s orbit about • the sun, which affect the distribution of solar radiation at the earth’s surface.

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The next ice age is expected to begin in about 50,000 years time. Figure shown below is a is a record of the • change in temperature at which ice was laid down (the change in global average temperature is about half the change at the poles) and of the atmospheric carbon dioxide content over the last 160,000 years.

Business-as-usualscenario for CO

2 in 2100

Double pre-industrialCO 2

Lowest possible stabilisation level by 2100

CO 2

nowCO 2

CO 2 concentration

CO

2co

ncen

tratio

n (p

pm)

Time before present(thousands of years)

Temperature

Now

Tem

pera

ture

diff

eren

ce fr

om n

ow (

C)

0

160 120 80 40

-10

0

10

100

200

300

400

500

600

Fig. 1.8 Record from an Antarctic ice core of temperature and carbon dioxide concentration

A strong correlation exists between atmospheric temperature and carbon dioxide content. This is partly because • the amount of carbon dioxide in the atmosphere is dependent on factors strongly connected to the average temperature. Also, the carbon dioxide content in its turn influences the temperature through the greenhouse effect. Over the past 200 years, human activities have increased the amount of carbon dioxide in the atmosphere by • over 30% - well beyond the range of its natural variation during the last million years or more. If the increase continues and if adequate action is not taken to stem it, the atmospheric carbon dioxide content • will reach double its pre-industrial value during the 21st century.As a result the average rate of warming of the climate is expected to be greater than at any time during the last • 10,000 years. This is not necessarily bad; some communities may experience a net benefit. But many ecosystems and humans will find it difficult to adjust to this rapid rate of change.

1.7.3 The Impact of Global Warming

In some locations, the impacts of global warming may be positive. For some crops, increased carbon dioxide • aids growth and at high northern latitudes winters will be less cold and the growing season longer. However, because humans and ecosystems have adapted closely to the current climate, most climate change, • especially if the change is fast, is likely to have negative impacts. The main impacts are likely to be changes in sea level, rainfall, and temperature extremes.


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First, largely because of thermal expansion of ocean water and accelerated melting of glaciers, sea level is likely • to rise by about half a meter by 2100. Sea defences in many coastal regions will need to be improved, albeit at considerable cost. However, such adaptation is not possible for countries with large river deltas such as Bangladesh, Southern • China and Egypt and for many islands in the Pacific and Indian Oceans.A second major impact of global warming is likely to be on water supplies. Warming of the earth’s surface • means greater evaporation and, on average, higher water vapour content in the atmosphere. Because the latent heat of condensation is the main energy source for the atmosphere’s circulation this leads to a more vigorous hydrological cycle. In many areas, heavy rainfall may become heavier while semi-arid areas may receive less rainfall. There will • be more frequent and more intense floods or droughts, especially in subtropical areas, which are vulnerable to such events. In many places, water is rapidly becoming a critical resource; a former Secretary General of the United Nations • said that he expected the next war to be about water not oil! Floods and droughts already cause more deaths, misery and economic damage than any other type of disasters. Any increase in their frequency or intensity could be the most damaging impacts of global climate change. Studies of food supplies in a globally warmed world suggest that the worldwide quantity of available food supply • might not be greatly affected. Some regions might be able to grow more while others grow less. However, the distribution of food production will change, not least because of changed water availability. The • regions likely to be adversely affected are those in developing countries in the sub-tropics. Here there are rapidly increasing populations and agricultural production will become inadequate to meet local • needs. Considering food supplies, sea level rise and the incidence of floods and droughts, a recent carefully researched study has estimated that there may be 150 million environmental refugees by 2050.

A

B

2100205020001950190018500

5

10

15

20

25

GtC

Fig. 1.9 Global carbon emissions from fossil fuel use(From 1850-1990 and as projected to 2100 - in billions of tones of carbon (GTC)

Region A shows the range of likely emissions under ‘business as usual’ and curve B an emission scenario that • would lead to stabilisation of the atmospheric carbon dioxide concentration.

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Other likely impacts are on human health (increased heat stress and more widespread vector borne diseases such • as malaria) and on the health of some ecosystems (e.g. forests), which will not be able to adapt rapidly enough to match the rate of climate change. Studies show that the necessary action to achieve a scenario of carbon dioxide emissions such as B in Fig. 1.9, • if carefully planned and phased, is likely to cost less than 1% of the Global World Product, much less than the likely cost of damage and adaptation would be if there were no action at all. The achievement of scenario B in Fig. 1.9 will require rapid development and deployment of appropriate technology and a great deal of determination on the part of the world community.

1.7.4 Actions to Tone Down Climate Change

To mitigate the effects of global climate change, action is required to reduce the human-induced emissions of • carbon dioxide. This has large implications, for the energy sector in particular. Technology is already available for much of what is required, for instance to generate and use energy much • more efficiently and to develop renewable energy sources such as solar, wind, water, biomass and others, which are not dependent on fossil fuels.Action can also be taken to increase the sinks, which remove carbon dioxide from the atmosphere (e. g. by • reducing deforestation and increasing forestation or by direct sequestration of carbon dioxide) and to reduce methane emissions from, for example, leakage from mines and landfill sites. The main role of governments and world agencies will be to stimulate markets, to encourage the development • and use of the most appropriate clean technologies. A challenge for everybody•

For scientists, to provide better information about likely climate change and its various local impacts �For governments, to set the necessary framework �For business and industry, to seize the opportunities for innovation and use of ‘clean’ technologies �For all communities and individuals in the world, to support the action being taken and contribute to it �

Role of an individual• Ensure maximum energy efficiency in the home (over 25% of CO � 2 emissions are from domestic energy use) through good heat insulation and through the use of high efficiency appliances (e.g. low energy light bulbs, Grade A or B appliances).Ensure maximum energy saving - do not overheat rooms and turn off lights when not required. �Support, where possible, the provision of energy from renewable sources; e.g. purchase ‘green’ electricity �now that this option is available.Use public transport, and walk and cycle where possible, and use a fuel efficient car (over 25% of CO � 2 emissions come from transport).Consider the environment when shopping; e.g. buy goods produced with low energy use and products that �originate from renewable sources.Through the democratic process, encourage local and national government to deliver policies that properly �take the environment into account.


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SummaryEnergy is defined as the capacity of a physical system to perform work. Energy exists in several forms such as • heat, kinetic or mechanical energy, light, potential energy, electrical, or other forms.The potential energy of the system is associated with the position of the system relative to earth’s surface. The • kinetic energy is associated with the motion of the body. Electrical energy is associated with the flow of electrons due to potential difference between two electrodes. Nuclear fusion is combining of two nuclei accompanied by release of heat. In case of nuclear fission reaction, • heavy element when bombarded with a neutron it breaks into two small elements. In such cases the mass of basic ingredient is different than that of the total mass of the final products.Heat is defined as a transfer (flow) of thermal energy across certain boundary (for example, from a hot body to • cold via the area of their contact.Sound is a mechanical wave which propagates through any mechanical medium and hence it has energy.• The mass of a body is a measure of its energy content. It is known that anything that has mass is a source of • energy.Exothermic reactions release heat. Endothermic reactions absorb heat. Heat evolved in the chemical reaction • is in the form of energy.The law of conservation of energy states that, “energy can neither be created nor destroyed, the total energy of • the universe is constant quantity, and only one form of energy can be converted into other forms of energy.”The conventional energy sources include the fossil fuels (coal, oil and natural gas), hydroelectric power and • nuclear power, while non-conventional energy sources include wood, animal waste and agricultural wastes.Renewable energy sources include both ‘direct’ solar radiation intercepted by collectors and indirect solar energy • such as wind, hydropower, ocean energy and bio-mass resources that can be managed in sustainable manner. Renewable energy is that indigenous resource available in considerable quantities to all developing nations. It • has significant local, regional or national economic impact. The use of renewable energy could help to conserve foreign exchange and generate local employment if conservation technologies are designed, manufactured, assembled and installed locally.Obstacles to the implementation of renewable energy systems are inadequate documentation and evaluation of • past experience, sceptical attitudes towards renewable energy systems on the part of the energy planners and a lack of qualified personnel to design, manufacture, market, operate and maintain such systems.Energy and environment are two sides of a coin. One thing to be noted is that, while man’s large-scale use of • commercial energy has lead to a better quality of life, it has also created many problems. The most serious of these is the harmful effect on the environment. Nuclear power plants, thermal power plants, chemical conversion plants etc. are emitting solid, liquid and gaseous • pollutants in the environment. Gaseous pollution is causing green house effect and global warming.

ReferencesBoyle, G., 2004. • Renewable Energy: Power for a Sustainable Future. 2nd ed., Oxford University Press publications.Hodge, B. K., 2009. • Alternative energy systems and applications. Wiley Publications.Twidell, J. & Weir, T., • Reneweable Energy Resources, [Pdf] Available at: <http://www.fme.aegean.gr/sites/default/files/cn/renewable_energy_resources_2nd_ed._-_john_twidell_and_tony_weir.pdf> [Accessed 5 July 2013].Energy Sources,• [Pdf] Available at: <http://www.need.org/needpdf/infobook_activities/PriInfo/SourcesP.pdf> [Accessed 5 July 2013].Diver, S., 2012. • New renewable energy source, [Video online] Available at: <http://www.youtube.com/watch?v=cYwOOz8dREU> [Accessed 5 July 2013].Diver, S., 2012. • New renewable energy source 1500 W, [Video online] Available at: <http://www.youtube.com/watch?v=BxQtqwDEbaI> [Accessed 5 July 2013].

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Recommended ReadingMasters, G. M., 2004. • RenewableandEfficientElectricPowerSystems. Wiley-IEEE Press.Kaltschmitt, M., Streicher, W. & Wiese, A., 2010. • Renewable Energy: Technology, Economics and Environment, 1st ed., Springer publication.El-Hinnawi, E., 1983. • New and Renewable Sources of Energy (Natural resources & the environment). Tycooly Publication.


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Self AssessmentThe _________ include the fossil fuels (coal, oil and natural gas), hydroelectric power and nuclear power.1.

commercial sourcesa. non commercial sourcesb. mechanical sourcesc. electrical sourcesd.

Match the following:2.

Potential energy1. associated with the flow of electrons due to potential difference between A. two electrodes.

Kinetic energy2. The magnetic field of a magnet can be used as a source of energyB.

Electric energy3. associated with the motion of the bodyC.

Magnetic energy4. associated with the position of the system relative to earth’s surfaceD. 1-A, 2-B, 3-C, 4-Da. 1-D, 2-C, 3-A, 4-Bb. 1-C, 2-D, 3-B, 4-Ac. 1-B, 2-A, 3-D, 4-Cd.

A transfer (flow) of thermal energy across certain boundary (for example, from a hot body to cold via the area 3. of their contact is called __________

electricitya. polarityb. potentialc. heat d.

Which of the following statements is false?4. Sound is a mechanical wave.a. Sound waves are longitudinal waves.b. The waves that produce a sense of sound on a human ear are called thermal waves.c. Only waves with frequencies lying in the range of 20 Hz to 20 KHz are audible.d.

The law of conservation of energy is nothing but _____________.5. first law of thermodynamicsa. second law of thermodynamicsb. third law of thermodynamicsc. fourth law of thermodynamicsd.

Which of the following is not an impact of global warming?6. In some locations, the impacts of global warming may be positive.a. The main impacts are likely to be changes in sea level, rainfall, and temperature extremes. b. In few areas, heavy rainfall may become heavier while semi-arid areas may receive more rainfall.c. There will be more frequent and more intense floods or droughts, especially in subtropical areasd.

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________ is defined as the capacity of a physical system to perform work.7. Electricitya. Powerb. Potentialc. Energyd.

________ reactions release heat, j ________ reactions absorb heat.8. Endothermic, exothermica. Exothermic, endothermicb. Electrical , magneticc. Chemical, physicald.

Which of the following is not an obstacle to the implementation of renewable energy systems?9. Adequate documentation and no evaluation of past experiencea. Sceptical attitudes towards renewable energy systems on the part of the energy plannersb. A lack of qualified personnel to design, manufacture, market, operate and maintain such systemsc. Weak or non-existent institutions and policies to finance and commercialise renewable energy systemsd.

Which of the following is not an independent primary sources providing energy to the earth?10. The suna. Geothermal forces b. Electromagnetismc. Planetary motion in the solar systemd.


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Chapter II

Solar Energy

Aim

The aim of this chapter is to :

explain the interior of the sun •

describe the process of energy generation in sun’s interior•

explain the types of radiations and sun-earth angles•

Objectives


describe interior of the sun and the process of energy generation in sun’s interior•

define the sun-earth angle•

explain different instruments for solar radiation measurement•

Learning outcome


understand various types of radiations and sun-earth angle •

describe different instruments for solar radiation measurement•

estimate the so• lar radiations falling over the flat or tilted surfaces

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2.1 Introduction Solar energy, radiant light and heat from the sun, has been harnessed by humans since ancient times using a range of ever-evolving technologies. Solar radiation, along with secondary solar-powered resources such as wind and wave power, hydroelectricity and biomass, account for most of the available renewable energy on earth. Only a minuscule fraction of the available solar energy is used. Sun is soul of every thing that exists on the earth. Since society is developing, at each step it utilises energy in various forms. The energy consumption per capita decides the degree of development of that country. This gives rise to more general statement that, larger the consumption of energy per capita, more developed be the country.

There are varieties of energy sources which can be classified in the category of conventional and non-conventional sources. However, looking towards the limitations and polluting behaviour of the conventional sources, switching over to non-conventional sources becomes a need of the time. Basically solar energy itself is a versatile source of energy since solar radiations are available worldwide, non-polluting, and eases of conversion of solar energy in to various required forms. The various aspects of solar energy are discussed including the basics of the sun, the energy available on the earth’s surface, measurements of solar radiations and estimation of solar energy on the earth’s surface.

2.2 Sun as Source of EnergyThe Sun is the star at the centre of the solar system. It has a diameter of about 1,392,000 km, about 109 times that of Earth, and its mass (about 2×1030 kilograms, 330,000 times that of Earth) accounts for about 99.86% of the total mass of the Solar System. About three quarters of the Sun’s mass consists of hydrogen, while the rest is mostly helium. Less than 2% consists of heavier elements, including oxygen, carbon, neon, iron, and others.

Fig. 2.1 Structure of the sun

2.2.1 Characteristics of the SunFollowing are the characteristics of the sun:

Sun is non- polluting and eternal source of energy for whole world. Being source of energy, its energy mostly • depends on the physical properties as mentioned below. Average physical properties of the sun are mentioned, as it is not uniform.•

Average mass of the sun (M) = (1.991 + 0.002) X 10 � 30 KgAverage radius of the sun (R) = (6.960 + 0.001) X 10 � 8 m3 Average density of the sun (ρ) = 1.41 + 0.002 g/cm � 3

Average temperature of the sun (T) = 5762 + 50 � oK


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Mass is one of the forms of energy. Mass of the sun consists of 80% of H• 2, 19% of He and 1% other elements. The chemical constituents of the sun are determined by the spectroscopic methods i.e., by analysing the spectrum one can find the chemical constituents and their percentages.

Gas Percent

Hydrogen 92.1000%Helium 7.8000%Oxygen 0.0610%Carbon 0.0300%Nitrogen 0.0084%Neon 0.0076%Iron 0.0037%Silicon 0.0031%Magnesium 0.0024%Sulphur 0.0015%All others 0.0015%

Table 2.1 Composition of the Sun

The fundamental reaction responsible for the energy generation process in the sun is the nuclear fusion reaction • as given below.

• 41H

1 2He4 + 2e0 + 26.7 MeV

In the above reaction, four protons combines to form one helium (helium nucleus). As mass of the helium nucleus is less than that of the four protons, the mass must be converted in to energy • given by the famous Einstein equation E = mc2. It is almost a continuous fusion reactor with its constituent gases as the “containing vessel” retained by gravitational forces. The sun is a hot body source. It consists of various regions such as •

Core �Radiative interior �Convective Zone �Photosphere �Chromosphere �Corona �

However, as it is not uniform, these different regions may have different properties.

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Fig. 2.2 Cross sectional view of the interior of the sunCore•

The central part of the sun spread over distance 0.23 R (Where R is the average radius of the sun) is called �Core. Volume of the core is 15 % of the total volume of the sun and about 40 % of the total mass is concentrated �in this region. Density of this region is very high i.e. of the order of 105 kg/m3. Temperature of this region is also very high i.e. of the order of 8-40 X 10 � 6K. Hence about 90 % of energy of the sun is generated in this region.

Radiative interior• After the core of the sun, the region extending from the distance 0.7 R from the centre that contains only �radiations is called radiative interior. In this region the energy is transferred from the core in the form of the X rays and ã rays etc. �The temperature and density has dropped to about 130,000 K and 70 Kg/m � 3 respectively.

Convective zone• Radiative interior is surrounded by the fluid (gases and liquids) of very low density of the order of 10 � -5 Kg/m3. Thus the zone from 0.7R to 1.0 R is called convective zone, as process of convection takes place in this �region. Temperature of this region is of the order of 5000 K. �

Photosphere• The layer occurring after the convective region is called photosphere. �In this region the gases are found to be strongly ionised. As these gases are able to absorb and emit the �continuous radiations, hence it acts as source of heat and light.This region is found to be opaque. Photosphere is the source of most of the solar radiations. �

Chromospheres• The transparent layer of several hundred kilometre deep is called reversing layer. This outside solar �atmosphere is observable during solar eclipses. The outside of the reversing layer is called Chromosphere ( � croma in Greek means colour). It is red in colour. This colour is originated due to the Há (H-alpha) line in hydrogen spectrum. This is the region with high temperature and low density than photosphere. �

Corona• Finally there is whitish glowing region called Corona. It consists of the ionised gases. �It is the region with the higher temperature of the order of 10 � 6K and very low density. It is also observed during solar eclipses. It is assumed that the temperature of this region is raised by shock waves. �


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2.2.2 Electromagnetic Energy Spectrum

The solar radiations are nothing but electromagnetic waves. It has the energy equal to:• E= hν Where, h is the Planks constant = 6.63 X 10–34 J s. ν is the frequency given by, ν = c/λ, Where, c is the velocity of light and λ is the wavelength. Thus E = hc/λ = 1.24/λ (μm) eV.

These radiations have different wavelengths and hence different energies as shown in Fig. 2.3•

10-12

Wavelength

GAMMA RAY

X-RAY ULTRA-VIOLET

Refl

ecte

d IR

The

rmal

IR

INFRAREDVISIBLE MICROWAVE

RADIO

1nm 1µm 1mm 1cm 1m10-11 10-10 10-9 10-8 10-7 10-6 10-5 10-4 10-3 10-2 10-1 100 101 m

Reflected Energy Peak, 0.5 µm

Radiant Energy Peak, 9.7 µm

Fig. 2.3 Electromagnetic energy spectrum

The spectrum of the electromagnetic waves is divided in to wavelength bands. The particular portion of the • electromagnetic spectrum for which human eye responds is called visible region, (4000 – 7000 Å).The radiations in the limit of wavelength 0.1 to 100 microns are called thermal radiation as the radiation cause • to heat the body. Visible region occupied very narrow band of the thermal radiations. The solar radiations being electromagnetic radiations travels with the velocity of light, given by,•

C= C0/n = νλ Where, C = Velocity of light in medium n = Refractive index of the wave travelling in medium C0 = Velocity of light in vacuum. = 3 X 108m/s

The sun is hot body source and the temperature of the outer surface of the sun is of the order of 5760 • + 500C.The radiations emitted from the hot source of the sun, are given by Planck’s radiation law, which states the intensity of radiation emitted by unit surface area into a fixed direction (solid angle) from the blackbody (a black body is an idealized physical body that absorbs all incident electromagnetic radiation) as a function of wavelength for a fixed temperature. The Planck’s radiation Law can be expressed through the following equation.•

wl = E

Where, wl = Spectral beam radiation h = Planck constant = 6.625 × 10-27 erg-sec k = Boltzmann constant = 1.38 × 10-16 erg/K c = Speed of light = 3 × 1010 cm/ sec

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T = the thermodynamic temperature of the black body λ = wavelength

The variation of w• l with λ for solar spectrum is given below.

2400

2000

1600

1200

800400

0.4 0.480.6 1.0 1.4 1.8 2.2 2.6

(W/m

m)

2

Fig. 2.4 The variation of wl with λ for solar spectrum

2.2.3 Solar Constant

The sun’s energy is radiated uniformly in all directions. Because the Sun is about 150 million kilometres from • the Earth, and because the Earth is about 6300 km in radius, only 0.000000045% of this power is intercepted by our planet. This still amounts to a massive 1.75 x 1017 watts. The power of the sun at the earth per square metre is called the solar constant. The solar constant is defined • as the energy of the sun received on a unit surface area perpendicular to the direction of propagation of the radiation per unit time, outside the earth’s atmosphere and measured at mean distance between sun and earth. The accepted value of solar constant is 1353 W/m2. Before invention of rocket and spacecraft, the estimation of solar constant was carried out from the ground-based • measurements of the solar radiations after transmission of radiation through the atmosphere. Now the availability of very high altitude aircrafts, spacecraft and balloons have made possible to measure solar • constant directly outside the earth’s atmosphere. The magnitude of the solar constant is determined theoretically as follows: Let’s assume sun is a hot black body source emitting radiations in all directions. The solar radiation intensity • leaving from the surface of the sun is called solar flux and is given by the Stefan–Boltzmann law.

Hs= σT4 ------------------------------------------------------------------------------(2.1) Where σ = Stefan- Boltzmann constant = 5.67 X 10-8 W/m2 - K4. Let T = Average temperature of the surface of the sun. = 5762 K Putting respective values of the notations in equation (2.1), Hs = 5.961 x 107 W/m2. --------------------------------------------------------------------(2.2) Hence total power emitted (PT) from the sun is given by, PT = Hs x 4Π R2 ------------------------------------------------------- ----------- (2.3) Where, R = radius of the of the sun surface. = 6.960 x 108 m. Putting value of Hs from eq. (2.2) gives PT = 3.630 x 1026 W

Thus power emitted by sun is transmitted in all directions over the sphere of radius ‘r’. The intensity of solar • radiations depends on r and is given by,

H0 = PT / 4Π r2 ------------------------------------------------------------------(2.4)If r is the distance between sun surface and the earth’s atmosphere, then, the intensity of solar radiations at the • earth’s atmosphere surface is,

H0 = PT / 4Π r02 -----------------------------------------------------------------------(2.5 )


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Putting the value of r0 = 1.5 x 1011 m in eq. (2.5), H0 = 1353W/m2

This value of the solar flux on unit area in unit time is called Solar constant.

2.2.4 Solar Energy Transfer

The sun is a hot body source and has tremendous amount of thermal energy. It emits this energy in all • directions. The type of energy received on the earth’s surface, however is not in the form of the heat. As per the law of • thermodynamics, the heat energy is transmitted from one place to other by three different mechanisms called

conduction �convection �radiation �

For conduction and convection mechanisms medium is required. For radiation no medium is required. There • is no medium between sun and earth. Thus the only way to transfer the energy of the sun to the earth is in the form of radiations. During their travel they pass through the atmosphere of the earth and are affected by various mechanisms such • as

absorptions �scattering �

Absorption occurs mainly due to the water vapour and CO• 2 molecules present in the atmosphere. Scattering is mainly due to the gas molecules and particulate matters in the atmosphere.

2.3 Solar Energy on Surface of the EarthSolar radiations received from the sun to the earth are classified as •

terrestrial radiations �extra terrestrial solar radiations �

The radiations received outside the earth’s atmosphere are called extra terrestrial radiations and radiations • received on earth’s surface are called terrestrial radiations. Radiations received on the earth’s surface without their change in direction are called direct solar radiations. While • radiations received on the earth’s surface, affected by scattering phenomena are called diffuse radiations. It is found that the intensity of the solar radiations received on the earth’s surface is not uniform. The rotational • motion of the earth around its own axis and motion around sun in elliptical orbit leads to day-night and season pattern such as summer, winter and rainy respectively. During the summer, earth is near to the sun and during the winter, it is comparatively away from the sun. Thus • the intensity of the solar radiations on the earth varies from season to season. The amount of the radiations received per unit time per unit area on the earth’s surface is called as intensity of • the radiations.

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Fig. 2.5 Earth’s season pattern

2.3.1 Geometry of the Sun and Earth

Sun is hot sphere with diameter D = 1.39 X 10• 6km. It is 1.5 X 108 km + 1.7 % away from the earth having diameter 1.27 X 104km. Earth revolves around sun in elliptical orbit rather than the circular orbits. Hence intensity of the solar radiation • (terrestrial and extra terrestrial) received on the earth surface varies with eccentricity of the ellipse. At mean earth-sun distance, the sun subtends an angle of 32’ with earth’s surface as shown in Fig. 2.6•

Earth

d

d= 1.27x10 km4

r= 1.5x10 km 1.7 %8 ±

32’

Sun

D

D= 1.39 x 10 km6

Fig. 2.6 Geometry of the sun and earth

2.3.2 Basics of Sun Earth Angles

For the estimation of the solar radiation at particular location at particular instant of time, it is necessary to • understand the solar radiation geometry. This solar radiation geometry involves various angles shown in Fig. 2.7. These angles are

Incident angle (θ) �Latitude angle (Φ � l)Declination angle (δ) �Hour angle (ω) �Solar azimuth angle (γ � s)Slope (s) �Altitude angle (α) �Zenith angle (θ � z)


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Sunrays

N

P

0

Equator

0

8

Sun W

N

E

S P

rsµ

2

P’

S

a

b

Fig. 2.7 Sun-earth angles

Incident angle (θ)• An angle made by the incident solar radiation with the normal to the plane surface on the earth is called incident angle.

Latitude angle (Φl)• Angle made by the radial line joining a point of location on the earth surface (OP) and projection of this line (OP’) on the equatorial plane is called latitude angle (Φl) as shown in Fig. 2.7 (a). This angular distance is the north to south of the equator, measured from the centre of the earth. This is taken as positive, measured for northern hemisphere.

Declination angle (δ)• The angular distance of the suns ray north or south of the equator is called declination angle, or simply it is an angle between the line joining the centre of the sun to centre of the earth and projection of this line on equatorial plane. It is direct consequence of tilt. This declination varies between 23.50 on June 22 to 23.50 on December 22.

Hour angle (ω)• An angle through which earth must turn in order to bring the meridian of the point directly in line with the sun’s ray is called hour angle. It is having the value 150 per hour since earth takes 24 hrs to rotate in 3600 around the sun. It is given by the Cooper’s relation: δ ( in degrees ) = 23.5 sin [(360 / 365 )( 284 + n )] Where, n = Julian day (day of the year by taking Jan. 1 as n = 1.It is measured on the basis of local solar time (LST) which does not coincide with the local clock hence in this standard time two corrections can be made.

Correction due to the difference in the longitude between location and meridian on which the standard �time is based. Hence correction of magnitude four minutes is necessary for each degree difference in the longitude.Equation of time correction, which is due to the earth‘s orbit and the rate of its rotation. These corrections �are based on experimental data.This local solar time is positive if measured in the morning and negative if measured after noon.

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Altitude angle (α)• It is vertical angle between the projection of the sun rays on the horizontal plane and the direction of the sun rays (passing through the point) is called altitude angle as shown in Fig. 2.7 (b)

Zenith angle (θ• z)It is vertical angle between the suns ray and a line perpendicular to the horizontal plane through the point i.e. the angle between beam from the sun and vertical. Simply it is a complementary angle of suns altitude angle (á). Therefore,

Zenith angle ( θz ) = (Π/ 2) – α

Solar azimuth angle (γs)• It is a horizontal angle measured from north to the horizontal projection of the sunray. West wise measurement of this angle is taken as positive.

The Slope• It is the angle made by the plane surface with the horizontal. Slope is taken positive for the surfaces sloping towards south and negative for the north slopping surfaces. Use of proper sign convention of the all angles is necessary for estimation of the solar energy on flat or tilted surfaces.

Generally, solar azimuth angle (γ• s), Slope (s), Altitude angle (α) Zenith angle (θz)can be expressed in terms of Latitude angle (Φl), Declination angle (δ),Hour angle (ω), with the help of expressions.

cos θz = cosΦl cosω cosδ + sinΦ sinδ cos γs = secα (cosΦl sinδ - cosδ sinΦ cosω) sin γs = secα cosδ sinω

In case of spherical geometry, relation between incidence angle (θ) and other angles can be given by following • relation:

cos θ = sin Φl (sinδ coss + cosδ cos γs cosω sins) + cosΦl (coss cosω cosδ - sins cosγs sinδ) + sins sinω sin γ cosδ ------------------------------------------------------------------------------------------- (2.6)

Thus from above discussion one can write equations for horizontal surface as well as for vertical surface.• For horizontal surface: As we are working with horizontal surface, the slope is zero i.e. s = 0 and Incident �angle is zenith angle. Thus above equation (2.6) can be modified as

cos θz = sin Φl sinδ + cosΦl cosω cosδ ----------------------------------------------------(2.7)For vertical surface: when working with vertical surface, s = 90 � 0, Hence equation (2.6) can be modified as,

cos θ = sin Φl cosδ cos γs cosω - cosΦl cosγs sinδ + sinω sin γ cosδ -------------------(2.8)Accordingly for vertical surfaces facing south one can write s = 90• 0 and γs = 0 and modify the eq (2.8).At time of sunrise and sunset Zenith angle is θ• z= 900 hence equation (2.7) becomes

cosωs = - sin Φ sinδ/ cos Φ cosδωs = cos -1(- tanδ tanΦ) ----------------------------------------------------------------------------------------( 2.9 )Thus complete rotation of the earth around the sun requires 24 hrs at the rate of 15 degree per hour. Hence day length (td) is given by, td = 2ωs / 15 --------------------------------------------------------------------------------- ( 2.10)Putting expression for ùs in to the equation (2.10) gives, day length, td = (2 /15 ) cos-1(- tanδ tanΦ)

2.3.3 Estimation of Solar Energy

Radiations on flat or horizontal surface• In 1924, Angstrom suggested an expression for monthly average horizontal solar radiations (Hav). Hav = H0’ [ a’ + b’ (n/ N)] ---------------------------------------------------------- (2.11)Where, a’ and b’ are arbitrary constants with values a’= 0.35 and b’ = 0.61 suggested by Freitz in 1951. H0 = Monthly averaged solar radiations for a clear day. N = Maximum number of hours in a day having bright sunshine for same period or is called day length.


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n = Average of hours in a day having bright sunshine for same period.Therefore day length (td) can be determined by the formula, N = td = (2 / 15) cos -1(- tanδ tanΦ)Monthly averaged solar radiations for a clear day (H0) can be determined by equation, H0 = 24 /π Isc[{1 + 0.033 cos (360 n/365) } (cosδ cosΦ sinωs +(2πωs sinΦ sinδ)/360)] Where, Isc = solar constant n = Julian day (day of the year considering Jan. 1 = 1) ωs = Sunrise hour angle = cos-1(- tanδ tanΦ)As all values have been determined, finally one can calculate the monthly averaged solar radiations on flat or horizontal surface using eq. (2.11)

Solar radiations on tilted surface• Vast data is available for radiations falling on the horizontal surfaces and less regarding the tilted surface. But, most of solar equipments are tilted to some angle to the horizontal. Orientation of the surface decides the amount of the solar radiations captured by the surface hence tracking systems are used so as to orient the radiation receiving surface perpendicular to the incident radiation.Hence amount of solar radiation flux falling on tilted surface is equal to the sum of direct radiations, diffuse radiations and reflected radiations on the surface from the surrounding.Summation of the radiations is as given below.Direct radiations: As solar equipments are always placed south facing then solar azimuth angle γs must be zero. Hence equation (2.7) becomes, cos θ = sin (Φl – s) sinδ + cos(Φl + s ) cosω cosδAlso for horizontal surface angle of incidence is equal to zenith angle (slope is zero) and hence equation (2.7) becomes, cos θz = sin Φl sinδ + cosΦl cosω cosδThe ratio between flux falling on tilted surface (Htilt ) and horizontal surface (H) is called tilt factor (Rbeam). Hence, Rbeam= Htilt/H

Rbeam = cosθz /cosθ --------------------------------------------------------------- (2.12)

Total radiations:• Total radiations are the combination of both direct radiations as well as the diffuse radiations. Tilt factor is referred as the correction factor for the direct radiations as discussed in equation (2.12). Assuming that diffused radiations are originated near sun hence scattering of the solar radiations will be mostly forward scattering. Correction for diffuse radiations can be applied during clear day.Thus correction factor for both radiation (R) may be equal; to tilt factor (Rbeam). Hence R = Htilt / H

Htilt = R (Hbeam + Hdiffuse)Assume that during cloudy days diffused radiations are distributed uniformly over sky. Therefore effective ratio of solar radiation, R = (Hbeam /H) Rbeam + (Hdiffuse / H) A surface tilted at slope‘s’ from the horizontal sees (1 + cos s)/ 2 of the sky dome.This allows correction that, R = (1 + cos s)/ 2If tilted surface is receiving radiations reflected by the surrounding or ground, with diffuse reflectance (ρ), then total radiations (RR) reflected from surrounding are given by, RR = (Hbeam + Hdiffuse )(1 - cos s ) ρ/2 Hence, overall sum of radiations, (Htilt) = Hbeam Rbeam + Hdiffuse (1 + cos s ) / 2 + RR And effective ratio of solar radiation, R = (Hbeam /H) Rbeam + (Hdiffuse / H )(1 + cos s )/ 2 + (Hbeam + Hdiffuse )(1 - cos s ) ρ/2

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Direct and diffuse radiations• The amount of energy that the earth receives from the stars and other celestial bodies is negligible. Hence radiant energy from the sun that strikes the earth is called insolation. The radiations received without change of direction are called beam radiations. The radiations received after suffering by the change in direction by scattering and reflection are called diffuse radiations. Finally, the sum of the beam and diffuse radiation flux is called total radiation or global radiation.

2.4 Solar Energy Measuring InstrumentsIn order to install any solar energy conversion system or device it is necessary to survey the site for the availability of the solar radiations in a year. Such surveys are necessary for the efficient working of the installed solar system. Otherwise failure of the system due to unknown seasonal variation may cause to loose large capital investment. These radiation measurements can be carried out with the help of the three basic instruments called Pyrheliometer, Pyranometer and Sunshine recorder etc.

2.4.1 Pyrheliometer

Principle: It is an instrument, which measures beam or direct solar radiations by obstructing diffused component • of solar radiations.Construction and Working: •

Mainly it consists of a black absorber plate (with hot junction of a thermopile attached to it) is located at �the base of the tube as shown in Fig. 2.8. This can be done by mounting a collimator tube of diameter and length such that radiation makes an angle �5o with the sensor plate. This collimator tube is aligned with the direction of the sunray with the help of two axis tracking mechanism �and alignment indicator. Thus the black plate receives only beam radiations and small amount of diffused radiations within the �acceptance angle of the instrument.A thermopile attached to this black absorber plate gives the output voltage which is measure of direct solar �radiations.


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Acceptanceangle

1

23

4

5

Fig. 2.8 Schematic of Pyrheliometer

(1.Tube blackened on inside surface, 2. Alignment indicator, 3. Black absorber plate, 4.Thermopile junctions, 5. Two axis tracking mechanism)

2.4.2 Pyranometer

This is an instrument used for measurement of total solar radiations (direct and diffuse). • Principle: •

Solar radiations are allowed to incident on the blackened detector. Detector absorbs radiations. �Thermopile is attached to the absorber act as hot junction end and shaded region acts as cold junction �end. Emf that developed as per Seebeck effect is measure of total solar radiations. �

Fig. 2.9 Schematic of Pyranometer

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Construction and working• It consists of blackened detector mounted on adjustable base as shown in Fig. 2.9. �The detector assembly is placed in a nearly spherical glass bulb with transmittance 0.90 over most of the �solar spectrum. This detector acts as the absorber of solar radiations, which results in the rise in temperature of the detector. Hot junctions of the thermopile are attached to the back surface of the detector while cold junctions are �screened from solar radiation to avoid its heating which results in temperature difference between two junctions leading to development of emf in the range of 0 to 10 mV.This emf is then integrated over a period of time and is measure of total radiations. �

2.4.3 Sunshine Recorder

Principle: Solar radiations are concentrated on a card strip marked with time scale. The bright sunshine radiations • burn the card stripe up to its available time.

Fig. 2.10 Schematic of sunshine recorder

Construction and working• The radiation of bright sunshine in a day measured by means of a sunshine recorder. It is simplest and more �rigid device to measure availability as solar energy at a given place.It consists of spherical glass bowl mounted concentrically on the brass bowl as shown in Fig. 2.10. To the �brass bowl three grooves were made. Card strips can be fixed in these grooves.There are two types of card strips. �

Curved card strips i. Straight card stripsii.

Long curved card strips are used in summer. A straight card strip is used at equatorial position and short �curved card strip are used in winter. The brass bowl has a rotating arrangement with a circular scale is kept as the latitude angle of place. For �Kolhapur, latitude angle is 16o 31’. On every card strip there are markings of hours of a day (6 a.m to 6 p.m). When solar radiations are received by a glass sphere it gets focused to a point on card strip held in grove �in spherical bowl. Whenever there is bright sunshine, the image is formed it has intensity enough to burn a spot on the card strip though out the day as the sun moves across the sky. The image (burning spot) moves along strip. A length of burnt trace is measure of bright sunshine for a �day.


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SummarySun is soul of every thing that exists on the earth. Since society is developing, at each step it utilises energy • in various forms. The energy consumption per capita decides the degree of development of that country. Solar energy, radiant light and heat from the sun, has been harnessed by humans since ancient times using a range of ever-evolving technologies.The Sun is the star at the centre of the solar system. It has a diameter of about 1,392,000 km, about 109 times • that of Earth, and its mass (about 2×1030 kilograms, 330,000 times that of Earth) accounts for about 99.86% of the total mass of the Solar System. About three quarters of the Sun’s mass consists of hydrogen, while the rest is mostly helium. Less than 2% • consists of heavier elements, including oxygen, carbon, neon, iron, and others.Mass is one of the forms of energy. Mass of the sun consists of 80% of H• 2, 19% of He and 1% other elements. The chemical constituents of the sun are determined by the spectroscopic methods i.e. by analysing the spectrum one can find the chemical constituents and their percentages.The sun is a hot body source. It consists of various regions such as core, radiative interior, convective Zone, • photosphere, chromospheres and corona. The spectrum of the electromagnetic waves is divided in to wavelength bands. The particular portion of the • electromagnetic spectrum for which human eye responds is called visible region, (4000 – 7000 Å).The power of the sun at the earth per square metre is called the solar constant. The solar constant is defined • as the energy of the sun received on a unit surface area perpendicular to the direction of propagation of the radiation per unit time, outside the earth’s atmosphere and measured at mean distance between sun and earth. The accepted value of solar constant is 1353 W/m2. As per the law of thermodynamics, the heat energy is transmitted from one place to other by three different • mechanisms called; conduction, convection and radiation Solar radiations received from the sun to the earth are classified as terrestrial radiations and extra terrestrial • solar radiations.This solar radiation geometry involves various angles. These angles include; incident angle (θ), latitude angle (Φ• l), declination angle (δ), hour angle (ω), solar azimuth angle (γs), slope (s), altitude angle (α), zenith angle (θz) The solar radiation measurements can be carried out with the help of the three basic instruments called • Pyrheliometer, Pyranometer and Sunshine recorder etc.Pyrheliometer is an instrument, which measures beam or direct solar radiations by obstructing diffused component • of solar radiations.Pyranometer is an instrument used for measurement of total solar radiations (direct and diffuse). • Sunshine Recorder is an instrument in which solar radiations are concentrated on a card strip marked with time • scale. The bright sunshine radiations burn the card stripe up to its available time.

ReferencesFoster, R., Ghassemi, M. & Cota• , A., 2009. Solar Energy: Renewable Energy and the Environment. 1st ed., CRC Press.Tiwari, G. N., 2002. • Solar Energy: Fundamentals, Design, Modeling and Applications. Narosa Publications.SOLAR RADIATION MEASUREMENT,• [Pdf] Available at: <http://www.edhenergy.com/downloads/solar%20radiation%20mesurement%20eng.pdf> [Accessed 5 July 2013].SOLAR RADIATION MEASUREMENT INSTRUMENTATION,• [Pdf] Available at: <http://www.tbcl.com.tw/Product/EPPLEB/EPPleb.htm> [Accessed 5 July 2013].NOVA PBS, 2012. • The Sun's Energy, [Video online] Available at: <http://www.youtube.com/watch?v=vwn0KGe8z3k&list=PLxk9t5jr_OgVdt3UspI0X5-jBYgfbyGn2> [Accessed 5 July 2013].NOVA PBS, 2012.• Solar Power, [Video online] Available at: <http://www.youtube.com/watch?v=m74bMrxhBkw> [Accessed 5 July 2013].

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Recommended ReadingSukhatme, S. P. & Nayak, J. K., 2009. • Solar Energy: Principles of Thermal Collection and Storage. 3rd ed., McGraw-Hill Education publication.Benduhn, T., 2008. • Solar Power (Energy for Today). Gareth Stevens publication.Scheer, H., 2004. • The Solar Economy: Renewable Energy for a Sustainable Global Future. Earthscan Publications Ltd.


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Self AssessmentThe whitish glowing region of the Sun called _____________.1.

Convective Zone a. Photosphereb. Chromospherec. Coronad.

Match the following:2. Core1. Temperature of this region is of the order of 5000 K.A. Radiative interior2. In this region the gases are found to be strongly ionised. B. Convective zone3. the region extending from the distance 0.7 R from the centreC. Photosphere4. The central part of the sun spread over distance 0.23 RD.

1-A, 2-B, 3-C, 4-Da. 1-D, 2-C, 3-A, 4-Bb. 1-C, 2-D, 3-B, 4-Ac. 1-B, 2-A, 3-D, 4-Cd.

The power of the sun at the earth per square metre is called the _____________3. electric constanta. temperature constantb. solar constant c. magnetic constantd.

Which of the following statements is false?4. The sun is a hot body source and has tremendous amount of thermal energy.a. The type of energy received on the earth’s surface, however is in the form of the heat.b. For conduction and convection mechanisms medium is required.c. For radiation no medium is required.d.

Scattering is mainly due to the _______ and _________ in the atmosphere.5. gas molecules , particulate mattersa. water vapour, COb. 2 moleculesgas molecules, COc. 2 moleculesparticulate matters, water vapourd.

Which of the following angles is not a Sun Earth Angles?6. Incident angle a. Declination angle b. Hour anglec. Longitude angled.

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________ is defined as the combination of both direct radiations as well as the diffuse radiations.7. Whole radiationsa. Sum radiationsb. Total radiationsc. Entire radiationsd.

________ is the radiant energy from the sun that strikes the earth. The radiations received without change of 8. direction are called ________.

Endothermic, exothermica. Insolation, beam radiationsb. Exothermic, endothermicc. Beam radiations , insolationd.

_________ is an instrument, which measures beam or direct solar radiations by obstructing diffused component 9. of solar radiations.

Pyrheliometera. Sunshine recorder b. Pyranometer c. Sunshine plotterd.

Solar radiations are concentrated on a card strip marked with time scale. The bright sunshine radiations burn 10. the card stripe up to its available time. This is the principle of _____________.

Pyrheliometera. Sunshine recorder b. Pyranometer c. Sunshine plotterd.


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Chapter III

Solar Energy Systems: Thermal and Photovoltaic

Aim


explicate the principle of conversion of solar radiation into heat•

elucidate the solar energy storage•

explain the types of collectors and solar cells•

Objectives


explain the principle of conversion of solar cells•

define solar energy storage and solar ponds•

enlist different solar thermal devices and solar cells•

Learning outcome

At the end of this chapter, students will be able to:

understand the principle of conversion of solar radiation into heat •

define the types of collectors and solar cells•

describe different solar thermal devices and solar cells•

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3.1 IntroductionCurrently, there are two different types of devices to convert solar energy into a useful form. The first type is the solar thermal collector in which a black absorber is heated by solar irradiance. This heat is then extracted by a heat transporting medium like water or air and made available for tap water or space heating. The second type is the photovoltaic solar panels in which solar cells convert solar energy into electricity. Solar thermal energy, or solar hot water, uses flat collector plates to harness the sun’s energy to heat water for use in businesses, homes, and pools. The installation and appearance are much like those of the PV panel, and the collectors are best installed facing south, under unobstructed sunlight.

Photovoltaic (PV) panels convert sunlight to electricity that can be used to supplement or replace the electricity supplied by the utility grid. PV panels are most commonly installed on rooftops, and are most effective with a southerly exposure that provides full sun. Other possible installations include a ground mount, a pole mount, and atop a porch, carport, or other shaded area. Unlike PV panels, solar thermal collectors do not convert sunlight to electricity, but transfer the energy directly to the water. Solar thermal systems displace the electricity or natural gas that would otherwise be required to heat water.

3.2 Principle of Conversion of Solar Radiation into HeatWhen a dark surface is placed in sunshine it absorbs solar energy and heats up. Majority of solar energy collectors • work on this principle and consist of a sun-facing surface, which transfers part of the energy it absorbs to a working fluid in contact with it.To reduce heat losses to the atmosphere and to improve its efficiency, one or two sheets of glass are usually • placed over the absorber surface, which shows the effect similar to that of Green house effect. This type of thermal collector suffers from heat losses due to radiation and convection. Such losses increase • rapidly as the temperature of the working fluid increases. Improvements such as the use of selective surfaces, evacuation of the collector to reduce heat losses and special • kind of glass are used to increase the efficiency of these devices.Direct Energy Conversion: (Photovoltaic Conversion)•

The direct energy conversion of solar energy into electrical energy by means of photovoltaic effect is �the conversion of light into electrical energy. The photovoltaic effect is defined as the generation of an electromotive force as a result of the absorption of ionising radiation. Energy conversion devices, which are used to convert sunlight into electricity by the use of the photovoltaic effect, are called solar cells. A single converter is called solar cell or more generally, a photovoltaic cell, and combination of such cells; designed to increase the electric power output is called a solar module or solar array.

3.3 Types of CollectorsSolar collectors can be classified according to their collecting characteristics, the way in which they are mounted and the type of transfer fluid they use.

Collecting characteristics• A non-concentrating or flat plate collector is one in which the absorbing surface for solar radiations is essentially flat with no means for concentrating the incoming solar radiation. A concentrating or focusing collector is one, which usually contains reflectors or employs other optical means to concentrate the energy falling on the aperture onto a heat exchanger of surface area smaller than the aperture.

Mounting• A collector can be mounted to remain stationary, be adjustable as to tilt angle (measured from the horizontal) to follow the change in solar declination or be designed to track the sun. Tracking is done by employing either an equatorial mounting or an azimuth mounting, for the purpose of increasing the absorption of the daily solar irradiation.


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Types of fluid• A collector will usually use either a liquid or a gas as the transfer fluid. The most common liquids are water or a water-ethylene glycol solution. The most common gas is air.

Solar collectors are classified basically in two different categories:• Flat plate or non-concentrating collectors �Concentrating or focusing collectors consists of : �

Cylindrical parabolic collector -Central receiver collector -Compound parabolic collector -

3.3.1 Flat Plate Collector (Non-Concentrating Collectors)

A simple flat plate collector consists of an absorber surface (usually a dark, thermally conducting surface); a trap • for radiation losses from the absorber surface (such as glass which transmits shorter wavelength solar radiation, but blocks the longer wavelength radiation from the absorber, a heat transfer medium such as air, water etc. and some thermal insulation behind the absorber surface. Flat plate collectors are used typically for temperature required upto 75• 0C although higher temperature can be obtained from high efficiency collector. These flat plate collectors are further classified into two basic types, based on heat transfer fluid.

Liquid type: where heat transfer fluid may be water, mixture of water antifreeze oil etc. �Air type: where heat transfer medium is air (used mainly for drying and space heating requirements.) �

Liquid type • Figure below shows a typical liquid-type flat plate collector. In general it consists of:

Glazing: One or more covers of transparent material like glass, plastics, etc. Glazing may be left out for �some low-temperature applications.Absorber: A plate with tubes or passages attached to it for the passage of working fluid. The absorber is �usually painted flat black or electroplated with a selective absorber.Header or manifolds: To facilitate the flow or heat transfer fluid. �Insulation: To minimise heat loss from the back and the sides. �Container: box or casing. �

Fig. 3.1 Typical liquid-type flat plate collectors

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Air Type/ Space Collector• Air type collectors are more commonly used for agricultural drying and space heating applications. Their �basic advantages are low sensitivity to leakages and no need for an additional heat exchanger for drying and space heating applications.However, because of the low heat capacity of the air and the low convection heat transfer coefficient between �the absorber and the air, a larger heat transfer area and higher flow rates are needed. Fig. 3.2 shows some common configurations of air heating collectors. Common absorber materials include �corrugated aluminium or galvanised steel sheets, black metallic screens or simply any black painted surface. Unglazed, transpired solar air collector offers a low-cost opportunity for some applications such as preheating �of ventilation air and agricultural drying and curing. Such collectors consist of perforated absorber sheets that are exposed to sun and through which air is drawn. �The perforated absorber sheets are attached to the vertical walls, which are exposed to the sun. The most important components, whose properties determine the efficiency of solar thermal collectors, are �glazing and absorber.

GlazingAir passage

Insulation Insulation

Corrugated sheet with selective surface

Polymer film

Clear glass

Blackened glass

GlazingGlazing

InsulationInsulation

Air flow parallel to sheet

Metal matrix

Air flow

Fig. 3.2 Some common configurations of air heating collectors

3.3.2 Concentrating or Focusing Collectors

Focusing collector is a device to collect solar energy with high intensity of solar radiation on the energy-absorbing • surface. Such collectors use optical system in the form of reflectors or refractors. A focusing collector is a special form of flat plate collector modified by introducing a reflecting (or refracting) • surface (concentrator) between the solar radiation and the absorber. Focusing collectors can have radiation increase from low value of 1.5 to 2, high values of the order of 10,000. Focusing collector comprises of receiver (absorber) and concentrator. The figure below shows the schematic • of a focusing collector.


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Insulation

Absorber or receiver

Cover Concentrator(reflector)

Orienting system

Fig. 3.3 Schematic of concentrating/ focusing collectorThere are wide varieties of means for increasing the flux of radiation on receivers. They can be classified on • the basis of

Lenses or reflectors �The types of mounting and orienting systems �The concentration of the radiation they are able to accomplish �Materials of construction, or by orientation �

Concentrator is a component used to increase the intensity of energy flux on a receiver. Concentration ratio • (CR), it is the ratio of the quantity = Aa/Ar, where Aa is the ratio of the area of the concentrator aperture and Ar is the energy absorbing area of the receiver. It determines the effectiveness of the concentrator.There are different types of concentrating or focusing collector depending upon the concentrator and receiver • geometries. These are as follows:

Cylindrical parabolic collector �Central receiver collector �Compound parabolic collector �

Cylindrical parabolic collector• It is a system consisting of a paraboloid or a parabolic reflector and having receiver at its focal point. The �concentration ratios are very high and therefore can be used where high temperatures are required. In a cylindrical system, the concentration ratio is lower than paraboloid counterparts. In both the cases the �receiver is placed at the focus, i.e. along the focal line in cylindrical parabolic system and at the focus point in paraboloidal system.The parabolic geometry is given by the relation �

Y2 = 4 a X Where a = Semi Major Axis.

The simple parabolic systems are shown in Fig. 3.4 (a) and (b) respectively for cylindrical line concentrator �and paraboloid point concentrator. However, there are variety of parabolic concentrators designed and are shown in Fig.. 3.5 (a to f). Modification to the parabolic system is shown in Fig. 3.4. In this case a double reflection system is used to shift the focus to a convenient point.Concentration ratios varies between 30 to 100 or higher which are needed to achieve temperatures in the �range of 300 to 5000C or higher. Collectors designed for such high concentration ratios necessarily have small angles of field of view and hence need to track the sun continuously.

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Absorber tube

Chain drive

Stay rods

Jack shaft

Absorber tube

Glass shield

Mirror stripsShield

Fig. 3.4 (a) Cylindrical parabolic collector

Opticalaxis

Focus

F

F = rim angle

Receiver

Principal point

4F

F

Focal line

4F

Oblique ray

Fig. 3.4 (b) Basic geometry of paraboloid mirror


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Receiver Reflector I

Reflector II(auxliary)

Parabolicmirror

Receiver

Parabolicmirror

(c) (d)

Reflector

Absorber

Receiver

Reflector

Reflector

(b)(a)

Reflector

Cylindricalreceiver

Fig. 3.5 Cylindrical parabolic collector

Central receiver collector• The concept of central receiver collector is simple. In order to avoid the cost and heat losses in transporting �a working fluid to a central location, use of sunlight itself as the transfer medium is proposed. To implement the concept, one needs a field of mirrors provided with the means of directing reflected �sunlight to a central location, or a location at one edge of the field of mirrors.In the typical central receiver, the reflector is composed of many smaller mirrors each with its own heliostat �to follow the sun. The heliostats are generally located in the horizontal plane, but when the situation is favourable, can simply follow the existing terrain. The basic difference between a single mirror concentrator and the heliostat system is that the heliostat �system has a dilute mirror. This means that the entire surface within the system is not covered with mirror surface. This diluteness is generally termed as the fill factor. A central receiver with a fill factor of about 40% means �that 40% of the land area is covered by mirrors.

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Solar radiation

Heliostats

Receiver

Ground surface

Fig. 3.6 Receiver collector

Compound parabolic collector (CPC or Winston collector)• It is possible to concentrate solar radiation by a factor of 10 without diurnal tracking using this type of �collector. A compound parabolic collector is as shown in Fig. 3.7.It consists of two parabolic reflectors which funnel the radiation from aperture to absorber. The right and �the left half belong to different parabolas (hence the name compound parabolic concentrator). The axis of the right branch, for instance, makes an angle θ � c, with the collector mid plane, and its focus is at A. At the end points C and D, the slope is parallel to the collector mid-plane.The compound parabolic collector developed by Winston represents what may be called as the most ideal �collector in the sense that, for a given field of view it achieves the maximum possible concentration ratio given by

CR = W / b =1 / sin θc Where, W = entrance aperture b = exit aperture, covered by the absorber θc = field of view (half angle)


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Parabola

AbsorberFluidpassage

Axis of parabola

D

Axis pfCPC

C

C

BA

Focus of

parabola

Solar radiationW

Fig. 3.7 Compound parabolic collector

3.3.3 Comparison of Flat Plate and Focusing Collectors

Following are the main advantages of the focusing systems over flat plate type collectors:• Reflecting surfaces requires less material and are structurally simpler than flat-plate collectors. For a �concentrator system the cost per unit area of solar collecting surface is therefore potentially less than that of the flat-plate collector.The absorber area of a contractor system is smaller than that of a flat-plate system of same solar energy �collection and the insolation intensity is therefore greater.As it is found that in case of solar energy concentrating collector the energy lost to the surrounding is less �than that for flat plate collector and the insolation on the absorber is more concentrated, the working fluid can attain higher temperatures in a concentrating system than that in a flat plate collector of the same solar energy collecting surface.Little or no antifreeze is required to protect the absorber in concentrator systems whereas the entire solar �energy collection surface requires antifreeze protection in a flat-plate collector.

The focusing systems also have some disadvantages, which are explained below:• Out of the beam and diffuse solar radiation, components, only beam component is collected in case of �focusing collectors because diffuse component cannot be reflected and is thus lost.Additional requirement of maintenance particularly to retain the quality of reflecting surface against dirt, �weather, oxidation etc.Non-uniform flux on the absorber whereas flux in flat-plate collector is uniform. �Additional optical losses such as reflectance loss and the intercept loss, so they introduce additional factors �in energy balances.

These disadvantages have restricted the utility of focusing collectors and no long time practical applications of focusing collectors other than for furnaces are being made.

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3.4 Solar Energy StorageSolar energy is a time dependent and intermittent energy resource. In general energy needs or demands for a • very wide variety of applications and also time dependent, but in an entirely different manner from the solar energy supply. There is thus a marked need for the storage of energy or another product of the solar process, if the solar energy is to meet the energy needs.The optimum capacity of an energy storage system depends, in general on the following factors:•

The expected time dependence of solar radiation availability. �The nature of the load to be expected on the process. �The degree of reliability needed for the process. �The manner in which the auxiliary energy is supplied. �An economic analysis that determines how much of the total usually, annual loads should be carried by �solar and how much by auxiliary energy source.

Energy storage may be in the form of sensible heat of solid or liquid medium as heat of fusion in chemical • systems or as chemical energy of products in the reversible chemical reaction.Mechanical energy can be converted to potential energy and stored in elevated fluids. Product of solar process • other than energy may be stored. For example, distilled water from a solar still may be stored in tanks until needed.The major characteristics of a thermal energy storage system are:•

Its capacity per unit volume or weight. �The temperature ranges over, which it operates i.e. the temperature at which heat is added to and removed �from the system.Temperature stratification in the storage unit. �The power requirement for addition or removal of heat. �The means of controlling the thermal losses from the storage systems. �Its cost. �

3.4.1 Types of Storages

Thermal storage• Thermal storage is subdivided into Sensible heat storage and Latent heat storage. �Sensible heat storage is further subdivided into Water storage and Pebble (rock) storage. �Energy can be stored by the heating, melting or vaporisation of material and the energy becomes available �as heat when the process is reversed. Storage by causing a material to rise in temperature is called sensible heat storage. Storage by phase-change, the transition from solid to liquid or from liquid to vapours is another mode of �thermal storage, known as latent heat storage, in which no temperature change is involved. It is possible for both sensible and latent heat storage to occur in the same material, as when solid is heated, �then melted, then raised further in temperature.

Electrical storage• Theoretically capacitors could store large amounts of electrical energy for long periods. The total energy �stored is

Hcap =1/2 Vε E2

Where, V = Volume of the dielectric ε = Dielectric constant E = Electric field strength


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Electric field strength is limited by the breakdown strength E � br of the dielectric. Therefore, the electric energy storable in a dielectric is limited. At present best dielectric material available is mica.Practically, because the conductivity of a dielectric is never nil, there will always be leakage losses. At �present, capacitive storage is economical for times no longer than 12 hrs. The capacitive storage on large scale is still uneconomical. The capacitors store electrical energy at high voltage and low current while inductor storage is at low voltage �and high current. The energy stored in an inductor is given by

Hind = ½ V μ HmWhere, μ = permeability of the material. Hm = magnetic flux density.

For H � ind to be large, both μ and Hm should be large. Consequently high magnetic fields are required. This will create large mechanical forces, which should be supported by strong structures. The reverse operation of discharging the stored energy creates another problem, since it involves the opening of circuits carrying large currents. Electrical energy can also be stored in chemical form in the primary cell. This is the simplest storage device. �It has no moving part, its efficiency is high and its output is in the form of electricity. It consists of two electrodes and an electrolyte, which is an ionic conductor. The energy stored by this cell �is very low for a given volume and the storing cost is expensive. These types of cells are of Hg, Ag-Zn, air cell etc.

Chemical storage• It is possible to device a storage battery in which the reactant is regenerated by a photochemical reaction �brought about by solar radiation. In this case the converter itself acts as a storage battery. The batter y is charged photochemically and discharged electrically whenever needed. Some of the reactions that could potentially be useful for the storage of solar energy are: �

2NOCl + Photons → 2 NO + Cl2 AgCl (S) + Photons → Ag(S) + ½ Cl2 NO2 + Photons → NO + ½ O2 H2O + ½ O2 → H2O2

Each one of these reactions has some shortcomings that could be overcome before it could be considered �for energy storage. It is possible to electrolyse water with solar generated electrical energy, store oxygen and hydrogen and recombine in a fuel cell to regain electrical energy. Hydrogen is one of the most efficient and practical fuels. Solar energy could be used by the anaerobic fermentation of algae for the production of methane (CH � 4). Methane is an excellent fuel, which is stable at room temperature and which reacts with oxygen to provide high temperature, releasing the stored energy in thermal form.

CH4 + 2 O2 hn→ 2 H2O + CO2

Solar energy has been converted into chemical energy of methane with 2% efficiency, because one square �km of the algae fields could produce an amount of methane storing 4MW of converted solar energy. Photosynthesis has been mentioned as a method of solar energy conversion. The products of the reaction are oxygen and H2CO, which is a fraction of carbohydrates, in the presence of light and chlorophyll.

CO2 + H2O hn → H2CO + O2The carbohydrates are stable at room temperature; but at high temperature the reaction is reversed, releasing �the stored energy in thermal form.

Thermo chemical energy storage• These systems are suitable for medium or high temperature applications. For storage of high temperature �heat, some reversible chemical reactions appear to be very attractive. Advantages of thermo chemical storage include high energy density storage at ambient temperature for long �periods without thermal losses and potential for heat pumping and energy transport over long distances.This type of storage is illustrated by a hypothetical reaction �

A + B ⇔ AB

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The forward reaction takes place with absorption of heat from solar energy and the heat is stored in the form of products. When heat is desired, the products are to be remixed to allow the reversible reaction to take place with liberation of heat. The original reactant is regenerated completing the cycle.

Both forward and reverse reactions take place at constant but different temperatures, the forward reaction �occurring at a higher temperature than the reverse reaction.

Hydro storage• It is possible to convert mechanical to potential energy and recover the potential energy on demand to provide �storage for mechanical systems. Such a system for example can pump water into an elevated reservoir during periods when solar radiation is available and recover the energy by running the water through a turbine when energy is needed. Other way of storing solar energy in the form of hydraulic energy is through heli-hydroelectric (HHE) �power conversion. Depending on the topography of the region, solar energy could be stored in hydraulic form in many ways. Hydro storage is reasonably efficient. The energy required to pump water uphill is reconverted with about �65 % to 75 % efficiency, because no thermodynamic cycle is involved. Hydro storage appears to be ideal for solar power storage with often production of power. The solar plant produces power at the maximum rate, during the day and is on standby during the night, �maintaining only adequate system temperature so that it is ready to turn out power the next day as soon as the collector sub-system reaches operating temperature.

3.4.2 Solar PondThe solar pond is a simple device for collecting and storing solar heat. Natural ponds convert solar radiation into heat, but the heat is quickly lost through convection in the pond and evaporation from its surface. A solar pond, on the other hand, is designed to reduce convective and evaporative heat losses so that useful amounts of heat can be collected and stored.

Fig. 3.8 Schematic diagram of solar pond

Principle of operation• An artificially constructed pond in which significant temperature rises are caused to occur in the lower �regions by preventing convection is called a “Solar pond”. The solar pond combines solar energy collection and sensible heat storage.


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Temperature inversions have been observed in natural lakes having high concentration gradients of dissolved �salts (i.e. concentrated solution at the bottom and dilute solution at the top). This phenomenon suggested the possibility of constructing large-scale horizontal solar collectors as ponds. Non-convective solar ponds have been proposed as a simple relatively inexpensive method of collecting and storing solar energy on a large scale. The two fundamental characteristics of solar energy, namely its diluteness and intermittent nature, are also �the reasons why it is not being harnessed on a large scale at present. First of all, collectors fabricated using materials such as glass, metals, wood etc. have size limitations and �therefore a large number of them with suitable interconnections are needed to collect large amount of solar energy. Secondly, to supply energy ‘on demand’ requires some sort of energy storage and re-conversion system to �smooth out the variation in the insolation due to cloud cover, seasonal and diurnal effects.Solar ponds promise an economical way around these two problems by employing a mass of water for both �collection and storage of solar energy. The energy is stored in low grade (60 to 1000C), thermal form that, in it, might be suitable for a variety of applications such as space heating and industrial process heat. Solar ponds may be classified into two types as convecting or non-convecting. �

Salt gradient solar ponds• This type of solar pond is a mass of shallow water of about 1 –1.5 m deep with a large collection area, which �acts as a heat trap. It contains dissolved salts to generate a stable density gradient. Salts have been dissolved in high concentrations near the bottom, with decreasing concentration toward �the surface. The salts most commonly used for salt gradient ponds are sodium chloride and magnesium chloride, although there are many other possibilities.Part of the incident solar radiation entering the pond surface is absorbed throughout the depth and the �remainder, which penetrates the pond, is absorbed at the black bottom. If the pond were initially filled with fresh water, the lower layers would heat up, expand and rise to the surface. Because of the convecting, mixing and heat loss at the surface, only a small temperature rise in the pond could be realised. On the other hand, convection can be eliminated by initially creating a sufficient strong salt concentration �gradient. In this case, thermal expansion in the hotter lower layers is insufficient to destabilise the pond. With convection suppressed, the heat is lost from the lower layers only by conduction. Because of its �relatively low thermal conductivity, the water acts as an insulator and permits high temperatures (over 900C) to develop in the bottom layers. Energy can be extracted from the pond by receiving the water in the hot layers of the pond through a heat exchanger.

Fig. 3.9 Salt gradient solar pond

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In the salt gradient solar ponds, dissolved salt is used to create layer of water with different densities – the �more the salt, the denser the water. The concentration of salt at the surface is low- usually less than 5 percent by weight and thus the water is relatively light. The salt concentration steadily increases with depth until at the bottom where it is very high, around 20 percent. Thus a solar pond has three zones with following salinity and depth: �

Surface convective zone or upper convective zone (UCZ) 0.3 –0.5, Salinity < 5% -Non-convective zone (NCZ) 1–1.5 m, salinity increases with depth. -Storage zone lower convective zone (LCZ) 1.5 to 2 m, salinity = 20% -

The deeper the zone, the more heat is stored. The lowest zone traps heat for the longer periods, damping the �effects of daily and even seasonal changes. This capacity for low cost storage is one of the chief advantages of salt-gradient solar ponds; they can be tapped for energy at night as well as during the day.Even during the longer periods of cloud cover or even ice cover, the stored energy is still available. As salty �water near the bottom heats up, it expands. However, it cannot rise because it is denser than the less salty water above. Thus the solar pond is a “non-convecting”, the warmed water stays trapper below. Some heat is still lost by conduction to the surface, but this process is much weaker than convection. �Lower water may even warm upto and above the boiling point of water. In order to reduce the cost of large solar thermal installations, it is necessary to devise more economical ways of collecting and storing solar energy. In this context, attention has been focused on the possibility of using large expanses of water of small depth �for absorbing and storing solar radiation instead of using flat-plate collectors and hot water storage tanks. However, experience shows that the water in such a pond usually heats up only a few degrees, because of �the natural convection currents, which are set into motion as soon as heat is absorbed at the bottom. One would obtain a significant rise in the water temperature only if the convection could be prevented. The usual method adopted to prevent convection is to dissolve a salt in the water and to maintain a �concentration gradient. For such ponds the more specific term ‘salt –gradient solar pond’ is used. One method of extracting heat from such a pond without causing undesirable mixing is to locate a heat exchanger just beneath the lower zone as shown in Fig. 3.9.

Classification of solar ponds• Solar ponds can also be classified as shallow solar pond, partitioned solar pond, viscosity stabilised solar pond, membrane stratified solar pond and saturated solar pond. A brief discussion is given below.

Shallow solar pond (SSP): A shallow solar pond is a body of water with shallow depth acting as large �collector and storage of solar radiation. It is large area, low cost collector where water is directly exposed to solar radiation and enclosed in a thermal insulating base material and one or two sheets of glazing.Partitioned Solar ponds: In a partitioned solar pond, the low convective zone and non-convective zone �is separated by a transparent portion and the process of operation remains the same as the conventional salt gradient solar pond. The idea of partitioned solar pond was given by Rabl and Nielson, so that lower convective zone can be used for seasonal storage of heat for house heating.Viscosity stabilised solar ponds: In the viscosity stabilised solar pond a kind of gel is used in water, making �it non-convective. The idea of viscosity stabilised solar pond was first given by Shafer and the phenomenon can be described as static rather than stable. It is known that the Rayleigh number, which relates buoyant forces and viscous drag, is responsible for the circulation and its critical value for the onset of natural convection for a layer of fluid bounded top and bottom, and heated from the bottom is 1707. The Rayleigh number is given as

Ra = g βΔ T d3/v a Where, b = Salt expansion coefficient ΔT = temperature difference between two layers. d = distance between two layers. v = kinematic viscosity of fluid. a = coefficient of salt diffusion.


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g = gravitational constant.Membrane stratified Salt less solar ponds: A possible alternative of the conventional salt gradient solar pond �is the membrane stratified solar pond.

3.5 Solar Thermal DevicesThere are two principle forms of energy into which solar radiations can be converted for practical applications. • One is the heat and the other is electricity. Heat is obtained when solar radiations are absorbed by a black surface. This heat may then be used in various • ways, which may be divided broadly into two classes:

Direct thermal applications like water heating, drying, distillation, cooking etc. �Applications involving the second law of thermodynamics like production of mechanical power and �refrigeration etc. This mechanical power production system is called the solar thermal power production system. As far as the conversion of solar energy into electrical energy is concerned it can either be done by solar thermal power production route or solar radiation can be directly converted to electrical power. In direct conversion of solar energy into electricity one may employ photovoltaic, thermoelectric, thermo ionic and photo chemicals.

Solar thermal power generation employs power cycles, which are broadly classified as low, medium and high • temperature cycles. Low temperature cycles generally use flat-plate collectors so that maximum temperature is limited to about • 1000C. Medium temperature cycles work at temperature ranging from 150 to 3000C, while high temperature cycles work at temperature above 3000C. For the low and medium temperature ranges, the thermodynamic cycles preferred are the Rankine cycle. • For the high temperature range apart from the Rankine cycle, the Brayton and Stirling cycles are also being considered. The spectrum of solar thermal power system technologies range from non-tracking, low temperature energy • conversion subsystems to point focusing collector (paraboloid) based on high temperature systems providing high density flux to an efficient power conversion system.

3.5.1 Solar Thermal Power Station

Fig. 3.10 Solar thermal power station

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There exist two major categories of thermal collection and power generation systems:• Centralised system (Tower concept) �Distributed system (Farm concept) �

The distributed system category has a number of variants and is generally differentiated from the centralised • system as follows:

The energy from each of discrete number of receiver is summed in either chemical or electrical form. �The receiver of a distributed system can generally be moved in accordance with the sun-tracking mechanism �adopted.

The solar farm is primarily intended for the decentralised supply of process heat and electricity in the range of • a few hundred to a few thousand KW.There exists an aerial layout of sun tracking concentrating collectors. Here a number of smaller heat engine/• generator units are associated with each collector unit at the focal point and the total output is combined. In the tower concept a field of heliostats, which follow the direction of sun, concentrates direct incident solar • radiation onto a central receiver mounted on the top of a tower. A heat transfer fluid (such as Hytherm 500) removes heat from the receiver and in some cases carries it to another heat storage unit. This heat source is used to operate an electrical power plant.

3.5.2 Solar Furnace

History• It is an instrument to get high temperature by concentrating solar radiations onto a specimen. Solar furnaces �have long been used for scientific investigations. French scientist Lavosier used it in 1774, with a lens as tall as man, for carrying chemical studies at high temperature. In 1921, a German scientist Strauble devised a solar furnace composed of a paraboloidal concentrator and lens. Then he built another furnace in which he used paraboloidal concentrator, 2 m in aperture and 86 cm in focal length was fixed facing downward and the solar radiation was conveyed upward by heliostat (turntable mirrors). The solar furnaces with high optical precision give temperature of over 30000C. The main advantage of solar furnace lies in the fact that heating is carried out without any contamination and �changing the position of the material in focus easily controls temperature. The solar furnace is an excellent means for studying properties of ceramics at high temperature above the range ordinarily measured in the laboratories with flames and electric currents. Main elements of a solar furnace are concentrator, heliostat and sun tracking system. The solar radiation �is nearly parallel as the angle a subtended by the sun is 32’ and a praboloidal mirror is adequate for a concentrator of the reflecting type. The geometry of a paraboloid is defined by its aperture ‘D’ and focal length ‘f’. The total solar energy �incident on a mirror is given by

¼ pD2. HbWhere, Hb is the intensity of direct solar radiation on the earth; which has the value of 1.3-1.4 Cal/cm2/min or 0.91-0.98 KW/m2. Hence the aperture governs the input power of the solar furnace size and intensity of the sun image is determined by the focal length and aperture ratio (f / D).

The paraboloidal mirror is necessary to have an optically smooth surface with accurate geometry and high �reflectivity. In some furnaces of the direct incident type, an aluminium mirror has been used to prevent an accident caused by the spill of melted material. However, it is very difficult to obtain an optically smooth surface of soft metals by grinding and polishing.

Types of solar furnaces: These Solar furnaces can be divided into two main types• Direct type: The concentrator itself is directed towards the sun as shown in Fig. 3.11. Reflection loss is not �introduced.Heliostat type: The radiation is converged into a fixed concentrator by means of a turntable mirror or heliostat �as shown in Fig. 3.12. There is a risk that melted material might spill during irradiation.


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Concentrator

Fig. 3.11 Schematic of direct incidence type solar furnace

Concentrator

Plane reflection

Fig. 3.12 Schematic of heliostat type solar furnace

Applications of solar furnaces• There are various applications of the solar furnaces, which include

Metallurgy: �Extraction of metals and smeltingi. Melting and casting processii. Heat treatmentiii. Sinteringiv.

Chemical and mineral synthesis �Crystal growth �Chemical reactions �Material testing �

Following are the characteristics of solar furnace:• It gives extremely high temperature. �In it contamination by reaction with crucible does not occur in fusion. �

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It provides very high heating and cooling. �

Major disadvantage of solar furnace is its operation only in the daytime and its high installation cost.•

3.5.3 Solar Distillation

The distillation of salt water to recover portable water is accomplished by exposing thin layers of the salt water • (usually in black shallow trays or basins) to solar radiation, and condensing the water vapours produced on a transparent cover in such a way that it can be collected in receiving troughs. The history of the basin type stills dates back to 1872, when a still, providing drinking water for animals and • used in nitrate mining in Chile, started its three decades of operation. Solar stills operating presently include about 20 basin-type installations with capacities varying from about 400 to 2500 lit. /day and an unknown number of smaller units.

Theory and Description of a Basin-type solar stillTwo basic approaches have been proposed for using solar energy to distil saline water. One approach reproduces on a small scale the pure water separation portion of the hydrologic cycle while the other uses the solar energy to energise distillers, which are also capable of operating on other types of energy.

First approach has been used to any great extent upto the present time. The hydrological cycle, from which all fresh water is produced, is large scale, natural, solar distillation. It has four essential phases viz.

The production of vapour from oceans, lakes and other large bodies of water. �The transport of this vapour as humidity of the air above the land to cooler regions by convective winds. �The condensation of this vapour and its precipitation as rain and snow. �The return of the water and melted snow by means of rivers to the ocean, lakes and other large bodies of �water.

For this hydrologic cycle, solar energy is the motivating energy. It penetrates the surface of the water, warms it and causes its evaporation. It also produces the winds, which transport the vapour to the cooler regions where the vapours condenses as liquids. Reproduced on a very small scale, the first three phases of hydrologic cycle produce pure water.

The simple solar still, generally known as the “basin type solar still” is as shown in Fig. 3.13. It consists of blacked basin containing saline water at a shallow depth, over which there is a transparent airtight cover that encloses completely the space above the basin.

FilterBasin liner

Transparentcover

InsulationOverflow

line

Filter distillation

trough

Fig. 3.13 Schematic diagram of solar still


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3.5.4 Industrial Applications of Solar Energy (Industrial Process Heat)

Solar water heating and air heating are rather well established and well known now. With all sophistications • and modern techniques, solar thermal energy systems are bulkier and still speak of only low efficiencies. In general in low grade thermal applications, where heat energy can as such be utilised, solar energy devices could be used with advantages.Rising oil prices, combined with combustion-generated pollution and fast depleting fossil fuels, have forced the • developing countries like India to adopt at least gradually, the low temperature applications of solar energy.Solar energy for thermal applications in industries has proved to be economically viable at present for temperatures • less than 1000C. With intensive development in the area of fixed and tracking concentrators, temperature upto 3000C will be feasible. In the present energy context, it is desirable to provide thermal energy below 300• 0C from sun. This is mandatory since high quality fuel, such as coal or oil, with high flame temperature when used for low temperature applications results in very low efficiency.It is estimated that a large fraction (60-70%) of all the energy consumed in industry is in the form of thermal • energy (process heat). Thus, there is a great potential for utilizing solar energy for industrial process heat (IPH), especially in tropical countries like India where solar radiation is abundant. Unlike the space heating or domestic hot water application, the demand for IPH is constant throughout the year. • Solar energy:

It is non polluting source of energy. �It is free from outside supply constraints. �

There are several factors, which have to be examined critically for making the best use of solar energy for IPH. • Firstly, solar radiation is a dilute source of energy with peak intensities of about 1 KW/m2. On the other hand, the energy use intensity is quite high in industry, which means that large areas are needed for collection of solar energy. The non-availability of large areas for installation of solar collectors can be the single major constraint for industries located in densely populated areas.Secondly, there is a wide fluctuation of solar radiation, both diurnal and seasonal, which necessitates storage of • thermal energy and/or a backup system using conventional energy sources. The need for a large area of collectors and the need for storage are mainly responsible for the high initial cost of solar systems. Thirdly, the complexity and costs of solar thermal system increase with the temperatures at which thermal energy • has to be delivered. Hence it is important to know the temperatures at which IPH is required and the quantity of heat needed at different temperatures.All the above factors point to the need for a proper study of the financial implication of installing solar energy • systems for IPH.There are several advantages of industrial applications over residential or commercial ones:•

Industrial loads are mostly on continuous basis throughout the year. �Industrial plants have maintenance crew, or in small plants skilled people, who can attend to smooth �operation of solar systems.Total quantum of energy replaced by solar is significantly more causing higher reduction in oil imports and �diversion of coal for high temperature tasks.

However there are some limitations too:• Intermittent availability of solar energy. �Industrial effluents can be harmful of the transparent covers and reflecting surfaces. �Though pay back period has come down to 3-5 years (hot water and air only), high initial capital investment �is a major impediment.

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3.5.5 Solar Cooking

In India energy consumed for cooking shares a major portion of the total energy consumed in a year. In villages • 95 % of the consumption goes only to cooking. Variety of fuel like coal, kerosene, cooking gas, firewood, dung cakes and agricultural wastes are used. The energy crisis is affecting everyone. Thus, solar cookers have a very relevant place in the present fuel consumption pattern.Various designs of solar cookers have been developed in our country. The first solar cooker has been developed • in the year 1945 by Mr. M. K. Ghosh of Jamshedpur, a freedom fighter. He developed a box type solar cooker with a reflecting mirror and a copper coil inside, on which the food materials are placed in pots.Basically there are three designs of solar cooker:•

flat plate box type solar cooker with or without reflector �multi-reflector type solar oven �parabolic disc concentrator type solar cooker �

Flat plate box type design is the simplest of all the designs. Maximum no load temperature with a suitable • reflector reaches upto 1600C. In multi-reflector, over four square or triangular or rectangular reflectors are mounted on the oven body. They all • reflect the solar radiations into the cooking zone in which cooking utensils are placed. Temperature obtained is of the order of 2000C. The maximum temperature can reach upto 2500C, if the compound cone reflector system is used. With parabolic disc concentrator type solar cooker, temperatures of the order of 450• 0C can be obtained in which solar radiations are concentrated onto a focal point. Principle of operation of a solar cooker is shown in Fig. 3.14.

Glasscovers

Sun’s ray

Rubberpacking

Woodenframe

Insulation Cookingutensils

Blackenedmetaltray

(a) Principle of box type cooker.

Solar radiation

Reflector

Blackened metaltray

(b) Reflector type solar cooker

Sun’s rays

Cooking pot

Parabolicmirror

(c) Principle of concentrating type cooker

Fig. 3.14 Principle and types of solar cookers


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Design principle and constructional details of a box type solar cooker:• The solar rays penetrate through the glass covers and absorbed by a blackened metal tray kept inside the �solar box. The solar radiation entering the box is of short wavelength. The larger wavelength radiation is not able to pass through the glass cover i.e. reradiaiton from absorber plate to outside the box is minimised by providing the glass cover. Two glass covers are provided to again minimise the heat loss. The loss due to convection is minimised by �making the box air tight by providing a rubber strip all round between the upper lid and box. Insulating material like glass-wool, paddy husk, saw dust or any other material is filled in the space between �blackened tray and outer cover of the box. This minimises heat loss due to conduction. When this type of cooker is placed in the sun, the blackened surface starts absorbing sunrays and temperature �inside the box starts rising. The cooking pots, which are also blackened, are placed inside with food material. The material get heat energy and food will be cooked in a certain period of time depending upon the actual temperature attained inside. The temperature attained depends upon the intensity of solar radiation and material of insulation provided. The amount of solar radiation intensity can be increased by providing mirror or mirrors. The solar cooker �is made up of inner and outer metal or wooden box with double glass sheet on it. Absorber tray (blackened tray) is painted black like boiler interior paint. This paint should be dull in colour �so that it can withstand the maximum temperature attained inside the cooker as well as water vapour coming out of the cooking utensil. The top cover contains two plain glasses each 3 mm thick fixed in the wooden frame with about 20 mm �distance between them. The entire top cover can be made airtight with padlock hasp. Neoprene rubber sealing is provided around the contact surfaces of the glass cover and the cooker box.A small vent for the vapour escape is provided in the sealing. Collector area of the solar cooker is increased �by providing a plane-reflecting mirror equal to the size of the glass frame. A 15 to 250C rise in temperature is achieved inside the box when reflector is adjusted to reflect the sunrays into the box. In winter when sunrays are much inclined to horizontal surface, reflector is a most useful addition. Overall dimensions of a typical model are 60 X 60 X 20 cm. This type of cooker is termed as family solar �cooker as it cooks sufficient dry food materials for a family of 5 to 7 people. The temperature inside the solar cooker with a single reflector is maintained from 700C to 1100C above the ambient temperature. This temperature is enough to cook food slowly, steadily and surely with delicious taste and preservation of nutrients.

Reflecting Mirror

Glass Covers Guide for Adjustment of Reflecting Mirror

Handle

Cooking Pots 60 cm

20 cm

60 cm

Fig. 3.15 Details of box type cooker

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Following are some advantages of solar cooker:• no attention is needed during cooking as in other devices �no fuel is required �negligible maintenance cost �no pollution �vitamins of the food are not destroyed and food cooked is nutritive and delicious with natural taste �no problem of charring of food and no over heating �

Following are some limitations of solar cooker:• One has to cook according to the sunshine; the menu has to be preplanned. �One cannot cook at short notice and food cannot be cooked in the night or during the cloudy days. �It takes comparatively more time. �For chapattis are not cooked because of high temperature for baking is required and also needs manipulation �at the time of baking.

3.5.6 Solar Water Heating Systems

Solar water heating is the direct use of solar energy that has been practised most extensively in the last two • decades. It is the most viable of all low temperature solar energy application. The demand for hot water is relatively constant throughout the year. Thus the collector and the other parts of the solar water heater will be working harder and longer to produce the savings in fuel that eventually must pay for the higher initial cost of the system. A solar water heater can also be sized to fit the demand more accurately. Although heating systems deal with • extreme loads only a few days of the year, they will have to be large enough to meet those extremes. A solar water heating system, on the other hand, will have roughly the same load day in any day out; except in unusual application, the design load (expected maximum load), should be close to the normal daily load.A problem common to all types of solar heating is the variable nature of sunshine. Solar water heater, however, • often have an additional advantage over solar space heating systems because the requirements for hot water are less rigid than those for space heating. For relatively constant hot water demands, the common solution is to provide an auxiliary heater. Solar water heaters are quite common in Australia, Israel, Japan and in Florida and Southern California of U.S.A.The basic elements of a solar water heater are:•

Flat plate collector �Storage tank �Circulation system and auxiliary heating system �Control of the system �

Types of solar water heaters• Black rubber hose as solar water heater: This is one of the simplest solar water heaters. By adjusting the �rate of flow through the hose, one can get a steady stream of hot water. The hot water can be stored in uninsulated or insulated storage tank, with or without auxiliary heater.Shallow trough of water: It consists of a shallow trough of water with a transparent cover sitting in the sun �as shown in Fig. 3.16.Japanese style plastic water bags: These are set on level platform. Some have reflectors below to reflect �additional energy upto the bottom side as shown in Fig. 3.17.Trough type: A variation of the type 3 is a simple wooden box with a sheet of plastic tacked to the inside �to hold the water (Fig. 3.18). The heater does not require a transparent cover, but it will be more efficient if it has one. In these heaters, the collector and the storage are one and the same.Japanese pipe solar water heaters: Japanese typically take bath in the evening because of the humidity and �therefore use water heaters which are nothing but glass, stainless steel, G.I. or plastic pipes blackened as shown in Fig. 3.19


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The major disadvantage of flat-basin heaters is that they must be horizontal. In the tropics, where, the sun is • high overhead all year, this disadvantage is minor, but in higher latitudes, where the sun is lower in the sky, a horizontal collector becomes less efficient, partly because much of the energy is reflected of the collector. In the winter when the sun is lower, efficiency decreases even more. An advantage of the flat basin type is that the depth of water is easily variable.

Shut offHot water

Over flow

Cold waterinlet

Fig. 3.16 Shallow trough of water

Fig. 3.17 Plastic water bags

Fig. 3.18 Trough type

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Reflectinginsulating cover

Hot wateroutlet

Cold waterinlet

Blackenedcylinder filledwith water

Fig. 3.19 Japanese pipe solar water heaters

Some typical and commercial designs of solar water heaters are:• Natural circulation solar water heater (pressurised) �Natural circulation solar water heater (non-pressurised) �Forced circulation solar water heater �

Natural circulation solar water heater (pressurised)• A natural circulation system is shown in Fig. 3.20. It consists of a tilted collector, with transparent cover �plates, a separate, highly insulated water storage tank, and well-insulated pipes connecting the two. The bottom of storage tank is at least a foot (0.4m) higher than the top of the collector, and no auxiliary energy is required to circulate water through it. Circulation occurs through natural convection, or thermosiphoning. When the sun heats water in the collector, �it expands (becomes less dense) and rises up the collector, through a pipe and into the top of the storage tank. This forces cooler water at the bottom of the tank to come out through another pipe leading to the bottom of the collector. This water in turn is heated and rises up into the tank. As long as the sun shines, the water will quietly circulate and got warmer. After sunset, a thermo siphon system can reverse its flow direction and lose heat to the environment during �the night. To avoid the reverse flow, the top heater of the absorber should be at least 1 ft (0.4 m) below the cold leg fitting on the storage tank, as shown. To provide heat during long, cloudy periods, an electrical immersion heater can be used as a backup for the �solar system. A non-freezing fluid should be used in the collector circuit. The thermosiphon system is one of the least expensive solar hot-water systems and should be used whenever possible.

Fig. 3.20 Schematic of a natural circulation solar water heater with auxiliary energy added to the storage tank (Pressurised)


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Natural circulation solar water heater (non-pressurised) • The pressurised system is able to supply hot water at locations above the storage tank. This creates �considerable stress on the water channels in the collector, which must be designed accordingly. The non-pressurised systems, supply hot water by gravity flow only to users lower than the tank. If pressurised �hot water is required (for showers or appliances) the difference in height will have to be large enough to meet the requirements. If the difference in height cannot be accommodated, the only solution is to install a separate pump and pressure �tank. The stresses within non-pressurised system are lower which allows cheaper and easier construction.

Tank Hot waterto load

Aux. Heater

One waycheck valve

Cold water

Collec

tor

Fig. 3.21 Non-pressurised solar water heater

Forced circulation solar water heater• Here there are no requirements for location of the tank above the collector. The additional components would �include a pump, motor and a pump controller (a differential thermostat between tank and collector). A check valve is needed to prevent reverse circulation and resultant night time thermal losses from the �collector. In this example, auxiliary heater is provided to the water leaving the tank and going to the load. A typical solar water heater with details is as shown in figure below.

Fig. 3.22 A typical solar water heater

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3.6 Energy Balance EquationThe thermal performance of any type of solar thermal collector can be evaluated by an energy balance equation • that determines the portion of the incoming radiation delivered as useful energy to the working fluid. For a flat plate collector of an area Ac , this energy balance on the absorber plate is

Ic Ac τs αs = qu + qloss + dec/dt Where, Ic = solar radiation on a collector surface. τs = effective solar transmittance of the collector cover(s). αs= solar absorbance of the collector –absorber plate surface. qu= rate of heat transfer from the collector- absorber plate to the working fluid. qloss = rate of heat transfer (or heat loss ) from the collector – absorber plate to the surroundings. dec/dt = rate of internal energy storage in the collector.

The instantaneous efficiency of a collector ç• c is simply the ratio of the useful energy delivered to the total incoming solar energy, or

ηc = qu / Ac Ic

In practice, the efficiency must be measured over a finite time period. In a standard performance test, this period • is on the order of 15 or 20 min., whereas for design, the performance over a day or over some longer periods is important. Then we have for the average efficiency

ηc = Integral ( qu dt) / Integral (Ac Ic dt) Where, ‘t’ is the time period over which the performance is averaged.

Thermal losses from the collector:• In order to obtain an understanding of the parameters determining the thermal efficiency of a solar collector, �it is important to develop the concept of collector heat –loss conductance. Once the collector heat-loss conductance U � c is known, and when the collector plate is at an average temperature Tc, the collector heat loss can be written in the simple form

qloss = Uc Ac (Tc - Ta)The simplicity of this relation is somewhat misleading because the collector heat-loss conductance cannot �be specified without a detailed analysis of all the heat losses. Fig. 3.23 shows a schematic diagram of a single-glazed collector, while Fig. 3.24 shows the thermal circuit with all the elements that must be analysed before they can be combined into a single conductance element shown in Fig. 3.24 (inset).

Glass Cover Sun

Working fluid

Insulation

Tube

Collector plate

l1

l2

Fig. 3.23 Schematic diagram of solar collector


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Ambient temperature(T )a

Cover temperature(T )g

R2t

Coverreflection

Coverabsorption

R1t

Is

Absorberplate reflection

Usefulenergydelivery(q )u

Collector bottom

housing temperature

Collectortemperature(T )

c

Ambient temperature (T )a

R2b

R1b

A

Ta

Is

Tsqu

B

R =1Uc

Fig. 3.24 Thermal circuits for flat plate collector

In order to construct a model suitable for a thermal analysis of a flat-plate collector, the following simplifying • assumptions will be made:

The collector is thermally in steady state. �The temperature drop between the top and bottom of the absorber plate is negligible. �Heat flow is one-dimensional through the cover as well as through the back insulation. �The headers connecting the tubes cover only a small area of the collector and provide uniform flow to the �tubes.The sky can be treated as though it were a blackbody source for infrared radiation at an equivalent sky �temperature.The irradiation on the collector plate is uniform. �

3.7 Economics of Solar Thermal SystemsOnce a solar heating system has been designed, its cost analysis should be done in order to:•

optimise the system �compare it with the total cost of a conventional heating system �

The additional amount of money invested in solar system is determined. The added cost of the solar system • calculated on per year basis is compared with the cost of fuel saved each year. From this comparison it can be determined whether or not solar heating is economically viable for a given system in a given location throughout the predicted life of the system. This approach is called life cycle costing. Life cycle costing, rather than initial investment costing, is the appropriate way to determine the costing benefit • ratio for a solar system because initial investment costing does not take into account the cost of the fuel saved during the life of the solar system.

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The annual cost of delivering solar heat, that is of owning, operating and maintaining the solar heating system • is given by equation.

Csa = [Cc Ac + Cst + CE] I + PCp + Cmm + CMLWhere, Csa = the annual cost of the solar energy system. Cc = Collector cost per unit area of collector. Ac = Collector area Cst = the capital cost of storage [Medium, container and insulation] CE = the capital cost of equipment pumps, piping, ducts, control and so on. I = Fraction of the investment to be charged per year, interest and depreciation. P = the annual power requirement of the solar energy system. Cp = the unit cost of the power. Cmm = the annual cost of the maintenance of materials CML = the annual cost of maintenance labour.

The first factor of cost is that of the owning it. This is usually the largest of all the items of annual cost. The • next important item is the annual power cost. It is usually the largest of all the items of annual cost. The next important item is the maintenance. It is usually small in well-designed system, particularly for water heating system. The maintenance costs are low for well-engineered system. The term I is given by

I = id (1 + id) t / (1 + id)

t –1Where, id = annual interest rate Rs/Re/yr. t= expected life time of solar system in years. The factor I is also called as the capital recovery factor.

The cost of each unit of energy delivered by the solar heating system can be determined by dividing the annual • cost of the solar system Csa by the total energy delivered by the system during the year.

3.8 Principle and Operation of Solar CellsSolar cell is the semiconductor device, which converts the radiant energy into the form of electrical energy. The • radiant energy is given by the relation E = hν. The electrical energy device consists of the source of electrons and the sink of electrons. When source and sink are connected through external circuit, there flows the current. So the requirement is to produce the sources of electrons and holes at the consumption of the radiant energy. • This can be done with a semiconductor. When a photon of energy hν, greater than the band gap energy Eg of a semiconductor is absorbed, a pair of electron and hole is generated. However, the electron with negative and hole with positive charge attract and annihilate immediately. As a result though the electrons and holes are generated within a single piece of a semiconductor by the absorption • of radiant energy one cannot get externally the electrons to flow. However, one creates a situation wherein the electrons and holes are immediately separated produces electricity. This is done with junction device. When a semiconductor junction is formed due to the transport of the charges across the junction to set the • equilibrium i.e. to make the Fermi levels at equal energy level, the donors are ionised at interface within the n-type semiconductor and acceptors are ionised ate the inter face within the p-type semiconductor. This gives rise to a local field, also called as built in potential VD. When the light of energy hν > Eg is made to incident within this interfacial layer the pair of electron and hole • are generated and are separated due to the influence of local electric field as shown in Fig. 3.25The n-type side of a junction acts as a source of electrons while the p-type as a sink of electrons and when these • two sides are connected through external circuit, the current flows through external circuit.


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Fig. 3.25 p-n junction with influence of local electric field

3.9 Solar Cell Electrical CharacteristicsEquivalent circuit•

By connecting a load across the terminals of a solar cell a current I � L can flow through the load and develop a voltage VL across it. The values of VL and IL, besides depending on the nature of the load, will be related to the photo generated current Iph and the properties of the diode. These relation ships can be established by reference to the simple equivalent circuit as shown in figure below.

IL

V VL

R1 RL

Rs

d

+Ve

Fig. 3.26 Equivalent circuit of solar cell including series and shunt resistances

The photo current I � ph is represented by a current generator and its magnitude depends on the wavelength and intensity of the incident light, optical absorption co-efficient of the solar cell material, the junction depth, the width of the depletion region and the life time and the mobility of the carriers on both sides of the junction. The polarity of the output photo-voltage VL is such that the diode is in forward bias condition. The current flowing through the diode is represented by Id and is given by

Id = Io Where, I0 is the reverse saturation current. K is the Boltzman constant. n is the junction ideality or perfection factor.

Shunt resistance paths are represented by R• sh; they can be caused by surface leakage along the edges of the cell, by diffusing spikes along dislocations or grain boundaries or possibly by fine metallic bridges along microcraks, grain boundaries or crystal defects such as stacking faults after the contact metallisation has been applied. The series resistance, Rs, can arise from contact resistances to the front and back, the resistance of the base region

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itself, and the sheet resistance of the thin diffused or grown layer. The load resistance is represented by RL and it is desirable to choose its magnitude such that maximum power is extracted from the solar cell. From the equivalent circuit of Fig. 3.26, a relation can be written between output photo current IL and output photo voltage VL as

IL = =

Methods for obtaining I - V characteristics of solar cells• Current voltage characteristics for solar cells can be obtained by three different methods namely �

Photo voltaic output with constant illuminationi. Photo voltaic output with variable illumination ii. Diode forward characteristics without illumination iii.

Photo voltaic output with constant illumination• This is the most commonly used method, applies a fixed illumination. Usually of known intensity, and a resistive load which is varied between short circuit and open circuit conditions, while measuring the voltage across the solar cell terminals and the current out of these terminals. Fig.3.26 shows the circuit diagram for this type of measurement including the generally applied equivalent circuit diagram for the solar cells. The I-V characteristics obtained in this manner are called as “photo voltaic output characteristics”. For an ideal solar cell the shunt resistance is very high and hence its contribution in I-V characteristics can be neglected. Since a solar cell acts as a generator in this method, the I-V characteristics are obtained in fourth quadrant of the I-V plane.

Photo voltaic output with variable illumination• This method appears more sophisticated than the previous one. Solar cell is illuminated with variable light intensity. The amount of illumination need not have to be known, if the value of the light generated current Iph can be determined. This condition is fulfilled when Rs is sufficiently is small. Figure below shows the circuit diagram for this type of measurement. It consists of a switch, a high resistance voltmeter and low resistance current meter.

V

Rs+Ve

IdIph

-Ve

Fig. 3.27 Circuit diagram for photovoltaic with variable illumination

The short circuit current, I � sc and open circuit voltage Voc are measured for every light intensity setting. Each pair of corresponding Isc and Voc values are plotted as one point in the first quadrant of the I-V plane and the series of measurements at different intensities give a curve similar to that of the forward bias I-V characteristics of a solar cell in dark.

Diode forward characteristics in dark• This method tests the solar cell like a diode in dark by applying external d. c. power supply in the forward �bias condition. I-V characteristics obtained by this method fall into the first quadrant of the I-V plane

The current- voltage characteristics of a solar cell obtained from above three methods are shown below


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Third method

Second method

First m ethodTe

rmin

al c

urre

nt (m

A)

3

2

1

Terminal voltage(volts)76543210

2.5

5.0

7.5

100

125

150

175

200

Fig. 3.28 The current voltage characteristics obtainable on the same solar cell by three different methods

Solar cell output parameters• In general, the photo-voltaic output properties are studied with the help of experimental circuit diagram shown in Fig. 3.26. Under constant illumination the load resistance RL is varied from zero to infinite ohms and corresponding IL and VL pairs are recorded. The pair of IL and VL at a load resistance RL gives point in the IL – VL plane. These points when plotted in fourth quadrant of IL – VL plane, give rise to a photo-voltaic output characteristic as shown in figure below. For an ideal solar cell, the series resistance Rs is minimum or zero.

VLVOC

Pmax

Imp

IL

Vmp

Isc

Fig. 3.29 Terminal properties of p-n junction solar cell in the dark and when illuminated

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Effect of Resistance on the Performance of Solar Cells• Resistive component in solar cells result in power dissipation and hence impair the performance of the cells. There are two principle resistive components:

series resistance R � s shunt resistance R � sh

The series resistance consists of mainly bulk resistance of the diode and contact resistance.

3.10 Types of Solar CellsThe key parameter involved in the principle operation of solar cell is the existence of the local field or the built • in potential needed to separate the electrons and holes generated due to incidence of photons on a semiconductor. This built in potential can be generated by making the junctions and by the virtue of the nature of the junction; the solar cells can be grouped into the following four categories namely:

3.10.1 Semiconductor - Semiconductor Junction Cells

There are two types of S-S junction cells namely homo junction and hetero junction cells, the homo junction • cell normally consists of a shallow p-n junction formed either by diffusion of dopant into a mono-crystalline semiconductor substrate or by growth of an epitaxial layer onto the substrate. Silicon and gallium arsenide are two common materials used with the diffused silicon cell, being the most • popular. A hetero junction consists of the inter face between two dissimilar semiconductors. Depending on the nature of the interface the hetero junction cells can further be classified as abrupt or graded • according to the distances during which the transition from one semiconductor to other is completed near the interface. For example, in the former case the transition occurs within a few atomic distances (d” 1 ìm) while in latter it • takes place over distances of the order of several diffusion lengths. Hetero junctions are fabricated by growth or diffusion of one semiconductor onto the other.

3.10.2 Semiconductor- Metal junction Cells (Schottky Barrier cells)

Semiconductor –metal junction cells which can be modelled, in first order approximation, as Schottky barrier • cells, are probably the simplest of all types to fabricate, requiring only an ohmic contact at the back and a semitransparent metal at the front, along with usual contact grid pattern to lower the series resistance. The transparent metal film is normally evaporated onto the carefully prepared semiconductor surface and films • of about 100 A0 thickness yield transmission of about 5-50 ohms/square cm. The thin semi-transparent metal layer permits passage of solar photons and forms the transition region in the • semiconductor for collection of generated carriers. Because of the presence of the built in field, the collection efficiency for carriers excited in the barrier transition region will be 100%. A major loss of photons is due to incomplete transmission of barrier metal layer, which is required to be thick enough for low series resistance.

3.10.3 Semiconductor-liquid Junction Cells

In recent years semiconductor-liquid junction cells have been attracting a great deal of interest in the field of • solar energy conversion as the photo-voltaic behaviour of a semiconductor liquid electrolyte interface can be utilised either to affect photo electrolysis, where light energy is converted into chemical energy in the form of the free energy of resultant products or to convert light energy onto electrical energy through the use of electrochemical photo-voltaic (ECPV) cells. It essentially consists of a semiconductor photo electrode and metallic counter electrode dipped into an electrolyte. • Charge transfer at the semiconductor electrolyte interface produces a band bending VB as a result of formation of space charge region, establishing a potential barrier. The direction of the field is such that for n-type semiconductor, holes generated in the space charge region • move towards the interface and excess electrons move towards the bulk. Though the semiconductor-liquid junction cells are superior to the p-n junction cells in many respects, there are two major problems associated


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with semiconductor –liquid junction cells namely the photo corrosion that occurs when the semiconductor is immersed in an electrolyte is subjected i. to illumination andthe over voltageii.

3.10.4 MIS and SIS Solar Cells

The most common way of forming a solar cell is to create a p-n junction by high temperature diffusion (800-• 11000C), where the conductivity type of the base semiconductor is changed to the positive type. Since this process is complicated and costly one, there has been a steady search for alternate and potentially • lower cost methods of forming a photovoltaic barrier or junction at low temperature. One of these techniques consists of inducing conductivity type change at the surface of semiconductor by the • application of an ultra thin metal or a relatively thick transparent (wide band) conducting semiconductor. This configuration is illustrated in figure below.

Sun light

Thin metal

Thin insulator Oxide semiconductor

P-type(Induced)

-

+

Fig. 3.30 Structure of MIS or SIS solar cell with thin interfacial layer

Such cells rely on thin interfacial layer (10 - 30 A � 0) between the top “inducing” contact (metal or conducting semiconductor) and the base semiconductor. This interfacial layer is generally oxide or some other compound which is normally an insulator in its bulk form. Hence these cells are referred to as metal-insulator-semiconductor (MIS) or semiconductor-insulator semiconductor (SIS) solar cells.

3.11 Solar PV SystemSolar cell module•

This module is smallest non-divisible, self contained, environmentally protected unit with a transparent cover. Several solar cells and with interconnected in series, parallel, series-parallel. This module has two terminals and delivers contain DC output when exposed to full sunlight.

Pm = n Pc …… Watts Where, Pm = power of one module, watts Pc = power of one cell, watts n= Number of cells in module.

Solar Array• An array has several modules connected in series, parallel, series/parallel and delivers DC power through two terminal leads.Power of an array Pp

Pp = n m Pc Where, n = number of cells in module m = number of modules in a panel

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Configuration of the solar PV array (solar collector) is selected to obtain desired voltage and current by suitable series, parallel connections of PV modules.

Single module Module voltage equal to load voltageSeveral modules in series Voltage of array is higher

Several modules in parallel to form an array voltage is same as of one module Higher current supplied to load

Several arrays in panel - Voltage to load same as that of an array- Current of arrays are added

Table. 3.1 Configuration of PV array and their resulting voltage current production

Battery Storage• The simplest means of storage on a smaller moderate scale is in electric storage batteries, especially as solar cells produce the direct electric current required for battery charging. The stored energy can then be delivered as electricity upon discharge. The common lead acid storage batteries, such as are used in automobiles, are not ideal for this purpose, but they are probably the best presently available. Extensive research in progress should lead to the development of more suitable batteries.

A possible alternative is to use the direct current from solar cells to decompose water (by electrolysis) into hydrogen and oxygen gases. These gases would be stored in a suitable form and utilised as needed to generate electricity in fuel cell.

Inverters and Power Conditioning• These are the devices usually solid state, which change the array Dc output to AC of suitable voltage, frequency and phase to feed photovoltaically generated power into the power grid or local load as shown in Fig..3.31. These functional blocks are sometimes referred to as power conditioning. A general type of inverter circuit, which is found best suitable for utility application in figure the current, can be used in two modes:

as an inverter changing Dc to AC or �as a rectifier changing AC to DC, thus changing the battery. �

SS

S

VSystem

reactance

3 - phaseA.C. System

D.C.Source

Smoothing inductor

Fig. 3.31 Current-field line commutated inverters

It is clear that the system photovoltaic offer the options of DC power, AC power, and hydrogen and oxygen fuels in either gas or liquid forms from which electricity can be generated.


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Each PV array has its inverter. The ratings of an inverter depend on the output of its array. The range of capacity of inverters for various arrays is 1.3 kW to 80 kW. The important features are:

Control and regulating features• Microprocessor base controls are used• Fundamental reactive power at 50 Hz• Harmonic filters•

The efficiency of inverter is 80 to 90 per cent for loads above 20 per cent. For smaller arrays self commuted inverters with pulse duration modulation and pulse frequency of several kHz is preferred.

Tracking Array• There may be tracking arrays or modules or fixed arrays. A tracking array is defined as one, which is always kept mechanically perpendicular to the sun array line so that all times it intercepts the maximum insolation. Such arrays must be physically movable by a suitable prime mover and are generally considerably more complex than fixed arrays. A fixed array is oriented east west and tilted up at an angle approximately equal to the latitude of the site.Fixed arrays are mechanically simpler than tracking arrays. Thus the array design falls in to two broad classes:

Flat-plate Arrays: Wherein solar cells are attached with a suitable adhesive to some kind of substrate �structure usually semi- rigid to prevent cells being cracked. This technology springs from the space-related photovoltaic technology, and many such arrays have been built in various power sizes.Concentrating Arrays: � Wherein suitable optics, e.g., Fresnel lenses, parabolic mirrors, compound parabolic concentrators (CPC), and others, are combined with photovoltaic cells in an array fashion. This technology is relatively new to photovoltaic in terms of hardware development, and comparatively fewer such arrays have actually been built.

3.12 Applications of Solar Photo-voltaic SystemGeneral applications (small scale)•

In spite of the high initial cost, photovoltaic systems are being used increasingly to supply electricity for many applications requiring small amounts of power. Their cost effectiveness increases with the distance of the location (where they are to be installed) from the main power gridlines. For example, studies shows it is more economical to install a stand-alone PV system instead of transmission line to a village having a load of 10 kW, if the village is more than 40 km from the grid line.Some applications for which PV systems have been developed are:

pumping water for irrigation and drinking �electrification for remote villages, for providing street lighting and other community services �telecommunication for the post and telegraph and railway communication network �radio beacons for ship navigation at ports �weather monitoring �battery charging �

In addition, in developed countries solar cells are being used extensively in consumer products and appliances.The major application of photovoltaic systems lies in water pumping for drinking water supply and irrigation in rural areas. The photovoltaic water pumping system essentially consists of

a photovoltaic array �storage battery �power control equipment �motor pump sets �water storage tank �

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Electricity generation• A basic photovoltaic system integrated with the utility grid is shown in Fig..3.32. It permits solar generated electrical power to be delivered to a local land. It consists of:

Solar Array, large or small, which converts the insolation to useful D.C. electrical power. �A blocking diode, which lets the array generated power flow only towards the battery or grid. Without a �blocking diode the battery would discharge back through the solar array during times of no insolation.Battery storage, in which the solar generated electrical energy may be stored. �Inverter or converter usually solid state which converts the battery bus voltage to AC of frequency and �phase to match that needed to integrate with the utility grid. Thus it is typically a DC, AC inverter. It may also contain a suitable output setup transformer, perhaps some filtering and power factor correction circuits and perhaps some power conditioning i.e. circuitry to initiate battery charging and to prevent over charging. Power conditioning may be shown as separate system functional block. This block is also used to function as a rectifier to charge a battery from the utility feeder when needed and when no insolation was present.Appropriate switches and Circuit Breakers, to permit isolating parts of the system, as the battery. One would �also want to include breakers and fusing protection (not shown) between the inverter output and the utility grid to protect both the photovoltaic system and the grid.

Blockingdiode

From utilityfeeder

Inverterconverter

Localload

Battery storage

Solar cell array

Fig. 3.32 Basic photovoltaic system integrated with power grid

3.13 Advantages and Limitations of Solar PV SystemsThe advantages of PV systems are•

uses clean, cheap, noiseless, safe, renewable solar energy to produce electrical energy at the location of the �utilisation, conservation of non-renewable fuelssuitable for remote loads away from main electrical network and at places where other fuels are scarce and �costly. cost of installation of long distribution lines, a distribution substation etc. is eliminatedsuitable for portable or mobile loads. e.g. radio sets, cars, buses, space crafts �reliable service, long life �mildest maintenance. �

The limitations of PV systems are• Irregular, intermittent supply of solar energy. �Need for storage batteries. �High capital cost due to large number of PV cells, low out put power, low efficiency and high technology �involved.


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Not economical for central power plants of MW rating due to very large areas of PV panels and very large �storage battery systems.Require storage batteries and or additional diesel generator sets for supplying power during night and during �cloudy periods.Do not generate power during cloudy seasons. Not suitable during rainy seasons. �Space for installing large PV panels is not available in large cities, industrial cities, etc. except on roofs of �buildings.Advanced PV technology is required for producing PV cells. �Very low efficiency of PV cells (10 to 14 %) �

3.14 Economics of Solar PV SystemsSolar PV systems as power sources are in use for a variety of applications.These include

Street lighting systems • Navigational aids• Community TV centers• Telemetry systems• On-site power supply • Space station power supply• Microwave repeater stations • Off shore oil-rigs• Cathodic protection installations for long gas pipe-lines•

Photovoltaic power supply systems have emerged as the chosen renewable technology for rural areas. While selecting the power supply system for the above applications at the planning stage, the economic analysis is usually carried out by comparing. The unit cost of supply by the following alternative supply systems for the particular remote site is.

Solar PV power supply system �Diesel electric power supply system �Grid supply to particular remote site �

Energy Requirements• The energy requirement of the particular remote load is estimated. For example,

Electrical energy requirement of a rural village = 3650 kWh / year �Microwave repeater station in remote area = 6300 kWh / year �

Comparison of Capital Cost• Cost factors to be considered in comparison should include the following:

Solar PV System Diesel Electric System Grid Supply System

1. Capital cost for equip-ment and installation -Interest-depreciation

- Solar PV systems- Battery system for storage

-Diesel genera-tor set

-Grid substation-Transmission and distribution lines upto the load site

2. Running cost -Fuel cost Nil -fuel cost at site - Cost of electric supply at site per unit- Operation and maintenance cost

Table. 3.2 Comparison of cost factor

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For remote locations, grid supply cost is higher because of additional capital costs of substations and transmission lines etc. are significant factors. For Diesel Electric Power Supply System the capital cost of diesel electric generators and running cost of fuel are significant factors. Solar PV systems are expected to be more and more popular during the coming decades and will become a very common source of electrical energy, for places where a solar panel can be installed.


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SummarySolar thermal energy, or solar hot water, uses flat collector plates to harness the sun’s energy to heat water for • use in businesses, homes, and pools. The installation and appearance are much like those of the PV panel, and the collectors are best installed facing south, under unobstructed sunlight. Photovoltaic (PV) panels convert sunlight to electricity that can be used to supplement or replace the electricity • supplied by the utility grid. PV panels are most commonly installed on rooftops, and are most effective with a southerly exposure that provides full sun. Other possible installations include a ground mount, a pole mount, and atop a porch, carport, or other shaded area. Energy conversion devices, which are used to convert sunlight into electricity by the use of the photovoltaic effect, • are called solar cells. A single converter is called solar cell or more generally, a photovoltaic cell, and combination of such cells; designed to increase the electric power output is called a solar module or solar array.Solar collectors are classified basically in two different categories: flat plate or non-concentrating collectors and • concentrating or focusing collectors consisting ofMechanical energy can be converted to potential energy and stored in elevated fluids. Product of solar process • other than energy may be stored. For example, Distilled water from a solar still may be stored in tanks until needed.The solar pond is a simple device for collecting and storing solar heat. Natural ponds convert solar radiation • into heat, but the heat is quickly lost through convection in the pond and evaporation from its surface. A solar pond, on the other hand, is designed to reduce convective and evaporative heat losses so that useful amounts of heat can be collected and stored.The main advantage of solar furnace lies in the fact that heating is carried out without any contamination and • changing the position of the material in focus easily controls temperature. The solar furnace is an excellent means for studying properties of ceramics at high temperature above the range ordinarily measured in the laboratories with flames and electric currents.The distillation of salt water to recover portable water is accomplished by exposing thin layers of the salt water • (usually in black shallow trays or basins) to solar radiation, and condensing the water vapours produced on a transparent cover in such a way that it can be collected in receiving troughsBasically there are three designs of solar cooker: Flat plate box type solar cooker with or without reflector, • multi-reflector type solar oven and parabolic disc concentrator type solar cookerSolar water heating is the direct use of solar energy that has been practised most extensively in the last two • decades.When a semiconductor junction is formed due to the transport of the charges across the junction to set the • equilibrium i.e. to make the Fermi levels at equal energy level, the donors are ionised at interface within the n-type semiconductor and acceptors are ionised ate the inter face within the p-type semiconductor. This gives rise to a local field, also called as built in potential VD

Current voltage characteristics for solar cells can be obtained by three different methods namely photo voltaic • output with constant illumination, photo voltaic output with variable illumination and diode forward characteristics without illumination. The solar cells can be grouped into the following four categories namely: Semiconductor - semiconductor • junction cells, semiconductor-metal junction cells, semiconductor-liquid junction cells and MIS SIS cellsA module is smallest non-divisible, self contained, environmentally protected unit with a transparent cover. • An array has several modules connected in series, parallel, series/parallel and delivers DC power through two terminal leads.Photovoltaic systems are being used increasingly to supply electricity for many applications requiring small • amounts of power. The major application of photovoltaic systems lies in water pumping for drinking water supply and irrigation in rural areas. The limitations of PV systems are irregular, intermittent supply of solar energy, need for storage batteries, high • capital cost due to large number of PV cells, low out put power, low efficiency and high technology involved

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Solar PV systems as power sources are in use for a variety of applications. These include Street lighting systems, • navigational aids, community TV centres, telemetry systems, on-site power supply etc.

ReferencesLorenzini, G., C. & Flacco, B. G., 2009. • Solar Thermal and Biomass Energy. 1st ed., WIT Press.Willeke, G. & Grassi, G., 1987. • Photovoltaic Power Generation (Solar Energy and Development). Springer Publications. Photovoltaic vs. Solar Thermal Energy• , [Online] Available at: <http://www.kashongenergy.com/photovoltaic-vs-solarthermal.html> [Accessed 5 July 2013].Abstract for Solar Thermal and Solar Photovoltaic Systems,• [Pdf] Available at: <http://www.ensymm.com/pdf/chemical_biotech/ensymm_solar_thermal_and_photovoltaic_systems_abstract.pdf> [Accessed 5 July 2013].TheInnovationAcademy, 2013. • Maria Browne - A photovoltaic Thermal System, [Video online] Available at: <http://www.youtube.com/watch?v=Nb4JhVYNXTM> [Accessed 5 July 2013].SolarEnergyWhich, 2012. What is the difference between Solar Thermal and Solar Photovoltaic Energy Systems • ?, [Video online] Available at: http://www.youtube.com/watch?v=1LXP_X2Oa98> [Accessed 5 July 2013].

Recommended ReadingAgrawal, B. & Tiwari, G. N., 2010. • Building Integrated Photovoltaic Thermal Systems: For Sustainable Developments (RSC Energy Series), 1st ed., Royal Society of Chemistry publication.Merrigan, J. A., 1980. • Sunlight to Electricity: Prospects for Solar Energy Conversion by Photovoltaics. The MIT Press publication.Duffie, J. A. & Beckman, W. A., 2006. • Solar Engineering of Thermal Processes. Wiley Publication.


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Self Assessment______________ uses flat collector plates to harness the sun’s energy to heat water for use in businesses, homes, 1. and pools.

Photovoltaic panels a. Solar thermal energy b. Solar radiationc. Electrical energyd.

Match the Following2.

Flat plate collectors1.

One needs a field of mirrors provided with the A. means of directing reflected sunlight to a central location, or a location at one edge of the field of mirrors.

Cylindrical Parabolic collector2. possible to concentrate solar radiation by a factor B. of 10 without diurnal tracking

Central receiver collector3. The concentration ratios are very high and there-C. fore can be used where high temperatures are re-quired.

Compound Parabolic collector4. used typically for temperature required upto 75D. 0C1-A, 2-B, 3-C, 4-Da. 1-C, 2-D, 3-B, 4-Ab. 1-B, 2-A, 3-D, 4-Cc. 1-D, 2-C, 3-A, 4-Bd.

Which of the following is not a type of storage system?3. Thermal storagea. Electrical storageb. Mechanical storagec. Hydro storaged.

Which of the following statements is false?4. An artificially constructed pond in which significant temperature rises are caused to occur in the lower a. regions by preventing convection is called a “Solar pool”.The solar pond is a simple device for collecting and storing solar heat.b. Solar ponds promise an economical way around these two problems by employing a mass of water for both c. collection and storage of solar energy. Solar ponds may be classified into two types as convecting or non-convecting.d.

__________ is a device to collect solar energy with high intensity of solar radiation on the energy-absorbing 5. surface.

Centre collectora. Concentrating collectorb. Focusing collectorc. Optical collectord.

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A component used to increase the intensity of energy flux on a receiver is called6. reflectora. concentrator b. optical devicec. paneld.

___________ is the semiconductor device, which converts the radiant energy into the form of electrical 7. energy.

Solar unita. Solar circuit b. Solar conductorc. Solar celld.

________ is smallest non-divisible, self contained, environmentally protected unit with a transparent cover.8. Solar circuit modulea. Solar chamber moduleb. Solar cell modulec. Solar unit moduled.

Which of the following is not an advantage of PV systems? 9. Reliable service, long lifea. Mildest maintenanceb. Advanced PV technology is required for producing PV cells c. Suitable for portable or mobile loadsd.

A single converter is called solar cell or more generally, a photovoltaic cell, and combination of such cells; 10. designed to increase the electric power output is called a ___________.

solar unita. solar module b. solar sectionc. solar divisiond.


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Chapter IV

Wind Energy

Aim


explain the origin and types of wind •

explicate wind speed measurement and wind data analysis•

explain the principle and operation of windmills•

Objectives


define wind speed measurement and wind data analysis•

explain the principle and operation of windmills•

explicate the comparative performance of windmills•

Learning outcome


understand the origin and types of wind •

define wind speed measurement and wind data analysis•

describe the principle and operation of windmills•

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4.1 History of Wind EnergyAfter the fuel crisis in 1973, a search was been started for a clean, safe and renewable energy sources. Out of the available sources wind energy is found to be one of the better options. In past centuries wind energy was used in many applications. One of the most popular uses of wind energy was for transportation in sailing ships. Other uses of the wind energy were in agricultural as well as in rural applications, such as grinding flourmills, wood cutting saw, stone crushers, mixers, water pumps, irrigation facilities etc. Apart from these applications wind energy is used for driving windmills. In the last few decades conversion of wind energy into useful electrical energy has attracted the attention of people.

Some small windmills were used for electricity production ahead of rural electrification programme but the development of the internal combustion engine and steam engine for electricity production led to rapid decline in the use of the wind as a source of power in most parts of the world. In India by 1990’s wind energy to electrical energy has become economically competitive in areas of favourable wind (e.g. Maharashtra, Gujarat and Tamilnadu) and wind electric energy systems are now on the forefront of renewable energy utilisation. Several wind turbine generators have been installed throughout the world. In this way wind energy is supposed to be the best resource of future energy.

4.2 Origin and Classification of WindThe origin of wind is based on the 2• nd law of thermodynamics, i.e. in a nature no system will remain in non-equilibrium state; it will try to regain its equilibrium state either by loosing or gaining the energy. The earth’s atmosphere consists of the particulates of gases and atmosphere is extended a few kilometres high • above the surface of earth. However, the distribution of air particulate with respect to height of atmosphere is not constant, i.e. the density of air at the surface is higher than the density of air at height ‘h’.Though vertically there is a non-equilibrium distribution of particles, there is no vertical motion of particles • because there are two forces acting on every particle; i.e. up thrust force due to non-equilibrium condition and second one is gravitational force acting downwards.So, as such, every particle above the surface of earth is vertically in equilibrium position. Due to this equilibrium • situation, there is no vertical motion of particulates. However, if you consider the horizontal distribution then due to uneven heating on the earth the density of air at one place can be higher than other place and this horizontal no equilibrium situation is removed by the horizontal flow of particles from one place to other; i.e. from high density to low density till it attains the equilibrium. The flow of particles in the air is called wind. The wind energy is the indirect form of solar energy; i.e. the solar • energy is absorbed by the earth’s atmosphere with non-identical situation and hence the wind. Based on this principle of the origin of wind there are variety of winds and these can be classified mainly into • two groups.

Planetary winds �Local winds �

4.2.1 Planetary Wind

Earth is considered as a single planet surrounded with few km of atmosphere. The Earth is receiving energy • from Sun at equator and at North and South Poles in uneven manner. At the equator intensity of solar radiation is higher than at the north and south poles. As a result the atmosphere at equator gets more energy, due to this air at equator moves up vertically in the sky. • As a result of this the density of the air at equator becomes lower than the north and south poles. So from 2nd law of thermodynamics there is flow of particulates from equator to poles. This cycle of motions of air from pole – equator – pole is affected by the rotation of the Earth and as a result • we get the planetary wind on the surface of the Earth as shown in fig 4.1 Planetary Winds -L= low pressure area, H= high pressure area


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L

H

H

L

L

Prevailing Westerlies

Polar Easterlies

Northeast Trade Winds

60 S0

60 N0

30 N0

00

30 S0

Pole

Pole

Doldrums

Fig. 4.1 Planetary winds

From the above Fig., it is seen that the planetary wind are not uniform over the globe as a whole. There are • north-easterly wind and south-easterly wind. At the horse latitude the wind pattern is different. In addition to non-uniform of wind pattern on globe as a whole, there is also uneven availability of the wind throughout the year. Though the wind pattern is periodic function, it is uneven because of two reasons:•

The Earth is rotating around the Sun in the elliptic orbit; as a result the distance between Earth and Sun is �changing continuously during the period of year.The axis of rotation of the Earth with its plane of rotation around the Sun is not perpendicular but it makes �some angle. This gives rise to the change in the intensity of solar radiations at north and south poles affecting wind pattern.

4.2.2 Local Winds

The local winds are produced by the local changes in the atmosphere. These can be broadly classified into three • types:

Coastal winds �Hilly and mountain winds �Micro winds �

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Fig. 4.2 Local winds

Coastal winds• At the coastal area, there is sharp boundary between land and ocean. The solar radiations are incident on both �land and ocean. The absorption coefficient of solar radiation for land (αL) is very large than the absorption coefficient for water (αw). Due to this, land gets hotter than the ocean giving more kinetic energy to the air particles; as a result, air �particulate moves up in the sky and creates non-equilibrium situation and thus there is flow of wind from ocean to land. On the contrary, at night the situation is reverse as shown in fig.4.3. This is a periodic phenomenon; every �day and night, we experience lot of winds across the coastal area.

Local windsSun

Day-wind

ColdHot

Moon

Night-wind

HotCold

Fig. 4.3 Coastal Winds•


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Hilly and mountain winds• In the morning, half part of the mountain facing the east is heated more than the west side and this creates non- �equilibrium situation. The high-density air from shadow side moves to Sun exposed site of the mountain. Reverse will be the situation in the evening and thus we do experience the breezes of the wind everyday. �

Fig. 4.4 Hilly and Mountain Winds

Micro level winds• Managing the local situation to create the non-equilibrium situation one can generate micro level winds. �For example, the micro level wind can be generated by making the garden around the house. Similarly, �using the concept of solar chimney, one can generate a good wind with better velocity.

4.3 Wind Energy and Assessment

4.3.1 Wind Energy BasicsWind energy is a manifestation of the solar energy. Wind is the air in motion and possesses the kinetic energy given by the relation:

E = ½ mVf2

Where m is the mass of air particulates and Vf is the velocity of free wind. The power associated with the wind is the rate of change of energy with time,

i.e.

----------------------- -------- (4.1)

Where is the mass of air flowing through unit area in unit time and is given by the relation

i.e. ------------------------------------------------------------------------ (4.2)

Where r is the radius of area of cross-section through which wind is flowing and is the density of air. Substituting (4.2) into (4.1) the power associated with the wind is given

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-------------------------------------------------- (4.3)

From the above equation (4.3) it is seen that the power of wind depends on the cube of velocity of wind and also vary with the density of air.

4.3.2 Wind Speed Measurement

A reliable prediction of the annual energy yield of wind farms is only possible, if it is based on accurate on-site • wind speed measurements. Wind energy applications require a higher standard of wind speed measurements than it is necessary for meteorological purposes. Particularly, critical aspects are the selection of the measuring site, the selection and calibration of anemometers, • and the installation of the sensors on met masts. The costs involved in high quality wind speed measurements are small as compared to the reduction of the financial risk of wind farm projects. Anemometer is the device for measuring wind speed; it is shown in figure below.•

Fig. 4.5 Cup type anemometer

Cup and propeller anemometers are the two types of commonly used anemometers. The propeller rotates to a • parallel axis in propeller-type anemometer. While cup anemometers are mainly used by meteorological station and normally consists of three or four cups mounted symmetrically at right angles to a vertical axis. The force exerted by the wind is greater on the inside surface of the cup than on the outside so the cups rotate. The rate of rotation is directly proportional to the wind speed and thus the wind speed can be measured. Other anemometer types include ultrasonic or laser anemometers that detect the phase shifting of sound or • coherent light reflected from the air molecules. Hot wire anemometers detect the wind speed through minute temperature differences between wires placed in the wind and in the wind shade. The advantage of non-mechanical anemometers is that they are less sensitive to icing. In practice, however, • cup anemometers tend to be used everywhere, and special models with electrically heated shafts and cups may be used in arctic areas. The units used to measure wind speed are Beaufort scale and m/s (SI Unit)•


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4.3.3 Wind Data Collection

From equation 4.3, it is noted that the wind power depends on cube of wind velocity. Thus, in order to estimate • the wind energy, it is essential to collect the data of wind velocity. The velocity of wind has to be collected on hourly and daily basis. The instrument called anemometer is used for this purpose. One can measure initial and final reading for every • hour in a day. After this measurement of hourly wind velocities the average wind velocity is calculated by using the relation:

Where ‘n’ is number of readings.

The wind velocity is also measured in terms of energy velocity and it is given by the relation:•

In order to collect the wind data at a given place, it is essential to decide the height of measurement. Only at a • certain height, there will be maximum power of wind. At height H=0 the wind velocity is zero due to the friction and dragging force acting on the wind at the surface. At large height, the velocity may be the higher but the density of air may be smaller. As a result the power is • smaller. The variation in wind velocity and density of air with respect to height is shown below.

Denc

ity

Velo

city

Height

Fig. 4.6 Variation in wind velocity and density of air with respect to height

At optimum height for maximum power, the data will be collected on daily average basis throughout the year. • Then plot graph of wind velocity versus time of the year. A typical plot of distribution of wind velocity is shown in figure below.

Jan Feb M ar Apr May June July Aug Sept Oct Nov Dec

Fig. 4.7 A typical month wise distribution of wind velocity

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4.3.4 Wind Data Analysis

From the wind velocity curve one can calculate the velocity distribution curve; i.e. for how many hours in a year • the wind is available with a certain speed. The velocity distribution curve is shown in Fig..4.8 (a).From the velocity distribution curve one can calculate the power distribution by substituting the values of air • density ρ (ρ= 1.185 kg/m3 at) and values of Vf.Then plot the graph of P vs. time. The typical plot of power distribution is as shown in Fig..4.8 (b). This is called • as the power distribution curve.

Fig. 4.8 (a) Velocity distribution curve (b) Power distribution curve

From the power distribution curve one can estimate the wind energy at that place. Using the relation:•

Modifying this equation for energy estimate, one gets:E = ∫ Pdt

E = Area under the curve of Power distribution

4.4 Theory of WindmillDescribed below is the theory of windmill.

4.4.1 Wind Power Conversion

The wind has kinetic energy ½ mV• 2. This energy can be converted into translated energy and rotational energy. Translated energy is used for sailing of the boats in ocean and separating the food grains in agricultural farms. The clouds formed on the ocean are transported to land side by using translation energy of the wind. The wind energy is converted into rotational energy by putting the rotor into the wind. The wind is struck to rotor and extracts the power.During this process, there is modification in wind flow as shown below. It is seen that the wind velocity gets • modified due to rotor.

(a) (b)


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VfVx Vw

Rotor

Wake filed region

Fig. 4.9 Modification of wind flow near the rotor

Let V• f is velocity of free wind, Vr the velocity of wind at the rotor and Vw the velocity of wind at wake field region. As per the magnitude of velocities

Vf > Vr > Vw

The wind velocity at the rotor is less than the free wind and wind velocity at the rotor (Vr) is given by

Vr = Vf (1 - a) --------------------------------------------------------------------------------------- (4.4)

Where ‘a’ is constant called as axial induction factor. In addition Vr is average velocity of the initial and final velocities, given by

Vr = -----------------------------------------------------------------------------------------(4.5)

From the Newton’s law of motion, the force acting on the rotor is given by the relation:F = m x acceleration

F = m

F =

F = πr2ρ (Vf – Vw) Vr

F = πr2ρ Vf (1 – a) (Vf – Vw) ----------------------------------------------------------------------(4.6)

The Power extracted by the rotor is nothing but the rate of change of work done by the rotor

P = P = Force x Vr

P = πr2 ρ Vf2 (1 - a) 2 (Vf - Vw) ------------------------------------------------------------------- (4.7)

In equation (4.7), all the parameters are experimentally measurable except the wake field velocity Vw. The expression

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for Vw can be obtained by considering equations (4.4) and (4.5). Vr = Vf (1-a) =

= ---------------------------------------------------------------------------- (4.8)

Putting Equation (4.8) in (4.7) we get

P = πr2 ρVf2 (1-a) 2 [Vf -Vf (1-2a)]

P = 2π r2 ρVf3 a(1-a)2 ----------------------------------------------------------------------------(4.9)

In equation (4.9) all the parameters except the wind velocity are constant for a given windmill.Hence it is seen that;

P α Vf3 ---------------------------------------------------- (4.10)

From equation (4.9) it is seen that the extraction of wind power is very much dependent on axial induction factor ‘a’ which lies between 0 and 1 from the above relation, it is seen thatat a = 0, P = 0 and at a = 1, P = 0i.e. one can extract maximum power for the value of ‘a’ lying between 0 and 1.Thus P is a function of ‘a’. At maximum power, the mathematical condition is

Equation (4.9) can be written as

P = a (1-a) 2. X

where X = 2π r2 ρVf3 = constant, for a given wind mill

Thus, at maximum power, = (1-a) (1-3a) ∴ (1-3a) = 0, or (1-a) = 0

The later solution is practically not viable. Thus only mathematical solution which is viable experimentally, is (1 – 3a) = 0. This gives the value of a,

∴ a = for Pmax ------------------------------------------------------------------------------------------ (4.11)

Substituting equation (4.11) in equation (4.9) one gets the expression for maximum power of windmill as

Pmax = πr2ρVf3 ------------------------------------------------------------------------------- (4.12)

4.4.2 Performance Characteristics of Windmill

The wind energy is converted into rotational energy by a rotor called as wind machine. The axial of rotor is • connected to the coil of generator and produces the electricity. The performance of wind machine is measured in terms of the efficiency of the wind machine. It is the ratio of output Power and input power.

• η = --------------------------------------------------------------------- (4.13)


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The maximum power output of an ideal machine with its axial induction ratio, a = • is given by equation 4.12The input power to wind machine is power associated with the free wind and given by the relation (5.5). • Substituting the values of equations (4.3) and (4.12) into equation (4.13), we get the expression for the efficiency of wind machine as

∴ Efficiency = η =

η = × 100

η = 59.3 %

From the above discussion it is seen that the maximum efficiency of windmill is 59.3%. This is theoretical • limitation to the performance of wind machine. The performance of the wind machine is also measured in terms of the power coefficient (Cp). It is defined as the ratio of actual power extracted from wind machine to the power input from the wind. It is given by

Cp =

Cp = 4a (1 – a)2 ----------------------------------------------------------------------------------- (4.14)

The above equation reveals that the power co-efficient C• p depends on the axial induction factor a. in practice, it varies from 0 to 1. The variation of Cp with ‘a’ is shown in figure below. It is seen that Cp maximises at ‘a’ =0.33.

Fig. 4.10 Variation of CP with ‘a’

Further, the performance of the wind machine is found to depend on the geometry of wind machine and the • velocity of wind. It is given by the relation:

Cp = Cp (λ)

Where, λ is the ratio of speed of tip of wind machine to the velocity of free wind. If D is the diameter of wind machine and N is number of rotations per unit time then

λ =

From the figure, it is seen that the performance of the wind mill is very much dependent on V• f. Therefore, while designing and fabrication of the wind machine for a given place all the crucial parameters should be taken into consideration.

‘a’

C P

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Wind velocityC

P

0.60.5

0.4

0.3

0.2

0.1

0 Vc Vd Vf

Cpmax

Fig. 4.11 Variation of CP with wind velocity

4.5 Wind Power Technology

4.5.1 Working of Windmill

The energy from the wind cannot be obtained directly. Therefore, to extract the energy from the wind suitable • device called the windmill is used. It is the device, which converts the kinetic energy of the wind into mechanical energy, and this mechanical energy is converted into electrical energy. The simplest way to think about this is to imagine that a wind turbine works in exactly the opposite way to a • fan. Instead of using electricity to make wind, like a fan, turbines use the wind to make electricity. Almost all wind turbines producing electricity consist of rotor blades, which rotate around a horizontal hub. • The hub is connected to a gearbox and generator, which are located inside the nacelle. The nacelle is the large part at the top of the tower where all the electrical components are located. Figure below shows the important parts of the wind machine.

Fig. 4.12 Important parts of wind machine

Important parts of the windmills• A typical wind turbine generator (aero-generator) consists of the following parts.

Tower: The tubular towers are more popular among modern turbines because of their lower airflow �interference and downstream turbulence creation. Also, they seem to be more aesthetically acceptable.


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Rotor blade: Three-bladed wind turbines are more common because they are better understood �aerodynamically and also have a lower noise level than the two bladed turbines. These blades are made of glass reinforced plastic.Nacelle: It is the most important component of the turbine, situated on the top of the tower and holds the �rotor blades in place. It also houses the gearbox, the blade pitch unit, generator and most of the sensors. The nacelle with rotor is electrically yawed into or out of the wind.Transmission system: The rate of rotation of wind turbine is low, about 40 to 50 rpm. Generators typically �require rpm of 1,200 to 1,800. As a result, most wind turbines require a gearbox transmission to increase the rotation of the generator to speed necessary for efficient electricity production.Generator: It converts mechanical energy to electrical energy. Wind turbines may be designed with either �synchronous or asynchronous generator, and with various forms of direct or indirect grid connections. Control System: The modern large wind turbine generator requires a versatile and reliable control system �in order to achieve optimum power output to ensure safety operation, to protect from damage in high wind speeds and to ease maintenance.Yaw control: Most of the wind turbines are ‘yaw active’; that means they ‘track’ the changing direction of �the wind. In this case, a motor rotates the turbine slowly about the vertical axis so as to face the blades into the wind. The area of the wind steam swept by the wind is then maximum.Hub: Central solid part of the wheel (Propeller) �Propeller: Revolving shaft with blades. The blades are set at an angle and twisted. (Like thread of a �screw)

Types of windmill• In order to make the use of wind energy, there are varieties of designs of wind machines. These wind machines are classified mainly into two groups.

Vertical axis wind machine �Horizontal axis wind machine �

4.5.2 Vertical Axis Wind Machine

In these types of wind machines the axis of rotation of windmill is perpendicular to the surface of Earth. In this • group of machines there is variety of designs. It can be further classified as:Savonius vertical axis wind machine•

This is the simplest type of windmill and easy to construct. Take a cylinder, cut it into two parts and then �just shifting one part over the other. One can make to rotate the wind machine along its axis. The simplest type of savonius wind machine is shown in fig.4.13 (a). The forces acting on the cylinders of the windmill are shown in fig.4.13 (b). The wind blowing inside the �cup tries to rotate the vanes of the wind machine. But the outside surface of the other vain opposes to rotate the wind machine. However, the force of rotation is higher than the force of opposition. Hence the resultant force rotates the windmill. In order to reduce the opposition forces and make maximum �use of wind energy multiple Savonius type of wind machine is designed and fabricated, which is shown in fig.4.13 (c). In case of multiple Savonius wind machines the oppositional force is reduced and part of wind energy for its conversion is used in better fashion.The reduction of the opposition force is further made possible by using split savonius type wind machine. �In this type of machine the axis of rotation is replaced from the centre of half cylindrical vain. The typical design of the split type savonius machine is shown in fig.4.13 (d). The distribution of wind and its direction at the split savonius wind machine is shown in fig.4.13 (e). The �performance of the savonius wind machine is also dependent on the aspect ratio of wind machine. It is defined as the ration of the diameter of wind machine and length of cylinder of savonius wind machine.

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Fig. 4.13 Vertical axis wind mills (a) Simple savonius wind mill (b) Forces acting on the cylinders of wind mill (c) Multiple savonius type wind mill (d) Split type savonius machine (e) The distribution of wind and

its direction at the split savonius wind machine

Darrious vertical axis wind machine• In case of savonius type of wind machine due to large amount of moment of inertia, it requires large �amount of wind energy and does not operate at low wind speed. This drawback is avoided in Darrious type windmill. In 1920, the Darrious has designed this machine, which is operated at low wind speed, and NASA in America �has done same research to develop such machines. There are two type machines F and D type machine. The schematic Fig. is shown in fig.4.14

(a) (b)

Fig. 4.14 Types of darrious vertical axis wind machine (a) Φ type and (b) Δ type

H –Rotor vertical axis wind machine• Two vertical blades are joined by a horizontal beam to form a H-shaped rotor. This is supported at the centre �of the horizontal beam by a hub mounted on a tall tower. The whole H - rotor revolves about its vertical axis due to wind force on vertical arm of H rotor designed as aerodynamic foil.


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This type of wind machines has been developed in two versions. First one is with fixed H frame without �change in shape and another one is with the variable shape of rotor in terms of angle and inclination with respect to the vertical plane to control speed and power. Fixed type design is used for smaller power (15 kW– 450 kW) application while variable shape blade design �is used for higher (500 kW – 1000 kW).

Fig. 4.15 H –Rotor vertical axis wind machine

Fig. 4.16 Schematic diagram of H –Rotor vertical axis wind machine

Magnus vertical axis wind machine• This type of wind machine consists of spinning cylinder. Magnus first demonstrated this concept �experimentally in 1912. When cylinders spun in a wind stream, as a result of these transnational forces are produced perpendicular to the wind stream by Magnus effect as shown in fig. 4.17. Such a device can be used as a sail to propel ships or land vehicles. The concept consists of several tall �vertical cylinders that are rotated about their axes in the presence of wind.

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The resultant of the lift and drag forces, more in the direction of lift because of its larger value propels the �cylinders horizontally along the track.

Fig. 4.17 Magnus type vertical axis wind machine

4.5.3 Horizontal Axis Wind Machine

There are four types of horizontal axis windmills (HAWM).•

(a) (b) (c) (d)

Fig. 4.18 Horizontal axis wind mills (a) Mono blade (b) Twin blade windmill (c) Three blade windmill and (d) Multi blade windmill


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Mono Blade HAWM• The mono blade wind turbines have lighter rotor and hence favourable in price. Smallest mono-blade designs �are of 15 kW to 30 kW capacities. This kind of wind machines may be used in Wind park or individually for water pumping, battery recharging and power supply system in the remote places. A typical mono blade horizontal axis wind machine is shown in fig.4.18 (a). Wind vanes are used for �orientation to the direction of wind. Due to lightweight, the installation and dismantling is easy. Blade length is in the range of 15 m to 25 m. Mono-blade designs experience minimum stresses on the �bearing and gears. Designs are aerodynamically optimised with high tip speed to wind speed ratio. Blades are made of metal, glass reinforced plastics, laminated wood, composite carbon fiber/fiberglass etc. �to obtain light, economical and strong design. Service life is about 30 years.

Twin Blade HAWM• Such units are built in large unit sizes of unit ratings, such as 1 MW, 2 MW and 3 MW. Such high capacity �units shown in fig.4.18 (b) are installed singly and feed power into the distribution network. Twin blade wind machines are cost effective and lighter than that of the equivalent three blade machines. �This machine requires tethering control because of the following reasons: The wind speed increases with �increasing height. When rotor is vertical, the blade in upper position experiences a greater force than the blade in lower position. A pivot within the hub allows the rotor to lean backwards to accommodate the extra force. Without such tethering arrangement, additional fatigue on the main shaft could seriously affect the life of a twin blade machine

Three Blade HAWM• The rotor has three blades assembled on a hub. The blade tip has a pitch control for controlling shaft speed. �The shaft is mounted on bearing. The gear chin changes the speed from turbine shaft speed to generator shaft speed. The blades are long and are mounted on a hub with aerodynamic profile as shown in fig.4.18 (c). Three �blade horizontal axis designs have been built for entire range from small to very large units (15 kW to 3 MW unit size). Three-blade design does not give vibration problem during yaw orientation. Three blades design gives most �favourable design compromise between cost, power rating and operational reliability.

Multi Blade HAWM• This type of design for multi blades is shown in fig.4.18 (d). The blade of this wind machine is made from �sheet metal or aluminium. The rotors have high strength to weight ratios and been known to service hours of freewheeling operation in �60 km/hr winds. They have good power coefficient, high starting torque and added advantage of simplicity and low cost.

Various manufacturers have analysed the economics of twin bladed design versus three bladed designs. It is • probable that the three bladed versions capture slightly more energy than the twin bladed version. However, it costs slightly more (about 1.3 times). The three blade designs are preferred considering higher energy captures simpler design.

4.5.4 Comparative Performance of WindmillThe performance of the wind machine is measured in terms of the power coefficient (Cp). Tip Speed Ratio (tsr) is the ratio of blade tip speed to wind speed (= Vr/V). Fig.4.19 shows the performance characteristics of Cp versus Tip Speed Ratio (tsr) different windmills.

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0.6Cp Betz 59% limits

Ideal propeller

Darrieus

High speed propeller

Dutch four arm

American multiblade

Savonius

0.5

0.4

0.3

0.2

0.1

0 1 2 3 4 5 6 7 8

tsr

Fig. 4.19 Performance characteristics CP versus Tip Speed Ratio, of different windmills


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SummaryIn India, by 1990’s wind energy to electrical energy had become economically competitive in areas of favourable • wind (e.g., Maharashtra, Gujarat and Tamilnadu) and wind electric energy systems are now on the forefront of renewable energy utilisation. Several wind turbine generators have been installed throughout the world. In this way wind energy is supposed • to be the best resource of future energy.The flow of particles in the air is called wind. The wind energy is the indirect form of solar energy; i.e. the solar • energy is absorbed by the earth’s atmosphere with non-identical situation and hence the wind. Based on this principle of the origin of wind there are variety of winds and these can be classified mainly into • two groups: planetary winds and local winds.The planetary winds are not uniform over the globe as a whole. There are north-easterly wind and south-easterly • wind. At the horse latitude the wind pattern is different. In addition to non-uniform of wind pattern on globe as a whole, there is also uneven availability of the wind throughout the year. The local winds are produced by the local changes in the atmosphere. These can be broadly classified into three • types: - coastal winds, hilly and mountain winds and micro winds.Wind energy is a manifestation of the solar energy. Wind is the air in motion and possesses the kinetic energy.• A reliable prediction of the annual energy yield of wind farms is only possible, if it is based on accurate on-site • wind speed measurements. Wind energy applications require a higher standard of wind speed measurements than it is necessary for meteorological purposes. Particularly, critical aspects are the selection of the measuring site, the selection and calibration of anemometers, • and the installation of the sensors on met masts. The costs involved in high quality wind speed measurements are small as compared to the reduction of the financial risk of wind farm projects. The wind energy is converted into rotational energy by a rotor called as wind machine. The axial of rotor is • connected to the coil of generator and produces the electricity. The performance of wind machine is measured in terms of the efficiency of the wind machine. It is the ratio of output Power and input power. The energy from the wind cannot be obtained directly. Therefore, to extract the energy from the wind suitable • device called the windmill is used. It is the device, which converts the kinetic energy of the wind into mechanical energy, and this mechanical energy is converted into electrical energy. In these types of wind machines the axis of rotation of windmill is perpendicular to the surface of Earth. In this • group of machines there is variety of designs. It can be further classified as Savonius rotor vertical axis wind machine, Darrious rotor vertical axis wind machine, H-rotor vertical axis wind machine and Magnus rotor vertical axis wind machine.There are four types of horizontal axis windmills (HAWM) : Mono blade HAWM , Twin blade HAWM, Three • blade HAWM and Multi Blade HAWM.

ReferencesAnaya-Lara, O., Jenkins, N., Ekanayake, J., Cartwright, P. & Hughes, M., 2009. • Wind Energy Generation: Modelling and Control. Wiley publication.Chiras,D., 2009. • Power From the Wind: Achieving Energy Independence. New Society Publication.NOVA PBS, 2012. • Wind Power, • [Video online] Available at: <http://www.youtube.com/watch?v=SQpbTTGe_gk> [Accessed 5 July 2013].ScienceOnline, 2012. • Energy from the Wind, [Video online] Available at: <http://www.youtube.com/watch?v=F3bZzOyMhKI> [Accessed 5 July 2013].ENERGY FROM THE WIND, • [Pdf] Available at: <http://practicalaction.org/docs/technical_information_service/energy_from_wind.pdf> [Accessed 5 July 2013].Wind Energy• , [Pdf] Available at: <http://www.need.org/needpdf/infobook_activities/IntInfo/WindI.pdf> [Accessed 5 July 2013].

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Recommended ReadingHau, E. & Renouar, H., 2010. • Wind Turbines: Fundamentals, Technologies, Application, Economics, 2nd ed., Springer Publications. Sathyajith, M., 2006. • Wind Energy: Fundamentals, Resource Analysis and Economics. Springer publications.Manwell, J. F., McGowan, J. G. & • Rogers, A. L., 2010. Wind Energy Explained: Theory, Design and Application. 2nd ed., Wiley Publication.


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Self AssessmentIn a nature no system will remain in non-equilibrium state; it will try to regain its equilibrium state either by 1. loosing or gaining the energy. Which law is this?

1a. st law of thermodynamics 2b. nd law of thermodynamics 3c. rd law of thermodynamics 4d. th law of thermodynamics

Match the following2. Coastal Winds1. The flow of particles in the air.A. Planetary Winds2. can be generated by making the garden around the houseB. Wind3. cycle of motions of air from pole – equator – pole.C. The micro level wind4. between land and oceanD.

1-A, 2-B, 3-C, 4-Da. 1-C, 2-D, 3-B, 4-Ab. 1-B, 2-A, 3-D, 4-Cc. 1-D, 2-C, 3-A, 4-Bd.

__________ is the device for measuring wind speed.3. Anemometer a. Electrical meterb. Mechanical meterc. Hydro meterd.

Which of the following is not a horizontal axis windmills?4. Mono blade HAWM a. Identical blade HAWMb. Three blade HAWM c. Multi Blade HAWMd.

The __________ are produced by the local changes in the atmosphere.5. terrestrial windsa. planetary windsb. local winds c. earthly windsd.

Wind is the air in motion and possesses the ___________.6. kinetic energya. potential energyb. mechanical energyc. electrical energyd.

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The performance of the wind machine is measured in terms of the ____________7. velocity coefficient a. potential coefficient b. work coefficientc. power coefficientd.

The wind energy is converted into rotational energy by a rotor called as ____________.8. wind appliancea. wind machine b. wind enginec. wind deviced.

___________ is the most important component of the turbine, situated on the top of the tower and holds the 9. rotor blades in place.

Transmission system a. Control system b. Nacelle c. Rotor bladed.

__________ converts mechanical energy to electrical energy.10. Transmission system a. Control system b. Nacelle c. Generator d.


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Chapter V

Bio and Hydrogen Fuels

Aim


explain the types of fuels and their properties•

explain biofuels as an alternative to fossil fuels•

elucidate hydrogen as a clean fuel•

Objectives


explain the types of fuels and their properties•

define fossil fuels and their limitations•

enlist the alternatives to fossil fuels•

Learning outcome


describe the properties of various fossil fuels •

understand limitations of fossil fuels •

identify applications of Bio-fuels•

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5.1 FuelsThe traditional definition of fuel is: Fuels are the substances, which can be burned to obtain heat. In the energy technology the term fuel denotes substances containing high energy density (J/Kg). Fuels contain energy in chemical form/nuclear form and which can be converted to the secondary energy such as thermal, mechanical, electrical, and chemical. With recent advancements in chemical technology, the fuels also include the substances used in nuclear reactions, fuel cells, thermonic generators, biogas plants, fuel processes etc. Thus fuels are primary intermediate and secondary substances having high energy density. More than 70% energy demand of the world is satisfied through the combustion of fuels. Figure below shows the world energy consumption of fuels.

120History of Consumption

Total

Industrialized

DevelopingEastern Europe/ Former Soviet Union

Source: Association for the Study of Peak Oil, www.asponews.org

Projected Future Consumption

WORLD OIL CONSUMPTION BY REGION 1970 – 2020

Mill

ion

Bar

rels

per

Day 100

80

60

40

20

01970 1980 1990 2000 2010 2020

250Quadrillion Btu

World marketed energy use by fuel type

History Projections

liquids

coalnatural gas

renewables

nuclear

200

150

100

50

01990 2000 2007 2015 2025 2035

Fig. 5.1 World energy consumption

5.1.1 Types of Fuels

There are varieties of fuels and these are grouped mainly into two types such as• Primary fuels �Secondary fuels �

These are briefly described as follows:Primary fuels/primary energy sources•

Primary energy sources can be defined as sources, which give a net supply of energy e.g. coal, oil, uranium etc. The energy required to obtain these fuels is less than what they can produce by combustion or nuclear reaction. Their energy yield ratio is very high. The yield ratio is defined as the energy fed back by the material to the energy received from the environment. Since their supply is limited it becomes very essential to use these fuels sparingly.

Secondary fuels• These fuels produce no net energy. Intensive agriculture is an example of this type of fuels where in terms of energy the yield is less than the input. The fuels are further grouped into:

Combustible/ conventional fuels �Chemical fuels �Nuclear fuels �

Combustible/conventional fuels• These fuels are extracted from earth. The fossil fuels have been formed during the million centuries in past �by slow decomposition and chemical reactions of buried organic mass e.g. biological plant, animal remains, marine organisms etc. These fuels are found in solid, liquid and gaseous forms, at various depths, in different locations. �Synthetic fuels (synfuels) are another type of combustible fuels. These fuels are liquid, semi liquid and �gaseous fuels derived from coal, petroleum oils, coal tar, oil shales, natural gas, wood, organic waste etc. e.g. coal gas, coke gas, producer gas, wood gas, methane blue gas, blast furnace gas, carburetted waste gas, by products from processing of crude petroleum oil.


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Renewable fuels �These fuels are generated periodically and their supply will last for millions of years. They include biomass generated from biological organisms, organic matter, organic waste etc.

Conventional renewable fuels �Conventional renewable fuels include wood, cow dung, agricultural waste. �Non-conventional renewable fuels �

These include Hydrogen and nuclear fossil fuels. They are not yet exploited on commercial scale. Urban waste is a non-conventional, renewable fuel. It is being used for producing methane, heat and electricity.

Non-renewable fuels �These are not regenerated and their reserves go on getting depleted due to consumption fossil fuels, nuclear fissile fuels are the conventional non-renewable fuels. Their reserves are getting deplete rapidly.

Chemical fuels• These fuels are used in fuel cells, batteries, exothermic reactions, and to obtain secondary fuels like hydrogen, �methane, chemical processing of raw coal and crude petroleum oil, natural gas gives several secondary fuels by-products. These are used as lubricating oils, raw materials for producing synthetic fibres, detergents, solvents, paints, �medicines, alcohols etc., e.g. hydrogen, ammonia, methane, benzene sulphur, carbon, n- butane, hexane, sodium, n-octane, propane, formic acid, glycol, naphtha, methanol etc.

Nuclear fuel• The energy can be harnessed from nuclear fission and fusion reactions. In these reactions the mass of �reactants is higher than the mass of products and there is change in mass Dm. From Einstein’s famous relation between mass and energy, E = MC2, one can calculate the energy released in nuclear fission and fusion reactions. The basic ingredients used in these reactions are called as nuclear fuels.Nuclear fissile fuel �

These are used in nuclear fission reactors to obtain thermal/ electrical energy. For example when a neutron causes fission of uranium235, a typical reaction produces barium, krypton, two or three neutrons and release of energy due to loss of mass as shown in the following reaction:

92U235 + 0N

1 → 56Ba97 + 56 Kr97 + 20N1 + g

The immediate products of a fission reaction, such as Ba137 and Kr97 above are called fission fragments. They, and their products, are called fission products.

Nuclear fusible fuels �These are used in nuclear fusion reactors to obtain thermal and electrical energy. For example, energy is produced in the sun and stars by continuous fusion reactions in which four nuclei of hydrogen fuse in a series of reactions involving other particles that continually appear and disappear in the course of the reaction, such as He3, nitrogen, carbon and other nuclei, but giving one nucleus of helium and two positrons, resulting in a decrease in mass of about 0.0276 amu, corresponding to 25.7 MeV. The heat produced in these reactions, maintains the temperature of the order of several million degrees in their cores and serves to trigger and sustain succeeding reactions.

41H1 → 2He4 + 2 +1e

0

5.1.2 Heating Value (Calorific Value) of Fuels

Energy per unit mass of fuel is called as heating value or calorific value of fuels. The Calorific value of the fuel • is measure of energy librated by the combustion of unit mass of fuel. It decides the energy density of the fuel. This measurement will be carried out with the help of an instrument called Oxygen Bomb Calorimeter. High energy density materials can be considered as efficient material for sustainable technology. The calorific • values of some fuels have been shown in table 5.1.

•

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Type of fuel FuelCalorific value

kJ/g kwh/kg

Solid

Charcoal 33 10.7Coal 25.33 8.1Wood 17 5.5Dung cake 6 – 8 2 – 2.6

Liquid

Kerosene 48 15.5Petrol 50 -Diesel 45 15.5Ethanol 30 9.7Biogas 35 – 40 11.3 – 20.9

GaseousButane (LPG) 50 -Methane 55 17.8Hydrogen 150 48.5

Table 5.1 Calorific values of different fuels

5.2 Fossil FuelsWorld’s energy supply comes mainly from fossil fuels. Fossil fuels are found in solid, liquid and gaseous forms, • at various depths, in different locations of the earth. The various types of fossil fuels contain organic and inorganic substances in different proportions and made of • various volatile materials, sterile materials, heating values, moisture. The heating values, ignition temperature, emission products also differ with each grade of fuel.Fossil fuel includes: coals of various grade, natural gas, petroleum oil, petroleum gases. Fossil fuels are non-• renewable source of energy.

5.2.1 Limitations of Fossil FuelsThe fossil fuels are originated out of biomass buried under the earth. Before 300 to 350 millions years the earth was covered with dense forest. The forest was grown by the process of photosynthesis in plants. The plants takes CO2 from the atmosphere, water from the ground and in the presence of solar energy, produce the biomass with the process of photosynthesis as

CO2 + H2O → (CH2O) n + ½ O2 ↑

It is estimated that with the help of solar energy, the plants reduce the biomass energy on the earth 3 x 1021 J per year. This is ten times higher than the total world’s energy consumption per year. Thus before the existence of human being on the earth, it was covered with full of forest and due to disasters, the whole of biomass was buried under the ground. Due to high pressure and temperature within the interior of the earth it was converted into fossil fuel consisting of coal, oil and gas. The estimated energy content of fossil fuel is of the order of 300 x 1021J.

5.2.2 Coal, its Origin and Limitations

Coal is a general term for a number of solid, blackish organic fossil minerals with widely differing compositions • and properties. Coal is essentially rich in amorphous carbon (carbon without regular structure) coals contains several liquids and gaseous hydrocarbons.Coals are found in different parts of earth at various depths. The various deposits of coal have been formed during • past millions of years and their maturity depends upon the period of formation and conditions of formation. The physical composition, chemical composition, heating value, combustion characteristic etc. of coals from different mines differ significantly the properties of coal in the same region but at different depths and states also differ. Coal is cleaned processed and then used in solid form as fuel; alternatively the coal may be gasified


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and then used as gaseous fuel. Liquefaction of coal gives several liquids, gaseous and solid fuels and chemicals. Thermal, electrical, work, chemical energy are obtained from the coal and the coal products. Coal and coal products are of vital importance in the energy sector. The steam thermal power plants use coal as • the input energy. Coal is principle fuel for combustion. Coal products are useful in industry.Limitations of coal:•

Coal is abundant but it will last for only 200 years. �Its calorific value is low. �Its shipping is expensive. �Coal is pollutant and when burnt it produces CO � 2, CO.Extensive use of coal as a source of energy is likely to disturb the ecological balance of CO � 2 since vegetation in the world would not be capable of absorbing such large proportions of CO2 produced by burning large quantities of coal.

5.2.3 Petroleum Oil, its Origin and Limitations

Petroleum and natural gas are important energy sources. The crude oil and raw material gas are obtained from • production wells. The crude oil and raw natural gas are refined in refineries. Refinery is the plant in which crude oil is distilled into usable substances.In India oil and natural gas exploration, processing and distribution has been established through ONGC, Gas • Authority of India, IOCL, HPL, BPCL, AOCL etc. The important products obtained from the petroleum and gas refineries are:

Fuel gases (butane, propane, methane) �Liquefied petroleum gas (LPG) �Liquefied natural gas (LNG) �Aviation fuel, car fuel �Gas turbine fuel, naphtha �Furnace oils, diesel �Kerosene fuel oils �Lubricating oils- sprites, solvents, paraffin’s, tar �

The energy route of petroleum and natural gas are similar. The steps include:• Exploration → Drilling → Discovery → Production well’s→ Production of crude /natural gas → Refining→ Transport and distribution→ Utilisation.

Petroleum and Naphtha (petroleum products)• Petroleum is the fossil fuel formed by decomposition and anaerobic reactions on buried vegetation and animal masses under favourable temperature and pressure and marine surroundings over several million centuries. Petroleum is an oily, bituminous liquid that may vary from colourless to black. Petroleum is a complex mixture of numerous hydrocarbons and a small amount of other substances.

Crude petroleum• It is a complex mixture of various hydrocarbons and other usable substances. The crude petroleum is refined in a refinery to obtain various fractions such as ether, methane, ethane, propane, butane, kerosene, fuel oil, lubricants, jelly, paraffin’s, tar etc.

Petroleum naphtha• It is a generic term applied to refined, partially refined, unrefined petroleum products and liquid products from natural gas. Naphtha is used for specific purposes such as cleaning, manufacture of rubber, paints, varnishes, etc. And naphtha has high volatility.

Petroleum spirit• These are refined petroleum distillates, which are suitable for using as thinners, solvents, for paints and varnishes and similar products.

Petroleum tar•

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Tar is a black, brown product obtained from petroleum refinery, when partly evaporated or fractionally distilled; petroleum tar gives a substituted solid residue.

Limitations of petroleum oil• Almost 40 % of the energy needs of the world are feed by the oil. The rising prices of the oil has brought the considerable strain to the economy of the world more, so in the case of developing countries that do not posses oil reserves enough for their own consumption with today’s consumption and with resource amount of 250,000 million tones of oil, it would be sufficient for about 100 years unless more oil is discovered.

India is not rich in petroleum reserves. The petroleum bearing areas are located in Assam, Tripura, Manipur, West Bengal, Ganga Valley, Punjab, Kutch, Himachal Pradesh, Andaman and Nicobar Island.

5.2.4 Natural Gas, its Origin and Limitations

A natural fossil gaseous fuel usually associated with petroleum oil. Composition methane 60–80%, ethane 5–8 • %, propane 3–18%, heavier hydrocarbons 2–14 % also non-hydrocarbons, which are sometimes present in natural gas, are: nitrogen, hydrogen sulphide, nitrogen, carbon dioxide. Propane, butane, pentane are the heavy hydrocarbons present in natural gas.They are removed from natural gas • and solid as LPG pipeline. Natural gas has density of 50 MJ/Kg.Limitations of natural gas•

Gas is incompletely used at present and huge quantities are burnt off in the oil production process because �of the non-availability of ready market. Gas is costlier to transport than oil. Since large reservoirs are located in the inaccessible areas. �

5.2.5 Pollution Due to Fuels

Combustion is combining the fuel rapidly with oxygen. The combustion is accompanied by released heat and • undesirable emissions such as CO, CO2, NOX, SOX, H2S, particulates etc. Also from thermal energy process, some heat and affluent are wasted and released to the environment resulting in pollution.Large-scale combustion in power plants and internal combustion engines and industrial plants has caused global • warming, pollution and environmental degradation.Nuclear fuels can also cause pollution by following ways:•

Danger of nuclear explosion in the reactor due to failure of controls can lead to disaster over a very large �area crossing even traditional borders.Radio active radiations from nuclear fuels, nuclear fission products, nuclear rectors etc., create health �hazards.Nuclear waste and useless spent fuels are stored for long duration and cannot be quickly destroyed or �dispersed in nature.

5.2.6 Alternative to Fossil Fuels: Fuel Substitution

Coal oil gas are depleting very fast. These fossil fuels are not distributed evenly throughout the world. Some • countries in the world have massive deposits of petroleum, while some countries, and do not have the kind of reserves that are sufficient to meet their domestic needs. This increases their dependence on other countries for a very vital need of mankind.In short the ill effects of the use of fossil fuels as energy source are as follows:•

They are not renewable. �Their use is responsible for climate change. �Their deposits are limited. Their use is increasing exponentially and costs are expected to rise in the future �when their production peaks out.The combustion products of these fuels are harmful to life on earth. �Dependence on other countries for fossil fuels is detrimental for the economic security of the nation. �


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The above reasons make it necessary to use alternative energy technologies to find a solution. Therefore, biomass • energy, fission, fusion, geothermal, hydroelectricity, hydrogen fuel cells, solar energy, tidal, ocean thermal energy conversion, wave, wind are most important to provide sustainable energy for the future. Biomass can be converted into transportation fuels such as ethanol, methanol, bio-diesel and additives for • reformulated gasoline. Biofuels are used in pure form blended with gasoline.Biomass energy is the utilisation of energy in organic matter. Organic matters are sometimes burnt directly to • produce heat. Sometimes they are refined to produce fuel like ethanol or other alcoholic fuels. In fact, using biomass energy is actually an indirect way of using energy from the sun.Biomass is virtually everywhere. In the agriculture industry, residuals like bagasse from sugarcane, straw from rice • and wheat, hulls and nutshells, as well as manure lagoons from cattle, poultry. Wood wastes like saw dust, timber slash and mill scrap are considerable organic materials. Even in cities paper and yard wastes are usable. Fully utilised biomass reduces pollution in underground water bodies by offsetting the amount of waste in • landfills.Methane and other poisonous gases that form from dead organic matters can be found in landfills and water • treatments plants. These can be captured and converted into fuels suitable for generating energy.Unlike other alternative energies, the burning method of biomass is not clean, similar to burning fossil fuels, it • produces large amount of CO2. However, it produces much less harmful pollutants (e.g. Sulphur). Thus biomass energy reduces or may even eliminate some of the pollution found in the atmosphere, land and water.

5.3 Bio-fuelsFuels derived by using biomass are known as biofuels. Biomass can be converted into transportation fuels • such as ethanol, methanol, bio-diesel and additive for reformulated gasoline. Biofuels are used in pure form or blended with gasoline. Petroleum resources are finite and therefore search for their alternative is continuing all over the world. Moreover, gases emitted by petrol and diesel driven vehicles have an adverse effect on the environment and • human health. There is universal acceptance of the need to reduce such emissions.In India, domestic supply of crude oil meets only about 22% of the demand and the rest is being met from • imported crude. Biofuels has been considered as one of the most preferred alternative fuel for petrol and diesel, particularly in the transport sector. Biofuels are fuels generated from biomass, which are renewable energy sources. There are different routs to use • biomass as energy source such as directly burning it, controlled combustion to generate producer gas, anaerobic digestion to generate methane and fermentation process to produce alcohol.Oil extraction from oil seeds plants, trans-esterification of oil with alcohol to produce Biodiesel is another way • of using biomass as a fuel. While all the above processes/methods generate biofuels, internationally alcohols and biodiesels have been named as biofuels. Brazil, USA, Canada, Australia, China are some of the countries using alcohol in transport sector.

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PoplarTree

Switchgrass

Organic Carbon

Detritus

Fossil Fuels

Microbes

Biofuels

Respiration

Photosynthesis

Respiration

Fig. 5.2 Biofuels cycle

5.3.1 Types of Bio-fuelsFollowing are the types of bio-fuels

EthanolEthanol (ethyl alcohol, grain alcohol, ETOH) is a clear, colourless liquid with a characteristic, agreeable odour. Ethanol, CH3CH2OH is an alcohol a group of chemical compounds whose molecules contain a hydroxyl group, -OH, blended to a carbon atom.

“Mix ethanol with petrol and drive away with low carbon dioxide emissions”, this green message is resounding in the petroleum corridors nowadays. Its very octane rating makes it an effective knock suppressor with an additional advantage of being a fuel in itself, its higher heat of evaporation, uniform composition. Stoichiometric air requirement, higher flash point etc. help to improve not only engine performance but also to reduce harmful engine emissions. The existing petrol-driven engines can run without any modification by blending ethanol with petrol up to 20% an 80–20 petro – alcohol mix is called gasohol.

The following facts stand in favour of ethanol addition:Ethanol contains 35% oxygen that can help complete combustion of the fuel thereby reducing harmful �tailpipe emissions by 30%.Ethanol is non-toxic, soluble in H2O and biodegradable. �Ethanol is renewable resource because of its photosynthetic origin. �Ethanol helps to reduce particulate emissions; especially fine particulates that pose a health treat to children, �senior citizens and those with respiratory elements.

This fuel has some limitations such as:Its low calorific value, higher surface tension, greater solvent power etc. restrict its use. �There is a view that the methanol addition increases aldehyde emission and �When aldehyde reacts with nitrogen in the air, it forms proxy acetyl nitrate, a carcinogen. �Widespread use of ethanol in fuel is said to be not viable because molasses (starting material to prepare �ethanol) would not be abundantly available.About 15 litres of H � 2O are required to manufacture one litre of ethanol from molasses.

MethanolMethanol is the simplest alcohol, containing one carbon atom. It is a colourless, tasteless liquid with a very faint • odour and is commonly known as “wood alcohol”.


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Methanol can be manufactured from a variety of carbon- based feedstocks such as natural gas; coal and biomass • (e.g. wood). Generally methanol is produced by steam reforming of natural gas in order to get synthesis gas (CO + H2), which is then fed into another rector in the presence of catalyst to get methanol. Ethanol and methanol exhibit similar chemical and physical characteristics when used as engine fuels. The • alternative methanol fuel in currently used is M-85. Neat methanol or M-100 may also be used in the future as an alternative fuel. But its toxic character and water-soluble nature, together with its easy volatility makes it less desirable • transportation fuel. It also suffers from the drawback that on partial combustion it gives rise to formaldehyde, which is classified as an air toxin.Methanol can be reformed to extract the hydrogen for use in fuel cells or can be used directly without reforming • in some fuel cells.

MethaneMethane is a colourless, odourless gas with a wide distribution in nature. It is the principle component of natural • gas, a mixture containing about 75% CH4, 15% ethane (C2H6) and 55 other hydrocarbons such as propane (C3H8) and butane (C4H10). It is also obtained from sewage treatment process and anaerobic digestion as follows:• Most of the waste we generate ends up in landfills, where it decomposes and produces landfill gas. Landfill • gas released into the air smells bad, contributes to local smog and is an explosive hazard. Additionally, landfill gas is about 50% methane a potential greenhouse gas that contributes to global climate change. However, this methane is also a reliable and renewable fuel source that, if not collected goes to waste.Anaerobic digestion is a biochemical process in which particular kinds of bacteria digest biomass in an oxygen • free environment. Different bacteria work together to break down complex organic waste in stages resulting in the production of biogas.Biogas is produced in a digester (also known as digester gas) is actually a mixture of gases, with methane and • carbon dioxide making up more than 90% of the total. Biogas typically contains 55–60% CH4, 35–40 % CO2, 5% hydrogen and traces of H2S and O2.

BiodieselBiodiesel is an alternative fuel, which is actually a monoalkyal ester of long chain fatty acids derived from • renewable lipid feed stocks, such as renewable oils and animal fats. It is made by the transesterification process, which help to remove glycerin from factor vegetable oil. Biodiesel can be used as a transportation fuel in compression –ignition diesel engines with little or no • modifications. Biodiesel is an efficient, clean, non-toxic, 100% biodegradable natural energy alternative to petroleum fuels. It is virtually free of sulphur and aromatics and contains about 11wt % oxygen. Again its high cetane number, • good- lubricating properties together with its built-in oxygen contain makes it an excellent fuel. Studies conducted with Biodiesel engines have shown substantial reduction in particulate matter (25–50 %). • However, a marginal increase in NO (1–6%) is also reported. Pure Biodiesel requires special treatment in cold whether because of its relatively high pour point. But it can be • handled easily, has a high energy density compared to that of mineral oils and substantially higher that natural gas or hydrogen. Biodiesel is currently used as 20 % blends (B-20) with petroleum diesel. Since Biodiesel blends dramatically • changed the distillation qualities of diesel, the cetane number equation cannot be used for calculating it ignition quality.

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5.3.2 Applications of Bio-fuelsUnlike the fossil fuels bio-resources have uniform distributions and are suited to decentralised applications. It is advantageous that these are available in solid, liquid and gaseous forms. These can be considered as best alternative to fossil fuels. Farmers will find it more beneficial to implement Silviculture than food production. The conversion of biomass to biofuels is aimed to enhance its quality and performance. Biofuels have gained credit as a replacement of the fuel for petroleum motors. The main two types of the biofuels are ethanol and vegetable oil methyl ester. Out of that ethanol is generated from sugar producing crops i.e. sugar cane or sugar beat and amylaceous plants like wheat, corn. All the process of production of biofuels requires a fermentation or distillation to separate water from alcohol. Vegetable oil methyl esters (VOME) are generally obtained from seeds of plants sunflower, soybean, rapeseed etc.

Household applications• Community in Rural areas of India still based on the firewood since firewood is available at low cost and �abundant. But it gives rise to pollution and hence biomass can be utilised in biogas plants to generate biogas. The generated biogas can be used for lightening houses and cooking applications. It can also be used for �charring fodder, grinding flower. A biogas lamp of luminosity 60- watt equivalent electrical light can function for six to seven hour if one cubic metre of gas is available. Some of the biogas equivalents are shown in table 5.2. �

Applications One meter cube of biogas equivalentCooking Can cook three meals for family of 5-6 membersLightening Can lighten 60–100 watt bulbs for 6 hoursFuel replacement 0.7 kg of petrolElectricity Generation Can produce 1.25 kilowatt hours of electricityShaft power Can run 1hp electric motor for two hours

Table 5.2 Some of the biogas equivalents(Source: adapted from Kristofferson (1991))

Automobile applications• Biofuels can be used as an alternative to the petrol and diesel with some modifications in the designs of the �internal combustion (I.C.) engines used for both for stationary and transportation applications. Modifications required such as provision should be made for the gas entry of biogas with air, advancing the �ignition timings, provision to storage and regulate biogas.

Sugar industry• Sugar industry requires uninterrupted electrical energy at large scale for number of appliances. Boilers are �used for heating sugar juice. A large amount of bagasse is available at sugar industry hence fermentation plants can be attached to �generate biogas. Generated biogas can be utilised to boil the sugar juice. Power co-generation helps to provide requirement of electricity.

Electricity generation• Biogas can be used to run electrical generator if biogas plant is capable to provide uninterrupted gas supply. �A fuel cell is a device, which converts chemical energy of the fuel into electrical energy. Biofuel can also �used as fuel for fuel cells.

Combustion device-chulha• Chulha is a combustion device with which one can carry out combustion of the biomass to produce heat �for cooking the food. The efficiency of traditional Chulha is very less and hence modified version of it based on the wood has �been developed by scientist at CTARA-IIT Bombay (Centre for Technical Alternative for Rural Area). With this stove, one can get blue flame without smoke.


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5.3.3 Biofuel Technology Worldwide Development

Brazil, USA, Canada, Australia, China are some of the countries using alcohol in transport sector. The ministry of • non-conventional energy sources has initiated a comprehensive programme on biofuels for surface transportation since 2002 –03 to develop the technology for converting vegetable oils, mainly non-edible oils to biofuels and promote the use of these biofuels in automotive sector after taking care of different aspects of the conventional diesel/ petrol engines.The ministry have also taken up a scheme on biofuels pilot demonstration project in rural areas for implementation • initially in one village in each state, namely, Menasina Adya, Kallugudda in Chikkamagalur district, Sipri and Sahariya in Lalitpur district, Basghari in Mandla district and Gadrdih in Bokaro district in the states of Karnataka, Uttar Pradesh, Madhya Pradesh and Jharkhand respectively.

5.4 Hydrogen FuelHydrogen is the simplest element, composed of one proton and one electron. It makes up more than 90% of • the composition of the universe. More than 30% of the mass of the sun is atomic hydrogen. It is the third most abundant element in the earth’s surface, and is found mostly in water under ordinary (earthly) conditions, hydrogen is colourless, odourless, tasteless and non-poisonous gas composed of diatomic molecules. In recent years hydrogen has been receiving worldwide attention as a clean and efficient energy carrier with • potential to replace liquid fossil fuels. Significant progress has been reported by several countries including India in the development of hydrogen energy as an energy carrier and an alternative to fossil fuels.Serious concerns to energy security, depleting fossil fuel reserves, green house gas emissions and air quality • are driving this global transformation effort towards a hydrogen – based economy.Properties of hydrogen•

Hydrogen at ordinary temperature and pressure is a light gas with a density only 1/14 � th that of air and 1/9th that of natural gas under the same conditions. By cooling to the extremely low temperature of -253ÚC at atmospheric pressure, the gas is condensed to a liquid with a specific gravity of 0.07, roughly 1/10th that of gasoline.The standard heating value of hydrogen gas is 12.1 MJ/cu.m compared with an average of 38.3 MJ/cu.m �for natural gas. Hence for producing a specific amount of energy, liquid hydrogen is superior to gasoline on a weight basis but inferior on a volume basis.The flame speed of hydrogen burning in air is much greater than for natural gas, and the energy required to �initiate combustion (i.e. the ignition energy) is less.Mixture of hydrogen and air are combustible over an exceptionally wide range of compositions, thus, the �flammability limits at ordinary temperatures extended from 4–74% by volume of hydrogen in air.

Some attractive features of hydrogen are as follows:• It can be produced from water, which is abundantly available in nature. �Hydrogen has the highest energy content per unit mass of any chemical fuel and can be substituted for �hydrocarbons in a broad range of applications.Its burning process is non-polluting �It can be used in fuel cells to produce both electricity and heat. �

5.4.1 Sources of Hydrogen

The hydrogen can be used as a fuel directly or it might be used as a raw material to produce methanol, ammonia • or hydrocarbons by using either carbon dioxide or nitrogen from the atmosphere. Hydrogen is chemically very reactive and hence it is not found in free states on the earth. However, combined • chemically with other elements; it is present in H2O, fossil hydrocarbons, biological materials such as cellulose, starch etc. and minerals such as bicarbonate rocks. Energy must be supplied to these compounds to break the chemical bonds to release hydrogen. The hydrogen • is a secondary fuel that is produced by using primary source.

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The combination of H• 2 with oxygen (from air) results in the liberation of energy, with water and heat as by-product.

H2 + ½ O2 → H2O + Energy5.4.2 Hydrogen Fuel Technology

Use of hydrogen as an energy source involved five basic issues:• production �storage and transportation �utilisation �safety and management �economy �

Commercial production of hydrogen today is carried out by the steam reformation or partial oxidation of hydrocarbons (natural gas, naphtha or crude oil) depending on supplies and economics.

5.4.3 Methods of Production of Hydrogen

Electrolysis or the Electrolytic Production of Hydrogen• In this process water is spitted into hydrogen and oxygen by means of direct electric current. This is the �simplest method of hydrogen production. The electrolytic cell for the production of hydrogen is shown in fig.5.3.

Fig. 5.3 Simple electrolytic cell

In principle, an electrolytic cell consists of two electrodes immersed in an aqueous conducting solution called �as electrolyte. A source of direct current voltage is connected to the electrodes so that an electric current can flow through the electrolyte from the positive electrode (or anode) to the negative electrode (or cathode) as a result, the water in the electrolyte solution is decomposed into hydrogen gas (H2) which is released at the cathode, and oxygen gas (O2), released at the anode. Although only water is split, an electrolyte (e.g. KOH solution) is required because water itself is a very poor conductor of electricity. Ideally, a voltage of 1.23 V should be sufficient for the electrolysis of water at normal temperature and pressure. For various reasons, especially the slowness of the electrode processes that lead to the liberation of hydrogen and oxygen gases, higher voltages are required to decompose water. The electrolysis efficiency can be increased by decreasing the decomposition voltage for a given current �density. To achieve this, the electrode surface must be able to catalyse the electrode processes. One of the best catalysts is platinum in a finely divided form, deposited on a metal base. However, because of the high cost of the platinum, other electrode surface materials are used commercially. For practical water electrolysis, the electrodes are generally of nickel- plated steel.In some electrolytic cells the diaphragm is present in between anode and cathode. It prevents electronic �contact between adjacent electrodes and passage of dissolved gas or gas bubble, from one electrode


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compartment to another (leading to a decrease in current efficiency and possible to explosions), without themselves offering an appreciable resistance to the passage of current within the electrolyte. Asbestos is the most common material for cell diaphragm.Advantages of this method over other are as follows: �

The efficiency of cell is high, related to the cells operating voltagei. Capital cost of the plant related to the hydrogen production from the cell of given size is lessii. Lifetime of the cell is goodiii. It requires less maintenanceiv.

Following two types of electrode arrangements are mainly used in industries for large-scale hydrogen �production.

Tank type electrolyseri. Filter press electrolyser (bipolar electrolyser)ii.

Tank type electrolyser• On a large scale the application of water electrolysis, industry used the tank electrolyser; it consists of a series �of electrodes. Alternating anode (+) and cathode (-) are suspended vertically and parallel to one another in a tank, filled most commonly with a 20 - 30 %, solution of potassium hydroxide in demineralised water. Alternate electrodes, usually the cathodes, are surrounded by porous diaphragms (e.g. asbestos) impermeable �to gas but permeable to the cell’s electrolyte, that prevents the passage of gas free from a series of gas collectors. All the anodes in the tank are connected to the same positive terminal of the direct current voltage source, and all the cathodes are connected to the same negative terminal as shown in fig.5.4

Electrolyte

Porousdiaphragm

Electrode

+ + + + - - - -

Hydrogen

O2

2V+

Cell battery voltage= number of cells X 2V

-

Fig. 5.4 Tank type electrolyser

By connecting the electrodes in parallel in this manner, the voltage required for a tank of several electrode �pairs, regardless of the number, is little more than for a single pair (or cell), that is, about 2 volts. As a rule, the numbers of tanks are connected in series and the operating voltage then roughly 2n volts, where n is the number of tanks so connected. The major advantages of tank type electrolyser are two- fold: Relatively few parts are required and those �needed are relatively inexpensive; and individual cells may be isolated for repair or replacement simply by short- circuiting the two adjacent cells with a temporary bus bar connection.The disadvantages of the tank electrolysers are: �

Inability to handle high current densities because of cheaper component parts.i. Inability to operate at high temperatures because of heat losses from the large surface areas of ii. connected cells.

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Filter press electrolyser (bipolar electrolyser)• The alternative and more widely used water electrolysis system is called the filter- press electrolyser because �of its superficial resemblance to a filter-press. Except at the ends of the cell, the electrodes are bipolar, that is, one face of each plate electrode is an anode and the other face is the cathode.

As in the tank system, porous diaphragms between adjacent electrodes prevent mixing of the hydrogen and oxygen gases. The schematic arrangements of electrodes in filter press electrolyser are shown in fig.5.5.

In the filter press electrolyser, the cells are connected in series with an anode at one end and a cathode at the �other end of the series. The total voltage required is then approximately 2C volts, where C is the number of cells in the series. The cells which are connected in series, their individual cell voltages are additive within a battery. Because �the cells can be made relatively thin, a large gas out put is achieved from relatively small volumes. The filter- press electrolyser is generally preferred, because it occupies less space and can be operated at a �higher current density than the tank type. The economics are thus more favourable, since a larger hydrogen production is possible in a plant size.

2V

Bipolarelectrodes

Electrolyte

2V 2V 2V

O2

H2

Porousdiaphragm

Fig. 5.5 Filter-press electrolyser

Thermo- chemical methods• In electrolysis of water, the electricity is used for hydrogen production. So conversion efficiency is less. Higher conversion efficiency might be possible if the heat produced by the primary fuel could be used directly to decompose water, without the intermediary of electrical energy. Such direct decomposition of water into hydrogen and oxygen is possible, but it requires temperature of at least 2500ÚC. Because of the temperature limitations and the conversion process equipment, direct single step water conversion cannot be achieved.

However, a sequential chemical reaction series can be devised in which hydrogen and oxygen is produced, water is consumed and all other chemical intermediates are recycled. This sequence of reactions is called a thermochemical cycle, because energy is supplied as heat at one or more of the chemical stages. In the reaction series, water is taken up at one stage, and hydrogen and oxygen are produced separately in different stages. The net result is the decomposition of water into hydrogen and oxygen.

Numerous candidate cycles have been suggested during past few years.Following are the few cyclic processes, for which demonstration models are already available, constructed mainly of glass/quartz and giving a continuous production of about 100 litre hydrogen per hour. These are the

Westinghouse sulphur cyclei. the Ispara mark ii. 13 bromine sulphur cycle iii. the general Atomic Co. iodine sulphur cycleiv.


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Westinghouse electrochemical thermal sulphur cycleThis is a two-step process, where in hydrogen and sulphuric acid is produced electrolytically by the reaction of sulphurous acid and water. SO2 is recovered by reducing SO3 obtained from sulphuric acid at high temperature, oxides of sulphur served as recycling intermediates within the process. Bio and Hydrogen Fuels. Hence in this process sulphur dioxide in sulphuric acid solution is oxidised electrochemically to sulphuric acid, hydrogen being formed at cathode. The sulphuric acid is highly concentrated, decomposed at a high temperature, and the sulphur dioxide and water formed are used again in the process.Electrolysis SO2 + 2 H2O → H2SO4 + H2 (reaction temperature 298 – 373 K) Thermolysis H2SO4 → SO2 + H2O + ½ O2 (reaction temperature 925–1125 K) The problem with this cycle is that for electrolysis diluted hydrochloric acid having concentration about 30 % is required but for thermolysis the concentration of hydrochloric acid should be 70-75 %. This makes it necessary to optimise the acid concentration for the two stages.

Ispara mark 13 cycleElectrolysis 2HBr→ H2 + Br2 (reaction temperature 353 – 473 K) Br2 + SO2 + 2H2O → H2SO4 (aq.) + 2HBr (R.T. 293 – 373 K) Thermolysis H2SO4 → SO2 + H2O + ½ O2 (reaction temperature 925–1125 K) In the first stage concentrated hydrobromic acid is electrolysed at about 373K, giving hydrogen and bromine. The bromine is separated by distillation and in the second stage reacts with sulphur dioxide giving an aqueous solution of hydrogen bromide and sulphuric acid. One thus obtains gaseous hydrogen bromide and fairly concentrated sulphuric acid (about 70-80%), which can then be decomposed at high temperatures, either directly or after further concentration.

Iodine sulphur cycleAmong the purely thermo chemical cyclic processes, those belonging to the iodine sulphur family are of most interest at the present time. The following three-stage process has been developed by the General Atomic Co. in particular:Electrolysis 2HI → H2 + I2 (reaction temperature 393 – 800 K) I2 + SO2 + 2H2O → H2SO4 (aq.) + 2HI (reaction temperature 370 K) Thermolysis H2SO4 → SO2 + H2O + ½ O2 (reaction temperature 1150 K)

The main difficulty lies in the fact that, if the side reactions are to be avoided, the two acids can be obtained only in dilute solution in the second stage. It is difficult to separate even on laboratory scale. Separation by distillation is impossible without decomposition.

The general atomic co. has developed a continuous process in which an excess of iodine is used for effective separation into approximately 56 % H2SO4 and hydroiodic acid rich in iodine. After phosphoric acid has been added to the later, treatment at a high temperature yields liquid iodine and gaseous hydrogen iodide.

Fossil fuel methods• Mostly a gaseous mixture of carbon monoxide and hydrogen is formed in the first stage, in the production �of hydrogen by using a fossil fuel. The processes in common use are steam reforming of methane or other hydrocarbon gas or light liquid hydrocarbon and partial oxidation of a heavier hydrocarbon in the presence of stem at a high temperature. In fossil fuel method hydrogen is produced from the reaction of coal or hydrocarbons in the presence of the steam at a high temperature. For example, in case of coal gasification hydrogen is produced as follows:

C+ H2O ↔ H2 + CO

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To remove the carbon monoxide, the mixture is submitted to the water gas shift reaction with the steam. The �carbon monoxide thereby converted to the carbon dioxide with the formation of additional hydrogen.

CO + H2O ↔ H2 + CO2 ------------------------ (5.1)Carbon dioxide is an acid gas that can be absorbed in an alkaline medium. If the small amounts of CO and �CO2 remaining are undesirable, they can be converted into methane, which can be separated as a liquid by cooling to a moderately low temperature.

CO + 3 H2 ↔ CH4 + H2OSeveral processes proposed for converting coal into hydrogen. Hydrogen would be made by reacting coal �or char obtained in the early stages of coal treatment, with steam and limited amount of oxygen. The heat generated when the carbon in the coal reacts with the oxygen produces the high temperature required for the carbon steam reaction. The product is mixture of carbon dioxide and carbon monoxide. The carbon monoxide can be removed by using (5.1) reaction.Air can be used instead of oxygen to supply the heat for the carbon steam reaction, but about half of the �product gas is inert nitrogen from the air. Although this procedure is more economical, the high proportion of nitrogen, which cannot be removed easily, is often a drawback. However, the iron steam process, described below is designed to use air, steam and coal char to make hydrogen �essentially free from nitrogen and also from carbon monoxide without the need for water gas shifts. This method depends on the reaction of steam with iron at a temperature of about 815 � 0C at a pressure of 70 atm. The products are fairly pure hydrogen gas and solid iron oxide as follows:

Fe + H2O → FeO + H2 (reaction temperature 8150C)The iron is recovered from the oxide in a separate vessel and returned for further reaction with steam. �The conversion of iron oxide to iron is achieved by means of a reducing gas mixture of carbon monoxide, hydrogen and nitrogen at a temperature of 10950C made by the air steam char process.Carbon monoxide and nitrogen are absent from the product, as the iron- steam reaction occurs in separate �vessel.In short the coal gasification process for the production of hydrogen can be summarised as: �

Steam gasification C+ H2O ↔ H2 + CO -------------------------- (5.2) CO + H2O ↔ H2 + CO2 ---------------------- (5.3) CO + 3 H2 ↔ CH4 + H2O -------------------- (5.4)Hydro gasification C + 2 H2 ↔ CH4 ------------------------------ (5.5)Combustion C+ O2 ↔ CO2 --------------------------------- (5.6) C + ½ O2 ↔ CO ------------------------------ (5.7)

First reaction is endothermic reaction and (5.3) and (5.4) are exothermic reactions. But since the first reaction (5.2) dominates over the (5.3) and (5.4) so the over all steam reformation process is endothermic.

Hydro gasification (5.5): direct reaction of hydrogen with the carbon of the coal, is of great importance for the production of methane as substitute natural gas from coal. Lastly, the reaction with oxygen (5.6) and (5.7) also play a part in gasification techniques, on the one hand serving as heat producing process in association with endothermic steam gasification

Solar energy methods• It includes �

Bio- photolysisi. Photo- electrolysisii.

Bio- photolysis �This method utilises living systems or material derived from such systems to split water into its constituent’s hydrogen and oxygen. This route makes use of the ability of microorganisms to produce hydrogen. This method has


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the advantage of producing hydrogen at lower temperatures, room temperatures, as opposed to the use of very high temperatures in routes such as direct thermal decomposition or thermo chemical hydrogen production. In normal photosynthesis in green plants, the green pigment chlorophyll takes up energy from sun light and in a complex series of reactions breaks up water molecules into oxygen gas, hydrogen ions and electrons the oxygen is evolved from the green plant but the hydrogen ions and electrons are removed by the interaction with carbon dioxide to produce simple sugar. Certain single cell green algae are able to make the enzyme hydrogenase. In these algae, second stage of photosynthesis can be circumvented by eliminating carbon dioxide. Hydrogen ions and electrons are then combining in the presence of hydrogenase to produce hydrogen gas. Thus the exposure of these algae to sunlight and water yields a mixture of oxygen and hydrogen gases that can be separated by various ways.

Blue green algae differ from green algae in several respects. In particular in addition to normal photosynthesis cells in which reaction with carbon dioxide occurs, they contain some larger cells (heterocysts) where hydrogen can be formed. In the presence of nitrogen from the atmosphere, the nitrogen combines with the hydrogen ions and electrons to produce ammonia. By preventing the access on nitrogen, blue green algae decompose water in sunlight to yield hydrogen and oxygen.

Instead of using living algae to obtain hydrogen from water, a more convenient approach is to utilise biological materials obtained from plants or bacteria. One advantage is the ability to vary the conditions to optimise hydrogen production.

Chloroplasts, the small bodies containing the chlorophyll in green plants, retain their photosynthetic activity when extracted from the plant. Hydrogen and oxygen can then be obtained from water by exposing chloroplasts to sunlight together with the enzyme hydrogenase and ferredoxin, an electron carrier, also of biological origin.

Photo-electrolysis �In photo-electrolysis a current is generated by exposing one or both anode and cathode electrodes to sunlight. In this route, light incident on a semiconductor electrode generates electron-hole pairs. The holes carry out the oxidation of water to oxygen and H+ ions, while the electrons reduce the H+ ions to H2 in gaseous form. In ordinary electrolysis water is decomposed into hydrogen and oxygen by passing an electric current from an outside source. In photo-electrolysis current is generated by exposing electrode to the light. Hydrogen and oxygen gases are liberated at the respective electrodes by decomposition of water.

Photo-electrolytic water splitting is based upon absorption of photons and generation of electron-hole pairs in a semiconductor in contact with aqueous electrolyte. The process of oxidation and reduction of species of the electrolyte can bring out to release H2 at cathode and oxygen at anode. Schematic of the photo-electrolysis of water is shown in fig.5.6.

Fig. 5.6 Photo-electrolysis of water

The photo-electrolytic reduction can be written as: H2O + Light → H2↑ + ½ O2 2H2O + Light → 2H2↑ + O2 (2.46 eV)

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The energy of 2.46 eV approximately corresponds to the wavelength of 500nm (wavelength available in solar radiations). Unfortunately water cannot absorb these radiations to form H2 and O2. For direct production of hydrogen and oxygen i.e. direct photo-decomposition the threshold energy is about 6.5eV i.e. l = 190 nm is required. Such high-energy radiation does not reach on the earth surface from sun. Therefore, semiconductor is used to absorb the photons and to decompose the water. A wide band gap semiconductor is used for this purpose for example titanium oxide (TiO2). The chemical reactions when TiO2 is used for the production follow: At anode: TiO2 + 2 hn → 2e- + 2h+ 2h+ + H2O → 2H+ + ½ O2 At cathode: 2e- + 2H+ → H2↑ Overall reaction: H2O + 2 hν → H2↑ + ½ O2

Theoretically possible efficiency of photo-electrolysis splitting of water is 24%.But the efficiency actually achieved with various semiconductor materials is only about 1 % or less.

The main advantage of this is it is economical and easy process. The disadvantages of this process are, Low conversion efficiency and instability of photo-electrode against corrosion.

5.4.4 Hydrogen StorageIn energy system there is a need to be able to store energy somewhere between the production point and the utilisation point. The need for storage is due to the mismatch between the optimum production rates of energy and then fluctuations in demand for energy by the user.The five principle methods for hydrogen storage are as follows:

Compressed gas storage• Hydrogen is conventionally stored for many applications in high-pressure cylinders. This method of storage is rather expensive and very bulky because very large quantities of steel is needed to contain quiet small amounts of hydrogen in the conventional industrial hydrogen system, compressed gas is used to supply relatively small amount of hydrogen but when hydrogen is considered as fuel then this method of storage was found to be costlier.

Liquid storage• Compressed gas storage would be too costlier for large-scale application. A more practical method to store it is in the form of liquid at low temperature. A liquid storage tank of hydrogen is shown in fig.5.7. This method has some disadvantages that are described as follows:

Considerable amount of energy is required to convert hydrogen gas into the liquid phase. �There is also inflammability danger from the fact that the liquefied atmospheric gas rich in oxygen would �concentrate in the vicinity of the liquid hydrogen tank.Liquefied hydrogen plant normally required some kind of primary refrigeration system such as liquefied �nitrogen gas to pre-cool hydrogen.About 25–30% of heating value of hydrogen is required to liquefy hydrogen. �

super-insulationlevel-probe

filling linegas extraction

liquid extractionfilling port

electric heaterreversing valve (gaseous/liquid)

cooling water heat exchanger

shut-off valve

gaseous hydrogen (+200c up to +800c)

safety-valve

liquid hydrogen (-2530c)

suspension

outer vessel

inner vessel

Fig. 5.7 Liquid storage of hydrogen


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Line packing• The use of line pack storage in the natural gas industry provides a relatively small- capacity storage system, but one with a very fast response time that can take care of minute by minute or hour by hour variations in demand. Hydrogen line packing system would have a similar capability that of natural gas but the capacity would be reduced by a factor of about 3 because the reduced heating value of the hydrogen compared with natural gas.

Underground storage• The cheapest way to store large amount of hydrogen for subsequent distribution would probably in underground facilities similar to those used for natural gas, these facilities would include, depleted oil and gas reservoirs and aquifers. Since the hydrogen gas tends to escape readily through a porous material, some geological formations that may be suitable for storing natural gas may not be suitable for hydrogen.

Metal hydrides: (storage in chemically bounded form)• Considerable interest has been shown recently in the possibility of storage of hydrogen in the form of a metal hydride by direct reaction with hydrogen gas in the form of a metal hydride.

A number of metals and alloys form solid compounds called metal hydrides, by direct reaction with hydrogen gas. Fig.5.8 shows the use of metal hydride for hydrogen storage when the hydride is heated, the hydrogen is released and the original metal is recovered for further use. Thus the metal hydride provides the possible means for hydrogen storage.

Fig. 5.8 Metal hydride for hydrogen

An important property of metal hydride is that the pressure of the gas released by heating a particular hydride depends mainly on the temperature and not on the composition. At a fixed temperature, the gas pressure remains essentially constant until the hydrogen content is almost existed. Three of the most promising metal hydrides are: lanthanum nickel (LaNi5), iron titanium (FeTi) and Magnesium nickel (Mg2Ni) alloys. The several studies are being made to find a metal hydride that would satisfy the requirements for the hydrogen storage. These requirements include the following:

The metal (alloy) should be fairly inexpensive. �The hydride should contain a large amount of hydrogen per unit volume and per unit mass. �The hydride should be formed without difficulty by reaction of the metal with hydrogen gas, and it should �be stable at room temperature.The gas should be released at the significant pressure from the hydride at a moderately high temperature �(preferably below 1000C)

The chemical reaction equation for exothermic formation of the hydride from hydrogen and metal is: H2 + Me ↔ MeH2 (hydride) + heat This means that during charging up with hydrogen heat is always produced at the same time, and in order to withdraw the hydrogen from the hydride it is always necessary to add heat.For the technical application of hydrides the temperature at which their dissociation pressures attain values above 1 bar is of particular interest.

There are two types of hydrides

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Low temperature hydridesIn low temperatures hydrides even at temperatures below freezing point their dissociation pressure is above 1 bar. Examples of these types are TiFeH2, CaNi5H6

High temperature hydridesThese are the hydrides for which the pressure limit of 1 bar is above the boiling point of water. Examples of these types are MgH2, Mg2NiH4Metal hydrides fulfil a duel role as follows:

Q + MeH2 → Me + H2Heat is stored and hydrogen is given off by the dissociation of the hydrides. The back reaction is as follows:

H2 + Me → MeH2 + QNow the hydrogen is absorbed and heat is released.

5.4.5 Transport of HydrogenPipelines

In the USA there is 720 km of hydrogen pipeline network and in Europe about 1,500 km. Over great distances, �pipeline transport of hydrogen could be an effective way of transporting energy. The energy loss in an electric power grid can be up to 7.5–8% of the energy it is transferring. This is about �double of what is needed to feed gas through a pipeline of the same length. Hydrogen pipes that are in use today are constructed of regular pipe steel and operate under pressure at 10–20 �bar, with a diameter of 25–30 cm. The oldest existing system is found in the Ruhr area. It is 210 km long and distributes hydrogen between 18 producers and consumers. This network has been in use for 50 years without any accidents. The longest hydrogen pipeline is 400 km and runs between France and Belgium.With little or no changes, the majority of existing steel natural gas lines can be used to transport mixtures �of natural gas and hydrogen. It is also possible, with certain modifications, to use pure hydrogen in certain existing natural gas lines. This depends on the carbon levels in the pipe metal.Newer gas pipelines such as those in the North Sea have low carbon content and are therefore suitable for �transporting hydrogen. If the speed is increased by a factor of 2.8 to compensate for hydrogen having 2.8 times lower energy density per volume than natural gas, the same amount of energy can be moved. The fact is that by using efficient hydrogen technology such as fuel cells, etc., the same amount of transported energy will yield increased output at final consumption.In the natural gas distribution network, pressure is low, around 4 bar, and so cheaper plastic pipe is usually �used. PVC (Poly Vinyl Chloride) and the newer HDPE (High Density Poly Ethylene) are too porous and not usable for transporting hydrogen. Gas pipelines, in addition to being used for transportation, can also be used to store great quantities of �hydrogen. By regulating the pressure in the pipes, it is possible to use the large volume a pipeline offers as storage during peak situations.

�Transport of liquid hydrogenLiquid hydrogen (LH2) is hydrogen, which has been cooled below -253ºC. The cooling process requires a great deal of energy, but for long-distance transportation and as fuel in certain applications used in air and space travel, LH2 still has obvious advantages over other fuels.

Roadway transportationHydrogen can be shipped with tank trucks in both liquid and compressed states. Several companies currently deliver these types of tank trucks.

Ocean transportationHydrogen can be transported as a liquid in tank ships. These are not too different from LNG tankers, aside from the fact that better insulation is required to keep the hydrogen cooled down over long distances. The Japanese WE-NET and the German-Canadian Euro Quebec have reported on the use of such tanks. The evaporated hydrogen may be


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used as fuel onboard. In 1990, the German institute for materials research declared that LH2 could be given the same safety rating as LPG and LNG, and transport of LH2 into German harbours was approved.

Air transportationThere are several advantages in transporting LH2 by air rather than by ship. LH2 is lightweight and the delivery time is much shorter, and evaporation is therefore not a big problem. Studies on this have been done by CDS Research Ltd. in Canada, with support from the WE-NET programme.

5.5 Applications of HydrogenThe possible areas of use for hydrogen in the near future are as follows:

It can be used in nitrogenous fertilisers.• Coal liquefaction as a basic chemical.• Electricity generation by using the fuel cell (A fuel cell is an electrochemical device that converts the chemical • energy of fuel to electrical energy. Fuel systems generally operate on pure hydrogen and air to produce electricity, heat and water).Production of electrolytic hydrogen for the full load exploitation of nuclear power stations.• Direct addition of hydrogen to the exiting natural gas distribution network. Use of the hydrogen in the processing • of the heavy oil.Use of hydrogen for the manufacture of synthetic liquid or gaseous fuels.• Direct use of hydrogen as a motor vehicle fuel in urban transport, particularly where air pollution problems are • already critical.Reduction of iron oxide by means of hydrogen in the steel industry.• Direct use of hydrogen as an aircraft fuel in air transport.•

5.6 Hydrogen Fuel Technology Worldwide DevelopmentsIn India

In July 2003, planning commission constituted a group on hydrogen energy under the chairmanship of �member. NHEB was set up in October 2003 under the chairman ship of minister for non-conventional energy sources for different aspects of hydrogen and fuel cell technologies. A steering group of NHEB under the chairman ship of Shri Ratan Tata, chairman Tata sons is in the process �of preparing the action plan define goals and time frame for the specific proposals on hydrogen energy powered vehicles, power generating systems and the hydrogen road map including modalities for public- private partnerships.During the year a demonstration of H2 powered systems including a motorcycle, a 3-wheeler and a Reva �car equipped with fuel cell was organised on 26th July 2004 at Vigyan Bhavan in New- Delhi. The ministry has been supporting a range of projects on research, development and demonstration of �hydrogen and fuel cell technologies including production, storage and utilisation of hydrogen as a fuel. Several research, scientific and educational institutions, laboratories, universities, industries etc. are involved in implementing the projects.A project on biological hydrogen production concluded at Shri AMM Murugappa Chettiar research Centre �(MCRC), Chennai. Under this project bioreactors upto a 125 m3 capacity have been set up to demonstrate production of hydrogen from distillery waste. During the year, a project on the development of metal hydride reactor progressed at IIT, Chennai. Under �this project carbon nano materials have been developed and characterised. International Advanced Research Centre (ARCI) for Power Metallurgy and New Materials, Hyderabad and IIT Chennai are working on a project on synthesis and evaluation of hydrogen absorbing alloys by mechanical alloying technologies. A project on demonstration of hydrogen fueled two wheelers transport developed at BHU. �SPIC Science Foundation (SSF), Tuticorin is working on development of portable hydrogen generator using �organic fuels such as methanol. IIT Kharagpur is working on a project on compressor driven metal hydride

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cooling and heating systems. Studies on development of hydrogen – fueled agricultural diesel engine by Jadhavpur University, Kolkata, are in progress.

In other countriesAmong the different storage systems of hydrogen, storage in the form of metal hydride is becoming a potential method to store the hydrogen. Research is going on all over the world at the various universities, national laboratories and private companies. Few research centres related to hydrogen research are listed below.

The Max –Plank Institute fur Kohlenforschung �The University of South Carolina (USC) �Savannah River Technology Centre (SRTC) �University of Hawaii in collaboration with Takara at UH, Gross and Thomas at Sandia National �LaboratoriesThe General Motors Research and Development Centre �Ames Laboratory and Lowa State University �The National Institute of Advanced Industrial Science and Technology �


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SummaryFuels are the substances, which can be burned to obtain heat. In the energy technology the term fuel denotes • substances containing high energy density (J/Kg). Fuels contain energy in chemical form/nuclear form and which can be converted to the secondary energy such as thermal, mechanical, electrical, and chemical.There are varieties of fuels and these are grouped mainly into two types such as primary fuels and secondary • fuels.The secondary fuels are further grouped into: combustible/ conventional fuels, chemical fuels and nuclear • fuels.Energy per unit mass of fuel is called as heating value or calorific value of fuels. The Calorific value of the fuel • is measure of energy librated by the combustion of unit mass of fuel. It decides the energy density of the fuel. This measurement will be carried out with the help of an instrument called Oxygen Bomb Calorimeter.Fossil fuels are found in solid, liquid and gaseous forms, at various depths, in different locations of the earth. • Fossil fuel includes: coals of various grade, natural gas, petroleum oil, petroleum gases. Fossil fuels are non-renewable source of energy.Coal is a general term for a number of solid, blackish organic fossil minerals with widely differing compositions • and properties. Coal is essentially rich in amorphous carbon (carbon without regular structure) coals contains several liquids and gaseous hydrocarbons.Petroleum and natural gas are important energy sources. The crude oil and raw material gas are obtained from • production wells. The crude oil and raw natural gas are refined in refineries. Refinery is the plant in which crude oil is distilled into usable substances.The energy route of petroleum and natural gas are similar. The steps include: Exploration → Drilling → • Discovery → Production well’s→ Production of crude /natural gas → Refining→ Transport and distribution→ Utilisation.Petroleum is a complex mixture of numerous hydrocarbons and a small amount of other substances. Crude • petroleum is a complex mixture of various hydrocarbons and other usable substances. Petroleum naphtha is a generic term applied to refined, partially refined, unrefined petroleum products and liquid products from natural gas. Petroleum spirits are refined petroleum distillates, which are suitable for using as thinners, solvents, for paints • and varnishes and similar products.Tar is a black, brown product obtained from petroleum refinery, when partly evaporated or fractionally distilled; petroleum tar gives a substituted solid residue.Combustion is combining the fuel rapidly with oxygen. The combustion is accompanied by released heat and • undesirable emissions such as CO, CO2, NOX, SOX, H2S, particulates etc. Also from thermal energy process, some heat and affluent are wasted and released to the environment resulting in pollution.Fuels derived by using biomass are known as biofuels. Biomass can be converted into transportation fuels • such as ethanol, methanol, bio-diesel and additive for reformulated gasoline. Biofuels are used in pure form or blended with gasoline. Petroleum resources are finite and therefore search for their alternative is continuing all over the world.Types of biofuels are ethanol, methanol, methane and biodiesel.• In recent years hydrogen has been receiving worldwide attention as a clean and efficient energy carrier with • potential to replace liquid fossil fuels.Use of hydrogen as an energy source involved five basic issues: production, storage and transportation, utilisation, • safety and management, and economy.

ReferencesGupta, R. B., 2008. • Hydrogen Fuel: Production, Transport, and Storage, 1st ed., CRC Press Publications.Drapcho, C., Nghiem, J. & Walker, t., 2008. • Biofuels Engineering Process Technology, 1st ed., McGraw-Hill Professional Publication.ixisuprflyixi, 2008. • hydrogen gas production through electrolysis, [Video online] Available at: <http://www.

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youtube.com/watch?v=wwsIkXdSqAg> [Accessed 5 July 2013].expertvillage, 2008. • How to Make Hydrogen : Why is Making Hydrogen Important?, [Video online] Available at: <http://www.youtube.com/watch?v=LXOUWxUcEDQ&list=PL9E7B58B681704FC6> [Accessed 5 July 2013].HYDROGEN PRODUCTION AND STORAGE,• [Pdf] Available at: <http://www.iea.org/publications/freepublications/publication/hydrogen.pdf> [Accessed 5 July 2013].Hydrogen Basics- Production, [Online] Available at: <http://www.fsec.ucf.edu/en/consumer/hydrogen/basics/• production.htm> [Accessed 5 July 2013].

Recommended ReadingBenduhn, T., 2008. • Ethanol and Other New Fuels (Energy for Today). Gareth Stevens Publication.Holland, G. & Provenzan, J., 2007. • Hydrogen Age, The: Empowering a Clean-Energy Future. Gibbs Smith publications.Ewing, R. A., 2007. • HYDROGEN - Hot Stuff Cool Science: Discover the Future of Energy. 2nd ed., PixyJack Publication.


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Self AssessmentThe substances, which can be burned to obtain heat are called _________.1.

fossilsa. fuelsb. hydrogenc. windd.

Match the following2. Primary fuels1. These fuels are used in fuel cells, batteries, exothermic reactionsA.

Conventional fuels2. In these reactions the mass of reactants is higher than the mass B. of products and there is change in mass

Chemical fuels3. These fuels are extracted from earth.C. Nuclear fuel4. sources, which give a net supply of energyD.


Energy per unit mass of fuel is called ____________.3. heating value a. warming value b. cooling valuec. temperature valued.

Which of the following is incorrect for fossil fuels?4. World’s energy supply comes mainly from fossil fuels.a. The heating values, ignition temperature, emission products are alike with each grade of fuelb. The various types of fossil fuels contain organic and inorganic substances in different proportions c. Fossil fuels are non-renewable source of energy d.

___________ is a complex mixture of numerous hydrocarbons and a small amount of other substances.5. Natural gasa. Coalb. Petroleum c. Fossil fueld.

Which of the following statements is true?6. Mix ethanol with petrol and drive away with low carbon dioxide emissions.a. Mix methanol with petrol and drive away with low carbon dioxide emissions.b. Mix propanol with petrol and drive away with low carbon dioxide emissions.c. Mix butanol with petrol and drive away with low carbon dioxide emissions.d.

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A monoalkyl ester of long chain fatty acids derived from renewable lipid feed stocks, such as renewable oils 7. and animal fats is known as___________.

Fossil fuelsa. Hydrogenb. Petroleumc. Biodieseld.

_________ is hydrogen, which has been cooled below -253ºC. 8. Solid hydrogen a. Liquid hydrogen b. Molten hydrogen c. Hydrogen d.

Which is not a type of bio-fuel?9. ethanola. methanol b. petroleumc. biodieseld.

__________ is combining the fuel rapidly with oxygen.10. Fissiona. Fusion b. Formationc. Combustiond.


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Chapter VI

Biogas

Aim


explain the principle, construction and operation of biogas plant•

explicate various types of biogas plants•

explain factors affecting performance of biogas plant•

Objectives


explain operation of biogas plant•

explicate fixed dome biogas plant•

enlist the applications of biogas•

Learning outcome


understand the principle, of biogas plant•

describe factors affecting performance of biogas plant•

identify various • applications of biogas

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6.1 IntroductionBiomass is organic matter from plants, animals and microorganisms grown on land and in water and their derivatives. Biomass includes forest crops, crops from agricultural fields, animal manure, urban and rural organic wastes etc. Fossil fuels are derived from biomass but are not renewable and hence not called biomass. Biomass can be converted into gaseous and liquid fuels called biogas and biochemicals respectively. Biogas and biochemicals can be transported over long distances and then used as secondary sources of energy.

Biogas and biochemicals have higher energy density (J/ Kg) than that of raw biomass. Most of the biomasses are used as fuels either directly or after obtaining biogas/biochemicals. Thus biomass is an important renewable source of for obtaining organic fuels. Biomass can be converted into biogas by anaerobic decomposition. This process takes place inside a plant/ device known as biogas plant.

6.2 Principle of Biogas PlantAnaerobic digestion process has been used for many years for conversion of biomass to gases and liquids. In • biogas technology biogas/ Methane (CH4) is produced from dung by anaerobic digestion in digestion plants. The size plants of these vary from 0.5 m3/day to 2000m3/day. In India anaerobic digestion plants are commonly known as biogas plants or gobar gas plants. Anaerobic decomposition of organic wastes by suitable bacteria produces biogas, which contains methane, • carbon dioxide, hydrogen and other impurities. Biogas can be produced from cow dung in a plant called gobar gas plant. Anaerobic decomposition takes place in two stages. In presence of mixed population of anaerobes, these anaerobes break down the complex substances and produces gas. Following three anaerobes help in anaerobic decomposition.•

Fermentative bacteria: These bacteria perform initial break down of polymeric materials. �Acid forming bacteria: These produce volatile fatty acids (e. g. Acetic acid) and �Methanogenic bacteria: These produce methane gas. �

The organic matter consists of carbohydrates, fats, and complex organic compounds. The first stage bacteria • convert complex organic matter to simpler organic compounds. These bacteria grow well in the pH range 6–7.The second group bacteria convert the various sugars, amino acids, and fatty acids to volatile fatty acids, carbon • dioxide and hydrogen. Varieties of fatty acids such as lactic acid, acetic acid, and propeonic acids are produced in the second step. In the third step methanogenic bacteria become active and decompose short chain fatty acids to give carbon dioxide and methane gas. Methanogenic bacteria are active only in the atmosphere of carbon dioxide and inactive in the presence of oxygen. They grow well in the pH range 7–8 (i.e. just greater pH than the first group of bacteria). These are very sensitive to a change of pH. Any drop in pH decreases the activity of these bacteria, resulting in cessation of methane generation. To maintain pH of slurry lime or ammonia are added to the digester. The whole process of biogas generation can be summarised as follows:•

Hydrolysis: The complex organic compounds are converted to low molecular weight soluble substances, �primarily carboxylic acids. Here low molecular weight soluble substances pass through the cell membrane of bacteria responsible for anaerobic digestion and extra cellular enzymes are released in medium. These enzymes hydrolyze the sludge particles.

(C6H10O5) x + XH2O → X (C6H12O6)Acid formation: During this process the organic matter is oxidised to volatile acid mainly acetic and propanic �acid.

X (C6H12O6) → 3X (CH3CO2H)Methane formation: volatile acids produced in previous step are converted to methane and carbon dioxide �by Methanogenic bacteria.

3X (CH3CO2H)) → 3XCH4 + 3X CO2The biogas is a mixture, generally consists of about 55–60 % methane, 35–40 % carbon dioxide, 5 % hydrogen and other impurity gases.


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6.2.1 Construction and Operation of Biogas Plant

In order to understand the principle of biogas plant a simple model is shown in fig.6.1.•

Gas:to combustion or other uses

SludgeFeed slurry

Dung Water Gas-holder (Tin A)(CH4 + CO2 +.......)

Digester (Tin B)

Fig. 6.1 Biogas plant

The biogas plant mainly consists of:• Digester �Gas holder �Distribution pipes �Gas appliances �

The biogas plant is fabricated simply with two tins (Tin A and Tin B); one of them is slightly smaller (Tin B) • than the other (Tin A), so that they can fit into one another. For the upper tin A, the funnel for inlet and pipes for outlet are fitted. Similarly for Tin B at the bottom, outlet tap is fitted. The Tin A is called gasholder and B is called digester.The Tin B or digester is completely filled with the slurry; Tin A is pressed downward so that it will tightly fit • into lower tin B. After two three weeks the upper tin A starts to move up due to production of biogas. This period varies from few weeks to two months and is known as retention time. The retention time varies with the size of biogas plant and rate of feed.The slurry is made up of 50 % cow dung and 50 % water. Purpose of using 50 % water is as follows: The • decomposition of biomass is easier when bond strength between its constituents is weakened. This has been done in case of cow dung by addition of water. Water is added to fresh cow dung to prepare slurry. In fresh cow dung 20 % solid matter and 80 % water is present. When 50 % water is added to it, the resulting • slurry consist of only 10 % cow dung and 90 % water which helps to weakened the bond and decompose the material. The approximate composition of gas is as follows 55-60% methane, 35-40 % CO• 2, 5 % Hydrogen, H2S and O2 traces. The gas burns with a hot blue and smokeless flame. Methane and hydrogen burn with oxygen give heat as follows:

CH4 + 2O2 → 2H2O + CO2 + Heat energy2H2 + O2 → H2O + Heat energy

The CO• 2 present in biogas does not contribute to heat energy and it is unfavourable to any burning process. Volume of digester can be calculated by using following relationship:

V = VSL . tr Where, VSL = Volume of slurry with which the plant should be charged daily for getting the desired gas production.

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tr = Retention time of slurry in plant.The daily gas production is given by:VCH4 = x. md Where, VCH4 = Daily Volume of methane production, x = Daily gas production rate, which is constant for particular temperature. md = Mass of the dung added.

6.3 Types of Biogas PlantsDepending on nature of gasholder biogas plants are classified into mainly two groups as follows:•

Floating gasholder biogas plant. �Fixed dome gasholder biogas plant. �

In floating type biogas plant the gasholder is separated from the digester where as in fixed dome plants the • gasholder and digester are combined.

6.3.1 Floating Gasholder Biogas Plant: (KVIC Model)

The floating dome gasifier first developed by Khadi and Village Industries Corporation and called KVIC Model. • It was initially investigated in 1952 and then extensively used by KVIC under different schemes for village. Modified version of KVIC model is called Ganesh model.

Digestion chamber

Dividing wall

200150

Outlet pipe for slurry1350

Geodesic dome

Gas outlet

Nylon cord

Hooks

Water-sealing

Mixing pitMixing pit

Heat-sealed PVC gas holder

Inlet pipe for slurry

100150125

Fig. 6.2 Floating gas holder biogas plant: (KVIC model)

Typical structure of floating gas holder biogas plant is shown in fig.6.2 in this plant biogas is stored in a drum • which moves up and down according to the quantity of gas stored in it. The pressure of gas in floating type plant is always constant. Since sides of the gasholder remains in the slurry there is no leakage of the gas to the atmosphere. The size of the gas holder depends on requirement of gas per day e.g. for cooking the food of one person gas • required is 0.24 m3 therefore for the family of six members total gas required per day is 1.44m3 and thus the size of gasholder must be more than this. It should be approximately 2m3. From experimental observations it has been observed that 1 kg of cow dung gives 0.036 m3 of gas 1 cow or cattle gives 10 kg dung every day. i.e. 0.36 m3 gas that is produced by the dung of one cattle. Thus to get sufficient biogas we need the dung of 5 cattle. The size of the digester depends on the retention time and for cow dung it varies from 20–55 days, depending • on temperature of location. Thus the size of digester varies from place to place to get same amount of gas. For example, the size of digester of biogas plant having capacity 2m3 will be smaller in Maharashtra than in Himachal Pradesh.


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6.3.2 Fixed Dome Biogas Plant

Janata Model• This type of biogas plant was constructed first time in 1978 by the planning research and action division �Lucknow. This model is known as Janata biogas plant and is relative cheaper than KVIC Model. Typical structure of Janata model is shown in fig.6.3.

Fig. 6.3 Fixed domed biogas plant (Janta design)

In this biogas plant the gasholder is dome type and fixed one. Digester is cylindrical with flat base cemented �at bottom. Inlet and outlet chambers are constructed for putting fresh slurry in and to take digested slurry out. The input and output slurry chambers are relatively bigger than KVIC plant. The gas is collected in between the dome and digester. When gas is generated, it develops the pressure within a dome and it suppresses the slurry down. This results in rise in the level of the input (i/p) and output (o/p) slurry chambers.On opening the pipe gas is released due to pressure of the slurry in i/p and o/p chambers. The gas releases �till the levels in i/p and o/p chambers and digester become equal. As a result the rate of gas released in this model is not constant and decreases with time. The size of the �biogas plant is decided by the gas generated or required every day i.e. 2, 4, 6, 8, 10 m3, accordingly the volume of gas collecting dome is decided. The size of the digester depends on retention time similar to KVIC Model. As retention time increases the volume of digester increases though volume of gasholder remains constant. �The construction prize of the Janata model is relatively less than KVIC model.

Deenbandhu model• The Janata model was further modified and the revised version is called Deenbandhu model, (the friend �of poor). It was developed by Action of Food production, New Delhi in 1952. It is low cost biogas plant. Schematic diagram of Deenbandhu model is as shown in fig.6.4. The major modification in Deenbandhu model is, in the bottom surface, which is shallow spherical disc rather than flat bottom shape in Janata model.

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Fig. 6.4 Fixed domed plant (Deenbandhu design)

Another difference is that there is no chamber at i/p slurry site. The asbestos cement pipe is used for inlet �and is embedded in digester wall at fixed position however the structure and nature of dome is same as that of Janata model. The principle and working of Deenbandhu model is similar to that of Janata model. Initially a level of slurry �in digester and out put slurry chamber is equal. As gas is produced there develops the pressure in the dome and it suppresses the slurry level in the digester with the increase in the level of the o/p chamber.When one opens the gas pipe, due to level difference between digester and o/p chamber, the gas comes out �till the levels in both chamber and digester become equal.

6.3.3 Greenhouse Multichamber Biogas Plant

Designing aspects• In India biogas production from sugar factory bagasse/ press mud, using a newly designed Green house multi-chamber (GHMC) biogas plant model has been examined. Experiments were carried out under batch system.

Anaerobic fermentation systems may be of standard or high rate designed. Mainly the designs are depending upon the control of the environmental factors, particularly whether there is provision for mixing or not. Most of the biogas plants are neither agitated properly nor do they include all the components of high rate digester, such as heat exchanger, supernatant and sludge draw off vacuum relief valves.The major problems observed in the existing biogas plants, are listed as follows:

high cost of plant installation �no control on temperature and hence biogas production falls in winter. the fall of temperature is considerably �less in new designlonger retention time �contamination due to the corrosion of the metallic gasholder �no guarantee of complete digestion of the material fed into the biogas plant �no proper agitation system �lack of scientific aspects �

V. Ranganathan recommended some improvements in existing biogas plants such as passive solar construction to maintain the temperature, simple insulations with soil or grass etc. considering all above problems in existing biogas plants and the suggestions made the new modified GHMC biogas plant is develop at School of Energy Studies, Department of Physics, Shivaji University Kolhapur laboratory to overcome these problems. The main purpose in the development of modified GHMC biogas plant is for its wide applicability in bio-solar co- generation projects based on the bagasse/ press mud utilisation.

The newly developed plant named Green House Multichamber (GHMC) biogas plant is found very efficient. It is a batch/ semi continuous plant in which three partition walls dividing the plant in four chambers have been incorporated.


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The agitator helps to avoid the scum formation in the digester. The plant fabricated and tested is of 12 litres prototype model. The layout of the GHMC plant is shown in fig.6.5. The digester consists of four chambers. The partition wall is made up of acrylic sheet of 5 mm thickness. Each chamber has 5 litres capacity. The digester has an inlet pipe for input with inclined slope with curvature at its bottom to facilitate the easy flow of the material fed.

Fig. 6.5 Modified green house multi-chamber (GHMC) biogas plant

An over flow arrangement is made for digested slurry, but not for gas which is provided a fixed top of transparent acrylic sheet from which the gas is piped to the gas measurement system. For the gas generated in each chamber, separated gas measurement system was provided. This helps to know the digestibility of the material under semi continuous digestion. The entire plant is constructed using plastic thick sheet, PVC pipes and acrylic sheet. Metallic contact is totally avoided to overcome the problem of corrosion.

Shape and size• For this biogas plant used plastic tray having size 52 cm x 29 cm x 25 cm. The size of four chambers is 13 cm x 29 cm x 25 cm for this purpose. The size of the partition wall is 20 cm x 25 cm is made up of acrylic sheet. For input PVC pipe size has diameter 1.5 inch having length 1.5 feet, and outlet PVC pipe of length of 1 feet. For easy flow of slurry, the special proper shape has been given to input and outlet as shown in fig.6.5. For semi continuous flow of slurry zigzag arrangement is made, which also a type of agitation.

Gas collection• For the gas collection special arrangement is made. Gas collection is done by down displacement of water with gas carrying pipe on which filled cylinder of water is held and placed in water tray.

Material and methods• Sugarcane press mud/ bagasse are waste material produced in sugar factories. Due to its biological compositions and dispersion ability in water, it is considered to be a better-feed material for biogas production.

Advantages• Cost of this plant is low compared to other biogas plant. �As solar radiation can pass through the acrylic sheet, there is no fall in biogas production in winter. �Retention time is low. �There is no contamination due to the corrosion of the metallic gasholder. �As there is presence of agitator scum formation is prevented. �As there is guarantee of complete digestion of the material fed into the biogas plant, large amount of gas �can be formed.

It is found that this newly developed GHMC biogas plant reduced the retention time drastically and is a great successful achievement in the field of biogas technology.

6.4 Performance of Biogas PlantThe performance of biogas plant is measured in terms of the quantity of biogas produced per day and the quality of the gas. It varies from model to model and also changes with the location of the biogas plant. The performance of the biogas plant is affected by various factors and these are discussed in the following section.

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6.4.1 Factors Affecting Biodigestion or Gas Production

The performance of biogas plant depends on following factors:• Temperature of slurry �pH of slurry �C/N ration of waste material �Loading rate �Uniform feeding �Seeding �Nutrients �Retention time �Size of biogas plant �

Temperature of slurry• It is observed that the rate of biogas production is higher in summer than in the winter. This is attributed �to the variation in temperature and the activity of various bacteria. In summer due to high temperature, the rate of breakdown of raw material increases and digestion period decreases. The variation of the period of digestion with temperature is shown in fig.6.6. This can further be understood from the activity of various bacteria present in the digester slurry. The �bacteria can be classified into two types depending on their temperature of operation such as thermophilic bacteria (requires temperature 50-600C), and mesophilic bacteria (30-400C). At higher temperature, as we increase the temperature the rate of break down of raw materials increases and digestion time decreases as shown in fig.6.6.From fig.6.6 it is seen that the digestion period is less. However, one cannot use this range due to the heating �of the plant at 50- 60ÚC. The optimum temperature for the good performance of biogas plant is in the mesophilic range. Methane gas production is found to be most efficient at 35-380C.

Fig. 6.6 Effect of slurry temperature on the digestion period

pH of slurry• The decomposition process proceeds in three steps such as hydrolysis, acid formation and methane formation. �These processes give rise to the change in the pH of the slurry. The methanogenic bacteria present in the slurry, however active in the pH range 7.2–8.2. Thus it is necessary �to monitor the pH range of the slurry to keep the methanogenic bacteria active.

C/N ration of waste material • Efficiency of methane production depends on the C/N ration in the waste material. The elements of the carbon �(in the form of carbohydrates) and the nitrogen (protein, ammonia, nitrates etc.) are main food for anaerobic bacteria. The carbon is used for providing energy and nitrogen is used for building the cell structure.


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The bacteria used up carbon about 30 times faster than the use of nitrogen. Therefore, the carbon and nitrogen �should be present in the proper proportions. When there is too much carbon in waste nitrogen will be used up first and carbon left over. So decomposition process is slowed down and comes to stop. Similarly too much of nitrogen lead to early use up of carbon and hence decomposition process stops and �remaining nitrogen will combine with hydrogen to form ammonia which kills or inhibit the growth of the methanogenic bacteria. Therefore, optimum carbon to nitrogen ratio for maximum microbiological activity is 30: 1.

Loading rate• It is defined as the amount of raw material feed to the digester per day per unit volume. �Loading rate depends on the size of the biogas plant. Both overloading and under loading reduce the rate �of biogas production.

Uniform feeding• If the digester is feed at the same time every day with a balanced feed of the same quality and quantity, the �micro-organisms will remain in a relatively constant organic solid concentration at all times. This will lead to the better performance.

Seeding• Raw cow dung has the less population of the bacteria, for accelerating the decomposition process; the �matter from the digested slurry is added to the fresh slurry in the digester. This is called the seeding. This is possible because the digested slurry has the more methanogenic bacteria.

Nutrients• The major nutrients required by the bacteria in the digester are carbon, hydrogen, phosphorous, oxygen, �and Sulphur of these nutrients. Nitrogen and phosphorous are always in short supply and therefore to maintain proper balance of nutrients �an extra raw material rich in phosphor (night soil) and nitrogen (chopped leguminous plants) should be added along with the cow dung to obtain maximum production of gas.

Retention time• The period of complete decomposition of the material inside the digester is known as retention time. It depends on the type of waste material and temperature. Normal value of retention time is between 30–45 days and in some cases it is of 60 days. In India retention time is found to be 30-40 days depending on the region.The different materials have different retention time.

Cow dung: 50 days �Poultry waste: 20 days �Pig dung: 20 days �Night soil: 30 days �

Size of biogas plant• In addition to the above factors size of the biogas plant also influences on its performance. When size of plant is large and i/p material feed to the plant is small then its efficiency will be low. The size depends on the factors like, the availability of raw material, requirement of biogas, environment etc.

6.4.2 Various Types of Inputs of Biogas Plant

Biogas is a gaseous fuel obtained from decomposition of biomass by the process of anaerobic digestion • (fermentation). The feed to the biogas plant includes:

urban waste (garbage) �urban refuse (human excreta) �rural, agricultural waste �cow dung �animal waste from butchery �

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Biogas production takes different time period depending on the raw material/ inputs, temperature, processes • adopted etc. The biomass used as a raw material feed/input can be classified into the following categories:

Agricultural waste �Straw of rice, wheat, other cereals or crops -Bagasse of sugar cane -Groundnut shell, rise husk, wheat husk etc. -Unused food grain of all types. -

Fruit waste and fruit tree waste �This includes wastes of all types of fruits e.g. coconut tree waste, coconut husk, coconut shells, unused and spoilt grapes, cashew, banana, mango etc.

Rural animal waste includes �Cow dung, horse dung, sheep dung -Slaughter house waste -Droppings, poultry waste -Piggery dung, piggery waste -Urban waste, municipal garbage -

Aquatic waste and aquatic crops �Waste from fishery -Harvested algae -Water hyacinth etc -

Industrial waste �Several industries process the agricultural, aquatic forest bio-resources. The waste produced during the process can be used to generate biogas. These wastes include: waste from sugar industry, tannery, furniture industry, food and fruit processing industries, marine food industry.

6.5 Applications of Biogas PlantThe main products of biogas plant are fuel gas and manure. Biogas can be utilised effectively for house hold • cooking, lighting, operating small engines, utilise power for pumping water, grinding flour by using the already known technology.The gas can be utilised in burners, and to operate both CI (diesel) and SI (petrol) engines.• In sewage treatment plants the gas is utilised as fuel for the boilers that supply hot water for heating the digesters, • for running gas engines, blowers, generators.Other byproduct is organic manure. It is rich in nitrogen and humus. It is in fully digested and finely divided • condition. It can be directly applied to the farm by mixing with irrigation water.When slurry cannot be used with irrigation water it can be used for rapid fermentation of compost. It is found • that the large content of bacteria and the nutrient material in the gas plant slurry accelerate the process of composting.The gobar gas manure can also form a good organic base for enriched manure. By enriching the manure with • chemical fertiliser like ammonium sulphate, super phosphate etc. A very fine organic base manure mixture could be produced.The biogas plants have urban applications for converting urban refuse and waste into useful energy biogas.• Biogas plants have rural applications for converting cow dung, agricultural waste etc. into biogas.•

Thus, biogas is emerging as a principle renewable energy form for village and communities. The biomass produce from cow dung and agricultural waste can save about 50 % needs of India’s villages.


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6.6 Future Prospects of BiogasThe Department of Non-conventional Energy Sources (DNES) of India under the ministry of Energy and Non-• conventional Energy Sources has given strategic priority to the gas plants. Biogas plants have been developed in India during 1930’s and 1940’s. The designs have been developed in India during 1950s, 1960s, 1970s and 1980s.Upto 1991 about 15, 00,000 family size biogas plants were installed in India. Also, about 415 biogas fired • internal combustion engines of total capacity of 5 MW were installed. More than 150 community biogas plants have been commissioned.Upto 2000AD about 150, 00,000 biogas plants are in operation in India. Biogas plants have a very significant role • in India’s rural development and energy strategy. Biogas is emerging as a principle renewable energy form for villages and communities. India is an agricultural and rural based country. India has about 0.60 million villages with about 600 million people living in villages. The cattle population of India is about 260 millions. The biomass production from cow dung and agricultural waste can serve about 50 % needs of India’s villages.Presently, fixed domed biogas plant: e.g. Janata biogas plant, Deenbandhu biogas plant and Floating gas holder • plant: e.g. Khadi and Village Industries type biogas plant, Pragati design, Ganesh biogas plant, Ferro- cement digester biogas plant are commonly used in India.Application of biogas in power generation has a potential in future by its use in a newly developing technology • of single chamber fuel cell (SCFC). A fuel cell is an electro-chemical device, which unlike storage cells can be continuously fed with fuel so that the electrical power output is sustained indefinitely. It converts liquid or gaseous fuels, such as hydrogen, hydrazine, hydrocarbons and coal gas, directly into electrical energy plus heat through the electrochemical reactions. There are varieties of the fuel cells developed depending on the availability of the fuels. It is known that the • 19th century is dominated by steam engine and IC engines govern 20th century. It is predicted that 21th century will be of fuel cells. Though the fuel cell has been invented 200 years before, it has not been commercialised and marketed. The limitations are mainly due to the availability of suitable fuel and appropriated design of fuel cells. In recent years, fuel cells with biogas as fuel have been predicted to be the potential candidate. Fig.6.7 shows • single chamber solid oxide fuel cell in which biogas can be utilised in energy conversion as follows:

Fig. 6.7 Schematic of a single chamber solid oxide fuel cell

Anode reaction: CH4 + ½ O2 → CO + 2H2 H2 + O— → H2O + 2e- CO + O— → CO2 + 2e- Cathode reaction: ½O2 + 2e- → O—

Ideally, simple chemical oxidation of hydrocarbon, which would yield CO• 2 and H2O, does not take place instead partial oxidation occurs at the anode and the products of this reaction are consumed electrochemically while oxygen is consumed electrochemically at the cathode as a consequence, the fuel and the oxidant need not be physically separated.

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Such SCFC’S have been investigated over the past few years by Hibino and co-workers at the National Industrial • Research Institute, Nagoya, Japan. Impressive performance of 467 mw/cm2 at 5000C has been reported for the fuel cells operating on methane/ air mixtures and incorporating a PdNiCeO2 cermet anode and SmSrCoO3 cathode. Because the complications resulting from sealing are eliminated.The SCFC greatly simplifies the system design and enhances thermal and mechanical shock resistance, there • by allowing rapid start up and cool-down. In general both conventional stack configurations, in which the fuel cell anode and cathode are placed on opposing sides of the electrolyte, and strip configurations, in which thin strip anodes and cathodes are placed on the same side of the fuel cell electrolyte as shown in Fig. are possible in a SCFC.


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SummaryBiomass is organic matter from plants, animals and microorganisms grown on land and in water and their • derivatives. Biomass includes forest crops, crops from agricultural fields, animal manure, urban and rural organic wastes etc. Fossil fuels are derived from biomass but are not renewable and hence not called biomass. Biomass can be • converted into gaseous and liquid fuels called biogas and biochemicals respectivelyAnaerobic digestion process has been used for many years for conversion of biomass to gases and liquids. In • biogas technology biogas/ Methane (CH4) is produced from dung by anaerobic digestion in digestion plants.Anaerobic decomposition of organic wastes by suitable bacteria produces biogas, which contains methane, • carbon dioxide, hydrogen and other impurities. Biogas can be produced from cow dung in a plant called gobar gas plant. Anaerobic decomposition takes place in two stages. In presence of mixed population of anaerobes, these anaerobes break down the complex substances and produces gas. The organic matter consists of carbohydrates, fats, and complex organic compounds. The first stage bacteria • convert complex organic matter to simpler organic compounds. These bacteria grow well in the pH range 6–7.The biogas plant mainly consists of: digester, gas holder, distribution pipes and gas appliances• Depending on nature of gasholder biogas plants are classified into mainly two groups as follows: Floating • gasholder biogas plant and fixed dome gasholder biogas plant.The main purpose in the development of modified GHMC biogas plant is for its wide applicability in bio-solar • co- generation projects based on the bagasse/ press mud utilisation.The newly developed plant named Green House Multichamber (GHMC) biogas plant is found very efficient. It • is a batch/ semi continuous plant in which three partition walls dividing the plant in four chambers have been incorporated.The performance of biogas plant depends on following factors: temperature of slurry, pH of slurry, C/N ration • of waste material, loading rate, uniform feeding, seeding, nutrients, retention time and size of biogas plant.Biogas is a gaseous fuel obtained from decomposition of biomass by the process of anaerobic digestion • (fermentation). The feed to the biogas plant includes: urban waste (garbage), urban refuse (human excreta), rural, agricultural waste, cow dung and animal waste from butchery.The main products of biogas plant are fuel gas and manure. Biogas can be utilised effectively for house hold • cooking, lighting, operating small engines, utilise power for pumping water, grinding flour by using the already known technology. The gas can be utilised in burners, and to operate both CI (diesel) and SI (petrol) engines.In sewage treatment plants, the gas is utilised as fuel for the boilers that supply hot water for heating the digesters, • for running gas engines, blowers, generators.There are varieties of the fuel cells developed depending on the availability of the fuels. It is known that the • 19th century is dominated by steam engine and IC engines govern 20th century. It is predicted that 21th century will be of fuel cells. Though the fuel cell has been invented 200 years before, it has not been commercialised and marketed. The limitations are mainly due to the availability of suitable fuel and appropriated design of fuel cells.

ReferencesMathur, A. N., 1992. • Biogas, production, management, and utilisation. Himanshu Publications. Karimov, K., 2009. • Potentialities of biogas technology: Production and utilization of biogas. VDM Verlag Publications.GreenpeaceUK, 2008.• Biogas: how it works, [Video online] Available at: <http://www.youtube.com/watch?v=but5ntRMQQc> [Accessed 5 July 2013].stevevidzi, 2012. • How to Make Home Biogas - An Introduction, [Video online] Available at: <http://www.youtube.com/watch?v=n25eEY92meo> [Accessed 5 July 2013].

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Ludwig, S., • Biogas Plants, [Pdf] Available at: <ftp://trabant.tr.fh-hannover.de/Schwarzes_Brett/Schwermer/KleinbiogasanlagenTD/Literatur_zu_Biogas/Sasse_Biogas_Plants.pdf> [Accessed 5 July 2013].Sources and Principle of Biogas Plants, • [Online] Available at: <http://www.tutorvista.com/content/science/science-ii/sources-energy/biogas-plants.php> [Accessed 5 July 2013].

Recommended ReadingDeublein, D. & Steinhauser, A., 2010. • Biogas from Waste and Renewable Resources: An Introduction, 2nd ed., Wiley-VCH publication. Khoiyangbam, R., Kumar, S., Jain, M. & Gupta, N., 2004. • Methaneemissionfromfixeddomebiogasplantsinhilly and plain regions of northern India [An article from: Bioresource Technology]. Elsevier publications.Khanal, S., 2008. • Anaerobic Biotechnology for Bioenergy Production: Principles and Applications. Wiley-Blackwell Publication.


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Self AssessmentIn India, anaerobic digestion plants are commonly known as biogas plants or___________1.

nitrogen gas plants a. gobar gas plants b. carbon gas plants c. bio-degradable gas plantsd.

Match the following2. Anaerobic digestion process1. Produce volatile fatty acidsA.

Fermentative bacteria2. Produce methane gasB.

Acid forming bacteria3. Perform initial break down of polymeric materialsC.

Methanogenic bacteria4. Conversion of biomass to gases and liquidsD. 1-A, 2-B, 3-C, 4-Da. 1-C, 2-D, 3-B, 4-Ab. 1-B, 2-A, 3-D, 4-Cc. 1-D, 2-C, 3-A, 4-Bd.

The _____ is made up of 50 % cow dung and 50 % water.3. slurry a. sludgeb. manurec. fatty acidsd.

Which of the following is not a part of biogas plant?4. Digester a. Gas blowerb. Distribution pipes c. Gas appliancesd.

Thermophilic bacteria require temperature around_______________5. 35–38a. 0C30–40b. 0C50–60c. 0C 70-80d. 0C

The period of complete decomposition of the material inside the digester is known as ______________6. retention time a. release time b. liberation timec. convention timed.

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Biogas is a gaseous fuel obtained from decomposition of biomass by the process of ____________7. aerobic digestion a. normal digestion b. unaerobic digestion c. anaerobic digestiond.

The _________ present in the slurry are active in the pH range 7.2–8.2.8. fermentative bacteria a. methanogenic bacteria b. aerobic bacteria c. bacteria d.

The complex organic compounds converted to low molecular weight soluble substances, primarily carboxylic 9. acids are called _____________.

radiationa. convectionb. hydrolysis c. conduction d.

In ________ type biogas plant the gasholder is separated from the digester, where as in _______ dome plants 10. the gasholder and digester is combined.

fixed, floating a. floating , movedb. moved, fixedc. floating , fixedd.


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Chapter VII

Biomass Energy Sources

Aim


explain various biomass sources•

explicate production of biomass with silvicultural and agricultural energy farms•

explain biomass energy conversion techniques•

Objectives

The objectives of this chapter are to :

explain silviculture (energy plantation), agriculture (energy crops), weeds and organic residues•

explicate intensive management•

elucidate rotation and spacing•

Learning outcome


understand high density planting, short growth (rotation) period•

identify rapid juvenile growth, adaptability to varying site conditions•

describe site preparation•

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7.1 IntroductionSolar energy captured by the plants through the process of photosynthesis produces biomass energy. The historical • origin of conversion and utilisation of biomass energy goes back to 1,50,000 years when man discovered fire from firewood. Since then, firewood has become the principal source of energy and even today 50 % of wood is put to its original use i.e. as fuel. Firewood and charcoal gave way to coal, coal to electricity and since 1859 oil has been the major source of • energy. The use of oil helped to usher in many significant transformations, particularly in the transportation and industrial sectors. However, the unilateral price hike of petroleum crude by the Organisation of Petroleum Exporting Countries • (OPEC) in 1973 by 300 % percent, the world is reeling under its impact. Since then serious thought began to be given to search alternatives renewable and non–polluting sources of • energy such as hydro, solar and bio-energy, which are particularly promising for countries like India with a large rural population. Biomass energy offers very great scope due to a wide spectrum of biomass available under different agro-climatic conditions of this country.

7.2 Biomass SourcesThe materials derived from biological activities, suitable for their conversion to produce energy, are collectively • referred to as a Biomass. As per the origin, the biomass sources may be classified into two broad categories: terrestrial biomass and aquatic biomass as shown in Fig.7.1.

Fig. 7.1 Biomass sources

Terrestrial biomass mainly consists of the materials derived from • silviculture (energy plantation) �agriculture (energy crops) �weeds and �organic residues (urban and rural waste) �

7.3 Silvicultural Energy FarmIn order to meet the increasing demands of biomass in paper industries, the concept of silvicultural energy farm • is introduced late in 1960. Since then experiments in intensive tree farming on short rotation are undertaken.


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The techniques employed in this farming are the combinations of forestry and agriculture. It is also referred to • agro forestry, and energy plantation. The typical characteristics of Silvicultural energy farms are:

High density planting �Short growth (rotation) period �Multiple harvests from each planting through coppice regeneration �Use of agricultural techniques of cultivation, fertilisation, irrigation and harvesting �

The success of Silvicultural energy farming depends on:• Selection of species �Intensive management �Rotation and spacing �Species recommended for energy plantation �

7.3.1 Selection of SpeciesA wide range of species have been studied for energy farms. The desirable characteristics of the species include:

rapid juvenile growth �adaptability to varying site conditions �ease of establishment �ease of regeneration (coppicing) �resistance to insects and fungi �

7.3.2 Intensive ManagementIntensive management is essential for successful silvicultural energy farming.

Site preparation• The land allotted for energy plantation is the wasteland and it is necessary to prepare the land by removal of existing vegetation, ploughing and disking. The planning of the plots and roads are needed considering the slope of the land, the ease of irrigation and harvesting. The energy farm is surrounded by the protection belt; this consists of trench (for fire and animal protection), fencing, wind protection belt, and fodder protection belt. The schematic representation of the energy plantation farm is shown in Fig..7.2.

Fig. 7.2 Energy plantation farm

Plantation• The energy plantation farm can be grown by sowing seedlings, plating bare samplings, and samplings with bags. For latter two, the development of nursery is essential. The survival rate of energy plants is more with bag sampling.

Weed control• The land used being the wasteland; it is full of weed and grows along with the energy plants. This affects on the growth rate of plants. The weed control during early growth improves plant survival and development and is necessary to obtain yields. Disking gives good results. Weed control can be major importance to energy farming.

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Irrigation• Moisture is essential for plant survival. It has been recommended that irrigation be applied at least during the first few years of growth.

Fertilisation• The yields are affected not only by water but also by soil quality and nutrients available. The nutrients are supplied by adding fertilisers to the soil.

7.3.3 Rotation and Spacing

There is a strong correlation between rotation age and planting density (spacing).Energy cropping depends upon • an appropriate range of rotation ages and spacing to meet the requirements of optimal biomass productivity and maximum economic profitability. The best combination of spacing and rotation will depend upon a variety of ecological, genetical and other factors, such as response to changes in water and nutrient availability, age-dependent wood density and the degree of urgency in efforts to reduce oil dependency.Spacing of medium to long rotation species should not differ much from that of conventional forestry; improved • ecophysical conditions and/or an intensive thinning programme may, however, justify denser spacing than normal - at least in regions with short transport distances.Typical short-rotation species and clones can be grown at varying intensities. Extremely short rotations (2–3 • years) are possible, but are sensitive to postponement of planned harvesting if spacing is too close.The geometry of spacing must be accommodated to the machines being used for planting, weeding, fertilisation • and harvesting to ensure efficient operation without causing damage to the stools or shoots. Close (70 to 100cm) double rows with safety margins in the space between the double rows corresponding to the width of the tracks seem promising regarding both biomass productivity and machine operation.The mixing of clones to avoid biological hazards of large monocultures deserves special attention, although • practical evidence is limited so far.

7.3.4 Species Recommended for Energy PlantationThere is variety of species recommended for energy plantation. To suit with the environment for a country like India, following are some of the species recommended

Eucalyptus tereticornis (Nilgiri) �Leucaena leucocephala (Subabul) �Prospis chilensis (Pardeshi babul) �Acacia auri culiformis (Bengali babul) �Tortilis (Israeli babul) �Albizzia lebbek (Sin’s) �Casuarina quisefifolla (Saru) �Dalbergia sissoo (Shisham) �

Leucaena for Energy Plantation • Out of the above species, Leucaena Leucocephala has become more popular and it has been considered here as a typical example as it has many more important characteristics: fast growth, drought resistance, strong tap root system, salt tolerance, nitrogen fixation, early flowering, easy plant protection, low cost of establishment, improvement of soil, cover crop and inter cropping with field crops. Leucaena is used as a good fodder. Leaves of Leucaena can be dried and stored as a leaf meal. The green leaves can also be used as green manure as the leaves are tender small in size and less in fibre. Leucaena wood is useful for fuel, charcoal, pole timber, paper rayon pulp production and gunpowder.Accessions of Leucaen Leucocephala are grouped into three types:

Hawaiian �Salvador �Peru type �


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Hawaiian• It is the most common dwarf type, grows slow and flowers 3–4 times in a year. It was introduced in India probably more than 100 years ago as a crop for soil reclamation.

Salvador• It is a fast growing, arboreal type popular known as the “Giant” or “Hawaiian Giant”. It is the most useful type for energy plantation and includes all the important accessions, such as K9 K28, K29, K67, K636.

Peru type• It is an intermediate between salvador and Hawaiian with basal branching habit.

7.3.5 Cultivation Practices for Salvador

Hawaiian giant is the most suited species for energy plantation and some of the popular accessions introduced • from the University of Hawaii are K8, K28, K24, K67 and K626. The cultivation practices for these plants are described in brief as follows: Before sowing, the seeds are treated with hot water and sulphuric acid for about 10 – 15 minutes. After scarification two seeds are dibbled in a polythene bag of 15 cm x 10 cm size, consisting of soil. If the soil for planting leucaena is acidic, a coating of lime and single super phosphate 0.5 kg each per kg seed • can be given before sowing. Thus, nursery can be developed with the samplings in the polythene bags. Seedling will be ready for planting in 8 – 10 weeks when they attain 10 – 20 cm height. It is beneficial to raise the nursery 2–4 months before the onset of monsoon and to plant during the early monsoon. The spacing between leucaena plants recommended are •

1 M x 1 M for energy plantation and windbreak and � 2-3 M for avenue plantation. �

However, it may vary depending on the site, availability of the soil and the method of irrigation. Phosphorus • is the most critical nutrient for leucaena. Basal application of 35-50 kg P2O5 per hectare is essential in low Phosphorus soil. Plots may be irrigated for better growth as per the availability of the water. The Jivarajbhai Patel Agroforestry Centre, Surendrabag, and Bharatiya Agro Industries Foundation, Pune, have • carried out systematic work on the growth and economics of the leucaena plant. The close spacings (45 cm x 45 cm, 60 cm x 60 cm, 100 cm x 100cm) were tried with more fertilisers and irrigation.It was found that one third of trees were the best one; another one third showed medium growth and the remaining • one third had poor growth. Thus selective cutting of the best trees at a tie was found more beneficial than clear felling of all trees at a time. The first cutting was started in the fourth year, when the tree poles were of saleable size. The second best were cut in the fifth year and the remaining one-third in the sixth year. Since, the leucaena has a vigorous coppicing growth; the trees cut in the fourth year were ready for cutting in • the seventh year. Thus a continuous cutting cycle of a short rotation ensures a continuous income from the same plot. On an average the biomass available from leucaena is listed in table 7.1.

Form of Biomass Biomass per tree (kg) Biomass per hector per yearSaleable timber 12 90Saleable Firewood 1.5 11.25Unsaleable Firewood 1.5 11.25

Table 7.1 Biomass available from leucaena

7.4 Agricultural Energy FarmIn silvicultural energy farm, trees and shrubs can produce biomass is 5–10 years of growth. On the other hand, in • agricultural energy farm, the crops attain their full growth seasonally. Thus, biomass for energy can be obtained in the same year of sowing. For energy crops, to grow expressly, a number of factors must be considered:

Crop yield, i.e., solar energy conversion efficiency �Energy yield, i.e., Ration of energy in the crop yield to those energy inputs for plantation, cultivation and �harvesting

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Water usage �Ease and method of conversion �

Considering the expenditure and income available, the year-wise profit per hectare has been worked out and • is listed below.

Year Accumulated income Rs.

Accumulated expenditure Rs.

Net income Rs. Net Av. In-come per year

Rs.

Const:Benefitratio

4th 30,000 43,750 -13,750 – 1:0.685th 60,000 54,025 5,975 1,195 1:1.16th 90,000 64,300 25,700 4,283 1:1.47th 1,20,000 75,575 44,425 6,346 1:1.58th 1,50,000 84,850 65,150 8,144 1:1.779th 1,80,000 95,125 84,875 9,430 1:1.8910th 2,10,000 1,05,400 1,04,600 10,460 1:2

Table 7.2 Economics of leucaena plantation

Considering all of the above factors, the species recommended for agricultural energy farm are: Cassava (Manihot • esculenta), Jerusalem artichoke (Helianthus tubevosus), Mahua flowers (Madhuca Indica), Sugarcane (Sacchraum Officinarum), Sunflower (Hellanthus annus), Sugarbeet (Beta Vulgaris) Sweet sorghum.Depending on the chemical constituents and their ultimate energy product, the energy crops can be divided into • two broad groups namely, carbohydrates crops and petro crops (hydrocarbon plants). Carbohydrates crops yield ethanol with fermentation and hydrocarbon plants give fuel oil after distillation.Sweet Sorghum: a multiple energy crop•

Experiments in India have shown that sweet sorghum (Jowar) as an ethanol could well be a liquid fuel for �tomorrow. Sweet sorghum provides fodder, grain from its earhead and sugar and ethanol from its stalk. Thus, it is the only crop known to the man, which gives food (grain), energy (ethanol) and fodder (bagasse). Besides the above advantages, sweet sorghum has a great tolerance to a wide range of climatic and soil �conditions. It is a short duration crop, maturing in 100 to 140 days, cheaper to grow and requires less water. Dr. Anil Rajavanshi and his colleagues at Nimbkar Agricultural Research Institute have done some pioneering work on the production and utilisation of ethanol from sweet sorghum. It is predicted that, with the land of one hectare, one can produce two to three tonnes of grain and about �2000 litres of ethanol. Thus it is possible for a farmer to get the net benefit between Rs. 4,000 – 6,000 /ha /year by growing sweet sorghum. The profit breakup consists of grain 60%, ethanol 30% and bagasse 10%. Thus, with the new concept of energy (ethanol) generation from sweet sorghum, one can find the solution for energy crisis. As a typical example, consider the case of Maharashtra. In 1984-85, the total area undergo rain sorghum �was about 6.5 million hectares. If the same area is brought under sweet sorghum, then, we will get 6.5 – 13.0 million tonnes of grain and 13 billion litres of ethanol per year. This ethanol is four time the need of kerosene for cooking the food of the population in rural Maharashtra. The excess of ethanol after the fulfillment of cooking and heating can be used in powering rural transportation �vehicles like two wheelers and modified diesel water pumping sets. Thus, this increased production of energy will have great ramifications for the rural economy and consequently for the state, as a whole.


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7.5 Aquatic BiomassAquatic biomass offers promising scope in view of their significant reproduction potential under the diverse • environmental conditions. The aquatic biomass is available from micro algae, african payal, water hyacinth, duckweeds, graularia, neogardhiella and hypnea. Among these, algae has made attractive “crop” in certain areas due to many advantages over terrestrial plants.In brief, algae grown on wastes, when compared with normal terrestrial plants will require 1/50 of the land area, • 1/10 the human resources for a water, 2/3 the energy, 1/5 the capital and 1/50 the human resources for equal amount of useful organic matter.For the good algae growth requires: •

illumination of sufficient intensity �CO � 2 supply temperature between 30–35 � 0C mineral supply and �agitation to prevent sedimentation and promote nutrient and light accessibility. �

There are four types of methods employed to grow the algae.• Clean algae grown on synthetic medium (for food). �Algae grown on wastes in conjunction with oxidising bacteria (for food, fodder, fuel or fertiliser). �Algae harvested from natural or semi natural blooms (for food or fodder). �Nitrogen fixing algae grown alone or in association with other plants (as nitrogenous fertiliser). �

The first three of these methods are summarised in Fig.7.3

Fig. 7.3 Alternative methods of algae production

The cultivation of algae by natural waterways or sea farming is highly attractive in many respects:• Ocean’s cover is 71% of the earth surface. �Half of the ocean area lies within 300C of the equator. �Little competition is efficient solar energy converters like most aquatic plants. �Ocean farming is not subjected to the normal climatic variations. �Terrestrial farming already employees the best arable land and fresh water. �

However, this method has not yet been popularised due to the following limitations:• The ocean bottom lies at depths where light intensity is nil and too deep for the growth of attached �seaweeds.Well-illuminated surface water has essentially no nutrients. �

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7.6 Biomass Energy ConversionBiomass i.e., plant products; consists mainly of carbon, hydrogen and oxygen. The same is true for virtually all • the fuels for which the energy content is inversely related to oxygen content. The familiar fuels that can be obtained from biomass conversion are two gases, hydrogen (H• 2) and Methane (CH4) and two liquids, methyl and ethyl alcohol (CH3OH and C2H5OH, respectively).There are various biomass energy conversion technologies, which can be classified into three fundamental types • namely, combustion, production of fuel from essentially dry biomass by chemical means and aqueous processing. The diagram below presents the principal conversion routes and the products involved.

Fig. 7.4 Biomass energy conversion processes and products

7.6.1 Combustion

Combustion of organic compounds in air is the oldest energy conversion process. It is the destructive biomass • energy conversion technique producing into heat energy. The maximum obtainable heats of combustion are listed in table 7.3. It can be seen from the table that the gasoline has incomparable superiority, 49 MJ/Kg-well over twice that of wood (with 20% water). Most fuels have low energy content because they are already partially oxidised. The empirical relation between heat of combustion and the weight percent of carbon, hydrogen and oxygen in • an organic fuel is given by Sopehr and Milner,

Heat of combustion (KJ/grams) =

Where, R = degree of reduction

From the above equation, it is seen that the fuel value of biomass is deteriorated by the content of oxygen. The • presence of water in biomass also affects on the heat of combustion. The typical water contents of various types of biomass are:

terrestrial plants – 60%, �fresh wood -- 47%, �Oven dried wood – 20%, �Aquatic plants - 95%, �For self-sustaining combustion the water content should be below 10 – 15%. �


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Fuel Type Source Heat of Combustion MJ/Kg (dry)

Solid fuels Carbon 32.5Charcoal (10-15%) 28–30Coal (lignite bituminous) 14.5–32.5(29)Wood (20% H2O) 20Garret pyrolytic char 21From urban refuse (20%) Urban refuse (as is) 10–11.5

Solid fuels Glucose 14–15.5Crops 13.5–16

Liquids Coal for fuels 37–41Number 4 petro fuel oil 31– 34Garret pyrolytic oil 241 Gasoline 49Methanol 21–24Ethanol 28–30Acetone 30N-Butanol 35

Gases Hydrogen 10–12Methane 37Carbon monoxide 12Natural gas 33–45Oil gas 11–41Producer gas(CO,N2,H2,CO2) 4.7–5.6Garret pyrolytic gas (27%) 20

Table 7.3 Heats of combustion of solid energy sources (after Edward 1975)

Heat of combustion is used for cooking, drying, producing high-pressure stem, electricity generation and in • industry. Due to the energy crises, the agricultural waste and urban waste are being tried nowadays for generating heat. One of the agricultural wastes is rice husk and used for heat of combustion with the specially designed furnaces. There are mainly two types of furnace namely.

flat grate furnace �cyclone furnace �

With flat grate furnace, the combustion efficiency (n) is 60% while with the cyclone furnace, it is 95%. The • cyclone firing technique utilises essentially the principle of vertical motion of fluid and the dynamics of fuel and ash particles in the fluid in the vertical motion. The technique ensures effective suspension burning of husk particles in condition of vigorous turbulence in a • cyclone chamber wherein combustion of both particles in the fluid in the vertical motion. The technique ensures effective suspension burning of husk particles in a condition of vigorous turbulence in a cyclone chamber wherein combustion of both particulate matter and gaseous combustibles takes place. Depending on the structure and geometry, the cyclone furnaces are grouped into: •

horizontal cyclone furnace �vertical cyclone furnace �

It is reported that the highest efficiency is obtained with horizontal cyclone furnace. It consists of two horizontal • cylindrical chambers. The fuel particles with primary combustion air are injected tangentially in the primary

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chamber for ignition of fuel particles and distillation of volatile components. The ignition temperature for rice husk lies between 4500C–5500C. The combustion process is completed in the secondary chamber with secondary air supply through tangential ejects. The rice husk burns to give a temperature around 12000C. The vertical cyclone furnace is shown in Fig.7.5. The rice husk is stored in a hoper. The hoper has a rotary • feeder to control the rate of discharge of rice husk. The fuel air mixture enters the chamber tangentially at the top. The mixture takes a spiral path towards the downward direction and reaches to the hearth where it ignites and burns to give a temperature around 12000C.The hot gas leaves the furnace in an upward direction, giving some of their heat to preheat the incoming air • and husks. The temperature of gases at the outlet point is reported to be 11000C. The combustion efficiency is as high as 95% compared to 60% achieved with stepped grate furnaces.

Fig. 7.5 Vertical cyclone furnace

The characteristics of cyclone type furnaces for rice husk are as follows:• Temperature of the furnace at the outlet – 600 � 0C – 11000C.Time required for the combustion of husk particles including preheating period. – 3 sec. �Air – fuel ratio: 125 cubic ft/kg. �Outside diameter of furnace: 2110 mm. �Critical velocity of husk particles: 6m/sec. �

7.6.2 Dry Chemical Processes

The dry chemical conversion processes can be grouped into pyrolysis (heating biomass in the absence of air), • gasification (heating biomass in presence of limited quantities of oxygen) and hydro gasification biomass in the presence of hydrogen).Pyrolysis•

In pyrolysis the organic material is destructively distilled in the absence of air yield a variety of energy rich �products like charcoal, oil, tar fractions and condensable gases. Thermal decomposition begins at about 1000C and increases with rising temperature. At about 2700C exothermic reactions set in with a rise in temperature (usually held at 400-5000C) bringing about complete carbonisation. The pyrolysis of wood gives rise to charcoal, and wood gas consisting of 28–33% carbon monoxide, 3.5–18% methane, 1.3% hydrocarbons and 1–3% hydrogen. The hydrogen content of the gas increases with increasing temperature of pyrolysis. The wood gas has a fuel value of 300 Btu/cuft.During the last few decades, the pyrolysis has been used primarily to produce charcoal. Recently, there �has been much research activity to produce wood gas and to dispose of agricultural and municipal waste for energy. The Georgia institute of Technology and Tech-Air Corporation have been intensively studying pyrolysis of biomass material for more than a decade and have built several pilot plants. Techno-Air pyrolytic process produces a liquid, solid and gaseous product.


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It is primarily designed for conversion of agricultural wastes like wood chips, pine bard and saw dust, Nut �hulls and cotton gin wastes have also been utilised. A simplified Techno-Air pyrolysis scheme is shown in here.

Fig. 7.6 The tech-air pyrolysis scheme

The mechanism and kinetic of the chemical reactions involved in the pyrolysis of wood is discussed at �length by Ferd Shafizadeh of Montana University employing thermo gravimetric technique. The wood or biomass contains mainly cellulose, hemicelluloses, lignin, extractive, mineral compounds and water. When the biomass is heated under controlled atmosphere, its weight reduces. The variation in weight of biomass with pyrolytic temperature is shown in Fig.7.7

Fig. 7.7 Thermogravimetry of wood

The dates are further analysed using Arrhenius plot for the rates of weight loss of wood in air and nitrogen. It �is seen that there are two distinct regions corresponding to two different pathways. The transition temperature lies at about 3000C.

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Fig. 7.8 Arrhenius plot for the first order reaction in the isothermal degradation of cellulose in air (-) and nitrogen (- - -)

In order to know the kinetics of the pyrolytic process the change in biomass with time at the fixed pyrolytic �temperatures below and above 3000C have been studied and the first order plots for the residual cellulose weight (normalised) versus time are obtained as show in Fig..7.9 both for air and N2 atmosphere.

Fig. 7.9 First order plot for the residual cellulose weight (normalised) versus time. Plots at 3100C and 3250C for air and nitrogen are similar.

It is seen that the rate of pyrolys is followed by weight loss under isothermal conditions, show and initial �period of acceleration and proceeds much faster in air than in an inert atmosphere. As the pyrolysis temperature is increased, the initiation period and the difference between pyrolysis under nitrogen and air gradually diminish and disappear at 3100C. At lower temperature, (region I), the reactions involve reduction in molecular weight or degree of polarisation �(DP) by bond scission, appearance of free radicals, elimination of water, formation of carbonyl, carboxyl and hydroperoxide groups (especially in air), evaluation of carbon monoxide and carbon dioxide and finally production of a charred residue. At by an alternative pathway, which provided a tarry pyrolyzate containing laevoglucose, about anhydroglucose �compounds, randomly linked oligosaccharides, and glucose decomposition products. Evaporation of the gaseous and volatile pyrolysis products leaves a carbonaceous char which fuels with solid phase (glowing) combustion. The char or charcoal is different from pure carbon compounds such as graphite. The cellulosis chars have been shown to have an empirical formula or approximately C � 6.7H3.3O. The yield and properties of the char are highly dependent on the pyrolysis temperature or heat treatment temperature (HTT), the nature of substrate and the presence of inorganic material.


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Gasification• Gasification is a partial oxidation process of biomass especially very promising and developing technology �for converting wood, o coal and municipal waste to gaseous products. The gas evolved consists of varying amount of carbon monoxide, carbon dioxide, hydrogen and depending on the type of process employed, large quantities of nitrogen. The mechanism of gasification can be understood from the constituents of wood. Wood mainly consists of �Holocellulose, celluolose, ligin, inorganic contents and moisture. The chemical formula of holocellulose and cellulose are shown in Fig.7.10.

(a) (b)

Fig. 7.10 The chemical formula for a) holocellulose and b) cellulose

There are various gasifiers developed for gasification of biomass and there are grouped into �up draught gasifier -down draught gasifier -crossed gasifier -

The schematic of up draught gasifier is shownbelow. The fuel descends through the three zones illustrated �and the air ascends through the oxidative combustion zone, pyrolysis zone and finally the drying zone, before being taken off.

Fig. 7.11 Schematic of up draught gasifier

The reactions occurring in different zones are as follows: �Zone A (Drying zone):The temperature of this zone lies between 1000C - 2000C

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Wet wood + Heat dry wood +steam

Zone B (Pyrolysis):The temperature of this zone lies between 2000C -5000CDry wood +Heat char+CO+CO+H2O+CH4+C2H5(illuminating gas)+pyro ligneous acids. + Tars.

Zone C (oxidation Zone):Temperature of this zone lies between 1100C–15000C.char + O2 + H2O ( steam either added or in fuel ) CO + H2 + CO2 + Heat.

7.6.3 Aqueous Processes

The biomass energy conversion by aqueous processes consists of mainly: anaerobic digestion, alcoholic • fermentation and chemical reduction. In an anaerobic digestion generally methane gas is produced from manure, algae and urban organic waste. The topic of anaerobic digestion is discussed at length separately in this seminar and reported in separate article. • In general the methane gas is produced from manure, algae and urban organic waste. Next to anaerobic digestion, the yeast fermentation route to alcohol production is the most important aqueous • process for bio-energy conversion. The ethanol can be obtained from starch, wood via acid hydrolysis, and from ligo-cellulose via enzymatic hydrolysis. There are four major states involved in microbial conversion of cellulosic materials to ethanol. These stages • are:

treating the raw material to render it susceptible to enzymatic attach; �producing the necessary enzymes �hydrolyzing the prepared substrate to sugars (chiefly glucose); and �fermenting the sugar formed to ethanol and distilling it out. �


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SummaryBiomass energy offers very great scope due to a wide spectrum of biomass available under different agro-• climatic conditions of this country.The materials derived from biological activities, suitable for their conversion to produce energy, are collectively • referred to as a Biomass. As per the origin, the biomass sources may be classified into two broad categories: terrestrial biomass and aquatic biomass.Terrestrial biomass mainly consists of the materials derived from Silviculture (energy plantation), agriculture • (energy crops), weeds and Organic residues (urban and rural waste).The techniques employed in this farming are the combinations of forestry and agriculture. It is also referred • to agro forestry, and energy plantation. The success of Silvicultural energy farming depends on: Selection of species, intensive management, rotation and spacing and species recommended for energy plantationIn silvicultural energy farm, trees and shrubs can produce biomass is 5–10 years of growth. On the other hand, in • agricultural energy farm, the crops attain their full growth seasonally. Thus, biomass for energy can be obtained in the same year of sowing. Aquatic biomass offers promising scope in view of their significant reproduction potential under the diverse • environmental conditions. The aquatic biomass is available from micro algae, African Payal, water hyacinth, duckweeds, graularia, neogardhiella and hypnea. Among these, algae has made attractive “crop” in certain areas due to many advantages over terrestrial plants. There are various biomass energy conversion technologies, which can be classified into three fundamental • types namely, combustion, production of fuel from essentially dry biomass by chemical means and aqueous processing. Combustion of organic compounds in air is the oldest energy conversion process. It is the destructive biomass • energy conversion technique producing into heat energy.Heat of combustion is used for cooking, drying, producing high-pressure stem, electricity generation and in • industry. Due to the energy crises, the agricultural waste and urban waste are being tried nowadays for generating heat. One of the agricultural wastes is rice husk and used for heat of combustion with the specially designed furnaces. The dry chemical conversion processes can be grouped into pyrolysis (heating biomass in the absence of air), • gasification (heating biomass in presence of limited quantities of oxygen) and hydro gasification biomass in the presence of hydrogen).In pyrolysis the organic material is destructively distilled in the absence of air yield a variety of energy rich • products like charcoal, oil, tar fractions and condensable gases.Gasification is a partial oxidation process of biomass especially very promising and developing technology for • converting wood, o coal and municipal waste to gaseous products. The gas evolved consists of varying amount of carbon monoxide, carbon dioxide, hydrogen and depending on the type of process employed, large quantities of nitrogen. The biomass energy conversion by aqueous processes consists of mainly: anaerobic digestion, alcoholic • fermentation and chemical reduction. In an anaerobic digestion generally methane gas is produced from manure, algae and urban organic waste. The topic of anaerobic digestion is discussed at length separately in this seminar and reported in separate article. • In general the methane gas is produced from manure, algae and urban organic waste.

ReferencesBiomass Resources,• [Online] Available at: <http://www.eere.energy.gov/basics/renewable_energy/biomass_resources.html> [Accessed 5 July 2013].Shukla, P. R., • BIOMASS ENERGY IN INDIA: TRANSITION FROM TRADITIONAL TO MODERN, [Pdf] Available at: <http://www.decisioncraft.com/energy/papers/ecc/re/biomass/bti.pdf> [Accessed 5 July 2013].

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Dr Vertis, A., Dr Qureshi, N., Yukawa, H. & Blaschek, H. P., 2010. • Biomass to Biofuels: Strategies for Global Industries. Wiley Publications. Brenes, M. D., 2006. • Biomass and Bioenergy: New Research. Nova Science Publications.YaleCourses, 2012. • 34. Renewable Energy, [Video online] Available at: <http://www.youtube.com/watch?v=lQuyMgwQqM0> [Accessed 5 July 2013].spot1333, 2010. • Biomass energy, [Video online] Available at: <http://www.youtube.com/watch?v=79nW2o-kYa4> [Accessed 5 July 2013].

Recommended ReadingLorenzini, G., Biserni, C. & Flacco, G., 2009. • Solar Thermal and Biomass Energy. 1st ed., WIT publication.Ravindranath, N. H. & Hall, D. O., 1995. • Biomass, Energy, and Environment: A Developing Country Perspective from India. Oxford University Press publications.Twidell, J. & Weir, T., 2009. • Renewable Energy Resources. 2nd ed., Taylor & Francis Publication.


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Self AssessmentThe materials derived from biological activities, suitable for their conversion to produce energy, are collectively 1. referred to as a ____________

biogasa. biomass b. biothermal c. biologicald.

Match the following2. Hawaiian1. an intermediate with basal branching habitA.

Salvador2. used as a good fodderB.

Peru type3. fast growing, arboreal type popular known as C. the “Giant”

Leucaena4. dwarf type, grows slow and flowers 3–4 times D. in a year


Leucaen Leucocephala are grouped into three types. Which of the following option is not one of its type?3. Ecuador a. Hawaiianb. Salvador c. Peru type sludged.

Which of the following is not a step in intensive management?4. Site preparation a. Deforestationb. Irrigation c. Weed controld.

____________ mainly consists of the materials derived from Silviculture (energy plantation), agriculture (energy 5. crops), weeds and Organic residues (urban and rural waste).

Aquatic biomass a. Globally biomassb. Terrestrial biomassc. Earthly biomassd.

___________ of organic compounds in air is the oldest energy conversion process.6. Combustion a. Decomposition b. Formationc. Incinerationd.

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In ________, the organic material is destructively distilled in the absence of air yield a variety of energy rich 7. products like charcoal, oil, tar fractions and condensable gases.

decompositiona. gasificationb. combustionc. pyrolysisd.

In _______ energy farm, trees and shrubs can produce biomass is 5–10 years of growth.8. sericulturea. silvicultural b. poultryc. fishery d.

_______ biomass offers promising scope in view of their significant reproduction potential under the diverse 9. environmental conditions.

Terrestriala. Globalb. Aquatic c. Aeriald.

The _________ technique utilises essentially the principle of vertical motion of fluid and the dynamics of fuel 10. and ash particles in the fluid in the vertical motion.

fixed floating furnacea. floating moved furnaceb. flat grate furnace c. cyclone firingd.


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Chapter VIII

Ocean Energy

Aim


explain ocean as a potential source of energy•

explicate origin of tides and ocean waves•

calculate working principle of the Ocean thermal energy conversion, ocean wave and tidal energy plants•

Objectives


explain ocean temperature difference•

explicate absorption of solar energy at different depth of ocean•

enlist types of harnessing energy from ocean•

Learning outcome


understand tidal energy•

identify aquatic ocean biomass•

describe ocean waves•

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8.1 Ocean as a Potential Source of EnergyInstead of depending on the land-based resources, there is a good potential with energy resources from ocean. The 1/4th part of the earth is occupied by land and 3/4th part is occupied by ocean. Hence to solve the problem of energy crises, the human being is now looking at the energy from the ocean. There are number of methods for getting energy from the ocean and these are:

Ocean thermal energy conversion (OTEC)• Ocean currents• Tidal energy• Hydroelectric energy• Aquatic ocean biomass•

8.2 Ocean Temperature Difference and its Use as a Source of EnergyThe heat energy from the sun stored in the ocean can be used to generate electricity. This electricity generation • makes use of temperature difference between the warm surface water and cold water deep inside the ocean. Ocean temperature difference is originated due to solar energy. The ocean thermal energy concept was proposed • in 1881 by the French Physicist Jacques d’ Arsonval.

Fig. 8.1 Absorption of solar energy at different depth of ocean

When solar radiations pass through ocean water, there is absorption of light as shown in Fig..8.1. The intensity • of solar radiation varies with depth of the ocean which is given by the relation,

Id = I0 e -α d

Where, ‘I � d’ is the intensity of solar radiation at depth‘d’ and ‘a’ is the absorption coefficient of water and Io is intensity of solar radiation in air.

These solar radiations are then converted into heat energy. The variation in intensity of radiation with depth • of ocean, give rise to temperature difference as shown in Fig..8.2 (a). A heat source is available at a higher temperature and a heat sink at lower temperature. The temperature difference across the depth of the ocean is maintained because the hot water has low density and • cold water has high density. Therefore hot water cannot move towards the bottom of ocean and cold water due to its high density cannot come up to the top of the ocean. This gives rise to permanent temperature difference between top surface and bottom of the ocean. The variations in density of water with temperature is shown in Fig.8.2 (b)


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Fig. 8.2 (a) Variation of temperature with respect to depth of ocean

Fig. 8.2 (b) Variation of density of water with respect to temperature

The maximum density of water occurs at temperature 3.98• 0C. Below this temperature, density decreases. Hence in no case, the 00C is observed at any depth of the ocean and no ice formation at the bottom of the ocean.There are two types of OTEC systems.•

Open Cycle (Claude cycle) OTEC system �Close Cycle (Anderson cycle) OTEC system �

8.2.1 OTEC System (Method of Conversion of Ocean Energy into Electrical Energy)

The concept of ocean thermal energy conversion (OTEC) is based on the utilisation of the temperature difference • in the ocean. Temperature of the surface of ocean is higher than its bottom. The surface temperature (and temperature difference) varies with both latitude and season. Both being maximum • in tropical, subtropical and equatorial waters i.e. between the two tropics, making these waters the most suitable for OTEC systems. According to thermodynamics principle, the temperature difference obtained can be converted in to mechanical • energy, which in turn converts into electrical energy using electrical generator. The concept of Carnot heat engine is used to generate power. The efficiency of the Carnot engine (Thermodynamic cycle) is given by, •

h= (T1–T2)/T1Where,T1 is the high temperature T2 is the low temperature

The process of OTEC requires that the warm surface water and cold water from depth (about 1000 m – 1500 m) • be brought into proximity so that they act as the heat source and the heat sink respectively for a heat engine.

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In other words, solar energy collected and stored as heat by the world’s major oceans, can be converted into • electricity through a generation process. Similar to that in case of OTEC no depletable fuel is required. Thus, OTEC power plants provide a potentially sustainable renewable source of electricity, located mainly at sea.There are two different methods for harnessing ocean thermal differences,•

Open cycle OTEC system also known as Claude cycle �Close cycle OTEC system, also known as Anderson cycle. �

8.2.2 Open Cycle (Claude Cycle) OTEC SystemThe first OTEC plant was built by the Frenchman Georges Claude in 1929 on the Mantanzas Bay in Cuba.

Principle• The hot water from the surface of sea is fed in evaporator, which acts as a source heat of energy. In the �evaporation chamber the low-pressure steam is produced and it is passed to the turbine. Due to the vapour pressure of steam turbines starts to rotate and electricity is generated. The cold water �at the bottom of the sea is used as heat sink. It is pumped from the bottom of the sea and brought into the direct contact condenser. A direct contact condenser is used to cool the turbine exhaust steam and condensed water is then allowed �to discharge back in to the ocean. This cycle is called as the open cycle because the condensate need not be returned into the evaporator. The whole system runs in the temperature range of 7OC to 27OC.

Working• The warm water on the top surface is serving as the source of heat energy. It is allowed to enter into �evaporator. Initially the pressure in the evaporator is lowered with vacuum pump. Vacuum pumps are also used to remove exhaust gases produced in the evaporator. The ocean hot water produces low-pressure steam in to the evaporator. This low-pressure steam is passed �through a turbine. This low-pressure steam exerts force on a turbine, which rotates it. Turbine is further connected to the electrical generator. Electrical generator generates electricity, which can �be given by the Faradays Law of electromagnetic induction.

e = -dφ/ dtAccording to this law rate of change of flux (f) is directly proportional to the emf generated in the coil of �generator. After passing through the turbine, steam is mixed with cold water pumped from the bottom of the ocean into the condenser. This mixture is then discharged back into the ocean. The Claude plant used as open cycle in which seawater itself plays the multiple role of heat source, working �fluid, coolant, and heat sink. The schematic flow diagram is shown in Fig..8.3 (a) and corresponding temperature versus entropy (T-S) diagram is shown in Fig.8.3 (b).

Fig. 8.3 (a) Open cycle OTEC system


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Fig. 8.3 (b) T-S diagram corresponding to Fig. 8.3 (a)

Though ocean temperature difference gives the energy flow free of cost and available plenty in the nature the • OTEC system is not popular due to following reasons.

Its energy conversion efficiency is very low (6.66%) -The electricity generated from this system is not economical -The maintenance cost is very high. -

Hence the system is not used widely. It is used only at isolated place like Iceland.

8.2.3 Close Cycle (Anderson Cycle) OTEC System

Principle• Closed cycle approach was first proposed by Barjot in 1926, but the most suitable design was proposed by �Anderson in 1960’s and hence it is also referred as the Anderson cycle. In this cycle, propane or ammonia was chosen as the working fluid. The temperature difference between hot surface water and cold water at bottom was 20 � 0C. The cold water was at about 600 m deep. Propane is vaporized in the boiler or evaporator and exhausted through condenser via turbine at about 5 bar and recycled. Hence turbine experience force and rotates the electrical generator to produce electricity.

Fig. 8.4 Close cycle (Anderson closed cycle) OTEC system

Working• The heat required for vaporisation is transferred from the warm ocean surface water to the working fluid �by means of a heat exchanger. The vapour leaving the evaporator drives an expansion turbine, similar to a steam turbine that it is designed to operate at lower inlet pressure.

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The turbine is connected to an electric generator in the usual manner. The low-pressure exhaust from the �turbine is cooled and converted back in to the liquid in the condenser. The cooling is achieved in to a heat exchanger by circulating cold deep ocean water, from a depth of 700 m to 900 m or more, around the steam-carrying pipe. The liquid working fluid is then pumped back as high-pressure liquid to the evaporator, thus closing the cycle. �Condenser and evaporator are the heat exchangers and are the major components of heat exchangers. In an ocean, the problem of biofouling mainly suffers the heat exchangers. This biofouling means accumulation �of a layer of slime due to action of the microorganisms on the waterside of the heat exchanger. This would resist exchange of heat between the working fluid and the cold deep ocean water. Large amount of the water must be circulated through this system. Widely used working fluids are ammonia, �propane, Freon, etc. This system is more popular than Claude cycle.

8.3 Ocean WavesWhen a stone is thrown on the steady surface of water, there develops waves on the surface of water and these • waves are propagating in outward direction. In the similar fashion the surface of the ocean when comes in contact with the wind produces waves.There is interaction between air molecules from the wind and the water molecules from the surface of the • ocean. At the costal area the temperature gradient is developed between land and ocean and there is flow of air molecules (wind) from ocean to the land and vice versa. When the air molecules flow over surface of water, it tries to shift the equilibrium position of the water molecules. • But due to gravitational force, the water goes again back and the waves are formed. Thus there are two forces acting on the water molecules at the surface of ocean due to the interaction between wind and surface of water and waves are formed as shown in Fig.8.5

Fig. 8.5 Ocean waves

Due to propagation of the wave there is displacement of water molecules with time t about its equilibrium • position and this is given by the equation.

Y = a sin [(2π x) /λ - (2πt)/T]The amplitude (a), period (T) and wavelength λ of the wave depend on the wind velocity.Since the water molecules moving up and down about its equilibrium position, it is associated with Potential • Energy and Kinetic Energy along x –direction. The energy associated with the ocean wave is given by the relation,

E/A= 1/2a2ρgWhere,

a= amplitude of the waveρ = densityg = gravitational constant.Then the energy associated with wave is,

Energy density/ T = 1/2a2ρg / TEnergy density = 1/2a2ρgf


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Where, f = frequency of the wave.

Fig. 8.6 Conversion of ocean energy to electrical energy

Then the energy associated with the ocean wave can be converted in to mechanical energy by using the float. It • can be wooden piece or light matter which can move upward and downward with the ocean wave. The float is connected to the piston via arm and the whole system is mounted in ocean. As the ocean waves • comes and goes out, the float will move up and down giving rise to motion in the piston and converts motion energy into mechanical energy. The shape of the down portion of the float is of typical size and it is such that the ocean waves are always • perpendicular to the surface of the float. Though the energy can be obtained from the ocean waves, it has not become popular due to the following reasons,

Installation is very costly. �It is not regular source of energy. �Cost is very high. �Efficiency is low. �

8.3.1 Energy and Power from the Waves

Wave at time, t

a= Amplitude = Wavelength

Trough

a

Crest

y

x

dx

ay

y

x

mnt

mnt

+2 m

nt+

Fig. 8.7 Two-dimensional progressive wave observed in an ocean

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Two-dimensional progressive wave as shown in Fig..8.7 is represented by the sinusoidal simple harmonic wave • shown at t = 0 and at time t = t. The wave may be expressed by the following relations.Y = a sin [(2π x) /λ - (2πt)/λ] -------------------------------------------------------------------- (8.1)Where,

Y = height above its mean level in metera = amplitude in metreλ = Wavelength in metret = time in secondsT = period in seconds

The ocean wave has the total energy comprising potential energy and kinetic energy is due to the vertical • displacement of the water molecules in the direction of propagation. Consider a small fraction of the wave between x and x+ dx. Then the potential energy associated with the small fraction of the wave is d (P.E.). The P.E. is depends on the relation,(P.E.) = m g h ----------------------------------------------------------------------------- --(8.2)For the ocean wave, height h = Y/2 and there is conversion factor g• c which is originated due to the frequent change in height with reference to the bottom surface.d (P.E.) = m (g/gc) (Y/2) ------------------------------------------------------------------------------- -(8.3)Since m is the mass of the fraction of ocean wave.

m =Y dx L ρ ------------------------------------------------------------------------------------------------ (8.4)Where,

Y = height above mean level.ρ = density of water in Kg/m3

L = arbitrary width of two-dimensional wave perpendicular to the direction of wave propagation.

d (P.E.) = Ydx L ρ (g/gc) (Y/2) ------------------------------------------------------------------------------- (8.5)

d (P.E.) = 1/2 ρL (g/gc) Y2dx -------------------------------------------------------------------------------- (8.6)

(P.E.) = ∫d (P.E.) ---------------------------------------------------------------------------------- (8.7)= ∫1/2 ρ L (g/gc) Y

2 dx --------------------------------------------------------------------------------(8.8)= 1 /2 ρL (g/gc) 0∫

l Y2 dx ------------------------------------------------------------------------------ (8.9)

Substituting, Y = a sin (mx – nt)Where, m = 2π/λ and n =2π/t

P.E. = 1/2 ρL (g/gc) 0∫l a2 sin2 (mx – nt) dx --------------------------------------------------------------- (8.10)

= 1/2 ρL (g/gc) a20∫

l sin2 (mx – nt) dx ------------------------------------------------------------- -- (8.11)Hence, 0∫

λ sin2 (mx – nt)dx = λ/2 = 0∫λ( 1-cos 2mx)/2 dx --------------------------------------------------- (8.12)

= λ/2 0∫λ - [(sin 2mx)/4m] 0∫

λ ------------------------------------------------------------- - (8.13)= λ/2 - (sin 2mλ)/4m ------------------------------------------------------------------ -- (8.14)

But, m = 2π/λ and n =2π/ t0∫

l sin2 (mx – nt)dx = λ/2 – [sin 2(2π/λ)λ]/4(2π/λ) ----------------------------------------------------------- (8.15)0∫

l sin2 (mx – nt) dx = λ/2 – [(sin 4π)/4(2π/λ)] --------------------------------------------------------------- (8.16)For a single wave there is no phase difference and hence second term is zero.

0∫l sin2 (mx – nt) dx = λ/2 -----------------------------------------------------------------------------------------(8.17)

Substituting, equation (8.17) in equation (8.11) we get,P.E. = 1 /2 ρL (g/gc) a

2 λ/2 -----------------------------------------------------------------------------------(8.18)= 1/4 ρL (g/gc) a

2 λ --------------------------------------------------------------------------------------(8.19)But, λL = area of unit ocean wave.Hence, P.E. = P.E. /λL = 1 /4 ρ (g/gc) a

2 --------------------------------------------------------------(8.20) = P.E.density

Similarly, one can work out the kinetic energy (K.E.) of the ocean wave and is given by the relation,K.E./area = 1/4 ρ (g/gc) a

2 (9.28)


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Total energy, (T.E.) = P.E. + K.E.= 1 /4 ρ (g/gc) a

2 + 1 /4 ρ (g/gc) a2

T.E. = 1 /2 ρ (g/gc) a2

Now,Power = E /t = [1 /2 ρ (g/gc) a

2]/tPower = 1 /2 ρ (g/gct) a

2

Advantages:• It is renewable form of energy. �It can be used repeatedly. �

8.3.2 Estimation of the Energy and Power from Ocean Tides

The expression of maximum energy that can be generated during one generation period can be derived with the • help of Fig..8.7, which shows the case of the basin beginning at high tide level, emptying through the turbine to the ocean, which is at low tide.

Area A

Basin Dam

High tide level

Range R

Ocean atlow tide

Reversible turbine and gates

dh

h

Fig. 8.8 Energy and power from ocean tides

During the emptying process the differential work done by the water is equal to its potential energy at that • time. Considering a tidal range R and intermediate head at a given time, the amount of work done is calculated considering small head dh, for a intermediate head dh as shown in Fig..8.8

dw = dm g h But, dm = ρA dhSo that,

dw = - ρ A dh g h Where,

w = work done by water Kcal/kg or Jouleg = gravitational constantm = mass flowing through turbine Kgh = head mρ = Water density Kg/m3

A = Basin surface area, considered constant m2

The total work done during full emptying period is obtained by integrating the expression.• W = R∫0 dw = -g r A R∫0 h dh = 1/2 r g A R2

Thus total work done is proportional to square of the tidal range R. The power is the nothing but rate at which • work is done. The power is generated during emptying (or filling) the basin and no power is generated during rest of time .The average theoretical power delivered by the water is the ratio of w to the total time (t) that it takes the period to repeat itself. This period is 6 hrs and 12.5 minutes.

6h 12.5min = 22, 350 secondsThus, average theoretical power (P) in watts can be given as,

Pav = w/ time = ρgAR2 / (2 x 22350)

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= 1/44700 g ρ A R2

Assuming an average sea water density = 1025 Kg/m2, the average power per unit basin area is given by,Pav = (1/44700) x 9.80 x 1025 R2

= 0.0225 R2 watts /m2 The actual power generated by a real tidal system would be less than the average theoretical power obtained by • the above expression due frictional losses and conversion efficiencies of turbine and electric generators. The actual power generated may be about 25 to 30 percent of the theoretical power.

8.4 Tidal EnergyTide is a periodic rise and fall of the water level of the sea, which is carried by the action of the attractive • gravitational force exerted by of the sun and moon on the water on the surface of earth. The first attempt to utilise energy of the ocean was made in the form of the tidal “mills” in the eleventh century in Grate Britain and later in France and Spain. The large scale up and down movement of the seawater represents an unlimited source of energy. If some part • of this vast energy is converted into electrical energy, it would be an important source of hydropower. The main feature of the tidal cycle is the difference in water surface elevations at the high tide and at the low • tide. At the time of the high tide water surface is elevated to high level and this elevated water can be allowed to store in the basin. During low tide ocean water level is low and hence water stored in the basin at high level is allowed to flow • through the turbine to the ocean. This flowing water rotates the turbine. Rotational motion of the turbine is given to the generator to produce electricity as shown in Fig.8.9.

Sea

High tide Tidal basin

Low tide

T.G. SeaT.G.

Fig. 8.9 Principle of tidal power generation

The first tidal power plant was commissioned by General De Gaulle at La Rance in 1966 which marked a break • through. The average tidal range available for the plant varies between 8.4 m (±4.2 m) to 13.5 m .In India there are possible tidal projects in the Gulf of Kutch and on smaller scale, in the sunder bans regions of the Bay of Bengal.

8.4.1 Origin of the Tides

The origin of the tide is due to interaction between the gravitational forces as well as the kinematic forces acting • on the water in the ocean. From Newton’s law of gravitation, we know that the gravitational force experienced by the to bodies is directly proportional to the product of the masses (m1 and m2) of two bodies and inversely proportional to square of the distance (r) between them,

F =Gm1m2/ r2

In order to understand the origin of the tide considers the interaction between small masses (H• 2O) placed at point P on the surface of the earth and the moon. The forces acting at a point p are as shown in Fig..8.10. At a point P there are mainly two forces responsible for the phenomenon of tides,


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Gravitational force of attraction denoted as F• G and,Centrifugal force (F• r), acting at a point P on the surface of earth because earth is rotating along with earth. It has two components,

Normal components �Tangential components. �

M

FG

E

FC

D

Fr

rF G

|

FG Fr

Fig. 8.10 Forces responsible for the phenomenon of tides

The normal component of Fr has no contribution of tidal phenomenon as it is balanced by the gravitational force • to the earth. Tangential component of force to the earth is called tractive force. The distribution of this tractive force over the surface is shown in Fig..8.11. It is seen that, the magnitude of the tractive force is maximum and is given in Fig.8.11.

South

North

Fig. 8.11 Distribution of tractive forces

Tidal range is maximum for the place, which lies on the earth’s latitude but it is not uniform all over the world. • The variety of interaction between the forces acting at a point on the ocean water on the surface of the earth tides occur. The rise and fall of the water levels follows a sinusoidal curve, shown with point ‘A’ indicating the high tide point and point ‘B’ indicating the low tide point. The average time for the water levels to fall from A to B and then rise to C is approximately 6 hours 12.5 minutes.

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+6

+4

+2

0

-2

-4

-6

12 h 25 min

R Tidal range

A

B

C

Fig. 8.12 Range of the tide

The difference between high tide and low water level is called the range of the tide. The tidal range R is defined • as,

R = Water elevation at high tide – Water elevation at low tide,Diurnal tides•

Diurnal means daily, pertaining to actions completed in 24 hours and recurring every 24 hours. These tides �include two low and two high tides for one rotation of the moon around the earth. For diurnal tide mean range is the same as diurnal tidal range. This phenomenon is originated due to the interaction between earth and moon. At the time near full or new moon, when sun moon and earth are approximately in a line, the gravitational �forces of sun and moon enhance each other. The tidal range is then exceptionally large. The higher tides are higher and low tides are lower than the average. These high tides are called spring tides.On the other hand near the 1st and 3rd quarters of the moon when the sun and moon are at right angles with �respect to the earth neap tides occur. When the tidal range is small, the high tides are lower and the low tides are higher than the average. Hence the range is not constant. It varies during the 29.5-day lunar month.

New Moon

First quarter

Full

moonThird

quarter

New Moon

12h 25 min.

Spring tide

Neap tide

Spring tide

Neap tide

Spring tide

29.51 d

Fig. 8.13 Relative high and low tides showing variation in the range during lunar month

Following points have to be specially noted in case of tidal phenomenon.• The tides are periodic phenomenon but no two tides in any cycle are alike. Since the relative position of �sun, moon, and their distance from earth continuously changes.The mean tidal range varies from place to place. �The shape of the tidal cycle depends upon the interaction of sea with the costal line of the earth. �


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8.4.2 Tidal Energy System

The power generation from tides involves flow between artificially developed basins at the sea. However, in • order to have a more or less continuous generation, this basic scheme can be elaborated by having two or more basin. Accordingly we can distinguish the following types of arrangements:One way single basin system•

In single basin system there is only one basin interacting with sea. Ocean and basin are separated by a dam �or barrage and the flow of water from ocean to basin through sluice gate, which is located along the dam. During high tide it is allowed to fill the dam and during low tide empting of the basin is accomplished �through single gate in which turbines are placed. At outlet turbines are placed. During emptying the basin potential energy of the water is converted into kinetic energy. This kinetic energy is converted in to rotational energy by using the turbines. These turbines rotate the �electrical generator to generate electrical energy. The variations of ocean level and water level in the basin are shown in Fig..9.14. This system is operated where the tidal range is not sufficient. In this system the power is generated only �when the water is flowing from basin to ocean. The disadvantage of this system is that the power station is operated only for (hour/ shorter period) and hence less efficient. The duration of power generation for this system is about 48 minutes per day.

High Level

Sluice Gate

Basin

T.G. Turbine and generator

G1

Fig. 8.14 One way single basin system

Two way single basin• In this system, the basin is filled during high tide by ocean water. Energy generation process of this system �is same as one-way single basin system but electrical energy is also generated during filling of the basin by placing turbines and generators in the gate, which is used to fill the basin. When the tidal range is sufficient, the power generated by filling as well as emptying the basin. Thus the �two way single basin system one can generate the power 12 hours in a day.

Ocean Sluice GGate

9

Basin G1

G2

Fig. 8.15 Two way single basin8.4.3 Multiple Basin System

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This system produces power for 24 hours. The water is always flowing from high level to low level. When the • ocean level is higher then gate G2 is closed and G1is open and high as shown in Fig..8.16.

Barrage

High level basin

Low level basin

G|

TG

G

P

Fig. 8.16 Multiple basin system

At high-level basin is filled up. When the ocean level goes down G1 is closed and G2 is open for emptying the • low level basin. There is always flow of water with sufficient way from high to low level basin and electricity is generated.


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SummaryThe 1/4• th part of the earth is occupied by land and 3/4th part is occupied by ocean. Hence to solve the problem of energy crises, the human being is now looking at the energy from the ocean. Different methods for getting energy from the ocean include ocean thermal energy conversion (OTEC), ocean • currents, tidal energy, hydroelectric energy and aquatic ocean biomass.The heat energy from the sun stored in the ocean can be used to generate electricity. This electricity generation • makes use of temperature difference between the warm surface water and cold water deep inside the ocean. Ocean temperature difference is originated due to solar energy. The maximum density of water occurs at • temperature 3.980C. Below this temperature, density decreases. Hence in no case, the 00C is observed at any depth of the ocean and no ice formation at the bottom of the ocean.There are two types of OTEC systems. They are open cycle (Claude cycle) OTEC system and close cycle • (Anderson cycle) OTEC systemThe concept of ocean thermal energy conversion (OTEC) is based on the utilisation of the temperature difference • in the ocean. Temperature of the surface of ocean is higher than its bottom. Tide is a periodic rise and fall of the water level of the sea, which is carried by the action of the attractive • gravitational force exerted by of the sun and moon on the water on the surface of earth. The main feature of the tidal cycle is the difference in water surface elevations at the high tide and at the low • tide. At the time of the high tide water surface is elevated to high level and this elevated water can be allowed to store in the basin. The origin of the tide is due to interaction between the gravitational forces as well as the kinematic forces acting • on the water in the ocean. are mainly two forces responsible for the phenomenon of tides, gravitational force of attraction denoted as FG and centrifugal force (Fr), acting at a point P on the surface of earth because earth is rotating along with earth. Diurnal means daily, pertaining to actions completed in 24 hours and recurring every 24 hours. These tides include • two low and two high tides for one rotation of the moon around the earth. For diurnal tide mean range is the same as diurnal tidal range. This phenomenon is originated due to the interaction between earth and moon.

ReferencesCruz, J., 2010. • Ocean Wave Energy: Current Status and Future Prespectives (Green Energy and Technology). 1st ed., Spinger Publications.Charlier, R. H. & Justus, J. R. 1993. • Ocean Energies: Environmental, Economic and Technological Aspects of Alternative Power Sources (Elsevier Oceanography Series). Elsevier Science Publications.2011. • Ocean energy, [Video online] Available at: <http://www.youtube.com/watch?v=NsDFYfJZsXs> [Accessed 5 July 2013].theAneja, 2009.• Ocean Energy - Wave Power Station, [Video online] Available at: <http://www.youtube.com/watch?v=gcStpg3i5V8> [Accessed 5 July 2013].Chapter 14: Ocean Energy• , [Pdf] Available at: <http://www.energyquest.ca.gov/story/chapter14.html> [Accessed 5 July 2013].OCEAN ENERGY, • [Online] Available at: <http://ec.europa.eu/research/energy/eu/index_en.cfm?pg=research-ocean> [Accessed 5 July 2013].

Recommended ReadingPeppas, L., 2008. • Ocean, Tidal, and Wave Energy: Power from the Sea (Energy Revolution). 1st ed., Crabtree Publishing Company.Charlier, R. H. & Finkl, C. W., 2009. • Ocean Energy: Tide and Tidal Power. Springer publications; 262 pages.Avery, W. H. & Chih Wu., 1994. • Renewable Energy From the Ocean: A Guide to OTEC. Oxford University Press Publication.

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Self AssessmentThe 1/41. th part of the earth is occupied by _______ and 3/4th part is occupied by_____.

ocean, land a. land, ocean b. rivers, farm c. farm, mountaind.

Match the following:2.

Ocean thermal energy conversion1. maximum for the place, which lies on the earth’s latitude A. but it is not uniform all over the world

Tidal energy2. due to interaction between the gravitational forces as well B. as the kinematic forces acting on the water in the ocean

Tidal range 3. The large scale up and down movement of the seawater C. represents an unlimited source of energy

The origin of the tide 4. based on the utilisation of the temperature difference in D. the ocean


Which of the following is not a type of OTEC systems?3. Release Cycle OTEC systema. Open Cycle OTEC systemb. Claude cycle OTEC systemc. Anderson cycle OTEC systemd.

Which of the following statements is true?4. Temperature of the surface of ocean is lower than its bottom. a. Temperature of the surface of ocean is higher than its bottom b. Temperature of the bottom of ocean is higher than its surface. c. Temperature of the bottom of ocean is elevated than its surface. d.

The concept of Carnot heat engine is used to generate ____________5. velocitya. voltageb. power c. potentiald.

___________ used, in which seawater itself plays the multiple role of heat source, working fluid, coolant, and 6. heat sink

The Claude Plant a. The Anderson Cycle b. The Close Cycle c. The Sealed Cycle d.


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Energy can be obtained from the ocean waves; it has not become popular due to the following reasons, which 7. of the following reasons is false?

Installation is very costlya. It is not regular source of energyb. Cost is very highc. Efficiency is highd.

Tide is a periodic rise and fall of the water level of the sea, which is carried by the action of the attractive 8. ____________ exerted by of the sun and moon on the water on the surface of earth.

kinetic forcea. gravitational force b. magnetic forcec. mechanical forced.

_______ means daily, pertaining to actions completed in 24 hours and recurring every 24 hours.9. Neuronal a. Multiunit b. Diurnal c. Monaurald.

The power generation from tides involves flow between artificially developed ______ at the sea.10. docksa. bridgesb. damsc. basinsd.

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Chapter IX

Geothermal Energy

Aim


explain the types of geothermal energy resources•

explicate advantages and disadvantages of geothermal energy•

elucidate environmental problems•

Objectives


explain hydrothermal•

explain vapour dominated and liquid dominated systems•

elucidate sources of hot water•

Learning outcome


understand electrification and mini-grid applications•

understand fumaroles•

analyse thermal features•


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9.1 IntroductionThe thermal energy contained in the interior of the earth is called the geothermal energy. Geothermal energy • is also one of the renewable energy sources. Geothermal energy has practically no intermittency, has highest energy density, and is economically not far removed from conventional technologies. The geothermal energy is enormous and will last for several millions of years. The evidence of the enormous • geothermal energy stored deep inside the earth is in the form of:Hot water springs• The geysers: Hot water and steam released periodically from small vents in the ground in volcanic region or • geothermal fields.Fumaroles: Hot steam and gases released from small vents in the ground in volcanic regions or geothermal • fields.Volcanic eruptions: Eruption of geothermal energy in large quantities releasing hot lava, rocks, ash, mud, forming • a typical conical hill or mountain.Geothermal energy is a proven resource for direct heat and power generation. In over 30 countries geothermal • resources provide directly used heat capacity of 12,000 MW and electric power generation capacity of over 8,000 MW. It meets a significant portion of the electrical power demand in several developing countries. For example, in the • Philippines geothermal provides 27% of that country’s total electrical generation, from power plant complexes as large as 700 MW.Individual geothermal power plants can be as small as 100 kW or as large as 100 MW depending on the energy • resource and power demand. The technology is suitable for rural electrification and mini-grid applications in addition to national grid applications. Direct use of geothermal heat can boost agricultural and aquaculture production in colder climates and supply heat for industrial processes that can add value to local primary products. Geothermal resources may be especially important and significant in developing nations where no indigenous • fossil fuel resources such as oil, coal or natural gas exist. For example, in Tibet, where no readily available fossil fuels exist, the Nagqu geothermal field provides a useful energy source for the local population. Costs of geothermal electric power are very much dependent on the character of the resource and project size. • The unit costs of power currently range from 2.5 to over 10 US cents per kilowatt-hour while steams costs may be as low as US $3.5 per ton. Major factors affecting cost are the depth and temperature of the resource, well productivity, environmental compliance, project infrastructure and economic factors such as the scale of development, and project financing costs. Geothermal energy is an important resource in volcanically active places such as Iceland and New Zealand. • How useful it is depends on how hot the water gets. This depends on how hot the rocks were to start with, and how much water we pump down to them. Water is pumped down an “injection well”, filters through the cracks in the rocks in the hot region, and comes • back up the “recovery well” under pressure. It “flashes” into steam when it reaches the surface. The steam may be used to drive a turbo generator, or passed through a heat exchanger to heat water to warm houses. A town in Iceland is heated this way. The steam must be purified before it is used to drive a turbine.

9.2 Origin of Geothermal EnergyThe temperature of earth increases with the depth. The average increase in temperature with depth is about 25 • to 300C per kilometre. This is called as average temperature gradient. Therefore it is necessary to drill 10 km deep production wells to obtain geothermal fluids at significant temperature of the order of 3000C. The planet earth originated from the sun several years ago and is cooling slowly. The earth was originally a mass • of hot liquids, gases and steam. As the fields cooled by loosing heat to the atmosphere, the outer solid crust; oceans and lakes were formed. The average thickness of cooler outer crust is about 30 km.

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Hot dry rocks, hot gases and liquids are deposited in the region below average depth of 2800 km. The magma • is in the temperature range of 1250 to 15000C. The centre of the earth is at temperature of 45000C. The inner core of the earth has several minerals including iron, nickel, silicon and magnesium. Fig..9.1 shows a typical geothermal field. The geothermal fields are the regions in which energy deposits are • available at a depth less than 1500 to 30000C. The hot magma near the surface solidifies into igneous rock. Igneous rock found at the surface is called volcanic rock. The heat of magma is conducted upward to this igneous rock. Ground water finds its way down to this rock • through cracks in it. This cold water will be heated by the heat of the rock or by mixing with hot gases and steam originating from the magma. The heated water will then rise convectively upward and into porous and permeable reservoir above the igneous • rock. A layer of impermeable solid rock, which traps the hot water in the reservoir, caps this reservoir. However, the solid rock has fissures that act as vents of the giant underground boiler. The vents show up at surface • as geysers, fumaroles or hot springs. A well taps steam from the fissure for use in a geothermal power plant.There are two kinds of geothermal steams. One originating from the magma itself, called as magmatic steam • and other from ground water heated by the magma, called as meteoritic steam. The later is the largest source of geothermal steam. All the geothermal sources do not produce steam. Some sources are lower in temperature so that there is only hot water; some receive no ground water at all and • contain only hot rock. The earth is loosing heat slowly through the outer crust with average energy loss of about 0.025 W/m2, which is too small compared with average solar radiation on the earth’s surface (25 W/m2).

A

C

BHeat

B

DD

E

F

G

H A - MagmaB - Hot rock (Igneous)C - ReservoirD - Solid rockE - Fissures (cracks)F - GeyserG - Hot springH - A well

Fig. 9.1 A typical geothermal field

9.3 Types of Geothermal Energy ResourcesThere are three types of geothermal sources namely:

Hydrothermal �Geopressured �Petrothermal �


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9.3.1 Hydrothermal Systems

Hydrothermal systems are those in which water is heated by contact with hot rock. The hydrothermal systems • are best resources for geothermal energy exploitation at present. Hydrothermal systems are sub divided into,

vapour dominated �liquid dominated systems �

Vapour dominated systems• In these systems water is vaporised into steam that reaches the earth surface in a relatively dry condition at �about 2000C and rarely above 7 kg/cm2 (7 bar). Fig.9.2 Vapour dominated power plant. This steam is most suitable for use in turbo electric power plants, with the least cost.However, it suffers from the presence of corrosive gases and erosive materials and environmental problems. �These types of systems are very less in number. There are only five known sites in the world. The geysers plant in the United States is the largest in the world today and other is Larderelo in Italy. Both are vapour-dominated systems. Fig.9.2 shows schematic diagram of a vapour dominated power system. Dry steam from the wells is collected, �filtered to remove abrasive particles and passed through turbines, which drives electric generators in the usual manner. The essential difference between this system and a conventional steam turbine generator system using fossil �fuel or nuclear fuel is that geothermal steam is supplied at a much lower temperature and pressure. The dry steam from the well at 2000C is used. It is nearly saturated at the bottom of well and may have a shut-off pressure up to 35 kg/m2 (35 bar). Pressure drops through the well causes it to slightly super heat at the wellhead. The pressure there rarely �exceeds 7 bars. It then goes through a centrifugal separation and then enters turbine after additional pressure drop.

Centrifugal separator

Turbine

Cooling Tower

WellGround

Reinjection Well

Condensate Pump

Generator

Steam jet

ejector

Direct contact

condenser

Alternate Reinjection 1

2

3

4

56

7

7

Fig. 9.2 Vapour dominated power plant

Liquid dominated systems• In this type of systems temperature of water is above normal boiling point (100 � 0C). However, as the water in the reservoir is under pressure, it does not boil but remains in the liquid state. When water comes to the surface the pressure is reduced and rapid boiling occurs so that liquid water flashes into a mixture of hot water and steam. The steam can be separated and used to generate electric power in usual manner. The remaining hot water �can be utilised to generate electric power or to provide space and processes heat, or it may be distilled to yield purified water. The water comes with various degrees of salinity, ranging from 3000 to 280,000 ppm of dissolved solids, �and at various temperatures. Therefore there are various systems for converting liquid-dominated system into useful work that depends upon these variables. The flashed-steam system suitable for water in the higher temperature range is one of the liquid dominated systems.

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The flashed-steam system• Fig.10.3 shows the schematic of the flashed-steam system. Water from the underground reservoir reaches �the wellhead at lower pressure. This process is essentially a constant enthalpy throttling process that results in two-phase mixtures of low quality at wellhead. This is throttled further in a flash separator resulting in a still low but slightly higher quality. This mixture �is now separated into dry saturated steam and saturated brine. The saturated brine is reinjected into the ground. The dry steam usually at pressures below 7 kg/m � 2 is expanded in a turbine and mixed with cooling water in a direct contact condenser with the mixture going to a cooling tower in the same fashion as the vapour dominated system. The balance of the condensate after the cooling water is recirculated to the condenser is reinjected into the ground.

Fig. 9.3 Liquid dominated single-flash steam system

The power generation from such system can be made more economical by associating chemical industry with • power plant to make use of the brine and the gaseous effluent. The flashed-steam system has following limitations as compared with the vapour-dominated system. This system requires much larger total mass flow rates through the well.• Due to large amount of flows, there is a greater degree of ground surface subsidence.• The system provides a greater degree of precipitation of minerals from the brine, resulting in the necessity for • design of valves, pumps, separator internals, and after equipment for operation under scaling conditions.Greater corrosion of piping and well casing.• Many times temperature and pressure of the water may not be sufficient to produce the flash steam.•

9.3.2 Geopressured Systems

Geopressured systems are sources of hot water. The water has been heated in the similar manner to hydrothermal • systems except that geopressured water is trapped in deeper underground aquifers at depths between 2400 to 9100 m. The water is thought to be at relatively low temperature of about 160• 0C and is under very high-pressure more than 1000 bar. It has relatively high salinity of 4 to 10 percent, and is often referred to as brine. A special feature of geopressured waters (or brines) is that their content of methane (natural gas), thought to be the • result of decomposition of organic matter. The energy value of the brine thus depends on their temperature. The solubility of methane in water at normal pressure is quite low, but it is increased at the high pressures of the • geothermal reservoirs. Such water is thought to have thermal and mechanical potential to generate electricity.

1

2 3

4

56

7

9

Brine

Flas

h Se

para

tor

Cooling Tower

Turbine

Direct contact condenser

Condensate pump8


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However, the temperature is not high enough and depth so great that there is a little economic justification of • drilling for this water. However, an attention can be drawn to recover the methane in the solution, which can be used for electric generation. Studies have been underway to determine the economic feasibility of generating electricity by a combined cycle i.e. combustion of methane as well as heat from the thermal content of the water.

9.3.3 Petrothermal Systems

Petrothermal systems are composed of hot dry rock (HDR) but no underground water. The known temperature of • HDR varies in between 150 to 2900C. This energy is called as Petrothermal energy. It represents by far the largest source of geothermal energy of any type. The rock, occurring at moderate depths, has very low permeability and needs to be fractured to increase its heat transfer surface. The thermal energy of the HDR is extracted by pumping water (or other fluid) through a well that has been • drilled to the lower part of the rock. The water moves through the fractures, picking up heat. It is then travels up a second well that has been drilled to the upper part of the rock and finally back to the surface. There it is used in power plant to produce electricity.Hot dry rocks exist because they are impermeable to water or no because water does not have access to them. • It is necessary for the heat transport mechanism that, the impermeable rock must be rendered into a permeable structure with large heat- transfer surface. A large surface is particularly necessary because of the low thermal conductivity of the rock. Rendering the • rock permeable is done by fracturing it, and water is introduced from the surface. The water is heated up by the rock and is returned to the surface where the heat is utilised as stated above. There are two fracturing methods that involve drilling of the well into the rock and then fracturing it. These • are, by using

high pressure water �nuclear explosive �

High Pressure water: Injecting water into HDR at very high pressures does fracturing by high-pressure water. • This water widens the existing fractures and creates new ones through rock displacement. This method is successfully used by oil industry to facilitate the path of underground. Nuclear explosives: In this method high nuclear explosives are introduced at the bottom of the well drilled into • the rock. Fracturing by nuclear explosives is the scheme that has been considered a part of programme for using such explosives for peaceful uses such as natural gas and oil stimulation, creating cavities for gas storage, canal and harbour construction. The principle hazards associated with this method are ground shocks, the danger of radioactivity releases to the environment and radioactive material that would surface with the heated water and steam. A feature of this Petrothermal system is that, as the reservoir heat is depleted with time, the temperature differences • within the rock result in stresses that causes the original fractures to propagate, thereby unlocking more HDR surface to the water and resulting in a pancake-shaped fracture zone. Fig.9.4 shows the schematic of Petrothermal concept, i.e. how heat is extracted from hot dry rocks. It is believed • that HDR systems offer more flexibility in operation and design than other geothermal systems. For example, the designer can have a choice of water flow rates and temperature by drilling to various depths, and operator can change pumping pressure and hence flow rates to suit load conditions.There are problems which are faced by developers include•

leakage of water (or fluid) underground �the effect of the water or fluid on rock composition �material carryover with the fluid �cost �

It should be noted that two wells are to be drilled instead of one and these wells are drilled deeper and in much • harder rock. This is expected to make Petrothermal exploitation very costly, unless the underground rock being developed is very hot.

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Pump

Heat exchanger

Power plant

3-5 km 2500C

Hot

gra

nite

Low

ther

mal

con

duct

ivity

Sed

imen

ts

Fig. 9.4 A schematic diagram of heat extraction from hot dry rock system

9.4 Advantages and Disadvantages of Geothermal EnergyAdvantages•

Geothermal energy does not produce any pollution, and does not contribute to the greenhouse effect. �The power stations do not take up much room, so there is not much impact on the environment. �No fuel is needed. �Once you’ve built a geothermal power station, the energy is almost free. �It may need a little energy to run a pump, but this can be taken from the energy being generated. �

Disadvantages• The big problem is that there are not many places where you can build a geothermal power station. One �needs hot rocks of a suitable type, at a depth where we can drill down to them. The type of rock above is also important; it must be of a type that we can easily drill through.Sometimes a geothermal site may “run out of steam”, perhaps for decades. �Hazardous gases and minerals may come up from underground, and can be difficult to dispose safely. �


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9.5 Environmental Problems: Impacts of Geothermal DevelopmentThe degree to which geothermal development affects the environment is, in most cases, proportional to the scale • of such development. For example, the environmental impacts associated with geothermal direct use projects are often minimal. Those associated with large-scale electrical generation projects may be very large. The direct use projects are often designed as closed loop use systems where the low- or medium-temperature • geothermal fluids are circulated through a heat exchanger or heat pump. Natural features such as hot springs, mud pools, geysers, fumaroles and steaming ground are associated with most geothermal systems. Because of their unique nature, these are often tourist attractions or are used by local residents. Geothermal development that draws from the same reservoir has the potential to affect these features. These • visible signs of geothermal activity are part of a country’s heritage and in any geothermal development; they must be taken into account during the environmental impact report. Another environmental problem caused by geothermal plants is land surface subsidence. This occurs because • of the extraction of the large quantities of underground fluids. Large extractions and reinjection also pose the possibility of seismic disturbances. Noise pollution is another problem. Exhausts, blow downs, and centrifugal separation are some of the sources of noise that necessitate the installation of silencers on some equipment. The thermal features may also hold significant cultural and spiritual importance to many indigenous people. The • Waiotapu Thermal Area in New Zealand is classified as Category A under New Zealand RMA: areas containing unique and outstanding hydrothermal features that must be completely preserved if a representative selection of features is to be retained. Therefore, under no circumstances is geothermal development allowed in this geothermal field. The potential • impacts of large-scale geothermal development are summarised in the table below.

Potential Impact Potential Effect Mitigation/Remediationmeasures

Land requirement Vegetation loss• Soil erosion wells• Landslides• Land ownership issues•

Single drill pads –several wells• Re-vegetation programmes• Adequate land compensation•

Water take from streams/wa-terways for drilling purposes

Impact on local • watershedDamming and diverting • local streams

Take from streams with high • flow ratesCoincide drilling with rainy • season not dry seasonBuild temporary reservoirs• Liaise with local farmers to • take their usage into account

Water take from reservoir Loss of natural features• Increase in steaming • ground hydrothermal eruptionsLowering of water table• Increase in steam zone• Subsidence• Saline intrusion•

Avoid water take from • outflowsAvoid areas where propensity • for Hydrothermal eruptions (which occur naturally also)Careful sustainable • management of resource, balancing recharge with take

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Waste (brine and condensate) d i s p o s a l i n t o s t r e a m s / waterways

Biological effects• Chemical effects• Thermal effects•

Effluent treatment and removal • of undesirable constituents Reinject all waste fluids• Cascaded uses of waste fluids • eg. Fish farms, pools

Reinjection Cooling of reservoir• Careful planning of reinjection • wells outside main reservoir

Induced seismicity• Monitor flow patterns before • reinjection e.g. Tracer tests

Scaling• Anti-scale treatment of fluids• Drilling effluent disposal into streams/waterways

Biological effects• Chemical effects•

Contain in soakage ponds or in • barrels for removal.

Air emissions Biological effects• Chemical effects• Localised slight heating • of atmosphereLocalised fogging•

Effluent treatment and removal • of undesirable constituentsMinimise emissions by • scrubbing H2S and treating other NCGs (Non Condensable Gases)

Noise pollution Disturbance to animals • and humansImpaired hearing•

Muffling of noise eg. Silencers•

Table 9.1 Impacts of large-scale geothermal development


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SummaryGeothermal energy has practically no intermittency, has highest energy density, and is economically not far • removed from conventional technologies. The geothermal energy is enormous and will last for several millions of years.Geothermal energy is an important resource in volcanically active places such as Iceland and New Zealand. • How useful it is depends on how hot the water gets. This depends on how hot the rocks were to start with, and how much water we pump down to them. There are two kinds of geothermal steams. One originating from the magma itself, called as magmatic steam • and other from ground water heated by the magma, called as meteoritic steam. The later is the largest source of geothermal steam. All the geothermal sources do not produce steam. Some sources are lower in temperature so that there is only hot water; some receive no ground water at all and • contain only hot rock. The earth is loosing heat slowly through the outer crust with average energy loss of about 0.025 W/m2, which is too small compared with average solar radiation on the earth’s surface (25 W/m2).There are three types of geothermal sources namely; hydrothermal, geopressured and petrothermal.• Hydrothermal systems are those in which water is heated by contact with hot rock. The hydrothermal systems • are best resources for geothermal energy exploitation at present. Hydrothermal systems are sub divided into, vapour dominated and liquid dominated systems.The degree to which geothermal development affects the environment is, in most cases, proportional to the scale • of such development. For example, the environmental impacts associated with geothermal direct use projects are often minimal. Those associated with large-scale electrical generation projects may be very large.

ReferencesGupta, H. K. & Roy, S., 2006. • Geothermal Energy: An Alternative Resource for the 21st Century. Elsevier Science Publications. Tabak, J., 2009. • Solar and Geothermal Energy (Energy and the Environment). 1st ed., Facts on File Publications.alternativeenergycom, 2007. • How a Geothermal Plant Works, [Video online] Available at: <http://www.youtube.com/watch?v=kjpp2MQffnw> [Accessed 5 July 2013].DTEEnergyCompany, 2009. • Geothermal Energy, [Video online] Available at: <http://www.youtube.com/watch?v=uVDBRQvBVso> [Accessed 5 July 2013].A Guide to Geothermal Energy and the Environment• , [Pdf] Available at: <http://geo-energy.org/reports/environmental%20guide.pdf> [Accessed 5 July 2013].Geothermal, [Pdf] Available at: <http://www.need.org/needpdf/infobook_activities/IntInfo/GeothermalI.pdf> • [Accessed 5 July 2013].

Recommended ReadingErnst Huenges & Ledru, P., 2010. • Geothermal Energy Systems: Exploration, Development, and Utilization. Wiley-VCH Publication.Glassley, W. E., 2010. • Geothermal Energy: Renewable Energy and the Environment. 1st ed., CRC Press publications.Watchel, A. & Voege, D., 2010. • Geothermal Energy (Energy Today). 1st ed., Chelsea House Publications.

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Self AssessmentGeothermal energy is an important resource in ____________ active places.1.

mountaina. volcanically b. icebergsc. waterfalld.

Match the following2.

Vapour Dominated Systems1. A. Maximum for the place, which lies water is trapped in deeper underground aquifers at depths between 2400 to 9100 m.

hydrothermal2. B. Composed of hot dry rock (hdr) but no underground water

geopressured3. C. Water is heated by contact with hot rock

petrothermal4. D. Water is vaporised into steam that reaches the earth surface in a relatively dry condition at about 2000c and rarely above 7 kg/cm2


Which of the following is not an advantage of geothermal energy?3. The power stations do take up much room, so there is much impact on the environmenta. Geothermal energy does not produce any pollution, and does not contribute to the greenhouse effectb. No fuel is needed.c. Once you’ve built a geothermal power station, the energy is almost freed.

magmatic steam and meteoritic steam are the types of ___________.4. thermal steamsa. geothermal steams b. electrical steams c. magnetic steams d.

In ___________, water is vaporised into steam that reaches the earth surface in a relatively dry condition at 5. about 2000C and rarely above 7 kg/cm2 (7 bar).

fluid dominated systems a. liquid dominated systems b. vapour dominated systemsc. solid dominated systemsd.


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A special feature of geopressured waters (or brines) is that their content of ______ (natural gas) is thought to 6. be the result of decomposition of organic matter.

methanea. ethaneb. propane c. butaned.

The solubility of methane in water at normal pressure is quite low, but it is increased at the high pressures of 7. the _____________________

thermal reservoirs a. hydrothermal reservoirsb. aquatic reservoirs c. geothermal reservoirsd.

Petrothermal systems are composed of _________, but no underground water.8. hot damp rock (HDR)a. hot dry rock (HDR) b. hot dehydrated rock (HDR) c. hot drenched rock (HDR)d.

Sometimes a geothermal site may “run out of_________”9. watera. vapour b. steamc. aird.

Large extractions and reinjection also pose the possibility of _______ disturbances10. minerala. fluidb. noisec. seismicd.

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Case Study I

Solar Power generating systems at the milk collection centres of Panchmahal dairyDairy industry in India is developing at faster rate. However rising cost of inputs in dairy industry such as milk, fuel and electricity is one of the major constraints. The milk collection centres cannot be operated in rural areas due to unavailability or lack of electricity. To overcome these difficulties a Solar Photovoltaic Power Generating System can be utilized for providing continuous uninterrupted power and operation of Milk Collection Centres.

In this direction Shri Bhupendrasinh P. Solanki, Member of Parliament (LS) and Chairman, Panchmahal Dairy, Godhra had already taken an initiative and contacted the Ministry of Non-conventional Energy Sources, Government of India, New Delhi with the proposal. The MNES had favourably considered the proposal and sanctioned 10 nos. of Solar Photovoltaic Power Generating Systems.

The total cost of the 525 Wp system is approximately Rs.1.70 lacs, including installation, commissioning, transportation, operation and one-year maintenance and monitoring. The MNES provides a subsidy of Rs 0.85 lacs for the system.

ResultsAll the systems were found to be performing excellent and the response was overwhelming from the secretary/ president of the Doodh Utpadak Shakari Mandli of these villages. The revenue is also increased. In this context, it was informed that in one of the villages some of the milk producers use to submit the milk to the centre when the power failure was there. Thus, the measurement of fat etc. was done manually and the milk was manipulated for high fat content, which lead to monetary loss to the collection centres.

Now, with the uninterrupted power supply with the help of Solar Photovoltaic Power Generating System such loss does not occur to the milk collection centre and they are able to provide better services. There is tremendous potential for Solar Photovoltaic Power Generating System as there are about 1000 milk collection centres per district and about 6 District are eager to install this type of Solar Photovoltaic Power Generating System in the State of Gujarat. It is the Doodh Utpadak Shakari Mandli – milk cooperative – a non-profit making organization, which has started the cooperative movement and now want to install Solar Photovoltaic Power Generating System at all milk collection centres.

QuestionsWhat is a Solar Photovoltaic Power Generating System?1. AnswerPhotovoltaic (PV) panels convert sunlight to electricity that can be used to supplement or replace the electricity supplied by the utility grid. PV panels are most commonly installed on rooftops, and are most effective with a southerly exposure that provides full sun. Other possible installations include a ground mount, a pole mount, and atop a porch, carport, or other shaded area.

Why was Solar Photovoltaic Power Generating System introduced in milk collection centres of Panchmahal 2. dairy?AnswerDairy industry in India is developing at faster rate. However rising cost of inputs in dairy industry such as milk, fuel and electricity is one of the major limitations. The milk collection centres cannot be operated in rural areas due to unavailability or lack of electricity. To overcome these difficulties a Solar Photovoltaic Power Generating System was installed for providing continuous uninterrupted power and operation of milk collection centres.


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What were the results observed on introduction of Solar Photovoltaic Power Generating System in the milk 3. collection centres?AnswerAll the systems were found to be performing excellent and the response was overwhelming from the president • of the Doodh Utpadak Shakari Mandli of these villages. The revenue also increased. In this context it was informed that in one of the villages some of the milk producers use to submit the milk to • the centre when the power failure was there. Thus the measurement of fat etc. was done manually and the milk was manipulated for high fat content, which lead to monetary loss to the collection centres. Now, with the uninterrupted power supply with the help of Solar Photovoltaic Power Generating System such • loss does not occur to the milk collection centre and they are able to provide better services.

What are the advantages of solar PV systems?4. AnswerThe advantages of PV systems are•

It uses clean, cheap, noiseless, safe, renewable solar energy to produce electrical energy at the location of �the utilisation, conservation of non-renewable fuels.Suitable for remote loads away from main electrical network and at places where other fuels are scarce and �costly. Cost of installation of long distribution lines, a distribution substation etc. is eliminated.Suitable for portable or mobile loads. E.g. radio sets, cars, buses, space crafts. �reliable service, long life. �mildest maintenance. �

What are the limitations of solar PV systems?5. AnswerThe limitations of PV systems are•

Irregular, intermittent supply of solar energy. �Need for storage batteries. �High capital cost due to large number of PV cells, low out put power, low efficiency and high technology �involved.Not economical for central power plants of MW rating due to very large areas of PV panels and very large �storage battery systems.Requires storage batteries and or additional diesel generator sets for supplying power during night and �during cloudy periods.Do not generate power during cloudy seasons. Not suitable during rainy seasons. �Space for installing large PV panels is not available in large cities, industrial cities, etc. except on roofs of �buildings.Advanced PV technology is required for producing PV cells. �Very low efficiency of PV cells (10 to 14 %) �

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Case Study II

The Appropriate Rural Technology Institute (ARTI), IndiaIntroduction:Pune is an affluent city in south India. However waste food is often discarded at the side of the road, attracting stray dogs, flies and rats and creating a public health hazard. ARTI has developed a biogas plant which uses food waste to supply biogas for cooking, replacing liquid petroleum gas (LPG) or kerosene. The plant is sufficiently compact to be used by urban households.

WorkingBiogas systems take wet organic material (feedstock) into an air-tight tank, where bacteria break down the material and release biogas – a mixture of methane with some carbon dioxide. A pipe takes the biogas to the kitchen, where it is used to cook with a biogas stove or for other purposes. Most biogas plants in India and elsewhere are designed to use animal manure as their main feedstock, and are therefore only used in rural areas. ARTI developed a compact biogas plant which uses organic materials available in urban areas, such as waste flour or kitchen waste, as feedstock. This feedstock has a higher energy density compared to manure, and digestion takes place much more quickly (typically 1 to 2 days, compared with 30 to 40 days for a manure-based plant), so a smaller quantity of decomposing material needs to be held in the plant at any one time. One kg (dry matter) food waste feedstock produces about 0.25 kg of methane, whereas 20 kg of cattle dung feedstock would be needed to produce the same quantity of methane.

How much does it cost and how do users pay?US$1 = Rs 47 (Indian Rupees) [November 2009]A compact biogas plant with a biogas stove costs about US$200 (Rs 10,000) to buy, but costs nothing to run if it uses only food waste. Even if waste flour is bought for feedstock, the running cost is only about US$0.04 (Rs 2) per day. There are no subsidies, so owners pay the full cost of the plant, although some suppliers accept payment in instalments.

How is it manufactured and maintained?ARTI trains local entrepreneurs also representatives of other NGOs to produce and install biogas plants. By 2006, 30 people had been trained and ten had established themselves as entrepreneurs.

BenefitsAbout 700 biogas plants were in use in 2006, in both urban and rural households in Maharashtra. A few have been installed in other parts of India and elsewhere in the world.

Environmental benefitsARTI estimated that using only household food waste in a biogas plant halves the use of LPG or kerosene for cooking, saving a typical urban household 100 kg/year of LPG or 250 litres/year of kerosene. This is equivalent to 300 to 600 kg/year CO2 . Further reductions in fossil fuel use and CO2 emissions arise from not having to transport LPG cylinders to be re-filled.

Social benefitsIndoor air pollution is reduced by cooking with biogas rather than wood or kerosene. This reduces respiratory and eye problems for those in the kitchen, most of whom are women. Using food waste in a biogas plant means that less of it is now discarded by the roadside, reducing the public health hazard.

Economic benefitsAn ARTI biogas plant and stove costs about US$200 (Rs 10,000), compared with only about US$100 (Rs 5,000) for an LPG bottle and stove. However, LPG costs about US$0.60 (Rs 30) per day. If biogas halves the amount of LPG used, then the cost of a complete biogas system is saved within two years. Some families who use a pressure cooker for cooking and collect food waste from their neighbours have replaced all their LPG use.


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QuestionsHow does ARTI biogas plant work?1. How much does the biogas plant cost and how do users pay?2. What are the environmental benefits of a biogas plant?3. What are the socio-economic benefits of a biogas plant?4. What are the different parts in a biogas plant?5.

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Case Study III

Geothermal District Heating Systems: Klamath Falls, OregonLocation: Klamath County, OregonOwner: City of Klamath FallsCapacity: 16 MBtu/hr (4.7 MW)Temperature: 210°F (99°C)Startup date: March 20, 1984Developer: the city of Klamath FallsCost: $ 2,580,000 (for the initial construction)

The city of Klamath Falls, Oregon, is located near a geothermal resource that has provided heating for homes, businesses, schools and institutions for many years. The district heating system was constructed in 1981 to initially serve 14 government buildings and 120 residents with some limited capacity for expansion. Total cost of the project was $2.58 million, consisting of 65% federal funds and the remainder from city, county and state funds.

The district heating system was originally designed for a thermal capacity of 20 million Btu/hr (5.9 MW thermal). At peak heating, the buildings on the system utilized only about 20% of the system thermal capacity and revenue from heating those buildings was inadequate to sustain system operation. This led the city to begin a marketing campaign in 1992 to add more customers to the system.

The city developed a flat rate for heat customers, which for most is about 50% of the costs for gas heat. New customers are connected directly into the distribution system with district loop water used as the building heating medium. This eliminates the customer’s need for a heat exchanger, thus reducing potential retrofit costs to participate. The state also operates two financial incentive programs. The first one offers businesses a 35% tax credit on the costs associated with connection to the district heating system and the second one provides loans to cover the entire cost of the energy project.

As well as being used to heat buildings, geothermal water is also piped under Klamath City’s roads and sidewalks to keep them from icing. The snowmelt systems have been incorporated into a downtown redevelopment project along Main Street which started in 1995. The heated sidewalk and crosswalk area currently served by the systems is over 60,000 ft2.

In 2003 and 2004, with assistance from the Department of Energy, the district heating system was upgraded to a thermal capacity of 36 MBtu/hr, allowing more customers to use the system. The control system has also been fully updated and integrated into the City Supervisory Control and Data Acquisition (SCADA) system, which monitors the water and wastewater systems.

QuestionsDefine geothermal energy with examples.1. What are the benefits from the Klamath Falls, Oregon to its residents?2. Apart from heating buildings, what are the other functions of the geothermal water district?3. When was the first district heating system constructed and what was its capacity?4.


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Recommended ReadingAgrawal, B. & Tiwari, G. N., 2010. • Building Integrated Photovoltaic Thermal Systems: For Sustainable Developments (RSC Energy Series), 1st ed., Royal Society of Chemistry publication.Avery, W. H. & Chih Wu., 1994. • Renewable Energy From the Ocean: A Guide to OTEC. Oxford University Press Publication.Benduhn, T., 2008. • Ethanol and Other New Fuels (Energy for Today). Gareth Stevens Publication.Benduhn, T., 2008. • Solar Power (Energy for Today). Gareth Stevens publication.Charlier, R. H. & Finkl, C. W., 2009. • Ocean Energy: Tide and Tidal Power. Springer publications; 262 pages.Deublein, D. & Steinhauser, A., 2010. • Biogas from Waste and Renewable Resources: An Introduction, 2nd ed., Wiley-VCH publication. Duffie, J. A. & Beckman, W. A., 2006. • Solar Engineering of Thermal Processes. Wiley Publication.El-Hinnawi, E., 1983. • New and Renewable Sources of Energy (Natural resources & the environment). Tycooly Publication. Ernst Huenges & Ledru, P., 2010. • Geothermal Energy Systems: Exploration, Development, and Utilization. Wiley-VCH Publication.Ewing, R. A., 2007. • HYDROGEN - Hot Stuff Cool Science: Discover the Future of Energy. 2nd ed., PixyJack Publication.Ewing, R. A., 2007. • HYDROGEN - Hot Stuff Cool Science: Discover the Future of Energy. 2nd ed., PixyJack Publication.Glassley, W. E., 2010. • Geothermal Energy: Renewable Energy and the Environment. 1st ed., CRC Press publications.Hau, E. & Renouar, H., 2010. • Wind Turbines: Fundamentals, Technologies, Application, Economics, 2nd ed., Springer Publications. Holland, G. & Provenzan, J., 2007. • Hydrogen Age, The: Empowering a Clean-Energy Future. Gibbs Smith publications.Holland, G. & Provenzan, J., 2007. • Hydrogen Age, The: Empowering a Clean-Energy Future. Gibbs Smith publications.Kaltschmitt, M., Streicher, W. & Wiese, A., 2010. • Renewable Energy: Technology, Economics and Environment, 1st ed., Springer publication.Khanal, S., 2008. • Anaerobic Biotechnology for Bioenergy Production: Principles and Applications. Wiley-Blackwell Publication.Khoiyangbam, R., Kumar, S., Jain, M. & Gupta, N., 2004. • Methaneemissionfromfixeddomebiogasplantsinhilly and plain regions of northern India [An article from: Bioresource Technology]. Elsevier publications.Lorenzini, G., Biserni, C. & Flacco, G., 2009. • Solar Thermal and Biomass Energy. 1st ed., WIT publication.Manwell, J. F., McGowan, J. G. & Rogers, A. L., 2010. • Wind Energy Explained: Theory, Design and Application. 2nd ed., Wiley Publication.Masters, G. M., 2004. • RenewableandEfficientElectricPowerSystems. Wiley-IEEE Press.

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Merrigan, J. A., 1980. • Sunlight to Electricity: Prospects for Solar Energy Conversion by Photovoltaics. The MIT Press publication.Peppas, L., 2008. • Ocean, Tidal, and Wave Energy: Power from the Sea (Energy Revolution). 1st ed., Crabtree Publishing Company.Ravindranath, N. H. & Hall, D. O., 1995. • Biomass, Energy, and Environment: A Developing Country Perspective from India. Oxford University Press publications.Sathyajith, M., 2006. • Wind Energy: Fundamentals, Resource Analysis and Economics. Springer publications.Scheer, H., 2004. • The Solar Economy: Renewable Energy for a Sustainable Global Future. Earthscan Publications Ltd.Sukhatme, S. P. & Nayak, J. K., 2009. • Solar Energy: Principles of Thermal Collection and Storage. 3rd ed., McGraw-Hill Education publication.Twidell, J. & Weir, T., 2009. • Renewable Energy Resources. 2nd ed., Taylor & Francis Publication.Watchel, A. & Voege, D., 2010. • Geothermal Energy (Energy Today). 1st ed., Chelsea House Publications


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Self Assessment Answers

Chapter Ia1. b2. d3. c4. a5. c6. d7. b8. a9. c10.

Chapter IId1. b2. c3. b4. a5. d6. c7. b8. a9. b10.

Chapter IIIb1. d2. c3. a4. c5. b6. d7. c8. c9. b10.

Chapter IVb1. d2. a3. b4. c5. a6. d7. b8. c9. d10.

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Chapter Vb1. d2. a3. b4. c5. a6. d7. b8. c9. d10.

Chapter VIb1. d2. a3. b4. c5. a6. d7. b8. c9. d10.

Chapter VIIb1. d2. a3. b4. c5. a6. d7. b8. c9. d10.

Chapter VIIIb1. d2. a3. b4. c5. a6. d7. b8. c9. d10.


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Chapter IXb1. d2. a3. b4. c5. a6. d7. b8. c9. d10.

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