Photovoltaic Powered Electrolysis Hydrogen Production System
166
1 Optimisation of Photovoltaic- Powered Electrolysis for Hydrogen Production for a Remote Area in Libya A thesis submitted to The University of Manchester for the degree of Doctor of Philosophy in the Faculty of Engineering and Physical Sciences 2011 Matouk M. Elamari School of Mechanical, Aerospace and Civil Engineering
Transcript of Photovoltaic Powered Electrolysis Hydrogen Production System
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Optimisation of Photovoltaic-
Powered Electrolysis for Hydrogen
Production for a Remote Area in
Libya
A thesis submitted to The University of Manchester for the degree of
Doctor of Philosophy
in the Faculty of Engineering and Physical Sciences
2011
Matouk M. Elamari
School of Mechanical, Aerospace and Civil Engineering
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List of Contents
List of Figures …………………………………………………...…………………..6
List of Tables………………………………………………………………….……..9
Abstract…………………………………………………………………….……….10
Declaration………………………………………………………………….…...….11
Copyright…………………………………………………………..……….…...….12
Dedication……………………………….…………………..…………………...…13
Acknowledgements………………………………………………………………...14
1 CHAPTER 1 INTRODUCTION ..................................................................16
1.1 General Background ................................................................................16
1.2 Project Aim and Scope ............................................................................19
1.3 Objectives of the Research.......................................................................22
1.4 Major contributions of the thesis ..............................................................23
1.5 Overview of the thesis .............................................................................23
2 CHAPTER 2 HYDROGEN AS A FUTURE ENERGY CARRIER............26
2.1 Energy Sources and Environmental Impacts ............................................26
2.2 Renewable Alternative Energy ................................................................28
2.3 Hydrogen Energy Aspects .......................................................................28
2.4 Hydrogen as an Energy Storage Medium .................................................30
2.5 Major Hydrogen Production Technologies ...............................................32
2.5.1 Steam Reforming .............................................................................32
2.5.2 Partial Oxidation ..............................................................................33
2.5.3 Auto Thermal Reforming .................................................................33
2.5.4 Coal Gasification .............................................................................34
2.5.5 Electrolysis ......................................................................................34
2.5.6 Thermo-Chemical Process ...............................................................35
2.5.7 Photo Processes ...............................................................................36
2.6 Hydrogen Storage ....................................................................................36
2.6.1 Hydrogen Storage in Gaseous Form .................................................36
2.6.2 Hydrogen Storage in Liquid Forms ..................................................37
2.6.3 Hydrogen Storage as Metal Hydrides ...............................................38
2.6.4 Underground (Geological) Storage...................................................38
2.7 Hydrogen Transportation .........................................................................39
2.7.1 Compressed Gas Transport ..............................................................39
2.7.2 Liquid Hydrogen Transport ..............................................................40
2.7.3 Metal Hydride Transport ..................................................................40
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2.8 Attractive Advantages for Hydrogen as an Energy Carrier .......................41
2.9 Safety Aspects Associated with Hydrogen ...............................................42
3 CHAPTER 3 PHOTOVOLTAIC SOLAR ENERGY AND HYDROGEN
PRODUCTION .....................................................................................................45
3.1 Background .............................................................................................45
3.2 Components of Solar Radiation ...............................................................46
3.3 Measurements of Solar Radiation ............................................................48
3.3.1 Pyranometer.....................................................................................48
3.3.2 Pyrheliometer ..................................................................................48
3.4 Harnessing and Using Solar Energy .........................................................50
3.4.1 Thermal Conversion ........................................................................50
3.4.2 Electrical Conversion .......................................................................51
3.5 General Description of PV Cell Technology ............................................51
3.5.1 Silicon Solar Cell Types and Their Efficiencies ...............................52
n-type silicon semiconductor .......................................................................53
p-type silicon semiconductor .......................................................................53
3.6 Photovoltaic Systems ...............................................................................55
3.6.1 Applications of PV Systems .............................................................57
3.6.2 Attractive Features of Photovoltaic System ......................................57
3.7 Solar Hydrogen Production Systems ........................................................58
3.7.1 Solar Photovoltaic-based Electrolysis ..............................................58
3.7.2 Solar Photoelectrolysis .....................................................................58
3.7.3 Hydrogen Production by Concentrated Solar Thermal Energy..........58
3.8 System Components of the PV-Electrolyser Hydrogen Production Process
59
3.8.1 PV Electricity Generation ................................................................59
3.8.2 The Electrolyser ...............................................................................60
3.8.2.1 Alkaline water electrolyser ...........................................................60
3.8.2.2 Proton Exchange Membrane (PEM) Electrolyser .........................61
3.8.2.3 High-Temperature Electrolyser ....................................................63
3.9 PV Electrolyser Coupled with Maximum Power Point Tracking ..............63
3.10 Maximum Power Point Tracking Technologies .......................................65
3.10.1 Perturb and Observation (PAO) Method ..........................................65
3.10.2 Incremental Conduction ...................................................................65
3.10.3 Fractional Open Circuit Voltage ......................................................66
4 CHAPTER 4 A POWER MATCHING SIMULATION OF A SOLAR
HYDROGEN PRODUCTION SYSTEM.............................................................68
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4.1 PSCAD Software .....................................................................................68
4.2 Model Components .................................................................................68
4.3 Input/Output Data ....................................................................................69
4.4 The PV Model .........................................................................................70
4.4.1 PV Equivalent Circuit ......................................................................70
4.4.2 PV PSCAD Model ...........................................................................72
4.4.3 Response of the Model to Changes in Insolation ..............................74
4.4.4 Response of the PV Model to Changes in Temperature ....................75
4.5 DC-DC Buck Converter PSCAD Model ..................................................76
4.6 The PEM Electrolyser PSCAD Model .....................................................77
4.7 PV-PEM Hydrogen Production Power Matching Model ..........................80
4.8 Simulations and Results ...........................................................................82
5 DESIGN DC/DC BUCK ONVERTER FOR PV-PEM HYDROGEN
PRODUCTION SYSTEM POWER MATCHING ..............................................89
5.1 Background .............................................................................................89
5.2 Buck Converter Theory and Operation ....................................................90
5.2.1 Purpose of Different Buck Converter Components ...........................90
5.2.1.1 Switch ..........................................................................................91
5.2.1.2 Pulse-Width Modulation Circuit...................................................91
5.2.1.3 Operating Frequency ....................................................................91
5.2.1.4 Inductor .......................................................................................92
5.2.1.5 Capacitor .....................................................................................92
5.2.1.6 Free-Wheeling Diode ...................................................................92
5.2.2 Circuit Description and Operation ....................................................92
5.3 PSCAD Simulation of a Buck Converter .................................................96
5.3.1 DC-DC Buck Converter Circuit Using IC TL494 Control Circuit ....98
5.4 Characteristics of the PV-PEM Electrolyser Test Rig ............................ 101
5.4.1 PV Characteristics ......................................................................... 102
5.4.2 PEM Characteristics ...................................................................... 103
5.4.3 Dependence of Hydrogen Production on the Operating Current of the
PEM Electrolyser .......................................................................................... 104
5.5 Design Buck Converter.......................................................................... 105
5.5.1 PWM Circuit ................................................................................. 105
5.5.2 Evaluation of Results ..................................................................... 107
6 REAL TIME EXPERIMENT OF A PV-PEM HYDROGEN
PRODUCTION SYSTEM USING A COMMERCIAL SPLIT-PI CONVERTER
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6.1 Background ........................................................................................... 112
6.2 System Components .............................................................................. 112
6.2.1 PV Module .................................................................................... 113
6.3 Measuring Solar Irradiance .................................................................... 115
6.4 Split-Pi DC / DC Converter ................................................................... 116
6.4.1 Control of Split-Pi Converter Software .......................................... 118
6.4.2 Maximum Power Point Tracking Algorithm .................................. 120
6.5 PEM Electrolyser .................................................................................. 122
6.6 Hydrogen Volume Measurement Device ............................................... 125
6.7 Results and Discussion .......................................................................... 126
7 CHAPTER 7 DESIGN A PV-HYDROGEN SYSTEM TO POWER A
FAMILY HOUSE IN THE SAHARA DESERT IN LIBYA ............................. 131
7.1 Background ........................................................................................... 131
7.2 Solar Energy Sources in Libya and the Hydrogen Option ...................... 131
7.3 Solar Hydrogen System as an Energy Supply for Libyan Remote Areas 132
7.3.1 Design of a Solar Hydrogen Power System for a family House in a
Remote Area Located in the Sahara Desert .................................................... 135
7.3.2 Energy Requirement ...................................................................... 137
7.3.3 Fuel Cell Specification ................................................................... 138
7.3.4 Hydrogen Storage .......................................................................... 141
7.3.5 PEM Electrolyser ........................................................................... 141
7.3.6 DC/DC Converter .......................................................................... 142
7.3.7 Size of the PV Array ...................................................................... 143
7.3.8 Monthly Average Energy Supplied and Consumed ........................ 143
7.4 Control and Monitoring of the PV-Hydrogen System ............................ 144
8 CHAPTER 8 CONCLUSIONS ................................................................... 147
8.1 Contributions Made During the Project .................................................. 148
8.2 Suggestions for Future Work ................................................................. 149
9 REFERENCES ............................................................................................ 152
10 CHAPTER 10 APPENDICES .................................................................... 159
10.1 Appendix A ........................................................................................... 159
10.2 Appendix B ........................................................................................... 160
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List of Figures
Figure 1.1: Map of Libya [9]. ................................................................................19
Figure 1.2: Electricity in Libya consumption and fuels used in power plants (a)
consumption by sector and (b) fuels used in power plants. ......................................20
Figure 1.3: Electric energy consumption per capita for Libya and other countries
[8]. ..........................................................................................................................21
Figure 2.1: Primary sources of hydrogen and its applications (The sectors are not
scaled) [12]. ............................................................................................................29
Figure 2.2: Solar hydrogen power system for a home [3]........................................31
Figure 2.3: Electrolysis of water.............................................................................35
Figure 2.4: Hydrogen gaseous storage and delivery in the USA [53] ......................37
Figure 2.5: Transmission cost comparison between electricity and hydrogen from
[20]. ........................................................................................................................39
Figure 2.6: Heavy duty truck at hydrogen production plant USA [54] ....................40
Figure 3.1: World solar map [26]. ..........................................................................45
Figure 3.2: Interactions of the Earth‟s atmosphere with incoming solar radiation [27]
...............................................................................................................................47
Figure 3.3: Typical pyranometer ............................................................................48
Figure 3.4: pyrheliometer [26]. ..............................................................................49
Figure 3.5: A pyranometer used for the measurement of diffuse radiation [26]. ......49
Figure 3.6: Flat plate solar thermal collector. ........................................................50
Figure 3.7: Schematic diagram of a photovoltaic cell .............................................52
Figure 3.8: Covalent bonds in a silicon atom. .........................................................53
Figure 3.9: Photovoltaic hierarchy [28]. .................................................................55
Figure 3.10: Effects of insolation and temperature on the characteristics of a PV
panel . .....................................................................................................................60
Figure 3.11: A schematic construction of alkaline water electrolyser [34]. .............61
Figure 3.12: Schematic diagram of a proton exchange membrane electrolyser [35].
...............................................................................................................................62
Figure 3.13: PV coupled with an electrolyser using a DC/DC converter for MPPT .
...............................................................................................................................63
Figure 3.14: Photovoltaic coupling with load using a DC/DC Buck converter ........64
Figure 4.1: PV- electrolyser power matching using DC/DC buck converter. ..........69
Figure 4.2: Equivalent circuit of the PV cell. ..........................................................70
Figure 4.3: I-V characteristics of the PV cell. .........................................................71
Figure 4.4: Effect of adding RS on the PV cell‟s I-V curve. ....................................71
Figure 4.5: Effect of adding RSh on the PV cell‟s I-V curve. ...................................72
Figure 4.6: Model of the PV module. .....................................................................72
Figure 4.7: The dependence of I-V characteristics on insolation. ............................75
Figure 4.8: Effect of temperature on the PV model curves. .....................................75
Figure 4.9: PV characteristics used in the PV-PEM PSCAD model. .......................76
Figure 4.10: PWM to produce different duty cycle generator. ................................77
Figure 4.11: PSCAD PEM electrolyser block. ........................................................78
Figure 4.12: I-V and P-V curves for the PEM electrolyser. .....................................79
Figure 4.13: PV-PEM electrolyser PSCAD model .................................................80
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Figure 4.14: V-P curves for the PV array and the PEM at different duty cycle values
(D). .........................................................................................................................82
Figure 4.15: Intersection with the rescaled I-V curve of the PEM. ..........................83
Figure 4.16: Intersection with the rescaled I-V curve of the PEM. ..........................84
Figure 4.17: Relationship between current, voltage, and duty cycle. .......................85
Figure 4.18: Relationship between hydrogen production rate and duty cycle. .........85
Figure 5.1: Buck converter circuit. .........................................................................90
Figure 5.2: Buck converter ON state. .....................................................................93
Figure 5.3: Buck converter OFF state. ....................................................................94
Figure 5.4: (a) CCM and (b) DCM for inductor current. ........................................94
Figure 5.5: ON and OFF waveforms of the buck converter [43]. ............................95
Figure 5.6: PSCAD buck converter simulation. ......................................................96
Figure 5.7: Load voltage and current ripples ..........................................................97
Figure 5.8: Diode voltage wave from (a) Vin = 20 V, D = 0.9 and Vo = 18 V and (b)
Vin = 20 V, D = 0.4 and Vo = 8 V. .........................................................................98
Figure 5.9: (a) DC-DC buck converter using IC TL494 and (b) the practical circuit
of the circuit shown in (a). ......................................................................................99
Figure 5.10: Diode voltage form (a) V in = 20 V and V out = 5V and (b) V in = 6 V
and V out = 5 V. ..................................................................................................... 101
Figure 5.11: PV –PEM test rig. ............................................................................ 102
Figure 5.12: Setup for determining the characteristics of a solar module. ............. 102
Figure 5.13: PV characteristics............................................................................. 103
Figure 5.14: Circuit diagram of the characteristics of the PEM electrolyser. ......... 103
Figure 5.15: PEM electrolyser characteristics. ...................................................... 104
Figure 5.16: Volume of hydrogen produced as a function of current over a 10-
minutes operational period. ................................................................................... 104
Figure 5.17: PWM circuit using IC SG3525. ........................................................ 105
Figure 5.18: PV –PEM electrolyser coupled by a buck converter circuit............... 106
Figure 5.19: Buck converter implemented on a PCB. ........................................... 106
Figure 5.20: Oscilloscope images of electrolyser voltage (red) at different duty cycle
values (blue). ........................................................................................................ 107
Figure 5.21: I-V curves for the PV array and the PEM electrolyser. ..................... 108
Figure 5.22: P-V curves of the PV array and the PEM electrolyser. ...................... 109
Figure 5.23: Relationship between the implemented efficiency of the Buck converter
and duty cycle. ...................................................................................................... 109
Figure 6.1: Hydrogen production system. ............................................................. 112
Figure 6.2: Power matching photovoltaic-electrolyser system using a Split-Pi
converter. .............................................................................................................. 113
Figure 6.3: PV modules facing the sun. ................................................................ 114
Figure 6.4: (a) I-V and (b) P-V curves of a single C21 module under different
insolation values. .................................................................................................. 115
Figure 6.5: Split – Pi DC/DC converter. ............................................................... 116
Figure 6.6: Split – Pi converter circuit. ................................................................. 117
Figure 6.7: Flowchart of visual basic control. ....................................................... 119
Figure 6.8: Visual basic software screen outlook. ................................................. 120
Figure 6.9: Hill-climbing MPPT method. ............................................................. 121
Figure 6.10: MPPT programme flow chart. .......................................................... 122
Figure 6.11: The 50-W, h-tec PEM electrolyser. ................................................. 123
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Figure 6.12: Characteristic curve of the h-tec PEM electrolyser. .......................... 124
Figure 6.13: Resistance-Power PEM electrolyser curve. ....................................... 124
Figure 6.14: Device for measuring the volume of hydrogen produced. ................. 125
Figure 6.15: I-V for PV and PEM electrolyser curves........................................... 126
Figure 6.16: P-V PV and PEM electrolysers power matching. .............................. 127
Figure 6.17: PEM electrolyser input power data during a clear, sunny day. .......... 127
Figure 6.18: PEM electrolyser input power under less favourable insolation
conditions. ............................................................................................................ 128
Figure 6.19: Electrolyser input power and PV maximum power relation .............. 128
Figure 6.20: Hydrogen production in relation to changes in insolation. ................ 129
Figure 7.1: Photovoltaic module for a water pump in the Libyan Sahara [51]. ...... 132
Figure 7.2: Daily solar irradiance on horizontal plane through the year in Ghadamis.
............................................................................................................................. 134
Figure 7.3: Complete solar hydrogen power system. ............................................ 136
Figure 7.4: I-V and I-P characteristics for Nexa 1200 from data sheet. ................. 140
Figure 7.5: Estimated amount of energy extracted from the PV system. ............... 144
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List of Tables
Table 2.1: Environmental impacts of conventional energy sources. ........................27
Table 2.2: Choices of hydrogen storage. .................................................................38
Table 2.3: Methods of hydrogen transportation. .....................................................41
Table 3.1: Wave lengths of solar radiation [28]. .....................................................47
Table 3.2: The efficiencies of the threee types of crystalline silicon cells [32]. .......55
Table 3.3: World‟s three largest PV systems as of June 2011 . ...............................56
Table 4.1: Parameters of a crystalline silicon solar cell [41]. ..................................74
Table 4.2: Constant K value of different solar modules tested [5]. ..........................87
Table 5.1: Voltage and current readings at both sides of the Buck converter. ........ 108
Table 6.1: Specifications of the PV module. ......................................................... 114
Table 6.2: Split – Pi converter switching duty cycle. ............................................ 117
Table 6.3: Specifications of the PEM electrolyser. ............................................... 123
Table 7.1: Climatic conditions of the project site at 30° N and 10
° E...................... 134
Table 7.2: Energy requirements for small family house in the Sahara Desert. ....... 137
Table 7.3: Nexa 1200 technical data from datasheet. ............................................ 140
Table 7.4: Specifications for the LM-10000 electrolyser. ..................................... 142
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Abstract
Hydrogen is a potential future energy storage medium to supplement a variety of
renewable energy sources. It can be regarded as an environmentally-friendly fuel,
especially when it is extracted from water using electricity obtained from solar
panels or wind turbines.
The focus in this thesis is on solar energy, and the theoretical background (i.e.,
PSCAD computer simulation) and experimental work related to a water-splitting,
hydrogen-production system are presented. The hydrogen production system was
powered by a photovoltaic (PV) array using a proton exchange membrane (PEM)
electrolyser. The PV array and PEM electrolyser display an inherently non-linear
current–voltage relationship that requires optimal matching of maximum operating
power. Optimal matching between the PV system and the electrolyser is essential to
maximise the transfer of electrical energy and the rate of hydrogen production. A
DC/DC converter is used for power matching by shifting the PEM electrolyser I-V
curve as closely as possible toward the maximum power the PV can deliver. By
taking advantage of the I-V characteristics of the electrolyser (i.e., the DC/DC
converter output voltage is essentially constant whereas the current increases
dramatically), we demonstrated experimentally and in simulations that the hydrogen
production of the PV-electrolyser system can be optimised by adjusting the duty
cycle generated by the pulse-width modulation (PWM) circuit. The strategy used was
to fix the duty cycle at the ratio of the PV maximum power voltage to the
electrolyser operating voltage.
A stand-alone PV energy system, using hydrogen as the storage medium, was
designed. The system would be suitable for providing power for a family‟s house
located in a remote area in the Libyan Sahara.
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Declaration
No portion of the work referred to in the thesis has been submitted in support of an
application for another degree or qualification of this or any other university or other
institute of learning.
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Copyright Statement
The author of this thesis (including any appendices and/or schedules to this thesis)
owns certain copyright or related rights in it (the „Copyright‟) and he has given The
University of Manchester certain rights to use such Copyright, including for
administrative purposes.
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copy, may be made only in accordance with the Copyright, Designs and Patents Act
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To my beloved family
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Acknowledgements
I wish to express my gratitude to my current supervisor Professor Peter Stansby and
my former supervisor, Professor Nick Jenkins, who both gave me valuable advice,
and excellent guidance that were essential in the successful completion of the project
work.
Immense thanks to Dr. Frank Thompson who contributed in the completion of this
thesis for his valuable notices and advices throughout the work.
My thanks also to the my country Libya for the opportunity they gave me to
complete my higher studies by providing the grants for this research. I am grateful
also to the Joule centre at the University of Manchester for providing facilities to
undertake this work.
My sincere acknowledgments and appreciations are to all my friends for their
valuable assistance and encouragement.
Finally I would like to express my gratitude to my family for their constant
encouragement, care, and patience during the production of this work.
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CHAPTER 1
INTRODUCTION
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1 CHAPTER 1 INTRODUCTION
1.1 General Background
Energy has always been and still is essential for human survival and social
development. In recent years, concern about energy resources has become a
worldwide issue for the following reasons:
Continuing increase in the world‟s population
Environmental problems associated with CO2 emissions from the combustion
of fossil fuels (The International Energy Agency (IEA) stated that the
projected use of fossil fuels will increase CO2 emissions by 57% from 2005
to 2030 [1]).
Growing demand for energy to improve living standards.
At the present time, a large proportion (about 65%) of worldwide energy demand is
met by liquid and gaseous fossil fuels (i.e., petroleum and natural gas) because they
are readily available and convenient to use. However, it is expected that the
worldwide production of fossil fuels will soon peak and, thereafter, begin to
decrease.
The need to identify and develop alternative types of clean and sustainable fuel is
becoming more and more urgent. The use of hydrogen produced by renewable
energy sources (solar, wind, and biomass) as an energy carrier could be the future
energy strategy that will replace conventional liquid and gaseous fuels in both
stationary and transportation applications.
Normally, elemental hydrogen exists as a gas, H2, and molecular hydrogen exists in
combination with many other atoms, e.g., H2O and CH4. Molecular hydrogen can be
separated from such molecular forms through chemical or physical methods, such as
electrolysis of water or by steam reforming of natural gas. Currently, the steam
reforming process is the least expensive of the various methods, but it depends on the
combustion of a fossil fuel, which produces CO2 emissions. However, the
electrolysis process can produce high-purity hydrogen from water without generating
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CO2. Therefore, hydrogen is an environmentally-attractive fuel, because it burns
without producing CO2. It is not a primary energy source, such as oil, coal, and gas,
but it can be used effectively as an energy carrier.
Energy storage is required with most renewable energy sources. For example, we
can consider the use of batteries, which have the following limitations:
(1) They are not presently used for seasonal storage (from summer to
winter).
(2) Their efficiency is low, and it decreases dramatically with time, causing
substitutions.
(3) Disposal creates an environmental problem.
(4) They are expensive, especially if we were to consider the large dimensions
required for seasonal energy storage [2].
Most of these limitations could be circumvented by replacing the batteries with a
system for using renewable energy sources to generate hydrogen as an energy carrier.
One of the most promising ways of hydrogen end use is to produce electricity by
recombining hydrogen with oxygen from the air in a fuel cell, which directly
converts the chemical energy into electricity. The only by-product of this process is
water. With the use of hydrogen in fuel cell systems, there are no harmful emissions,
such as CO2, nitrogen oxides, or SO2. Fuel cells are silent electricity generators, so
they can be used as auxiliary power generators in hospitals, IT centers, and
submarines. Electricity from fuel cells can be used to run appliances and motors,
provide light, and to power cars.
We can use the energy provided by the sun to generate hydrogen, which, in turn, can
be used as an abundant, clean, and efficient source of energy for local use or longer-
term, distributed energy supplies.
One of the challenges in producing hydrogen by using solar energy (a PV-hydrogen
system) is the high cost. Therefore, it is important that such a system be designed and
operated in a way that allows it to achieve maximum power output, i.e., by matching
the power generated by the PV array and the power used to produce the hydrogen.
A large amount of modeling and experimental work on solar-hydrogen production
systems has been conducted in the last few decades. Much of the research has
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focused on optimal coupling between the photovoltaic source and electrolysers based
on their current, voltage, and power characteristics. Mismatch problems arise due to
variations in the output power from PV systems that are operating under different
incident solar radiation (insolation) conditions. Two ways of connecting the PV
source with the electrolyser are listed below:
1. Direct coupling between PV array and electrolyser [3][4]. In this method,
the key strategy is to find the series-parallel combination of PV modules
and electrolyser stacks. This method should meet the following
conditions:
The PV array should supply a minimum working voltage to the
electrolyser in order to split water into hydrogen and oxygen.
The PV – electrolyser system should contain the minimum number of
electronic devices (power conditioning equipment) to decrease losses.
2. Connection through a DC/DC converter, with maximum power point
tracking to ensure optimal power transfer between the PV panels and the
electrolyser. Ideally, output from the PV source would be held at the
maximum power point in order to achieve the greatest overall efficiency
[5].
L. Arriaga and Martinez [3] showed the results obtained by direct coupling of a 2.7-
kW PV array with 25 cells of PEM electrolyser at different insolation and
temperatures. They confirmed that the electrolyser stack was working near the
maximum power point at a good range of irradiance (600-800 W/m2).
In their experimental work, G. Ahmed and E. El Shenawy [4] compared the
hydrogen flow rate obtained by direct coupling of the PV array and electrolyser with
the use of a maximum power point tracker. The results showed that greater hydrogen
flow rates were achieved by using the maximum power point tracker.
Gibson and Nelson [5] developed a comprehensive mathematical model for
optimising the efficiency of PV-electrolyses systems. The strategy of the model was
to match the maximum power voltage of the PV device and the operating voltage of
the proton exchange membrane (PEM) electrolyser. The authors concluded that the
optimised PV-electrolyser system increased the hydrogen generation efficiency from
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2.6% to 12.4%, and this clearly results in minimising the cost of hydrogen
production.
Sulaiman and Veziroglu [6] developed a solar hydrogen power model for a solar-
hydrogen energy system in Saudi Arabia.
1.2 Project Aim and Scope
Libya is an oil-exporting country located in the middle of North Africa (Figure 1.1),
with the Mediterranean Sea as its northern border. It has six million inhabitants and a
land area of 1,750,000 km2
. For the last four decades, Libya has been dependent
mainly on fossil fuels (petroleum and natural gas) for its supply of energy; only a
very small amount of energy has come from renewable sources. Since Libya has no
biomass or geothermal energy sources, it will have to develop wind and solar energy
if it is to reduce its dependence on fossil fuels.
Figure 1.1: Map of Libya [9].
This thesis is concerned only with electrical energy, fuel use for transportation and
domestic heating will not be discussed.
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The General Electricity Company of Libya (GECOL) is the only Libyan company
that produces and distributes electricity in the country. According to GECOL, its
installed electricity generating capacity was 6.28 GW in 2008, and this capacity
depended entirely on the use of oil and natural gas. The Libyan national grid has an
extensive high-voltage network of about 12,000 km spread across the country. In
spite of this, Libya, like other North African countries, has remote areas where
people live but, as yet, are not connected to the grid.
Figure 1.2(a) shows the Libyan consumption of electrical energy by sector during
2008, with the services and residential sectors consuming around 70% of the total.
Also, Figure 1.2(b) shows that many power plants have been converted to use natural
gas instead of oil so that the export of oil volume (and the associated revenue) could
be maximized [7].
Figure 1.2: Electricity in Libya consumption and fuels used in power plants (a)
consumption by sector and (b) fuels used in power plants.
Libya has a high per capita consumption of electrical energy compared with other
North African countries, (UK data included for comparison) [8] as seen in Figure
1.3.
According to data from GECOL, the per-capita energy consumption increased from
330 kWh in 1970 to 3920 kWh in 2008, and the peak load of electrical power in
Libya has increased continuously at the relatively high rate of approximately 10%
per year, while the population growth rate has averaged just over 2% per year. This
21
shows that energy demand is largely controlled by the very rapid improvement of
living standards in the country.
Figure 1.3: Electric energy consumption per capita for Libya and other countries
[8].
Every nation in the world is aware of the two main problems associated with the use
of fossil fuels, i.e., 1) they are limited and will be depleted within the next century
and 2) using fossil fuels produces severe environmental problems from the effects of
CO2 and other emissions.
To avoid a future crisis in both of these areas, it is important for Libya to begin the
rapid development of renewable energy as a strategic energy policy. Both wind and
solar sources can produce significant amounts of electrical energy, and both
technologies should be developed as rapidly as possible.
This thesis is concerned with one section of solar energy, namely, photovoltaic
electrical energy production in which solar radiation is converted into electricity by a
silicon p-n junction. This is particularly attractive for Libya, which has abundant
sunshine for most of the year.
Hydrogen storage offers an alternative to battery storage for solar energy for power
systems in remote areas. Hydrogen production via water electrolysis using
photovoltaic as a power source and later using the hydrogen produced to generate
electricity using fuel cells is an ideal power source for remote areas.
22
The benefits to be obtained when solar hydrogen technology is installed in the
remote areas of Libya are:
1- The standard of living will be increased for the people who live in the
remote villages.
2- Sufficient electrical energy will be produced to meet local needs.
3- The settlement of people in these areas will be encouraged so they can
avoid migration to the crowded cities.
4- CO2 emissions will be reduced, thereby contributing to the solution of
the global warming issue.
5- Additional oil can be exported rather than burned in Libya, thereby
contributing to the country‟s economic growth.
1.3 Objectives of the Research
1- To review previous work on renewable energy, especially hydrogen
systems, with special emphasis on the technology of solar hydrogen
production systems with power matching to maximize the efficiency.
2- To use a laboratory-scale, photovoltaic–electrolyser test facility to
validate the concept of power matching using a DC/DC converter.
3- To use software engineering, namely PSCAD, to develop a computer
code capable of simulating physical processes of solar hydrogen
production system components.
4- To experimentally demonstrate a real-time, outdoor, solar hydrogen
production system using a computer-controlled DC/DC converter.
5- To design a solar hydrogen power supply system for a Libyan family
house in a Saharan remote area.
23
1.4 Major contributions of the thesis
Modelling photovoltaic powered electrolyser hydrogen production system
using PSCAD software. This developed a system model to incorporate
appropriate control strategy for system power matching using DC/DC buck
converter.
A visual basic software code was developed to controlee Split-PI DC/DC
converter. Areal time experiment was conducted to achieve PV-PEM
hydrogen production system power matching. The system was examined in
real time experiment and the results show the system was working at its
maximum power.
A stand alone PV energy system using hydrogen as a storage medium was
designed. The system provides a power for family house located in a remote
area in Libyan Sahara.
1.5 Overview of the thesis
The thesis contains eight chapters, and the first chapter provides general background
information related to the solar-hydrogen system and its importance, followed by the
aim and scope of the thesis.
Chapter two provides a summary of the available knowledge that has resulted from
the investigation of hydrogen as an attractive energy carrier; hydrogen production
methods and technologies; aspects of the use of hydrogen for in electricity generation
and transportation; the storage, distribution, and transportation of hydrogen; and the
safety features that must accompany the use of hydrogen.
Chapter Three illustrates solar irradiance and photovoltaic electricity generation; the
main components of the solar hydrogen production system, including the
photovoltaic power generator, electrolyser, and DC/DC converter as a power
conditioner between the source and the load; the construction of each component;
and the operational principles associated with each component.
Chapter Four describes the use of PSCAD software engineering methodology to
develop a computer model that is capable of exploring the modelling of power
24
matching in a photovoltaic–hydrogen production system. The evaluation shows the
different factors that affect the I-V characteristics based on theoretical equations that
describe the operation of both the photovoltaic device and the PEM electrolyser. The
simulation proved that the value of the duty cycle of the DC/DC converter influences
power matching and the hydrogen production rate.
Chapter Five describes a number of laboratory experiments with the small
photovoltaic-hydrogen production system that were conducted using a test facility. A
DC/DC buck converter was designed and implemented on a PCB to match the power
between the photovoltaic device and the proton exchange membrane (PEM)
electrolyser. The open-loop control method, using analogue IC, was used. The ratio
of output to input voltage ratio was given by the duty cycle control in the switching
converter.
Chapter Six describes the implementation and evaluation of a real-time PV-PEM
system scaled up by a factor of 10 from the laboratory unit described in chapter five,
using a commercial, computer-controlled, DC/DC converter. Visual basic software
code was created to control the converter and to track the maximum power point
(MPPT) of the photovoltaic device under different levels of irradiance to maximize
the efficiency of the system. The results showed that the readings at the converter
terminals followed the maximum power that the photovoltaic device delivered.
Chapter Seven includes the design of a photovoltaic–electrolyser system to produce
hydrogen that fuels a fuel cell capable of generating the electricity needed for a
family‟s house. The location was chosen in Libya, which is characterized by large
desert areas, scattered populations, and remote communities.
Chapter Eight presents the conclusions that were drawn as a result of the work and
suggestions for future work.
Appendices of computer codes used in PSCAD and Visual Basic software are
included at the end of the thesis.
25
CHAPTER 2
HYDROGEN AS A FUTURE
ENERGY CARRIER
26
2 CHAPTER 2 HYDROGEN AS A FUTURE
ENERGY CARRIER
2.1 Energy Sources and Environmental Impacts
Global demand for energy has grown significantly in the last five or six decades
because of industrial development, increases in people‟s living standards, and
population growth.
Energy sources can be classified into two categories:
1- Renewable energy sources are those that are sustainable and that are renewed
by nature, for example solar, wind, tidal, and biomass sources.
2- Conventional or non-renewable energy sources are exhaustible and limited to
definite periods of time, depending on the extent of their usage.
Fossil fuels, such as oil, natural gas, and coal, have been considered as the main
source of world energy due to their convenience and flexibility of use. It is these
properties that are responsible for the ever-growing demand for fossil fuels.
Unfortunately, non-renewable fuels are always limited, because they were formed
many eons ago from the carbon-rich remains of plants and animals, and, at some
time in the future, they inevitably will be used up. Another disadvantage of fossil
fuel is that, during the process of combustion, significant quantities of many toxic
materials are emitted into the air. These emissions pollute the atmosphere, land, and
water, and some of them cause global warming, which is becoming a serious,
worldwide issue.
The environmental impact linked to using conventional energy sources has received
widespread attention all over the world in recent years. The global warming effect
from using conventional energy sources has been investigated internationally. The
result of this investigation is a set of guidelines known as the Kyoto Protocol. This
Protocol was adopted initially in December 1997 in Kyoto, Japan, and it was
implemented on 16 February 2005. As of 2010, 187 states had signed and approved
this Protocol. A part of the agreement requires industrialised countries to reduce their
27
emissions of a "basket of greenhouse gases" by around 5 % between 2008 and 2012,
as compared to 1990 levels. To give an overview of the impacts of greenhouse gas
emissions, Table 2.1 from [10] provides the environmental impacts of conventional
energy sources, including pollution. As can be seen in the table, coal and oil have
significant environmental impacts for all of the pollutants presented. It should be
noted that about 80% of the world's energy supplies are provided by fossil fuels [10].
(Data for pollution resulting from the use of nuclear energy are included for the sake
of completeness).
Table 2.1: Environmental impacts of conventional energy sources.
To know the problems caused when we are dependent on fossil fuels and how we can
find alternative ways of energy production, we must know the ways in which energy
is currently being used or consumed. There are four main sectors for using energy in
human activities:
1- The private household sector.
2- The transport sector (including private and public transportation).
3- The industrial sector (e.g., manufacturing and agriculture).
4- The commercial and institutional sector (e.g., commercial offices, educational
facilities, and health facilities).
Thus, the world has to look for new alternative ways of energy production for
28
increasing the use of renewable energy sources to replace conventional energy
sources.
2.2 Renewable Alternative Energy
Renewable energy is any energy source that is sustainable and doesn't contribute to
global warming. Most renewable energy sources are derived from solar radiation,
including the direct use of solar energy for heating or photovoltaic electricity
generation, and indirect forms, such as wind energy, wave energy, and hydroelectric
energy. Tidal sources of energy result from the gravitational pull of the moon and the
sun, and geothermal energy comes from the heat available within the Earth.
Although the latter is finite on a geological time scale, we can classify geothermal
energy as a renewable energy source. The biggest obstacle to renewable energy
technology is that these sources of energy are not often convenient or flexible. At the
present time, the cost of renewable energy is significantly higher than the cost of
energy from fossil fuels. In addition to cost, one of the greatest challenges in utilising
renewable energy, particularly solar and wind energy, is the discontinuous or
irregular nature of these types of power; the wind does not always blow, and sunlight
is not always available. So, it is imperative that energy storage be combined with
these renewable energy sources. Renewable technology becomes much more
practical when it is used in conjunction with a storage system. Hydrogen could
provide energy storage for either wind or solar installations as an alternative to
electrical batteries. As a comparison, the best of all possible chemical battery cells,
namely a beryllium/air cell, has the capacity to store 24.5 megajoules per kg of
reactants, while hydrogen can store about 120 megajoules per kg [11, 12]. Several
aspects of hydrogen storage are discussed in the following paragraph.
2.3 Hydrogen Energy Aspects
Hydrogen is the simplest element, with an atom consisting of only one proton and
one electron. Hydrogen exists as a gas, H2, or in combination with other elements,
forming, for instance, water, ammonia, and hydrocarbon compounds. Hydrogen can
29
be separated from hydrocarbon compounds using the heat reforming process, and it
can be separated from water by electrolysis.
Hydrogen has the highest energy to weight ratio of all fuels; one kilogram of
hydrogen contains the same amount of energy as 2.1 kg of natural gas or 2.8 kg of
petrol.
Hydrogen is not a primary energy source, such as coal and gas, but it can be referred
to as an energy carrier. Initially, it has to be produced using existing energy systems
based on different conventional primary energy sources.
Hydrogen is considered as one of the most promising alternative fuels for the future
because of its capability of storing energy of high quality and because of its potential
to become an important energy carrier in the future. The ability of hydrogen to
improve energy security results from the wide range of options for sources,
converters, and applications. Figure 2.1 [12] shows the variety sources for hydrogen
production and its utilization aspects. The figure illustrates that hydrogen has a high
energy flexibility compared with any other alternative fuel.
Figure 2.1: Primary sources of hydrogen and its applications (The sectors are not
scaled) [12].
Hydrogen can be produced chemically from hydrocarbons, but the most attractive
30
option is to produce hydrogen by the electrolysis of water, because water covers 2/3
of the Earth and is, therefore, abundant in many parts of the world.
The generation of hydrogen is equivalent to the storage of energy in chemical form
as a fuel. Ultimately, hydrogen fuel can be used to produce thermal energy by
combustion or to produce electricity using fuel cells. If we considered moving
towards the large-scale use of hydrogen as fuel, there would have to be a significant
investment in infrastructure. All sectors in Figure 2.1, e.g., transport, buildings, and
industry would have to be modified extensively [12].
A small-scale development could be established quite simply with the production of
hydrogen locally using some renewable energy source and the electrolysis of water.
One such example of renewable hydrogen is a solar hydrogen system for a household
or small village to supply off-grid applications with electricity for cooking and
transportation. Figure 2.2 shows a solar PV–electrolyser fuel cell system to
demonstrate the use of hydrogen as an energy carrier in stand-alone applications.
Fuel cells operate in a converse manner to an electrolyser; they combine hydrogen
with oxygen from the air in an electrochemical process to produce electricity, and the
by-product is water.
Fuel cells are used to generate power for cars, and they are much more efficient than
a car running on petrol. A family car powered by a fuel cell would need around 5 kg
of hydrogen to achieve a range of 500 km [14].
2.4 Hydrogen as an Energy Storage Medium
Hydrogen can be attractive electricity storage medium; electricity can be used to
produce hydrogen through water electrolysis, while hydrogen can then produce
electricity using fuel cells. Therefore, when there is low-demand for energy
hydrogen could be stored and later used during high-demand periods.
31
Figure 2.2: Solar hydrogen power system for a home [3].
Such a method is particularly good for an off-grid system in rural regions, where
renewable energy is the only energy option. An example of a house powered by
hydrogen is given in the following paragraph [15].
A “modular house” has been running on solar power and stored hydrogen in the state
of New Jersey in the USA since 2006; the home owner has generated all the power
for the home and fuel for his car and other mechanical items used around the house.
The system uses solar PV panels to generate electricity using sunlight, and this
electricity is used to extract hydrogen from water. Although the system was costly to
construct, it avoids all the costs for purchasing electricity, purchasing fuel for heating
the house, and purchasing fuel for the car. In addition, no greenhouse gas emissions
are produced. For a larger-scale project, one finds that the whole island community at
Uist, Shetland Isles, Scotland, relies on hydrogen produced from wind power. This,
indeed, demonstrates the flexibility of a hydrogen/renewable source combination.
For a solar hydrogen power system, solar panels could be installed in sunny regions,
such as desert areas, and the energy produced could be stored and transported as
hydrogen over long distances.
32
Hydrogen has a higher energy density versus typical battery materials. Also, the long
charging time for rechargeable batteries requires regulators with controls in order to
avoid overcharging.
In the U.S. and Japan, hydrogen and fuel cells are considered to be core technologies
for the 21st century. There is strong investment and industrial activity in the hydrogen
and fuel cell arena in these countries, which are driving the transition to hydrogen
rapidly with funding for research and development [12].
In many countries, due to the awareness of carbon dioxide emissions, greenhouse
gases, and rapidly increasing oil prices, more hydrogen-fuelled automobiles are
being produced.
2.5 Major Hydrogen Production Technologies
2.5.1 Steam Reforming
Steam reforming of natural gas is currently the least expensive method for hydrogen
production, and it is responsible for more than 90% of the worldwide hydrogen
production. This is a chemical process in which a mixture of water and natural gas
(methane) is used to produce hydrogen from the natural gas. First, the natural gas is
cleaned and combined with steam at very high temperature (1100 to 1300 oK). Then,
the mixture of gases is passed over a nickel-alumina catalyst, where they are
converted to carbon monoxide (CO) and hydrogen (H2). The final step is a catalytic
water-gas reaction in which carbon monoxide and water are converted into carbon
dioxide (CO2) and hydrogen (H2).
The reforming reaction is:
CH4 + H2O + 206 kJ/kg => CO + 3 H2……………………………………..2.1
It is then followed by the exothermic shift reaction:
CO + H2O => CO2 + H2 + 41 kJ/kg………………………………………....2.2
The overall reaction is:
33
CH4 + 2H2O + 165 kJ/kg => CO2 + 4H2………………………………..….2.3
Unfortunately, production of hydrogen from natural gas has the following
disadvantages:
1- It accelerates the depletion of natural gas as a fossil fuel resource.
2- Carbon dioxide is a major byproduct of this process, and, so, in carbon
terms, it is only slightly better then burning the original methane.
3- The heating process to form steam creates additional carbon dioxide.
Although this process is well established, commercially viable, and is presently
being used to meet the demand for hydrogen, it is not sustainable because it is
based on the use of fossil fuels.
2.5.2 Partial Oxidation
In this process, liquid or gaseous hydrocarbons are mixed with oxygen in a high-
pressure reactor. The carbon monoxide is reacted with water to form CO2 and H2,
and the CO2 is captured and the H2 is purified. The equation for the partial oxidation
of natural gas is:
CH4+1/2O2 → CO +2H2 …………………………..……………………… 2.4
CO + H2O → CO2 + H2…………………………………………………. .2.5
Hydrogen production by partial oxidation has a theoretical efficiency similar to that
of conventional steam reforming, but less water is required.
2.5.3 Auto Thermal Reforming
In the auto thermal reforming process, the two previous methods are combined, i.e.,
natural gas, steam, and oxygen are reacted in a single vessel that consist of two
zones, one for combustion and the other for reforming.
34
2.5.4 Coal Gasification
Coal gasification is used in large hydrogen production plants and is used
commercially for hydrogen production. The coal is partially oxidised with oxygen in
a high-pressure reactor. The reason for using oxygen at high pressure and
temperature is to reduce the production of nitrogen oxides in the process. The
method has advantages in that there are large coal reserves in many parts of the
world. The disadvantage of this method is that it produces larger amounts of carbon
dioxide than other methods. In addition, slag and ash are the waste products of this
process, and these may contain heavy metals that could pollute air, water, and land.
2.5.5 Electrolysis
Primary energy sources can be used to produce hydrogen by electrolysis, and the
hydrogen can be converted to electricity. Producing hydrogen by water electrolysis
has a greater appeal over those processes using hydrocarbons because there are no
emissions. Also, electrolysis is preferred for the following reasons:
1. It is a potentially effective way of producing hydrogen locally so it can be located,
say, at fuel station.
2. Electrolysis offers a way to use electrical energy generated by renewable sources.
3. Electrolysis, operating in combination with fuel cells, can establish stand-alone
energy generator systems.
The electrolysis of water is a very simple process in which electricity is used to split
water molecules (H2O) into hydrogen (H+) ions and oxygen (O
-) ions, as illustrated
in Figure 2.3 from [17]. These hydrogen and oxygen ions migrate through the water
towards the cathode and the anode, respectively. This process is an efficient method
of producing high-purity hydrogen in large quantities with little or no adverse
environmental impacts, assuming that the electrical energy required to operate such a
process comes from renewable power sources, such as wind, photovoltaic, and
hydroelectric systems.
35
Figure 2.3: Electrolysis of water
The electrolysis process currently has an energy efficiency of approximately 75%,
and, theoretically, the efficiency could be increased to more than 90% in the future
[17].
Hydrogen produced by electrolysis results in no greenhouse gas emissions,
depending on the source of the electricity used.
2.5.6 Thermo-Chemical Process
In this process, water is superheated to a very high temperature (around 2500 oK),
whereupon it dissociates into its original components, hydrogen and oxygen. This
process has two problems, i.e., 1) a high temperature source is required and 2) the
reaction vessel must be made of materials that can withstand such high temperatures.
A third problem is the difficulty of separating the hydrogen and oxygen products.
Relatively complex chemical methods are used to accomplish this [16].
36
2.5.7 Photo Processes
Sunlight is used in these processes to produce hydrogen from water. The processes
can be divided into three main categories, i.e., 1) photo electrochemical, 2)
photochemical, and 3) photo biological. This method has low efficiency, so it is used
only when small quantities of hydrogen are required [14].
2.6 Hydrogen Storage
There are several different methods and techniques for storing hydrogen. They are
dependent on two parameters, i.e., 1) the quantity of gas/liquid and 2) the duration of
storage. For example, for long periods (seasonal storage), the hydrogen can be
converted to liquid hydrides (e.g., ammonia) or stored as pressurized gas in
underground tanks. The underground-tank system has been used since 1971 in the
city of Kiel, Germany, where a hydrogen gas tank with a hydrogen content of 65%
has been stored in a 32,000-m3 cavern at a pressure of 80 to 160 bar at a depth of
130 m. Hydrogen must be stored in large quantities for seasonal periods in order to
regulate and ensure continuity of supply.
In general, there are three well-developed methods for storing hydrogen, i.e., 1)
gaseous hydrogen storage, 2) liquid hydrogen storage, and 3) metal hydride storage.
2.6.1 Hydrogen Storage in Gaseous Form
Compressed gas storage of hydrogen is the simplest storage solution. The only
equipment required is a compressor and a pressure vessel. The only problem with
compressed gas storage is low storage density, which depends on the storage
pressure. Higher storage pressures result in higher capital and operating costs [21].
Compressed hydrogen gas tanks are the most popular because, unlike liquid
hydrogen storage, they do not require refrigeration with the attendant insulation.
Since the hydrogen molecule is small, compressed hydrogen systems demand greater
care against leakage as compared with pressurized natural gas installations. Also,
hydrogen tanks often are made from lighter materials, such as aluminium or carbon/
graphic compounds, than is the case for other gases.
37
There are two categories for pressurized hydrogen storage:
Moderately pressurized hydrogen (1-1.5 Mp), which would be used,
typically, from underground caverns or large stationary vessels at ground
level.
Small, high-pressure (20 Mp), cylindrical vessels for industrial applications,
where the cylinders are transportable.
Figure 2.4: Hydrogen gaseous storage and delivery in the USA [53]
2.6.2 Hydrogen Storage in Liquid Forms
In order to reduce the volume required to store a useful amount of hydrogen,
particularly for vehicles, liquefaction may be employed. Since hydrogen does not
liquefy at temperatures above -253 °C, there is a large amount of energy needed in
the liquefaction process [18].
The advantage of liquid hydrogen is that its energy/mass ratio is three times greater
than that of gasoline. It has the greatest energy density of any fuel in use (excluding
nuclear fuels), and that is why it is employed in all space programs. However, its
energy/volume ratio is low.
Most liquid hydrogen tanks are spherical, because this shape has the lowest surface
area for heat transfer per unit volume. As the diameter of the tank increases, the
volume increases faster than the surface area, so a large tank will have proportionally
less heat transfer area than a small tank, reducing boil-off. Cylindrical tanks are
38
sometimes used because they are easier and cheaper to construct than spherical tanks
and because their volume/surface area ratio is almost the same [18].
The worldwide transport of hydrogen could be conducted in liquid form using ships.
Common stationary liquid hydrogen tanks have capacities ranging from 1500 L up
to 75,000 L of liquid hydrogen with radial dimensions of 1.4-3.8 m and heights of 3-
14 m. The largest hydrogen tank was announced by NASA; this tank is located at
ground level, and it has a hydrogen storage capacity of about 270 tones of liquid H2.
Another liquid hydrogen tank exists at a bus refueling station in London and is
operated by British Petroleum. This tank is an underground storage tank [21].
2.6.3 Hydrogen Storage as Metal Hydrides
Metal hydride hydrogen storage uses a specific metallic compound that acts as an
absorber and releases hydrogen at constant pressure. The purity of hydrogen used has
a direct relationship with the life of the metal hydride storage cylinder. This type of
storage is suitable for hydrogen fuel cell cars, where empty cylinders can be
exchanged easily for full cylinders [18] .
2.6.4 Underground (Geological) Storage
Underground hydrogen storage is one of the most promising technologies for large-
scale storage of low-pressure gas. Hydrogen can be stored in excavated rock caverns,
salt domes, and depleted oil/gas fields. Table 2.2 summarizes the choices available
for hydrogen storage with regard to storage times and hydrogen quantities.
Method General use
Underground Large quantities , long term storage times
Liquid Large quantities , long term storage times
Compressed Gas Small quantities , short term storage times
Metal hydride Small quantity
Table 2.2: Choices of hydrogen storage.
39
2.7 Hydrogen Transportation
In energy terms, hydrogen has the potential to be a cost-competitive method of
transmission over long distances. As shown in Figure 2.5, for any distance greater
than 3000 km, the transmission of an equivalent amount of energy using hydrogen
transmission would be cheaper than using electricity transmission by wires. There is
always a need to have the capability for the long-distance transmission of energy,
because the energy sources of the future are likely to be far from the industrial and
population centres. This is the case for coal deposits and for nuclear energy, both of
pose potential and serious pollution hazards. Similarly, for the case of using solar
energy, the areas of maximum solar irradiation in North Africa, Saudi Arabia,
Australia, and other tropical areas are generally far removed from populated areas
[19, 20].
Figure 2.5: Transmission cost comparison between electricity and hydrogen from
[20].
2.7.1 Compressed Gas Transport
Like natural gas, hydrogen can be transported by pipelines. In Germany, a 512-km
hydrogen gas pipeline has been operational for several years, providing evidence that
transporting hydrogen by pipelines is technologically feasible; the pipeline in
40
Germany varies in diameter from 80 to 150 mm, and the pressure in the pipeline is
150 psi [21, 22].
As an alternative to pipelines, hydrogen can also be transported in various sizes of
pressure trucks and vessels, but, for long-distance transport, it is more practical and
less expensive to transport hydrogen as a liquid [22].
.
2.7.2 Liquid Hydrogen Transport
The liquification of hydrogen is achieved by cooling hydrogen gas below its boiling
temperature of -253 °C. When gaseous hydrogen is changed to the liquid form, there
is a vast reduction in the volume required to store a useful amount of hydrogen –
particularly for vehicles. Figure 2.6 shows a heavy duty truck for a liquid hydrogen
transport in a hydrogen production plant in the USA [Ref].
Figure 2.6: Heavy duty truck at hydrogen production plant USA [54]
2.7.3 Metal Hydride Transport
Metal hydrides have the advantage of having a low volume/energy density. Hydrides
are unique because some can adsorb hydrogen at or below atmospheric pressure and
41
then release the hydrogen at significantly higher pressures when heated. Depending
on the alloy chosen, there is a wide range of operating temperatures and pressures for
hydrides. This hydrogen transportation process has the advantages that no
liquefaction is required and leakage and safety problems are minimised.
Hydride transportation has the disadvantage of being limited to very small-scale
usage; thus, it has poor mass/energy density values, and the metal alloys that must be
used have relatively high costs. Table 2.3 compares the methods for transporting
hydrogen and their general use.
Method General use
Pipeline Large quantities , long distance power transmission
Liquid Large distances
Compressed Gas Small quantities over short distance
Metal hydrides Short distance
Table 2.3: Methods of hydrogen transportation.
2.8 Attractive Advantages for Hydrogen as an Energy
Carrier
There are many advantages of hydrogen as an energy carrier:
1- Hydrogen has the highest energy content per unit weight of any known fuel.
2- When hydrogen is burned in an engine, it produces zero emissions; also,
when it combines with oxygen from air in a fuel cell, it produces electric
energy and the by products are water and heat. So, no greenhouse gases or
other harmful emissions are produced.
3- Hydrogen and electricity are interrelated; they may be substituted for each
other fairly easily. This could be valuable in most of our existing equipment
and infrastructure if a hydrogen economy were to become commonplace.
4- Hydrogen can be produced locally from numerous sources; it can be
produced in domestic places where it is used, or it can be produced at a
central location and then distributed in gaseous or liquid form.
42
5- When hydrogen is produced by the electrolysis of water using renewable
energy sources, the energy system can be deemed to be sustainable and
secure; renewable energy sources, such as solar photovoltaic technology,
wind, and hydro, can provide the power needed to produce hydrogen from
water.
6- The use of hydrogen requires no new technological breakthroughs. Hydrogen
production and its end use as an energy carrier already have been
demonstrated; electrolysis and fuel cell technologies are well-established
technologies that are currently being used. Hydrogen storage and
transportation technologies would be similar to those used in natural gas
supply systems. Hydrogen can replace oil and natural gas in most of their end
uses, such as for vehicles and electric energy generator systems.
7- Hydrogen can be used in some applications with high efficiency, e.g., in fuel
cell cars and in power system applications, in which hydrogen can be
converted to electricity and heat at efficiency levels of around 80%.
8- When hydrogen is produced in large quantities and stored in large-scale
facilities, it can be transported as an energy carrier over long distances at less
cost transmitting electricity by wire.
2.9 Safety Aspects Associated with Hydrogen
Hydrogen, like petrol and natural gas, is flammable and can be dangerous under
specific conditions. But it can be handled safely if the safety precautions and
guidelines are carefully followed.
1- The flammability limits of hydrogen with oxygen are wide. Thus, hydrogen
combines explosively with oxygen when the limits are between 4 and 75%,
whereas methane combines explosively with oxygen when the limits are
between 5 and 15%.
2- Leakage can be a problem in hydrogen systems. The escape velocity of
hydrogen is three times that of methane on a volume basis. However,
because hydrogen only has about one-third of the energy per unit volume
43
that methane has, leakages of the two gases would amount to approximately
the same amount of energy per unit time [20].
3- Hydrogen is lighter than air and diffuses rapidly. At 25 0C and atmospheric
pressure, the density of air is 1.225 kg/m3, while the density of hydrogen is
only 0.083 kg/m3. The diffusivity of hydrogen is 3.8 times greater than that
of natural gas. Since hydrogen is the lightest element in the universe, it is
very difficult to confine. Engineers who design hydrogen systems must
ensure that there is adequate ventilation in any installation where hydrogen
is being produced, processed, or stored.
There are commercially available combustible gas detectors combined with alarms
that can sense hydrogen concentrations between 0% and 50% of the lower
flammability limit (LFL) [23]. A meter, manufactured by Senko, was purchased for
use in the laboratory. The alarm was set at an LFL of 4%, so that safe working
conditions could be maintained throughout the experimental work.
Summary
Hydrogen could be an energy carrier of the future. It is a sustainable fuel option and
one of the potential solutions for the current energy and environmental problems.
Existing commercial production methods, such as steam methane reformation
depends on the combustion of a fossil fuel, which produces CO2 emissions. The
electrolysis process however, can produce high purity hydrogen from water without
generating CO2 .
44
CHAPTER 3
PHOTOVOLTAIC SOLAR ENERGY
AND HYDROGEN PRODUCTION
45
3 CHAPTER 3 PHOTOVOLTAIC SOLAR
ENERGY AND HYDROGEN PRODUCTION
3.1 Background
Nuclear fusion is the energy source that heats up the sun. Although the sun‟s interior
temperature is in excess of a million degrees Kelvin (oK), its surface temperature is
approximately 5700 oK, and it behaves approximately as a black body radiator with
this temperature. It can be termed an inexhaustible energy source since it has been
estimated that it will maintain its present stable state for another billion years [24].
The sun is the largest available energy source in our solar system; it supplies the
earth with an annual energy of 1.5 x 1018
kWh; the energy received by the Earth from
the sun in one hour is adequate for all human energy needs for nearly a year [25][26].
Incident solar radiation (insolation) is fundamental to most other sustainable energy
sources, such as wind, waves, biomass, and hydropower.
Figure 3.1 [26] shows the average insolation intensity for the entire world with an
annual average provided for each location. The regions of maximum insolation are
depicted in red, and the areas that receive minimum insolation are shown as blue
regions. Both values have units of kWh/m2.
Figure 3.1: World solar map [26].
46
Insolation is attenuated partially as it crosses the atmospheric layers. A considerable
portion of solar radiation is reflected back into space, preventing it from reaching the
Earth's surface. This happens due to absorption, scattering, and reflection of the
incoming radiation in the upper layer of the atmosphere (stratosphere), and the
radiation can be attenuated further in the lower layer of the atmosphere (troposphere
), due to clouds and weather conditions.
Solar energy can be transformed into a storable chemical fuel in the form of
hydrogen. The use of solar energy to drive the electrolysis process in which
hydrogen and oxygen are produced from water is a very promising process because it
produces no harmful pollutants, it is easy to operate, and maintenance requirements
are minimal.
Photovoltaic, electrolyser, and fuel cell power systems could be used as an
alternative for a photovoltaic system and batteries to provide a supply of power in
remote areas.
3.2 Components of Solar Radiation
Incoming solar radiation is categorized based on its various modes of interaction
with the Earth‟s atmosphere. The four categories are:
1. The radiation passes directly the atmosphere, in which case it is called direct
radiation.
2. Be reflected in which solar is reflected after it strikes an atmospheric
particle.
3. The radiation can be absorbed by the atmosphere.
4. The radiation can be scattered by the contents of the atmosphere; when this
occurs, it is called diffuse radiation, because the small particles and gas
molecules it encounters scatter the radiation in random directions [24].
Figure 3.2 illustrates the four possible effects that the Earth‟s atmosphere can
exert on incoming radiation from the sun.
47
Figure 3.2: Interactions of the Earth‟s atmosphere with incoming solar radiation [27]
The total global radiation received on the Earth is the combination of the direct
radiation (also called sunlight) and the diffuse radiation that reaches the ground.
Direct radiation comes directly from the sun to the Earth. Diffuse radiation comes to
the Earth after being reflected by the ground or by other surfaces it may have
encountered. The reflected radiation from the ground is a function of the albedo (or
reflectiveness ratio) of the Earth‟s surface [27].
As stated before, the sun has a very high surface temperature and the radiation
corresponding to this temperature commonly is divided into various regions or bands
on the basis of wavelength, as shown in Table 3.1 [28]. Spectral bands are divided
into high frequency, visible frequency, and low frequency bands.
Frequency Radiation
type
Wavelength
m
Fraction of
energy
High frequency Ultra-violet < 0.38 6.4 %
Visible frequency Light 0.38 - 0.78 48 %
Low frequency Infra-red > 0.78 45.6 %
Table 3.1: Wave lengths of solar radiation [28].
48
3.3 Measurements of Solar Radiation
Insolation depends strongly on the location and local weather. Solar radiation
measurements are taken using either a pyranometer (measures global radiation)
and/or a pyrheliometer (measures direct radiation).
3.3.1 Pyranometer
A pyranometer measures the total radiation arriving from all directions, including
both direct and diffuse components. It measures all the radiation that is of potential
use in the solar energy system.
A typical pyranometer, shown in Figure 3.3 has a thermocouple mounted on a black
carbon disc. The amount of voltage generated is related directly to the value of the
insolation. Normally, the device is covered by one or two hemispherical glass covers
to protect it from sand, rain, and other contaminants, which might affect the radiation
measurements.
Figure 3.3: Typical pyranometer
3.3.2 Pyrheliometer
A pyrheliometer, Figure 3.4 [26], is an instrument that is used to measure insolation
49
resulting from direct solar radiation at a given location through a narrow collimating
tube. Since the instruments must be pointed directly at the sun, pyrheliometers are
typically mounted on a tracking device that follows the sun‟s movements [26].
Figure 3.4: pyrheliometer [26].
Measurements of diffuse radiation on horizontal surfaces also are made using a
pyranometer. This can be achieved by shading the instrument to block the direct
beam of sunlight. This is usually done by means of a shading ring as shown in Figure
3.5 [26].
Figure 3.5: A pyranometer used for the measurement of diffuse radiation [26].
50
An alternative method that can be used to measure insolation has a photodiode sensor
that produces current through a calibrated resistance to produce a voltage that is
proportional to the insolation. This method is less expensive, but also less accurate,
than the method based on thermopiles (devices that convert thermal energy to
electrical energy). An example of this technique is the Daystar meter that was used in
the present work.
3.4 Harnessing and Using Solar Energy
Converting solar energy into useful power is based on capturing solar radiation and
preventing it from radiating back into the atmosphere. Two methods are available,
i.e., 1) thermal conversion by using solar thermal panels and 2) electrical conversion
by using photovoltaic panels.
3.4.1 Thermal Conversion
Solar thermal energy refers to technologies that convert radiant energy into usable
heat energy to heat up water in a flat plate arrangement. Figure 3.6 illustrates a
typical thermal collector. It consists of a large, insulated box with a glass or plastic
cover that allows short-wave radiation to pass through and fall onto a dark, heat-
absorbing metal plate. A coil of metal tubing through which water is circulating is
located on the back of this plate.
This solar thermal system could replace other energy sources, such as natural gas and
electricity, as a provider of hot water to buildings in sunny regions.
Figure 3.6: Flat plate solar thermal collector.
51
Solar thermal systems have four main components: the solar collector panels, water,
the storage tank, and the controller. When solar collector exposed to solar radiation it
heats up the water passages through the pipes. The water is circulated by pump
through the thermal panel to the storage. Solar desalination thermal system can be
used to produce drinking water solar by purifying the water in remote areas. It can be
used through removing impurities as fluoride and salts to produce drinking water. In
this system glass or transparent is used to cover plate of water which is mounted in
front of black backing to trap solar energy. When the sun radiation heats the water in
the still the water evaporates which then condensed and used as pure water.
3.4.2 Electrical Conversion
The second way to use solar radiation is by converting sunlight into electricity. This
is achieved by the use of a photovoltaic cell. In 1839, Edmond Becquerel discovered
the photovoltaic (PV) effect when he immersed a silver chloride electrode in an
electrolytic solution, connected the electrode to a counter metal electrode, and used
white light for illumination. Under these conditions, he observed that a voltage and a
current were produced. However, the birth of the modern era of PV solar cells
occurred in 1954, when D. Chapin, C. Fuller, and G. Pearson at Bell Labs
demonstrated solar cells based on p-n junctions in single crystal Si with efficiencies
of 5-6% [32]. This original Si solar cell still works today, and single-crystal Si solar
cells dominate the commercial PV market [33].
3.5 General Description of PV Cell Technology
Figure 3.7 [27] shows a schematic representation of a photovoltaic cell. The incident
photons cause electrons in the photovoltaic material to be freed from atoms, and a
current flows from the p-side to the n-side, as explained in the next section.
A PV cell is made of semiconductor materials and behaves like other solid-state
devices, such as diodes and transistors. The most widely used material for PV cells is
silicon. There are two commercially available types of PV cell technologies, i.e.,
52
crystalline silicon and thin film. In crystalline-silicon technologies, individual PV
cells are cut from large single crystals. In thin-film PV technologies, the PV material
is deposited on glass or thin metal that mechanically supports the cell or module.
Figure 3.7: Schematic diagram of a photovoltaic cell
3.5.1 Silicon Solar Cell Types and Their Efficiencies
Solar cells can be made of many different semiconductors. A crystalline silicon solar
cell was used as an example for the theoretical and modelling study in this thesis for
two reasons. First, crystalline silicon was the material used in the earliest successful
PV devices. Second, and more importantly, crystalline silicon is still the most widely
used PV material. Crystalline silicon has band-gap energy of 1.1 electron volts (eV).
To produce a solar cell, the semiconductor must be “doped.” Doping is the
intentional introduction of chemical elements that can obtain a surplus of either
positively charged carriers (p-conducting semiconductor layer) or negatively charged
carriers (n-conducting semiconductor layer) from the semiconductor material. A p-n
junction results when two differently-doped semiconductor layers are combined and
located at the boundary between the two layers. When light is incident on a p-n
junction, charge carriers (electrons and holes) are released at its two sides.
A key parameter for the charge collection of solar cells is the hole and the mobility
of the electrons. The most common solar cell material is silicon. A silicon atom has
53
14 electrons, located in three deferent shells; the outer orbit contains four valence
electrons, so it can share with its neighbouring atoms to complete the outer shell with
eight electrons. In intrinsic silicon (no impurities added), each atom forms covalent
bonds with four adjacent atoms. The results of the covalent bonding are 1) the atoms
are held together to form a solid substance and 2) the silicon is a poor conductor of
electricity because the four outer electrons are not free to move.
n-type silicon semiconductor
N (negative)-type semiconductors, Figure 3.8, are formed by conducting the process
of doping impurities that have +5 valences, such as phosphorous (P) or arsenic (As),
into the silicon semiconductor, with each of the two electrons of silicon and
phosphorous atoms forming a covalent bond and one electron remaining free. This
process is to increase the numbers of free electrons that have negative charges. The
extra electron is only weakly bound to the atom, and it easily can be excited into the
conduction band in the p-n junction.
p-type silicon semiconductor
The purpose of p (positive)-type, Figure 3.8, doping is to create abundances of holes,
so impurities, such as boron (B) or aluminium (Al), are added to the silicon, and the
result is that one electron is missing from one of four covalent bonds. Thus, the
silicon atom can accept an electron from neighbouring atoms to complete the fourth
bond.
Figure 3.8: Covalent bonds in a silicon atom.
54
These free electrons and holes in the semiconductor are called carriers because they
carry a charge from one place to another.
When a photon of sunlight enters a semiconductor material, it can free an electron
from its position in a p-type semiconductor, and the electron will obtain enough
energy from the photon to move freely. The amount of energy required to free an
electron is called the band gap of the material, and each material has its own
distinctive band gap, which is referred to as the energy difference between the
conduction band and valence band.
Silicon cells are classified into three categories based on their crystal types, i.e.,
monocrystalline, polycrystalline, and amorphous.
Pure semiconductor material is necessary for the production of monocrystalline cells.
Monocrystalline rods are extracted from molten silicon and then cut into thin plates
for further processing to form a p-n junction.
Polycrystalline cells are produced by pouring liquid silicon into blocks; during this
process, crystal structures of varying sizes are formed within a block of material.
Again, thin plates are cut from these blocks, and further processing produces a p-n
junction.
An amorphous or thin-layer cell is formed by depositing a silicon film on glass or
another substrate material; the thickness of the layer is less than 1 m. In this case,
the p-n junctions are formed during the deposition process.
The efficiency of each type of cell is shown in Table 3.3 [32]. This clearly illustrates
that monocrystalline cells have a superior performance, but the cost of these cells is
higher than that of the other two cells.
A typical monocrystalline photovoltaic cell produces less than five amperes per
square meter at approximately 0.5 volt DC. Consequently, cells must be connected in
series and parallel configurations to produce enough power for any appliance,
whether it is to be used for an industrial or a domestic application.
55
Material Approximate level of
efficiency in the
laboratory %
Approximate Level of
efficiency in Production,
%
Monocrystalline Silicon 24 14 to 17
Polycrystalline Silicon 18 13 to 15
Amorphous Silicon 13 5 to 7
Table 3.2: The efficiencies of the threee types of crystalline silicon cells [32].
3.6 Photovoltaic Systems
The photovoltaic hierarchy is shown in Figure 3.9 [28]. Electricity is produced by an
array of individual PV modules connected in series and parallel to deliver the desired
voltage and current. Each PV module, in turn, is constructed of individual solar cells
that are connected in the same manner.
Photovoltaic (PV) systems are clean, renewable sources of energy that have been
used in stand-alone applications for many years. However, with the growing concern
over greenhouse gas emissions and other environmental issues, renewable energy
sources, such as PV systems, increasingly are being connected to the electricity grid.
Europe and Japan are at the forefront of the development of grid-connected PV
systems.
Figure 3.9: Photovoltaic hierarchy [28].
56
According to Solarbuzz solar photovoltaic (PV) panels cost an average of $3.05 per
watt in Europe, and they have a 20-year lifetime, with an average output of
approximately 10.6 W/ft2 (114 W/m
2).
The price of the active material and cost of the manufacturing are the main elements
that determine the total price of PV technologies. New processes are being
researched for producing low-cost wafer silicon (both monocrystalline silicon (sc-Si)
and polycrystalline silicon (pc-Si)) so that low-cost materials can be used for thin-
film PV applications.
The development of PV systems has been a major focus in Europe and in North
America, and highly successful projects have been completed, as shown in Table
3.3[28]. Ontario Canada‟s expanded Sarnia PV plant with a peak power of 97 MW is
the largest PV power plant that has been commissioned so far in 2011 [28].
Plant Name Size (MW) Location
Sarnia 97 Ontario, Canada
Montalto di Castro 84.2 Italy
Solarpark 80.7 Germany
Table 3.3: World‟s three largest PV systems as of June 2011 .
The three most common types of photovoltaic systems are:
Photovoltaic systems that feed power directly to into the utility grid. The
photovoltaic systems deliver DC power to a power conditioning unit (PCU) that
converts the DC to AC and sends the AC power to the load. If the photovoltaic
array does not supply enough power to satisfy the demand of the load, the PCU
draws additional power from the utility grid.
Stand-alone photovoltaic systems, which are effective in remote areas where
there is no grid. These systems avoid the need for expensive (and possibly high
maintenance) generators. However, such a stand-alone PV system require a
means of storing electricity so that power can be maintained during the night
when there is no solar insolation.
57
A solar tracker is occasionally used to make a PV array more efficient, but the added
cost is often prohibitive.
3.6.1 Applications of PV Systems
There are many applications in which PV systems can be used. Some examples of
these applications are provided below:
Remote site electrification
Pumping water and operating treatment systems
Healthcare systems
Communications
Disaster relief applications
Security systems
Cathodic systems
3.6.2 Attractive Features of Photovoltaic System
The photovoltaic systems are known to have many attractive features such as:
1- In many small applications, such as in sunny, rural areas, PV systems
are more viable economically than other alternatives.
2- PV systems do not require a fuel to generate electricity.
3- PV systems are more reliable than diesel and wind generators because
they do not have moving parts.
4- PV systems consist of individual solar panels and modules. This makes
it relatively easy to provide the appropriate size for any particular
installation.
5- The expected life of PV cells is about 20 years.
6- No harmful pollutants are created during the operation of PV systems,
so there are no harmful emissions.
7- Simple, routine, low-cost cleaning is adequate for maintaining a PV
system.
58
3.7 Solar Hydrogen Production Systems
Based on the types of energy inputs, hydrogen production using solar energy can be
classified into the three types as described below:
3.7.1 Solar Photovoltaic-based Electrolysis
This method is based on using electricity produced by PV panels to produce
hydrogen by the electrolysis of water. Electrolysis is conducted by passing direct
electric current (DC) through the water to generate hydrogen and oxygen. One
advantage of PV- electrolyser technology is that it does not emit greenhouse gases.
The efficiencies of modern photovoltaic systems and electrolysers are about 20% and
80%, respectively, and the total efficiency of transforming solar radiant energy to
energy in the form of hydrogen is nearly 16% [33].
3.7.2 Solar Photoelectrolysis
Photoelectrolysis, which integrates solar photovoltaic energy combined with the
electrolysis of water into a single photoelectrode, uses solar energy to extract
hydrogen directly from water. Photoelectrolysis uses photoelectrochemical (PEC)
light-collecting systems to power the electrolysis of water. When a semiconductor
photoelectrode submerged in an electrolyte is exposed to sunlight, the semiconductor
generates a voltage that is high enough to extract hydrogen and oxygen from water.
3.7.3 Hydrogen Production by Concentrated Solar Thermal
Energy
This method uses the thermal energy produced by concentrated solar radiation in
heating up a fluid or a chemical source of hydrogen, such as water or fossil fuels,
respectively. The most familiar method of hydrogen production is the thermal
decomposition of natural gas (NG) in a high-temperature solar chemical reactor.
59
3.8 System Components of the PV-Electrolyser Hydrogen
Production Process
The energy generated by a photovoltaic array must be combined with a storage
system due to the irregular availability of solar radiation. The conventional energy
storage method in PV stand-alone systems is batteries. In this thesis, we describe
how solar energy can be stored in an alternative energy carrier form by using the
electrolysis of water to produce hydrogen. Hydrogen generation and storage is
gaining importance in the emerging “hydrogen economy,” and, therefore, it is
important to assess the feasibility of a PV - hydrogen electrolysis system.
For the particular situation of remote areas in Libya, replacing a diesel generator by a
system that consists of a photovoltaic array, an electrolyser, and a hydrogen tank/fuel
cell combination could eliminate the difficulties associated with the use of generators
, in remote areas, such as the expense and the difficulty of transporting fuel to these
areas.
If the “hydrogen economy” gains greater acceptance in the future, the face of
manufacturing and industry may change significantly. In industrial development,
there is a continuing challenge in using fuels and in the demand to lower CO2
emissions. If hydrogen were to be used on a large scale to replace fossil fuels, many
benefits would be realised.
3.8.1 PV Electricity Generation
The characteristics of a 60-W, commercial PV panel under different levels of
irradiance (kW/m2) are shown in Figure 3.10. The irradiance has a large effect on the
short-circuit current (horizontal part of the I–V curves), while its effect on the open-
circuit voltage (vertical arm of the curves) is rather weak. The maximum power
(Pmax) output of a photovoltaic cell changes with irradiance, i.e., the cell generates
more power when the irradiance is higher. Another factor that affects the PV power
is temperature, as shown in Figure 3.10. As temperature increases, both voltage and
power decrease. This is a particularly severe problem, since the cells are often
operated at the maximum power point (mpp).
60
Figure 3.10: Effects of insolation and temperature on the characteristics of a PV
panel .
3.8.2 The Electrolyser
Water electrolysis is a process in which immersed electrodes are used to pass
electricity through to split it into positive hydrogen ions (H+) and negative oxygen
ions (O-), which collect at the cathode and anode, respectively. This process is an
efficient method for producing high-purity hydrogen without emitting any harmful
pollutes or causing any negative environmental impacts. The process must have an
electrical input, which can be provided by renewable sources, such as solar, wind,
hydroelectric, and geothermal sources.
3.8.2.1 Alkaline water electrolyser
Alkaline water electrolysers usually use an electrolyte that contains an aqueous
solution of potassium hydroxide (KOH). Figure 3.11 [34] shows a schematic
construction of alkaline water electrolyser. Alkaline electrolysers operate at
relatively low current densities of <0.4 A/cm2, and their conversion efficiencies
range from 60-90%.
61
Figure 3.11: A schematic construction of alkaline water electrolyser [34].
The chemical reactions that occur in an alkaline electrolyser are shown below:
The reaction at the anode is:
4H2O + 4e- 2H2 + 4OH
- …………………………….................3.1
The reaction at the cathode is:
4OH- O2 + 4e
- + 2H2O ………………………………………….3.2
The overall reaction is:
2H2O 2H2 + O2 …………………………………………………3.3
3.8.2.2 Proton Exchange Membrane (PEM) Electrolyser
The operation of a PEM electrolyser depends on the use of costly metal catalysts
(platinum, platinum/ruthenium) and a solid polymeric electrolyte for transferring
protons as schematically shown in Figure 3.12 [35]. The main advantages of a PEM
electrolyser over an alkaline electrolyser [12] are:
No requirement to pass a liquid electrolyte
Much smaller mass and overall dimensions
Much lower power consumption
The production of higher purity gases
62
The production of compressed gases in the plant with a higher level of safety
Operation at a high current density and the built-in ability to operate with
transient variations in electrical power input (Hence, it has excellent
application flexibility with respect to capturing variable renewable electricity
supplies, such as wind and solar power.)
Figure 3.12: Schematic diagram of a proton exchange membrane electrolyser [35].
In a PEM electrolyser, the following steps occur to produce hydrogen:
Water reacts at the anode to form oxygen and positively charged hydrogen
ions (protons).
The electrons flow through an external circuit and the hydrogen ions
selectively move across the PEM to the cathode.
At the cathode, the hydrogen ions combine with electrons from the external
circuit to form hydrogen gas.
Anode Reaction:
2H2O → O2 + 4H+ + 4e
- ……………………………………………..3.4
Cathode reaction
4H+ + 4e- →2H 2 ………………………………………………………….3.5
63
3.8.2.3 High-Temperature Electrolyser
A high-temperature electrolyser is a highly efficient method that is used for massive
hydrogen gas production from steam at high temperatures by utilizing both heat and
electric power. The advantage of the high-temperature system is its ability to
substitute heat for part of electrical energy required to split the water.
3.9 PV Electrolyser Coupled with Maximum Power Point
Tracking
The I-V photovoltaic characteristics vary considerably with solar insolation and
temperature, as shown in Figure 3.13. The operating point at which the PV panel
generates its maximum power, Pmax, at a particular voltage, Vmp, and current, Imp, is
called the maximum power point (MPP). In some applications, a PV array and load
can be coupled directly because they work on the same DC voltage; this method is
simple and reliable, but it does not operate at the maximum power point of the PV
array. Other applications may require a DC/DC converter to adjust the PV voltage, as
shown in Figure 3.13.
Figure 3.13: PV coupled with an electrolyser using a DC/DC converter for MPPT .
The DC/DC converter may have facilities for computer control, and, then, both
voltage adjustment and MPP tracking can be achieved by a software routine. Such a
system is described in the following paragraphs.
Three different types of DC/DC converters were used in coupling the PV array with
64
the load. The Buck converter, shown in Figure 3.14, is commonly used to step down
the voltage from the PV source to the operating load voltage, while a Boost converter
is used to step up the voltage. The third type is a combination of the previous two
converter types, and it is called the Buck-Boost converter. A Buck-Boost converter is
capable of increasing or decreasing the PV voltage to any voltage that is needed by
the load.
IPV
VPV
D
S1
Rload
C2
L iL
C1 V
load
IPV
IPV
VPV
VPV
D
S1
S1
Rload
Rload
C2
C2
L iL
iL
C1
C1 V
loadV
load
Figure 3.14: Photovoltaic coupling with load using a DC/DC Buck converter
A DC/DC converter uses transistor switches as simple on-off switches, which allows
the current to pass through the converter circuit or blocks it from doing so.
The relationship between the input and output voltages of the converter is controlled
by the duty cycle of the switch itself. The duty cycle D (0 < D < 1) is the fraction of
the off-time duration of the converter switch cycle, which is referred to as pulse-
width modulation.
In the case of a photovoltaic–electrolysis hydrogen production system, the DC/DC
converter steps down the PV voltage to the electrolyser operating voltage. As
mentioned earlier, the electrolyser cell is capable of producing hydrogen at high
efficiency under high current density conditions. The consumption of power is
proportional to the instantaneous current density, so the main point to consider is the
amount of power or current that can flow to the PEM cell from the DC-DC
converter, and this is the task of the control unit of the converter.
Since current depends on solar insolation and voltage depends on temperature, the
MPP changes as ambient conditions change. The maximum power point tracker is
used to make the PV deliver its maximum power. For this purpose, an analog or
65
digital circuit implemented in the DC/DC converter, known as the maximum power
point tracker (MPPT), provides an interface between the PV panel and the load. This
is usually done by slightly varying the duty cycle of the converter.
The current in a PV array varies significantly as insolation varies, whereas the
voltage of the PV array varies far less. For a silicon solar cell, the voltage variation at
the maximum power point is about 8%, whereas the current at the maximum power
point varies by as much as 80% [3]. Moreover, the voltage of the PV array at the
maximum power point has a fixed linear relationship with the open circuit voltage,
so it is better to choose the voltage as a controlling variable of the duty cycle of the
converter.
3.10 Maximum Power Point Tracking Technologies
In PV systems, significant efficiency gains can be achieved by keeping the load
operating point near the knee of the PV panel‟s I-V curve. The three main methods
of maximum power point tracking are described below.
3.10.1 Perturb and Observation (PAO) Method
In the perturb and observation method, the output current and voltage of the PV array
are measured and the resulting power is calculated. The algorithm is based on
comparing this calculated power with previous values. A detailed analysis of the
PAO method is given in Chapter 6.
A drawback of the PAO method is that the operating point oscillates around the
MPP. Also, it takes considerable time to track the MPP. In addition, the PAO
algorithm can be confused during rapidly changing atmospheric conditions [3].
3.10.2 Incremental Conduction
This method consists of using the slope of the derivative of the power with respect to
the voltage in order to reach the maximum power point. The mathematical
relationships of power and voltage that are used to track the MPP can be expressed
66
as:
dP/dV > O, left of MPP
dP/dV = O, at MPP
dP/dV < O, right of MPP
3.10.3 Fractional Open Circuit Voltage
A PV cell's open circuit voltage will vary under irradiance and temperature
conditions with approximate similarity to an array under load. This is the principle
behind fractional open circuit voltage (FOCV) and pilot cell methods. It can be
represented as simply as:
VMPP ≈ k ·VOpen
The proportionality constant k depends on the qualities of the particular PV cells
being used. In the fractional open-circuit voltage scheme, the array is momentarily
disconnected from the converter at regular intervals, and the open circuit voltage is
measured. Of course, this results in a temporary loss of power. An alternative is to
use one or more pilot cells, which are selected to have the same qualities as the cells
in the array. In this case, the main array is always connected to the converter, and the
pilot cells are available continuously for voltage circuit optimization.
Summary
We can conclude that the combination of a photovoltaic array with water electrolysis
can transform solar energy into hydrogen. The method is attractive since it requires
little maintenance and is environmentally friendly. Furthermore, future cost reduction
of PV cells is expected on the industrial scale. As the insolation varies during the day
and the year the direct coupled between a photovoltaic source and the electrolyser is
unlikely to be at its optimum most of the time. Hence a power conditioning device is
needed for the system to maximize hydrogen production. The following chapters will
examine DC/DC converters and their role in helping to optimise a photovoltaic –
PEM electrolysis hydrogen production system.
67
CHAPTER 4
A POWER MATCHING
SIMULATION OF A SOLAR
HYDROGEN PRODUCTION
SYSTEM
68
4 CHAPTER 4 A POWER MATCHING
SIMULATION OF A SOLAR HYDROGEN
PRODUCTION SYSTEM
4.1 PSCAD Software
PSCAD is a simulation tool for Power System Computer-Aided Design and
Electromagnetic Transient for DC. This simulation package has been used often in
many renewable power system simulation and design studies. The advancement-of-
time simulation has reduced the effort required significantly.
One of the advantages of computer simulation is its flexibility and convenience,
because parameters of a system model can be easily manipulated; also, computer
simulation is essential during the first stages of design to avoid the cost of errors
being detected in the later stages of design.
In this part of this thesis, the use of PSCAD to develop a simulation model for the
solar hydrogen production system is discussed. The system model simulated the
performance of power matching between the PV module and the proton exchange
membrane (PEM) electrolyser using a DC/DC Buck converter. One of the challenges
in producing hydrogen using solar energy (PV-Hydrogen production system) is to
keep costs down. Therefore, it is important that the system operate at maximum
power. This operation is achieved by matching the power generated by the PV cell
with the power required to produce hydrogen.
4.2 Model Components
Only the essential components of the solar hydrogen production system were
included in this simulation programme. As seen in Figure 4.1 the components
included were a photovoltaic module, a DC-DC Buck converter with a duty cycle
control circuit, and a PEM electrolyser.
69
Figure 4.1: PV- electrolyser power matching using DC/DC buck converter.
4.3 Input/Output Data
The input data to the simulation programme were the solar radiation hitting the
photovoltaic module and the ambient temperature.
The output results of the simulation are:
Characteristics of the photovoltaic current, voltage, and power under standard test
conditions (1000 W/m2 and 25
oC).
Characteristics of the current and voltage of the electrolyser.
Current and voltage readings at the input and output of the DC-DC buck converter
under different duty cycle values of the converter switch.
Definition of the duty cycle at maximum power matching.
To match a PV module with an electrolyser, the first requirement is to know the
current (I)–voltage (V) characteristics for both the power source and the load, since
each of them is being modelled in PSCAD, and they are coupled with a DC/DC buck
converter that acts as the power conditioner of the system.
70
4.4 The PV Model
4.4.1 PV Equivalent Circuit
Figure 4.2 shows the equivalent circuit of an ideal PV cell, which is a current
generator connected in parallel with a diode. The photocurrent, Iph, represents the
current generated by light in proportion to the photons of solar flux hitting the PV
cell.
In the simplified model, a series resistance was added to represent the voltage losses
occurring at the boundary and external contacts. For the practical PV cell model, a
shunt resistance Rsh was added to represent the leakage of current that occurred in the
cell. In this case, the Iph delivers current to the diode, the parallel resistance Rsh, and
the load.
Figure 4.2: Equivalent circuit of the PV cell.
For the actual PV cell and its equivalent circuit, there are two conditions of particular
interest, as shown in Figure 4.3, i.e., the current that flowed when the PV terminals
were shorted together (Ish short circuit current) and the voltage across the PV terminal
when its leads were kept open (Voc open circuit voltage). The I-V characteristics for
the PV cell are shown in Figure 4.3.
71
Figure 4.3: I-V characteristics of the PV cell.
The working point of the solar cell depends on the load and the solar insolation. The
maximum power point (mpp), at which the PV cell delivers its maximum power (Vmp
and Imp), occurred near the knee point along the characteristic I-V curve.
Adding a series resistance RS to the PV equivalent circuit (Figure 4.4) caused the
voltage in the I-V curve to shift by SIRV .
For the current leakage, the value of Rsh was generally kept high, and, so, adding the
parallel resistance RSh caused the current to decrease by a small amount, V/RSh as
seen in Figure 4.5.
I
V
Rsh = 0, Rs
IRsV
Figure 4.4: Effect of adding RS on the PV cell‟s I-V curve.
72
Slope=1/Rsh
I
V
Rsh
VI
Rsh = 0, Rs
Figure 4.5: Effect of adding RSh on the PV cell‟s I-V curve.
4.4.2 PV PSCAD Model
Figure 4.6: Model of the PV module.
The model takes into consideration the variation in insolation and temperature.
Changes in insolation affect the photon-generated current and had a relatively
insignificant effect on the open circuit voltage, whereas temperature variations
caused the open circuit voltage and the short circuit current to vary only a marginal
amount.
The following general equation gives the relationship between the current and
voltage of a PV cell [38]. The output current (I) is equal to the difference between the
photocurrent and the diode current, Id. This output current (I) is given by:
1exp
KT
qVIII oph …………………………………..……... 4.1
73
where V is the voltage; Io is the dark saturation current; q is the electron charge
(1.602 × 10-19
C); k is Boltzmann‟s constant (1.38 × 10-23
J/K); and T is the
temperature.
The open circuit voltage was measured at the terminals by setting the output current,
I, equal to zero. The open circuit voltage is given by the equation:
1ln
o
ph
ocI
I
q
KTV ……………………………………………..…...4.2
The current-voltage characteristics of the crystalline silicon cell module can be
described by the following formulae [41], which were implemented in FORTRAN
codes (appendix A) inside the PSCAD model:
800
)20(
NOCTSTT a ..........................................................................4.3
8.4..................................................................................................
7.4............................................................................).........1exp(
6.4..............................................................).........(1000
5.4...................................................))........11
(exp()(
4.4...............................................).........1108(1002.716.1
0
3
0
24
dph
d
refoscph
ref
g
ref
do
g
III
nkt
qVII
TTJs
II
TTnK
qE
T
TII
TTE
where :
T = cell temperature in oK
T a = ambient temperature, oX
I 0 = dark saturation current, A
E g = energy gap of cell semiconductor, ?
I d = diode current, A
74
I ph = photo current or light-generated current, A
V = cell output voltage, V
In this PV cell model, a crystalline silicon solar cell was considered, the parameters
used in modelling of this type of silicon PV cell are tabulated in Table 4.1 [41].
Symbol/Value Description Unit
q = 1.602 x1019
Electron charge C
k = 1.38x1023
Boltzmann constant J/oK
n = 1.792 Non-ideality factor
T Input data Ambient temperature oK
T ref = 293 Reference temperature oK
I sc Short circuit current at reference state A
NOCT = 49 Nominal Operating Cell Temperature oC
J o = 1.6 103 Temperature coefficient A/
oK
S Input data Insolation W/m2
I do do = 71.1109 Diode reversal current A
Table 4.1: Parameters of a crystalline silicon solar cell [41].
4.4.3 Response of the Model to Changes in Insolation
The characteristics of the solar cell at different levels of insolation (S) are shown in
Fig 4.7. The insolation has a large effect on the short-circuit current (the horizontal
part of the I–V curves). If the insolation level increases, the short circuit current
increases, keeping the open circuit voltage almost constant (where S is the value of
insolation in W/m2).
According to the voltage and power curves, the maximum output power of a
photovoltaic module changes as insolation changes. When the insolation is greater,
the cell generates more power.
75
Figure 4.7: The dependence of I-V characteristics on insolation.
4.4.4 Response of the PV Model to Changes in Temperature
The cell temperature varies because of the changes in the ambient temperature and
changes in the levels of irradiance. Since only a small fraction of the insolation on a
cell is converted to electricity, most of that incident energy is absorbed and converted
into heat.
Figure 4.8 shows that the cell temperature (in oK) of the PV module increases, while
the open circuit voltage decreases, and the short circuit current is almost constant.
Figure 4.8: Effect of temperature on the PV model curves.
There is a maximum power generated by a PV module occurring at a point called the
maximum power point (mpp) with the coordinates V = maximum voltage Vm and
76
maximum current Im.
The PV power source characteristics used in this PSCAD simulation are shown in
Figure 4.9.
Figure 4.9: PV characteristics used in the PV-PEM PSCAD model.
4.5 DC-DC Buck Converter PSCAD Model
If the PEM electrolyser is connected directly to the PV module, the operating point is
the result of the intersection between the I-V curves of the electrolyser and the PV
module. However, if a DC/DC buck converter is placed between the load and the PV
module, the operating point depends on the duty cycle of the converter switch. Direct
coupling may result in somewhat lower efficiency, due to the losses related to
power/voltage matching.
A DC-DC buck converter was used for the PV-PEM electrolyser to step down the
output voltage of the PV to the level of the PEM operating voltage and to make the
PV and PEM electrolyser work at their maximum power.
The DC-DC buck converter (described in more detail in Chapter Five) was used in
this PSCAD model as a power conditioner between the PV model and the PEM
electrolyser model, as shown in Figure 4.13.
The converter was built using passive and active elements available in a PSCAD
package library. All the elements were considered to be ideal. Also, the converter
77
was designed to operate in continuous conduction mode.
A Pulse-Width Modulation (PWM) circuit was used to control the duty cycle D
duration (ON and OFF time durations) of the converter switch. The pulse-width
modulator was generated by comparing a signal-level DC voltage with a repetitive
sawtooth wave form that had a constant peak, as shown in Figure 4.10.
Figure 4.10: PWM to produce different duty cycle generator.
The duration of the on and off states controls the relationships between the voltage
and current on both terminals of the DC-DC buck converter, as the following
equation indicates:
Ipem
Ipv
Vpv
Vpem
ToffTon
TondutycycleD
)( …………………………………. 4.9
where ( pvV and PVI ) are the PV voltage and current and ( pemv and pemI ) are
the PEM electrolyser voltage and current, respectively .
4.6 The PEM Electrolyser PSCAD Model
A PSCAD model of a two-cell PEM electrolyser was developed. This kind of water
electrolyser has advantages over the traditional water electrolyser, i.e., it operates at
high current density; it avoids using a liquid electrolyte as the alkaline electrolyser
does; and it produces a high purity gas that is ideal for use as a fuel for the fuel cell.
78
Figure 4.11: PSCAD PEM electrolyser block.
As shown in Figure 4.11, the PEM electrolyser PSCAD model had two inputs and
two outputs, i.e., the inputs are current and temperature, and the outputs are voltage
and hydrogen volume. As mentioned in Chapter Three, the production rate of
hydrogen depends on the current flowing into the electrolyser and the temperature of
the water. If the temperature increases, the operating voltage of the electrolyser will
decrease; more current flows at a higher temperature. The equations below govern
the relationships between the input and output variables.
The I-V relationship of a PEM cell at a cell current density of (1 A/m2) is given by
the following equations [39][42]:
IRVV ac 0 , …………..………………………………..4.10
where V is the cell voltage of the PEM, and V 0 is the theoretical dissociation
voltage, which depends on absolute temperature T (oK), as shown:
2853
0 8.9)ln(5.95.15.1 TeTTeTeV …………………….4.11
The term ηc is an excess voltage on the cathode side with a value varying from 0.05
to 1 V; ηais an excess voltage on the anode side with a maximum value of 0.3 V; R is
cell resistance of the PEM electrolyser at the water temperature of 293 OK (25
OC),
and its approximate relationship with current [42] is:
79
1
131.0
)7.5/1(
)7.5/1(
Ipem
Ipem
e
eR ……………………..………………………..4.12
where I is the current flowing through the PEM electrolyser. The PEM temperature T
was adjusted to room temperature (294 0K).
The volume of hydrogen produced was calculated by applying Faraday‟s first law of
electrolysis:
ZPF
tTIRmVH
..
...)( 3
………………………………………………………..4.13
Where R =8.314 Joule /(mol Kelvin), I = current in A, T is the temperature in K , t =
time in sec , F Faraday‟s constant = 96485 C/mol, P ambient pressure in P, Z =
number of excess electrons = 2 (for hydrogen) and 4 (for oxygen).
Figure 4-12 shows the I-V and P-V characteristics of the PEM cell electrolyser
model. The voltage-current graph shows that, for the PEM electrolyser, the current
only starts to flow at a certain voltage, after which it rises continuously. The slope of
the curve is dependent on the electrolyser equivalent ohmic cell resistance.
The applied voltage must be at least above the threshold PEM cell voltage in order
for current to flow, which leads to a release of hydrogen at the cathode and oxygen at
the anode.
Figure 4.12: I-V and P-V curves for the PEM electrolyser.
80
4.7 PV-PEM Hydrogen Production Power Matching
Model
The PSCAD model of the PV electrolyser hydrogen production system with a buck
converter for power matching is shown in Figure 4.13.
The power supply of the circuit is a PV module, and its characteristics are shown in
Figures 4.9 and 4.12. The short-circuit current was 0.7 A, and the open-circuit
voltage was 20 V; the operating voltage of the two-cell PEM electrolyser was
approximately 5.3 V.
Figure 4.13: PV-PEM electrolyser PSCAD model
The component sizes for the DC/DC buck converter were selected as follows [40]:
The inductor L value was calculated as:
LMs
omom
If
DVL
)1( ,………………………………………………………..4.14
where:
81
fs = sT
1 is the switching frequency
Dcm is the duty cycle at maximum converter output power
∆ILm is the peak-to-peak ripple of the inductor current
Vom is the maximum of the DC component of the output voltage
Iom is the DC component of the output current at maximum output power
In this simulation, the inductance value L was selected as 0.6 mH.
The output capacitor value calculated to give the desired peak-to-peak output voltage
ripple was:
oms
omcmO
Vrf
IDC , …………………………………………………………4.15
where r is the output voltage ripple defined as r = (∆Vom / Vom), and ∆Vom is the peak-
to-peak value of the output voltage at maximum power (assumed to be equal to 0.05
V). In this simulation, Co is selected at 500 F.
If we need the ripple of the PV output current to be less than 2% of its mean value,
then the input capacitor value can be calculated by:
spvmpvm
cmomcmin
fRI
DIDC
02.0
)1( ,………………………………………………….4.16
where Iom is the converter input current at maximum input power, and Rpvm is the
internal resistance of the PV array at the maximum power point, which is defined as:
pvm
pnm
pvmI
VR
, ………………………………………………………4.17
Cin was selected as 2000 F.
82
where Vpvm is the output voltage of the PV array at the maximum power point. The
series resistance Rs was set as 0.08 , and the shunt resistance Rsh was set as 200
.
To generate different duty cycle values for the DC/DC converter switch, a fixed-
amplitude, sawtooth signal was compared with a changeable voltage level. A
comparator produced pulses with different duty cycles, and the pulses switched the
buck converter on and off.
4.8 Simulations and Results
The following PSCAD simulation results were obtained for the standard irradiance
(1000 W/m2) and the standard temperature (25
0C). The measured values are in volts
for voltage readings and in amperes for current values.
The DC/DC buck converter model had no losses, and it acted as an ideal step-down
DC transformer, making the input and output power of the converter the same value.
Figure 4.14 shows the V-P characteristics of both the PV array and the PEM
electrolyser. The PV array was able to supply up to 8.3 W. In order for the PEM
electrolyser to operate at this power, its voltage had to be 14.7 V. The values of the
voltage of the PV array and the PEM electrolyser were not equal to each other.
Figure 4.14: V-P curves for the PV array and the PEM at different duty cycle values
(D).
83
With a duty cycle value of one, the PEM electrolyser was connected directly to the
PV array. The operating point corresponding to this duty cycle was determined by
the intersection between the V-I characteristics of both the PV array and the PEM
electrolyser, as shown in Figure 4.15. This corresponded to a voltage of 5 V and a
current of 0.66 A. The power supplied to the electrolyser in this case was 4 W. This
is less than the maximum value of 8.3 W.
Figure 4.15: Intersection with the rescaled I-V curve of the PEM.
The lossless buck converter modelled in this chapter operated as an ideal transformer
for DC power. The voltage at its input terminals was equal to the voltage at its output
terminals divided by the value of the duty cycle. On the other hand, the current at its
input terminals was equal to the current at its output terminals multiplied by the
value of the duty cycle.
With the PEM electrolyser supplied from a buck converter, its I-V characteristics, as
seen at (referred to) the input terminals of the converter were scaled by the duty cycle
of the converter. Each value of a duty cycle would produce a different characteristic
at the input terminal of the buck converter. Examples of these characteristics are
shown in Figure 4.16 for some possible values of duty cycle.
84
PV maximum
power
Figure 4.16: Intersection with the rescaled I-V curve of the PEM.
Figure 4.17 also shows the I-V characteristics of the PV array superimposed on the
set of I-V characteristics of the PEM electrolyser referred to the PV array side of the
converter. The intersection between the I-V characteristics of the PV array and those
of the PEM electrolyser corresponding to a certain value of duty cycle is the
operating point for this duty cycle. As shown by Figure 4.16, reducing the duty cycle
will move the operating point to the right.
With a duty cycle of D = 0.4, the I-V characteristics of the electrolyser, referred to
the PV array side of the converter, intersected the I-V characteristics of the PV array
at 14.7 V. This is the voltage at which the PV array supplied its maximum power.
With this operating point, the voltage at the electrolyser side of the converter was 5
V, and the current was 1.4 A.
Reducing the duty cycle below 0.2 would cause the I-V characteristics of the
electrolyser not to intersect those of the PV array. This means that, if the PEM
electrolyser is to operate, it would need a voltage on the PV array side of the
converter that is higher than the open-circuit voltage of the PV array. The PV array
would fail to supply this voltage.
Figure 5.18 shows the variation of the current and voltage of both the PV array and
85
the PEM electrolyser with the change of duty cycle. The electrolyser voltage was
almost constant, whereas the electrolyser current increased from 0.56 A at a duty
cycle of 1.0 to 1.4 A at a duty cycle of 0.4, before it decreased again to 0.005 A at a
duty cycle of 0.1.
Figure 4.17: Relationship between current, voltage, and duty cycle.
0
0.02
0.04
0.06
0.08
0.1
0.12
0.14
0.16
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
Duty cycle D
Hyd
rog
en
pro
du
cti
on
rate
in
m3
Figure 4.18: Relationship between hydrogen production rate and duty cycle.
86
The relationship between hydrogen production rate and duty cycle value is shown in
Figure 4.18. The theoretical hydrogen production volume (m3) was calculated by
applying Faraday‟s first law of electrolysis, which showed that, at a D value of 0.4,
the maximum hydrogen production rate occurred, while direct connection (where D
=1) has less hydrogen production rate.
The power matching duty cycle D can be calculated by using voltage and current
values, as follows:
The duty cycle needed to achieve maximum power point operation is equal to the
ratio between the voltage of the PEM electrolyser and the PV array voltage at its
maximum power point.
mpp
PEM
V
VDutycycle ………………………………………….……….4.18
In the equation below, it is apparent that the maximum power point voltage (Vmpp)
of the PV array has an almost linear relationship with the open-circuit voltage (Voc)
of the solar photovoltaic module:
ocmpp KVV …………………………………………………….….4.19
where K is a constant that has different values for different solar panels, and Voc is
the open-circuit voltage. The open-circuit voltage (Voc) can be measured by
disconnecting the PV at regular intervals.
From the practical measurements and characteristics Table 4.2 of different
commercial photovoltaic modules undertaken by Thomas and Nelson [6]. The
4.040.1
56.0
)(
)(
71.14
88.5
)(
)(
AIpem
AIpv
VVpv
VVpemdutycycleD
87
constant factor K of various PV modules were shown to be similar.
Model number Model
manufacture
Type of silicon
cells
Voc
(V)
Vmpp
(V)
K
SQ-75 Shell Monocrystalline 21.7 17 0.78
ND-NOECU Sharp Multicrystalline 24.9 20 0.80
Hip-J54BA2 Sanyo Amorphous 66.4 54 0.81
NT-185U1 Sharp Single crystalline 44.9 36.2 0.80
Table 4.2: Constant K value of different solar modules tested [6].
Summary
A PSCAD simulation model was developed for photovoltaic (PV) module and proton
exchange membrane (PEM) electrolysis hydrogen production system. The system
model simulates the performance of power matching between PV and PEM using a
DC/DC buck converter. The results show that by adjusting the duty cycle of the buck
converter, the system could be optimized and operate at its maximum power.
88
CHAPTER 5
DESIGN DC/DC BUCK CONVERTER
FOR PV-PEM HYDROGEN
PRODUCTION SYSTEM POWER
MATCHING
89
5 DESIGN DC/DC BUCK ONVERTER FOR PV-
PEM HYDROGEN PRODUCTION SYSTEM
POWER MATCHING
5.1 Background
A DC/DC converter is an electronic circuit that is used to convert a direct current
(DC) source from one voltage to another.
Conventionally, linear regulators have been used to regulate voltage; the resistance
of the linear regulator varies in accordance with the constant output voltage level.
Losing power (i.e., voltage drop across resistance multiplied by the current flow) is
one of the main disadvantages of this type of voltage regulation, as the dissipated
power is in the form of heat. Also, this type of regulator only can be used for cases in
which the required output voltage is lower than the input voltage. It is not possible to
have an output voltage that is greater than the input voltage.
A switched regulator, which is an electronic circuit that uses a power switch, a diode,
a capacitor, and an inductor, is much more versatile than the resistive regulator. The
time that the switch remains open during each cycle is varied to maintain a steady
output voltage that can be varied as desired. The advantage of a switched regulator is
that the inductor stores energy in the “ON” phase and then gives back most of the
energy during the “OFF” phase. Also, unlike linear regulators, switched power
supplies can step down (buck converter) or step up (boost converter) the input
voltage.
Step-down buck converters usually are used in solar hydrogen production systems.
They are used to step down the voltage of the photovoltaic power source to the lower
operational voltage of the electrolyser.
A DC/DC buck converter is described in this chapter. Features of the design are
discussed, and the circuit is fabricated on a printing circuit board (PCB). The buck
converter was designed to match the power for a small laboratory PV-PEM
90
electrolyser test system.
5.2 Buck Converter Theory and Operation
The Buck converter circuit components are shown in Figure 5.1, the average load
voltage is lower than the input source voltage, and Pulse-width modulation (PWM) is
used to control the converter switch.
Figure 5.1: Buck converter circuit.
5.2.1 Purpose of Different Buck Converter Components
As seen in the buck circuit diagram in Figure 5.1, the buck converter consists of five
components:
Switch
Pulse-width modulation circuit
Inductor
Capacitor
Free-wheel diode
We will now provide additional details concerning the function and selection of these
components.
91
5.2.1.1 Switch
The power switch in a buck converter is for turning the converter on or off, and, in
general, this switch must have very fast switching times and be able to withstand the
voltage spikes produced by the inductor. Transistors are, typically, used for
switching. The signal from the pulse-width modulator (PWM) is connected to the
gate of the transistor to determine the on and off time. The load current and the off-
state voltage capability decide the size of this power switch.
The transistor can be a metal oxide silicon field effect transistor (MOSFET), an
insulated gate bipolar transistor (IGBT), a bipolar junction transistor (BJT), or a
junction field effect transistor (JFET). MOSFETs are used for high frequency power
systems. In newer designs, MOSETs have replaced BJTs for use at higher
frequencies and lower voltages.
The IGBT is also used for power switching. It uses low power to produce the same
characteristics as the BJT. The IGBT is highly suited for high power and high
voltage applications, because it has a switching speed that is much slower than the
switching speed of a MOSFET. Therefore, circuits using an IGBT have lower
switching frequencies.
5.2.1.2 Pulse-Width Modulation Circuit
The key objective of a pulse-width modulator (PWM) circuit is to obtain a fixed
value of the output voltage of the converter. The PWM circuit controls the ON and
OFF time durations of the switch. The desired value of the output voltage is achieved
by varying the duty cycle of the switch,
The pulses of the PWM circuit are generated at a constant switching frequency by
comparing a DC voltage level with a repetitive sawtooth wave form that has a
constant peak. An error amplifier is used as a comparator, and, when the level of the
DC signal is less than the sawtooth waveform, the switch control signal becomes
high, causing the switch to turn on; otherwise, the switch is off.
5.2.1.3 Operating Frequency
The performance of the buck switch is determined by the operating frequency. The
higher the switching frequency, the smaller the physical size of the components
92
becomes. However, there are always losses in each of the components, and these
losses tend to increase as frequency increases. Therefore, there will always be some
optimum frequency that gives the greatest efficiency of the converter.
5.2.1.4 Inductor
The inductor limits the current slew rate through the power switch when the switch is
on. When the current through the inductor tends to decrease, the inductor will act as
an energy source with negative polarity. Also, the inductor controls the current
ripple.
The smaller the inductor value, the faster the transient response becomes, and large
current ripple is produced, causing higher conduction losses in the switches and in
the inductor. Also, smaller inductor values require a larger filter capacitor to
minimize ripple in the output voltage.
5.2.1.5 Capacitor
The capacitor provides a filtering action for the inductor, and it provides a means of
removing the harmonic current from the load. Therefore, the output capacitor
minimizes the voltage ripple produced at the output of the buck converter. In
addition, voltage ripple is caused to a smaller extent by a high equivalent series
resistance in the capacitor.
Thus, for maximizing the performance of a step-down converter, a capacitor should
be selected that minimises the losses caused by internal series resistance and
inductance.
5.2.1.6 Free-Wheeling Diode
The free-wheeling diode provides a path for the current from the inductor when the
switch is off, so there is always a path for current flow to the load. The diode must be
able to turn on and off relatively rapidly to enable the delivery of the energy stored in
the inductor to the load.
5.2.2 Circuit Description and Operation
The equations that govern the operation of the circuit in the on state and the off state
93
are shown below.
ON state:
When the switch is ON for time duration DT, the switch conducts the inductor
current IL, causing energy stored in it to increase, and the diode becomes reverse
biased. This results in a positive voltage, VL = Vin - VO, across the inductor. The VL
causes an increase in the inductor current IL, as shown in Figure 5.2.
Figure 5.2: Buck converter ON state.
1.5.....................................................................L
VV
dt
dioinL
OFF state:
In the off state ( Figure 5.3) the switch is open and diode D conducts because of the
stored inductive energy, which causes the current through the inductor to decrease
linearly. At the off state, a negative induced voltage drop across the inductor causes
the inductor voltage VL = -VO .
2.5............................................................................L
V
dt
diOL
94
where Vin and Vo are the converter input and output voltages, respectively; L is
inductance; iL is the inductance current; VL is the inductance voltage; ton is the time
duration of the ON state; and Ts is the total cycle time.
Figure 5.3: Buck converter OFF state.
During the ON state and then during the subsequent OFF state, the buck converter
can operate in two state modes, i.e., continuous current mode (CCM) and
discontinuous current mode (DCM). The difference between the two modes is that
the inductor current IL in conduction does not fall to zero in the CCM mode, as is
shown in Figure 5.4.
The overall performance is usually better in the CCM mode, and the maximum
power generated by the source can be obtained. In the DCM mode, the current in the
inductor falls to zero for some portion of the switching cycle. This case is used when
the maximum load current is fairly low, and, as a result, the overall converter size
will be smaller because a smaller inductor can be used.
Figure 5.4: (a) CCM and (b) DCM for inductor current.
IL
t
IL
t (b)
IL
t
IL
t (a)
95
Since, in steady-state operation and in an ideal component, the waveform must repeat
from one time period to the next, the integral of the inductor voltage V L over one
time period must be zero, where:
3.5.........................)1( TDDTttToffons
Typical waveforms are shown in Figure 5.5 a and b.
Figure 5.5: ON and OFF waveforms of the buck converter [43].
5.5......................................).........()(
4.5...........................................00
onsoonoin
T
t
L
t
L
T
L
tTVtVV
odtVdtVdtVs
on
ons
6.5...............................................).........(dutycycleDI
I
T
t
V
V
o
in
s
on
in
o
96
In this steady state mode and with ideal components, the output voltage power is
equal to the input power, as shown in equation (5.7).
7.5..................................................................outoutinin
VIVI
5.3 PSCAD Simulation of a Buck Converter
The PSCAD circuit of a Buck converter is shown in Figure 5.6. The input voltage of
the converter is 20 V, and it is connected via the Buck converter to a resistive load of
2 ohms. A sawtooth waveform (minimum of 0 V and maximum of 1 V) is fed to
terminal B of a comparator. A steady voltage with a range of 0 V to 1 V is fed to
terminal A. By varying this steady voltage, pulses with a variable duty cycle can be
generated, and the pulses are then applied to the transistor. The sawtooth frequency
was chosen as 5 kHz. This simulation is to show that the buck converter output
voltage value is controlled by the duty cycle value of the switch.
Figure 5.6: PSCAD buck converter simulation.
The inductor and capacitor values for CCM were calculated using the flowing
formulae:
97
8.5......................................................2
)1(min
f
RDL
9.5........................................................................0
minfV
IC L
where the component values are those illustrated in Figure 5.6.
Figure 5.7 shows the simulation results of load voltage ripple and CCM current
ripple. It shows the converter output voltage and current ripples, which depend on the
values of the inductor, capacitor, and operating converter frequency
.
Figure 5.7: Load voltage and current ripples
It is clear that the waveforms are very similar to those given in Figure 5.5.
0.0
2.0
4.0
6.0
8.0
10.0
12.0
14.0
16.0
18.0
20.0
22.0
y
VD (V) VO ( D=0.9) V
(a)
98
Figure 5.8: Diode voltage wave from (a) Vin = 20 V, D = 0.9 and Vo = 18 V and (b)
Vin = 20 V, D = 0.4 and Vo = 8 V.
5.3.1 DC-DC Buck Converter Circuit Using IC TL494 Control
Circuit
The circuit was built as described in the step-down converter Application Note in the
IC TL494 data sheet, where the PWM control method was used to control the output
current of the converter. Some modifications were made to the original circuit
available in the data sheet to increase the maximum output current. The input voltage
range was from 10 to 40 V, the output voltage was fixed by the control loop at 5 V,
and a maximum current of 1 A was delivered at the output. A Darlington Tip 129
transistor was used as the switching transistor. The values of the inductance and
capacitance in the filtering parts were modified from those given on the data sheet,
and they are shown in Figure 5.9.
. For increased current drive to the Tip129 switching transistor, the internal transistor
collectors (pins 8 and 12) were connected in parallel. The TL494 has two error
amplifiers; one was used to adjust output voltage, and the other was used to control
maximum current.
(b)
99
Figure 5.9: (a) DC-DC buck converter using IC TL494 and (b) the practical circuit
of the circuit shown in (a).
The pulse width was adjusted by the TL494 to maintain the converter output voltage
at 5 V. This was achieved by comparing the output voltage to a reference voltage
generated inside the IC by simply connecting the output voltage to the (+) input of
the error amplifier of the IC through a 5.1-kΩ resistor and the (-) input was
connected to the reference voltage pin.
Overcurrent protection was achieved by using a second amplifier, so the output
(b)
(a)
100
current was sensed across the current sensor. In the present circuit, a 0.1-ohm resistor
was used, so the voltage across this resistor was Iout 0.1 ohm, which is then
compared with the reference voltage. Because of the low value of the sensor voltage,
the divider chain of 5.1-kΩ and 150-Ω resistors reduced the reference voltage.
Figure 5.10 a and b shows the waveforms of the diode voltage VD and output voltage
Vo taken by using a dual-channel, USB picoscope (Pico Technology, Ltd., Country).
The input voltage reading is on the left-hand side of the graph (blue), and the output
voltage is on the right-hand side (red). It can be seen from the experimental results
that the variation of duty cycle was subjected to the changing input voltage. The
output voltage was constant and was controlled by the load when the load current
was greater than the input current.
(a)
101
Figure 5.10: Diode voltage form (a) V in = 20 V and V out = 5V and (b) V in = 6 V
and V out = 5 V.
5.4 Characteristics of the PV-PEM Electrolyser Test Rig
When a buck converter is used in a PV-PEM hydrogen production system, the input
voltage V in , from the PV source, is variable and depends on the temperature and
irradiation conditions. The output voltage Vo must remain almost constant at the
working voltage of the PEM electrolyser. The buck converter duty cycle D controls
the input and output voltage values at converter terminals so it can be fixed at the
maximum power point voltage of the PV array.
It was indicated in Chapter Three that the maximum power matching between the PV
and PEM electrolyser was achieved by adjusting the converter duty cycle. A DC/DC
buck converter was designed and implemented in PCB to provide practical
verification of the power matching between the PV-PEM electrolyser test rig
available in our laboratory, which is shown in Figure 5.11. The test rig consisted of a
low-power PV panel, Solarex LD 664-431, and a one-cell PEM electrolyser. As the
electrolyser is operating at a low voltage, the DC/DC buck converter steps down the
PV module voltage to that required by the electrolyser.
(b)
102
Figure 5.11: PV –PEM test rig.
5.4.1 PV Characteristics
The PV module consisted of two small crystalline solar panels connected in series.
The circuit shown in Figure 5.12 was used to obtain the I-V and V-P characteristics.
The PV module was illuminated by an artificial light source. Voltage and current
readings were obtained as functions of the load resistance value by using two
TENMA digital multimeters.
Figure 5.12: Setup for determining the characteristics of a solar module.
The experimental I-V and V-P curves, given in Figure 5.13, are for a fixed insolation
of 700 W/m2. This light level was achieved with a 500-W halogen lamp at a distance
of 40 cm from the PV panel.
.
PV
LD 664431
A
V R
Light
103
0
0.1
0.2
0.3
0.4
0.5
0.6
0 1 2 3 4 5 6
Voltage (V)
Cu
rren
t (A
)
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
2
0 1 2 3 4 5 6
Voltage (V)
Po
wer
(W)
Figure 5.13: PV characteristics.
5.4.2 PEM Characteristics
This experiment was conducted to obtain the voltage and current requirements to
produce hydrogen gas from PEM electrolysis.
The circuit is given in Figure 5.14 where the PEM electrolyser is type D–666 484.
A
VPEM
Elec.
Figure 5.14: Circuit diagram of the characteristics of the PEM electrolyser.
The parameters of the LD-666 484 PEM electrolyser were:
Operating voltage: approximately 3 V
Operating current: 3 A
Maximum current: 4 A
Maximum temperature: 80 °C
104
Gas generation: approximately 35 ml H2/min at 4 A
The purpose of drawing the I-V characteristics of the electrolyser was to determine
the operating voltage at which hydrogen production commences. Below this voltage,
there is insufficient energy to cause water molecules to dissociate.
The input I-V characteristics (response) of the PEM electrolyser in Figure 5.15 were
obtained by adjusting the power source. The input currents were defined at different
applied voltages (0 - 3.3 V). The current only began to flow at a threshold voltage
(Vthreshold ≈ 2 V), and then it increased exponentially.
0
0.5
1
1.5
2
2.5
3
3.5
4
0 1 2 3 4 5 6
Voltage (V)
Cu
rre
nt
(A)
0
2
4
6
8
10
12
14
0 1 2 3 4 5 6
Voltage (V)
PO
we
r (W
)
Figure 5.15: PEM electrolyser characteristics.
5.4.3 Dependence of Hydrogen Production on the Operating
Current of the PEM Electrolyser
To determine the relationship between the volume of hydrogen produced (ml) and
the current I (A), the amount of current was varied over a fixed process duration,
and the volume of hydrogen produced for that duration was measured.
0
10
20
30
40
50
60
70
80
90
0 0.2 0.4 0.6 0.8 1 1.2 1.4
Current (A)
Hyd
rog
en
(m
l)
Figure 5.16: Volume of hydrogen produced as a function of current over a 10-
minutes operational period.
105
By plotting the volume of hydrogen gas as a function of current, Figures 5.16, it was
apparent that the volume of hydrogen produced had a linear relationship with the
current.
5.5 Design Buck Converter
5.5.1 PWM Circuit
The use of a pulse-width modulation (PWM) circuit is a way to control the On- OFF
time duration of a buck converter switch. The IC SG3525 was used to generate the
PWM output. Figure 5.17 shows the PWM circuit used in this converter. The level of
an external DC voltage source was compared with the continuous sawtooth signal
generated by the IC oscillator. When the sawtooth signal was at a greater voltage
than the voltage of the input signal, a comparator was used to produce a pulse for the
on duration. When the input signal level was greater than the sawtooth signal, the
comparator turned off for the off duration. The duration of the on and off periods can
be controlled by varying the level of the input voltage signal by using a voltage
source with a variable range from 0 to 5 V.
Figure 5.17: PWM circuit using IC SG3525.
106
The buck converter circuit was implemented in PCB combined with the PWM duty
cycle circuit as seen in Figure 5.19. The value of the buck converter components
were selected in order to have the same values of the Buck converter PSCAD model
in Chapter Four, as follows:
L = 0.6 mH; Cin = 2000 uF; and Cout = 500 uF, and the main transistor switch,
Tip36, was selected, and Darlington Tip 122 transistors were used to increase the
current gain. The circuit is shown in Figure 5.18.
Figure 5.18: PV –PEM electrolyser coupled by a buck converter circuit.
Figure 5.19: Buck converter implemented on a PCB.
107
5.5.2 Evaluation of Results
Figures 5.20 (a) and (b) show the oscilloscope images of two duty cycle waveforms
(blue) and the operating voltage value of the PEM electrolyser for the designed buck
converter. The red line is the DC voltage value (right-hand side) of the PEM
electrolyser operating voltage. The results illustrate that, although the duty cycle
changed, the voltage of the operating electrolyser remained almost constant. It was
noted that the electrolyser current changed, so the electrolyser consumed additional
power.
Figure 5.20: Oscilloscope images of electrolyser voltage (red) at different duty cycle
values (blue).
µs0 20 40 60 80 100 120 140 160 180 200
V
-5
-4
-3
-2
-1
0
1
2
3
4
5V
-5
-4
-3
-2
-1
0
1
2
3
4
5
ch A: Frequency(kHz) 12.5209Aug2009 15:31(b)
µs0 20 40 60 80 100 120 140 160 180 200
V
-5
-4
-3
-2
-1
0
1
2
3
4
5V
-5
-4
-3
-2
-1
0
1
2
3
4
5
ch A: Frequency(kHz) 12.5209Aug2009 15:48(a)
108
The following readings Table 5.1 are obtained during the using the Buck converter
as power conditioner between the PV module and PEM electrolysers.
D Vpv(V) Ipv(A) Vpem(V) Ipem(V) Ppv(W) Ppem(W)
0.05 5.45 0.02 2.4 0.03 0.1 0.07
0.12 5.29 0.03 2.5 0.05 0.15 0.12
0.22 5.19 0.05 2.6 0.08 0.25 0.2
0.31 5.11 0.07 2.7 0.11 0.35 0.29
0.35 5.07 0.08 2.7 0.12 0.4 0.32
0.46 5 0.12 2.8 0.16 0.6 0.44
0.55 4.94 0.14 2.8 0.19 0.69 0.53
0.64 4.81 0.21 2.85 0.27 1.01 0.76
0.72 4.71 0.26 2.85 0.3 1.22 0.85
0.8 4.6 0.31 2.85 0.33 1.42 0.94
0.87 4.49 0.35 2.85 0.36 1.57 1.02
0.93 4 0.46 2.9 0.4 1.84 1.16
1 2.9 0.5 2.8 0.5 1.45 1.4
Table 5.1: Voltage and current readings at both sides of the Buck converter.
Figure 5.21 shows the I-V curve obtained at the Buck converter terminals that
represents the I-V curves for PV (blue) and PEM electrolyser (red).
0
0.1
0.2
0.3
0.4
0.5
0.6
0 1 2 3 4 5 6
Voltage (V)
Cu
rren
t (A
)
PV I-V curve PEM I-V curve
Figure 5.21: I-V curves for the PV array and the PEM electrolyser.
109
From the experimental curves, it can be seen that each duty cycle D had its
corresponding current and voltage values on the I-V curves of the PV array and the
PEM electrolyser as seen in Figure 5.21. As the on time duration of the duty cycle
increased, the converter output current increased, resulting in the production of
additional hydrogen.
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
2
0 1 2 3 4 5 6
Voltage (V)
Po
wer
(W
)
PV P-V curve PEM P-V curve
Figure 5.22: P-V curves of the PV array and the PEM electrolyser.
From the power-voltage curve of Figure 5.22, it can be determined that the direct
connection (D = 1) had less power value than the PV module can deliver, which
matched up with D = 0.9 at maximum power voltage of nearly 4 V.
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 1.1
Duty cycle D
Con
vert
er e
ffeci
ency
%
Figure 5.23: Relationship between the implemented efficiency of the Buck converter
and duty cycle.
110
The increase in the input power of the PEM electrolyser was determined the electrolyser
current and the duty cycle until a maximum power point, after which the input power
decreased following the I-V characteristics of the PV array.
Although the circuit provided a general idea about the operation of the buck
converter, it could be regarded only as an initial design. The converter was found to
have efficiencies between 65% and 70% depending in the duty cycle, as shown in
Figure 5.23. To increase this efficiency, greater care would be needed in the selection
of the switching transistor and the inductance.
Summary
A DC/DC buck converter was designed and implemented for power matching of a
PV-PEM hydrogen production laboratory test rig. Although the circuit gives us the
general idea about buck converter operation it could only be regarded as an initial
design. The converter was found to have an efficiency between 65% and 70%
depending on the duty cycle . To increase this efficiency, greater care would be
needed in the selection of the switching transistor and the inductance.
Since a commercial Buck-Boost converter, UDIO 60.25.L, was available from
Greenenergy Technology [45] it was decided to use this converter for further
experiments as described in Chapter 6.
111
CHAPTER 6
REAL TIME EXPERIMENT OF A
PV-PEM HYDROGEN PRODUCTION
SYSTEM USING A COMMERCIAL
SPLIT-PI CONVERTER
112
6 REAL TIME EXPERIMENT OF A PV-PEM
HYDROGEN PRODUCTION SYSTEM USING A
COMMERCIAL SPLIT-PI CONVERTER
6.1 Background
The PV-hydrogen production system was scaled up by a factor of 10 compared to
that of the system described in chapter Five. Two Solar Century PV modules (type
C21) were connected in series so that a peak power of 100 W (with 100% insolation)
could be delivered to the input terminals of a Split-Pi Buck-Boost converter. A
seven-cell, proton-exchange membrane (PEM) electrolyser (h-tec type E107-230)
with a power capacity of 50 W was connected to the output terminals of the Split-Pi
converter. This latter unit was a commercial DC/DC converter from Green Energy
Technologies and was used as a power interface allowing optimal energy transfer
from the PV modules to the PEM electrolyser.
6.2 System Components
The photovoltaic (PV) powered proton-exchange membrane (PEM) electrolyser
hydrogen production system is shown schematically in Figure 6.1.
Figure 6.1: Hydrogen production system.
The PV-Hydrogen production system consists of two PV modules, connected in
series; a computer-controlled DC/DC converter; a PEM electrolyser; and a device to
113
measure the volume of hydrogen produced. Each item is shown in Figure 6.2 and
described in more detail below.
Figure 6.2: Power matching photovoltaic-electrolyser system using a Split-Pi
converter.
6.2.1 PV Module
The two solar modules were attached to an aluminium A-frame, as illustrated in
Figure 6.3.
Castors allowed movement in the horizontal plane, and adjustable support bars
allowed the tilt angle of the modules to be changed. Only manual adjustments were
made in the present experiments, although, for future work, the support structure
could be engineered to provide automated tilt and azimuthal angle variations.
.
114
Figure 6.3: PV modules facing the sun.
The PV module specifications used in this experimental work are shown in Table
6.1. These readings were taken under standard conditions, i.e., an insolation of 1000
W/m2 and a cell temperature of 25
0C (100% insolation as referred to above).
Model type C21-M52D from Solarcentury
Silicon crystalline
The sizes 1240mm 240 mm
Peak power 50 W
Peak power voltage Vmax 10 V
Peak power current Imax 4:45A
Open circuit voltage Voc 11 V
Short circuit current Isc 5A
Table 6.1: Specifications of the PV module.
The measured results of current versus voltage and power versus voltage for each of
the PV modules under different values of solar insolation are given in Figure 6.4.
115
Figure 6.4: (a) I-V and (b) P-V curves of a single C21 module under different
insolation values.
6.3 Measuring Solar Irradiance
Solar insolation was measured using a meter from Daystar, Inc. (type Daystar). This
meter provided an insolation scale from 50-1200 W/m2. The sensor responded to a
spectral bandwidth of approximately 0.3-1.1 m (i.e., a major part of the solar
spectrum), and it had a digital LCD display.
(b)
(a)
116
6.4 Split-Pi DC / DC Converter
The UDIO.60.25.L Split-Pi converter from Green Energy Technologies (Fig. 6.5)is a
type of switch-mode power supply. It consists of two MOSFET bridges and two Pi
filters, as shown in the circuit diagram below in Figure 6.6. The circuit configuration
is similar to the circuit topology for a step-up boost converter followed by step-down
buck converter. The magnitude of the output voltage can be greater than or less than
the magnitude of the input voltage, as determined by choosing the on and off states
of the selecting switches, as shown below.
The features of the Split-Pi converter are as follows:
1. 0-60 V input.
2. 0-60 V output.
3. 25 A maximum bi-directional current flow.
4. 1.5 kW maximum common GND non-isolated.
5. Seamless UP/DOWN conversion.
6. Serial digital control.
7. 256-code voltage control.
8. > 98% power efficiency.
9. Exceptionally low voltage ripple (< 10 mV).
Figure 6.5: Split – Pi DC/DC converter.
The basic circuit (Figure 6.6) consists of four semi-conductor switches and input and
output LC filters. The capacitors must be large to control the voltage ripple at the
117
converter terminals. Capacitor C3 is not connected to any external terminal and
provides energy storage during switching.
Figure 6.6: Split – Pi converter circuit.
Only one bridge switches at any time to provide voltage conversion. A straight-
through, 1:1 voltage output was achieved with the top switch of each bridge switched
on with the bottom switches off. The output voltage was adjustable on the duty cycle
based on the ratio of the switching MOSFET bridge, as shown in Table 6.2 below.
Closed-loop control was achieved through an RS 232 input on the Buck-Boost
converter. Using visual basic software, a computer was used to specify the input-to-
output voltage ratio, and, by the same serial link, data regarding input and output
voltages and currents were fed back to the computer.
Code S1 S2 S3 S4 State Comments
0 OFF OFF OFF OFF Open All open circuits no current
can flow
1
to
127
OFF ON PWM* PWM Buck
L to R
PWM is linear with code
and PWM =code/128 and
PWM* is the inverse and =
(128- code)/128
128 OFF ON OFF ON Short LH and RH voltages are
equal
129
to
255
PWM* PWM OFF ON Boost
L to R
PWM ratios are set to
laniaries the inverse
relationship
Table 6.2: Split – Pi converter switching duty cycle.
118
By careful selection of components (proprietary Intellectual Property Rights of the
company), the Split-Pi technology provides high efficiency (> 98%) direct current
(DC-DC) up and down (boost, buck) voltage conversion with the ability to
seamlessly sink and source electrical current with identical forward and reverse
transfer characteristics.
6.4.1 Control of Split-Pi Converter Software
The Split-Pi DC/DC converter from Green Energy Technologies has many
advantages over other converters, i.e., users can:
-create a software programme to control the converter switches
-alter duty cycle pulse-width modulation (PWM) settings by sending a decimal code
from 0 to 255 for the voltage ratio control between the converter terminals and
record the values of input and output voltage and current.
A software computer programme was developed (Appendix B) to control the Split-Pi
converter through a serial, digital-control port. This facility was very useful for
monitoring and controlling the real-time measurements and for utilizing optimal
energy transfer from the PV modules to the PEM electrolyser at all times.
The Split-Pi converter uses one side of the device as a reference, which is referred in
this work as the left-hand side (LHS), and the PEM electrolyser was connected to
this side. The PV power source was connected to the right-hand side (RHS).
The RHS voltage can be calculated using the following equation:
1.6........................................................................255
2
RVV
LHSRHS
where VRHS is the right-hand side voltage value; VLHS is left-hand side voltage; and
R is value of the ratio, which is controlled by the computer.
For a simple demonstration of the interfacing procedure, a preliminary Visual Basic
119
program was written. A parameter Rmax (in the range of 0 to 255) was specified, and,
then, the program incremented a counter, starting at zero or some other value, Rinitial,
until the counter value became greater than Rmax, at which point the program was
terminated. At each incremental point, the current and voltage at the input and output
terminals were measured so that input and output power could be calculated.
The flow chart for the visual basic programme is shown in Figure 6.7.
Initialize
Ratio initial set
Ratio max set
Start
Read Iin ,Vin, Iout
and Vout
For ratio value
Store Data
Increment Ratio
Is Ratio > Ratio
max
Store Data in file
End
yes
No
Figure 6.7: Flowchart of visual basic control.
With the screen output, Figure 6.8, displaying current, voltage, and power at both
input and output terminals, the procedure can be followed as the ratio, R, is
incremented increased from Rinitial to Rmax. The program was initiated by the START
120
button, and it ends when Rmax is reached. At this point there are three options:
Press START again to obtain more readings.
Press SAVE to save data to file.
Press EXIT to leave the routine
Figure 6.8: Visual basic software screen outlook.
6.4.2 Maximum Power Point Tracking Algorithm
Maximum power point tracking (MPPT) techniques are employed in solar hydrogen
production systems to make full utilization of the solar insolation.
The main advantage of the maximum power point tracker is to adjust the current and
voltage of the photovoltaic module for optimum electrolyser performance. This, in
turn, maximizes the hydrogen production rate from the electrolyser. There is an
additional influence of the fluctuations of the ambient temperature of the PV
modules, but this is generally smaller than the changes in solar insolation. In
principle, the temperature changes could be incorporated into the optimization
program by having a temperature sensor fixed to the solar modules, but this was not
included in the present investigation.
A maximum power point tracking strategy, based on the climbing-hill method, was
121
applied in the visual Basic programme code to control the Split-Pi DC/DC converter.
The hill-climbing method, Figure 6.9, is widely used in practical PV systems and
MPPT controllers due to its simplicity and easy implementation.
.
Figure 6.9: Hill-climbing MPPT method.
In the hill-climbing method, the input power to the converter, PK, is calculated from
the input voltage and current values at each PWM duty cycle (ratio). It is then
compared with a previous power value, PK-1. If the current power value is greater
than the previous power value, then the ratio is incremented, and readings are taken
again for a higher ratio.
Just after the MPP point, PK is less than PK-1. The “greater than” condition in the flow
chart is now false, and the program comes out of the loop and passes to the next
stage. At this point, the ratio is decremented to yield a point to the left of the MPP
point so that the hill climbing can start over again. The strategy adopted here is that
the power will fluctuate closely about an optimum value over and over again until
some other condition is set to terminate the program. The flow chart in Figure 6.10
shows that termination occurs when a pre-set “Final Count” is exceeded. This may,
typically, be 1000 for a measurement trial of about two hours.
In order to start the programme, for example at count 10, we must set an initial
power value for PK-1 so that a comparison with the reading P10 can be made. It can
122
simply be set to zero since any reading will be greater than this value.
Subtract 2 from
Ratio
Start
Is Count <
Final count
Initialize
Set ratio initial
Set power max
Set Final count
Is P in < power
max
Read Iin Vin
Iout Vout
Calculate Pin and Pout
End
Store Data in file
yes
no
yes
no
Figure 6.10: MPPT programme flow chart.
6.5 PEM Electrolyser
A h-tec PEM electrolyser of Figure 6.11 was used to produce hydrogen from the
power generated by the PV module. It consists of seven PEM cells connected in
series and is filled with distilled water. This electrolyser was chosen so that the input
power requirement would be a reasonable match to the power delivered by the PV
module.
123
Figure 6.11: The 50-W, h-tec PEM electrolyser.
Table 6.3 shows the PEM electrolyser specifications.
Model name E107 (230) (h-tech)
Electrode area 7 cells of 16cm2 each
Rated power 50W at 14V DC
Permissible voltage 10:5 - 14:0V DC
Permissible current 0 - 5:0 A DC
H2 production 230cm3/min (8.33 x 10
-8 kg/min)
Table 6.3: Specifications of the PEM electrolyser.
An I-V characteristic curve for the electrolyser was plotted to ensure that the
specifications provided by the manufacturer were approximately in agreement with
measured values, Figure 6.12.
124
0
0.5
1
1.5
2
2.5
3
3.5
4
4.5
0 1 2 3 4 5 6 7 8 9 1011121314151617181920
Voltage (V)
Cu
rren
t (A
)
Figure 6.12: Characteristic curve of the h-tec PEM electrolyser.
From the characteristic curve, it is possible to calculate the internal resistance of the
electrolyser. This is shown in Figure 6.13, which shows that the resistance is high at
points below cut-off.
0
50
100
150
200
250
300
0 20 40 60
Power (W)
R (
oh
m)
Figure 6.13: Resistance-Power PEM electrolyser curve.
125
6.6 Hydrogen Volume Measurement Device
This device is a simple water displacement arrangement similar to the one described
in reference [47].
A gas holder with greater capacity than [47] was required for the present system, and
this is shown in Figure 6.14.
When hydrogen gas enters through the filling tube, the inner cylindrical container is
raised, and the level is monitored with an ultrasonic level indicator (Pepperl+Fuchs,
model UB 300-18GM-U-V1). This sensor gives an analogue signal that is monitored
using a standard multi-meter (e.g., a Thurlby 1905a multi-meter) fitted with a serial
port.
Figure 6.14: Device for measuring the volume of hydrogen produced.
Since a standard RS232 serial interface is fitted to the Split-PI converter and a serial
port is also provided by the Thurlby multimetre, provision had to be made for two
serial cables to be applied to the computer. The present computer was equipped with
only one serial input port, so a four-way RS232 router [48] gave a time-sharing
126
method of interfacing both the Split-PI and the level monitor into the computer.
6.7 Results and Discussion
The recorded data points were presented graphically, Figures 6.15 and 6.16. The
maximum power output from the PV modules is clearly shown to occur at
approximately 15 V.
0
1
2
3
4
5
6
7
0 5 10 15 20 25
Voltage (V)
Cu
rren
t (A
)
PV I-V curve PEM I-V curve
Figure 6.15: I-V for PV and PEM electrolyser curves.
Figure 6.16 shows the P-V curve matching of photovoltaic module and electrolyser
at all possible code value starting from 0 to 255 during Buck- boost converter
operation.
127
0
10
20
30
40
50
60
70
80
90
0 50 100 150 200 250 300
Voltage ratio
Po
wer
(W)
PV output power curve PEM input power curve
Figure 6.16: P-V PV and PEM electrolysers power matching.
Figure 6.17 shows the power delivered by the PV modules to the electrolyser using
the Split-Pi converter controlled with the MPPT software. With the exception of a
small interval within a two-hour period, the sky was clear, and an irradiation of
850W/m2was occurring. The graph shows that the Split-Pi converter gives good
tracking of the maximum power point, and, even when a small cloud covered the sun
at a count of 170, maximum power tracking was restored within the next two or three
counts. If a smaller “ripple” in the optimum output is required, then the ratio should
be decremented by a smaller amount.
0
10
20
30
40
50
60
70
80
90
0 200 400 600 800 1000
Counter
PE
M e
lectr
oly
ser
inp
ut
po
wer
(W)
Figure 6.17: PEM electrolyser input power data during a clear, sunny day.
128
Tracking on a cloudier day is given in Figure 6.18, where the solar insolation was in
the range 550-800 W/m2. This indicates the performance of the proposed system
during more continuous and larger variations in solar irradiance.
0
10
20
30
40
50
60
70
80
0 200 400 600 800 1000
Counter
PE
M e
lec
tro
lys
er
inp
ut
po
we
r (W
)
Figure 6.18: PEM electrolyser input power under less favourable insolation
conditions.
The results shown below illustrate that the input power for the PEM electrolyser is
very near to the path of the maximum power points of the PV module characteristics
during the tracking routine.
0
10
20
30
40
50
60
70
80
0 2 4 6 8 10 12 14 16 18 20 22 24 26
Voltage (V)
Po
wer
(W)
120 W/m2 220 W/m2 370 W/m2
773 W/m2 PEM input power
Figure 6.19: Electrolyser input power and PV maximum power relation
129
Hydrogen production is given in Figure 6.20.
0
2
4
6
8
10
12
14
16
0 200 400 600 800 1000 1200
Count
Vo
lum
e o
f g
as
(litr
es
)
Insolation 800 Watts/square metre
Insolation 500 Watts/square metre
Figure 6.20: Hydrogen production in relation to changes in insolation.
The gas output of 220 litres per minute over the latter part of the curve (insolation
800 Watts/m2 ) compares well with the manufacturer‟s data sheet for the h-tec 230
electrolyser. Clearly, this insolation was maximised over the two-hour period by
manually tracking the sun and ensuring that the PV modules were operating at their
maximum power point by the hill-climbing routine in the software.
Summary
Finally, we can conclude that the system that was composed of two C21 PV
Modules, a Split-Pi Buck-Boost converter and an h-tec electrolyser provided a real-
time generation unit for converting solar energy into hydrogen. Optimum tracking
was achieved using computer control.
130
CHAPTER 7
DESIGN A PV-HYDROGEN SYSTEM
TO POWER A FAMILY HOUSE IN
THE SAHARA DESERT IN LIBYA
131
7 CHAPTER 7 DESIGN A PV-HYDROGEN
SYSTEM TO POWER A FAMILY HOUSE IN
THE SAHARA DESERT IN LIBYA
7.1 Background
Since electricity plays a substantial role in everyday life, sustainable, rural
electrification is needed that can at least be capable of providing necessary light and
energy. In this chapter, the design of a solar hydrogen power system for a family
house in the remote Sahara Desert is discussed. The design incorporates materials
and well-known equipment that are currently commercially available. This system
uses relevant technologies to harness the sun‟s radiation and covert it into electricity
using photovoltaic panels as the power source. The system uses water electrolysis
and fuel cells to use hydrogen as a storage medium to power the house 24 hours per
day.
7.2 Solar Energy Sources in Libya and the Hydrogen
Option
Solar energy is the most abundant renewable natural resource in Libya. The daily
average of solar radiation on a horizontal plane is 7.1 kWh/m2/day in the coastal
region and 8.1 kWh/m2/day in the southern region, with an average sun duration of
more than 3500 hours per year (Saleh Ibrahim, 1993).
In 1976, the first photovoltaic system, a cathodic protection station, was established
in Libya. This station had a peak power of 650 kW. Since then, photovoltaic systems
have been widely used for many applications, such as stand-alone systems to pump
water , Figure 7.1, and communication repeater stations in rural areas.
132
Figure 7.1: Photovoltaic module for a water pump in the Libyan Sahara [51].
Since solar energy is only available during the day, it is important to have energy
storage facilities. The most common storage facility is batteries, but an alternative is
to generate hydrogen gas that can be stored for longer periods of time. Hydrogen has
the advantage over batteries as energy storage medium because it is transportable,
can be stored indefinitely, and does not create environmental pollution when energy
in the hydrogen is converted to electricity or when it is burned to produce heat.
Most of southern Libya averages over 6 KWh/m2 per day of global radiation,
whereas northern Mediterranean countries receive less than 3 KWh/m2
[51]. This
makes hydrogen produced in Libya using solar energy via electrolysis an attractive
energy source for domestic use in remote areas as well as for export to Europe in
either liquid or gaseous form. Pipelines could be used to transport the hydrogen gas
in much the same way as natural gas is presently being transported to Europe.
Special refrigerated tanks would be necessary for transporting liquid hydrogen.
7.3 Solar Hydrogen System as an Energy Supply for
Libyan Remote Areas
There are many villages and remote areas located in Libya and other parts of North
Africa as well. These areas are far away from the electricity grid. Economically, the
small populations in these remote areas cannot be connected to the grid at
133
competitive costs because of long distances and related line losses.
In some Libyan remote areas, diesel engines are used to generate electricity, but
some remote areas cannot use diesel engine generators because of difficulties
associated with fuel transportation and maintenance.
For this reason, the renewable energy sources should be utilized to generate
electricity locally for a household and/or water pump applications.
Due to the intermittent nature of wind and the low wind speed in the Sahara Desert,
photovoltaic systems are the preferred option. They have proven to be more reliable
than wind power due to the high solar radiation in these remote desert regions.
In this study, the technological focus was on a solar PV-hydrogen power system.
This system is based on a PV array, a DC/DC converter, a proton exchange
membrane (PEM) electrolyser, and a PEM fuel cell; in combination with a low–to-
medium pressure hydrogen storage installation, this would service the power needs
of a remote homestead situated in the southwest Libyan desert.
For such locations where the transportation of fuel is problematic and costly, it is
better to have a stand-alone system using a photovoltaic electricity generator.
In this study, a solar-hydrogen system was used to supply the energy needs of a
remote household with a daily electrical demand. The system was located in a remote
area that has a high level of solar radiation but no access to a central grid that can
provide power.
A small desert town called Ghadamis was chosen for the study. It is located at
latitude of 30° N and a longitude of 10° E. The geographical location is at the
intersection of the borders of three countries, namely, Libya, Algeria, and Tunisia.
Ghadamis is very old town, and it has unique architectural features; recently, it has
become a tourist haven, attracting people from many parts of the world.
Table 7.1 shows the climate conditions of Ghadamis. The average global irradiation
is 2200 kWhm-2
year-1 for
3500 h/year. Hence, the climate condition of this town was
considered to be an ideal place for the application of solar hydrogen systems. To
optimise the sizes of the different components in the solar hydrogen power system,
the weather data and load demand were considered as input data. The sizes of the
different components depend on these input data.
134
Sunshine duration 3500h/year
Irradiance 2200 kWh/ m 2/year
Relative humidity 34
Rainfall 20mm/year
Wind speed 8.71 knots
Extreme maximum temperature 36.71°C
Mean maximum temperature 29.67°C
Extreme minimum temperature 8.12°C
Mean minimum temperature 14.10°C
Table 7.1: Climatic conditions of the project site at 30° N and 10
° E.
As stated above, the aim of the project was to supply electrical energy to a family
house in Ghadamis using three main components, i.e., a photovoltaic source, a
DC/DC converter, and an electrolyser-hydrogen storage-fuel cell system.
According to data from the Libyan Solar Research Centre (SRC), Ghadamis has an
excellent solar profile. Figure 7.2 shows that the daily energy varies from about 4 to
8.5 kWhr for each square metre of photovoltaic cell. Also, Ghadamis has a high
number of daylight hours each day; the minimum is 10 hours/day in January and
December, and the maximum is 14 hours/day in July.
Figure 7.2: Daily solar irradiance on horizontal plane through the year in Ghadamis.
135
Thus, as indicated in Figure 7.2, Ghadamis is an ideal place to utilize solar panels to
produce electric energy directly from the sun, and this energy could be stored in the
form of hydrogen fuel through water electrolysis.
7.3.1 Design of a Solar Hydrogen Power System for a family
House in a Remote Area Located in the Sahara Desert
In this thesis, the solar-hydrogen power system to electrify a house in a remote area
of the Libyan Desert has the following major components, i.e., PV array, DC/DC
converter, PEM electrolyser, hydrogen storage, and PEM fuel cell.
Typically, such a house would be occupied by a small family consisting of four
members, and the system would provide electrical power for all household
applications 24 hr/day.
There are two ways to connect the solar hydrogen system to the household load.
First, the electricity produced by the PV panels can be used to provide DC power
directly for the household applications. Second, the electricity could be used to
produce hydrogen (stored energy) for use in running the fuel cell to meet the
household demands for electricity day and night. In this project, we used the second
type of system due to its advantages over the first type of system, as indicated below:
1. It was a more energy-efficient system, so it required a smaller hydrogen
storage capacity.
2. It requires the electrolyser and the fuel cell to work fewer hours, thereby
extending their lifetimes.
The only disadvantage of the first approach is that it uses more electronic devices
rather than the second type.
136
Figure 7.3: Complete solar hydrogen power system.
The system, shown in Figure 7.3, demonstrates a means of continuously supplying
electrical energy day and night for a family house in the Sahara region. During the
daytime, it is estimated that half of the solar electricity will be used directly to
provide the energy needs for the house, and the other half will create hydrogen via
the PEM electrolyser for storage. At night, the stored hydrogen will power the fuel
cell, producing the needed supply of electricity.
All the electrical loads in this house were assumed to be DC loads operating with a
constant voltage of 24 V. In order for the solar panels to convert sunlight to DC
electrical power, a power conditioning and control is required, i.e., a DC/DC
converter, and its task is power matching. The PEM electrolyser uses the DC power
to produce hydrogen via water electrolysis. Stored hydrogen and oxygen from the air
provide the inputs to the fuel cell to generate DC electric power.
Provided there was sufficient hydrogen stored, a portion of it could be used as fuel
for transportation.
The system is required to supply electricity to operate the essential household
applications and a water pump, if necessary, as shown in Table 7.2.
137
7.3.2 Energy Requirement
The load was assumed to be lighting, cooling fans, refrigerator, television, and
computer. The load does not include air conditioning, but it could be added by
increasing the load‟s power consumption by a factor depending on BTU rating of the
air conditioner used. Table 7.2 describes the energy requirements for the household
applications. Appliance usage time and the loads were estimated for daytime usage
(D) and night-time usage (N).
Application
Day(D)
Night(N)
No of
Units
Power
Per unit
(W)
Total
current
(A)
Operating
hours per
day (H)
Energy
consumption
Wh/day
Living room light (N) 1 60 2.5 5 300
Dining room light (N) 1 60 2.5 5 300
Kitchen light (N) 1 60 2.5 3 180
Bedrooms light (N) 2 60 5 2 240
Bathroom light (N) 1 40 1.6 2 80
Dining room and living
room fans (D and N)
2 60
5 5 1200
Bedrooms Ceiling fans
(D and N)
2 60 5 5 600
Refrigerator (D and N) 1 100 4.16 12 1200
TV and sat.&rece.
(D&N)
1 60 2.5 5 300
Computer and
accessories (D and N)
1 100 4.16 5 500
Small applications (D
andN)
60 2.5 2 120
Water pump (D) 1 300 8.33 2 600
Total 860 (D)
900 (N)
36 (D)
38 (N)
5620
Table 7.2: Energy requirements for small family house in the Sahara Desert.
138
Based on Table 7.2, during daylight, the PV array must be capable of providing 860
W for household requirements plus the electrolyser load. Fuel cell output power
should not be less than 900 W to ensure that the electricity demand of the house can
be met at night. To ensure a modest surplus of power, it is suggested that 1 kW of
power will feed the home during the day and an additional 1 kW of power will be
used provide for the generation of hydrogen. Thus, a 2-kW solar array will be
necessary.
7.3.3 Fuel Cell Specification
A fuel cell is an electrochemical energy converter that converts chemical energy into
electrical current (DC). The present work is concerned with the combination of
hydrogen and oxygen to produce electricity. The only product of this reaction is
water.
In general, fuel cells have many advantages over conventional electrical generators,
including a wide range of applications, easy maintenance because there are no
moving parts, silent operation, high power density, and clean energy production.
Selecting an appropriate fuel cell for a given application must be based on
consideration of the I-V curve, rated power, hydrogen consumption rate, and size and
volume. Usually, the manufacturer provides the I-V curve in the data sheet.
From the chemical equations that describe the operation of a fuel cell, the hydrogen
consumption of the fuel cell can be derived.
The chemical reactions that occur in a fuel cell are:
At the anode:
eHH 442 2 ………………………………………………………...7.1
At the cathode:
OHHeO 22 244 …………………………………………………..7.2
In a fuel cell, when one mole of hydrogen is consumed, two electrons are freed from
139
the hydrogen. Thus, the charge involved in the reaction for the total amount of
hydrogen consumed will be:
Charge = (2 electrons) x (the charge of each electron) x (moles of 2H consumed)
The charge of the electron is equal to 96485, which is the Faraday‟s constant, F.
Amount of 2H consumed = xF
ech
2
arg moles
Dividing by time:
Amount of 2H consumed = xF
current
2 moles/sec
Multiplying by the molar mass of H2 which is 2.02 x 10-3
kg/mole
Amount of 2H consumed = xF
xcurrentx
2
1002.2 3
kg/sec
By substituting the value of F, the equation becomes:
Amount of 2H consumed = Ix 81005.1 kg/s
To get the amount of hydrogen in m3/s, we divide by the density of hydrogen, which
is 0.084 kg/m3. The amount of hydrogen consumed = I x Z x 1.05x10-8 kg/sec,
where I is the current withdrawn, and Z is the number of cells in the stack.
The commercial Nexa® 1200 fuel cell stack from Ballard, with an output of 1.2 kW,
was an ideal fuel cell for producing the electricity required by household application
during the night in this project. The technical data shown below from it‟s the
company‟s data sheet provide more details.
140
Table 7.3: Fuel cell Nexa 1200 technical data from datasheet.
Since the household appliances operate at 24 V, the output voltage of the Nexa 1200
was adjusted to this voltage using a built-in regulator.
Figure 7.4: I-V and I-P characteristics for Nexa 1200 from data sheet.
Figure 7.4 shows the output voltage, current, and power characteristics for the Nexa
1200 fuel cell. From the graph, we can determine that the 24 V, 52 A, and rated
power of 900 W are suitable for meeting the load requirements of the house at night.
The main fuel cell reaction is an exothermic reaction, so it produces heat that must be
removed to keep the fuel cell at constant temperature. Consequently, a cooling
141
system is required to remove the heat. In the Nexa 1200 fuel cell, cooling achieved
by using a fan to blow air across the fuel cell.
Delivering a power of 900 W over a 10-hour period with a hydrogen flow of 15 litres
per minute (from Table 7.3) will require a total of 10 m3 of hydrogen gas (at 1 bar).
Thus, a storage facility for this amount of gas is required, as described in the next
paragraph.
7.3.4 Hydrogen Storage
Hydrogen that is produced by electrolyser must be stored for later use by the fuel cell
to produce electricity. Storage methods and techniques are discussed in Chapter Two.
In the solar hydrogen power system, the simplest and most practical way to store the
hydrogen gas is by compressing it, so we used cylindrical storage tank for the
compressed hydrogen. The main parameters in designing the hydrogen gas storage
were volume, pressure, and temperature. The commercial electrolysers available on
the market can deliver hydrogen up to a pressure of 30 bar. The electrolyser chosen
pumped hydrogen into storage at a pressure of 3 bar.
As indicated above, a gas volume of 10 m3 was required, but this can be reduced to
3 m3 if the gas is stored at a pressure of 3 bar. Since an additional quantity of gas will
be required for other uses, e.g., cooking or to power vehicles, the total storage is
envisaged to be closer to 4 m3 at a gas pressure of 3 bar.
A cylindrical hydrogen storage tank was chosen. The storage volume was calculated
based on a diameter of 1.5 m and a height of 2.60 m. Such a tank could be situated
underground for safety reasons and to avoid excessive temperature fluctuations.
7.3.5 PEM Electrolyser
The electrolyser converts the electrical energy produced by the PV array into
hydrogen to store in a tank. The electrolyser should be large enough to fill the tank
with hydrogen. A PEM electrolyser rated at a pressure of 3 bar was selected from the
manufacturer (Hgenerators, type LM-20000) to produce 20,000 ml/min of hydrogen
at 1 bar. Thus, the gas output from 10 hours of sunlight will be approximately 12 m3
142
at 1 bar. This amount of hydrogen will almost fill the storage cylinder (at 3 bar
pressure).
The specifications for the LM-10000 are shown in Table 7.4.
Outflow pressure 3 Bar
Hydrogen purity 99.99%
DC power 32-36 V and 25 A
H2O consumed 1000 ml/h
H2 gas production 10,000 ml/min
Table 7.4: Specifications for the LM-10000 electrolyser.
7.3.6 DC/DC Converter
The DC/DC converter has a varying input voltage from the PV array and the
operating voltage of the electrolyser.
Two DC/DC converters were used in this system to match the output power of the
PV panels to the input power of the load, with one connected to the PV panels to the
household load during daylight, and the other connected to the PV with the PEM
electrolyser to extract the available maximum power from the PV and supply it to the
load. In this work, the Split-pi converter, described in Chapter Six, was an ideal
DC/DC converter for use in achieving this purpose due to its ability to match the
output characteristics of the PV array to the input characteristics of the PEM
electrolyser. This ensured that the maximum power would be transferred from the
PV array to the load continuously, thereby maximizing the hydrogen production rate.
The specifications of the Split-pi converter (Chapter Six) met our design
requirements, and it could be used as a maximum power point tracker and for power
matching between the PV array and the load.
Since the Split-PI converter was a relatively low-cost item compared to the other
items in this system, it may be useful to have two such converters, as shown in
Figure 7.3, for added control.
143
7.3.7 Size of the PV Array
PV arrays are built up with series-connected and/or parallel-connected combinations
of solar cells in order to produce 32 V and 35 A at the input terminal of DC/DC
converter. Therefore, for an array of Ns * Np (number of panels in series by the
number of panels in parallel), the PV current, voltage, and power can be given,
respectively, by the following equations:
IPV = Np IPV ………………………………………………………..…….7.3
VPV = Ns VPV ……………………………………………………………7.4
A total power of 2 kW is required, and a suitable solar module would be C21-M52D,
manufactured by Solar Century. These modules have a peak power of 50 W, so that
40 modules of 1174 mm x 318 mm (total area of 15 m2) will be required. A line of
10 modules in parallel with a series connection of four lines will give a suitable
output to both the electrolyser and the domestic dwelling.
7.3.8 Monthly Average Energy Supplied and Consumed
The C21-M52D module has 14.9% efficiency, and the temperature coefficient of
open circuit voltage is -0.034V/0C, making it suitable for use in the hot Sahara Desert
region.
Since we know the average monthly insolation in Ghadamis, Figure 7.2, we can
estimate the electric energy extracted from the PV panels and compare it with the
load demand.
The electricity supplied each month can be estimated as:
PV electricity per month = KWhAS 30 ………………………….7.5
Where, S = average annual irradiation, kWh/m2-day
A = array area, m2 and = module efficiency (14.9%)
144
Figure 7.5 shows the estimated energy extracted from PV panels and energy
consumed; the amount of extra energy reaches its maximum value during the
summer months, and it can be used to generate more hydrogen for other purposes
other than generating electricity or long-term storage.
0
100
200
300
400
500
600
700
Janu
ary
Febr
uary
Marsh
April
May
June
July
Aug
ust
Sep
tembe
r
Octob
er
Nov
embe
r
Dec
embe
r
Months
KW
/h p
er
day
KWh/month extracted from PV KWh/ month household load energy difference
Figure 7.5: Estimated amount of energy extracted from the PV system.
7.4 Control and Monitoring of the PV-Hydrogen System
In the solar hydrogen power system (SHPS), some important parameters to control
energy efficiency would be monitored. The parameters include input and output
currents and voltages of the DC/DC converters, hydrogen flow rate, and hydrogen
pressure in the storage tank. Since the Split–pi converter has the advantages of
measuring and controlling input and output electrical parameters using software, a
computer could be used to monitor the system. Also, additional sensors could be
used to record hydrogen pressure and gas flow rate. Suitable software would be
devised to provide both monitoring and control. In case of an emergency, the
software would include the capability of shutting the system down.
145
Summary
The feasibility of using solar energy and hydrogen production in remote areas in
Libya looks to be promising even with existing technology. Future advances in PV
cell manufacture and electrolyser design will make this form of energy provision
even more attractive and it should be implemented as part of the government‟s
energy plans in future years.
146
CHAPTER 8
CONCLUSIONS
147
8 CHAPTER 8 CONCLUSIONS
Hydrogen is a clean fuel that produces only water on combustion or when combined
with oxygen in fuel cells to produce electrical power. Like electricity, it is an energy
carrier, and it has potential for energy storage, transportation, and electricity
generation for countless outlets, such as lighting, heating, and powering motor
vehicles.
This thesis has been concerned with the transformation of solar energy into hydrogen
as a storable fuel.
The successful production of hydrogen via water electrolysis using photovoltaic
electricity as a power source and the subsequent use of stored hydrogen to produce
electricity using a fuel cell support the proposal that this technology be used as an
ideal stand-alone system, particularly for remote, desert areas in Libya.
One of the challenges in producing hydrogen by using solar energy (PV-hydrogen
system) is to reduce the overall costs. Therefore, it is important that the system
operate at maximum power. This thesis has demonstrated, by mathematical
simulation and experimental results, a method of achieving power matching between
the photovoltaic array and a proton exchange membrane electrolyser.
The use of hydrogen as an energy carrier was thoroughly and critically analysed
(Chapter Two). The environmental impacts of non-renewable sources of energy,
such as coal, oil, natural gas, and nuclear power, also were presented. Hydrogen
production methods and technologies and the aspects of hydrogen transportation and
use in electricity generation also were included. Hydrogen storage, distribution, and
transportation were discussed in some detail, and, finally, the safety aspects of
hydrogen production using relatively small-scale systems were addressed.
Aspects of solar irradiance and the basic principles of the photovoltaic process were
given in Chapter Three, in which the main components of a solar hydrogen
production system were described, along with the general principles of operation for
each component.
In Chapter Four, a PSCAD software computer model was developed that was
148
capable of exploring modeling for a photovoltaic-hydrogen production system with
power matching using a DC/DC Buck converter. The evaluation took into account
the different factors that affect the I-V characteristics of a PV array. The simulation
proved that the operating voltage of the electrolyser and the PV voltage at maximum
power were the key elements in power matching.
In Chapter Five, the results of a number of laboratory experiments with a small
photovoltaic-hydrogen production test facility were described. A DC/DC Buck
converter was designed and implemented on a PCB to match the power between the
photovoltaic array and a proton exchange membrane (PEM) electrolyser.
In Chapter Six, field trials of a PV-PEM system were conducted. The power capacity
of this system was approximately 10 times that of the laboratory unit shown in
Chapter Five. In these trials, a commercial, computer-controlled DC/DC converter
(Split-pi unit from Green Energy Technologies, Ltd.) was used. Visual Basic
software code was developed to control the converter and to track the maximum
power point (MPPT) by adjusting the input and output voltage ratio. This allowed us
to maximise the efficiency of the system. The results showed that the power output
closely follows the maximum output power of the photovoltaic array.
A photovoltaic-electrolyser system to produce hydrogen that fuels a fuel cell capable
of generating electricity was described in Chapter Seven. Even with components that
are currently available, the power would be adequate for the demands of a small
family‟s house. The location in Libya that was chosen for the demonstration of the
technology was a desert area with scattered populations and remote communities.
8.1 Contributions Made During the Project
(1) Optimisation of a photovoltaic-PEM electrolyser hydrogen production
system using a DC/DC buck converter. There are non-linearities in both the PV
array and the PEM electrolyser in any hydrogen production system. These features
were investigated with a PSCAD model and by conducting experimental work in
order to determine how to achieve optimal power matching. The key point was to
adjust the duty cycle of the DC/DC converter at a value equal to the maximum ratio
of PV maximum power point voltage and electrolyser operating voltage. Then, we
149
can be assured that both the PV array and the PEM electrolyser operate at their
maximum power when the duty cycle of the converter is set at this value.
(2) Design of a stand-alone solar hydrogen power supply system for a family
house in a remote area in Libya. The system was designed with currently available
components, i.e., solar modules, PEM electrolysers, fuel cells, and the associated
electronic control devices. It is understood that many developments are being made
in “hydrogen” technology, and, therefore, revisions to this initial design will likely be
necessary in the future.
8.2 Suggestions for Future Work
The results of solar hydrogen research conducted, as reported in this thesis, have
identified some points that must be addressed, and the following recommendations
are made for future investigations:
(1) Implementing design of the solar hydrogen power system that is described in
chapter seven. The components of the system used in this research were selected
from available commercial products. The technical and economical issues for a
stand-alone solar hydrogen power system for household use should be thoroughly
investigated in a house of appropriate size, so, it is proposed that a prototype house
be constructed and utilized for future research so that better control of all essential
variables can be adequately controlled.
(2) Large central solar hydrogen power system to electrify a small, remote
community in the Sahara desert and schemes for exporting PV energy. Since
the Sahara Desert has a source of clean and inexhaustible solar energy, it is suggested
that a large-scale solar hydrogen power system be designed, built, and evaluated in
this area. The large-scale system would contain all the components used in the
domestic house, but the power ratings would be increased significantly. When
proven, such a system could provide electricity from renewable sources for many
remote villages and settlements in the Sahara Desert. This type of investigation
150
would require a high capital investment, and it would require government support. At
an even higher level, a feasibility study should be conducted to assess the possibility
of exporting electricity (stored in the form of hydrogen) from North Africa to
Europe.
(3) Solar hydrogen fuel station in North Africa. To reduce the consumption of
fossil fuels and contribute to solving global energy-environmental problems, it is
suggested that a study be conducted to investigate the prospects of a network of solar
hydrogen fuel stations in the North African region. The stations would utilize the
available solar radiation in the region to produce hydrogen to power fuel cell vehicles
(FCV) and to power other small devices.
151
CHAPTER 9
REFERENCES
152
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158
CHAPTER 10
APPENDICES
159
10 CHAPTER 10 APPENDICES
10.1 Appendix A
(1) Crystalline solar cell parameters FORTRAN code
q = 1.602e-19
ak = 1.38e-23
an = 1.792
Ta = 293.0
Tref = 293.0
aIsco = 2.0
Noct = 49.0
aJo = 1.6e-3
aIdo = 71.1e-9
T = Ta+$S*(aNoct-20)/800
Eg = 1.16-(7.02e-4*(T**2)/(T+1108))
aIo = aIdo*(T/Tref)**3*(exp(q*Eg/an*k)*(1/Tref-1/T))
aIph = aIsco*$S/1000.0+aJo*(T-Tref)
aId = aIo*(exp((q*$V)/(n*ak*T))-1)
$aI = aIph-aId
(2) PEM electrolyser FORTRAN code
Vo = 1.1
Ac = 0.05
Aa = 0.05
R = 0.013
$V = Vo+Ac+Aa+$I*R
$H = (8.314*$I*($T)*600)/((96485.0*101325.0)*2)
160
10.2 Appendix B
Visual Basic Program Code for Split-PI Converter Control
Private Sub EXIT_Click()
End
End Sub
Private Sub Command1_Click()
Text4.Text = Hex$(CInt(Text5.Text))
End Sub
Private Sub ReadData_Click()
Dim Res As String
Dim Ratio As Integer
Dim R As String
Dim V_in As Double
Dim I_in As Double
Dim V_out As Double
Dim I_out As Double
Dim Result As String
Dim Count As Integer
Dim Count1 As Integer
Dim Count2 As Integer
Dim Mass(100)
'Print Hex$(255)
With MSComm1
.Handshaking = comXOnXoff
.RThreshold = 1
.RTSEnable = False
.Settings = "19200,N,8,1"
.CommPort = 1
.SThreshold = 1
.InputLen = 50
.InBufferSize = 4096
.PortOpen = True
End With
'direct serial with parallel switch
161
'send string message
'counting loop for delay
Call vbOut(888, 0)
For Count = 1 To 2
Ratio = Text3.Text
R = Hex$(Ratio)
If Len(R) < 2 Then
R = "0" + R
End If
MSComm1.Output = "NFEV" + R
'MSComm1.Output = "NFEV" + CStr(Hex(Text3.Text))
'Print Hex$(CInt(Text3.Text))
'Delay by counting
For Count1 = 1 To 900
For Count2 = 1 To 800
Next Count2
Next Count1
' End delay
Text1.Visible = True
Text1.Refresh
Text1.Text = CStr(Count)
Result = MSComm1.Input
Text2.Visible = True
Text2.Refresh
Text2.Text = Result
Next Count''
MSComm1.PortOpen = False
End Sub
Option Explicit'
Dim Ratio As Integer
Dim R As String
Dim V_in(1000) As Double
Dim I_in(1000) As Double
Dim P_in(1000) As Double
Dim V_out(1000) As Double
162
Dim I_out(1000) As Double
Dim P_out(1000) As Double
Dim Result As String
'Dim Count As Integer
Dim Count1 As Integer
Dim Count2 As Integer
Dim Mass(100)
Private Sub Command1_Click()
Text4.Text = Hex(Text5.Text)
End Sub
Private Sub Command2_Click()
Text8.Text = Val("&H" + CStr(Text7.Text))
End Sub
'BUCK_boost March 30th 2010
Private Sub EXIT_Click()
End
End Sub
Private Sub ReadData_Click()
'Print Hex$(255)
Call vbOut(888, 0)
Dim Count As Integer
With MSComm1
.Handshaking = comXOnXoff
.RThreshold = 1
.RTSEnable = False
.Settings = "19200,N,8,1"
.CommPort = 1
.SThreshold = 1
.InputLen = 50
.InBufferSize = 4096
.PortOpen = True
End With
'send string message
'counting loop for delay
'START LOOP FOR CHANGING RATIO
'***************************************
Ratio = 1
163
Do
Ratio = Ratio + 1
Text3.Refresh
Text3.Text = Ratio
R = Hex$(Ratio)
If Len(R) < 2 Then
R = "0" + R
End If
MSComm1.Output = "NFEV" + R
'Print Hex$(CInt(Text3.Text))
'Delay by counting
For Count1 = 1 To 1000
For Count2 = 1 To 1000
Next Count2
Next Count1
' End delay'
For Count = 1 To 2'
'*****************************LEFT VOLTAGE
MSComm1.Output = "NFEI00"
'Print Hex$(CInt(Text3.Text))
'Delay by counting
For Count1 = 1 To 1000
For Count2 = 1 To 1000
Next Count2
Next Count1
' End delay
Text1.Visible = True
Text1.Refresh
Text1.Text = CStr(Count)
Result = MSComm1.Input'
If Len(Result) > 10 Then
Result = Right$(Result, 10)
End If
Text2.Visible = True
Text2.Refresh
Text10.Refresh
'Text2.Text = Mid$(Result, 7, 10)
164
Text10.Text = Int(100 * Val("&H" + Mid$(Result, 8, 10)) * 0.015258)/100
V_in(Ratio) = Val("&H" + Mid$(Result, 8, 10)) * 0.015258
'*********************************RIGHT VOLTAGE
MSComm1.Output = "NFEI01"
'Print Hex$(CInt(Text3.Text))
'Delay by counting
For Count1 = 1 To 9000
For Count2 = 1 To 800
Next Count2
Next Count1
' End delay
Text1.Visible = True
Text1.Refresh
Text1.Text = CStr(Count)
Result = MSComm1.Input'
Text2.Visible = True
Text2.Refresh
Text2.Text = Int(100 * Val("&H" + Mid$(Result, 8, 10)) * 0.015258)/100
V_out(Ratio) = Val("&H" + Mid$(Result, 8, 10)) * 0.015258
'****************************LEFT CURRENT
MSComm1.Output = "NFEI04"
'Print Hex$(CInt(Text3.Text))
'Delay by counting
For Count1 = 1 To 1000
For Count2 = 1 To 1000
Next Count2
Next Count1
' End delay
Text1.Visible = True
Text1.Refresh
Text1.Text = CStr(Count)
Result = MSComm1.Input'
'Print Len(Result)
Text11.Visible = True
Text11.Refresh
Text11.Text = -Int(100 * (Val("&H" + Mid$(Result, 7, 10)) - 0.0625 * 32800) *
165
0.021286)/100
I_in(Ratio) = -(Val("&H" + Mid$(Result, 7, 10)) - 0.0625 * 32800) * 0.021286
P_in(Ratio) = Int(100 * V_in(Ratio) * I_in(Ratio))/100
Text12.Visible = True
Text12.Refresh
Text12.Text = CStr(P_in(Ratio))
'*******************************RIGHT CURRENT
MSComm1.Output = "NFEI05"
'Delay by counting
For Count1 = 1 To 1000
For Count2 = 1 To 1000
Next Count2
Next Count1
' End delay
Text1.Visible = True
Text1.Refresh
Text1.Text = CStr(Count)
Result = MSComm1.Input
'
'Print Len(Result)
Text6.Visible = True
Text6.Refresh
Text6.Text = Int(100 * (Val("&H" + Mid$(Result, 7, 10)) - 0.0625 * 32800) *
0.021286)/100
I_out(Ratio) = (Val("&H" + Mid$(Result, 7, 10)) - 0.0625 * 32800) * 0.021286
P_out(Ratio) = Int(100 * V_out(Ratio) * I_out(Ratio))/100
Text9.Visible = True
Text9.Refresh
Text9.Text = P_out(Ratio)
Next Count
For Count1 = 1 To 1000
For Count2 = 1 To 1000
Next Count2
Next Count1
Loop While Ratio < 20
Print " "
Print " "
166
Print V_in(5)‟
MSComm1.Output = "NFEV00"
MSComm1.PortOpen = False
End Sub
Private Sub Command3_Click()
'Saving data
Open "A:\Loop1.csv" For Output As #1
Dim i As Integer
For i = 0 To 150
Print #1, Str$(i) + "," + Str$(V_in(i)) + "," + Str$(I_in(i)) + "," + Str$(P_in(i)) + "," +
Str$(V_out(i)) + "," + Str$(I_out(i)) + "," + Str$(P_out(i))
Next i
Close #1
End Sub
