Energy Chapter06 2012

Environmental Physics

Chapter 6:Solar Energy: Characteristics and Heating

Copyright © 2012 by DBS

Introduction

• Renewable energy provides around 8 % of the world’s energy

• Wind energy is the fastest growing energy resource, followed by photovoltaics

• Studies suggest renewables could rise to 30-40 % share by 2050

Solar derived

RES

radiant

wind

waves

hydro

biomass

geothermal

tidal

Fundamental Sources of Energy

FUSION(SOLAR)

FISSION GRAVITATIONAL(PE/KE earth-

moon-sun)

Fossil fuels

Wind

Waves

Biomass

Hydro

Radiant

Nuclear energy

(man-made)

Geothermal

(natural)

Tides

Introduction

Fig. 6-1, p. 162

3 % of total energy use

Wood and agricultural wastes

Figure 6.1: U.S. renewable energy consumption (by source), 2003.

Introduction

• Renewable energy resources (RES) do not produce CO2

• Biomass does produce CO2 when burnt but is carbon neutral

• Each RES still has an environmental impact (but it is minimal compared to FF)

Introduction

• The potential of RES:

– Earth receives thousands of times more energy from the sun daily than is used in all other resources

– N and S Dakota, and Texas have enough wind energy potential to meet all US electricity needs

– A 140 x 140 mile parcel of land in Arizona covered with solar cells could meet the entire electricity needs of the US

• The problem with RES:

– Seasonal and time dependent

– Storage problems

– Price

Characteristics of Incident Solar Radiation

The energy from the sun reaching the earth per day:

Insolation = incident solar radiation

N. Europe 600 Btu/ft2/d 6800 kJ/m2/d79 W/m2

Equator2000 Btu/ft2/d 23000 kJ/m2/d 266 W/m2

Question

Show that 600 Btu/ft2/d = 79 W/m2

A conversion factor is 1 W = 3.41 Btu/h

600 Btu x 1 W = 79 W/m2

ft x 12 in x 2.54 cm x 1m 2 x 24 hr x d 3.41 Btu ft 1 in 100 cm d h


• With current technology, the sunlight falling on a typical single house can provide from 1/3 to ½ of the heating needs anywhere in the US, even with cloud present

Solar heated house near Chicago, IL.


• Energy released from the fusion of hydrogen nuclei to produce helium nuclei

Core 40 x 106 ° C

Surface ~ 6000 °C


• Intensity of EM radiation from the sun received at the top of the earth’s atmosphere

9 % UV, 40% visible, 50 % IR

• Only ½ of this reaches surface

• Absorbed by atmospheric gases

Figure 6.2: Spectrum of solar radiation reaching the earth at the top of the atmosphere and at ground level.

Fig. 6-3, p. 165

Figure 6.3: Energy balance for the earth. The earth receives about 50% of the incident solar radiation: 21% is from direct radiation and 29% is scattered through the clouds. The energy leaving the earth’s surface comes from evaporation and conduction to the atmosphere (33%), and infrared radiation (noted here as terrestrial radiation). Most of the infrared radiation (113%) is absorbed by the atmosphere and reradiated back to the surface (the “greenhouse effect”). In order to have temperature equilibrium at the earth’s surface, the energy input must equal the energy output. For this figure, 50% (incident radiation) = 3% (reflected) + 33% (evaporation) + 14% (net terrestrial radiation: 113% + 6% − 105%).

Reflected3 %

Scattered29% 105%Conduction/

convection33%

113%

6%

Direct21%

Energy in = Energy outAbsorbed by clouds

+ atmosphere19 %

Incoming solar energy

100%Radiated from

clouds + atmosphere

60%Radiated

from earth6%

Reflected3 %

Net terrestrial radiation

8%

Characteristics of Incident Solar RadiationAlbedo

• What Happens to Sunlight?

Fig. 2.13

30% Albedo 19% Absorbed51% ??


• Relatively constant temperature of the earth is a result of the energy balance between incoming solar radiation and the energy radiated from the earth

• Most of the IR radiation emitted from the earth is absorbed by CO2 and H2O (and other gases) in the atmosphere and then reradiated back to earth or into outer space

• The reradiation back to earth is called the atmospheric greenhouse effect

• Earth temperature is maintained ~ 40 °C higher than it would be with no atmosphere (-15 °C)


Insolation at the top of the earth’s atmosphere solar constant = 1354 W/m2 = 429 Btu/ft2/h

1kWh/m2 / day = 317.1 Btu/ft2/day


• Insolation at earth’s surface varies between 0 and 1050 W/m2

• Depends on latitude, season, time of day, cloudiness

Figure 6.4: Motion of the earth around the sun, illustrating the seasons and the tilt of the earth’s axis. (controls latitude and season)


- Incoming solar radiation spread out

- More atmospheric scattering

- More direct incoming solar radiation

- Less atmospheric scattering

Fig. 6-5, p. 167

Figure 6.5: Insolation values for a clear day on a horizontal surface located at 40°N latitude, as a function of the month and the hour of the day.

Insolation is lowest in winter when the need for heat is highest


• Sun’s elevation, or angle above the horizon is called its altitude

• Altitude is a function of latitude

• Further north you are the lower in the sky the sun will be

Figure 6.6: Yearly and hourly changes in the sun’s position in the sky for 40°N. Also shown are the solar altitude θ (angle above the horizon) and the solar azimuth φ (angle from true south).

As fall moves into winter the sunrise and sunset points of the sun’s motion across the sky move gradually southward


• Insolation reaching the surface is composed of direct, diffuse and reflected components

• Insolation is usually measured on a horizontal surface

Figure 6.7: Components of solar radiation.

Fig. 6-8, p. 169

Figure 6.8: Daily clear-day insolation as a function of month and collector orientation.

Insolation on a vertical surface in winter is greater than on a horizontal surface

Fig. 6-9, p. 169

Figure 6.9: Mean daily solar radiation (on an annual basis) for radiation incident on a horizontal surface, in units of Btu/ft2/d.

Table 6-3, p. 170

To calculate space heating requirements need data on average insolation and outdoor temperatures (Climate Atlas)

End

• Review

History of Solar Heating

Archimedes ‘death-ray’ c. 212 BC

Anasazi Indianas c. 12000 BC

National Solar Test Facility, NM - 2200 °C

Can melt quarter-inch-thick steel plate in 2 minutes.


• 19th century - Solar steam boilers produce steam to run engines

• 1878 - Mouchot (French) ran a printing press using solar driven steam power

Figure 6.11: Solar steam engine, Paris, 1878. Water was heated by the sun at the focus of the concentrating dish (A). The steam produced was used to run a steam engine (B) whose mechanical output ran a printing press. The water was supplied from tank (C).

Early 20th Century Egyptian Solar Power Plant

• 1912 Shuman (American) put into operation the first large scale solar power plant in Egypt

• Provided irrigation water from the Nile

• Trough-like parabolic collector which focused the sun’s rays onto a black metal pipe

• Find peak output if the total area of the collector is1207 m2

e.g. average solar insolation for June = 1200 W/m2, calculate the efficiency of the plant

1207 m2 x 1200 W/m2 = 1448 kW

Assuming:

(i) all solar energy converted to thermal energy of the steam, heating it to 100 ° C

(ii) air temp. = 20 °C

Efficiency = (TH-TC)/TH = 80/373 = 0.21 = 21%

Max. useful work output = 1448 kW x 0.21 = 304 kW


• 1872 – Wilson (Sweden) built a 4700 m2 solar still for the desalination of sea water in Chile

• Produced more than 23,000 liters per day

Figure 6.12: Solar desalination project using a cup and plastic wrap.


1860s - Mouchot’s solar pot was able to bring 3 liters of water to a boil in 1.5 hours.

1870s - Adam’s solar cooking apparatus, India, 1878. Sunlight is reflected to the blackened metal container, containing the food, as shown in the insert. The metal container is enclosed in a glass jar.

1767 - DeSaussure (Swiss) obtained temperatures high enough for cooking in a glass covered insulated ‘hot box’

Solar Cooking


1950s - Telkes’ (American) oven. The design features a fixed cooking pot and a moveable reflector.

Heating of the pot via radiation and convection

Solar Cooking

Question

Suppose the solar radiation is 850 W/m2 and you can collect 20 % of the energy that falls on the reflecting surface of a solar hot dog cooker. If you need 240 W for the cooker, what is the minimum collector area required?

Power required = 240 W

850W/m2 x 20/100 x Area = 240W

A = 1.41 m2

Overview of Solar Heating Today

• Used primarily for swimming pools and domestic hot water (DHW), also space heating

• Active solar system – fluid heated by the sun is circulated by a pump or fan

• Passive solar system – used no external power, fluid circulates naturally

Figure 6.17: General features of a solar heating system (active or passive).

Solar Domestic Hot Water

• 5% of collectors sold today are for DHW, 95% for pools

• Three types:

– Active flat-plate collectors (FPCs)

– Batch water heaters

– Passive (thermosiphoning systems


Flat-plate collector to preheat water for domestic hot water uses. The house also uses passive solar heating.

Figure 6.18: Cross-section of a flat plate collector (FPC) showing heat losses and gains.

Temperatures of around 160-180 °F.

Fig. 6-19b, p. 179

Figure 6.19: Solar collector absorber plates.


• Solar DHW system

• FPC on roof

• Backup system

Figure 6.20: Solar domestic hot water system with heat exchanger.

Question

What size flat plate collector (FPC) is needed to supply a family’s domestic water needs in March in Denver, Colorado? Assume 80 gallons per day (1 gal = 8.3 lb), ΔT = 70 °F for the water, and that the collector-heat exchange system has an average efficiency of 40 %. The collector tilt angle is equal to the latitude (see Appendix D).

Heat needed, Q:

Q = mc ΔT = 80 gal x 8.3 lb/gal x 1 Btu/lb.°F x 70°F = 46,480 Btu/d

Heat available from FPC = insolation x area x efficiency

46,480 Btu/d = 2060 Btu/d.ft2 x 0.40 x Area

Area = 56 ft2


• Batch water heaters

– Black tank inside an insulated box with a glass cover

– Output usually flows into conventional water heater for further heating

• Thermosiphon

– Water flows form the collector to the tank under natural circulation

– Less dense hot water rises

Batch water heater Thermosiphoning

End

• Review

Passive Solar Space Heating Systems

• Passive solar space heating – house acts as solar collector and storage facility

Figure 6.23: The Brookhaven house: an energy conservation house at the Brookhaven National Laboratory in New York State uses a greenhouse as a major passive solar feature. Fuel consumption is about one-fourth the normal usage of a house of similar size in the same climate.


• Passive solar space heating – heat flows by natural means, no mechanical devices such as pumps or fans

• Sunlight collected through south-facing windows and the energy is stored in the thermal mass of the building (concrete, water, stone etc.)

• More solar energy transmitted through glass than is lost through the same windows over 24 hrs

• Sunlight is kept out during summer using roof overhangs (sun is higher in the sky)


• Essential elements of a passive solar system:

– Excellent insulation

– Solar collection (south-facing windows)

– Thermal storage

• 3 Types of passive systems

– Direct gain

– Indirect gain

– Attached solar greenhouse


• Direct gain

– Large south-facing windows admit solar radiation

– Thermal mass exposed to direct radiation absorbs radiation

– Thermal mass radiates heat back into the room at night

Figure 6.24: Passive solar system—direct gain. South-facing windows act as solar collectors. Moveable insulation is used to cover the windows at night to reduce heat loss. A massive concrete floor acts as a storage device and prevents overheating. The overhang blocks the summer sun.


• Temperature performance

Figure 6.25: The performance of a passive solar commercial building (the Conservation Center, Concord, New Hampshire) during three sunny but cold winter days. Heating was with direct gain (large double-glazed, south-facing windows, with no night insulation). Thermal storage consists of a dark slate floor over a 4-inch concrete slab and phase change materials in the walls. Even though the outside temperature ranged from 20°F down to –15°F, no auxiliary heat was used.


• Adobe houses of the SW US utilize solar gain and thermal mass principles

• Adobe brick – sand, clay, water, sticks/straw/dung

Arg-é Bam, Iran c. 500 BC


• Indirect gain

– Collects and stores solar energy in one part of the house and uses natural heat transfer to distribute this heat to the rest of the house

e.g. Trombe wall

Figure 6.26: Indirect gain. The concrete wall acts as a solar collector and a heat storage medium. At night the vents are closed to prevent heat loss.


• Attached greenhouse

– Greenhouse on south-side of house

– Acts as expanded thermal storage wall

– Windows must be insulated at night

– Concrete floors and water filled drumsused for energy storage


• Thermosiphoning air panel (TAP) collector

– Powered by pressure differences

– Air flows behind corrugated metal absorber to reduce convective heat loss

– Easily retrofitted addition

Figure 6.28: Thermosiphoning air panel collector.

Table 6-4, p. 189

Active Solar Space Heating Systems

• Active system

– Flat plate or evacuated tube collectors (thermal storage) and mechanical means of delivering heat into the living space

– Working fluid may be water or air

– FPC usually roof-mounted, storage tank in the basement

– Auxillary heaters (electric) may be added for days with poor insolation

– May be vertical mounted (~60% less insolation than roof)

Figure 6.29: Basic space heating and domestic hot water system.


• Active system

– May be vertical mounted (~60% less insolation than roof)

Figure 6.30: Active solar space heating and domestic hot water system integrated into the façade of this house in Austria, a so-called “solar combisystem.”


• Active system

– e.g. air system with rock storage

Figure 6.31: Hot-air flat plate system. Air transfers heat from the collector either directly into the rooms or into the rock storage bin (solid line). When heat is being removed from storage (dashed line), the air flow is in the opposite direction so that as much heat as possible can be picked up from storage. Water for domestic use is preheated in the storage bin.


Air vs. water

• Pros

– Air system costs less to install

– Air doesn’t freeze

• Cons

– Not as efficient

– Larger storage facility

– Costs more over time (running costs)

– Difficult to retrofit (size of ducts)


• Sun is lower in the sky during winter months

• Collector must be positioned at a large angle (local latitude + 10)

Figure 6.32: Calculating collector tilt angle from the horizontal for space heating.


• Optimum angle for Pittsburgh?


• To calculate area of flat panel collector required:

Require quantity of heat needed (Q), average insolation (I), efficiency of the collector (ε)

Q = I x ε x A

• E.g. How many square feet of FPC are required to provide all thermal energy needed to heat a home for one day when the heat load is 20,000 Btu/hr? Mean daily insolation is 1800 Btu/ft2.d and efficiency is 50%

Q = I x ε x A

20,000 Btu/hr x 24 hr/d = 480,000 Btu/d = 1800 Btu/ft2.d x 0.50 x A

A = 533 ft2

h~ one half of the roof!

At $45 /ft2 = $24k

Thermal Energy Storage

• Solar energy heating system must be able to store energy for nighttime use and cloudy days

• Require materials with large specific heat (Q=mcΔT) (e.g. rock in the case of air heating systems)

Thermal Energy Storage

• Other media include phase-change materials, melting during day, freezing at night (releases heat)

Summary

• Solar energy system consists of collector, storage and distribution systems

• Active systems use FPC through which fluid moves to transfer collected energy, pumps or fans move the fluid between collector and storage systems

• Passive systems use large south-facing windows as the collector and natural means of heat transfer, thermal mass (water, rock) within the house stores the energy

• Size of collector depends on solar insolation, amount of heat needed (DHW or space heating), and collector efficiency

• Collectors should be tilted at angle from the horizontal equal to latitude + 10

Energy Chapter06 2012

Documents

Transcript of Energy Chapter06 2012