Environmental Physics
Chapter 6:
Solar Energy: Characteristics and Heating
Copyright © 2008 by DBS
Introduction
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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
Fossil fuels
Nuclear energy
Wind
(man-made)
Waves
Geothermal
Biomass
(natural)
Hydro
Radiant
GRAVITATIONAL(
PE/KE earthmoon-sun)
Tides
Introduction
Wood and
agricultural wastes
3 % of total
energy use
Figure 6.1: U.S. renewable energy consumption (by source), 2003.
Fig. 6-1, p. 162
Introduction
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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
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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
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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/d
79 W/m2
Equator
2000 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
ft x 12 in x 2.54 cm x 1m
ft
1 in
100 cm
2
x 24 hr x d
d
1W
3.41 Btu
h
= 79 W/m2
Characteristics of Incident Solar Radiation
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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.
Characteristics of Incident Solar Radiation
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Energy released from the fusion of
hydrogen nuclei to produce helium nuclei
Surface ~ 6000 °C
Core 40 x 106 ° C
Characteristics of Incident Solar Radiation
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Intensity of EM radiation from the sun received
at the top of the earth’s atmosphere
9 % UV, 40% visible, 50 % IR
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Only ½ of this reaches surface
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Absorbed by atmospheric gases
Figure 6.2: Spectrum of solar radiation reaching the
earth at the top of the atmosphere and at ground level.
Reflected
3%
Incoming
solar energy
100%
Radiated from
clouds +
atmosphere
60%
Energy in = Energy out
Radiated
from
6% earth
6%
Absorbed by clouds
+ atmosphere
19 %
Reflected
3%
Direct
21%
Scattered
Conduction/
29%
convection
33%
Net
terrestrial
105%
113%
radiation
8%
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%).
Fig. 6-3, p. 165
Characteristics of Incident Solar Radiation
Albedo
Fig. 2.13
• What Happens to Sunlight?
30% Albedo
51% ??
19% Absorbed
Characteristics of Incident Solar Radiation
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Relatively constant temperature of the earth is a result of the energy balance between incoming
solar radiation and the energy radiated from the earth
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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
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The reradiation back to earth is called the atmospheric greenhouse effect
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Earth temperature is maintained ~ 40 °C higher than it would be with no atmosphere (-15 °C)
Characteristics of Incident Solar Radiation
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
Characteristics of Incident Solar Radiation
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Insolation at earth’s surface varies between 0 and 1050 W/m2
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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)
Characteristics of Incident Solar Radiation
- Incoming solar radiation spread out
- More atmospheric scattering
- More direct incoming solar radiation
- Less atmospheric scattering
Insolation is lowest in winter when the
need for heat is highest
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.
Fig. 6-5, p. 167
Characteristics of Incident Solar Radiation
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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
As fall moves into winter the
sunrise and sunset points of the
sun’s motion across the sky
move gradually southward
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).
Characteristics of Incident Solar Radiation
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Insolation reaching the surface is composed of direct, diffuse and reflected components
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Insolation is usually measured on a horizontal surface
Figure 6.7: Components of solar radiation.
Insolation on a vertical
surface in winter is
greater than on a
horizontal surface
Figure 6.8: Daily clear-day insolation as a function of month and
collector orientation.
Fig. 6-8, 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.
Fig. 6-9, p. 169
To calculate space heating requirements need data on average insolation and
outdoor temperatures (Climate Atlas)
Table 6-3, p. 170
End
• Review
History of Solar Heating
Anasazi Indianas c. 12000 BC
Archimedes ‘death-ray’ c. 212 BC
National Solar Test Facility,
NM - 2200 °C
Can melt quarter-inch-thick
steel plate in 2 minutes.
History of Solar Heating
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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
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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 x 1200 W/m2 = 1576 W/m2
Assuming:
(i)
(ii)
all solar energy converted to thermal energy of the steam,
heating it to 100 ° C
air temp. = 20 °C
Efficiency = (TH-TC)/TH = 80/373 = 0.21 = 21%
Max. useful work output = 1576 kW x 0.21 = 330 kW
History of Solar Heating
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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.
History of Solar Heating
Solar Cooking
1767 - DeSaussure (Swiss) obtained
temperatures high enough for cooking in a
glass covered insulated ‘hot box’
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.
History of Solar Heating
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
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 x 20/100 x Area = 240W
A = 1.41 m2
Overview of Solar Heating Today
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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
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5% of collectors sold today are for DHW, 95% for pools
Three types:
– Active flat-plate collectors (FPCs)
– Batch water heaters
– Passive (thermosiphoning systems
Solar Domestic Hot Water
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.
Figure 6.19: Solar collector absorber plates.
Fig. 6-19b, p. 179
Solar Domestic Hot Water
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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
Solar Domestic Hot Water
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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
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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 Systems
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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)
Passive Solar Space Heating Systems
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Essential elements of a passive solar system:
– Excellent insulation
– Solar collection (south-facing windows)
– Thermal storage
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3 Types of passive systems
– Direct gain
– Indirect gain
– Attached solar greenhouse
Passive Solar Space Heating Systems
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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.
Passive Solar Space Heating Systems
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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 doubleglazed, 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.
Passive Solar Space Heating Systems
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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
Passive Solar Space Heating Systems
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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.
Passive Solar Space Heating Systems
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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 drums
used for energy storage
Passive Solar Space Heating Systems
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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
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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 Solar Space Heating Systems
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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 Solar Space Heating Systems
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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.
Active Solar Space Heating Systems
Air vs. water
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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)
Active Solar Space Heating Systems
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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.
Active Solar Space Heating Systems
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Optimum angle for Pittsburgh?
Active Solar Space Heating Systems
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To calculate area of flat panel collector required:
Require quantity of heat needed (Q), average insolation (I), efficiency of the collector (ε)
Q=IxεxA
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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=IxεxA
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
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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
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Other media include phase-change materials, melting during day, freezing at night (releases heat)
Summary
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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