CHAPTER 4
RENEWABLE - SOLAR
PHYX 1020
2002
ENERGY
USU 1360
CHAPTER 4
RENEWABLE ENERGY I - SOLAR ENERGY
What do we see related
to solar energy?
Economics
Extent of underprivileged countries
Light
Electromagnetic wave energy
Latitude effects
Seasons
Weather
Wind
Clouds
Potential for rain
Oceans
Temperature gradient
Potential for waves
Biomass
Potential for plant growth
Politics
The world’s oil resources
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CHAPTER 4
RENEWABLE - SOLAR
PHYX 1020
2002
ENERGY
USU 1360
The Source of Solar Energy
• Inside the sun (as in other stars) the pressure and temperature is
sufficient for nuclear fusion reactions to occur
• Part of the high energy release in the form of heat energy converted
from the mass energy of the fusion is used to maintain the high
temperature for fusion to occur (c.f. Burning)
• The rest is conducted and convected to the surface of the sun where an
equilibrium temperature of ~6000K results
• This is much less than the temperature in the interior needed to
maintain fusion
• The hot surface of the sun radiates electromagnetic wave energy
• This form of energy travels through the near vacuum of space in all
directions
• A small fraction of this is intercepted by the earth and provides almost
all of our energy either directly or indirectly
• Bottom line the source of most of our energy is Nuclear
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ENERGY
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Solar Renewable Energy (1)
• Channel of energy flow
– Direct
– Indirect
• Direct
– Active heating panels
• Electromagnetic wave energy
Heat energy
– Passive heating systems
• Electromagnetic wave energy
Heat energy
– Solar boilers
• Electromagnetic wave energy
Heat energy
– Solar photoelectric conversion
• Electromagnetic wave energy
Electrical energy
2002
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CHAPTER 4
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PHYX 1020
ENERGY
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Solar Renewable Energy (2)
• Indirect
–
–
–
–
–
Hydro electricity
Wind
Biomass
Waves
Ocean temperature gradient
• These will be considered later in the section
on alternative energy
2002
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CHAPTER 4
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PHYX 1020
ENERGY
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Renewable Energy
• Note:
– Renewable sources
only 8%
– Of these Solar related
energy sources make
up 96%
– Direct solar energy is
only 1%
2002
DOE Annual Energy Review, 1999
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RENEWABLE - SOLAR
PHYX 1020
ENERGY
Solar Energy at Top of Atmosphere
UV VIS
Infrared
Solar Spectrum
Above atmosphere
Earth’s Surface
2002
USU 1360
• Spectra of solar electromagnetic
wave emission over wavelength
range 0.2 - 3.0 mm
– Energy emitted at different
wavelengths
– Wavelength band associated
with UV, visible, IR
– Broken line at top of
atmosphere
– Solid line at earth’s surface
• Broken line characteristic of
emission from a black body at
~6000K with superimposed line
emissions
• Solid line shows strong
absorption bands by atmospheric
atoms and molecules
• Area under the curve is the total
energy in the wavelength range
• Over all energies it is called the
SOLAR CONSTANT
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CHAPTER 4
RENEWABLE - SOLAR
PHYX 1020
ENERGY
USU 1360
Atmospheric Modification of Solar Energy
47%
• Absorption and scattering are wavelength dependent
– Causes the dips in the previous curve
2002
• Above the atmosphere the power from the sun is ~1300W/m2
– At the earth the average over a full (24 hr) day is ~164W/m2
• Mid-latitude, av. cloudiness, accounts for rotation of earth, horizontal surface
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PHYX 1020
ENERGY
USU 1360
CHAPTER 4
RENEWABLE - SOLAR
Distribution of Annual Insolation in USA
Daily energy averaged over one year
2002
•
•
Regions depend on clear skies, latitude, low humidity.
Energy figures are for an 8-hour day while sun is up.
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ENERGY
USU 1360
CHAPTER 4
RENEWABLE - SOLAR
PHYX 1020
Geometrical Effects of Solar Energy Plate Collectors
•
•
•
•
2002
•
Total energy collected per m2 are areas under curves
Note peak power ~1000W/m2, but average 500W/m2 or less for 8-hour day
Slanted stationary plate is inclined to point at sun at noon
Actual angles depend on day of year
Steering is an expensive option for domestic use
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CHAPTER 4
RENEWABLE - SOLAR
PHYX 1020
ENERGY
USU 1360
Flat Plate Collectors
• This is an example of an ACTIVE solar energy
conversion system.
– Active because energy is used to distribute the heat
from where the conversion occurs to where it is needed
(forced convection)
• Heat energy at surface of sun is transferred to a
collecting plate by electromagnetic radiation.
2002
– Surface of collecting plate absorbs the electromagnetic
wave energy and converts it to heat energy
– Heat energy is conducted through the plate to a fluid in
contact with the plate and increases the heat energy of
the fluid
– A pump keeps the fluid moving past the plate to a
HEAT EXCHANGER where the energy is extracted
and stored in another fluid for future use.
– The original fluid with a lower level of heat energy is
then returned to the collector plate to gain more energy.
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CHAPTER 4
RENEWABLE - SOLAR
PHYX 1020
ENERGY
USU 1360
Solar Energy Collectors
The Greenhouse Effect (1)
• The greenhouse effect depends on
two physics principles:
– Stefan’s Law
– Wien’s Law
2002
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CHAPTER 4
RENEWABLE - SOLAR
PHYX 1020
ENERGY
USU 1360
Solar Energy Collectors
The Greenhouse Effect (2)
– Stefan’s Law
• This describes how much power an
object emits at a given temperature
when all wavelengths are considered
• P/A = e s T4
e = emissivity between 0 and 1.0 (shiny at
low end, dull black at high end, 1.0 is
called a “black body”)
s = Stefan’s constant (5.76 x 10-8 W/m2.K4)
– T is the temperature of the object on the
Kelvin scale
2002
• Note the very strong dependence on
temperature - often determines the
steady temperature of objects
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CHAPTER 4
RENEWABLE - SOLAR
PHYX 1020
ENERGY
USU 1360
Solar Energy Collectors
The Greenhouse Effect (3)
– Wien’s Law
• This describes the wavelength at which the
peak emission occurs
lmax = 2898 / T mm
– Thus for solar energy from the sun
lmax = 2898/6000 = 0.48 mm
– While for the collector
lmax = 2898/360 = 8.1 mm
2002
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CHAPTER 4
RENEWABLE - SOLAR
PHYX 1020
ENERGY
USU 1360
Solar Energy Collectors
The Greenhouse Effect (4)
• The effect occurs because the “greenhouse”
glass is transparent to 0.48 mm but is opaque
to 8.1 mm. Thus little energy can escape by
radiation and the heat energy (temperature)
increases in the enclosure compared to no
enclosure
• This is the basis for solar energy collectors
and has relevance to the earth’s temperature
increase causing global warming to be
discussed later.
2002
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RENEWABLE - SOLAR
PHYX 1020
ENERGY
USU 1360
Schematic Solar Heating System (Collector)
• Insulation at base to
reduce heat loss by
conduction
• Black collector on
absorber
– low loss by reflection
– High absorption of solar
radiation
• Water pumped through
collector (Active System)
– Heat transfer by forced
convection
• Double paned glass
2002
– Reduced loss by
conduction
– Blocks loss by radiation
(greenhouse effect)
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PHYX 1020
ENERGY
USU 1360
Schematic Solar Heating System (System)
2002
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CHAPTER 4
RENEWABLE - SOLAR
PHYX 1020
2002
ENERGY
USU 1360
Calculation for Active Solar Heating (1)
• What area of flat plate solar collector
oriented to point at the sun at would be
needed to maintain the temperature at a
comfortable level in an average house on a
cold winter day in northern Utah?
– Assume the heat energy needed to be supplied to
the house is 106 Btu/day
– Referring to fig 4.3 the average daily solar
energy input in N. Utah is 4.4 kWh/m2
– The flat plate collector will not be 100% efficient
- assume an efficiency of 0.7 (70%)
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CHAPTER 4
RENEWABLE - SOLAR
PHYX 1020
ENERGY
USU 1360
Calculation for Active Solar Heating (2)
• Calculation
– Solar energy needed accounting for efficiency
• Eff = Heat energy to house / Heat energy from sun
• Substituting known values:
0.7 = 106 / solar heat
energy
• Thus by rearranging: solar heat = 106 / 0.7 = 1.4 x 106
Btu
– Convert kWh/m2 to Btu/m2
• Using table in book we find 1 kWh = 3413 Btu
• Thus 4.4 kWh/m2 = 4.4 x 3413 = 15,017 Btu/m2
– Calculate area of solar collector needed to supply
the house thermal energy
2002
• Area = Solar heat energy needed / solar heat energy/m2
•
= 1.4 x 106 / 15,017 = 93.2 m2 ~1000 ft2
• This is could be constructed as a panel 50 x 20 ft (quite
large!)
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CHAPTER 4
RENEWABLE - SOLAR
PHYX 1020
ENERGY
USU 1360
Passive Solar Collection (1)
• Three components to passive solar
heating of buildings
– Collection
– Storage
– Insulation
2002
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CHAPTER 4
RENEWABLE - SOLAR
PHYX 1020
ENERGY
USU 1360
Passive Solar Collection (2)
• Collection
– Solar energy must penetrate the building and
will be trapped due to the greenhouse effect.
– Provision must be made to reduce solar energy
input during the summer.
• Use of roof overhangs to shield windows from the
sun in summer and around noon
• Use of sun blinds on S and W windows
2002
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CHAPTER 4
RENEWABLE - SOLAR
PHYX 1020
ENERGY
USU 1360
Passive Solar Collection (3)
• Storage
– The heat energy from the absorbed solar
radiation must be used to heat up a large
mass to provide heat energy after sunset or
when cloudy.
– Heat energy storage varies with material
• Water
• Iron
• Brick
• Concrete
62 Btu per cubic foot per °F
54
“
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25
“
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22
“
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2002
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CHAPTER 4
RENEWABLE - SOLAR
PHYX 1020
ENERGY
Passive Solar Collection (4)
• Insulation
– The windows, walls, roof and floors must be
constructed to reduce the loss of heat energy from
the building by conduction.
– Use of layers of low thermal conductivity material
•
•
•
•
2002
USU 1360
Glass wool
Styrofoam sheets
Shredded paper
Air (as in double/triple glazed windows)
– Make layer as thick as possible (except air gaps)
– Reduce inevitable heat loss from outside walls by
windbreaks to reduce the forced convection of the
air flow associated with the wind.
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CHAPTER 4
RENEWABLE - SOLAR
PHYX 1020
ENERGY
USU 1360
Passive Solar Collection
Practical systems
Trombe Wall
passive solar heating system
Direct Collection
Passive solar heating
system
2002
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CHAPTER 4
RENEWABLE - SOLAR
PHYX 1020
ENERGY
USU 1360
Solar Boilers
• Solar thermal electric power generation
– Solar energy is used to generate the heat source for a heat
engine to subsequently rotate an alternator
– High temperatures are required for high thermodynamic
efficiency of the heat engine.
• Hard to do with direct solar energy - even in a car the temperature
is below boiling point of water
– The solar radiation energy need to be concentrated in a
small volume
• Use of curved (parabolic) mirrors
• Use of arrays of plane mirrors (heliostats)
– Water pumped through the region of solar energy
concentration and the high heat flow turns it to steam
used as the heat source for a heat engine.
2002
• Boiler temperatures of 1000 - 2000 °C are achievable now
• Depends on ratio of mirror collecting area to boiler surface area
(concentration ratio)
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CHAPTER 4
RENEWABLE - SOLAR
PHYX 1020
2002
ENERGY
USU 1360
Parabolic trough collectors
Barstow CA
Colorado
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CHAPTER 4
RENEWABLE - SOLAR
PHYX 1020
ENERGY
USU 1360
Heliostat Collector
10 MW installation at
Barstow CA
2002
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CHAPTER 4
RENEWABLE - SOLAR
PHYX 1020
2002
ENERGY
USU 1360
Heliostat Calculation (1)
• A University decides to convert its football
field to a heliostat array to generate electricity
by producing steam from the collected and
directed solar energy. Assume all of the solar
energy falling on the field is concentrated on
the solar boiler and calculate the peak watts of
power input to the boiler and electrical power
generated.
– Size of football field (including end zones) 160 x
360 ft
– Peak solar power at site 1000 W/m2
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CHAPTER 4
RENEWABLE - SOLAR
PHYX 1020
ENERGY
USU 1360
Heliostat Calculation (2)
• Calculation
– Convert area of field to m2 1m = 3.28 ft
• Field area = (160 / 3.28) x (360 / 3.28) = 5354 m2
– Calculate total power at 1000 W/m2 peak power
• Total power at boiler = 1000 x 5354 = 5.354 x 106 or ~
5.4 MW
– Account for efficiency of a modern heat
engine/generator ~ 0.35
• Electric power delivered = 0.35 x 5.4 = 1.9 MW
2002
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CHAPTER 4
RENEWABLE - SOLAR
PHYX 1020
ENERGY
USU 1360
Solar Photo-Electricity (1)
• It is possible to cause charge separation (and hence
electrical energy) directly from electromagnetic
waves by the wave-electron interaction in materials.
– This was discovered as the photo-electric effect in which
light was found to cause the emission of electrons from
surfaces.
– A threshold effect was seen in the wavelength of the
electromagnetic radiation necessary for the emission to
occur
– This led to Einstein’s explanation of the effect that the light
could be considered as a stream of particles called photons
– The energy of each photon is proportional to 1/l
• Where l = wavelength of the light
2002
– Note the shorter the wavelength the more energy carried by
the photon.
– The photon energy must then exceed the minimum energy
to extract the electron from the surface accounting for the
wavelength threshold.
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CHAPTER 4
RENEWABLE - SOLAR
PHYX 1020
ENERGY
USU 1360
Solar Photo-Electricity (2)
• The photo electric effect requires a vacuum to
allow the emitted electrons to be captured and
maintain the charge separation.
• In 1954 another method of employing the energy
of photons to energize electrons in material was
developed as the solar cell.
2002
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CHAPTER 4
RENEWABLE - SOLAR
PHYX 1020
ENERGY
USU 1360
Solar Cells (1)
• Solar cells are semi-conductor devices formed into a p-n junction in
silicon.
• p refers to silicon doped with a substance which results in the absence of a
chemical bond electron
• n refers to and excess electron to chemical bonding requirements
• When these doped mixtures in silicon are brought into contact charge
migrates from one side of the junction to the other and sets up a voltage of
~0.5 volts
• The effect of the light photons is to release bound electrons in the junction
so that the 0.5 V can drive a current in an external circuit.
– The energy comes from interaction with light photons
– There is again a threshold set by the minimum energy to release a bound
electron
– The more photons/second (brighter) the more current can be driven by the
solar cell
2002
Schematic
Solar Cell
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CHAPTER 4
RENEWABLE - SOLAR
PHYX 1020
ENERGY
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Solar Cells (2)
• Practical information
– Solar cells are fabricated on slices of crystals of silicon
– Mounted in glass they are typically 2 inches by 1/16 inch thick
– Each cell generates about 0.5 volts
• To produce high voltages they are connected in series
• To produce high currents they are connected in parallel
• In practice large arrays are a combination of series/parallel connection
– Power output related to solar power input
• Expressed in peak watts out for 1000W/cm2 solar energy input
• Typical efficiency is 10%
• Rest of solar power input results in increased heat energy of cells
– i.e. higher temperature increases which reduces efficiency of electrical
power generation
– Costs in 1997 were $4.16/peak watt in a module ($2.78 for cell)
2002
• Equivalent to $0.50 - $1.00 per unit of electricity (but no anti-pollution
costs)
• Needs to fall to <$0.50/peak watt to compete with fossil fuel power.
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CHAPTER 4
RENEWABLE - SOLAR
PHYX 1020
ENERGY
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Solar Cells (3)
• What will bring down costs?
– Price reductions in fabrication
• Extract a thin ribbon of crystallized silicon from a molten mass
• Amorphous silicon deposited as a film of very small crystals
– Increased efficiency
• Use of gallium arsenide instead of silicon
• Concentrating solar energy using mirrors
SHIPMENTS OF SOLAR CELLS
TRADE IN SOLAR CELLS
EXPORTS
51,000kW peak
IMPORTS
DOE annual Energy Review, 1999
2002
Production of Solar Cells 1982-98
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CHAPTER 4
RENEWABLE - SOLAR
PHYX 1020
2002
ENERGY
USU 1360
Solar Cells (4)
DOE annual Energy Review, 1999
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PHYX 1020
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CHAPTER 4
RENEWABLE - SOLAR
Solar Cells (5)
Solar Cell modules
2002
ENERGY
Solar Cell
Power Station
Solar Cell
Street lamp
Solar Cell powered buildings
35
CHAPTER 4
RENEWABLE - SOLAR
PHYX 1020
ENERGY
USU 1360
Solar Cell calculation (1)
• Suppose the household consumption of
electrical energy is an average of 25kWh per
day. What area of solar cells would be needed
to supply this amount of energy during 8 hours
of daylight if the solar power received during
those 8 hours is 600W/m2 and the solar cells
are 10% efficient at converting solar energy to
electrical energy? What is cost of cells?
2002
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CHAPTER 4
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PHYX 1020
ENERGY
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Solar Cell calculation (2)
• Calculation
– Calculate the solar energy received / m2 during the 8 hours
• Energy = power x time
•
= 600 x 8 = 4,800 Wh/m2 or 4.8 kWh/m2
– Calculate energy from cells at 10% (0.1) efficiency
• Energy out/Energy in = 0.1
• Thus: Energy out/4.8 = 0.1 or Energy out = 4.8 x 0.1 = 0.48
kWh/m2
– Calculate area required
• Area = Electrical energy needed / Electrical energy from cells/unit
area
•
= 25 / 0.48 = 52 m2
– Calculate cost of cells at $4.20 per peak watt
2002
• The cells being considered produce 100 peak watts ($420) per
square meter (based on peak solar power input of 1000 watts/m2)
• Cost = 420 x 52 = $21,840 at 1997 prices for solar cell modules
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CHAPTER 4
RENEWABLE - SOLAR
PHYX 1020
2002
ENERGY
USU 1360
Learning Objectives (1)
• Understand that renewable solar energy may be used directly or indirectly
• Be aware of four techniques used in the direct conversion of solar energy.
• Know that renewable energy provides only 8% of the US energy
requirements.
• Be aware that solar energy accounts for 96% of renewable energy.
• Be aware that only 1% of renewable energy is direct solar energy.
• Understand what is meant by the solar spectrum
• Be familiar with the form of the solar spectrum and the approximate
wavelength at which it peaks
• Know what is meant by the the term “Solar Constant”
• Be aware of the effect of the atmosphere on the spectrum as the solar
energy penetrates it to the earth’s surface.
• Be familiar with the major processes in the atmosphere which reduce the
total solar energy at the earth’s surface compared to that above the
atmosphere.
• Know approximately what percentage of the sun’s radiant energy reaches
the earth’s surface compared to that above the atmosphere.
• Know the approximate power per unit area from solar radiation at the
earth’s surface on a sunny day near noon (1000W/m2).
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CHAPTER 4
RENEWABLE - SOLAR
PHYX 1020
2002
ENERGY
USU 1360
Learning Objectives (2)
• Understand that the flat plate is an example of active, direct solar energy
conversion.
• Know the principal of converting solar energy using flat plate collectors.
• Know the principal of the “Greenhouse Effect”.
• Be aware of the relevance and predictions of Stefan’s & Wien’s laws in
rlation to the “Greenhouse Effect”
• Know the parts of a flat plate solar energy collector and their purpose.
• Understand what is meant by passive, direct solar energy conversion.
• Know the three components that are used in buildings to facilitate passive
solar energy conversion to heat the buildings.
• Be aware of practical examples of passive solar heating such as the
Trombe wall and construction techniques for south windows.
• Understand how the collecting area for radiant solar energy can be
enhanced by focussing collecting mirrors.
• Be aware of parabolic trough collectors to generate steam from solar
radiation
• Know what is meant by a heliostat used to generate steam from solar
radiation
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CHAPTER 4
RENEWABLE - SOLAR
PHYX 1020
2002
ENERGY
USU 1360
Learning Objectives (3)
• Know what is meant by the effect known as photo electricity.
• Understand that electromagnetic radiation has an energy associated with
its wavelength.
• Know that short wavelengths are more energetic than long wavelengths
• Be aware of the development of the photo sensitive semiconductor
junction to form the solar cell.
• Know that photo electronic effects have a threshold effect with respect to
the wavelength of the radiation.
• Know the general shape and size of a solar cell
• Understand that series connections can be used to increase the voltage of a
solar cell array.
• Understand that the current which can be drawn can be enhanced by
parallel connection.
• Know that the voltage of one cell is ~0.5V and the power it can deliver is
~10% of the incident solar radiation power.
• Be aware of the increasing production of solar cells in recent years.
• Know some uses of solar cells as commercial electrical energy sources.
40