Solar Collection and Energy Transport.
Solar Energy: Collection, Energy
Generation and Heat Transfer
How Solar Energy is Used:
- Heat Water into Steam
- Turn photons into electrons
Solar Thermal Power:
Photons into Electrons: PhotoVoltaic Devices
- Focus sunlight on a bucket of water
This requires about 2000 heliostats
Maintenance, initial costs make energy
expensive (25-50 cents per KWH).
- Direct conversion into electricity
Photovoltaics; conversion of solar photons into electrons that
flow down a semi-conductor. Demonstrated in class last time.
Main problem is low efficiency (about 10%).
When photons strike a metal, their energy is used to liberate loosely
bound electrons and therefore induce a current.
Efficiency of this process depends upon the material
This is the principle behind many digital cameras, otherwise
known as CCD cameras .
These kinds of cameras are used in astronomy to take digital pictures . Incoming photons are converted into units
of electric charge and stored at individual pixel locations. Different
amounts of charge represent different intensity levels. This encoded
information is a digital image.
To make use of the photoelectric effect, we need material that is a
good conductor of electricity and which can be manufactured in bulk
at reasonable cost. This conditions strongly constrain the available
choices. For most practical aspects, Silicon is the material of choice.
Schematic structure of energy bands in Silicon:
- is abundant on the earth and readily found in the crust.
It is a direct product of fusion inside stars. It can be easily
recovered from the crust and mass-produced.
are cheap because silicon works well for circuit boards and is
an easily recoverable material from the earth's crust.
The result is a world-wide economy centered around semi-conductor technology.
- Has four outer (valence) electrons to bond silicon atoms together
in a crystal
- Under normal circumstances, there are no free electrons available
in silicon to conduct electricity. All the electrons are used to
bind the atoms in place to form the crystal.
- The conduction band is empty and therefore no current can be
carried by the material.
Hence, if a silicon atom receives at least 1.11 Electron
Volts from some source, a valence electron will move
to the conduction band. Once an electron is in the conduction
band, the material can carry a current and the material is
now a conductor.
So much energy is 1.11 Electron Volts?
This does not mean that the efficiency of silicon in converting
solar photons to electrons is 77%!
- 1.11 eV corresponds to the energy that a photon of
wavelength 1.12 microns has.
- 77% of the energy from the sun is carried in photons
with wavelength less than this and therefore can move
a valence electron in silicon into the conducting band
The efficiency is strongly temperature dependent. As the temperature
is raised, the internal resistance of the material increases and
the electrical conductivity decreases.
- Photons with energy greater than 1.11 eV heat the crystal
- 43% of average absorbed photon energy goes into heating
- Some photons are reflected by the exposed surface of the crystal
- There is some internal resistance in the crystal that inhibits
the flow of electrons. This internal resistance increases as the
crystal is heated.
- At 0 degrees C silicon has efficiency of 24%
- At room temperature the efficiency is 12%
- Highest efficiency is achieved (at 0 degrees) in
CdTe (Mercate-Telluride) but this is only 28%
The fundamental physical limitation in production photovoltaic cells
is then this decrease in efficiency as the temperature of the cell
increases. Because of this, for a material like silicon, the
operating efficiency of a photovolatic array will probably never
be higher than 20% and will most likely be between 5 and 15%.
This doesn't mean that production is not possible. It does mean
that relatively large collection areas must be obtained which
means high capital costs. If those costs can not be subsidized,
then PV arrays can never be competitive in the commercial energy
To have a production photovoltaic cell, one must mix impurities
into silicon (like boron). This will create an internal electric
field which will allow the liberated electrons to move down
Over the last 40 years, the effort has gone into increasing the
efficiency of PV cells and bringing down the manufacturing costs.
- $100 per kilogram of pure silicon
then the crystals
have to be grown in a carefully controlled environment from the
molten silicon. impurities lower overall conductance and
reduce the efficiency
- Present Costs of solar cells is about $5,000 per Kilowatt
compared to $1,000 per Kilowatt from a coal-fired plant.
- Typically solar PV cell grids have enough components to produce
20-25 Volts per grid
- In California, there are some
Production line PV facilities 350 Megawatts are generated
in PV arrays which are about 11% efficient.
Advances in Amorphous
Silicon technology has led to continuous thin-film deposition
- Uses very little silicon
- Inexpensive manufacturing process
- used in watches and calculators today
Can increase efficiency by using solar cells in conjunction with
focussed systems (parabolic collectors).
- Increase gain by a factor of 100 per unit area
- For a given energy requirement, can then reduce collecting
area by a factor of 100.
- Heat load on cells causes temperature to rise and
efficiency to lower
- Need a cooling system (water - could then be used as alternative
space heating source).
- Price of focusing system is quite high
Consumer cost for energy from newly constructed coal-fired plant
in the US ranges from 8-20 cents per KWH
PV power generation would cost the consumer 25-50 cents per KWH.
Costs might be equivalent when pollution from coal is also considered
but still, the structure is not here for the consumer to pay the
true cost of energy