Solar Energy: Conversion into Electricity
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.
- Energy Losses:
- 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 efficieny 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 likley 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 $10,000 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 --> efficiency
is about 11%
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.
Read More About Solar Power Towers
- 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 focussing 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 50-100 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
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