Solar Energy: Conversion into Electricity So much energy is 1.11 Electron Volts?

• 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

This does not mean that the efficiency of silicon in converting solar photons to electrons is 77%!

• 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.

The efficiency is strongly temperature dependent. As the temperature is raised, the internal resistance of the material increases and the electrical conductivity decreases.

• 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 market place.

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 the material.

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%

Recent Advances:

Advances in Amorphous Silicon technology has led to continuous thin-film deposition process.

• 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.

BUT

• 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

Read More About Solar Power Towers

Current Economics:

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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The Electronic Universe Project

e-mail: nuts@moo.uoregon.edu