Solar Energy: Collection, Energy Generation and Heat Transfer

How Solar Energy is Used:

Two Choices:
• Heat Water into Steam
• Turn photons into electrons

Solar Thermal Power:

• 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%).

Photons into Electrons: PhotoVoltaic Devices

Charge Generation Photoelectric Effect

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.

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. Computers 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.

Schematic structure of energy bands in Silicon:

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?

• 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 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 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 $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.

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 focusing system is quite high

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 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

Previous Lecture Next Lecture Course Page ___________________________________________________________________ ___________________________________________________________________

The Electronic Universe Project

e-mail: nuts@moo.uoregon.edu