• Direct conversion into electricity Photovoltaics; conversion of solar photons into electrons that flow down a semi-conductor. Class Demonstration: Solar Cells and Solar Panel.
• Main problem is low efficiency (about 10%).

Charge Generation

Photons:

• Light particles that have a discrete energy.
• Energy is proportional to the frequency of the light E=hf
• h = Plancs Constant = 6.63 *10^-34Joule-seconds
• Energy is inversely proportional to the wavelength l. (Red Light has less energy than violet)

Photoelectric Effect: Classroom Demonstration

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

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 Electron-volt = 1.6*10^-19Joules, 1.11eV = 1.76 *10^-19Joules
• 1.11 eV corresponds to the energy that a photon of frequency 2.65*10¹⁴Hertz, or a wavelength of 1120*10^-9m.
• 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 photovoltaic 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.

To obtain the proper Voltage and Current from an array of photovotaic cells or modules, you have to conect them in series and paralell combinations. To obtain the proper voltage, you connect cells in series. To increase current capapbility you connect cells, or combinations of cells in series, in paralell.

Cell Connection Activity to help understand is available here.

An example of Module Connections to gain proper voltages and currents is available here.

Remember, cosuming electricty is about Power = Voltage x Current. devices are made to run at a certain power and voltage and your system will need to be designed to meet those demands.

Info about Solar Modules

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 - $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 350 Megawatts are generated in PV arrays which are about 11% efficient.

Calculation for Solar Energy which shows that relative inefficiency can be compensated for with collecting area.:

• A site in Eastern Oregon receives 600 watts per square meter of solar radiation in July.
• Assume that the solar panels are 10% efficient and that the are illuminated for 8 hours.
• How many square meters would be required to generate 5000 KWH of electricity
  • each square meter gives you 600 x.1 = 60 watts
  • in 8 hours you would get 8x60 = 480 watt-hours or about .5 KWH per square meter
  • you want 5000 KWH
  • you therefore need 5000/.5 = 10,000 square meters of collecting area

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.

Now and Beyond in Solar Cells

First Generation: Single-crystal, single layer p-n junction diode, capable of generating usable electrical energy from light sources with the wavelengths of sunlight.

Second Generation: Thin epitaxial deposits of semiconductors on lattice-matched wafers. There are two classes of epitaxial photovoltaics - space and terrestrial. Space cells typically have higher AM0 efficiencies (28-30%) in production, but have a higher cost per watt. Their "thin-film" cousins have been developed using lower-cost processes, but have lower AM0 efficiencies (7-9%)

Third Generation: o not rely on a traditional p-n junction to separate photogenerated charge carriers. For space applications quantum well devices (quantum dots, quantum ropes, etc.) and devices incorporating carbon nanotubes are being studied - with a potential for up to 45% AM0 production efficiency.

Review

1) To get work out of electricity you have a way to separate charges. They like to get back together and they do that by flowing through wires connected to a light bulb, refrigerator, tv set, stereo, etc.

2) A battery separates charges by chemical reactions. When the stuff that is making a chemical reaction runs out, the battery is dead and no current can flow, and you can't listen to Puddle of Mud (or Hendrix!)

3) A PV Cell separates charges when light is incident on the cell. If there is no light, charges don't separate, no current flows and again, no Jimi....

Current Economics:

 Current Price $0.48/kWp

Need to get to $0.15 -) $0.20 per kW

Consumer cost for energy from newly constructed coal-fired plant in the US ranges from 8-20 cents per KW

Today PV power generation would cost the consumer around 30 cents per KWH.

55% of the world population living in remote areas, PV is actually competitive. (Lifestyle with electricity does not exist. Demand on a system would be less. Hopefully we are the experiment that shows what not to do!)

Lifestyle needs to be changed here. For Example

Energy use in U.S. is 9500kWhr/person each year. Germany, 3270kWhr/person each year.

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.

Only currently strongly applicable in remote locations or the RV Industry.

More on Solar Photo Voltaics

Solar PV Myths

Million Roofs???

Solar Info and Energy Cost comparisons

Environmental Impacts from Manufacturing PV Cells

Energy Payback Time (4 years? 2 years?)

Examples of Solar Cells Available.

Crystal Silicon Panels Like the one used in class (Astropower Model 7105)

Amorphous Silicon Panel

PV Roof Shingles

Scientific American Article: Required Reading!