Photovoltaics work on the principle that when a photon of light with sufficient energy hits an atom with an extra valence electron, energy will be imparted to that electron and it will move from the valence band to the conduction band and will be free to move within the solid.
Making Photovoltaic Materials
First, pure silicon is diffused with either Boron (or Aluminum as in the picture) to make p type silicon. In this case "p" is for "positive". This is because Silicon has 4 valence electrons while Boron has only 3, leaveing a "hole" in the crystal structure.
To make n (negative) type silicon, a similiar process is used with Phosphorous. Phosphorous has 5 valence electrons and thus creates atoms with an extra valence electron in the crystal structure. Typically, both n and p type silicon is made through a diffusion process, taking two thin sheets of silicon and either boron or phosphorous together, applying heat and pressure, allowing the boron or phosphorous atoms to naturally diffuse into the silicon
The PV Junction
To then construct a PV cell, n type and p type silicon is sandwiched together and again allowed to diffuse. After some time, the diffusion force and the magnetic force due to the positive and negative charges of the two types of silicon become equal and the diffusion ceases.
Building a PV Cell
As noted above, when a photon hits an atom with an extra valence electron, that electron moves to the conduction band. Since there is a natural magnetic force created between the two types of silicon, this free electron is pushed toward the positive terminal of the cell.
Every material, however, requires a different amount of energy to free a valence electron, some of which are pictured below.
To create ever more efficient cells, several PV junctions having different bandgap energies can be stacked on top of one another as pictured below. While commercially available single junction silicon PV cells have an efficiency of approximately 15%, multi-junction experimental cells have reached efficiencies as high as 40%.
One of the first things to look into when considering a photovoltaic installation is the cost.
Government and Other Incentives
The government and several power companies offer incentives for renewable power generation, including photovoltaics. The Metropolitan Edison Company offers SEF Grants to residential users interested in installing a PV system. A map of where the incentive is offered is pictured below.
Aside from local and state grants and loan programs, the federal government also offers a tax break to residents installing "green" energy producing products. A 30% tax credit can be claimed up to a maximum amount of $2,000., thus reducing the initial cost of the system.
Power Usage
Before looking for a PV system, the power usage of the residence first must be known. The Energy Informaiton Administration keeps statistics of the average US household utility usage as well as the average over several regions. For example, the average household in the Middle Atlantic region (NY,PA,NJ) in 2001 used about 7733.3 kWh of electricity.
Available Solar Energy
NREL has published data, seen below, on the average amount of solar radiation hitting the earth's surface that can be collected with a flat plate, latitude tilt, south facing PV panel. Notice that most of the Middle Atlantic region receives between 4 and 4.5 kWh per meter squared of solar radiation per day.
Knowing that the commercially available PV cell distributed by Solar Power Industries, Inc. has an efficiency of 15.75% (pictured below), that the average PV system is approximately 75% efficient, there is 4.0 kWh/m2/day available, the average house requires 7733.3 kWh per year, and the 200W modules offered by DM Solar (pictured below) are 1.47 m2 each, we need approximately 30.55 modules to supply power to the average house in the middle atlantic region.
The 7.2 kW Grid-Tie system offered by DM Solar comes with all the equipment neccessary to connect to the grid, a 25 year warranty, and is $32,269. Subtracting our $2,000 tax credit leaves $30,269. The system, based on the previous assumptions should create approximately 9,115.29 kWh of electricity per year, which is 1,381.96 more than the average middle atlantic household requires. Selling this excess power to the grid at $0.153056 per kWh will net $211.52 per year. Subtracting 25 years worth of extra power from the cost of the system leaves $24,981.10. If you where to buy the same amount of electricity over 25 years for the same price it would cost you $29,590.70, meaning that a PV system will save the average middle atlantic household $4,609.60 over 25 years.
Other Factors Not Accounted For
The above assumes the PV cells are kept clean and in perfect working order
The quoted PV system is not a latitude tilt system, thus less solar radiation will be available to it than used in the calculations
Inflation and price increases are not accounted for
Environmental Impact
On average, producing 1,000 kWh of solar power reduces greenhouse emissions by:
8 pounds of SO2
5 pounds of NOx
1,400 pounds of CO2
Assuming a 2.7 year energy buyback period, the system described above over 22.3 years of use would save approximately:
1,379.6 pounds SO2
862.3 pounds NOx
241,434 pounds CO2
total = 121.8 tons of greenhouse emissions
As an example, a 2003 Mitsubishi Eclipse, under avearge driving conditions, emits approximately 8.7 tons of greenhouse gas each year; which over 22.3 years is 194 tons of emissions.
Texts
Martin A Green; Solar Cells; Operating Principles, Technology, and System Applications; University of New South Wales, Australia; Prentice-Hall, Inc., Englewood Cliffs, NJ; (C) 1982
Stephen J. Fonash; Solar Cell Divice Physics; Engineering Science Program, The University of Pennsylvania, University Park, PA; Academic Press, (C) 1981
Stefan C.W. Krauter; Solar Electric Power Generation - Photovoltaic Energy Systems; Springer-Verlag Berlin Heidelberg, (C) 2006
Tetsuo Soga; Nanostructured Materials for Solar Energy Conversion; Department of Environmental Technology and Urban Planning, Nagoya Institute of Technology, Nagoya, Japan; Elsevier B. V., (C) 2006
Informational Websites
BP Solar
UC Irvine household energy study
Photovoltaic Timeline
Spectrolab
US Dept. of Energy - Energy Efficiency and Renewable Energy
Solar Cell Government Incentives
Mitsubishi Eclipse Greenhouse Gas Emissions
DMsolar (residential PV supplier)
Energy Information Administration statistics
NASA
Wikipedia
NREL (National Renewable Energy Laboratory
Interesting Papers
PV FAQs - What is the energy payback for PV
NEW METHODS FOR MEASURING PERFORMANCE OF MONOLITHIC MULTI-JUNCTION SOLAR CELLS
PVSIM Ó : A Simulation Program for Photovoltaic Cells, Modules, and Arrays
ON THE PERFORMANCE LIMITING BEHAVIOR OF DEFECT CLUSTERS IN COMMERCIAL SILICON SOLAR CELLS
PHOTONIC CRYSTALS FOR THERMOPHOTOVOLTAIC ENERGY CONVERSION
Presentation Materials
See "Photovoltaics Noties" and "Photovoltaics Presentation" below under "files"
