Photovoltaic (or PV) systems convert light energy into electricity. The term "photo" is a stem from the Greek "phos," which means "light." "Volt" is named for Alessandro Volta (1745-1827), a pioneer in the study of electricity.
In 1839, Edmond Becquerel discovered that certain materials produced small amounts of electric current when exposed to light. The first solar electric module was built by Charles Fritts in 1883. It was made of Selenium coated with a thin layer of Gold. The output however was not efficient. In 1954, D.M. Chapin, C.S. Fuller and G.L. Pearson, of Bell Laboratory, patented a way of making electricity directly from sunlight using silicon-based solar cells. The next year, the Hoffman Electronics-Semiconductor Division announced the first commercial PV product, that was 2.0-percent efficient, priced at $25 per cell, at 14 milliwatts each, or $1,785 per watt (in 1955 dollars). By the mid-1960s, efficiency levels were nearing 10 percent.
We call modern day devices that convert sunlight into energy photovoltaic cells, or "PVs" for short. More commonly, they're known as solar cells. We can find them on calculators, sidewalk lighting systems and along side freeways to power phones for stranded motorists.
Solar photovoltaic cells are small, square shaped panel semiconductors manufactured in thin film layers from silicon and other conductive materials. When sunlight strikes the PV cell, chemical reactions release electrons, generating electric current. This current, together with the cell's voltage (which is a result of its built-in electric field or fields), defines the power (or wattage) that the solar cell can produce.
PHOTONS AND ELECTRONS
A semiconductor material consist of pure silicon that has impurities added to it in order to increase its’ ability to conduct electricity. One side of the semiconductor (n-type) contains additional electrons while the other side contains fewer (p-type), referred to as holes . This creates an electric field potential between the two halves.
The semiconductor cell builds up an equilibrium at the junction of the two types. As photons strike the material they free more electrons and holes. The semiconductor’s electric field forces the electrons to move in a defined direction, thus creating a current. The current is drawn off by adding metal contacts to the semiconductor cell to create a current path. This current is combined with the current from all the other interconnected cells to give defined output.
SEMICONDUCTOR MATERIALS
Different semiconductor materials can be used to make a solar cell. Single crystalline is the most efficient but also the most costly to manufacture. It is costly because the silicon is extruded from a single vat in a cylindrical form. The wafers are then cut in extremely thin layers by a laser.
Polycrystalline is less efficient but has a lower manufacturing cost since it can be cast rather than the expensive extrusion process of single crystalline. The inefficiency comes about because there are many more inclusions that reduce the charges that can find there way to the cells edge.
Amorphous silicon is sputter evaporated onto glass, metal or other materials. It is less efficient than Polycrystalline because of the increased boundary conditions compared to the other materials.
Different semiconductor materials absorb different colors of light. The range within which a material can absorb light is called the band gap. In order to improve efficiencies for a PV cell, layers of differing semiconductor materials, each absorbing different spectrums of light, can be sandwiched together. This forms a higher band gap and allows more light to be converted into electrical energy thus improving the overall efficiency.
Light energy that is not converted to electrical energy is dissipated as heat.
Factoid : PV modules covering one fourth of the area occupied by US railroads could provide all of the electricity needs for the US.
PUTTING IT ALL TOGETHER
Diagram (No. 2) represents a Utility-Tied Solar Electric system that provides power to a home or business and feeds back power to the utility grid thus running your meter backwards. This system does not provide backup electricity but is an effective implementation in reducing or negating your energy costs.
COMMON TERMS
Solar Electric panels produce electricity from the sun’s light energy.
Solar Thermal panels produce hot water from the sun’s heat energy.
Power: A "watt" is a measurement of power ("horsepower" is another). A 100-watt light bulb requires more power than a 60-watt light bulb, which is why it is brighter. A "kilowatt" or kW is 1,000 watts (kilo means 1,000). A "megawatt" or MW is 1,000,000 watts (mega means 1,000,000).
Energy: A "kilowatt-hour" or kWh is a measurement of energy ("calorie" is another). A kilowatt-hour of energy is used if you turn on a 100-watt light bulb for ten hours:
(100 watts) x (10 hours) = 1,000 watt-hours or 1 kilowatt-hour
A megawatt-hour or mWh is 1,000 kilowatt-hours.
If a PV Panel has a rated power or nameplate power of 120 kW, that tells you that the panel will produce 120 kilowatt hours (kWh) of energy per hour of operation, when running at its maximum performance.
Net Metering: Electricity produced by a business or residence is purchased back by the utility company at the same rate that it is sold to the customer.
