Photovoltaics
Photovoltaic (PV) cells use semiconductor materials similar to those used in computer chips to convert sunlight directly into electricity. The solar resource for generating power from Photovoltaic systems is plentiful. For instance, enough electric power for the entire country could be generated by covering about 9% of Nevada — a plot of land 100 miles on a side — with PV. The primary uses for Photovoltaics today are for homes, telecommunications, security and lighting systems, water pumps, and load management.
The Photovoltaic effect is the basic physical process by which a PV cell converts sunlight into electricity. Sunlight is composed of photons, or particles of solar energy. These photons contain various amounts of energy that correspond to the wavelengths of the solar spectrum. When photons strike a PV cell, they may be reflected or absorbed, or they may pass right through. Only the absorbed photons generate electricity. When this happens, the energy of the photon is transferred to an electron in an atom of the cell material. With its newfound energy, the electron is able to escape from its normal position associated with that atom to become part of the current in an electrical circuit. By leaving this position, the electron causes a hole to form. A special electrical property of the photovoltaic cell — a built-in electrical field — provides the voltage needed to drive the current through an external load such as a light bulb.
A typical PV or solar cell might be a thin square wafer that measures about 4 inches on a side. A cell can produce about 1 watt of power — more than enough to power a watch, but not enough to run a radio. When more power is needed, some 40 PV cells can be connected together to form a module. A typical module is powerful enough to light a small light bulb. For larger power needs, about 10 such modules are mounted in PV arrays, which can measure up to several yards on a side. The amount of electricity generated by an array increases as more modules are added. Ten to 20 PV arrays can provide enough power for a household; for large electric utility or industrial applications, hundreds of arrays can be interconnected to form a single, large PV system. Flat-plate PV arrays can be mounted at a fixed-angle facing south or on a tracking device that follows the sun, allowing them to capture more sunlight over the course of a day.
The simplest PV system generates direct-current electricity when the sun is shining and runs equipment such as water pumps or fans. A power inverter can convert direct current to alternating current. PV systems also may include batteries that store electricity for use at night or when the sun isn't shining.
When power must be available on demand or the electricity required exceeds the PV system's capacity, an electric generator can work effectively with PV to supply the load. Remote locations, where loads are currently supplied by diesel generators, are good candidates for PV systems.
Some PV cells are designed to operate with concentrated sunlight, and a lens is used to focus the sunlight onto the cells. This approach has advantages and disadvantages compared with flat-plate PV arrays. The main idea is to use very little of the expensive semiconducting PV material while collecting as much sunlight as possible. The lenses cannot use diffuse sunlight, but must be pointed directly at the sun. Therefore, the use of concentrating collectors is presently limited to the sunniest parts of the country.
The amount of power generated by a flat-plate PV array at a particular site depends on how much of the sun's energy reaches it from all directions. This is called global solar radiation. PV arrays usually are tilted at an angle equal to the site's latitude, which allows the array to capture the most sunlight over the course of a year.
The situation is different for concentrating PV collectors, which use only the direct-beam sunlight rather than global solar radiation. However, because they are mounted on tracking devices that follow the sun, they gather as much energy or more than a flat-plate PV array that doesn't follow the sun. The higher efficiency of concentrating PV collectors means that they produce more electricity per square foot than does a flat-plate PV array. See the Concentrating Solar Power page for more information.
The main advantages of PV systems are their modularity, portability, high reliability, and low environmental impact. These systems have no (or few) moving parts, which means operating and maintenance costs are low. Another obvious benefit of PV systems is that the sun provides abundant and free fuel.
Grid-connected PV provides a distributed generation resource to augment the electricity grid and is now the world's largest PV market. In the past decade, incentives in Japan and Germany have hastened the dramatic growth in this market. The PV system may tie to the grid at the substation level, to relieve transmission line load. Or it may tie at the industrial, commercial, or residential customer site. Capacity constraints in generation, transmission, and distribution are usually caused by solar-related loads such as air conditioning, so it makes sense to use solar energy to alleviate sun-induced problems.
Previously, the largest market for PV was the off-grid market, which takes advantage of PV's ability to function as a stand-alone electrical system. Telecommunications and transportation warning signage are the two largest segments of the off-grid market. Most of the off-grid market is located in remote locations and inaccessibility to the utility grid. However, in many instances, the grid may be near a well-developed area, but it is still more cost-effective to install a modular PV system, rather than to cross roadways or sidewalks. Some utilities offer PV systems as alternatives to expensive construction.
The same values that drive the PV system market also set the wide range of PV costs. The high range of capital costs of $5-$12 per watt is offset by low operating costs. The 20-year life-cycle cost is $0.20-$0.50per kWh.
A remote home installation that requires batteries, a generator, or both may need 2-5 kilowatts of power as high as $12 per watt, or a high cost of $60,000. However, the cost of a rural distribution line now averages $60,000 per mile. With the additional advantage of lower land costs in remote areas, PV shapes up as the best value.
For more information on PV technologies, visit DOE's Solar Energy Technologies Program Web site. The National Center for Photovoltaics, headquartered at the National Renewable Energy Laboratory (NREL) is another good source of information on photovoltaics.