Photovoltaic (or PV) cells are devices that use semiconductor materials—similar to those used in computer chips—to convert sunlight directly into electricity. The electric current can either be used immediately, or it may be stored, as in a battery, for later use.
A typical PV or solar cell might be a square that measures about 4 inches (10 centimeters) 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 meters on a side. The amount of electricity generated by an array increases as more modules are added.
'Flat-plate' PV arrays can be mounted at a fixed-angle facing south, or they can be mounted on a tracking device that follows the sun, allowing them to capture more sunlight over the course of a day. 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.
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 both 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 limited to the sunniest parts of the country.