Solar Cells and Photovoltaic Arrays
Solar cells convert sunlight directly into electricity and are made of semiconducting materials similar to those used in computer chips. When sunlight is absorbed by these materials, the solar energy knocks electrons loose from their atoms, allowing the electrons to flow through the material to produce electricity. This process of converting light (photons) to electricity (voltage) is called the photovoltaic effect. A typical solar cell measures 10 centimeters by 10 centimeters (about 4 inches square) and generates about 1 Watt of power at about 0.5 volts.
Individual solar cells can be connected in series in order to increase the voltage, or in parallel in order to increase the current into a module. Solar cells are typically combined into modules that hold about 40 cells, and about 10 of these modules are mounted in a photovoltaic (PV) array that can measure up to several meters on a side. These 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 the most sunlight over the course of a day. About 10 to 20 PV arrays can provide enough power for a typical U.S. household, although some tribal residences may use less power. PV arrays can also be used for large electric utility or industrial applications. Hundreds of arrays can be interconnected to form a single, large PV system.
PV systems have few moving parts and are highly reliable. In fact, many PV arrays come with warranties that are good for 20 years or more. Flat-plate PV arrays without tracking have no moving parts, and even two-axis tracking requires only a relatively small number of low-speed moving parts. This tends to keep operation and maintenance (O&M) costs down. Indeed, some early kilowatt-scale first-of-a-kind plants demonstrated O&M costs around half a cent per kilowatt-hour, which is minimal.
In many PV systems, energy will not be used as it is produced but may be required at night or on cloudy days. If tapping into the utility grid is not an option, a battery backup system will be necessary. About 80% of the energy channeled into the battery backup can be reclaimed. Like PV cells, batteries are direct-current devices and are directly compatible only with dc loads. However, batteries can also serve as a power conditioner for these loads by regulating power; this allows the PV array to operate closer to its optimum power output. Most batteries must also be protected from overcharge and excessive discharge, which can cause electrolyte loss and can even damage or ruin the battery plates. Protection is usually achieved using a charge controller, which also maintains system voltage. Most charge controllers also have a mechanism that prevents current from flowing from the battery back into the array at night.