PV in Simple, 'Stand-Alone' Systems
Thanks to a PV-powered water pumping system installed in 1992, this young girl in Cardeiros, Brazil, can fill her jug from the school's water tank. Additional PV systems with batteries power the school's lights, refrigerator, and television.
Stand-alone PV systems are often best in places where utility-generated power is either unavailable (because the area is so remote from power plants), undesirable (because of a possible utility power outage in an emergency), or too costly to hook up to (because of the price of extending power lines). Stand-alone systems are also excellent for uses that don't require a lot of power.
The same sunny days that dry out plants, make animals thirsty, and heat up buildings and cars just happen to be very good days for generating electricity with photovoltaics. And that's exactly what stand-alone PV systems (those not connected to a utility power grid) do every sunny day, and on some cloudy days, too, all over the world. The electricity is then used to power water pumps for irrigation and drinking wells, for example, or ventilation fans for cooling. For this reason, the most simple PV systems are those that generate direct-current (dc) electricity so it can be used right away to run water pumps, fans, and many other appliances that use dc electricity.
These basic PV systems have several advantages that make them suitable for these jobs. First, they produce energy where and when it's needed, so complex wiring, storage, and control systems aren't needed. Second, small systems that produce less than 500 watts and weigh less than 68 kilograms (150 pounds) are easy to transport and install. Most installations take only a few hours. And, although pumps and fans require regular maintenance, PV modules require only an occasional inspection and cleaning. See the case study on PV-powered water for cattle for an example of a stand-alone system in action.