Off-the- grid applications can be large or small and include powering satellites and space vehicles, and powering electric equipment, such as water-level sensors, meteorological stations, and emergency telephones in remote areas. Off-the-grid photovoltaics are emerging as a major contributor to developing countries that can’t afford to build electrical grids or large central power plants that burn fossil fuels. One company in the United States is manufacturing photovoltaic systems that power lights and televisions at an installed cost of less than $400 per household. About half a million homes, mostly in villages not linked to a countrywide electrical grid, now receive their electricity from photovoltaic cells. Solar thermal generators focus sunlight onto water- holding containers. The water boils and is used to run such machines as conventional steam-driven electrical generators.
The first large-scale test of using sunlight to boil water and using the steam to run an electric generator was “Solar One,” funded by the U.S. Department of Energy. It was built in 1981 by Southern California Edison and operated by that company along with the Los Angeles Department of Water & Power and the California Energy Commission. Sunlight was concentrated onto the top of the tower by 1,818 large mirrors (each about 20 feet in diameter) that were mechanically linked to each other and tracked the sun.
At the end of 1999, this power tower was shut down, in part because the plant was not economically competitive with other sources of electricity. New solar thermal generators are being built with very large output. More recently, solar devices that heat a liquid and pro- duce electricity from steam have used many mirrors with-out a tower, each mirror concentrating sunlight onto a pipe containing the liquid. This is a simpler system and has been considered cheaper and more reliable.