Generation of electricity
Electricity can be generated by a variety of technologies that ultimately depend on the effects of solar radiation. Windmills and waterfalls (themselves very old sources of mechanical energy) can be used to turn turbines to generate electricity. Most existing windmill installations are relatively small, containing ten or more windmills in a crisscross configuration that takes advantage of wind shifts. In contrast, most electricity from hydroelectric installations comes from giant dams. Many of the sites suitable for large dams have already been tapped, especially in the industrialized nations, but during the 1970s small dams used years earlier for mechanical energy were retrofitted to generate electricity.
Large-scale hydro projects are still being pursued in many developing countries. The simplest form of solar-powered electricity generation is the use of an range of collectors that heat water to produce steam to turn a turbine. Several of these facilities are in existence, producing approximately 200 megawatts of power.
Other sources of sun-derived electricity involve high-technology options that remain unproven commercially on a large scale. Photovoltaic cells, which convert sunlight directly into electricity, are currently being used for remote locations such as orbiting space satellites, unattended railroad crossings, and irrigation pumps; but progress is needed to lower costs before widespread use is possible. The commercial development of still other methods seems far in the future. Ocean thermal conversion (OTC) generates electricity on offshore platforms; a turbine is turned by the power generated when cold seawater moves from great depths up to a warm surface. Also still highly exploratory is the notion of using space satellites to beam electricity via microwaves down to the earth.