Concentrated solar power, also called solar thermal, is a means of gathering solar energy distinct from the use of photovoltaic (PV) panels. Instead of directly converting solar energy to electricity, as in PV panels, concentrated solar power concentrates sunlight onto a relatively small point, which heats a medium. The heat from the medium is then either transferred directly to the target to be heated, such as a swimming pool, or connected to a generator for producing electric power.
Unlike commercial PV panels, which have an efficiency of about 17-19%, concentrated solar power efficiency ranges from 40-60%. There are three types of concentrated solar power devices -- low temperature devices (used to heat pools), medium-temperature devices (used to heat water for commercial or residential use), and high-temperature devices (used to generate electricity).
Concentrated solar power is cheaper than solar panels, as it primarily consists of mirrors to concentrate the sun's rays, rather than photovoltaics, an electronic system which requires high-purity silicon to produce. Both must regularly be cleaned, or they loose their energy-generating power.
In recent years, several large-scale concentrated solar power plants have been built. SEGS (Solar Energy Generating Systems), a solar thermal plant run by the Israeli company Solel, covers 1000 acres. The largest solar power plant of any kind, this plant produces 90% of all commercially available solar power -- 354 megawatts, about 1/6th of a major nuclear power plant, and about 1.5% of the Three Gorges Dam, the world's largest power plant of any kind. At the Solel plant, mirrors concentrate solar power onto tubes filled with oil, which are heated to generate electricity.
In 2006, about 21 million square feet of concentrated solar power systems were built in the United States, 16 million of it the low-temperature variety used to cheaply heat swimming pools. These use either warm water or air as a medium to transfer heat.
The term "concentrated solar power" is most often associated with high-temperature solar thermal collectors, such as those used in the Solel plant. These use steam or gas turbines to generate electricity from superheated fluid.