PV costs run nearly double those of solar thermal for a power plant of a similar size, but PV has the advantage of modularity;panels can be incorporated into individual homes, companies, and buildings or installed in small spaces. This micropower approach has helped the market for PV explode in the past five years, while solar thermal remained moribund. With gas costs rising and the world sharpening its focus on global warming, and governments around the world making a concerted attempt to invest in alternative energy sources on a larger scale, solar thermal is attracting new attention. In Spain in particular, the technology has been assisted by Royal Decree 436, implemented in March 2004, which approved a feed-in tariff (a guaranteed price) for solar thermal power. The feedin tariff made building this type of power plant economically viable. The government also recognizes that, as with wind, support is necessary at the beginning to enable the creation of new plants—which will most likely drive down prices, as has happened in Spain with wind power.

Technologies

The most common technology so far, and the one in use at Andasol 1, is based on a series of parabolic troughs, huge curved mirrors about 18 feet wide that collect the sun’s energy and focus it on a receiver pipe in the middle. Oil streams through that pipe along a long loop of troughs. The mirrors slowly track the sun from east to west during daytime hours, and the oil reaches about 400 ºC (about 750 ºF). The heat transfer fluid then travels to a steam generator, where the heat is transferred to water, immediately turning the water into steam.

That steam powers a turbine, the same technology used in conventional power plants. The tower technology works on the same principle as the troughs—the sun’s heat—but uses curved mirrors called heliostats, mounted on trackers that shift position with a slight mechanical groan every few seconds. The heliostats direct the sun’s light to a central receiver at the top of the tower. Testing towers have been built in Spain, the United States, and Israel, but the Solúcar PS10 site is the first commercial application of the technology.

TAt PS10, 624 heliostats, 120 square meters each (nearly 1,300 square feet), concentrate solar radiation at the top of a 115-meter tower (about 377 feet). A receiver at the top transfers the heat directly to water, and the pressurized steam reaches 250 ºC.