The heliostats concentrate solar radiation so effectively that temperatures on the surface of the receiver tubes could potentially reach 3,600 degrees Fahrenheit, about one-third of the sun’s surface temperature. “Unfortunately, the steel would melt and the receiver would break down,” Fernández said dryly. Ceramic materials could perhaps withstand such heat, he went on, but they haven’t yet been developed and tested. So on bright days plant operators adjust the heliostats to limit the sunlight directed at the towers and maintain the temperature around 570 degrees—plenty hot to make steam out of water, which boils at 212 degrees.
As Fernández drove me around Solúcar’s campus, we stopped at the construction site for Solnova 1, the first of five planned CSP facilities here to use so-called parabolic trough technology. Solnova 1 will deploy mirrors shaped like shallow troughs, each several hundred yards long and about two yards high, in parallel rows like ranks of immense shiny ribbons. At the focal point of each mirror will run a transparent tube filled with a synthetic oil. The mirrors will direct sunlight to the tubes, and the heated oil will be used to boil water for turbine-driving steam.
Leaving Solnova 1, I caught sight of a vast array of photovoltaic panels. Although the panels supply around three megawatts of electricity—enough to power 2,200 households—Solúcar has not emphasized photovoltaics. “Photovoltaics are not leading to a technology that can store energy except in batteries,” Fernández said. That is too costly.
A more efficient storage system is on display at Solúcar’s TES (for Thermal Energy Storage) facility, which uses concentrated solar power to heat huge containers of molten salts, chiefly sodium and potassium nitrates, to some 570 degrees. The salts retain heat for up to six hours, meaning that it can be released after the sun goes down. This storage method could solve, at least partially, solar energy’s toughest challenge: providing power at night.
Spain’s booming solar capacity has depended on hefty government support. (CSP plants, for example, cost about twice as much to build and operate as conventional coal-fired plants.) The Spanish government’s subsidy to solar energy providers—among the world’s most generous—is running to more than €1.5 billion, or almost $1.9 billion, a year.
In the United States, federal and state governments are providing smaller financial incentives to individuals, solar power firms and utilities. Parabolic trough facilities in California and Nevada already generate more than 370 megawatts, and Abengoa is planning a 282-megawatt plant using similar technology near Phoenix, said Reese Tisdale, solar research director for Emerging Energy Research in Cambridge, Massachusetts. New parabolic trough, photovoltaic and CSP plants are proposed in California, Nevada, New Mexico, Colorado, North Carolina and Florida.
One massive solar farm, using towers and heliostats and promising to generate 392 megawatts of power, is slated for construction in, yes, the Mojave Desert in Southern California, if it passes state and federal environmental reviews.
“If you want to be optimistic,” Tisdale said, “there could be as much as eight gigawatts supplied by solar power plants [in the United States] by 2025.” That would be enough to power a U.S. city of six million (Americans use more electricity than Spaniards) and save 37,260 barrels of oil or 11,000 tons of coal a day.
Richard Covington writes from his home near Paris.