An alternative to the direct conversion of sunlight into electricity is to concentrate the sun to produce heat. CSP plants use mirrors to concentrate the sun, similar to the way that kids take a magnifying glass to concentrate the sun to burn wood—or other things! These are utility-scale thermal plants that in principle are no differ­ent from a coal-fired plant or a nuclear plant, but they use the concentrated heat of the sun to produce high-pressure steam, which drives a turbine and a generator to produce electricity. Because of this, they use large amounts of water—twice as much per unit of electricity produced as coal-fired power plants (12)—which can be a severe problem since the best locations for solar power are in deserts. Several variations exist to concentrate the sun.

Parabolic trough reflectors use expensive parabolic mirrors 300 to 450 feet long and 15 to 20 feet tall to concentrate the sun onto a receiver tube that runs down the middle carrying a high-temperature heat transfer fluid that heats up to 700°F. The fluid then runs through a heat exchanger to produce steam from water. Many parallel rows of the parabolic mirrors constitute a power plant. A less expensive variation known as Compact Linear Fresnel Reflector uses long rows of flat mir­rors to focus the sun on tubes containing water to produce steam directly (13, 14). The world’s largest parabolic trough solar power plant, with a capacity of 350 MWac, has been running for more than 20 years in the Mojave Desert at Kramer Junction, California. A 64 MWac parabolic trough plant was built in Nevada in 2007 (15). By far the world’s largest parabolic trough plant—two phases of 500 MW each—was planned at Blythe, California, but the high cost of these plants led its developer to convert it to a PV plant (16).

A solar dish uses concave mirrors of about 40 feet in diameter to focus the sun on a small area, resulting in a very high concentration of the sun—typically by a factor of over 2,000—to heat a working fluid to about 1300°F. These systems have a Stirling engine that converts the working fluid to mechanical energy, which then drives a generator. Each dish generates 5-30 kW, depending on the system (14). This technology is lagging behind the other CSP approaches because of the high cost of building the dishes. At the time of writing, there are no large-scale Dish Stirling projects in operation and few, if any, in the pipeline (17). This technology is not ready for prime time.

The most efficient CSP plants use a circular series of flat mirrors that follow the sun and focus its energy on a central tower—the Power Tower. A transfer fluid is heated to temperatures of 800-1000°F, which then produces steam to drive a turbine (14). There is only one operating power tower in the United States—in Antelope Valley, California—that has a 5 MW capacity. Two experimental 10 MW power towers were operated by Sandia National Laboratory in the Mojave Desert for several years before being closed down (18). Spain has been the world leader in solar tower technology, due to a feed-in tariff system similar to that in Germany. At least Spain has good solar resources, similar to Florida (see Table 4.1). The Gemasolar plant near Seville, Spain, has a capacity of17 MW. It uses thermal stor­age to even out the variation in the sun’s intensity and to generate power even after the sun has set. The concentrated solar energy heats a molten salt to a temperature of 565°C (1050°F), which then goes through a heat exchanger to turn water to steam to drive a turbine and generator. The hot salt can store the heat for up to 16 hours (19).

The United States will soon take the lead in concentrated solar power, though, doubling the global capacity in a single plant—Ivanpah—at the edge of the Mojave Desert in California about 40 miles southwest of Las Vegas. The world’s largest solar power tower is scheduled to be finished in 2013 and will provide a nominal capacity of 377 MW. It is huge. It has three 459-foot tall towers surrounded by about 3,500 acres of reflecting mirrors (heliostats) that follow the sun. The mirrors focus the sun on the solar receivers at the top of the towers, where it boils water to make steam to drive a turbine to make electricity (20, 21). It uses air cooling to condense the steam, making it 95% more efficient than wet-cooled solar thermal plants (21). Its overall efficiency for electricity production is about 29%, much higher than PV electricity production (22).

Another large solar power tower project is scheduled for completion in 2013— the Crescent Dunes Solar Energy Project near Tonopah, Nevada. It is rated at 110 MW and covers 1,600 acres of desert land. The 540 foot tall central tower is more than a hundred feet taller than the towers at Ivanpah. It uses molten salt as the heat transfer fluid, similar to the Gemasolar plant in Spain. This will allow it to store solar energy for up to 10 hours (23).