A technology that uses mirrors to concentrate solar power could provide millions of homes with clean energy.

Think of solar energy and you’ll probably visualize a photovoltaic (PV) panel on a rooftop supplying power when the sun shines. But for many people, solar power means mirrors spread over thousands of acres in the desert. Deployed by concentrated solar power (CSP) plants, these reflective surfaces focus heat from the sun to create steam that, in turn, generates electricity by powering turbine engines.

thus sparing consumers the expense of a PV system that can cost tens of thousands of dollars per building. By storing solar heat in tanks filled with molten salt, CSP plants generate electricity for up to 6 hours even when the sun isn’t shining.

Nearly 300,000 homes in the sun-drenched U.S. Southwest already get their electricity from CSP, and the numbers are set to rise. Federal funding for CSP R&D grew from $2 million in 2005 to $30 million in 2008, according to George Douglas, spokesperson for the U.S. Department of Energy’s (DOE’s) National Renewable Energy Laboratory (NREL). What’s more, 4500 megawatts (MW) of CSP-enough to supply more than 3.5 million homes with emissions-free power-are now planned for development in California, Florida, Hawaii, and Arizona. “Under the right scenarios, CSP could meet a significant portion of today’s energy needs,” says Nate Blair, a senior energy analyst with NREL.

CSP plants come in four design categories. Of these, parabolic troughs-curved troughs made of mirrors that gather sunshine-are the oldest and best established. Nine parabolic-trough plants, collectively dubbed the Solar Energy Generating Systems (SEGS), were built in California’s Mojave Desert during the 1980s. These plants deploy more than 1 million parabolic mirrors, arranged in half-pipe rows and spread over 1600 acres. Sunshine bounces off the mirrors toward a central tube filled with synthetic oil and heats the oil to about 600 °F. The heated oil flows into a heat exchange system, a device that transfers heat from the oil to water, which is separated from the oil by a solid surface. The resultant steam runs a turbine cycle to create electricity that feeds directly into the local power grid. According to NREL, trough plants that will yield 1100 MW of power are now planned for U.S. development.

“Power tower” plants make up a second CSP category. Unlike trough designs that run parallel to the ground, power towers focus light onto an oil-filled receiver, which is located up to 80 meters above the land surface. An array of flat, movable mirrors (known as heliostats) surrounds the tower, reflecting solar radiation at 600 times its normal intensity…
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