radiation at 600 times its normal intensity. The towers generate temperatures in excess of 1800 °F, thus enhancing the efficiency by which mirrors that gather sunshinesare the Think of solar energy and you’ll probthe plants produce steam turbine oldest and best established. Nine ably visualize a photovoltaic (PV) panel electricity, says Blair. About 900 MW parabolic-trough plants, collectively on a rooftop supplying power when of power tower CSP plants are now dubbed the Solar Energy Generating the sun shines. But for many people, in the planning stages, NREL estiSystems (SEGS), were built in Calisolar power means mirrors spread mates show. over thousands of acres in Parabolic troughs and power the desert. Deployed by towers are both amenable to concentrated solar power thermal storage. Instead of dissi(CSP) plants, these reflective pating excess heat into the air, surfaces focus heat from the the plants store it in tanks of sun to create steam that, in molten salt. During the evening, turn, generates electricity by or when clouds obscure the sky, powering turbine engines. that heat can be released to run CSP plants dump solar the steam turbine for 6 h or power priced at roughly more, depending on the electric15¢ per kilowatt-hours ity demand. comparable in cost to coal Linear Fresnel and parabolic and nuclear powersdirectly dishes are two newer CSP catinto the electricity grid, thus egories. Although the potentially sparing consumers the exinexpensive designs of the linear pense of a PV system that Fresnel type have yet to be tested can cost tens of thousands Many areas across the southwestern U.S. are already geton a large scale, about 1600 MW of dollars per building. By ting their electricity by using concentrated solar technology. of “dish-engine” CSP plants are storing solar heat in tanks now slated for development in filled with molten salt, CSP California, according to NREL. fornia’s Mojave Desert during the plants generate electricity for up to 6 Proposed CSP projects in the U.S. 1980s. These plants deploy more hours (h) even when the sun isn’t are ready to go, technically. But pothan 1 million parabolic mirrors, arshining. litical bickering over a proposed 8 ranged in half-pipe rows and spread Nearly 300,000 homes in the sunyear extension to the federal solar over 1600 acres. Sunshine bounces drenched U.S. Southwest already get investment tax credit has so far kept off the mirrors toward a central tube their electricity from CSP, and the these projects in limbo. According to filled with synthetic oil and heats the numbers are set to rise. Federal Carol Werner, executive director of oil to about 600 °F. The heated oil funding for CSP R&D grew from $2 the Environmental and Energy Study flows into a heat exchange system, a million in 2005 to $30 million in Institute, a Washington, D.C., think device that transfers heat from the 2008, according to George Douglas, tank, investors won’t back CSP deoil to water, which is separated from spokesperson for the U.S. Departvelopment without being assured the oil by a solid surface. The resultment of Energy’s (DOE’s) National that a tax creditsworth up to 30% of ant steam runs a turbine cycle to Renewable Energy Laboratory production costswill be available create electricity that feeds directly (NREL). What’s more, 4500 megaonce plant construction is cominto the local power grid. According watts (MW) of CSPsenough to suppleted; that can take up to 6 years. to NREL, trough plants that will yield ply more than 3.5 million homes “We keep lurching around on the 1100 MW of power are now planned with emissions-free powersare now proposed extension, and that makes for U.S. development. planned for development in Califorit hard to put financing together,” “Power tower” plants make up a nia, Florida, Hawaii, and Arizona. Werner says. Meanwhile, CSP develsecond CSP category. Unlike trough “Under the right scenarios, CSP opment is advancing in Spain, Portudesigns that run parallel to the could meet a significant portion of gal, and Australia, where renewableground, power towers focus light today’s energy needs,” says Nate energy tax credits have a longer onto an oil-filled receiver, which is Blair, a senior energy analyst with timeline. located up to 80 meters above the NREL. land surface. An array of flat, movCSP plants come in four design —CHARLES SCHMIDT able mirrors (known as heliostats) categories. Of these, parabolic surrounds the tower, reflecting solar troughsscurved troughs made of NREL
Sunlight on mirrors
10.1021/es802114k
2008 American Chemical Society
Published on Web 08/13/2008
October 1, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 9 7031
