The most obvious criticism of solar energy is that it doesn't work very well when the sun is down. The new Gemasolar heliostatic plant doesn't have that problem, on account of a vat of molten salt that keeps it running through 15 hours of dark.
More than 2,600 concentrically-arranged mirrors at the Gemasolar installation just outside of Seville, Spain concentrate solar energy towards a centrally located molten nitrate salt tank. As the rays converge, they super-heat the salt to over 900 degrees Celsius, causing water around the tank to boil and drive steam turbines. In addition, any superfluous heat generated during the day is stored within the liquefied salt. It acts like a giant thermal battery for driving the turbines at night and during overcast days — up to 15 hours at a time with no sunlight. Seville, Spain, however, is one of the sunniest areas in Europe, so that doesn't happen very often.
The £200 million Gemasolar plant just opened and has a potential output of 20 megawatts, though it is currently operating below that capacity (officials expect it could reach 70 percent capacity by 2012). It's the largest solar power station of its type in Europe, and it has an annual production total or roughly 110 GWh/year — enough to power 25,000 homes and reduce atmospheric CO2 emissions by more than 30,000 tons a year.
The combination of thermal energy storage and sunny weather guarantees that the Gemasolar plant can operate for at least 6,500 hours a year, up to three times longer than other renewable sources.
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