Sunday, November 27, 2011

Solar power that flows all night

By Jamie Condliffe

It is the engineering endeavour that could finally make solar-power production a 24/7 activity

Just outside Seville in Spain, a tower the size of a modest skyscraper stands proud amid a sea of mirrors, sprawling over 185 hectares. These adjustable mirrors, called heliostats, focus the sun's rays on a central receiver atop the tower, lighting it up like a beacon. But unlike conventional solar technology, this plant produces power all night long.

This is Gemasolar, the world's first commercial-scale solar power plant to keep its energy flowing for 24 hours. It stores solar energy by heating up molten salts. This heat is then converted into electricity in the daytime and during the hours of darkness.

"In the past, prototype plants have been designed to generate electricity when the sun is shining," says Greg Glatzmaier, senior engineer at the National Renewable Energy Laboratory in Golden, Colorado. "But Gemasolar uses large storage tanks [for molten salt], and a very large heliostat field, so when the sun is shining it collects enough solar energy to create electricity after the sun's gone down."

Concentrated power
The huge array of 2650 heliostats reflects sunlight onto a receiver perched on the tower. Its energy is transferred to the salts – a mixture of sodium and potassium nitrates – which circulate in the receiver. This concentrated solar power raises the temperature of the salts to 500 ºC, which in turn is used to raise steam to drive turbines. The plant boasts a maximum output of 20 megawatts.

The super-hot salt is stored in insulated tanks, providing capacity to produce electricity for up to 15 hours without sunlight. During a sunny spell in June 2011, the plant's owners Torresol Energy announced it had achieved the holy grail of solar energy production: generating electricity for 24 hours straight.

The plant had only been operating for a month at that point, and the eventual goal is to supply uninterrupted electricity throughout the summer. Over a whole year Gemasolar aims to produce the equivalent of 6400 full-capacity hours, says Torresol Energy's Santiago Arias Alonso. That's 73 per cent of a full year, with the gap accounted for by reduced sunlight in winter. "Our expected output will be more than 100 gigawatt-hours a year," he says. That's enough to provide 25,000 homes with electricity.

 Sunny outlook
The best locations for these plants are hot, sun-baked landscapes where land-use is limited. Better, by using dry cooling to lower the temperature of the turbine exhaust, similar plants can be built in areas with little water.
Energy from the plant does not come cheap: it is costly compared with burning fossil fuels. Yet experts expect future plants to be competitive. "The cost of electricity generated from a plant like Gemasolar, if it were built in southwest US, would probably be in the range of 13 to 16 cents per kilowatt-hour," says Glatzmaier. That's thanks to the sunnier weather in the region, and government tax incentives.

Trailblazing technology
Gemasolar has set a precedent that others are following. "We have a [similar] plant under construction right now inNevada, which will come online December 2013," says Bill Gould ofSolarReserve. "That plant has a 120-megawatt capacity – the equivalent of 200,000, maybe 300,000 homes," he says.

Electricity from that site would be close to competitive with fossil-fuel power, which costs between 6 and 10 cents per kilowatt-hour. Such prices would also bring it into line with other renewables, such as photovoltaics.
The cost of photovoltaics is set to drop over the next few years, so why not just cover the land with solar cells instead? "A lot of people are asking that," says Glatzmaier. But once again the capacity for energy storage wins out. "You can build plants that generate electricity 24 hours a day," he says. "And there's no equivalent to that with photovoltaics."