Molten Salt Makes Solar Thermal Energy Available 24×7
Wednesday, April 29, 2009
In the high desert of southern Spain, not far from Granada, the Mediterranean sun bounces off large arrays of precisely curved mirrors that cover an area as large as 70 soccer fields. These parabolic troughs follow the arc of the sun as it moves across the sky, concentrating the sun’s rays onto pipes filled with a synthetic oil that can be heated to 750 degrees Fahrenheit. That super-heated oil is used to boil water to power steam turbines, or to pump excess heat into vats of salts, turning them a molten, lava-like consistency. The salts are just fertilizers — a mix of sodium and potassium nitrate — but they represent a significant advance in the decades-old technology of solar thermal power production, which has traditionally used mirrors to heat water or oil to generate electricity-producing steam. Now, engineers can use the molten salts to store the heat from solar radiation many hours after the sun goes down and then release it at will to drive turbines. That means solar thermal power can be used to generate electricity nearly round-the-clock…..Some of the recent claims for solar thermal power have been stunning. Researchers at the German Aerospace Center have estimated that 16,000 square kilometers of solar thermal power plants in North Africa — paired with a new infrastructure of high-voltage, direct-current transmission lines — could provide enough electricity for all of Europe. And scientists have estimated that constructing solar thermal power plants on less than 1% of the world’s deserts — an area roughly the size of Austria — could meet the entire world’s energy needs…..The case for solar thermal power hinges on economics. The sun bathes the Earth with an average of 6 kilowatt-hours of power per square meter over the course of a day, and a concentrated solar power plant like Andasol is the cheapest way to harvest a portion of that. Photovoltaics — semiconductor panels that convert sunlight to electricity — deliver power at roughly 40 cents per kilowatt-hour, while conventional solar thermal power plants can do so for around 13 cents per kilowatt hour, according to the U.S. National Renewable Energy Laboratory. This is only marginally more expensive than the average U.S. price for coal-generated electricity in 2008 of 11 cents per kilowatt hour. The cutting-edge technology of using molten salts to store solar-generated heat is considerably more expensive, but experts expect that price to fall steadily as the technology improves and is mass-produced. Roughly 612,000 megawatt-hours of electricity from the sun were produced in 2007, according to the Energy Information Administration (EIA), and solar thermal collectors sufficient to cover more than 15 million square feet were shipped and ready for installation that year — more than double the amount in 1998. In the United States, some 3,100 megawatts of solar thermal power are planned by 2012, and capacity worldwide is expected to reach 6,400 megawatts within 3 years — roughly 14 times the current amount. Still, electricity from the sun contributes just 1% of the renewable energy generated in the U.S., and all renewables taken together only provide 7% of U.S. energy needs.
…..The Andasol power plant uses more than 28,000 metric tons of sodium and potassium nitrates to store some of the sun’s heat for use at night or on a rainy day. The molten salts are stored in enormous hot and cold vats, able to be employed on command to soak up extra heat or drive the generation of electricity. “The turbine is running more hours every day because we have storage and we have the possibility to plan our electricity production,” said Sven Moormann, a spokesman for Solar Millennium, the German company building Andasol…..But molten salts don’t have to be just used for storage, as they are at Andasol and will be at Solana. They can also be used directly as a fluid in solar thermal power plants that operate at a much higher temperature, replacing the synthetic oil or water used in power towers. In this variation of the solar thermal technology, large fields of mirrors concentrate the sun’s heat on a central tower that glows with intense light. Such plants operate at more than 1,000 degrees Fahrenheit — closer to the temperatures employed at a coal-fired power plant — and therefore can use the salts directly as a heating medium. At night, when temperatures begin to drop, the cooling salts that have already transferred their heat to drive a turbine simply drain to the bottom of the tower, where they are stored in tanks, ready to be heated again the next sunny day…..In addition, there is another way to use this technology for capturing the sun’s heat — cleaning up existing fossil fuel-fired power plants or other operations that burn a lot of CO2-emitting fossil fuels. By employing the mirrors of a solar thermal array to pre-heat steam, the amount of natural gas, oil, or coal that must be burned can be reduced…..Ausra argues that it can generate the same steam without any CO2 emissions by employing its solar thermal technology…..“People need to look at this as a hedge against fossil fuel prices,” says Murphy. “You could start deploying a new type of power plant. We used to burn coal and natural gas — now we can use the sun to make steam.”
Reference : http://e360.yale.edu/content/feature.msp?id=2144