CATCH THE SUN

How to Use Solar Energy at Night; Molten salts can store the sun's heat during the day and provide power at night
David Biello, February 18, 2009 (Scientific American)

“…[M]ore than 28,000 metric tons of salt is now coursing through pipes at the Andasol 1 power plant. That salt will be used to solve a pressing if obvious problem for solar power: What do you do when the sun is not shining and at night?

”The answer: store sunlight as heat energy for such a rainy day…Part of a so-called parabolic trough solar-thermal power plant…Because most salts only melt at high temperatures…and do not turn to vapor until they get considerably hotter—they can be used to store a lot of the sun's energy as heat…use the sunlight to heat up the salts and put those molten salts in proximity to water via a heat exchanger. Hot steam can then be made to turn turbines without losing too much of the original absorbed solar energy."

storage schematic (click to enlarge)

”The salts—a mixture of sodium and potassium nitrate, otherwise used as fertilizers—allow enough of the sun's heat to be stored that the power plant can pump out electricity for nearly eight hours after the sun starts to set…Using mirrors to concentrate the sun's energy is an old trick—the ancient Chinese and Greeks both used it to start fires—and modern power plants employing it might provide a significant source of renewable energy without any greenhouse gas emissions…engineers have tried a number of different technologies to store the sun's energy…

“The Andasol 1 power plant, which cost around $380 million (300 million euros) to build, is the first to actually use the technology, so it remains to be seen how it will work in commercial practice. But U.S. government laboratories—NREL as well as Sandia National Laboratory in Albuquerque, N.M.—have already proved the technology can work in demonstration projects that employed it, like the Solar Two power tower outside Barstow, Calif…

“Solar Millennium is so confident the technology will work that a twin solar-thermal power plant (Andasol 2) is already near completion…And Arizona Public Service Co. (APS) has contracted with Abengoa Solar to build a 280-megawatt solar thermal power plant—dubbed Solana or "sunny place"—70 miles (110 kilometers) southwest of Phoenix…thermal energy storage at Andasol 1 or power plants like it costs roughly $50 per kilowatt-hour to install..Electricity from a solar-thermal power plant costs roughly 13 cents a kilowatt-hour…"

solar power tower (click to enlarge)

“As efficient as solar-thermal power plants using parabolic troughs with molten salt storage systems like Andasol 1 or Solana are, they don’t capture as much of the sun's heat as is possible…To allow the salts to get hotter, some companies, such as SolarReserve in Santa Monica, Calif., are developing so-called power towers—vast fields of mirrors that concentrate sunlight onto a central tower. Because of the centralized design such a structure can operate at much higher temperatures—up to 1,000 degrees F (535 degrees C)—and use molten salts directly as the fluid transferring heat in the power plant…But such a power plant…would cost as much as $800 million for a 200-megawatt power tower.

“…researchers are also looking into salts that could be used instead of the oil in parabolic trough power plants…long-term research projects are looking at other thermal storage technologies, such as storing heat in sand or creating single-tank molten salt storage…Ultimately, it will come down to how much value policymakers and consumers put on electricity that is renewable and emissions-free.