Dec 22 2009

Solar Power--With a Pinch of Salt

Posted by: Jonathan Marshall

Solar energy is great--clean, renewable and reliable--except when the sun goes down. Too bad that happens every day, for hours at a time.

Now a handful of companies in the solar industry are using a high-tech version of thermal storage to stabilize the output of their power plants when clouds pass overhead or the sun goes down.

PG&E today disclosed that it has contracted with a subsidiary of Santa Monica-based SolarReserve, LLC for 150 megawatts of clean solar power, augmented by a proven energy storage system based on molten salt. The proposed Rice Solar Energy project is sized to produce as much renewable energy as consumed by more than 60,000 average homes, starting in 2013.

If approved by state regulators, SolarReserve's project will be located at the site of the World War II-era Rice Airfield, near the unincorporated community of Rice in San Bernardino County, Calif.

The project will use thousands of large, movable mirrors to focus the sun's rays onto a receiver in a central tower to heat four million gallons of molten salt to more than 1,000 degrees Fahrenheit. The hot, liquid salt will flow into a storage tank and used to heat water for a steam generator to produce electricity. The stored molten salt can also provide energy during periods when sunlight dims or is not available.

The ability to store heat and tap it at any time for power generation is thus like having a huge battery or backup generator on hand to smooth out peaks and valleys of solar generation.

The storage technology was demonstrated successfully over several years in the 1990s at the Department of Energy-sponsored Solar Two power plant in Southern California.The owner of some of the patents, United Technologies, licensed the process to SolarReserve.

Molten salt--28,500 tons of it--is currently being used for thermal storage at the 50 MW Andasol 1 solar thermal power plant in Spain, the first of three sister plants designed by Solar Millennium. Abengoa Solar has a molten salt demonstration project and plans to use the technology in a 280 MW solar thermal power project in Arizona. Other companies that have expressed an interest in molten salt thermal storage include the Spanish company Sener and SkyFuel, based in Albuquerque.

Sandia National Laboratory in Albuquerque, home of the National Solar Thermal Test Facility, says molten salt is ideal for capturing solar energy in power towers "because it is liquid at atmosphere pressure, it provides an efficient, low-cost medium in which to store thermal energy, its operating temperatures are compatible with todays high-pressure and high-temperature steam turbines, and it is non-flammable and nontoxic."

Best of all, a well-insulated storage tank for molten salt can be 99 percent efficient, so it loses heat only very gradually.

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