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Concentrating solar power systems under construction in the Southwest can economically store heat and provide electricity for up to four hours without sunlight, according to a leading solar technology executive who is bullish on their storage potential. Based on that promise, his company, Acciona Solar Power of Spain, already is planning to beef up the storage system at its upcoming 64 MW Nevada Solar One plant near Boulder City. The plant is the first major concentrating solar power plant being built in the world in 15 years, according to Nevada Power, which is under contract to purchase the electricity from the facility. It is expected to begin producing power this fall. Initially, the plant was planned to incorporate a half hour of storage, explained Don Points, Acciona vice president, to a group of Western states’ public utilities commissioners who met on renewable energy in Santa Fe, New Mexico, September 14. Acciona did this by incorporating extra tubing for heat collecting fluid, explained Points. Essentially, the system is to operate by using parabolic mirrors to aim the sun’s rays on a tube that contains synthetic fluid. That so-called “solar fluid” is to heat up to 735 degrees Fahrenheit and then run through a heat exchanger, where steam is to be produced to drive a turbine to generate power. The tubing consists of an external pipe made of clear glass, which is to be put under a vacuum, and an inner stainless steel pipe which is to hold the fluid. The vacuum in the glass tube is to help insulate the stainless steel pipe to maximize the fluid’s energy retention. To increase the plant’s energy storage capacity to four hours, the company plans to add a molten salt system that would use a new solar field of mirrors, according to Points. The fluid in the tubes from those mirrors is to be used to heat molten salt in a secondary heat exchanger. The hot molten salt then is to be stored. When the sun is not shining, but power is still needed from Nevada Solar One, the molten salt then can be dispatched to the secondary heat exchanger to heat the solar fluid. The heated fluid then would run through the plant’s primary heat exchanger to make steam to drive the turbine. At this time, according to Points, four hours is about the maximum storage capacity that is economical in the Southwest with solar trough concentrating plants, although it is technically feasible to store enough solar energy to provide eight hours of power output. He said that the plant typically will be able to produce at peak capacity without tapping storage from about 8 a.m. until 7 p.m. Points added that the facility sometimes is expected to burn a small amount of natural gas to keep the solar fluid from freezing and damaging the system during sustained desert cold snaps that are accompanied by cloudy conditions. Not surprisingly, the plant would not produce much electricity in such weather. Overall, though, both the company and National Renewable Energy Laboratory have high hopes for storing solar energy with molten salt and other fluids. The federal laboratory notes that two other major parabolic mirror solar projects are being planned in the Southwest. These include a 553 MW Solel plant that would sell its power to Pacific Gas & Electric (Circuit, July 27, 2007) and a federal project that could have up to 500 MW of capacity in New Mexico and sell its power to PNM (Public Utilities New Mexico).

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