A $1.45 billion loan guarantee has been finalized for Abengoa Solar Inc.’s Solana project, the world’s largest parabolic trough concentrating solar plant. The announcement was made by U.S. Department of Energy Secretary Stephen Chu. Located near Gila Bend, Arizona, the 250-megawatt (MW) project is the first large-scale solar plant in the United States capable of storing energy it generates. Large insulated tanks filled with molten salt will be used with concentrating solar power (CSP) to store the heat. This stored heat can then be used to produce energy during periods of low or no sun, including the evening hours. Solana will produce enough energy to serve 70,000 households and will avoid the emissions of 475,000 tons of carbon dioxide per year compared to a natural gas burning power plant.
Molten salt is used in solar power tower systems 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. In addition, molten salt is used in the chemical and metals industries as a heat-transport fluid, so experience with molten-salt systems exists for non-solar applications.
Generally speaking (according to a report from Sandia) the molten salt is a mixture of 60 percent sodium nitrate and 40 percent potassium-nitrate, commonly called saltpeter. The salt melts at 430°F and is kept liquid at 550°F in an insulated cold storage tank. The salt is them pumped to the top of the tower, where concentrated sunlight heats it in a receiver to 1050°F. The receiver is a series of thin-walled stainless steel tubes. The heated salt then flow back down to a second insulated hot storage tank. The size of this tank depends on the requirements of the utility; tanks can be designed with enough capacity to power a turbine from two to twelve hours. When electricity is needed from the plant, the hot salt is pumped to a conventional steam-generating system to produce superheated steam for a turbine/generator.
The uniqueness of this solar system is in de-coupling the collection of solar energy from producing power, electricity can be generated in periods of inclement weather or even at night using the stored thermal energy in the hot salt tank. The tanks are well insulated and can store energy for up to a week. As an example of their size, tanks that provide enough thermal storage to power a 100-megawatt turbine for four hours would be about 30 feet tall and 80 feet in diameter. Studies show that the two-tank storage system could have an annual efficiency of about 99 percent.
Abengoa Solar Inc., the project sponsor, estimates that the Solana project will create between 1,600 to 1,700 new construction jobs and over 60 permanent jobs. The jobs created by the project will be located in Arizona and in neighboring states. To accommodate the project’s need for over 900,000 mirrors, a mirror manufacturing facility will be built outside of Phoenix.
U.S. providers and manufacturers will supply 70 percent of Solana’s components, such as mirrors, receiver tubes, and the heat transfer fluid. Electricity from the project will be sold through a long-term power purchase agreement with Arizona Public Service Company.
“The finalization of the DOE loan guarantee is a major milestone for both Abengoa and Arizona Public Service, which will purchase the power output of the plant,” said Julia Hamm, President and CEO of the Solar Electric Power Association. “The addition of 250MW of solar electric capacity to the APS portfolio will be a great thing for the company’s customers as well as the whole state of Arizona,” said Hamm.
“Storage technologies can play a crucial role in modernizing our nation’s electric grid, including increasing the value of solar electricity to the purchasing utility. The storage capacity of the Solana plant will allow it to provide power to the grid during times of passing cloud cover and help serve APS’s peak demand by providing power even after the sun goes down,” Hamm added.
Abengoa Solar is currently building the largest solar platform in Europe. Located just outside of Sanlúcar la Mayor, Seville, this solar thermal and photovoltaic installation complex will have a nominal power output capacity of 300MW, obtained through tower technology, parabolic trough collectors, and photovoltaic technology. The photo above shows the copany's PS10 and PS20 solar plants there which incorporates thermal storage that allows full production for 30 minutes, even after the sun goes down.
For more information on Energy Storage, see www.energystoragetrends.com.
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