Molten salt thermal energy storage, the technology that extends a concentrated solar power station’s daily operating life by up to six hours, is gathering momentum in the United States.

Molten salt thermal energy storage, the technology that extends a concentrated solar power station’s daily operating life by up to six hours, is gathering momentum in the United States with the likely completion of a second utility-scale power plant later this year.

The first large-scale concentrated solar power (CSP) plant to use molten salt storage, the $US2 billion Solana 280 MW parabolic trough plant near Gila Bend in Arizona, successfully passed commercial operation tests last October in what the project developer, Abengoa of Spain, called a “major accomplishment” for the concentrating solar power (CSP) industry.

Arizona’s largest utility, Arizona Public Service, will buy all of Solana’s electricity output under a 30-year power purchase agreement with Abengoa.

Solana’s thermal storage system, which stores heat generated during the day in tanks of molten salt and uses this at night to drive steam-powered turbines, can produce energy for six hours at maximum power after the sun goes down.

Abengoa is not involved in the second project, known as Crescent Dunes, which is a 110 MW solar thermal tower plant with integrated molten salt storage at Tonopah in Nevada.

The developer, California-based renewable energy company SolarReserve, announced on February 12 that it had entered the plant commissioning stage and said full commercial operation was scheduled for “later in 2014.”

When completed, Crescent Dunes will produce 500,000 megawatt-hours annually of electricity, enough to power 75,000 homes during peak demand periods.

Las Vegas-based power utility NV Energy will take all of the Crescent Dunes output under a 25-year power purchase agreement with SolarReserve.

According to SolarReserve, when Crescent Dunes becomes operational it will be the world’s largest solar thermal plant with “fully integrated” energy storage. Work began on the plant in September 2011.

“SolarReserve’s industry-leading solar thermal energy storage technology solves the intermittency issue that limits the use of other renewable energy projects, company CEO Kevin Smith said in a February 12 statement.

Whatever the merits of competing claims about the biggest or best storage technology, Spain’s Abengoa is regarded as an industry leader in molten salt technology, having set up a pilot plant in January 2009 at its Solucar facility — Europe’s largest solar power complex — near Seville in January 2009.

Last month Abengoa announced it had won the tender to build a 110 MW solar power plant with molten salt storage in Chile’s Atacama Desert for the country’s Ministry of Energy and the Chilean economic development agency Corfo. It will be the first solar-thermal plant for direct electricity production in South America. Work on the plant is expected to start in the second half of this year, and according to Abengoa it will have 17.5 hours of storage capacity.

The Atacama Desert has the highest solar radiation concentration in the world and is already known as a major lithium-producing areas through salt-pan evaporation.

In a statement last month, Abengoa said the power plant in Chile would use solar-thermal tower technology similar to the Solana project.

“A series of mirrors (heliostats) will track the sun on two axes, concentrating the solar radiation on a receiver on the upper part of the tower where the heat is transferred to the molten salts. The salts then transfer their heat in a heat exchanger to a water current to generate superheated and reheated steam, which feeds a turbine capable of generating around 110 MW of power,” it said.

Like Abengoa’s Solana, the Crescent Dunes project in Nevada has a loan guarantee from the US Department of Energy. Private funding comes from SolarReserve and its partners, engineering and construction company ACS Cobra and the equity capital arm of Spanish banking group Santander.

SolarReserve also hopes to start construction later this year on a 150 MW concentrating solar power plant with molten salt storage at the former Rice army airfield in Riverside County, California.

The technology to heat molten salt was first demonstrated in the 1980s in France and the United States, but it is only in recent years that it has been used commercially. There are at least five 50 MW solar power plants using molten salt storage in Spain and another half dozen or more under construction there.

However the world’s biggest operating solar thermal power facility, the 392 MW Ivanpah “power tower” plant in California developed jointly by BrightSource Energy, NRG Energy and Google, does not have a storage system. NRG announced in mid-February that the plant had reached commercial operation at the end of 2013. At full capacity its three 140-metre towers will generate enough electricity to power 140,000 Californian homes. Ivanpah uses 173,500 heliostats that follow the sun’s trajectory, aided by solar field integration software and a solar receiver steam generator.

BrightSource has said previously it will look at using molten salt storage for future projects.

Geoff Hiscock writes on international business and is the author of “Earth Wars: The Battle for Global Resources,” published by Wiley.