Torresol Energy, S.A. began operating the world’s first utility-scale concentrating solar power plant that uses flat heliostats and stores heat using molten salt, allowing it to sustain power through the night for around

The 19.9-megawatt Gemasolar concentrating solar power plant uses 2,650 flat mirrors called heliostats arranged over 185 hectares of land to heat molten salt. The heliostats focus sunlight on a tower where liquid is heated up to 900 degrees centigrade. It is then stored for later use at above 500 degrees centigrade in tanks beneath the tower.

Compared with water, salt retains heat longer, thus extending the plant’s daily operating hours. Stored heat is used later to generate steam and run the power generator, even at night. The Gemasolar facility can produce electricity for 15 hours solely using stored heat, the company said.

“This accordingly allows for generation of electricity 24 hours a day for many months of the year, even during the hours of darkness or poor daylight during winter,” Torresol said in a statement. Able to absorb roughly 95 percent of the sun’s heat, the company said Gemasolar can generate electricity for 6,500 hours a year, 1.5 to three times more than other renewable energy power plants.  In total, the power plant can generate roughly 110 gigawatts-hours of energy a year while avoiding more than 30,000 tons of carbon dioxide.

Torresol is a joint venture 60 percent owned by Spain’s Sener Grupo de Ingeniería, S.A. and 40 percent by Masdar, a subsidiary of Abu Dhabi’s Mubadala Development Company. Sener supplied the molten salt storage system and the receiver.

Construction on the project began in the province of Seville in Spain’s sunny Andalucía region in 2008. It was partly financed by a 171 million-euro ($241 million) loan from Banco Popular, Banco Español de Crédito or Banesto, the European Investment Bank and Instituto de Credito Oficial, Spain’s official credit institute. 

Torresol has invested $1.4 billion dollars to build three concentrating solar plants in Spain, including Gemasolar. In 2009, the company began work on the construction of two more solar plants, Valle 1 and Valle 2, in San Jose del Valle in the province of Cadiz.

Unlike Gemasolar, the Valle project is comprised of two adjacent 50-MW solar power plants using rows of curved parabolic or curved mirrors that focus sunlight onto a heat collector tube directly in front of it. The collector is filled with synthetic oil.

Several concentrating plants already operate around the world using parabolic-trough technology, mainly in the United States and Spain. However, older designs can only operate at daytime – when sunlight is available – an issue dealt with in recent years by introducing heat storage using molten salts.

Torresol fitted the Valle solar power plants with a separate molten salt heat storage system that absorbs heat from the oil using a machine called a heat exchanger. This enables the power plants to generate power for seven hours even without sunlight, similar with Gemasolar.

In January last year, the company secured loans totaling $760 million to finance the construction of its twin solar plants. Torresol said the total cost of the project is $1 billion dollars.

Santa Monica, California-based SolarReserve is currently building what could be the world’s largest concentrating solar power plant using molten salt. The 110-megawatt Crescent Dunes Solar Energy Project is similar to Torresol’s Gemasolar design of using molten salts to transfer and store heat.

The United States Department of Energy offered a conditional loan guarantee of $737 million to SolarReserve last week to finance the plant’s construction in Tonopah, Nevada.

Molten salt offers a number of advantages over oil used in rival parabolic trough solar power plants, according to Solar Power and Chemical Energy Systems, a think-tank on concentrating solar power managed by the International Energy Agency.

Molten salt can operate at higher temperatures than oils – above 500 degree Celsius instead of around 400 degrees Celsius – therefore increasing efficiency and power output of a plant. In addition, using oil requires additional facilities to transfer the energy from the oil to heat storage, which increases capital costs.

“This causes a loss in cycle efficiency of up to 7 percent,” says Julie Way, development director at SolarReserve. However, molten salt does have some drawbacks. Special materials are required to avoid salt corrosion which could mean more construction and maintenance costs. In addition, salt has a higher freezing point than other heat transfer fluids, which means that pipes must be kept warm to keep the salt molten.

Oliver M. Bayani,