Molten salts are able to retain much of the energy they absorb and give back 93% of that energy to generate electricity.
Making days last forever and stopping the sun from setting have long been romantic ideals reserved for poetic ballads. Not anymore. In the world of clean technology, particularly solar energy, emerging expertise is ensuring that the sun’s munificence continues through the night.
Solar energy is one of the most conventional sources of renewable power and has been used since ancient times. But though convenient, solar energy’s biggest drawback has been that its availability depended on the whims of the sun. Storing solar energy has been a tricky endeavor so far. Engineers have attempted to pack solar energy into batteries for a rainy day, but that has proved too expensive and technologically inefficient.
Then, another more efficient system, known as Concentrated Solar Power (CSP), was developed. Here, sprawling parabolic mirrors are used to direct the brilliant rays of the sun, enough to heat up a special synthetic oil. Known as the solar thermal method, the heat from the oil is transferred to water placed in overhead troughs, and the resultant steam powers a turbine that generates electricity. The same oil, which is a heat transfer medium, stores its heat for later use. But there’s a downside; the oil still fails to capture the sun’s heat entirely. After reaching a peak of about 350 degrees Celsius, the oil breaks down and loses heat.
To combat that problem, large scale solar plants have begun to replace the oil with molten salt, which is much more heat resistant. Molten salts, comprised of potassium and sodium nitrates, can withstand temperatures up to 550 degrees Celsius, which makes them hardier than the synthetic oil.
Both the solar thermal system and the usage of molten salts are ideas that have been around for two decades. The U.S. Department of Energy (DOE) developed these concepts as far back as the early 1980s as part of a test program. Named Solar One, the pilot solar thermal plant stored heat in a tank filled with rocks and sand, which was used in the night. Later in 1995, a subsequent test program, Solar Two, was redesigned to include the molten salt system. The plant was decommissioned in 1999 after successfully demonstrating the efficiency of molten salt to produce energy.
Spain is the current leader of CSP with more than 50 government
approved solar plant projects awaiting construction. By 2015, the
government hopes to generate more two megawatts of power
from CSP sources.
Despite the success of Solar Two, the world’s first power plant to use this technology was in Europe, and not the United States. The plant, named Archimede, was opened in July 2010 in Sicily. It can take in sufficient heat to generate five megawatts of electricity and can store enough for nights or cloudy days. Italy is not alone in pursuing the salty path to clean electricity. About 28,000 tons of molten salt races through the pipelines of Spanish power plant Andasol, located near Granada. The salts are “enough for 7.5 hours to produce energy with full capacity of 50 megawatts,” explains Sven Moormann, a spokesman for the German firm Solar Millennium, which designed the Andasol plant.
Now the U.S. is playing catch up. The first major plant that uses molten salts for solar energy storage in the country is expected to begin construction by mid-2011. If all goes well, Spanish utility company Abengoa’s Solana plant, located in Arizona, should begin functioning by 2013.
The 250 megawatt plant worth $2 billion can light up 70,000 homes and will be able to meet heating demands in winter before sunrise, and cooling needs during summer after sunset. And in October 2010, the U.S. Interior Department approved the world’s biggest solar power project, conceived by Solar Millennium. On the anvil is a 1,000 megawatt CSP plant located near Blythe, California, which is expected to power 750,000 homes. “The Blythe Solar Power Project is a major milestone in our nation’s renewable energy economy and shows that the United States intends to compete and lead in the technologies of the future,” noted Ken Salazar, U.S. Interior Secretary.
As countries buckle under pressing deadlines to reduce their carbon emissions, solar energy storage plants are fast emerging as a viable option. According to a study by Greenpeace, CSP technology will account for 7% of the world’s electricity generation by 2030. Investment in CSP is expected to touch $12.3 billion by 2050, preventing the emission of 2.1 billion tons of carbon dioxide. “Due to the feed-in tariff in Spain and a few schemes in the U.S., this technology is actually taking off,” says Sven Teske, co-author of the report to The Guardian. The forecast sure seems sunny for CSP’s future.