In Spain, German scientists working for Solar Millennium AG, an international installer of solar thermal plants with headquarters in Germany, has developed a revolutionary power plant.

Called Andasol 1, the plant produces energy from sunlight. More importantly, it can produce this energy even when the sun isn't shining, thanks to a surprising new process that uses a cousin of common table salt.

Table salt, or sodium chloride, melts at 1,452 degrees Fahrenheit (788.88 degrees Celsius). Many salts, or salt derivatives, though, have lower melting points (as well as lower atomic density). Using these low-density salts, the Andasol 1 aims to store solar energy more effectively, and more efficiently, than solar thermal installations using oil as the heat carrier/exchanger, since oils break down above 750 degrees while melted salt is still going strong. In fact, the more temperature molten salt takes on, the more efficient it becomes.

The principle behind this molted (or melted) salt technology is quite simple. Using a base of 60-percent sodium nitrate and 40 percent potassium nitrate, Andasol salts are melted at 430 degrees Fahrenheit and stored in an insulated tank at 550 degrees. This pre-heated salt is then pumped to the top of a tower fitted with another tank called a receiver, where the sun heats it to 1,050 degrees. This heated, molten salt flows back down to a second insulated storage tank, and can be used on cloudy days to run a steam turbine. (Other solar thermal facilities are investigating salt compounds with even lower melting points).

The use of salt allows the generating plant to run at full bore (50 megawatts) for almost 8 hours after the sun is set, and the technology is reportedly 93-percent more efficient than using photovoltaic (solar) panels to convert sunlight into electricity. If the technology could be applied to residential energy, cooled salt from the turbine could also be used to generate heat in a hot-water baseboard (or radiant) heating system.

Andasol 1, the first solar-thermal plant to be built anywhere in the world, cost $380 million (U.S. dollars) to build, and the company is so confident of the technology it has already started construction of Andasol 2, which should be online by summer.

In fact, this molted salt technology seems to be taking off all over the world. Southwest of Phoenix, Arizona, a joint effort between Arizona Public Service Co. (APS) and Abengoa Solar will build a 280-megawatt solar thermal power plant called Solana ("sunny place").  Solana will complement the Deming plant when the two come into operation in 2011. The Deming plant, a solar-powered electric generation facility, is located in Luna County, and will become, at inception, the world's largest solar power plant at 300 megawatts, or enough energy to power 240,000 homes.

And in Santa Monica, California, a company called Solar Reserve has developed a series of tracking mirrors, or heliostats, that sit above a receiver and raise molten salt temperatures to over 1,000 degrees Fahrenheit in imitation of the "Rankine cycle" used in coal-burning generation plants, but without the emissions. 

Supplanting Andasol 1 and 2, at least in terms of commercial viability, is the proposed construction, in Spain, of the Gemasolar plant, a joint effort between Spanish engineering group SENER and Torresol Energy, a spinoff from Abu Dhabi's renewable- energy initiative Masdar (which is itself an offshoot of Abu Dhabi Future Energy Company).

Gemasol, near Seville, will consist of thousands of mirrors reflecting solar radiation onto a collection tower filled with molten salt. Torresol Energy claims that Gemasolar, producing enough power to light 30,000 homes, will be the first commercial plant in the world to use this technology when it is operational in 2011. Clearly they haven't heard of Solana.

Torresol Energy was established in March of 2008. SENER has 60 percent ownership and Masdar has the balance. The company recently secured a little over $214 million U.S. dollars from three Spanish-based banks.

Quibbling over who got there first aside, molten salt technology, when applied to solar power, appears to be the first really clean, efficient and effective use of solar technology that is not directly tied to the sun's daily output. As such, it is truly groundbreaking, and promises economies of scale that may finally put solar energy on parity with fossil fuels in terms of reliability.

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