How can one use solar energy after the sun sets? Simple: store the sun's heat in molten salts.

The world's first solar power plant to employ such technology—a thermal power plant that concentrates the sun's rays with mirrors on long, thin tubes filled with the molten salt—opened in Syracuse, Sicily, on July 14. Dubbed Archimede—after the famous Syracusan scientist Archimedes who supposedly coined the term "Eureka" for scientific discovery and reputedly repelled a Roman fleet through the use of mirrors to concentrate the sun's rays and burn the invading ships—the power plant can harvest enough heat to generate five megawatts of electricity, day or night, and can store enough energy to keep producing power even at night or during cloudy daytime hours.

In addition to the benefit of storage, molten salts also operate at a higher temperature (roughly 550 degrees Celsius) enabling them to capture more of the sun's energy—as well as create the steam for turbines in conventional power plants. Meaning that such solar thermal power plants could be swapped in for fossil fuel-burning ones. This power plant is on the grounds of a natural gas combined-cycle power plant.

The molten salts also don't burn like the oils used as working fluids in other concentrating solar power plants operating today. If Archimede springs a leak, it will end up with piles of fertilizer (the salts in question are potassium and sodium nitrates, which are literally used as fertilizer in agricultural applications). Of course, the freezing point of such salts is a balmy 220 degrees C so Archimede will have to capture a lot of heat to keep the salts fluid. That's why the plant's owner—Italian energy giant Enel—is supplementing the sun's rays with a little old-fashioned natural gas burning as well.

Of course, it requires 30,000 square meters of special parabolic mirrors and 5,400 meters of high heat-resistant pipe to collect the sun's rays in the molten salts, even in Syracuse. All that adds up to a building cost of roughly $80 million for just 5 megawatts of electricity. But Italy is hardly alone in pursuing such plants: the Andasol power plants in Spain use more than 28,000 metric tons of such salts to store thermal energy from its otherwise conventional concentrating solar power plants and the U.S. company SolarReserve plans to deploy such molten salt technology in its "power towers" coming soon to the Nevada desert. Eureka!

Image: Courtesy of Archimede Solar Energy