Storing solar power

The biggest handicap with solar power was the restriction that it can generate power only when there is sunshine. This meant that expensive solar power plants remained idle for more than half the day. Now, a possible solution to this problem comes in the form of storing energy within molten salts.

The world's second solar power plant to employ such technology — a thermal power plant that concentrates the sun's rays with parabolic mirrors on long, thin tubes filled with the molten salt — opened recently in Syracuse, Sicily. Archimede, developed by the Italian energy giant Enel, can harvest enough heat to generate 5 MW of electricity, day or night, and can store enough energy to keep producing power even at night or during cloudy daytime hours. Another plant, Andasol 1, near Granada, Spain, developed by a German solar company, Solar Millennium, AG, has been in operation for more than an year.

Because most salts - a mixture of sodium and potassium nitrate, otherwise used as fertilizers - only melt at high temperatures (table salt, for example, melts at around 800 degrees Celsius) and do not turn to vapor until they get considerably hotter, they can be used to store a lot of the sun's energy as heat. The sunlight heats up the salts and these salts are then put in proximity to water using a heat exchanger. The hot steam so generated can then be made to turn turbines without losing too much of the original absorbed solar energy.

Such plants can store enough of the sun's heat to produce electricity for nearly eight hours after the sun starts to set. Unlike previous efforts to store sun's energy, using batteries or compressing air or pumping water uphill, round-tripping of energy from molten-salts is very efficient. Melting salts at temperatures above 435 degrees Fahrenheit (224 degrees Celsius) can deliver back as much as 93 percent of the energy (round-trip efficiency), plus the salts are ubiquitous because of their application as fertilizers.

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.

There are of course several formidable challenges to be overcome before these technologies can be exploited commercially. First, since the melting point of such salts is high, they will have to capture a lot of heat to convert the salts to fluid. This raises the need to have other sources of energy just to get salts to reach the melting point.

Further, apart from the large space required by parabolic mirrors, there is need for very large lengths of heat-resistant pipes to collect the sun's rays in the molten pipes. The Archimede requires 30,000 square meters of special parabolic mirrors and 5,400 meters of high heat-resistant pipe to collect the sun's rays, adding up to a building cost of roughly $80 million for just 5 megawatts of electricity. The Andasol 1 plant covers 126 acres (50 hectares) with long rows of troughs and pipe.

Also, current technology limitations with the materials used to fabricate solar troughs means that they cannot capture as much of the sun's heat as is possible. While salts can be heated much beyond their melting points (and to the extent used to store heat energies), the synthetic oils used to capture the sun’s heat in the troughs begin to break down beyond 400 degrees C.

These difficulties mean that thermal energy storage at Andasol 1 or power plants like it costs roughly $50 per kilowatt-hour to install, though the turbines can be generating power for longer periods and those costs can be spread out over more hours of electricity production. Electricity from a solar-thermal power plants across Europe costs roughly 13 cents a kilowatt-hour, still nearly twice as much as electricity from a coal-fired power plant.

Scientists are also exploring technologies that enable them to capture more heat energy in the salts by heating them to more than 500 degrees C. In this context, molten salt power towers, which are vast fields of mirrors that concentrate sunlight onto a central tower, use molten salts directly as the fluid transferring heat in conventional power plants.

Researchers are also looking into salts that could be used instead of the oil in parabolic trough power plants, such as those that melt at lower temperatures and therefore would not freeze as readily during cold nights.