The Andasol 1 plant in Spain started generating power in November 2008, and as long as the sun is shining, it operates pretty much like any other solar-thermal power plant. Sunlight strikes some sort of solar collector -- in this case, a field of parabolic-trough mirrors focused on tubes filled with oil, which warms to more than 752 degrees Farenheit (400 degrees Celsius). That hot oil is used to boil water, which produces steam, which spins a turbine.
It's only when the sun isn't shining that the storage system affects power generation. The setup goes like this:
The field of solar collectors at Andasol 1 is big enough to collect almost twice as much sunlight as the plant needs to operate during sunny times. The extra heated oil is sent to a heat exchanger running between giant vats of molten salt. One vat holds relatively cool molten salt (about 500 degrees F or 260 degrees C). That salt is pumped into the heat exchanger, where it picks up heat from the oil. The now hotter molten salt (752 degrees F or 400 degrees C) flows into the second vat, where it waits until the sun dips behind a cloud.
When the power plant needs the stored heat, the hotter molten salt is pumped back through the heat exchanger. There, it transfers its heat to the oil that will generate steam. The hotter oil travels to the power center, and the now-cooler molten salt flows back into the cooler tank. The process then starts all over.
Using salt to store the sun's heat, the plant can operate without sunlight, running almost twice as long as other solar power plants. The salt-storage setup lets Andasol 1 generate 50 percent more energy than it would without it -- 178,000 megawatt-hours of electricity [source: Fairly]. That extra generating ability lowers the overall cost of the plant's electricity. It could eventually rival the cost of natural-gas power.
This type of salt storage isn't the only design on the table for storing the sun's energy. Some plants are looking at using a more direct approach that skips the oil -- they would both collect and store the sun's heat in salt. Sand is another potential heat-storage material.
And another group has developed a system that mimics the molecular effects of photosynthesis to store solar power: It uses sunlight to split water molecules into hydrogen and oxygen, which are then put back together in a fuel cell.
For more information on solar energy storage and related topics, look over the links on the next page.