A recently opened solar power plant near Seville in southern Spain, is the first of its kind to be able to generate electricity from sunlight during the day and still retain enough heat to continue generating energy all the way through the night. The $410 million Gemasolar plant has a output of 20 megawatts, although at the moment, it does not produce at full capacity when the sun isn't out.
The plant is of the heliostatic variety, meaning that it doesn't derive its power from photovoltaics, but rather from the raw heat energy of sunlight. A series of concentric mirrors, 2,650 in this case, direct the sunlight at centrally located salt tanks. The heat melts the salt, which boils water around it, and the steam generated turns the turbines. The salt tanks' ability to retain heat is what affords the plant up to 15 hours of sunless energy generation.