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Generating Electricity From Solar Power Even After Sun Down
 
One of the biggest problem with solar energy is how to sustain energy generation after sunset. It boils down to finding a way to store solar energy for use on demand. This has lead to the creation of a different form of power generation - solar thermal. Solar thermal generates electricity by using the sun’s heat to boil water. The water is then used to heat salt that will store the energy until it is required for use. 
 

Two California companies are building power plants based on this principle. Solar Reserve is building a plant in the Nevada desert scheduled to start operation in 2012. The plant will generate a peak power of 110 megawatts and store enough energy to run for eight to 10 hours when the sun is down.

Bright Source will build three power plants in California that would begin operations in 2016 and 2017. Already, companies like Google, Chevron and Good Energies are investing in the two companies. Utilities companies NV Energy and Southern California Edison have also signed long-term contracts to buy power from these new concept power plants.

These power plants will complement solar panels, which produce electricity directly from sunlight. After dusk, the solar thermal plants will start operation, drawing on the stored thermal energy to generate electricity.

Technical details of the solar thermal plants for Solar Reserve and Bright Source vary slightly. However, both will use thousands of computer-controlled mirrors to reflect sunlight at a tall tower that absorbs the heat. Solar Reserve;s tower will store the heat in molten salt, which can be used immediately to boil water, generating steam that turns a conventional turbine and generator. The hot salt can also be used to retain the heat for many hours for later use. Bright Source's tower heats water that can be used immediately as steam power or to heat salt for thermal storage.

The solar thermal plants use salt for heat storage because salt can store more heat than water.

The “round-trip efficiency” of the solar thermal system is around 95 percent. That refers to the ratio of energy recovered compared with the energy used. This is far greater than the efficiency for the biggest conventional form of energy storage - pumped hydro. Pumped hydro involves pumping water into a reservoir located on a hill and generate electricity by using the water to turn a turbine when the water flows down.