One of the biggest problems with solar energy is that the sun doesn’t shine 24 hours a day. This means that unless users are only planning on using electricity when the sun is shining, some form of energy storage system is required. Since storing excess electricity in rechargeable batteries isn’t really practical for large-scale solar power plants, another storage system is needed. U.S. utility-scale solar project developer SolarReserve has now received approval for the first solar power plant in California that uses molten salt technology to store the sun’s thermal energy as heat so it can generate electricity when needed, at any time of the day or night.

The Rice Solar Energy Project is designed as a solar power tower, with thousands of tracking mirrors (heliostats) focusing concentrated sunlight on a receiver that sits at the top of a central tower to collect the thermal energy. Whereas most solar power towers use this thermal energy to heat water into steam to power a turbine, SolarReserve’s system uses the thermal energy to heat molten salt to store the energy. The molten salt is a mixture of sodium nitrate and potassium nitrate that is non-flammable and non-toxic and is an efficient and inexpensive energy storage medium.

In a Rankine cycle, the sunlight heats this molten salt to over 1,000 degrees Fahrenheit (537.8 degrees Celsius) before it flows through to a thermal storage tank where the company says it maintains 98 percent thermal efficiency. When needed, it is pumped to a steam generator to produce steam and drive a standard turbine to generate electricity. SolarReserve says the Rice Solar Energy Project has the ability to collect and store enough thermal energy each morning to operate at full power all afternoon and for up to eight hours after sunset.

Molten salt technology was demonstrated in the Solar Two project in the Mojave Desert that had the ability to produce 10 MW of electricity. Its molten salt system allowed the plant to store energy in large tanks and continue running for up to three hours after the sun had set. Starting in 1995, it was decommissioned in 1999 having successfully demonstrated the technology.

In an indication of California’s commitment to solar and other renewable energy solutions, SolarReserve received approval for the construction and operation of the 150 MW solar tower project in a record time of 13 months from the initial application. The Rice Solar Energy Project will be located 30 miles (48 km) northwest of Blythe in eastern Riverside County and will supply approximately 450,000 MWh of zero-emission electricity annually – enough to power up to 68,000 homes during peak electricity periods.

SolarReserve is aiming to start construction of the plant in the third quarter of 2011.

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