BrightSource Energy, Inc. will now add energy storage facilities that use molten salt in three solar thermal power plants it will build, and will supply electricity to Southern California Edison.
Energy storage will allow the power plants to increase them power output and dependability by enabling them to generate electricity after dark or when electricity demand spikes.
Under the original power purchase agreements with Southern California Edison, the California-based company needed to build seven power plants to meet its obligation to generate 4 million megawatt-hours of electricity annually for the utility.
Having solar power plants running ideally 24/7, BrightSource can now forgo building one 200-megawatt solar station.
The amended contract is pending approval from the California Public Utilities Commission. It will now consist of two BrightSource solar thermal plants scheduled to deliver electricity in 2015 and three BrightSource plants with energy storage scheduled to deliver electricity in 2016 and 2017.
In addition, BrightSource and its partners – NRG Energy, Google Inc., and Bechtel – are currently constructing the 392-megawatt Ivanpah solar project in southeast California. Half of its output will be delivered to Southern California Edison while rest to Pacific Gas & Electric.
BrightsSource builds and designs concentrating solar power plant that focuses sunlight on a water-filled reservoir on top of a tower, creating steam for running a generator.
The company’s energy storage system called SolarPLUS diverts some of this steam to a heat exchanger and heat molten salt, a mixture of sodium and potassium nitrate, which traps heat better compared with water. The system was just announced in August this year.
Molten salt can also operate at higher temperatures than oils used as a heat transfer medium – above 500 degree Celsius instead of around 400 degrees Celsius – therefore increasing efficiency and power output of a plant.
Molten salt is becoming a sought-after technology for concentrating solar thermal project developers such as SolarReserve L.L.C. and Abengoa Solar, now also offering molten salt storage together with their solar thermal plants.
However, the technology does have some limitations.
Since salts can only retain heat for up to 10 hours according to the United States Department of Energy, it is unlikely that heat stored from summer sunlight can be used to generate electricity in the winter.
Special materials are required to avoid salt corrosion in molten salt systems, which could mean more construction and maintenance costs. In addition, salt has a higher freezing point than other heat transfer fluids, which means that pipes must be kept warm to keep the salt molten.