In June 2019, China’s Dunhuang molten salt solar thermal power plant hit 100MW, its maximum power levels. The plant will provide 390 million kWh of clean power each year and reduce carbon dioxide emissions by more than 3500 tons. And it can run 24-hours a day, even when the sun does not shine. It does this by storing energy in a molten salt solution for later use.
More and more solar power plants are turning to molten salt to solve solar energy’s number one problem – where to get energy at nighttime or when the sun is behind clouds. Solar energy could be stored in normal batteries but batteries are expensive and not very good for the environment. Instead, the energy can be stored in molten salt.
The process of storing energy in molten salt is called “thermal storage”. Solar plants use large “power towers” which generate power by focusing the sun’s energy at a central tower containing a mechanism called a “heat exchanger” or “heat receiver”. Solar radiation is concentrated onto the tower by varying the angle of solar mirrors (called heliostats) to direct the sun’s energy to the receiver.
Inside the receiver are pipes containing molten salt. Molten salt is usually composed of sodium and potassium salts. The salt solution melts at 460 degrees F. When melted, it looks and feels like water which lets it be easily moved around through pipes. The molten salt solution is very good at storing heat energy. It has a very high “heat capacity”. It can be heated to very high temperatures, up to more than a thousand degrees F. A lot of heat energy can be stored in molten salt solutions.
The hot molten salt is routed to a thermal storage tank, similar to a giant thermos bottle. The tanks are designed to store the energy with minimal loss. When electricity is needed and the sun is not shining, the hot molten salt is routed to a heat exchanger (or steam generator) where it is used to produce steam that powers a steam turbine.
Steam turbines are used in normal electric power plants. Hot steam is used to turn giant turbines which generate electricity as they rotate. Since molten salt can be stored at very high temperatures, it doesn’t take much energy from the molten salt to start creating steam needed to turn the turbines. In fact, the molten salt can be stored for a very long time without losing very much heat.
After generating the steam, the molten salt is redirected to a “cold salt thermal storage tank”. The molten salt sits in the cold storage tank until it is ready to be routed back to the thermal receiver on the solar tower where the process begins all over again.
Molten salt is not toxic and would not cause much harm if it leaked from the tanks. This is a big improvement over other technologies such as batteries or nuclear energy. With molten salt thermal storage, a solar power plant can use all-natural materials to generate electricity day or night and on sunny or cloudy days.