A variety of fluids was tested to transport the sun’s heat, including water, air, oil, and sodium, before molten salt was selected as best.
Molten salt is used in solar power tower systems because it is liquid at atmosphere pressure, it provides an efficient, low-cost medium in which to store thermal energy, its operating temperatures are compatible with todays high-pressure and high-temperature steam turbines, and it is non-flammable and nontoxic.
In addition, molten salt is used in the chemical and metals industries as a heat-transport fluid, so experience with molten-salt systems exists for non-solar applications.
The molten salt is a mixture of 60 percent sodium nitrate and 40 percent potassium-nitrate, commonly called saltpeter. The salt melts at 430 F and is kept liquid at 550 F in an insulated cold storage tank. The salt is them pumped to the top of the tower, where concentrated sunlight heats it in a receiver to 1050 F.
The receiver is a series of thin-walled stainless steel tubes. The heated salt then flow back down to a second insulated hot storage tank. The size of this tank depends on the requirements of the utility; tanks can be designed with enough capacity to power a turbine from two to twelve hours. When electricity is needed from the plant, the hot salt is pumped to a conventional steam-generating system to produce superheated steam for a turbine/generator.
The uniqueness of this solar system is in de-coupling the collection of solar energy from producing power, electricity can be generated in periods of inclement weather or even at night using the stored thermal energy in the hot salt tank. The tanks are well insulated and can store energy for up to a week.
As an example of their size, tanks that provide enough thermal storage to power a 100-megawatt turbine for four hours would be about 30 feet tall and 80 feet in diameter. Studies show that the two-tank storage system could have an annual efficiency of about 99 percent.
The Department of Energy invested $737 million into the Crescent Dunes Solar Energy Project in Nevada, which will generate energy well into the night by using molten salt as an energy storage medium. To do this the plant will focus nearly 20,000 heliostats upon a solar power tower filled with salt, heating the material to 1,050 degrees Fahrenheit. Once it has been heated the salt will retain its thermal energy for a long time, and it can be mixed with water to produce steam on demand, which can be used to drive turbines to produce electricity.
Nevada’s new molten salt solar plant will be run by rocket scientists from Pratt and Whitney. Companies such as SolarReserve have championed the use of molten salt in solar thermal projects because unlike water, it retains heat for a very long time. In this case, it is hoped that the plant will continue to produce power for 12 hours after the sun has set. The National Solar Thermal Test Facility has conducted several studies and concluded that molten salt is the most efficient material when it comes to storing the sun’s heat.