Advantages of Using
Molten Salt
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.
Lisa Sena-Henderson
Last
Modified: 01/10/06