Solar towers use many large, computer controlled, sun tracking mirrors (heliostats) to focus the suns energy on a receiver located at the top of a tower. A heat transfer fluid, usually molten nitrate salt, is heated in the receiver and used either to drive a turbine/generator to produce electricity or to provide high temperature thermal heat. The molten salt can be used to store the thermal energy for producing electricity at night or during cloudy weather. Commercial power plants would be sized from 50 MW to 200 MW each.
A ten MW plant, Solar One, located near Barstow CA operated for six years demonstrating the viability of solar towers. It used a heat transfer fluid to transfer the heat to the generator. Solar Two, shown left, was a retrofit of solar one, built to demonstrate the advantages of molten salt for heat transfer and solar storage. At one point it delivered power to the grid for seven days, 24 hours a day during cloudy weather. Molten salt solar towers are well suited to peaking power applications, being able to generate power when most needed, day or night, cloudy or sunny.
In a molten-salt power tower, the molten nitrate salt, which is a clear liquid with properties like water at temperatures above its 240oC (464oF) melting point, is pumped from a large storage tank to the receiver, where it is heated in tubes to temperatures of 565oC (1049oF). The salt is then returned to a second large storage tank, where it remains until needed by the utility for power generation. At that time, the salt is pumped through a steam generator to produce the steam to power a conventional, high-efficiency steam turbine to produce electricity. The salt at 285oC (545oF) then returns to the first storage tank to be used in the cycle again.
The following paragraphs were excerpted from the Solar Power Tower report referenced at the end of this post.
The Solar Two receiver is comprised of a series of panels (each made of 32 thin-walled, stainless steel tubes) through which the molten salt flows in a serpentine path. The external surfaces of the tubes are coated with a black Pyromark™ paint that is robust, resistant to high temperatures and thermal cycling, and absorbs 95% of the incident sunlight. The receiver design has been optimized to absorb a maximum amount of solar energy while reducing the heat losses due to convection and radiation. The design, which includes laser-welding, sophisticated tube-nozzle-header connections, a tube clip design that facilitates tube expansion and contraction, and non-contact flux measurement devices, allows the receiver to rapidly change temperature without being damaged. For example, during a cloud passage, the receiver can safely change from 290 to 570ºC (554 to 1,058ºF) in less than one minute.
The salt storage medium is a mixture of 60 percent sodium nitrate and 40 percent potassium nitrate. Molten salt can be difficult to handle because it has a low viscosity (similar to water) and it wets metal surfaces extremely well. Accordingly, Solar Two is designed with a minimum number of gasketed flanges and most instrument transducers, valves, and fittings are welded in place.