Molten Salt
The salts are heated and stored in an insulating container during off-peak hours. When energy is needed, the salt is pumped into a steam generator that boils water, spins a turbine, and generates electricity. The conversion of thermal energy to electricity can proceed by different cycles such as the Rankine, Brayton, and Air-Brayton cycles. The Brayton gas cycle, for example, involves (1) adiabatic expansion of the high-pressure and high-temperature gas across a turbine to do work, (2) isobaric cooling, (3) adiabatic compression to high pressure, and (4) isobaric heating whereupon the cycle is repeated. Other applications include using the stored heat directly for high temperature processes (such H2 production and coal-to-liquid conversion), which avoids the thermodynamic cost incurred from converting to electricity.The cooled salt is pumped back into the storage tank to be heated and reused.
There are two different configurations for the molten salt energy storage system: two-tank direct and thermocline. The two-tank direct system, using molten salt as both the heat transfer fluid (absorbing heat from the reactor or heat exchanger) and the heat storage fluid, consists of a hot and cold storage tank.The thermocline system (see Fig. 1) uses a single tank such that hot and cold salt are separated by a vertical temperature gradient (due to buoyancy force) to prevent mixing.There are two cycles in the thermocline system: charging and discharging. To charge, salt flows out of the cold side, is heated by the heat exchanger (reactor), and flows into the tank's hot side. To discharge, salt flow out of the hot side, transfers heat to generate power (turbine), and flows into the tank's cold side.The thermocline system reduces costs through a single tank and cheap filler material in the tank to act as thermal storage; the estimated cost relative to the two-tank direct system is about 35%.
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