Molten Salt Accumulators
Different thermal energy storage systems are used in solar plants. In some cases molten salt tanks are used. The basic idea is to use the heat from salt fusion; that is, the amount of heat required to melt a solid salt at its melting point into a liquid without increasing its temperature. The heat capacity of a body is defined as the heat needed to increase the temperature of the body by one degree and since the temperature is not increased during melting, the heat capacity is very high at this stage. The liquid salt releases the same amount of heat when it solidifies.
The energy stored at a molten salt energy storage system can be modeled by the following equations:
, . -Tst
C^EAt))-^ — ps+ – ps – – Lst(Tst – Ta)
0 < Ps+< Ps+
0 < Ps-< Ps-
Ps+ — Hst+(qst, Tstin — Tst) ps – — HSt-(qSt, TSt – Tstin)
where cs(Est(t)) is the thermal capacity of the molten salt accumulator, Tst is the temperature of the salts, Tstin is the temperature of the fluid entering the accumulator. Ps+ and Ps – are the power stored and extracted from the accumulator, respectively. Lst(Tst – Ta) are the rate of energy losses in the accumulator which can be expressed as a function of Tst – Ta. The function Hst+(qst, Tstin – Tst) corresponds to the maximum power that can be stored, which depends on the temperature differences between the inlet oil flow and the temperature of the accumulator. The maximum power that can be extracted from the accumulator can be computed as a function, Hst-(qst, Tst – Tstin), of the oil accumulator flow (qst) and the difference of temperature between the molten salt and the inlet oil flow.
The thermal capacity of the molten salt accumulator, cs(Est(t)), can be expressed as a function of the stored energy Est(t). When the stored energy is below a certain level, the thermal capacity corresponds to that of the salt in a solid state. When the salt is melting, because enough energy has already been stored, the incoming energy is used to melt the salt without increasing its temperature. This can be modeled as a very high accumulator thermal capacity. Once all the salt is melted, the thermal capacity of the accumulator corresponds to that of the salt in a liquid state.
The outlet HTF temperature, Tstout, can be computed from the following power balance equation:
pfcfqst (Tstin — Tstout)) = Ps+ — Ps – — Lst (Tst — Ta) (8.11)