Abstract
In this paper, a mathematical model for the overall exergy efficiency of combined charging–discharging processes of three phase change materials (PCMs) named PCM1, PCM2, PCM3 and different heat transfer fluid (HTF, the solar field HTF and thermal energy storage (TES) system HTF are different) has been developed. The model takes into consideration the effects of inlet temperatures and the number of heat transfer units (NTUs) of the solar field HTF and the TES system HTF on the maximum overall exergy efficiency and the optimum melting temperatures of PCM1, PCM2 and PCM3. The analysis is based on a lumped model for the PCMs which assumes that a PCM is a thermal reservoir with a constant temperature of its melting point and a distributed model for the solar field HTF and the TES system HTF which assume that their temperatures vary in their flow path. The results show that the maximum overall exergy efficiency can be improved by increasing the NTUs of either the solar field HTF or the TES system HTF which is not more than 5. It is found that, for the TES system HTF, increasing its inlet temperature can increase the maximum overall exergy efficiency, however, for the solar field HTF, only when the NTUs of the solar field HTF and the TES system HTF are both more than 2, increasing its inlet temperature can increase the maximum overall exergy efficiency. It is also found that, compared to increasing the NTUs of the TES system HTF, increasing the NTUs of the solar field HTF is more efficient in improving the maximum overall exergy efficiency. Considering actual application of solar thermal power, we suggest that inlet temperature ranges of the solar field HTF and the TES system HTF should be 800–1200K and 350–400K, respectively, and the ranges of the NTUs of PCM1, PCM2 and PCM3 should be 4–5, correspond to the ranges of Tm1.opt, Tm2.opt and Tm3.opt are 750–850K, 550–600K, 450–500K, respectively. The present analysis provides theoretical guidance for application of three PCMs storage system for solar thermal power.
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