Heat transfer characteristics of a molten-salt thermal energy storage unit with and without heat transfer enhancement

Eutectic molten salt can be used as the latent thermal energy storage (LTES) medium in solar energy applications. In the present study, eutectic salt (50 wt% NaNO3, 50 wt% KNO3) with a melting temperature of about 220 °C was employed as the PCM for the middle-temperature solar energy application, which can be powered by the parabolic-trough solar collector using oil as the heat transfer fluid. There are many LTES units in which the molten salt is encapsulated in the thermal energy storage tank, where the heat transfer characteristic of the LTES unit is very important for the overall performance of the entire thermal energy storage tank. We experimentally and numerically investigated the heat transfer characteristics of the molten-salt in a LTES unit with and without heat transfer enhancement. Various heating temperatures of 240 °C, 250 °C, and 260 °C and cooling temperatures of 30 °C, 70 °C, and 110 °C were employed in the study, so as to extensively reveal the heat transfer characteristics during heat storage and retrieval. It was found that natural convection was very dominant during heat storage in the case of pure molten-salt, especially when the heating temperature was higher, and it was weakened in the case of molten-salt with metal foam; while the heat retrieval process was enhanced by the presence of the metal foam. The numerical results were compared with the experimental results, showing reasonable agreement, which indicated that such numerical model could be used for the further study of the performance of the LTES system.