Molten-salt thermal energy storage in thermoclines under different environmental boundary conditions

Operation during the charge and discharge cycles of molten-salt thermoclines used for solar thermal energy storage depends strongly on the environmental boundary conditions to which the tanks are exposed. A comprehensive model which accounts for thermal transport in the molten-salt heat transfer fluid and the filler material in the tank is developed for exploring the effects of boundary conditions on thermocline performance. Heat loss from the tank under non-adiabatic boundary conditions is found to distort the temperature and salt flow distributions relative to the uniform conditions found in adiabatic thermoclines; as a result, the outflow temperature drops more rapidly in the former case. Such effects of non-adiabatic boundaries become insignificant at large salt-flow Reynolds numbers. As the Reynolds number increases beyond 250, the discharge efficiency of non-adiabatic thermoclines approaches that of the adiabatic counterparts. In the case of significant heat loss at the walls, the discharge efficiency of thermoclines increases with increasing Reynolds number, a trend that is opposite to that in adiabatic thermoclines.