Cyclic operation of molten-salt thermal energy storage in thermoclines for solar power plants

The cyclic operation of molten-salt thermal energy storage thermoclines for solar thermal power plants is systematically investigated. A comprehensive, two-temperature model is first developed for the cyclic operation of a thermocline operating with a commercially available molten salt as the heat transfer fluid and quartzite rock as the filler. Volume-averaged mass and momentum equations are employed, with the Brinkman–Forchheimer extension to the Darcy law used to model the porous-medium resistance. Energy equations for the molten salt and the filler are coupled by an interstitial Nusselt number representing the heat transfer between the phases. A finite-volume approach is employed to solve the governing equations. The model is validated against experiments from the literature and then used to systematically study the cyclic behavior of the thermocline thermal storage system. Thermal characteristics including temperature profiles and cycle efficiency are explored. Guidelines are developed for designing the dimensions and molten salt flow rates for solar thermocline systems of different power capacities. The cycle efficiency is found to be improved at smaller melt Reynolds numbers, larger length ratios (molten salt flow distance in a half-cycle to the filler particle diameter) and larger tank heights. The filler particle diameter and the tank volume are found to strongly influence the cycle efficiency.