Simplified model of a dual-media molten-salt thermocline tank with a multiple layer wall

Abstract

Thermal ratcheting is a critical phenomenon associated with the cyclic operation of dual-media thermocline tanks in solar energy applications. To study this phenomenon, it is necessary to develop a comprehensive model of a thermocline tank that includes both the heterogeneous filler region and the composite tank wall. Because CFD models require a high computational cost to simulate a thermocline tank considering transient state operation, a simplified dual-phase model that includes the unsteady heat transfer through a multiple layer wall has been developed. The filler region consists of a rock bed with interstitial molten salt, and the tank wall is composed of a steel shell with two layers of insulation (firebrick and ceramic). In this simplified model, the fluid flow inside the tank is considered to be one-dimensional along the tank axis direction, whereas the heat conduction in the composite wall is considered to be two-dimensional. Therefore, a convective heat transfer coefficient from the bed to the wall is necessary to couple the molten salt flow with the heat transfer in the tank shell. In this work, the effects of both convective heat transfer from the bed to the wall and molten salt flow rate on the time-dependent thermal response of both the steel shell and molten salt have been analyzed. The simplified model is able to predict the temperatures of the molten salt, filler material and layer wall as well as the mechanical stress in the tank shell.