A theoretical and experimental investigation of heat transfer involving melting and freezing in encapsulated lithium hydride salt

Thermal energy storage may be used in space-based power generation systems to store waste heat. In systems where the power generation cycle is active only during brief periods, a system employing thermal energy storage can be lighter and more compact than a system that has enough radiator surface to dissipate all of the waste heat as it is generated. Lithium hydride (LiH) salt is well suited for this application, because it has the highest heat of fusion of any ionic salt and melts at a temperature compatible with high-temperature power generation cycles. In one proposed concept for an energy storage system, LiH would be stored in spherical containers arranged in a packed bed, with an externally circulating liquid metal used as the heat transfer fluid.

The purpose of the present study was to investigate, both theoretically and experimentally, the heat transfer characteristics of the individual spherical LiH capsules, including effects due to the solid-liquid phase change. A numerical heat transfer model was developed to simulate the heat transfer performance of spherically encapsulated LiH under both zero-gravity and normal-gravity conditions. The model employs a finite difference formulation and uses an enthalpy-based method to account for phase change. The model also predicts the void that forms as the salt freezes, as well as the effect of natural convection flow in the melted salt.

Experiments were carried out in which instrumented spherical containers filled with LiH were heated rapidly in a high-flux induction furnace to simulate as closely as possible a typical heating cycle in a real thermal energy storage system. The data obtained during these experiments was used to determine the physical behavior of the capsules during melting and solidification, and thus to validate the numerical model. The performance of the numerical model in predicting IV temperature-time histories of thermocouples embedded in the salt was generally satisfactory, but the model underpredicted the total time required for melting by ten to fifteen percent. Numerical results for zero-gravity and normal-gravity cases were also compared, and conclusions were drawn about the effects of gravity on the thermal behavior of these systems.