Alkali Metal/Halide Thermal Energy Storage Systems Performance Evaluation

Materials compatibility and durability of advanced salt/alkali metal slurry thermal energy storage systems has been demonstrated [1]. Applications are being evaluated for both space and terrestrial solar thermal power conversion [2]. High energy density of these thermal storage systems is achieved by colocation of heat input and extraction within the slurry mixture which is overwhelmingly phase-change salt. This paper addresses performance testing of these systems. Understanding of mechanisms of both micro and macro stratification of the slurry is necessary to fully predict system performance as a function of gravity and system geometry. If it can be shown the gravity stratification effects are secondary to a combination of: (1) liquid metal film adhesion (wetting) to the heat exchange surfaces and solidified salt particles, (2) solubility of alkali metal in the salt-rich phase, and (3) stirring produced by liquid to vapor conversion of the alkali metal, then system geometry limitations are greatly relaxed for space application. Performance testing was accomplished using a sodium heat pipe to transfer heat from the slurry canister to a gas gap calorimeter. Testing was accomplished with the heat pipe installed only in the vapor space above the alkali metal/salt slurry and with an increase heat pipe and minimum vapor space. This testing conclusively demonstrated the effectiveness of the pseudo-heat-pipe type heat transfer mechanism operating in the slurry system under terrestrial conditions.