High temperature thermal energy storage in the CaAl2 system

Abstract Concentrated solar power (CSP) has been gaining significant attention as a method to produce electricity, but requires a suitable thermal energy storage (TES) system for continuous operation when the sun is unavailable. Thermochemical energy storage systems based on metal hydrides have advantages due to their high energy densities, potential for cost reductions, and higher operating conditions versus commonly utilized molten salts. Higher operational temperatures are desired for CSP power plants to achieve higher efficiencies and therefore lower costs. Herein, we demonstrate the feasibility of CaAl2 as a high temperature thermal energy storage material for concentrated solar power applications. CaAl2 is demonstrated to operate at 600 °C and have excellent reversibility, reasonable thermal conductivities, and equilibrium pressures practical for metal hydride based thermal energy storage systems. CaAl2 was determined to hydrogenate in a two-step process mechanism to form CaH2 and Al and dehydrogenate to reform CaAl2 at 600 °C by adjusting hydrogen pressure. To our knowledge, this material represents the lowest cost metal hydride TES material capable of operating at or above 600 °C.