Exploring halide destabilised calcium hydride as a high-temperature thermal battery

Abstract

© 2019

CaH2 is a metal hydride with a high energy density that decomposes around 1100 °C at 1 bar of H2 pressure. Due to this high decomposition temperature, it is difficult to utilise this material as a thermal battery for the next generation of concentrated solar power plants, where the currently targeted operational temperature is between 600 and 800 °C. In this study, CaH2 has been mixed with calcium halide salts (CaCl2, CaBr2 and CaI2) and annealed at 450 °C under 100 bar of H2 pressure to form CaHCl, CaHBr and CaHI. These hydride-halide salts incur a thermodynamic destabilisation of their hydrogen release, compared to CaH2, so that they can operate between 600 and 800 °C within practical operating pressures (1–10 bar H2) for thermochemical energy storage. The as-synthesised metal hydrides were studied by in-situ synchrotron X-ray diffraction, temperature programmed desorption and pseudo pressure composition isothermal analysis. Each of the calcium hydride-halide salts decomposed to form calcium metal and a calcium halide salt after hydrogen release. In comparison to pure CaH2, their decomposition reactions were faster when heated up to 850 °C, and the experimental values of the desorbed hydrogen gas were very close to the theoretical ones. All samples after their decomposition showed signs of sintering, which hindered their rehydrogenation reaction.