Thermochemical energy storage performance of zinc destabilized calcium hydride at high-temperatures

Abstract

CaH₂ has 20 times the energy density of molten salts and was patented in 2010 as a potential solar thermal energy storage material. Unfortunately, its high operating temperature (>1000 °C) and corrosivity at that temperature make it challenging to use as a thermal energy storage (TES) material in concentrating solar power (CSP) plants. To overcome these practical limitations, here we propose the thermodynamic destabilization of CaH₂ with Zn metal. It is a unique approach that reduces the decomposition temperature of pure CaH₂ (1100 °C at 1 bar of H₂ pressure) to 597 °C at 1 bar of H₂ pressure. Its new decomposition temperature is closer to the required target temperature range for TES materials used in proposed third-generation high-temperature CSP plants. A three-step dehydrogenation reaction between CaH₂ and Zn (1 : 3 molar ratio) was identified from mass spectrometry, temperature-programmed desorption and in situ X-ray diffraction studies. Three reaction products, CaZn₁₃, CaZn₁₁ and CaZn₅, were confirmed from in situ X-ray diffraction studies at 190 °C, 390 °C and 590 °C, respectively. The experimental enthalpy and entropy of the second hydrogen release reaction were determined by pressure composition isotherm measurements, conducted between 565 and 614 °C, as ΔH_des = 131 ± 4 kJ mol⁻¹ H₂ and ΔS_des = 151 ± 4 J K⁻¹ mol⁻¹ H₂. Hydrogen cycling studies of CaZn₁₁ at 580 °C showed sufficient cycling capacity with no significant sintering occurring during heating, as confirmed by scanning electron microscopy, demonstrating its great potential as a TES material for CSP applications. Finally, a cost comparison study of known destabilized CaH₂ systems was carried out to assess the commercial potential.