Thermochemical energy storage performance of zinc destabilized calcium hydride at high-temperatures
Sruthy Balakrishnan, M. Veronica Sofianos, Terry D. Humphries, Mark Paskevicius, Craig E. Buckley
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 ΔHdes = 131 ± 4 kJ mol-1 H2 and ΔSdes = 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.