Destabilized Calcium Hydride as a Promising High-Temperature Thermal Battery

Abstract

Calcium hydride (CaH2) is considered an ideal candidate for thermochemical energy storage (thermal battery) due to its high energy density and low cost. Its very high operating temperature and poor cycling stability are the main factors that hinder its development and implementation as a thermal battery for concentrated solar power (CSP) plants. In this work, CaH2 was thermodynamically destabilized with aluminum oxide (Al2O3) at a 1:1 molar ratio to release hydrogen at a lower temperature than the hydride alone. Temperature-programmed desorption measurements showed that the addition of Al2O3 destabilized the reaction thermodynamics of hydrogen release from CaH2 by reducing the decomposition temperature to ∼600 °C in comparison to ∼1000 °C for pure CaH2 at 1 bar of H2 pressure. The experimental enthalpy and entropy of this system were determined by pressure composition isotherm measurements between 612 and 636 °C. The enthalpy was measured to be ΔH des = 100 ± 2 kJ mol–1 of H2, and the entropy was measured to be ΔS des = 110 ± 2 J·K–1 mol–1 of H2. The XRD after TPD and in situ XRD data confirmed the main product as Ca12Al14O33. The system exhibited a loss of capacity during hydrogen cycling at 636 °C, which was found to be due to sintering of excess Al2O3, as confirmed by X-ray diffraction and scanning electron microscopy. The hydrogen cycling capacity was significantly improved by reducing the initial amount of Al2O3 to a 2:1 molar ratio of CaH2 to Al2O3, deeming it as a highly promising high-temperature thermal battery for the next generation of CSP plants.

Hydride-based thermal energy storage

“…This capacity loss was attributed to the evaporation of Zn from the alloys. The enthalpy and entropy of formation of reaction (23) was found to be ∆H des = 131 ± 4 kJ mol −1 H 2 , ∆S des = 151 ± 4 J mol −1 K −1 H 2 respectively, providing a T dec ( [67], does not take part in the reverse reaction but rather acts as a catalyst enabling a 90% energy capacity retention over 500 CO 2 cycles [24]. Thus, ternary aluminium-containing compounds seem to have a positive effect on TES materials and may be worth further investigation in connection with MHs.…”

“…1 bar) = 597 ± 35 • C.Alumina is also an efficient additive for the thermodynamic destabilisation of CaH 2[67]. When mixed in a 2:1 ratio (CaH 2 :Al 2 O 3 ) multiple reversible hydrogen cycles are achieved, with the formation of Ca 12 Al 14 O 33 .…”

Calcium hydride with aluminium for thermochemical energy storage applications

“…22 Similarly, the addition of Al 2 O 3 to CaH 2 (1 : 1 molar ratio) showed sintering resulting in poor cycling capacity, but reducing the initial amount of additives may promote the sorption stability of this system operating at 636 °C and 1 bar of H 2 . 23 In 1981, Veleckis investigated Ca-Al alloys and reported the formation of CaAl 4 and CaAl 2 in a two-step reaction process (eqn (1) and ( 2)), along with the conclusion that the system has potential as a high temperature hydrogen storage medium due to its reversible hydrogenation capabilities at 450-650 °C. 24 An absorption enthalpy of −83.1 kJ mol −1 H 2 was also determined for the reaction of CaAl 2 with H 2 , giving a theoretical energy density of 865 kJ kg −1 CaAl 2 .…”

“…22 Similarly, the addition of Al 2 O 3 to CaH 2 (1 : 1 molar ratio) showed sintering resulting in poor cycling capacity, but reducing the initial amount of additives may promote the sorption stability of this system operating at 636 °C and 1 bar of H 2 . 23…”

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

“…This method successfully showed by adding Al2O3 to CaH2, where the enthalpy of the reaction was found to be ΔH =100 kJ mol −1 H2 at 636 °C and 1 bar of H2 pressure. 23 CaH2-3Zn is an alternate system that exploits the feasibility of Zn as a high-temperature thermochemical TES material for the third generation CSP plants. The aim of this study focused on thermal analysis, and thermodynamic characterization of the CaH2-3Zn system using mass spectrometry, temperature programmed desorption and in-situ X-ray diffraction studies.…”