• Energy Research

Magnesium hydride owns the largest share of publications on solid materials for hydrogen storage. The Magnesium group of international experts contributing to IEA Task 32 Hydrogen Based Energy Storage recently published two review papers presenting the activities of the group focused on magnesium hydride based materials and on Mg based compounds for hydrogen and energy storage. This review article not only overviews the latest activities on both fundamental aspects of Mg-based hydrides and their applications, but also presents a historic overview on the topic and outlines projected future developments. Particular attention is paid to the theoretical and experimental studies of Mg-H system at extreme pressures, kinetics and thermodynamics of the systems based on MgH2,nanostructuring, new Mg-based compounds and novel composites, and catalysis in the Mg based H storage systems. Finally, thermal energy storage and upscaled H storage systems accommodating MgH2 are presented.

Cycling of a high temperature thermal battery using a pair of metal hydrides to store thermal energy and the associated evolved gaseous hydrogen.

Hydrated and anhydrous LiB11H14salts are prepared through a facile approach. Liquid-like Li+conductivity (10−2S cm−1) is observed for a-LiB11H14·(H2O)nat 60 °C. LiB11H14·2H2O is classified as a new class of ionic liquid as it melts near 70 °C.

A high temperature thermal energy storage system has been developed for concentrating solar thermal applications.

For renewable energy sources to replace fossil fuels, large scale energy storage is required and thermal batteries have been identified as a commercially viable option. In this study, a 3.2 kg prototype (0.82 kWhth) of the limestone-based CaCO3-Al2O3 (16.7 wt%) thermochemical energy storage system was investigated near 900 °C in three different configurations: (i) CaCO3 was thermally cycled between 850 °C during carbonation and 950 °C during calcination whilst activated carbon was utilised as a CO2 gas storage material. (ii) The CaCO3 temperature was kept constant at 900 °C while utilising the activated carbon gas storage method to drive the thermochemical reaction. (iii) A mechanical gas compressor was used to compress CO2 into volumetric gas bottles to achieve a significant under/overpressure upon calcination/carbonation, i.e. ≤ 0.8 bar and > 5 bar, respectively, compared to the ∼1 bar thermodynamic equilibrium pressure at 900 °C. Scenarios (i) and (iii) showed a 64% energy capacity retention at the end of the 10th cycle. The decrease in capacity was partly assigned to the formation of mayenite, Ca12Al14O33, and thus the absence of the beneficial properties of the expected Ca5Al6O14 while sintering was also observed. The 316L stainless-steel reactor was investigated in regards to corrosion issues after being under CO2 atmosphere above 850 °C for approximately 1400 h, and showed no significant degradation. This study illustrates the potential for industrial scale up of catalysed CaCO3 as a thermal battery and provides a viable alternative to the calcium-looping process.

Abstract The thermochemical energy storage properties of calcium hydride (CaH2) destabilised with either silicon (Si) or CaxSiy compounds at various molar ratios, were thoroughly studied by a combination of experimental and computer assisted thermodynamic calculations. Particularly, the destabilisation effect of Si on CaH2 at five different molar ratios (1:1, 1:2, 2:1, 3:4, 5:3 CaH2 to Si) was extensively investigated. Theoretical calculations predicted a multi-step thermal decomposition reaction between CaH2 and Si forming CaxSiy at varying temperatures, which was confirmed by in-situ synchrotron X-ray diffraction, differential scanning calorimetry, thermogravimetric analysis and mass-spectroscopic measurements. The most suitable destabilisation reactions between CaH2 and Si or CaxSiy that meet the criteria of a thermal energy storage system for the next-generation of concentrated solar power (CSP) plants were identified. The CaH2 and CaSi system (in a 2:3 molar ratio of CaH2 to CaSi) showed desirable operating conditions with a decomposition temperature of 747 ± 33 °C at a hydrogen pressure of 1 bar. Pressure composition isothermal measurements were conducted on this system to determine its practical enthalpy of decomposition to form Ca5Si3. The calculated value (107.3 kJ mol−1 H2) was lower compared to the experimentally determined value (154 ± 4 kJ mol−1 H2). This mismatch was mainly due to the formation of CaO and a CaSi solid solution in addition to the desired Ca5Si3 phase.