Search

• The SrH 2 and 2Al system was studied for thermochemical energy storage applications. • Thermodynamic destablization was observed compared to pure strontium hydride. • The system reversibly stores H 2 at a temperature of 846 °C at 1 bar H 2 pressure. • After 50 cycles mild degradation in H 2 storage capacity was observed. • The material could be a thermal energy storage material upon further modification. [Display omitted] Thermal batteries are ideal for storing renewable energies or excess electricity from the grid. The most efficient thermal batteries utilize reversible thermochemical reactions where the heat produced during discharge drives a heat engine. Metal hydrides can be used as the thermal energy storage (TES) material in these batteries, since when heated, hydrogen is released in an endothermic process, charging the battery. When this hydrogen is reintroduced to the metal the metal hydride is reformed during the exothermic reaction (discharge). The optimal thermal battery would have a high operating temperature, low operating pressure and low material cost. SrH 2 could meet these demands except its operating temperature is above 1000 °C. Adding aluminum to strontium hydride causes thermal destablization allowing an operating temperature of 1 bar hydrogen at 846 ± 36 °C, providing ideal properties as a TES material. The SrH 2 -2Al system reacts in two stages with the second step exhibiting only a 32% reduction in capacity over 50 cycles. Pressure-composition isotherm analysis of the second step determined the thermodynamics of H 2 desorption to be Δ H des = 132 ± 2 kJ/mol H 2 and Δ S des = 118 ± 2 J/K/mol H 2. Further studies by scanning electron microscopy have determined changes in morphology over cyclic activity, while simultaneous thermal analysis and powder X-ray diffraction have identified the reaction pathways of the process. A cost analysis of the system has shown that a reduction in materials cost would enhance technological application of this material. [ABSTRACT FROM AUTHOR]

The thermochemical energy storage properties of calcium hydride (CaH 2) destabilised with either silicon (Si) or Ca x Si y 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 CaH 2 at five different molar ratios (1:1, 1:2, 2:1, 3:4, 5:3 CaH 2 to Si) was extensively investigated. Theoretical calculations predicted a multi-step thermal decomposition reaction between CaH 2 and Si forming Ca x Si y 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 CaH 2 and Si or Ca x Si y that meet the criteria of a thermal energy storage system for the next-generation of concentrated solar power (CSP) plants were identified. The CaH 2 and CaSi system (in a 2:3 molar ratio of CaH 2 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 Ca 5 Si 3. The calculated value (107.3 kJ mol−1 H 2) was lower compared to the experimentally determined value (154 ± 4 kJ mol−1 H 2). This mismatch was mainly due to the formation of CaO and a CaSi solid solution in addition to the desired Ca 5 Si 3 phase. ga1 • Calcium hydride has been thermodynamically destabilised with silicon. • A multistep decomposition pathway was determined by theoretical calculations. • The experimental thermal decomposition pathway and thermodynamics were established. • The Ca–Si system is a viable thermal energy storage material. [ABSTRACT FROM AUTHOR]