Thermodynamic characterization of Mg-50 wt% LaNi5 composite hydride for thermochemical energy storage application

In the present work, composite material, Mg-50 wt% LaNi5, is thermodynamically characterized for thermochemical energy storage application, and its mechanism of hydrogen absorption is studied by using Johnson-Mehl-Avrami (JMA) equation. Pressure concentration isotherms (PCI) of the composite during hydrogen absorption and desorption are measured at 250 degrees C, 300 degrees C, 350 degrees C, and 375 degrees C. The enthalpy of absorption and enthalpy of desorption are calculated as 62.082 and 66.009 kJ/mol H-2. Absorption kinetics is measured at 250 degrees C, 300 degrees C, and 350 degrees C with hydrogen supply pressures of 10, 20, and 30 bar. The activation energies are calculated at different supply pressures using Arrhenius equation. Based on activation energy, the rate-limiting steps are identified. A thermochemical energy storage method is proposed based on equilibrium pressure-temperature relation obtained during absorption and desorption. The amount of thermochemical energy stored and recovered per kg of the composite is calculated based on reaction enthalpies and hydrogen concentration. The maximum theoretical thermochemical energy stored and recovered at a temperature of 350 degrees C is calculated as 1452.198 and 1365.804 kJ/kg respectively. The maximum theoretical energy storage efficiency is estimated as 0.94. Hence, the composite, Mg-50 wt% LaNi5, can be used as thermochemical energy storage material in the temperature range of 250 degrees C-350 degrees C with a thermochemical energy storage capacity of 0.403 kWh/kg composite.