Destabilization of lithium hydride and the thermodynamic assessment of the Li–Al–H system for solar thermal energy storage

Abstract

Lithium hydride destabilised with aluminium, LiH–Al (1 : 1 mole ratio) was systematically studied and its suitability as a thermal energy storage system in Concentrating Solar Power (CSP) applications was assessed. Pressure composition isotherms (PCI) measured between 506 °C and 652 °C were conducted to investigate the thermodynamics of H₂ release. Above the peritectic temperature (596 °C) of LiAl, PCI measurements were not consistently reproducible, possibly due to the presence of a molten phase. However, below 596 °C, the hydrogen desorption enthalpy and entropy of LiH–Al was ΔH_des = 96.8 kJ (mol H₂)⁻¹ and ΔS_des = 114.3 J (K mol H₂)⁻¹, respectively LiH_(s) at 956 °C, ΔH_des = 133.0 kJ (mol H₂)⁻¹ and ΔS_des = 110.0 J (K mol H₂)⁻¹. Compared to pure LiH, the Li–Al–H system has a reduced operating temperature (1 bar H₂ pressure at T ∼ 574 °C) that, combined with favourable attributes such as high reversibility, good kinetics and negligible hysteresis, makes the Li–Al–H system a potential candidate for solar thermal energy storage applications. Compared to pure LiH, the addition of Al can reduce the cost of the raw materials by up to 44%. This cost reduction is insufficient for next generation CSP but highlights the potential to improve the properties and cost of high temperature hydrides via destabilisation.