Extended investigation of LiOH–LiBr binary system for high-temperature thermal energy storage applications
Résumé
LiOH–LiBr binary system is thoroughly investigated by means of DSC and XRD experimental analysis. Observed discrepancies compared to previous existing studies relate to temperature values of phase equilibria as well as stoichiometric compounds present in the system. From our experimental results, a modified LiOH–LiBr phase diagram is proposed which gives satisfactory explanation to all observations carried out. It includes stoichiometric compounds Li2(OH)Br (peritectoid plateau at 250 °C, x ≤ 0.666), Li3(OH)2Br (stable between 230 and 280 °C, melting peritectically for x ≥ 0.5) and Li4(OH)3Br (peritectic plateau at 289 °C, x ≥ 0.5). It also displays a eutectic transition at 254 °C approx., which extends over the composition range x > 0 to x = 0.66–0.67, with eutectic point at x = 0.40. The disagreements with previous studies also concern the enthalpies of transition. Whatever the transition is considered, the enthalpies measured in this work are much lower than those predicted before. However, the peritectic compound Li4(OH)3Br is still an attractive candidate for TES applications around 300 °C such as Direct Steam Generation CSP technology. In particular, when compared to NaNO3, which the reference material at that temperature, the advantages of using Li4(OH)3Br as heat storage material lie in the higher volumetric latent heat storage capacity (+ 54%) and lower volume changes during phase transitions (3% vs. 11%). This would result in smaller storage tanks, lower size heat exchangers, contributing to decrease the cost of the storage system.