Evaluation of a three-phase sorption cycle for thermal energy storage

• A three-phase sorption cycle including crystallization and hydration processes is suggested for thermal energy storage.
• LiCl/H2O, CaCl2/H2O and LiBr/H2O are selected to evaluate the proposed cycle.
• Presence of the crystallization and hydration processes could extensively enhance the storage density.
• LiCl/H2O is an excellent choice for both cold and heat storage.

In recent years, sorption processes are increasingly gaining attention in the field of TES (thermal energy storage), owing to their advantages of high storage density and no significant heat loss during storage process. In this paper, a novel three-phase sorption cycle is proposed and evaluated for TES, to provide cold storage in summer and heat storage in winter. The uniqueness of the cycle is that it allows the solution to reach its crystallized state or even dehydrated state during the charging stage. Three working pairs, namely LiCl/H2O, CaCl2/H2O and LiBr/H2O, are considered for the proposed cycle based on their pressure–temperature-phase diagrams. Detailed analysis reveals that presence of the three-phase crystallization process could extensively improve the energy storage performance, as heat storage densities increase by 43%, 79% and 38% for LiCl/H2O, CaCl2/H2O and LiBr/H2O respectively. Overall, LiCl/H2O is determined to be the best choice among the three couples, with a cold storage density of 519 kWh/m3 (1242 Wh/kg, charged at 90 °C in summer) and a heat storage density of 618 kWh/m3 (1250 Wh/kg, charged at 75 °C in winter). This theoretical evaluation suggests that the three-phase sorption cycle can be a promising solution for TES.