Investigation of water adsorption characteristics of MgCl2 salt hydrates for thermal energy storage: Roles of hydrogen bond networks and hydration degrees

Abstract

Thermochemical energy storage holds great promise in solar energy applications, and MgCl2 hydrate salt is considered a promising material for medium and low-temperature thermochemical energy storage. Understanding the adsorption behavior of water molecules in MgCl2 hydrate salts and uncovering the underlying mechanisms are crucial for designing efficient thermochemical energy storage systems. This paper investigated the water adsorption characteristics of MgCl2 hydrate salt utilizing the Grand Canonical Monte Carlo (GCMC) method and captured the adsorption isotherm and concentration distributions of water molecules in MgCl2 hydrate salts pores at various temperatures. The results demonstrate that at low temperatures, particularly when the pore size is small (D < 0.8 nm), the interconnected hydrogen bond network plays a crucial role in enhancing the water adsorption capacity of pure MgCl2. As the temperature increases, the influence of hydrogen bond networks diminishes, and their impact on the water adsorption capacity becomes negligible. Moreover, the hydration degrees of MgCl2 salt significantly influence their water adsorption capacity. MgCl2·6H2O exhibits the best water-uptake performance, followed by MgCl2·4H2O, while MgCl2·2H2O displays the worst water-uptake performance. These findings shed light on the adsorption behavior of water molecules in MgCl2 hydrate salt and offer valuable insights into the design and optimization of efficient thermochemical energy storage systems. Such knowledge is essential for advancing the utilization of thermochemical energy storage in solar energy applications.