Hydration kinetics of K₂CO₃, MgCl₂ and vermiculite-based composites in view of low-temperature thermochemical energy storage

Thermochemical energy storage (TCES) may store heat for a theoretically indefinite amount of time at high energy storage density. It is an ideal means to achieve seasonal thermal energy storage (TES). Hydration at atmospheric pressure of inorganic hygroscopic salts has attracted much attention from the scientific community: TES in the range 30 °C-150 °C is achievable and suitable for domestic heating applications such a space-heating. While progress at both material and reactor scales have been made, there is still a lack of fundamental understanding of the relationships connecting the two, which is necessary in order to enable full TCES potential and develop technical solutions. We investigated inorganic salts K₂CO₃ and MgCl₂, and composites consisting in these salts impregnated into vermiculite. Experimental measurements (dynamic vapour sorption) and numerical optimization of known solid-state kinetic models relevant for sorption were used to derive kinetic coefficients for different solid-state reaction kinetic models and to shed light on the possible rate-limiting mechanisms of the hydration of each material. Potassium carbonate (K₂CO₃) hydration was found to be kinetically hindered by what appears to be a diffusion barrier at the interparticle level. Impregnation of K₂CO₃ lead to a significantly improved hydration, controlled at 25 °C by nucleation and 40 °C by phase-boundary control according to the best fitting kinetic models. MgCl₂ hydration was best modelled by first-order model and diffusion-type models, pointing towards intraparticle diffusion control. Finally, the hydration of MgCl₂ impregnated into vermiculite was best modelled by phase-boundary control models, with no notable rate-limiting step change at different temperatures.