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 degrees C-150 degrees 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 degrees C by nucleation and 40 degrees 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.

Automated summary

Thermochemical energy storage (TCES) provides a high energy storage density for storing heat indefinitely, making it ideal for seasonal thermal energy storage (TES). Research on the hydration of inorganic salts such as K2CO3 and MgCl2 sheds light on the kinetic differences between materials, improving hydration efficiency and understanding rate-limiting mechanisms. Expanding this fundamental understanding is crucial for unlocking the full potential of TCES and developing technical solutions.