Assessing the suitability of alkali-activated metakaolin geopolymer for thermochemical heat storage

This study investigates the suitability and potentials of using alkali-activated metakaolin geopolymers as sustainable low-cost thermochemical heat storage materials via water sorption and/or hydration reactions. Four different alkali-activated metakaolin geopolymer formulations were assessed. The cyclic water sorption capacity and moisture diffusion coefficients were assessed via two continuous water vapor sorption/desorption cycles using the dynamic water vapor sorption (DVS), proving the regeneration ability of geopolymers for sorption thermal energy storage. The thermochemical properties of geopolymers, including the dehydration enthalpy and activation energy, were determined. For the amorphous sodium aluminosilicate hydrate gel (N-A-S-H) with bulk Si/Al ratio of 1.5, the mass and volumetric energy storage capacity of 827.9 J/g_{hydrate sample} and 350 kW h/m³ were achieved, with a charging temperature of around 120 °C. The outcomes of this study suggest that alkali-activated metakaolin geopolymers have the potential to be used for both low-temperature water sorption thermal energy storage and medium temperature hydration/dehydration thermochemical energy storage. The energy storage performances of metakaolin geopolymers are closely related to their aluminosilicate framework structures and surface textural properties. The partial crystallisation of the amorphous N(K)-A-S-H gel, resulting in the formation of micropores, which does not seem to affect the maximal water uptake but increases the proportion of the chemically bound water. The presence of the amorphous mesoporous aluminosilicate gel N(K)-A-S-H gel in geopolymers leads to desorption hysteresis at relative humidity higher than 30 %. Further optimisation of the synthesis approach, aluminosilicate framework structures, micro and mesopore structures will be required to optimise their overall thermal energy storage performances.