Multi-field coupling analysis of thermochemical energy storage reactor with prepared Mg(OH)2/expanded graphite composites

Mg(OH)₂/MgO thermochemical energy storage(TCES) system becomes more attractive in the field of solar thermal utilization, but the coupling mechanism between the reaction kinetics and heat and mass transfer has not been fully explored. In this paper, Mg(OH)₂/expanded graphite (EG) thermochemical energy storage composites were first prepared using the wet ball milling method, focusing on the reaction kinetics of the dehydration process. It was found that the rehydration rate remained above 80 % after multiple cycles. Next, numerical simulations of the dehydration and hydration processes in the reactor filled with the Mg(OH)₂/EG composites were carried out to reveal the mechanism of the multi-physical field coupling of the chemical reactions. The results show that the improvement of the thermal conductivity of the composites enhances the heat transfer in the reactor and makes the temperature distribution of the composites more uniform, leading to significant reduction of the reaction time and an increase of 68.3 % in the thermal charge power compared to that of pure Mg(OH)₂ used. By introducing a gradient porosity structure to optimize the mass transfer characteristics of the reactor, the dehydration/hydration performance can be significantly improved compared to a fixed porosity. The direct benefits include reduced reaction time by 19.7 % and 39.5 % and reduced average pressure by 7.5 % and 82.2 %. Particularly, the average thermal charging/discharge power is increased by 11.1 W and 1.9 W, respectively.