Influence of mesoporous Wakkanai siliceous shale and microporous zeolite 13X on the thermal energy storage performance of MgCl2/2CaCl2 composites

Thermochemical energy storage (TCES) using composite salt hydrates is a promising technology for low-grade heat utilization. However, a systematic comparison linking the mass transfer behavior and structural stability of representative mesoporous and microporous matrices to packed-bed reactor performance under identical salt and operating conditions is still lacking. In this study, composites based on mesoporous Wakkanai Siliceous Shale (WSS) and microporous 13X zeolite, both impregnated with MgCl 2 /2CaCl 2 , were systematically compared under identical preparation and operating conditions. The thermophysical properties, sorption characteristics, structural stability, and packed bed thermal energy storage performance were experimentally investigated to clarify the influence of pore structure on TCES performance. The mesoporous WSS composites exhibited higher water uptake and better structural stability than the 13X composites, whereas pore blockage and pellet pulverization occurred in 13X at high salt loadings. In experiments using an open TCES system with a packed bed reactor, the 13X composites produced a higher initial temperature rise, but their heat output decayed rapidly because of restricted mass transfer and structural degradation. In contrast, the interconnected mesoporous structure of WSS maintained pellet integrity, facilitated water-vapor transport, and thereby sustained hydration. Consequently, WSS20 achieved an average water uptake exceeding 0.6 g/g, sustained a temperature rise above 10 °C for 140 min, and provided the highest effective volumetric thermal energy storage density of 0.86 GJ m −3 . These results demonstrate that mesoporous matrices can provide higher water uptake, sustained heat release, and improved structural stability in composite salt hydrate TCES systems.