Impact of the impregnation method on the volumetric behaviour of vermiculite–CaCl2 composites under packed-bed cycling for thermochemical energy storage

Abstract

Thermochemical energy storage (TCES) based on salt hydrates offers high energy density and negligible standby losses. However, repeated hydration–dehydration in packed beds can cause salt redistribution, pore restructuring, and substantial changes in settled packed volume, which may alter vapour transport and thermal output. This study compares atmospheric impregnation (AP), wet vacuum-assisted impregnation (WVac), and dry vacuum-assisted impregnation (DVac) for vermiculite–CaCl 2 composites over 20 hydration–dehydration cycles at 25 °C and 50 ± 2% relative humidity. The initial estimated volumetric salt contents were 0.073, 0.208, and 0.249 g cm −3 for AP, WVac, and DVac, respectively. At cycle 20, DVac retained the highest dynamic temperature response and showed a 43% increase in settled packed volume. After 20 cycles, the DVac composite retained a more open mercury-accessible pore structure than in the as-prepared state. The MIP, microscopy, and static-sorption measurements are consistent with improved vapour accessibility in the cycled DVac composite. Using the measured outlet-flow information, the estimated 3 h cumulative sensible heat carried by the air stream during hydration was approximately 480 kJ for DVac, 390 kJ for WVac, and 290 kJ for AP. The results show that impregnation-route screening should consider not only initial salt loading but also the evolution of pore accessibility and packed-bed volume during cycling.