Further improvement of the synthesis of silica gel and CaCl2 composites: Enhancement of energy storage density and stability over cycles for solar heat storage coupled with space heating applications

Composite materials based on a silica gel loaded with CaCl2 are of great interest for seasonal thermochemical heat storage. In order to improve the performance of these materials for this application, and to evaluate their multi-cycle stability, a new synthesis protocol is proposed, based on successive impregnation/drying steps by using a matrix with a broad pore size distribution. Through this method, a CaCl2 content of 43 wt%, a high cycle loading lift of 0.40 g/g and an unprecedented energy storage density for this type of material of 211 kW h/m3 of packed bed composite, in conditions of a solar heat storage system (adsorption at 30 °C, desorption at 80 °C, and water vapor pressure of 12.5 mbar) can be reached. Moreover, the distribution of the salt inside the pores and the absence of any salt crust outside the matrix prevent salt leakage, leading to an outstanding preservation of the cycle loading lift after 10 cycles. Based on Polanyi theory, it can be assumed that the energy storage density can exceed 350 kW h/m3 for water sorption at 20 °C, desorption at 80 °C, with both steps at a water vapor pressure of 12.5 mbar.