A new composite sorbent based on SrBr₂ and silica gel for solar energy storage application with high energy storage density and stability

Highlights: • Synthesis of a new composite material based on SrBr₂ and silica gel. • High energy storage density of 203 kW h/m³ is reported for thermal energy storage applications. • High cycling stability is reported. - Abstract: The excellent matching between the sorption and desorption temperatures of hexahydrated SrBr₂ and those required for solar heat storage for building applications, the high heat of reaction (67.5 kJ/mol of water) coupled with the gain/loss of 5 mol of water per mole of salt make this salt an appealing sorbent for solar thermal energy storage applications coupled to space heating. Due to the morphological instability of this salt, it is necessary to incorporate it in a porous matrix as a composite sorbent. A new composite material for thermochemical energy storage applications was developed. It consists of a mesoporous silica gel impregnated by strontium bromide with salt content equal to 58 wt.%. The structure and the sorption properties of the composite were characterized by SEM-EDX, temperature dependent XRD, XRF, and N₂ sorption measurements. The salt is homogeneously distributed inside the pores of the silica gel. Water sorption isotherms were measured between 20 °C and 80 °C, which enabled us to understand the sorption mechanism. A mathematical model was developed and used to fit the experimental data in order to predict the sorption behavior of the composite at different conditions (influence of temperature and pressure conditions on the cycle loading lift and energy storage density). The interest of using such a composite for thermal energy storage application is then discussed (thermal energy produced by solar collector and used for space heating). A high cycle loading lift of 0.22 g/g is obtained corresponding to an energy storage capacity of 230 W h/kg and an energy storage density of 203 kW h/m³ of packed bed composite (between 30 °C and 80 °C at 12.5 mbar) is reported, with an excellent stability over 14 sorption/desorption cycles. The sorption kinetics of this composite is enhanced compared to pure salt. Test on a laboratory scale open type reactor gives a maximum specific thermal power of 200 W/kg and a mean specific thermal power of 92 W/kg at 30 °C and 12.5 mbar for an extent of reaction of 0.68.