Development and characterization of silica gel-LiCl composite sorbents for thermal energy storage

The development of renewable energy conversion systems closely depends on the progress in efficient thermal energy storage (TES) processes. Recently, sorption thermal energy storage (STES) is perceived as a promising option for TES owing to the advantages of high energy storage density, long-term heat preservation ability and flexible working modes. This study aims to find a suitable silica gel-LiCl composite sorbent for the application of STES via the optimization of some key parameters, among which, the mass concentration of the impregnating solution and the pore structure of the matrix are most important. According to the measured sorption isobars and the pore volumes, it could be concluded that the SLi30 sample is the most appropriate composite as it offers the largest water uptake under typical working conditions for closed SETS systems and meanwhile it is free of the worrying issue of solution carryover. Another advantage of the composite sorbents is that complete water desorption can be reached at relatively low temperatures, from 60 to 100 °C. The Dubinin-Astakhov (DA) equations derived from the Polanyi theory were used to simulate the sorption properties of five sorbents. The calculation results showed that the energy densities for the composite sorbents have been remarkably improved as more hygroscopic LiCl was added into the silica gel matrix. The SLi30 sample could achieve a cold storage density of 108 kWh/m3 and a heat storage density of 163.6 kWh/m3 at a desorption temperature of 80 °C. Overall, the results demonstrate that the prepared silica gel-LiCl composite sorbents in this paper are good candidates for low-temperature driven STES systems.