Study on consolidated composite sorbents impregnated with LiCl for thermal energy storage

LiCl/H2O shows a great potential for sorption thermal energy storage with its large water sorption capacity. However, as a highly hygroscopic salt, LiCl is likely to turn into liquid solution when it contacts with water vapor. A new type of consolidated composite sorbent was developed, by using activated carbon (AC) as a porous host matrix to carry LiCl, mixing with expanded natural graphite treated with sulfuric acid (ENG-TSA) to increase heat transfer and adding silica solution (SS) as a binder to enhance mechanical strength. Samples with different densities and varied mass ratios between different elements were prepared and related characteristics including equilibrium water uptake, thermal conductivity, TGA/DSC simultaneous thermal analysis and kinetic performance were investigated. Results reveal the existence of an obvious sorption plateau corresponding to formation of LiCl ⋅ H2O, as well as two sharp increase of water uptake related to hydration reaction and three-phase deliquescence processes respectively. Activated carbon is a better choice of matrix compared with silica gel, with a water uptake up to 0.97 g/g at 30 °C and 1660 Pa. It is proved that among the four elements in the consolidated composite (AC, LiCl, ENG-TSA, silica solution), LiCl is responsible for almost all the water uptake effect. Addition of other elements do not influence the equilibrium water uptake of LiCl. Variation of bulk densities from 462 to 820 kg/m3 displays thermal conductivities from 2.0 to 2.83 W/(m K), improving that of loose-packed AC–LiCl composite by at least 14 times.