Thermal conductivity of composite sorbents "salt in porous matrix" for heat storage and transformation

The thermal conductivity of composites "inorganic salt in porous matrix" was measured as function of the water uptake w by a "hot wire" method. Three hygroscopic salts (CaCl₂, MgCl₂, LiBr) were confined to the pores of a commercial KSK silica gel. For CaCl ₂, an alumina was used as host matrix, too. Similar dependencies λ(w) were found for the three silica based sorbents: a smooth rise at w < w* = 0.4-0.55 and a sharp increase within the narrow w-range near the threshold uptake w*. Regardless the salt confined, this increase occurs at the same fraction ν of the pore volume occupied by the salt solution, ν* = 0.60-0.64. At ν > ν* the solution starts to leak out of the pores to form a liquid film on the external surface of silica particles that strongly facilitates the heat transfer between adjacent particles and enhances the heat conductivity of the whole bed. No such threshold behaviour was found for CaCl₂/alumina composites. The data obtained can be used for analysing open and closed adsorptive units for heat transformation, gas drying, extraction of potable water from the atmosphere, etc. For user friendliness, the experimental λ(w)-dependences were approximated by polynomial equations.