Humidity dependence of transport properties of composite materials used for thermochemical heat storage and thermal transformer appliances

Water sorption thermochemical heat storage is a promising way to provide dwellings with renewable central heating. It requires the use of several cubic meters of materials per dwelling. Depending on the design of the heating system, specific heat and mass transfer issues occur. For instance, the heat transfer rate in reactive medium and the kinetics of sorption process determine the system thermal power. In addition, the moisture propagation during inter-seasonal storage must be understood. In this paper, the influence of the water mass uptake on the apparent thermal conductivity and apparent mass diffusivity of solid material were studied. The studied material was a composite of calcium chloride (CaCl2) encapsulated in mesoporous silica with a salt content of 40-43 wt.%. The thermal conductivity was measured by the transient hot bridge method and varied from 0.13 to 0.16 W m−1 K−1, having a threshold at 0.14 g/g of water mass uptake. The apparent water mass diffusivity was studied using a diffusion column. The water diffusivity - concentration dependency was established by using the modified Hall method. The apparent diffusion coefficient ranged from 3 × 10−10 to 2 × 10−8 m2 s−1 in experimental conditions.