In-situ thermal conductivity of sorption composites: An analytical approach

A new analytical quasi-steady-state model to estimate the effective thermal conductivity and diffusivity of wetted sorbent composites containing thermally-conductive additives is developed based on the effective medium theory, or unit cell approach, and covers all salient morphological parameters, material properties, and operating conditions. The proposed closed-form solution is validated with experimental data using several consolidated salt-in-matrix sorbents with silica gel, CaCl2, polyvinyl alcohol (PVA) binder, and 0-15 wt.% of graphite flakes (GF), as well as 1-5 wt.% of expanded natural graphite (ENG) as thermally-conductive additives fabricated and characterized in our lab. The addition of 15wt.% of GF, and 5 wt.% of ENG to the composite sorbent when tested with a relative humidity (RH) of 2%, resulted in a ~ 536%, and a ~ 572% enhancement in the effective thermal conductivity. Also, increasing the sorbed water content in the composite with no thermally-conductive additives from 0.02 to 0.9 g∙g-1 caused a 318% increase in the thermal conductivity.