Heat and mass transfer in porous spherical pellets of CaCl2 for solar refrigeration

An analysis of heat and mass transfer in porous spherical pellets with vapour absorption/desorption is presented. The size, shape and thermophysical properties of the pellet is assumed invariant with time, and the volume reaction model is utilised. The temperature of the solid and vapour phases are assumed to be the same at any location. The resulting partial differential equations for the coupled heat and mass transfer processes are solved numerically. Quantitative values are obtained using data specific to the CaCl2–NH3 absorbent–refrigerant system under conditions typical for solar refrigeration applications. The generation, diffusion and absorption kinetics of the system are studied by computer simulation for pellet porosity, ϵ, in the range 0.02≤ϵ≤0.10. It is found that, for pellet sizes ranging from 4 to 20 mm diameter, the intra-pellet free ammonia diffusion resistance is negligible. Furthermore, the intra-pellet temperature and absorption level gradients are only significant for pellet sizes greater than about 20 mm. Thus, a lumped parameter analysis of the single pellet problem is adequate for solar refrigeration applications, for which absorbent pellet sizes are normally in the range of 3–10 mm. Also, where the absorbent volume is nodularized for numerical solutions of the heat and mass transfer equations, a node size limit of 20 mm should be observed.