Characterization of transport limitations in SAPO-34 adsorbent coatings for adsorption heat pumps

Abstract

Adsorption heat pumps have become an increasingly viable technology to use waste heat or renewable thermal energy for heating and cooling. The power density of this emerging technology is limited by the rate of heat and/or mass transfer in the adsorption heat exchanger (AdHEX) which drives investment costs. This work presents an experimental analysis of the mass and heat transfer during water sorption on SAPO-34 coatings to determine the limiting transport mechanism in state-of-the-art AdHEX. Isochoric temperature swings were carried out and evaluated using a recently introduced method to determine the relative importance of heat and mass transport impedances. Coatings with thicknesses between 60 and 460 µm were investigated and in all cases the sorption dynamics were limited by mass transport. Ragone plots were used to characterize the power and energy trade-off during thermal cycling of SAPO-34 in water vapor to identify the pareto-optimal cycle time for a specific coating thickness. With the knowledge of the rate-limiting mechanism, the overall transport rates of adsorbent coatings can systematically be improved to enhance transport rates in next-generation AdHEX.