Optimization of the cycle durations of adsorption heat pumps employing zeolite coatings synthesized on metal supports

Abstract

In order to remove the limitations originating from inefficient heat and mass transfer in adsorption heat pumps an arrangement involving zeolite 4A coatings synthesized on stainless steel heat exchanger tubes was recently proposed and a related mathematical model was presented. In this study, the same mathematical model is employed to optimize the cycle durations in order to obtain the maximum amount of power from the adsorption heat pumps. Accordingly, the temperature and concentration profiles across the adsorbent layer are computed for distinct cases employing various cycle durations. The optimum cycle durations that maximize the energy obtained during a specific period of time for a given adsorber volume, corresponding to the employment of various zeolite layer thicknesses as well as two distinct wall thicknesses of the heat exchanger tubes are determined. The ratios between the optimum cycle durations and those obtained previously by allowing the operation of a cycle to be completed only when the temperature difference between the heat exchange fluid and the surface temperature of the adsorbent layer decreases to 1°C, remain around 0.2–0.4 for the cases investigated. The employment of the optimum cycle durations results in an increase of almost twofold in the power obtained from the adsorption heat pumps while the optimum zeolite layer thickness value obtained in this case is observed to increase by about 50% and is found to be in the range 75–150 μm depending on the wall thickness of the heat exchanger tubes utilized in this study.