When do thin zeolite layers and a large void volume in the adsorber limit the performance of adsorption heat pumps?

Abstract

In order to allow for the required pressure change in the adsorption heat pumps during the isosteric heating and cooling periods, a sufficiently high ratio between the zeolite mass and the void volume should be provided in the system. In this study, the effect of the percentage of void volume in the adsorber on the p– T relationship of the zeolite 4A-water pair during the heating period was investigated for various thicknesses of zeolite coatings and radii of heat exchanger tubes. As a result, it was determined that a significant reduction in the performances of the adsorption heat pumps does not occur for the parameter values of practical concern. Even very thin zeolite coatings (5 μm) may be used with a high void volume (e.g. 80%) in the adsorber. Calculations in a recent study, leading to the claim that coatings of thicknesses<500 μm with a relatively large void volume in the adsorber (⩾50%) may not allow for the required pressure change in the adsorption heat pumps, seem to be in significant error. Utilizing zeolite 13X instead of 4A may lead to some improvement in the power and thermodynamic efficiency of the adsorption heat pumps, but this favorable effect loses its significance for relatively thick coatings.