Determination of characteristics of adsorbent for adsorption heat pumps

Abstract

Adsorption heat pumps, which are environmentally friendly and operating with thermal sources, have gained attention in recent years. Although they have higher primary energy efficiency than traditional heat pumps, adsorption heat pumps require improvements due to the low COP and SCP/SHP values. The selection of appropriate working pair, the determination of adsorption equilibrium and kinetics of the pair is quite important in the design of adsorption heat pumps. In this study, the working pairs used in adsorption heat pumps were discussed, and the models used in adsorption equilibria and kinetics were explained. In the experimental part, the effect of the adsorption and desorption temperatures on adsorption capacity and mass diffusivity were investigated. Type RD silica gel-water and zeolite 13X-water were selected as working pairs in the adsorption experiments. Accordingly, two volumetric systems were constructed; adsorption experiments were conducted and pressure changes were recorded against time. The experimental studies showed that the adsorption capacity was decreased with increasing adsorption temperature and with decreasing desorption temperature, and zeolite 13X-water pair had higher adsorption capacity than type silica gel-water pair under the same conditions. Type II and Type I isotherms were observed for type RD silica gel-water pair and zeolite 13X-water pair, respectively. The effective diffusivity of zeolite 13X-water pair was found in the range of 2x10-8-9x10-9 m2/s for the short time period and in the range of 7x10-10-10-8 m2/s for the long time period. In addition, it was seen that the effective diffusivity was effected from the initial adsorptive concentration and the effective diffusivity was decreased with increasing adsorbate concentration. This may be related to the effects of the heat transfer resistance, surface resistance or hydration and migration of the cations in the structure of the zeolite 13X.