A study on isotherm characteristics of adsorbent-adsorbate pairs used in adsorption heat pumps

Abstract

Adsorption heat pumps are promising systems due to their ability to recover heat at low temperature levels and to provide cooling and/or heating effects. It has advantages such as using renewable energy sources, being environmental friendly, having no vibration and lower operation costs. In this study, the most common pairs used in adsorption heat pumps were reviewed and their thermophysical and adsorption behaviors were discussed. The adsorption equilibrium phenomena were explained in details. The different experimental methods for determination of adsorption equilibrium for pairs were explained and compared. The change of heat of adsorption according to the varying adsorbed amount was studied numerically for two different equilibrium equations. It is found that, the heat of adsorption for pairs fitting Dubinin-Astakhov equation has a decrease with increasing adsorbed amount, while the heat of adsorption for pairs fitting Freundlich equation does not depend on adsorbed amount. A numerical study was performed to investigate the performances and cooling capacities of three different adsorption chillers for six different pairs. The results showed that when S40 silica gels - water pair is used, the chiller cooled with low temperature source gives the highest performances.In the experimental section of the present study, a volumetric setup was designed and constructed. Experiments were performed for a conventional pair of silica gel - water. It was observed that the maximum adsorption capacity of the experimented silica gel -water pair was 21% (kg water vapor /kg silica gel) at 35 °C, 19% (kg water vapor/kg silica gel) at 45 °C and 11% (kg water vapor /kg silica gel) at 60 °C.