Comparative analysis of promising adsorbent/adsorbate pairs for adsorptive heat pumping, air conditioning and refrigeration

In this study, the most promising working pairs are evaluated for utilization in thermal driven adsorptive heat pumping, air conditioning and refrigeration applications employing water, ethanol and methanol as refrigerant. With this aim, a comparative study was carried out for different currently available (silica gels, zeolites, aluminophosphates, activated carbons) and recently developed materials (composite adsorbents). A simple mathematical model was developed in order to evaluate the performance of various working pairs for different sorption cycles. Among the considered adsorbents, the Mitsubishi product AQSOA-FAM-Z02, the composite adsorbents LiBr-silica and CaCl2-silica appeared the best water adsorbents for air conditioning and heat pumping purpose, providing heating/cooling COP up to 1.62/0.71 and heating/cooling enthalpy up to 1080/570 kJ kg-1. Also the LiCl-silica/methanol working pair showed high performance for air conditioning cycles, especially in terms of cooling enthalpy (Qev = 640 kJ kg-1). The composite LiBr-silica showed to be the most promising methanol and ethanol sorbent for refrigeration purpose, permitting cooling COP in the range 0.53-0.59 and cooling enthalpy in the range 180-360 kJ kg-1. The noticeable influence of the metal-to-adsorbent mass ratio on the sorption cycle performance was also demonstrated, showing that utilization of compact finned tube aluminum heat exchanger types (typical mmet/mads = 0.9-1.6) allows a 15-30% cooling COP higher than a traditional stainless steel tube-and-shell exchanger (mmet/mads = 2.4-3.1). Additionally, some brief dynamic considerations are done for most interesting working pairs.