Review of regenerative adsorption heat pumps

NASA is again considering a permanent lunar base as part of its new Exploration Vision. Because the lunar surface reaches 100 degrees C near the equator during the lunar day (14.75 Earth days) and -100 degrees C during the lunar night (also 14.75 Earth days), some type of heat pump is needed to elevate the temperature of absorbed sunlight and interior waste heat in order to reject it to space. An earlier investigation by M. A. Lambert (Conceptual Design of a Regenerative Adsorption Heat Pump for a Lunar Base, AIAA Paper 99-0465, 1999) identified adsorption (solid-vapor) heat pumps as one of two technologies best suited to this application, the other being a mechanical vapor compression device with a high-efficiency centrifugal compressor. In order to properly preface the creation of a detailed design for an adsorption heat pump, this investigation reviews the state of the art in adsorption heat pumps, specifically the recently developed, much more efficient, regenerative variants. The principal figures of merit are the coefficient of performance for cooling (COPC) and the specific cooling power. Nonregenerative adsorption heat pumps exhibit a COPC of only 0.3 to 0.4, whereas regenerative designs have achieved a COPC of 1.1 with 70% regeneration of waste heat. Attention has focused on maximizing overall heat transfer coefficient and minimizing nonadsorbent mass of the most critical component, the adsorption bed, while maintaining adequate vapor permeability. This involves establishing a thermal wave temperature gradient traversing the bed, which minimizes the temperature difference for heat transfer and entropy generation. A number of novel designs have resulted. The most studied adsorbents are zeolites (a class of highly nanoscopically porous minerals), activated graphite, and silica gel. The refrigerants receiving the greatest attention are ammonia, methanol, and water.