MINIATURIZATION OF AN AMMONIA-WATER ABSORPTION CYCLE HEAT PUMP USING MICROCHANNELS

Small-scale, portable vapor compression cooling systems are encumbered with the need for electricity from batteries or a portable generator in order to operate the mechanical compressor. Heat actuated technologies, such as the absorption cycle heat pump, offer an alternative that can substantially reduce or eliminate the reliance on electricity. Heat can be provided by combusting high energy density liquid fuels or by recovering waste heat from other processes, such as fuel cell systems or vehicle exhaust. With support from the U.S. Army, collaboration between the Pacific Northwest National Laboratory (PNNL) and Oregon State University (OSU) is endeavoring to develop miniaturized heat-actuated technologies for portable and personnel cooling. Progress on a miniaturized ammonia-water cycle is reported, including the recent operation of a 250 Watt breadboard system. Microchannel technologies offer a unique approach for intensifying these transport processes by reducing the characteristic length-scale to less than a millimeter. In applying these techniques to miniaturize the absorption cycle heat pump, the major technical hurdles are associated with the absorber and desorber, because of the requirements for two-phase processing and for balancing heat and mass transfer. The PNNL approach for absorbers utilizes thin wicking materials within microchannels. OSU is developing desorbers using fractals that operate similar to a thermal inkjet printer. The principles of the concepts are presented along with experimental results from testing prototype devices operating with ammonia-water at conditions consistent with ammonia-water heat pump cycles. Potential for additional size and weight reductions is discussed. One application target calls for a 150 Watt portable cooling system for the individual soldier that weighs less than 6 pounds and more preferably less than 4 pounds.