Optimizing the Compression/Absorption Heat Pump System at High Temperatures
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Large amounts of low grade waste heat from industrial processes are not utilized, due to lack of heat integration equipment. Industrial processes tend to have specifications at high temperature lifts, that not are suited to be operated by conventional technology from residential heat pumps. Standard vapor compression heat pumps have undesirable high pressure ratios that are inefficient at high temperature lifts. Compression-absorption heat pumps use zeotropic working fluid mixtures that are suitable for temperatures between - 10 and + 160 ℃ at system pressures below 20 bar, which make them applicable for delivering heat to high temperature processes. The advantages of the compression-absorption heat pumps, also known as the hybrid heat pump are the use of non-ozone depleting working fluid mixtures, reduced irreversibilities due to heat transfer with temperature glides, high temperature lifts, low pressure ratios and flexible capacity control. Two separate simulation models were developed comprising a two-stage CAHP system and an absorber model. The two-stage CAHP system used waste heat water at 50 ℃ as heat source and heat sink temperatures, with the objective of achieve maximum supply temperature at four different compressor discharge temperature limitations. The absorber model compared five different compact heat exchangers heating air in a cross-flow, where the main goal was to minimize the absorber height and the fan work. The two-stage process investigated the benefits of the desuperheater, where the supply temperatures with and without the desuperheater where nearly the same. Maximum supply temperatures were obtained at 171.8 ℃ with a COP of 2.08, when the maximum discharge temperature was set to 250 ℃. A correction factor was used for the intermediate pressure as K∙√(P_LP∙P_HP ) The optimum K-factor increased at elevating absorber pressure from 1.16 to 1.35 at absorber pressure from 17 to 47.5 bar. Simulations from the absorber model yielded much larger mass flow rate for the air than for the mixture. The heat exchange between the air and the mixture was sensitive to the absorber height and the air mass flow rate, which resulted in large pressure drops and fan work. Finned at tube heat exchangers gave the best results with respect to the absorber height and fan work. There is suggested to conduct further work with other heat sink and source temperaturesand also optimize the temperature lift in the two stage model. Finned at tube heat exchangers could be further investigated in an absorber model with other dimensions and more accurate approaches for thermal resistance and fin efficiency.