Conference Year

2014

Keywords

high-temperature heat pump, heat recovery, coefficient of performance, primary energy consumption

Abstract

To reduce energy consumption utilizing heat recovery systems is increasingly important in industry. The screening of refrigerants and primary energy efficiency are mainly discussed with a case study for high-temperature heat pump cycles in this study. The overall coefficients of performance of four different cycle configurations to increase temperature of compressed water up to 160 oC using waste heat of 80 oC are calculated and thermodynamically compared for selected refrigerants, R717, R365mfc, R1234ze(E), and R1234ze(Z). The calculation results suggest that a multiple stage cycle drastically reduces throttling loss in expansion valve and exergy loss in condensers, and consequently achieves the highest overall coefficient of performance (COP) among the calculated cases with a refrigerant R1234ze(Z) having the critical temperature about the target outlet water temperature. A cascade cycle using R1234ze(Z) and R365mfc performs relatively high COP and also brings many practical benefits, such as varied combination of refrigerants and lubricant oils and prevention of the liquid-back caused by cold start. At a compressor efficiency of 0.7 and an approach temperature difference in heat exchangers of 2 K, the calculated overall COP is ranging from 4.3 to 4.94. This is corresponding to the primary energy efficiency 1.62 to 1.83, when the transmission-end-efficiency of electric power generation is 0.37. Even where the compressor efficiency of 0.61 and the approach temperature difference of 8 K, the efficiency is above 1.3. As remarked above, the thermodynamic assessment demonstrated that the high-temperature heat pumps to recover waste heat are promising system to reduced primary energy consumption for industrial applications.

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