Energetic and exergo-environmental analysis of transcritical high-temperature heat pumps with low GWP refrigerants for industrial waste heat recovery

Abstract

Sub-critical mechanically driven industrial heat pump technologies have gained momentum in recent years with systems capable of supplying temperatures up to 150°C. Extending the operating envelope using heat pump technologies to and beyond 200°C requires an innovative shift towards transcritical techniques to meet the demands of a broader range of industrial processes. Transcritical high-temperature heat pumps (TC-HTHPs) pose many technical and operational challenges requiring research and development to evaluate operational performance. This paper assesses the feasibility of operating a HTHP system to achieve 200°C heat sink temperatures using suitable low GWP refrigerants. The work investigates three different TC-HTHP cycle configurations using a steady-state theoretical model to compare and evaluate energetic, exergetic and environmental performance. In addition, cycle performances for this study are obtained at the optimum pressure for the gas cooler in which the maximum COPs are achieved. The results reveal that the basic cycle with dual internal heat exchanger (IHX) was the most efficient configuration, with HFO-514A and HFO-1234ze(Z) refrigerants identified as the most promising candidates for TC-HTHP systems. Configurations employing HCFO-1233zd(E) exhibited a trade-off between high energetic and exergetic efficiency while providing an A1 safety group classification. A pinch point analysis of the gas cooler demonstrated the need to optimise the overall length to achieve increased operational performance at very high glide temperatures. An evaluation of the environmental impact identified reductions in TEWI value by up to 20 % for the refrigerants tested over HFC-245fa. This study provides a basis for future practical activities of TC-HTHPs using eco-friendly candidates.