Theoretical evaluation of a transcritical high temperature heat pump using eco-friendly refrigerants

Decarbonisation of the industrial sector in developed countries is an important element in addressing climate change worldwide. Many high temperature industrial heat processes up to 150°C could consider high temperature heat pump (HTHP) technologies as a viable replacement to existing fossil fuel equipment. Higher process temperatures up to 200°C would require a shift from subcritical HTHPs (SC-HTHPs) to transcritical HTHP (TC-HTHPs). These systems operate at much higher pressures and temperatures resulting in several technical and operational challenges to be solved. This paper investigates three different operating configurations for a TC-HTHP cycle to achieve 200°C using suitable low GWP refrigerants. A steady-state theoretically model was developed to compare and assess the energetic, exergetic and environmental performance for the TC-HTHP cycle. Employing the LMTD method, the gas cooler's heat transfer coefficient, overall length, and heat transfer area were calculated Analysis of the gas cooler indicated that compressor discharge pressure, gas cooler pinch points, and the temperature of waste heat supplied to the evaporator all significantly affect cycle performance. The energetic and exergetic results show that a TC-HTHP cycle with two additional IHX proved to be the most efficient cycle across a variety of heat source temperatures ranging from 75 to 90°C. In comparison to other candidates, the refrigerant R514A had the best performance with a high COP, high exergetic efficiency, lowest TEWI and lowest gas cooler pressure demand. Analysis of the gas cooler indicated that the compressor discharge pressure, the pinch point of the gas cooler, and the temperature of waste heat provided to the evaporator all significantly affect the overall cycle performance.