Exergetic performance of CO2 and ultra-low GWP refrigerant mixtures as working fluids in ORC for waste heat recovery

Abstract

Environmental regulations target refrigerants with global warming potential (GWP) above 150. Meanwhile, zeotropic mixtures are proven to increase the exergetic efficiency of Organic Rankine Cycles (ORCs). The present study investigates the exergetic performance of binary mixtures of R32 and 8 ultra-low GWP fluids (n-pentane, NOVEC649, R1233zd, isobutane, R1234ze, R1234yf, propylene, and CO2) in standard and recuperative ORCs. The ORCs are optimized with respect to the molar concentrations of their components and evaporation pressure. The relative exergetic efficiency improvement of zeotropic (ZORCs) compared to pure fluid ORCs (PORCs) is negatively correlated with the heat source temperature and ranges from a maximum of 36.39% (at 100 °C) to less than 5% at temperatures above 200 °C. For each mixture, ZORCs are favorable over PORCs of their components for heat source temperatures primarily below and secondarily between the critical temperatures of their components, while the average relative performance improvement is about 30–50%. At increasing heat source temperatures, ZORCs operating with isobutane, NOVEC649, R1233zd and n-pentane are successively optimal. Although recuperative ZORCs are superior to standard ZORCs for dry mixtures of components with a large critical temperature difference at higher temperatures, standard cycles are ultimately superior considering the global optimization results.