Ultra-low GWP Refrigerant Mixtures as Working Fluids in ORC for Waste Heat Recovery

orc2019 Tracking Number 168

The F-gases regulations of the European Union have introduced restrictions and bans on refrigerants with global warming potential (GWP) above 150 in order to reduce the use of fluorinated gases (such as HFCs) by two-thirds compared to 2014 levels. As a result, significant research efforts have been lately oriented towards ultra-low GWP refrigerants in Vapor Compression and Organic Rankine Cycles (ORC). Meanwhile, the use of zeotropic mixtures in ORCs has been suggested as a promising second law efficiency improvement strategy in ORC applications. Considering the above, in the present study binary zeotropic mixtures of R32 and 6 ultra-low GWP fluids (R1234yf, R1234ze(E), n-pentane, propylene, isobutane and CO2) at variable molar concentration ratios, corresponding to a total of 21 working fluid combinations, are evaluated as working fluids in ORCs for waste heat recovery at heat source temperatures ranging from 100 to 300 °C. In each case, the cycles are optimized with respect to the molar concentration ratio of the mixture components and the evaporation pressure. The relative exergetic efficiency improvement attained by the use of mixture fluid ORCs (ZORCs) compared to the optimized pure fluid ORCs (PORCs) running with their constituent components strongly depends on the mixture and the heat source temperature and typically ranges from a minimum of 0.56 % to a maximum of 62.92 %. For each binary mixture, ZORCs are mostly favourable compared to the PORCs of its components within a region between the critical high critical temperature component (HTC) and LTC critical temperatures. The application of ZORCs running with ultra-low GWP fluids is primarily appealing at very low heat source temperatures (100 and 120 °C), for which the relative exergetic efficiency benefits are maximized. Among the mixtures examined, n-pentane/isobutane exhibits the highest exergetic efficiency within a broad range of heat source temperatures.