Exergy efficiency potential of dual-phase expansion trilateral and partial evaporation ORC with zeotropic mixtures

The exergy efficiency improvement potential of dual-phase expansion (trilateral and partial evaporation) Organic Rankine Cycles (ORC) with zeotropic mixtures of R1233zd(E), R1234ze(E) and R1234yf as well as isobutane and propane for waste heat temperatures from 80 °C to 200 °C is investigated. For each fluid pair, standard ORCs are compared to trilateral and partial evaporation cycles (T-ORC and P-ORC) of mixtures (T-Z and PE-Z) and pure fluids. For isobutane-propane and R1233zd(E)-R1234ze(E), PE-Z and T-Z cycles result in the highest exergy efficiency for most temperatures. For R1233zd(E)-R1234yf, PE-Z and T-Z cycles are superior below 140 °C. For R1234ze(E)-R1234yf, PE-ORCs and standard ORCs with R1234ze(E) are most efficient at temperatures below and above 140 °C, respectively. PE-Z cycles of R1233zd(E)-R1234ze(E) exhibit the highest efficiency at all temperatures except for 100 °C, at which T-Z cycles are superior. Generally, PE-Z cycles have slightly higher exergy efficiency compared to PE-ORCs with pure fluids of high critical temperature. Given the technical challenges of zeotropic cycles, the latter could be more appealing. For expansion isentropic efficiencies around 60%, dual-phase expansion cycles remain competitive against saturated vapor zeotropic (S-Z) and saturated ORCs at lower temperatures. However, isentropic efficiencies above 50% are necessary for them to be competitive against S-Z cycles.