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 degrees C to 200 degrees 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 degrees C. For R1234ze(E)-R1234yf, PE-ORCs and standard ORCs with R1234ze(E) are most efficient at temperatures below and above 140 degrees C, respectively. PE-Z cycles of R1233zd(E)-R1234ze(E) exhibit the highest efficiency at all temperatures except for 100 degrees 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.