Thermodynamic analysis of carbon dioxide blends with low GWP (global warming potential) working fluids-based transcritical Rankine cycles for low-grade heat energy recovery

Carbon dioxide is a promising natural working fluid that can be used in transcritical Rankine cycles due to environmental and safety concerns. However, the high operation pressure has to be reduced and the relatively low efficiency of the system has to be increased. Traditional working fluids have been widely investigated to reclaim low-grade heat energy, and most of them have high GWPs (global warming potentials) or are flammable or even toxic. Consequently, to mitigate the above disadvantages, we studied zeotropic mixtures of carbon dioxide blends with 7 low GWP working fluids for use in a TRC (transcritical Rankine cycle) for low-grade heat conversion. The results revealed that these zeotropic mixtures can help improve the thermal efficiency of the TRC and decrease the operation pressure compared to that of pure CO2. Owing to the perfect thermal match in the heat transfer process, higher exergy efficiencies were achieved for the entire system when zeotropic mixtures were used than pure CO2. Maximum exergy efficiencies exist for the TRC at the corresponding optimal pressures for each mixture. Finally, the mixture CO2/R161 is recommended for small capacity instruments for its high efficiency, in spite of its high flammability; the mixtures CO2/R1234yf and CO2/R1234ze can be used in TRCs with larger capacities due to their lower flammability.