Technical and environmental analysis of transcritical Rankine cycles operating with numerous CO2 mixtures

Highlights: • Use of CO₂-based, zeotropic binary mixtures applied to power generation. • Mixture mass fraction selection based on technical and environmental parameters. • Power plant performance normalized with the mixture's GWP. An exploratory study is presented considering the effect of eight different binary mixtures that serve as working fluid for a transcritical Rankine cycle operating with or without recuperator – the binary mixtures considered are composed by CO₂, a natural fluid, and a secondary fluid (i.e., R134a, R32, R152a, R41, R161, R1234ze(E), R1234yf, R1270). The secondary component selection was based on its environmental impact (i.e., GWP < 1500 and ODP = 0) and glide temperature (< 40 °C). In addition to the working fluid composition, the study also addressed the effect of the mixture's mass fraction, which ranged from nearly pure refrigerant to nearly pure CO₂, turbine inlet pressure and the heat source's temperature on the thermal performance of the cycle. The results obtained show that, while mixtures with large mass fractions of refrigerant tend to thermodynamically outperform nearly pure CO₂ mixtures, compositions with high CO₂ content are capable of delivering competitive performance levels when technical and environmental parameters of the power plant and working fluid are considered. For instance, the GWP normalized net power produced by a basic transcritical Rankine cycle operating with pure CO₂ is significantly higher than that of the same cycle operating with pure refrigerant.