Systematic performance assessment of double-stage high-temperature heat pumps based on zeotropic mixtures of natural hydrocarbons

This study investigates, for the first time, the performance potential of zeotropic mixtures of natural hydrocarbons in double-stage heat pumps (DSHPs). A parametric analysis is conducted for heat source temperatures of 20–80 °C, heat sink temperatures of 80–140 °C and source and sink temperature differences of 10–40 K. A screw compressor model is employed to predict isentropic efficiency as a function of pressure ratio and optimize the built-in volume ratio. Multiple practical design constraints are incorporated, while internal heat exchangers (IHXs) are examined. The COP improvement potential of zeotropic mixtures compared with pure fluids is highly case-specific and strongly dependent on mixture properties and heat source/sink characteristics. For single-stage heat pumps (SSHPs), the maximum COP improvement ranges from 37% to 117%, although typical values remain below 30%. For DSHPs, maximum improvement ranges from 18% to 145%, with most cases below 20–30%. DSHPs outperform SSHPs at higher temperature lifts and lower source and sink ΔΤ , whereas SSHPs are superior at lower temperature lifts. The maximum relative COP improvement of DSHPs over SSHPs ranges from 37% (pentane-isopentane) to 113% (pentane-butane), while typical values remain below 30–40%, showing strong variability across fluids and operating conditions. Overall, DSHPs operating with zeotropic mixtures can achieve substantial performance improvements compared with SSHPs and DSHPs using pure fluids. However, there is a strong dependence of performance benefits on heat source and sink temperatures and temperature difference. Finally, near-optimum performance can often be obtained with different mixture compositions.