**5. Conclusions**

The organic Rankine cycle (ORC) is a popular and promising technology that has been widely studied and adopted in renewable and sustainable energy utilization and low-grade waste heat recovery. The use of zeotropic mixtures in ORC reduces the irreversibility in the evaporator and the

condenser, thereby improving the thermodynamic performance. The selection of working fluid and expander are strongly interconnected. In order to make better use of the wet zeotropic mixture, a single screw expander, which is a novel expander with a tolerance of vapor-liquid two-phase expansion, was used in this paper for efficient utilization of the wet zeotropic mixtures listed in REFPROP 9.1 in a low-temperature subcritical ORC system.

Five indicators, namely net work, thermal efficiency, heat exchange load of condenser, temperature glide in evaporator, and temperature glide in the condenser, were used to analyze the performance of an ORC system with wet and isentropic zeotropic mixtures as working fluids.

Through calculation and analysis, it can be seen that R441A, which is a wet zeotropic mixture, can be selected as a suitable working fluid in subcritical ORC using a single screw expander without considering the isentropic efficiency of the expander. It is suitable for both open and closed type heat sources. Its reasonable and feasible expander exit temperatures range from 310 K to 320 K. When considering the isentropic efficiency of a single screw expander, R441A with an expander outlet temperature of 320 K may be the suitable zeotropic mixture for both open and close type heat sources. R436B may be selected for an expander outlet temperature of 315 K. R432A may be selected for an expander outlet temperature from 295 K to 310 K.

All three selected zeotropic mixtures belong to the A3 safety group classification which means a lower toxicity but a higher flammability. These three mixtures have very low GWP due to their components. In future practical applications, it is necessary to consider the ORC system complexity that may be caused by the use of the mixture.

**Author Contributions:** Conceptualization, X.Z.; Data curation, Y.Z., X.Z., and Z.L.; Formal analysis, X.Z. and Y.Z.; Funding acquisition, X.Z.; Methodology, X.Z. and Y.Z.; Resources, Y.W., C.M., and J.W.; Writing—original draft, X.Z.; Writing—review & editing, X.Z. and J.W. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by the National Natural Science Foundation of China (Grant No.51506001) and Beijing Municipal Education Commission (KM201710005029). The authors gratefully acknowledge them for financial support of this work.

**Conflicts of Interest:** The authors declare no conflict of interest.
