**5. Conclusions**

The organic Rankine cycle (ORC) is a popular technology used in waste heat recovery and medium- to low-temperature heat utilization. The working fluid plays a very important role in the thermodynamic cycle. The expander is a key device in ORC. In order to give full play to vapor–liquid two-phase expansion tolerance, which is an important characteristic of single-screw expanders, two ORC models—an ideal subcritical cycle model and an ideal transcritical cycle model—were established. On this basis, the isentropic e fficiency of the expander was also considered for the purposes of analyzing thermodynamic performance.

Three indicators—namely, net work output, which was used to evaluate open heat source, thermal efficiency, which was used for closed heat source, and the heat exchange load of the condenser, which was used to evaluate economic performance—were chosen to analyze the performance of the ORC systems.

On the basis of our calculations and analyses, it can be seen that, without considering the isentropic efficiency of the expander, ORC systems that use single-screw expanders and undergo a vapor–liquid two-phase expansion are able to obtain higher thermal efficiency, greater net work output, and smaller heat exchange loads of the condenser. Cis-butene may be the best candidate when working in the subcritical cycle. HFO working fluids are more suitable for working in the transcritical cycle, and HFO-1234ze(E) may be the best.

When considering the isentropic efficiency of the expander, cis-butene is still the best choice for working in the subcritical cycle. Meanwhile, for the transcritical cycle, HFO-1234ze(E) may still be the best.

**Author Contributions:** Conceptualization, X.Z.; Data curation, Y.Z., X.Z., and M.C.; Formal analysis, X.Z. and Y.Z.; Funding acquisition, X.Z.; Methodology, X.Z.; Resources, Y.W., C.M., and J.W.; Writing—Original Draft, X.Z.; Writing—Review & Editing, J.W.

**Funding:** This research was funded by the National Natural Science Foundation of China (Grant No.51506001) and the 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.
