**5. Conclusions**

Referring to the applicable environmental parameter limits set on refrigerants, four new refrigerant mixtures have been proposed in this work. The optimal weight shares of the individual components were estimated by analyzing the GWP, thermodynamic, and operational parameters.

Theoretical tests were performed using the REFPROP 10.0 program, and the collected data allowed for a preliminary estimation to be made. On the basis of theoretical analyzes, it was shown that all the proposed compositions, except for the R161–R41–R1234ze(E) mixture, can be classified as near-azeotropes or even azeotropes, because their temperature glide in a wide range of evaporation pressure does not exceed 1K. At optimal compositions, the share of HFOs in all mixtures does not exceed 20%. After considering the advantages and disadvantages of the refrigerants proposed, it was determined that the most optimal composition in high-temperature (air conditioning) systems was the R1234yf–R152a–RE170 mixture with a weight share of 0.1/0.5/0.4. This is argued by its low GWP, equal to 63, the relatively high COP of 5.61, the relatively low normal boiling point of -27.27 ◦C, and a lower weight share of the most flammable components (flammability class 3) than in the case of R429A.

The analyzes also show that it is extremely difficult to find a blend with a negligible impact on the greenhouse effect and at the same time good thermodynamic properties, which could be used as a replacement for R404A or R507A in low-temperature systems. Both of the proposed mixtures have disadvantages compared to currently used refrigerants. Although the R32–R41 achieves high volumetric cooling capacity, it is also characterized by high evaporating and condensing pressures and high discharge temperatures, which will result in higher thermal and force loads of the compressor working elements and may lead to their shorter life span. On the other hand, the R161–R41–R1234ze(E) mixture, despite the high coefficient of performance, shows nearly ten times higher temperature glide than R404A, which may cause evaporator malfunctions. Components with extremely different vapor pressures can cause excessive frosting to the initial sections of the evaporator due to the evaporation of low-boiling components. On the other hand, components with a high boiling point may not completely evaporate, which can lead to fractionation of the refrigerant inside the system and change its operating parameters. In case of extreme temperature glides, it is necessary to increase the vapor superheat set point to prevent the compressor from sucking in liquid refrigerant, which obviously affects the efficiency of the system. Therefore, further research should be directed towards this application. It should be emphasized that the presented analyzes do not explore the issue of using these mixtures in cooling cycles completely. Above all, further studies of the flammability and safe use of the presented mixtures are required, as all the components used are flammable, and a significant part of them belong to the highest flammability class.

**Author Contributions:** Conceptualization: B.G. and A.S.; methodology: B.G.; validation: B.G.; investigation: A.S. and B.G.; writing—original draft preparation: B.G., A.S. and S.R.; writing— review and editing: S.R. and B.G.; supervision: B.G. and S.R.; project administration: B.G. and S.R.; funding acquisition: B.G. and S.R. All authors have read and agreed to the published version of the manuscript.

**Funding:** Financial support for this work was provided by the Polish National Science Center under the MINIATURA3 project (2019/03/X/ST8/01192) as well as by the Polish National Agency for Academic Exchange under the RadMAT project (PPN/PPO/2018/1/00042/U/00001).

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