**4. Discussion**

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Summarizing the results presented in this study, seven optimal mixture compositions were selected, of which six are ternary mixtures and two are binary:

R32–R41–R1234ze(E) 0.9/0/0.1 air-conditioning

R32–R41–R1234ze(E) 0.1/0.9/0 low-temperature cycle

 cycle



 cycle •R1243zf–R152a–RE1700.1/0.5/0.4air-conditioningcycle

TheirdistributionontheexperimentplanisshowninFigure17.

**Figure 17.** Marking the optimal mixtures in a triangular pattern.

There were many criteria for proving the applicability of a given refrigerant. The main ones were the temperature glide values achieved at *p*e and within the whole range of working pressures. Based on the temperature differences obtained for the evaporation process, for which the limit value was set as 10 K, a significant proportion of the points for the first two mixtures were rejected. The temperature drop in the evaporation process for the assumed cooling cycle and variable evaporation temperatures for selected mixtures are presented in Figure 18. The figure shows that only for the mixture R161–R41–R1234ze(E), significant changes in the evaporation temperature are obtained, and therefore this mixture must be classified as zeotropic (ZEO). At the same time, it is clearly visible that with the increase in the evaporation temperature, the temperature glide increases significantly.

**Figure 18.** Temperature glide of the new mixtures depending on the evaporating pressure.

On the other hand, all the mixtures containing the R152a and RE170 fluids are characterized by a small temperature glide, the value of which does not exceed 0.5 K. The binary mixture R32–R1234ze(E) shows a temperature glide of almost exactly 1 K (limit for near-azeotropic mixtures). Figure 18 also shows that among the selected mixtures, R32–R41 achieves much higher evaporating pressures.

Table 2 shows a comparison of the newly defined mixtures with the reference refrigerants dedicated to low-temperature installations. The parameters that are favorable to the new mixtures are the GWP and specific cooling capacity. Compared to currently used refrigerants, it was found that the most optimal is a R32–R41 binary mixture with a mass fraction of 0.9/0.1, for which the specific cooling capacity is 243 kJ/kg and is more than twice as high as for the R404A or R507A. The grea<sup>t</sup> advantage of the R32–R41 binary mixture is the increase in volumetric cooling capacity by over 2700 kJ/m3. The disadvantages are the slightly higher temperature glide and high working pressures. The biggest problem seems to be the much higher discharge temperature exceeding 110 ◦C, which is typical for the currently implemented and used R404A substitutes. Conversely, R161–R41–R1234ze(E) obtains a lower volumetric cooling capacity, which is dictated by a much higher specific vapor volume, at the same time showing a much higher temperature glide, which can eliminate this refrigerant from systems requiring precise evaporator temperature. The use of this mixture, however, allows to significantly improve the COP; compared to R404A, the increase is as much as 15.8%. However, the use of these mixtures requires a profound change in the refrigerant market, as currently R41 is not widely available.

**Table 2.** Comparison of the newly defined mixtures to the reference refrigerants suitable for low-temperature systems; properties based on [27].


Table 3 shows the properties of the mixtures considered as refrigerants in air conditioning cycle. R410A, R134a, R32, and R429A were used as reference. An indicator that definitely favors the new refrigerants is the GWP, the value of which for R32-free mixtures does not exceed 63 and is less than half the permissible limit. Of the reference substances, only R429A has the same low GWP. The COP for the new mixtures are also favorable, since they all exceed a value of 5.34. When comparing the temperature glide, it can be observed that it is almost zero for the mixtures containing RE170, which is a slight advantage over R429A. These mixtures achieve a volumetric cooling capacity almost identical to that of R134a and approximately 6–12% higher than that of R429A. However, they are not in competition with R32 or mixtures containing it, so a higher amount of refrigerant in the system will be required. In terms of the obtained volumetric cooling capacity, the mixtures R32–R1234ze(E) and R161–R41–R1234ze (E) may be an interesting proposition. For both mixtures, the obtained values are much higher than for R134a, and in the case of a binary mixture R32–R1234ze(E) also at a similar level as for R32 and R410A. By analyzing all the variables, it can be summarized that the mixture R1234yf–R152a–RE170 with the weight shares of 0.1/0.5/0.4 seems to be the most promising for the implementation in air-conditioning cycles.

The conclusions drawn from the theoretical analysis should be confirmed by means of experimental studies of the various evaporation and condensation temperatures. To do this, mixtures with similar compositions should be tested, with a smaller jump in the weight shares of the individual components. However, this is future research work, as the scope of this work only included theoretical considerations regarding the new mixtures.


