*3.3. R1234yf–R152a–RE170*

The third mixture has a completely different composition to the previous ones. It was decided to check the working parameters of the mixture, which in addition to fluorinated HFC/HFO refrigerants also contains dimethyl ether, known as RE170. The proportion of dimethyl ether in the mixture is likely to influence the toxicity and flammability class of the new refrigerant. The mixture's flammability class is assumed to be 3, as to be expected from a mixture with a high proportion of RE170. The representative from the HFC group is R152a. This is often used in existing mixtures because of its low GWP and its relatively high normal boiling point, thus prompting its use in medium to high temperature circuits. The last mixture component is the most common and most easily available fluid from the HFO group—R1234yf. As in the R161–R41–R1234ze(E) mixture, all the constituents have a GWP lower than 150; therefore, the GWP of the mixture itself does not exceed this value (Figure 11a). The flaws of this blend is that normal boiling point never reached −30 ◦C, which proves third mixture is not predisposed for use in low-temperature units (Figure 11b). The device's operation under negative pressure is associated with certain dangers and requires the use of a larger compressor. Considering all of the above, in the case of the R1234yf–R152a–RE170 mixture, it was decided to analyze only the high-temperature cycle.

**Figure 11.** Summary of the basic properties for the R1234yf–R152a–RE170 mixture: (**a**) GWP and (**b**) normal boiling point.

When analyzing the third mixture (R1234yf–R152a–RE170) and the temperature glides obtained (Figure 12), the conclusion is that the temperature difference during the phase change does not exceed 1 K for the assumed evaporating pressure level regardless of the composition; thus, it can be considered at least as near-azeotropic. The achieved volumetric cooling capacity and COP play the greatest role in the assessment (Figure 13a,b). The highest COP value determined was 5.67, but it was for pure RE170. Given the importance of the fluid share from the A3 group being as small as possible, this point was not analyzed. It is worth noting that, with a binary and ternary mixture, as many as 54 points out of 63 have a COP > 5.48, which is the reference value obtained by R134a. There are two ternary mixtures that meet all the assumptions of the work and appear to be good potential substitutes for phased-out fluorinated greenhouse gases. The first is the 0.1/0.1/0.8 mixture, which has a COP of 5.65. The second mixture with a very similar COP has a mass fraction of 0.1/0.5/0.4, but is additionally characterized by a lower normal boiling point and temperature glide well below 0.1 K. In this term, it can be considered as azeotropic. The second composition also has a volumetric cooling capacity that is 124 kJ/m<sup>3</sup> higher than that of the first mixture.

**Figure 12.** Temperature glide at the evaporation pressure for air-conditioning system (*t*e/*t*c = 0/30 ◦C) with R1234yf–R152a–RE170 mixture as refrigerant.

**Figure 13.** Volumetric cooling capacity (**a**) and (**b**) the COP for air-conditioning system (*t*e/*t*c = 0/30 ◦C) with R1234yf– R152a–RE170 mixture as refrigerant.
