Thermodynamic analysis of auto-cascade refrigeration cycles, with and without ejector, for ultra low temperature freezing using a mixture of refrigerants R600a and R1150

•A mixture of R600a and R1150 was thermodynamically evaluated in auto-cascade cooling cycles.•The influence of the mass fraction, the discharge pressure and the phase-separator temperature were evaluated.•The results are especially sensitive to the temperature at the phase-separator and the mass fraction of the refrigerants.•The ejector as an expansion device can improve the COP by up to 12% with regard to the reference case.