Circulation concentration of CO2/propane mixtures and the effect of their charge on the cooling performance in an air-conditioning system

CO2 and propane mixtures are considered as alternative refrigerants due to their negligible direct global warming potentials and favorable thermodynamic properties. To properly evaluate the system performance using zeotropic mixtures, the circulation concentration was measured and the cause for its shift from the charged concentration was discussed. The circulation concentration of CO2/propane mixtures has increased CO2 fraction than its charged concentration. In addition, the effect of refrigerant charge on the cooling performance was tested for the transcritical cycle of CO2 and the subcritical cycle of CO2/propane mixtures of 75/25 and 60/40 by the charged mass percentage. It is shown that CO2 refrigeration system could operate without a significant impact on its COP over a relatively wider range from the optimum charge.