Theoretical study of the optimal intermediate pressures for a two-stage vapor-compression refrigeration system with ecofriendly refrigerants

• Optimal intermediate pressures are obtained with ecofriendly refrigerants. • R717 and RE170 exhibit low GWP and comparatively high COP of 4.73 and 4.69. • R1234yf can improve coefficient of performance by up to 85 % in two-stage systems. • New correlations using the evaporating and condensing temperatures are proposed. The growing importance of low-GWP refrigerants arises from their role in mitigating climate change and supporting global sustainability goals. This study investigates the optimal intermediate pressure of a two-stage vapor-compression refrigeration system using eco-friendly refrigerants, aiming to reduce energy consumption, carbon emissions, and environmental impact. For evaporating temperatures of -20, -40, and -60 °C and condensing temperatures of 30, 40, and 50 °C, the minimum total compressor work and maximum coefficient of performance of a flash-intercooled system are determined. The results show that the optimum intermediate pressure increases with condensing temperature at a fixed evaporating temperature, and likewise increases with evaporating temperature when the condensing temperature is fixed. Among the refrigerants assessed, R717 and RE170 yield the highest coefficient of performance of 4.73 and 4.69, whereas R1234yf provides the lowest (4.37) under optimal conditions. However, R1234yf achieves the largest performance enhancement ratios of 33.6 %, 53.6 %, and 85.1 % at evaporating temperatures of -20, -40, and -60 °C and 50 °C condensing temperature when applied to a two-stage system. The optimal intermediate pressure corresponding to minimum total compressor work is compared against existing predictive correlations, with discrepancies increasing as both temperatures rise. Finally, correlations for the optimum intermediate pressures of the two-stage refrigeration systems are developed as functions of the evaporating and condensing temperatures.