A novel auto-cascade refrigeration system integrated with vapor injection and fractionation: Energy efficiency improvement and economic evaluation

• A vapor injection fractionation auto-cascade refrigeration cycle is proposed. • Energy, exergy and exergo-economic analysis are conducted. • Single-objective optimization is conducted to obtain the optimal solution. • The theoretical performance of the proposed cycle is superior to the baseline cycle. • The total cost rate of the proposed cycle is reduced. The auto-cascade refrigeration cycle is widely recognized as an effective technical solution for achieving ultra-low temperatures due to its simple structure and reliable operation. However, compressor performance and the separation efficiency of refrigerant mixtures remain key limiting factors. To address these challenges, this paper proposes an improved cycle integrating vapor injection technology with fractionation separation. Through comparative analysis of environmentally friendly refrigerant mixtures, R600a/R170 is selected as the working fluid. Under optimal design parameters, the coefficient of performance increases from 0.45 to 0.82, exergy efficiency improves from 24.9% to 36.9%, and the total economic cost rate decreases from 2.8358 $/h to 2.4320 $/h. A variance-based sensitivity analysis reveals that the condenser outlet vapor quality contributes the most to system performance with a share of 57.01%, followed by the fractionation temperature difference at 16.88%. Multi-objective optimization further identifies a balanced design where the coefficient of performance reaches 0.72 and the total economic cost rate is 2.413 $/h. These findings offer valuable references for refrigerant selection and structural design of auto-cascade refrigeration systems.