Thermodynamic performance optimization and analysis of an auto-cascade refrigeration cycle with vapor injection for ultra-low temperature freezer

To improve the performance of the conventional auto-cascade refrigeration cycle, a modified auto-cascade refrigeration cycle with vapor injection is proposed for the application of a -80 ℃ freezer in this study. The particle swarm optimization algorithm is used to obtain the cycle performance at different operation conditions. The mixture concentration, compression ratio and condenser outlet quality are optimized globally. The binary mixture selection is carried out among the R1150/R290, R150/R600 and R1150/R600a. The results indicate that R1150/R600 is proposed as the alternative refrigerant. The performance comparisons between conventional and vapor injection auto-cascade refrigeration cycles show that the vapor injection technique efficiently improves system performance, especially at high condensation and low evaporation temperatures. Compared with the conventional auto-cascade refrigeration cycle, the optimal COP in the modified auto-cascade refrigeration cycles show a 9.3∼53.8% improvement in COP and the maximum improvement in the volumetric refrigeration capacity reaches 58.2% at low evaporation temperature. The statistical results of the optimal concentration of the mixture R1150/R600 at the selected operating conditions show that 88% of the optimal R600 concentration in the VARC cycle is in the range of 0.657∼0.699. The optimal concentration of R600 in the CARC cycle varies in a wide range of 0.57∼0.75. The optimal intermediate pressure is 38.1∼13.4% lower than the geometric mean of discharge pressure and evaporation pressure. The exergetic analyses illustrate that the largest exergy destruction of the VARC cycle occurs in the recuperator-2 instead of the compressor in CARC, contributing to 22.6∼23.5% of the total exergy destruction.