Performance evaluation of an ejector-enhanced modified cascade–auto-cascade refrigeration system for ultra-low-temperature cooling

Auto-cascade refrigeration (ACR) systems can reach ultra-low temperatures but often suffer from two major drawbacks: high compressor exit (discharge) temperatures and low system performance. Many previous studies have focused on performance enhancement while neglecting the critical issue of compressor discharge temperature. Although techniques such as vapor injection, two-stage compression, and additional heat exchangers can enhance system efficiency, they also introduce structural complexity and higher costs. To address these challenges, this study proposes a novel hybrid refrigeration cycle that integrates a cascade cycle with an ACR system. The objective is to lower the compressor discharge temperature and improve the overall performance while maintaining simplicity through the use of environmentally friendly refrigerant mixtures. Four different configurations were studied: three modified cascade refrigeration cycles and one ejector-enhanced cascade refrigeration (MECR) cycle. The MECR system achieved the best results, with a coefficient of performance of 0.894 and an exergy efficiency of 30.1% at evaporator exit and condensation temperatures of −60 °C and 30 °C, respectively. The compressor discharge temperature was reduced by 36.9%, improving safety when using low-global warming potential refrigerants. Relative to comparable systems reported in the literature, the proposed design improved exergy efficiency by up to 58.2% without compromising structural simplicity. These findings demonstrate that the MECR configuration provides a practical balance between performance improvement, safety, and system simplicity, offering meaningful advantages for ultra-low-temperature applications without increasing the system complexity.