Ultra-Low temperature Cascade refrigeration systems: Refrigerant selection, cycle configurations, and performance enhancement techniques.

Ultra-low-temperature (ULT) refrigeration has become a necessity in many industrial applications such as cryogenic food handling, biomedical cold-chain logistics, and precision scientific systems. This review critically evaluates recent advances in ultra-low-temperature cascade refrigeration systems by integrating refrigerant selection, refrigeration cycle configurations, performance enhancement techniques, and environmental assessment methodologies. Synthetic and natural refrigerants are evaluated based on their thermophysical and environmental characteristics relevant to deep-cold applications. Special emphasis is placed on low-GWP refrigerants for cascade refrigeration systems. The review identifies R744A (N₂O), R170, and R1150 as promising refrigerants for ULT applications based on reported thermodynamic and environmental performance. The literature consistently identifies cascade vapor-compression systems as the most practical configuration for achieving ULT refrigeration. Performance enhancement techniques, including internal heat exchangers, expanders, subcooling, and ejector-assisted cycles, are critically compared. The review underscores the necessity of integrated refrigerant-system optimization and identifies research needs in accurate thermophysical property data, low-charge component design, and unified energy–exergy–environmental evaluation frameworks.