Review of Systems and Refrigerants for Ultra-low Temperature Refrigeration

ULTR has drawn more attention recently due to its increasing need and use in many industries, such as vaccine storage, freeze drying in food processing, and liquefaction of natural gas. This paper comprehensively reviews the current research on ULTR technology. Two main system architectures were identified as the current state-of-the-art: two-stage cascade and auto-cascade systems. A major challenge for ULTR is that most of the refrigerants used have high GWP values; R23 and R508B have GWP of 14800 and 13396, respectively. Some refrigerants have low GWP, such as R290 (3), R600a (3), and R1150 (4), but they are all within the A3 class of flammable refrigerants, which is not suitable to use in some applications requiring fire prevention. An alternative technology is the sublimation cycle system using CO2 as the working fluid. The benefit is the low GWP of CO2, but the technical challenges in handling solid CO2 during the refrigeration process need further research. Refrigerant mixtures optimization is also analyzed to obtain the optimum composition (mass fraction) of the mixtures with low GWP refrigerants for maximizing COP. R600/R600a/R744 mixtures with a mass fraction of 77.4%/8.5%/14.1% produces the maximum COP of 0.98. Conducting refrigerant mixtures optimization is very valuable to deal with the challenge that most refrigerants of ULTR systems have high GWP values.