A new study has found that hydrocarbons are the optimal refrigerant for operating ultra-low-temperature refrigeration systems from −40 to −80°C (−40 to −112°F), including those used in vaccine storage, tuna storage and liquified natural gas (LNG) pre-cooling.
The study examined different configurations for refrigeration systems, including auto-cascade refrigeration (ACR), cascade refrigeration systems (CRS), multi-stage refrigeration systems (MRS) and air refrigeration cycles (ARC). These systems were evaluated using ethane (R170), ethylene (R1150), Propylene (R1270), propane (R290), ammonia (R717), Nitrous oxide (R744A) and CO2 (R744). Additionally, mixtures like R744/R290 and R744/R1150 were analyzed to determine their efficiency and applicability across different ultra-low-temperature applications.
For vaccine storage, particularly in applications requiring temperatures of −70°C (−94°F) or below, like storage of the Pfizer COVID-19 vaccine, the study indicates that the most energy-efficient freezers use hydrocarbons such as R170 and R290. In this setup, ACR, CRS and ARC systems were analyzed.
ACR systems using R1150/R600 and R1150/R290 mixtures achieve COPs of 0.4 to 0.5 at −80°C (−112°F) for small-sized refrigerators with cooling capacities below 1 kW (0.28 TR). At similar condensing temperature of 15°C (59°F) the other two mixtures, R744/R290 and R170/R290, were evaluated, and they achieved COPs of 0.303 and 0.326, respectively.
The ARC offers a COP of around 0.4 at a warehouse temperature of −70°C, making it suitable for larger capacities. In comparison, CRS configurations using R744/R1150 or R744/R170 mixtures for the low-temperature circuit and R290 in the high-temperature circuit achieved COPs of 1.2 to 1.3, balancing temperature glide and safety by minimizing the flammability risk.
“The CRS and ARC are viable options for larger cooling capacities, with hydrocarbons as high-temperature refrigerants leading to the best performance,” the study noted.
The study, “Ultra-Low-Temperature Refrigeration Systems: A Review and Performance Comparison of Refrigerants and Configurations,” was conducted by researchers from the Norwegian University of Science and Technology. The research team included Yosr Allouche, the International Institute of Refrigeration (IIR) Director General.
Tuna storage, LNG pre-cooling
The study also examined the most effective ultra-low temperature refrigeration system and refrigerant for use in tuna storage. A distinction between onshore and offshore storage was made, with the study noting that R22 is still widely used to store tuna offshore.
“For storing tuna [offshore] at −60°C (−76°F), a CRS with a refrigerant pair R1270 in the HTC (high temperature circuit) and R170 in the LTC (low temperature circuit) or R290 in the HTC and R170 in the LTC results in the best COP of around 1.4,” the study wrote.
CRS using R744 in the HTC and R744A in the LTC had COPs around 0.9 which is lower than hydrocarbon pairs. Furthermore, the study suggests further research into using these two refrigerants in combination due to their significant cooling capacity per unit volume and climate-neutral properties.
The process of creating LNG involves cooling natural gas to a temperature from -30°C (-22 °F) to -50°C (-58 °F) or even lower, depending on the pre-cooling method used. LNG pre-cooling is performed using a CRS with various mixtures like R744/R290, R170/R290, R170/R600, R1150/R600, R1150/R290 in the LTC to improve efficiency by matching temperature glide. These glides can be adjusted according to the requirements of the gas-processing unit by altering the compositions of the refrigerant mixtures.
The highest temperature glide is achieved with the R1150/R600 mixture, while the lowest temperature glide is achieved with the R170/R290 mixture. R717 and R290 were evaluated for the HTC and R290 was found to have more potential because of its better pressure ratio and temperature after compression. The highest COP of 1.059 was calculated using an R170/R600 mixture in the LTC with R290 in HTC at a condensing temperature of -80°C (-112°F).
“The CRS and ARC are viable options for larger cooling capacities, with hydrocarbons as high-temperature refrigerants leading to the best performance.”
Allouche et al.
