‘Hybrid ejector’ found to boost efficiency of transcritical CO₂ refrigeration by up to 42%

May 02, 2023

A new study has found that an ejector can improve the COP of a conventional transcritical CO₂ (R744) refrigeration system between 5% and 42% across all ambient conditions, including high ambient temperatures where efficiency can be compromised.

The study, “Next generation of ejector-supported R744 booster systems for commercial refrigeration at all climates,” was published in the International Journal of Refrigeration’s April 2023 issue.

The researchers – Ángel Á. Pardiñas, Researcher at ITG (Galician Institute of Technology); Håkon Selvnes, Research Scientist, SINTEF Energi; Krzysztof Banasiak, Research Scientist at SINTEF Energi; and Armin Hafner, Professor Engineering at the Norwegian University of Science and Technology (NTNU) – presented a “novel hybrid configuration” that simplifies ejector-supported R744 systems while maintaining their benefits and addresses practical challenges at low and elevated ambient temperatures.

“Improvements in the ejector system were larger at the higher end of the temperature range due to the combined effect of the ejector and slightly increased medium-temperature (MT) evaporation temperature and at the low end due to the reduced pressure ratio for the MT compressors,” the study says. When compared to a more advanced parallel compression-supported booster system, the efficiency benefits were still as high as 22%.

An experiment conducted in the SINTEF laboratory by the researchers demonstrated “superior energy efficiency” compared to booster systems, the study says. With an improvement in the COP of around 40% at the most extreme gas cooler outlet temperature tested (40°C/104°F in summer mode and 10°C/50°F in winter mode), this configuration “outperformed booster systems in any condition.”

Simplified diagram of the ejector-based refrigeration system and secondary loops in the NTNU/SINTEF laboratory (Trondheim, Norway)

Three key components

The proposed hybrid solution, developed by Pardiñas and his team, is based on three key components:

• Medium-temperature and low-temperature (LT) compressor suction groups.
• Non-superheated MT evaporation with increased evaporation temperature.
• Year-round ejector utilization.

During the summer mode (elevated ambient temperatures), the ejector actively operates as a high-pressure control device. In the winter mode (low ambient temperatures), the ejectors remain passive, functioning as a check valve. Additionally, the utilization of a low-pressure accumulator layout in subcritical mode “eliminates the practical challenges associated with a booster layout operation at low ambient temperatures,” the study says.

The ejector design for this system accommodates high entrainment ratios and significant pressure lifts in summer mode while enabling active control of the high-side pressure and efficient performance under varying conditions. In winter mode, the passive ejector ensures a low-pressure drop between the suction and discharge ports.

Furthermore, expansion valves impact high-pressure levels in winter mode when the ejector motive port is closed, allowing liquid to be supplied directly to cabinets from the gas cooler, the study notes. Conventional control methods could result in incomplete condensation of the gas, depending on the degree of subcooling at the gas cooler outlet.

The market appears to be ready to supply such state-of-the-art ejectors, given that similar ejectors have already been introduced by Danfoss and are being used by the OEM Fenagy.