Rotary Gas Pressure Exchanger Lifts Transcritical CO2 Supermarket COP by 19%

Saeed and colleagues published an Applied Thermal Engineering paper in 2024 that California supermarket refrigeration contractors should read with CARB rules in mind. The team built and tested a transcritical CO2 refrigeration system that pairs a rotary gas pressure exchanger with two low-lift ejectors. The rig demonstrated up to 60 bar of free pressure lift and 18.2% COP improvement across gas cooler exit temperatures.

Transcritical CO2 has been gaining ground in commercial refrigeration for years. It's the leading low-GWP architecture for supermarkets under CARB's phasedown rules. The efficiency problem everyone has been working on is what happens in warm ambient conditions, when the gas cooler exit temperature rises and system COP falls off.

The rotary pressure exchanger is the workaround the study puts numbers on.

What a pressure exchanger does

A rotary gas pressure exchanger is a compact rotating device that transfers pressure from one fluid stream to another without requiring a mechanical compressor. In the Saeed rig, it recovered pressure from the high-pressure side that would otherwise have been throttled away in an expansion valve, and returned that pressure to the low-pressure side.

Up to 60 bar of free pressure lift means the main compressors had less work to do. That's where the 18.2% COP number comes from.

The two low-lift ejectors complemented the pressure exchanger by handling the rest of the expansion work efficiently. The combined arrangement performed best at the high gas cooler exit temperatures where transcritical CO2 traditionally suffers most.

Why California specifically

CARB's refrigerant rules are pushing supermarket operators toward low-GWP systems, and transcritical CO2 is the dominant candidate for store-level refrigeration. The rules have timelines that most operators are already working against, which means retrofit and new-build projects are competing for the same supplier capacity.

Warm California climates are the hardest environment for transcritical CO2. Coastal San Diego isn't the problem. Sacramento in August is. Palm Desert in September is worse. Any technology that lifts COP specifically in that operating window affects total cost of ownership directly, and supermarket operators run the numbers carefully.

The study's rig was a proof-of-concept scale, not a full supermarket rack. Commercial products incorporating rotary pressure exchangers are still emerging, though ejector-equipped systems are already shipping from several major suppliers. Expect the next generation of supermarket CO2 systems to blend both approaches.

For contractors, the practical step is to get familiar with the technology vocabulary. Customers who have read about low-GWP mandates will ask about ejectors, parallel compression, flash gas bypass, and now pressure exchangers. Being able to speak intelligently about the trade-offs is worth a few evenings with the trade literature.

For more, see our EPA refrigerant transition update and California right-to-repair law coverage.

Source

Saeed, Z., et al. (2024). "Experimental investigation of a transcritical CO2 refrigeration system incorporating rotary gas pressure exchanger and low lift ejectors." Applied Thermal Engineering, Vol. 254, 123913. https://www.sciencedirect.com/science/article/pii/S1359431124015813