CNR study: ejector-enhanced CO₂ cycle boosts COP by up to ~25 % in refrigerated-transport prototype

A team from the Italian National Research Council (CNR) has published experimental results showing that a two-phase ejector can raise the coefficient of performance of a transcritical CO₂ (R-744) refrigeration cycle by up to approximately 25 % at high ambient temperatures, a finding the authors say is particularly relevant for refrigerated transport where cooling demand rises in hot conditions.

From laboratory to vehicle

The study, published in the International Journal of Refrigeration [1], presents the design and experimental assessment of a CO₂ refrigeration unit developed for light and medium-sized refrigerated transport. The work was carried out within the EU-funded ENOUGH project [2], in which the International Institute of Refrigeration (IIR) participated as a consortium partner. The authors describe the stationary prototype as an intermediate step between earlier numerical investigations and the eventual installation of the technology on a refrigerated vehicle.

The prototype was installed and tested at the CNR Construction Technologies Institute (CNR-ITC) laboratories in Padova, Italy. The research team included Silvia Minetto (President), Antonio Rossetti (Secretary) and Francesco Fabris (member) of IIR Commission D2 – Refrigerated Transport, along with co-authors W. Shah and S. Marinetti.

Test conditions and configurations

The experimental campaign covered refrigerated-space temperatures from −5 °C to +5 °C and ambient temperatures from 20 °C to 40 °C. Two operating configurations were assessed:

• a conventional back-pressure cycle;
• an ejector-enhanced cycle using a two-phase ejector.

Ejector advantage at high ambient temperatures

At lower ambient temperatures the simpler back-pressure configuration delivered higher efficiency. As ambient temperature rises, however, the low critical temperature of CO₂ drives operating pressures and expansion losses up under transcritical conditions, significantly penalising the simple cycle. Under these high-ambient-temperature conditions the ejector configuration proved highly beneficial, with the experimental campaign demonstrating COP improvements of up to approximately 25 % compared with back-pressure operation.

The ejector configuration also helped maintain or increase cooling capacity as ambient temperature rose. According to the authors, this behaviour is especially valuable for transport applications, where refrigeration demand typically increases under hotter outdoor conditions and must be accounted for when designing and sizing mobile units.

Benchmarking and next steps

The CO₂ unit prototype showed competitive energy performance when benchmarked against published results for systems using synthetic refrigerants, although the authors stress that such comparisons should be interpreted cautiously because of differences in system architectures, controls and operating conditions.

Following the laboratory investigations, an optimised and more compact version of the CO₂ system is under implementation on a refrigerated vehicle that has successfully obtained ATP certification and is ready for field testing. Future research will compare the performance of the final mobile system with that of the stationary laboratory prototype.

Figure. The CO₂ refrigeration unit installed in CNR-ITC laboratories [1].

For more information, the scientific paper is available in open access in the International Journal of Refrigeration and in FRIDOC.

Sources:

[1] Shah, W., Fabris, F., Minetto, S., Marinetti, S., Rossetti, A. (2026). Experimental analysis of a R-744 refrigeration unit for light commercial vehicles. International Journal of Refrigeration 188 (2026) 106985. http://dx.doi.org/10.1016/j.ijrefrig.2026.106985

[2] ENOUGH Project. https://enough-emissions.eu/