CO₂ refrigeration in transport: ejector improves efficiency at high temperatures

Refrigeration of refrigerated vehicles represents one of the most challenging applications of the cold chain : the systems must guarantee stable temperatures during transport, but operate in highly variable environmental conditions and with stringent constraints in terms of weight, size and energy consumption.

In this scenario , interest in CO₂ (R744), a natural refrigerant with a GWP of 1, is growing. Recent experiments have analyzed R744 transcritical units intended for light and medium-sized refrigerated vehicles, comparing a traditional cycle with a configuration equipped with an ejector.

The results show that, precisely at higher external temperatures, the ejector can recover part of the losses typical of the transcritical cycle and significantly improve efficiency . This is particularly interesting for urban distribution and the European cold chain : when the external temperature rises, the refrigeration load required of the vehicle increases simultaneously, and it becomes more complex to maintain high system performance.

R744 in refrigerated transport: why high temperatures are a challenge

CO₂ has particularly interesting characteristics from an environmental perspective , but the operation of transcritical systems is strongly influenced by the external temperature. The critical point of CO₂ is just above 31°C . When the system must release heat at higher temperatures, the cycle enters transcritical operation and the operating pressures and losses associated with expansion increase. It is precisely under these conditions that efficiency can decrease.

The experimental tests evaluated the refrigeration unit's performance at refrigerated compartment temperatures of -5°C, 0°C, and 5°C, and ambient temperatures between 20°C and 40°C. In the traditional configuration, with the compartment at 0°C, the COP decreased from approximately 2.2 at 20°C outside to approximately 1.1 with the ambient temperature at 40°C. In the same range, the cooling capacity decreased from approximately 6.6 to 4.2 kW.

Dynamics are particularly important for refrigerated transport because harsher climate conditions often coincide with increased vehicle cooling requirements. Hence the need for architectures capable of maintaining high performance even on the hottest days.

How the ejector works in CO₂ systems

One strategy studied to increase the efficiency of transcritical systems involves the insertion of an ejector instead of, or in addition to, the traditional expansion valve . In a conventional circuit, a significant portion of the energy available during the reduction of refrigerant pressure is dissipated in the expansion valve. The ejector, however, allows part of this energy to be recovered, using it to increase the pressure of the refrigerant drawn from the low-pressure side.

The result can be a reduction in the work required by the compressor and therefore an improvement in COP . Tests on the refrigerated transport prototype have shown that the advantage increases especially with increasing ambient temperature. With the refrigerated compartment kept at 0°C, the increase in COP compared to the traditional configuration was:

• +14.3% at 30°C outside;
• +25.1% at 35°C;
• +22.2% at 40°C.

With the compartment temperature at -5°C, the maximum improvement observed was still 25.1% at 40°C outside.

However, the ejector does not prove advantageous in all conditions : at lower external temperatures, the traditional configuration has shown superior performance. This suggests the interest of systems capable of adapting their configuration to operating conditions, using the most efficient mode based on temperature and load.

Efficiency and sizing of mobile refrigeration units

Using an ejector can have implications beyond simply improving COP. In refrigerated vehicles, the unit is typically sized to accommodate the most severe climate conditions. If refrigeration capacity decreases at high temperatures, it may be necessary to install a unit oversized for normal operating conditions.

Simulations performed on the system show that integrating the ejector can improve the COP up to approximately 28.6% at 40°C, also helping to limit the drop in cooling capacity.

For units installed on electric vehicles, the issue takes on an even greater weight: any consumption intended for cold production in fact subtracts energy from traction and can affect range.

Design must therefore find a balance between:

• cooling capacity;
• energy efficiency;
• unit weight;
• encumbrances;
• circuit complexity;
• reliability;
• ease of maintenance.

From experimentation to road testing

The available results are promising, but they still concern a technology in the testing and validation phase . The prototype was first tested in the laboratory. An optimized and more compact version was subsequently installed on a refrigerated vehicle, also obtaining ATP certification and thus moving on to the field testing phase.

Road testing will be important to evaluate performance and reliability in real distribution conditions:

• continuous load variations;
• frequent door openings;
• stops;
• urban traffic;
• variable outside temperatures;
• non-stationary operating cycles.

CO₂ could therefore play an increasingly important role in the mobile cold chain. The ejector represents one of the technologies that can help overcome one of the main limitations of transcritical R744 systems: maintaining high efficiency and cooling capacity when the outside temperature rises.