A prototype CO2 (R744) high-speed rail air-conditioning system offers a 14% increase in cooling capacity and a 16% increase in COP over a system using R407C according to Stefan Elbel, a Professor and Researcher at Technische Universität (Technical University) Berlin and CEO of U.S.-based Creative Thermal Solutions.

Elbel, whose research centers around sustainable heating and cooling systems, spoke about CO2 as a refrigerant in high-speed rail and refrigerated container systems during a presentation on transport market trends at the ATMOsphere (ATMO) Europe Summit 2023. The conference took place September 19–20 in Brussels and was organized by ATMOsphere, publisher of R744.com.

Elbel said that CO2-based systems present a more efficient alternative to traditional options and noted that his findings of the prototype AC unit’s performance were “conservative” as the customer funding its development requested a design similar to traditional HFC units without ejectors and additional modifications. The high-speed rail AC system was a CO2 standard booster system, which performed better than an HFC unit at similar ambient conditions.

Elbel explained the advantages of using CO2 over conventional refrigerants like R407C and R1234yf in rail applications.

“When comparing the density and capacity at low temperature and pressure, CO2 proves more effective,” he stated. “This efficiency becomes crucial in applications where maintaining above atmospheric pressures is essential.”

The cooling capacity of the AC system using CO2 rather than R407C increased by 14% from 39.1 to 44.7kW (8.6 to 12.7TR), said Elbel.

In addition to high-speed rail, Elbel discussed the potential of CO2 transport refrigeration systems in military applications. The military often requires multi-temperature containers for different operational needs, such as transporting fresh food and frozen goods simultaneously.

“While current CO2 systems are primarily single temperature, we are exploring multi-temperature options for specialized applications, including those for military use,” said Elbel. “These systems would have to be robust, adaptable to various climates and capable of maintaining different zones within the same container.”

Elbel said the goal of this area of his research is to have one part of the container chilling foods at around 3°C (37.4°F) and another freezing them at temperatures as low as −20°C (−4°F).

“Our prototype aims to exceed base capacity by 30% with a target COP of around 1.0 even at a high temperature of 57°C [134.6°F],” said Elbel.

The system under development comprises a multi-stage transcritical compressor with an intercooler paired with a subcritical booster compressor and ejector.

“With the modified standard CO2 booster system, we found an 18% higher COP compared to systems using R404A,” said Elbel. “The COP was calculated considering compressor power at the high-pressure side, neglecting sub-critical compressor power consumptions. This is done to make a relevant comparison with single circuits or the temperature level of the R404A system.”

COP variations

Ebel’s analysis of CO2 refrigerated container systems combined theoretical cycle estimates with experimental data. A theoretical cycle analysis was conducted to estimate the coefficient of performance in scenarios lacking specific vital components.

“We used our prototype, which achieved a COP of 0.75 in experiments, as a benchmark,” said Elbel. “By maintaining the same pressure levels and compressor efficiencies but changing the cycle layout, we could estimate the COP under different conditions.”

An internal heat exchanger accounted for a 31% improvement in COP, making it feasible to be used with a multi-stage transcritical compressor.

With the internal heat exchanger, the low-temperature compressor was found to be responsible for a 24% improvement in COP. Low-temperature compressors play a crucial role in achieving high-efficiency levels, particularly in systems that require maintaining low temperatures over extended periods of time.

“While current CO2 systems are primarily single temperature, we are exploring multi-temperature options for specialized applications, including those for military use.”

Stefan Elbel, a Professor at Technische Universität (Technical University) Berlin and CEO of U.S.-based Creative Thermal Solutions

Author Saroj Thapa