A team of researchers in Italy have found that the life cycle carbon footprint of a transport refrigeration unit using CO2 (R744) was 26.2% less than that of a unit employing R134a.

The researchers are from the National Research Council’s Construction Technologies Institute and its Energy and Transportation Department.

One of the researchers, Francesco Fabris, presented a research paper, “Comparison of the environmental impact of HFC and natural refrigerant transport refrigeration units from a life cycle perspective,” at the 10th International Institute of Refrigeration (IIR) Conference, held in Ohrid, North Macedonia, April 27–29.

The study received funding from the European Union’s Horizon 2020 research and innovation program under a project named ENOUGH.

Fabris and his team focused on a medium-temperature (MT) application at 0°C (32°F), and compared a traditional HFC refrigerant, R134a, to a natural refrigerant, R744.

A dynamic numerical simulation in the study revealed that the R744 unit outperformed the R134a unit during the hottest months of the year, demonstrating a higher COP. However, the R744 unit’s performance dipped during the colder months due to a low-duty cycle (less need for use). Despite this variation, the study found that the R744 unit’s COP was still 27.5% higher than that of the R134a unit on an annual basis.

The evaluation of the carbon footprint of the cooling unit’s entire life cycle also revealed that the R744 unit presented higher indirect emissions (6.7%) due to its higher weight, “highlighting the importance of components and weight optimization,” said Fabris. Indirect emissions derive from the fuel used in the vehicle.

On the other hand, the high 100-year GWP of R134a (1,526) resulted in significant direct emissions, making up 30.9% of total emissions. In contrast, direct emissions were negligible for the R744 unit.

As a result, “the R744 unit demonstrated a substantial reduction of 26.2% of the total life cycle carbon footprint compared to the baseline R134a unit,” said Fabris.

Dynamic numerical modeling

The carbon footprint was evaluated over the entire life cycle of the transport refrigeration system. Two different cooling units designed to provide MT refrigeration were compared – the baseline R134a unit and the R744 unit.

A dynamic numerical modeling approach was used for the monthly evaluation of the performance and energy consumption of the refrigeration systems during operation. The overall environmental impact of the life cycle of the two units was assessed, including:

  • refrigerant production and disposal;
  • refrigerant leakage;
  • cooling unit manufacturing and disposal; and
  • additional fuel to carry the cooling unit’s weight.

A simple R134a vapor compression unit is used as a baseline since the HFC system still represents most of the commercial transport refrigeration market. The R134a system has a charge of 2kg (4.4lbs) and a total weight of 68kg (150lbs). Meanwhile, R744 had a refrigerant charge of 4.6kg (10.1lbs) and a total weight of 156kg (344lbs). A novel ejector supported the R744 vapor compression unit.

“The R744 unit demonstrated a substantial reduction of 26.2% of the total life cycle carbon footprint compared to the baseline R134a unit.”

Francesco Fabris, Postdoctoral researcher at the Italian National Research Council

Author Saroj Thapa