Researchers have experimentally evaluated a transcritical R-744 refrigeration unit designed for light and medium-sized refrigerated transport. The laboratory prototype was tested in conventional back-pressure and ejector-enhanced configurations at refrigerated-space temperatures of −5 °C, 0 °C and 5 °C, with ambient temperatures from 20 °C to 40 °C (68 °F to 104 °F).
At the design condition of 0 °C (32 °F) refrigerated-space temperature, COP ranged from 2.2 at 20 °C (68 °F) ambient to 1.1 at 40 °C (104 °F) in the back-pressure configuration. Cooling capacity under the same conditions decreased from 6.6 kW (approx. 22,500 BTU/h) to 4.2 kW (approx. 14,300 BTU/h).
The ejector configuration improved COP at higher ambient temperatures but performed worse than the back-pressure cycle at lower temperatures. At −5 °C (23 °F) refrigerated-space temperature, the ejector increased COP by up to 25.1% at 40 °C ambient. At 0 °C refrigerated-space temperature, COP improvements reached 14.3% at 30 °C (86 °F), 25.1% at 35 °C (95 °F) and 22.2% at 40 °C.
The researchers also compared the experimental R-744 results with R-404A, R-410A and R-452A systems reported in previous literature. Against averaged R-404A and R-410A results, the R-744 system showed about 15.1% higher COP at 20 °C ambient in back-pressure operation and about 17.7% and 16.9% higher COP at 30 °C and 40 °C, respectively, using the ejector. The authors caution that the reference systems may differ in architecture, operating conditions and control strategies.
The prototype was developed within the EU-funded ENOUGH project. The study reports that implementation of R-744 on a refrigerated vehicle has been finalized, the system has received ATP certification and is ready for field testing.
