Applying an ejector could lead to further efficiency improvements of transcritical CO2 (R744) systems, test results by the leading Scandinavian research institute SINTEF and others have shown.
The use of an ejector could not only simplify transcritical CO2 system processes, but also considerably improve the system's performance at high ambient temperatures, when AC systems are generally known to show the lowest efficiency. On top of that, the simpler design of an ejector, compared to advanced work recovery machines, could significantly cut the cost of transcritical systems working with the natural refrigerant CO2. The ejector could replace expansion valves in different applications, such as domestic air conditioning, MAC, etc.
This key findings will be highlighted by Dr. Armin Hafner, Research Scientist at the Norwegian research institute SINTEF, at the International Congress of Refrigeration in Beijing. Based on highly promising test results suggesting the future use of ejectors in transcritical processes, SINTEF is going to further investigate design issues of ejectors suitable for different R744 air conditioning and heat pump systems.
Experimental investigations & results .
The ejector manufactured and tested by SINTEF has been integrated into an oil-free test facility applying a CO2 liquid pump to focus on the ejector behavior. For all test series the ejector nozzle inlet temperature was kept at 30, 35 and 45°C, while varying the total mass flow rates and the mass flow fractions through the evaporator. Test results have shown that the ejector could provide a pressure lift of more than 7 bar, which improves the COP (Coefficient of Performance) of transcritical systems significantly, due to a reduction in required compressor work.
For more detailed information on the test facility an updated paper including the latest test results will be available on R744.com soon.
"A well designed ejector for an R744 system can utilize and recover some of the expansion work which is normally lost, as shown by Elbel et al. from the University of Illinois and our own measurements. High ambient temperature operation of R744 systems can be performed more efficiently when applying such a system configuration. However, the diffuse patent protection of transcritical ejector systems used for this application should be better understood and also some more work on system control aspects should be further investigated, before one could expect such units to be used extensively," says Armin Hafner.