31 December 2008
CO2 is being advocated as one of the natural refrigerants to replace CFCs and HCFCs in vapor compression cycles based on its environmentally friendly characteristics. However, the perceived lower efficiency of the basic transcritical CO2 refrigeration cycle compared to vapor compression systems using HFC and HCFC refrigerants is a major hindrance for the technology to make progress towards practical applications. In order to recover the expansion losses and increase the cycle efficiency, it has been proposed to replace the expansion valve with either a work producing expansion device or an ejector expansion device. A comparison of the two devices shows that the ejector expansion device offers the advantages of simplicity, reliability, and availability while work producing expansion devices have the potential to yield higher cycle efficiencies. In the paper presented here, it is shown that the transcritical CO2 cycle with an ejector expansion device outperforms a basic transcritical CO2 cycle in both COP and cooling capacity by up to 38 and 41%, respectively, for the application of a military Environmental Control Unit at high ambient temperatures. It is also shown that a careful design of the ejector and control of the motive nozzle mass flow rate are critical in order to improve the performance of ejector expansion transcritical CO2 cycles.