Due to the low critical temperature of CO2, refrigeration systems utilizing this natural refrigerant often operate transcritically, which often results in lower efficiency values than systems that use conventional refrigerants, caused by the associated high pressure. One common method used to increase the efficiency of CO2 refrigeration systems are ejectors, with which a part of the cycle expansion losses can be recovered. In this work the efficiency increases which can be achieved with a high-lift ejector system have been investigated both numerically and experimentally. Currently used ejectors are designed for a low to medium pressure lift, which results in low efficiency values when being operated outside of their design parameters. Additionally, operation outside of the design conditions leads to the risk of backflow in the ejector. Thus, and due to the complexity of the control strategy, previous ejector systems have only achieved a low level of popularity.
In order to further improve the maximum efficiency of the ejector technology based on the investigated high-lift system, the development of a novel ultrahigh-lift-ejector (prec > 1000 kPa) cycle with a subcooling heat exchanger was the focus the work presented in this paper.
To carry out this work, measurement data of the high-lift ejector system were taken to validate the developed dynamic simulation results. An ejector map for the tested high-lift ejector was created and validated, which enabled the new ejector cycle with the additional subcooling heat exchanger to be simulated. The potential of the novel cycle to increase the efficiency of the refrigeration system was demonstrated and directly compared to the high-lift cycle, as well to a baseline system without an ejector. Both circuits showed efficiency increases of up to 9 % compared to the baseline circuit, whereby the non-optimized ultrahigh-lift cycle had slightly lower COPs compared to the high-lift cycle (ΔCOP < 2 %).