Determination of the optimum high pressure for transcritical CO2-refrigeration cycles

Modern refrigeration systems have to fulfil many requirements, which include a rapidly growing number of environmental aspects. For compression systems, this entails demands for low energy for propulsion of the systems and a high coefficient of performance (COP) respectively. Compression systems using the ecological refrigerant carbon dioxide (CO2) are often run at transcritical conditions. This means that the evaporation of the refrigerant takes place at subcritical conditions, whereas the heat rejection into the environment occurs at supercritical pressures and temperatures. For such transcritical CO2-systems with the main components compressor, gas cooler, expansion unit and evaporator, a maximum COP exists, depending on system, environmental conditions and operating conditions. The COP is influenced mainly by high pressure. A graphical method from the literature to determine the optimum high pressure is too time-consuming if several operating conditions are investigated. With a simulation model, the optimum high pressure can easily be determined for different operating conditions as a function of the refrigerant temperature at the gas cooler outlet. With these data it is possible to determine a control function. This function is used to adjust the high pressure so that the system can be run with a COP that deviates from the maximum values by less than 5.8 %. (C) Elsevier, Paris.