CO2, CO2-expander-compressor-unit, transcritical
One of the most attractive low GWP and zero ODP refrigerants is carbon dioxide (R744). The use of CO2 in low temperature stages of cascade systems is nowadays state of the art. If it is used in refrigeration systems with higher heat sink temperatures, e.g. supermarket systems, CO2 has a disadvantage in terms of energy efficiency compared to other refrigerants. One possibility to improve the system COP is the introduction of a work extracting expander linked with a compressor for a second compression stage. Such an option, an expander-compressor-unit, provides two advantages: the system COP is improved and the discharge pressure and temperature of the main compressor become reduced. Until now several expanders and expander-compressor-units have been proposed. In this paper experimental results from several operation modes with an expander-compressor-unit (ECU) in supermarket systems will be compared with a standard throttle process.