vehicle air conditioning, R744, flammable refrigerant, hybrid refrigerat
According to the motor vehicle directive of the European Community, as of January 2017 new cars will only be allowed to use air conditioning systems with refrigerants of GWP below 150. A moratorium to significantly extend this deadline is not to be expected considering the recently negotiated F-Gas-Regulation. One option will be flammable refrigerants of the ASHRAE classification A3 or a future classification A2L. However they are subject to safety reservations when used in systems with direct evaporation. R744 requires for a wide use and a complete realization with development, industrialization and validation probably more time than the remaining available time horizon. Therefore an option shall be discussed, which is based on a hybrid solution with R744 and a flammable refrigerant and which may allow a sufficiently rapid implementation. The utilization of R744 refrigerant mixtures shall not be considered. Instead a small and compact refrigerant loop will be operated with A3 or A2L refrigerants. Compactness, leak tightness and significant reduced charge amounts already contribute to the enhanced safety. R744 will be used as a secondary refrigerant. For the distribution in branched systems, new approaches have to be discussed. Additional safety aspects arising from the use of R744 as secondary refrigerant and at the same time in the function as fire extinguishing medium. Efficiency, safety, time-to-market and scenarios for the possible full transition to R744 as a refrigerant will be discussed.