(387f) Estimations of Direct and Indirect Emissions From Mobile Air Conditioner Units in Automobile Transportation: The Cool Mile Functional Unit

Hydrofluorocarbons (HFCs) may become regulated in the near future by the US Environmental Protection Agency because of their contributions to global climate change.  HFCs are currently used as refrigerants and solvents because they are relatively inexpensive and inert.  This work focuses on the use of HFCs in automobile air conditioning applications because of the large number of vehicles in use in the US, and the rapid rise of the application of refrigeration in mobile air conditioning systems (MACs) worldwide, with R134a being the predominantly used refrigerant.  R-134a (1,1,1,2-tetrafluoroethane,) has emissions of about 38.6 x 10⁶ kg/year per year from all sources, with approximately 47% of those due to emissions from vehicles. If R-134a is banned, replacements need to be evaluated in order to determine if greenhouse gas (GHG) emissions are indeed reduced when both direct emissions, in the form of leaks, and indirect ones from changes in fuel mileage, are accounted for. 

 Papasavva, et al, hafe already analyzed emissions patterns for a new replacement compound, HFc-1234hf, which is a hydrofluoro-olefin from fleet-wide use of the refrigerant in different locations around the world under ambient weather conditions, but did not consider indirect emissions impacts.  This refrigerant has also been technically evaluated for use in passenger cars and it appears to be a suitable drop-in replacement.  Other refrigerant choices are possible and are investigated in this work, using the "number of cool-miles" as the functional unit in order to capture the changes in emissions due to air conditioning use during drawdown (initially cooling of a vehicle from a hot start) and/or stable maintenance of temperature on longer drives so that the importance of different variables can be explored. 

 This work shows that some refrigerants have reduced direct emissions due to the refrigerant having a lower global warming potential (GWP), but that indirect emissions due to running the refrigerant through the system sometimes outweigh those direct emissions reductions.  Future work should explore how to capitalize on opportunities to minimize the sum of the direct and indirect emissions.