Study Finds That CO2 Emissions in Europe from Mobile AC May Be An Average Six Times Higher Than Predicted by the EPA MOBILE6 Model
A study by researchers at Empa in Switzerland on the impact of mobile air conditioning (MAC) activity on CO2, fuel consumption, and pollutant emissions of diesel cars representing the European fleet confirmed that, as determined in an earlier study of gasoline cars, MACs remain running at ambient temperatures far below the desired temperature of the passenger compartment, unless manually switched off. (This dehumidifies intake air to prevent possible windshield misting, although, the authors note, this is seldom.)
The team developed a simple model to calculate extra emissions and fuel consumption resulting from MAC activity as a function of ambient temperature and humidity: EEMAC, Empa Emission model for Mobile Air Conditioning systems. When applied to the average meteorological year of a central European region and compared with the US EPA MOBILE6 model, the EEMAC model found estimated average annual CO2 output six times higher than that of MOBILE6.
A paper on their work was published online 8 June in the ACS journal Environmental Science & Technology.
...up to two-thirds of these emissions are caused by operating the MAC at temperatures below 18 °C, where mere ventilation is sufficient, and the MAC could thus be switched off without any discomfort to the vehicle occupants. When the MOBILE6 model is applied, the resulting extra emissions are lower by a factor of 3 to 10 and thus greatly underestimate the European situation.
If the actual fleet of a country is modeled, the overall influence of MAC activity on relative emissions is somewhat smaller, as the total fleet is not equipped with a MAC system. However, the discrepancies between the two models remain, and 2% of CO2 and fuel could be saved without any discomfort to the vehicle occupants. This is twice the amount of fuel than is saved annually by renewing the fleet.—Weilenmann et al.
MOBILE6, developed by the US EPA, is applied all over the world, including Europe. MOBILE6 is based only on measurements above 22 °C (71.6 °F) and assumes that MACs do not run at lower temperatures. The Empa team showed that all systems in European cars, if left switched on, keep running except below 5 °C to prevent freezing. In this range of 5 °C-18 °C, it is not necessary to cool incoming air to maintain a comfortable temperature. However, cooling to about 3 °C dehumidifies the air for the rare case of windscreen misting.
This load on the refrigerant circuit must be taken into account when calculating annual fleet emissions, which EEMAC does. In addition, no data are available for diesel vehicles, where the power for the MACs is delivered with a different efficiency than for gasoline cars.—Weilenmann et al.
Based on the study, the authors suggest that (a) these findings are used to educate the public and (b) to propose discussions with manufacturers to optimize new MACs such that they turn off completely if no cooling is needed and turn on only when the windscreen mists. Legislation could also support this process, the authors suggest.
Martin F. Weilenmann, Robert Alvarez and Mario Keller (2010) Fuel Consumption and CO2/Pollutant Emissions of Mobile Air Conditioning at Fleet Level - New Data and Model Comparison. Environ. Sci. Technol., Article ASAP doi: 10.1021/es903654t