April 13, 2012
With the increased interest in electric vehicles, there is also increasing attention being paid to the air emissions benefits of displacing gasoline-fired vehicles with vehicles “fueled” by electricity. The extent of air emissions benefits depends on the fuel source used to generate the electricity used to charge the vehicle. If gasoline is displaced with electricity that is 100% generated from coal, for example, are there any reductions in greenhouse gas emissions associated with the move to “electrify” the transportation system?
The level of greenhouse gas reductions from substituting electricity for gasoline to fuel vehicles depends upon the region of the country, and the composition of the electric generation mix in that particular region. According to a report by the Union of Concerned Scientists, electric vehicles (EVs) charged from the electricity grid nationwide produce lower global warming emissions than the average compact gasoline-powered vehicle (with a fuel economy of 27 miles per gallon)—even when the electricity is produced primarily from coal in regions with the “dirtiest” electricity grids. The U.C.S. report, which takes into account the full cycle of energy production (often called a well-to-wheels analysis), demonstrates that in areas where the electric utility relies on natural gas, nuclear, hydroelectric or renewable sources to power its generators, the potential for electric cars and plug-in hybrids to reduce carbon dioxide emissions is great. In regions with the “cleanest” electricity grids, EVs produce lower global warming emissions than even the most fuel-efficient hybrids. EVs charged entirely from renewable sources like wind and solar power produce virtually no global warming emissions.
The report divides the United States into 26 regions. Each region comprises a single interconnected electricity grid, though several utility companies may operate within a region. Because the utilities sell power among themselves, the emission levels for one city or utility cannot be pinpointed for every hour of every day, but regional analysis provides an approximation of average emissions over time. On this map, the numbers following the city names represent, as a miles-per-gallon equivalent, the amount of greenhouse gases generated in charging the battery of an electric Nissan Leaf in that city. The darkest regions on the map are served by utilities burning a high percentage of coal to generate power; in those regions, charging an electric car sends as much carbon dioxide into the atmosphere as driving a car rated at 31 to 40 m.p.g., about the same as a current compact model. In the lightest areas of the map the electricity is generated by cleaner fuels, so the equivalent miles per gallon is higher than the best of today’s hybrids. In Seattle and Portland, for example, where the electricity is provided primarily by hydropower, the greenhouse gas emissions for an electric vehicle are equivalent to a gasoline-powered vehicle achieving 73 miles per gallon. The comparable figure for this region (Pittsburgh, Columbus, Cleveland, Nashville, Indianapolis and Memphis) is 41 miles per gallon.