Which Vehicle Holds Smallest Carbon Footprint?-CMU News - Carnegie Mellon University

Thursday, July 16, 2015

Which Vehicle Holds Smallest Carbon Footprint?

Electrics in Some Regions, Hybrids in Others, Says Carnegie Mellon Study

By Tara Moore / 412-268-9673 / tararaemoore@cmu.edu
Leaf vs. PriusA Nissan Leaf (left) and Toyota Prius get a charge at Carnegie Mellon's Electric Garage.

A new study by Carnegie Mellon University researchers finds that in some regions of the United States electric vehicles have the smallest carbon footprint, but in other regions, hybrids do.

Carnegie Mellon Professor of Engineering and Public Policy and Mechanical Engineering Jeremy Michalek, Associate Professor of Engineering and Public Policy Inês Azevedo and University Professor of Civil and Environmental Engineering Chris Hendrickson, together with Mili-Ann Tamayao, a professor at the University of the Philippines who earned her Ph.D. in engineering at CMU, studied carbon dioxide emissions of different vehicles in different regions.

“Electricity is produced from different sources in different regions of the U.S. and at different times of day. Different emissions are produced depending on where and when an electric vehicle is charged,” Michalek explained.

An electric vehicle charged in the Northern Midwest can produce two to three times as much carbon dioxide as the same vehicle charged on the West Coast, according to the study, which is being published by the journal Environmental Science and Technology. In the Northern Midwest electricity is largely produced by coal-fired power plants, while on the West Coast more of the electricity is produced from natural gas.

“Electricity is produced from different sources in different regions of the U.S. and at different times of day. Different emissions are produced depending on where and when an electric vehicle is charged.” — Jeremy Michalek

“We find that in the Western U.S. and in Texas, the Nissan Leaf battery electric vehicle has a smaller carbon footprint than the gasoline Toyota Prius. However, in the Northern Midwest, the Prius has a smaller carbon footprint,” Azevedo said.

In other regions the comparison is uncertain.

“Some past estimates of electric vehicle carbon footprints have decided that an electric vehicle should be responsible for the average emission rates of power plants in the state, grid region, sub-region or country where the vehicle is charged,” Michalek said. “But if you want to know the emissions consequences of owning an electric vehicle instead of a gasoline vehicle, you have to look at the way the electricity grid responds to electric vehicle charging load compared to how it would behave without that load.”

Because the electricity grid is strongly interconnected, it is difficult to know exactly which power plants respond to changes in load, the researchers explained.

“The system is complicated, so any estimate will have uncertainty,” Azevedo said. “In many regions of the U.S., it’s not clear whether the Leaf or the Prius has the smaller carbon footprint, and it can depend on what time of day the vehicle is charged. However, both the Leaf and the Prius are low-carbon compared to most other vehicles.”

The study recommends that electric vehicles be promoted most strongly in the regions where they do the most good.

“Luckily, California, which has the lion’s share of electric vehicle sales, also has a relatively clean electricity grid,” Michalek said. “A gasoline hybrid can still produce lower carbon emissions in some other parts of the country. We’ll have to continue to clean up the electricity grid before electric vehicles can offer the largest benefits in reducing greenhouse gas emissions everywhere.”