Prius or Leaf? Carbon footprint hinges on location

"The system is complicated, so any estimate will have uncertainty," Inês Azevedo says. "In many regions of the US, 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." (Credit: Tony Webster/Flickr)

Which has the smaller carbon footprint: hybrids or electric vehicles? A new study shows that the answer varies depending on where you live.

Researchers studied carbon dioxide emissions of different vehicles in different regions.

“Electricity is produced from different sources in different regions of the US and at different times of day. Different emissions are produced depending on where and when an electric vehicle is charged,” explains Jeremy Michalek, professor of engineering and public policy and mechanical engineering at Carnegie Mellon University.

Northern Midwest or West Coast

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 will appear in 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.

“We find that in the Western US 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,” says coauthor Inês Azevedo, associate professor of engineering and public policy.

In other regions the comparison is uncertain.

‘The system is complicated’

“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 says.

“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 explain.

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“The system is complicated, so any estimate will have uncertainty,” Azevedo says. “In many regions of the US, 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 says.

“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.”

Source: Carnegie Mellon University