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Environmental policy favors electric vehicles (EVs) in the United States, with up to $7,500 in tax credits for battery-powered cars. However, determining greenhouse gas (GHG) emissions is tough. A fuel-efficient gasoline-powered car may emit less than a large EV charged with coal-derived electricity.

Researchers at Cleveland, Ohio’s Case Western Reserve University, in collaboration with Argonne National Laboratory and the University of Wisconsin-Milwaukee, recently published a report that suggests a one-size-fits-all federal policy isn’t the best approach. EV cleanliness, researchers found, varies widely depending on typical commutes, climate, and the source of electricity for EV charging.

In 12 states, including three in the 20 most populous states – Ohio, Indiana, and Missouri – GHG emissions for EVs top 140g of CO2 per km (g/km) driven. A fuel-efficient, non-hybrid gasoline car that gets 40mpg such as the Chevy Cruze or Honda Fit averages slightly less. In coal-heavy Kentucky, a compact crossover getting 29mpg would produce less carbon (190g/km) than an EV after 10 years of use (192g/km). EVs emit less than gasoline-powered cars in most locations, but the difference between the two is low in many regions.

Chris Yingchun Yuan, associate professor at Case Western’s Department of Mechanical and Aerospace Engineering, says others have modeled fuel source, noting that coal-burning states produce less GHG benefit from EVs than areas with hydropower or natural gas. However, an often-overlooked factor is battery life. The more frequently a lithium-ion battery charges and discharges, the less efficient it becomes.

“For the same number of miles driven per year, the hotter states have the shorter battery lives – Arizona, Texas, New Mexico,” Yuan says. “To figure out which states are best to deploy battery-powered vehicles, we need to understand which states have the lowest emissions per kilometer driven.”

The researchers estimated battery degradation based on duty cycle and temperature for each state, showing significantly longer life in cold states. In hot climates, batteries lasted 5 years or less before falling to less than 70% capacity. Warm-state batteries will require more electricity to drive fewer miles, increasing the amount of carbon burned to power them.

“We modeled the lithium-ion process inside of the battery to figure out the degradation rate based on time of use and mileage. We also compared our results with some experimental data provided by the National Renewable Energy Laboratory (NREL),” Yuan says, adding that mathematical models matched 5-year, real-world experiments.

He adds that EVs can play a big role in fighting GHG-driven climate change, but regulators should take a more targeted approach and only support EVs in climates that benefit from such vehicles.

“Regulators should encourage or discourage EVs based on how they perform in each area, not averaging the entire country,” Yuan says.

Argonne National Laboratory
http://www.anl.gov

Case Western Reserve University
http://www.case.edu

National Renewable Energy Laboratory
http://www.nrel.gov

University of Wisconsin-Milwaukee
http://www.uwm.edu

About the author: Robert Schoenberger is the editor of TMV and can be reached at 216.393.0271 or rschoenberger@gie.net.