Off-peak overnight plug-in car charging has long been promoted, but could it actually be worse for the environment?

According to a new study published in the journal Environmental Research Letters by researchers at Carnegie Mellon University, this is the case for electrified vehicle owners living in coal-intensive regions.

That finding came after CMU’s researchers modeled the “PJM” grid supplying Washington, Philadelphia, Pittsburgh, Cincinnati and Chicago.

Overnight is when most people are sleeping so grid demands are typically much lower, and at that time utilities may switch their power from cleaner sources at their disposal to coal, which can be problematic.

“We looked at how power plant operations would change in response to electric vehicle charging load, and we modeled emissions from those plants and their downwind air pollution consequences for human health and the environment,” said Michalek. “We found that charging [an electrified car] late at night reduces power generation costs by a quarter to a third, largely by shifting to cheaper coal-fired power plants. But the extra emissions released as a result can cause 50 percent higher costs to human health and the environment.”

Specifically, the source of the most damage stems from sulfur dioxide emissions from coal-fired power plants. These form airborne particles that people breathe, according to the study which delved into where things have been and where they are headed.

“The most recent year for which all of the necessary data are available to make this assessment is 2010,” said Professor of Engineering and Public Policy and Mechanical Engineering Jeremy Michalek. “We find that in 2010 a battery electric vehicle like the Tesla Model S could cause two to three times as much damage to human health and the environment as an ordinary gasoline vehicle.”

Never mind that the Model S was released in 2012, the point he makes by naming a well-known EV is the grid had issues in 2010, and still does.

While utility companies may switch to dirtier coal sourced power, and as long as the demand is low, this was not stated as a major problem.

“As coal is phased out and the grid becomes cleaner, the emissions implications of charging at night will be mitigated,” Michalek says, “and the benefits of late-night charging for the electricity grid may be good reasons to delay charging. For now, if you live in a coal-heavy region like the Chicago, Washington, D.C., Philadelphia area, delaying charging until late at night can cause more harm than good.”

So, the prognosis, while adjusted from some assumptions, is still good.

“Air emissions damages resulting from electric vehicle charging hinge primarily on the amount of coal in the system, not the amount of wind or solar power,” said Carnegie Mellon Assistant Professor of Engineering and Public Policy Paulina Jaramillo.

The findings of the study summed up the solution as retiring coal plants is needed if electrified vehicles are to maximize their pollution reducing benefits.

CMU via Gas2