Charging electric vehicles (EVs) at night is more cost effective but increases air emissions, a study by Carnegie Mellon University researchers has found.

Charging EVs late at night, when demand is low and electricity is cheapest to generate, is generally preferred by both grid operators and EV owners. However, CMU researchers found that it produces substantially higher greenhouse gas emissions and air pollution than simply charging as soon as the driver returns home.

Charging late at night reduces power generation costs by a quarter to a third. Image credit: Pixabay.Charging late at night reduces power generation costs by a quarter to a third. Image credit: Pixabay.The study, carried out by Jeremy Michalek, professor of engineering and public policy and mechanical engineering, and colleagues modeled the PJM region, which includes Washington, D.C., Philadelphia, Pittsburgh, Cincinnati and Chicago. It examined how power plant operations would change in response to EV charging load—and modeled emissions from those plants and their downwind air pollution consequences.

“We found that charging late at night reduces power generation costs by a quarter to a third, largely by shifting to cheaper coal-fired power plants," says Michalek. "But the extra emissions released as a result can cause 50% higher costs to human health and the environment.”

According to the study, coal-fired power plants often operate below full capacity at night, so they are available to be dispatched in response to new nighttime load, like EV charging. These coal-fired power plants produce sulfur dioxide, which is the largest threat to human health resulting from EV charging.

In a separate study, Michalek and colleagues looked at greenhouse gas emissions from EV charging across the United States. “In nearly all U.S. regions, charging later at night increases greenhouse gas emissions,” he says.

Michalek adds that the picture could change in the future, as many coal-fired power plants are expected to be retired in response to recent regulations. “As coal is phased out and the grid becomes cleaner, the emissions implications of charging at night will be mitigated,” he says, “and the benefits of late-night charging for the electricity grid may be good reasons to delay charging.”

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