Smart charging EVs may reverse power system emissions

Shifting from gasoline to plug-in electric vehicles (EVs) reduces tailpipe emissions, but it also increases demand for electricity, which can increase power system emissions. So just how clean are EVs really?

Jeremy Michalek sought to answer that question in a new study published in ACS Environmental Science & Technology. 

“Electricity is most expensive to generate when we get home from work in the early evening, turn on the lights, and start cooking even while the lights are still on at our workplaces,” said Michalek, professor of engineering and public policy and mechanical engineering. “It is least expensive in the middle of the night when everyone is sleeping.” 

“Which power plants are available to provide additional energy when you demand it depends on the time of day,” Michalek explained. “We looked at how that answer might change if EVs were smart-charged at the times of lowest cost.”

The exciting finding is that smart charging your EV can actually result in negative charging emissions, even in a coal-heavy region.

Jeremy Michalek, Professor, Engineering and Public Policy, Mechanical Engineering

By studying PJM Interconnection—the largest U.S. grid operator, which spans 13 states, including Pennsylvania—researchers found that when EV charging is timed to minimize costs, economic incentives for solar and wind investment can increase to the point of reversing emissions consequences of the charging itself. 

“This happens because the extra charging load at just the right times makes it profitable to build a lot more wind and solar infrastructure,” Michalek said. “Once wind and solar are built, they are cheap to operate since no fuel needs to be purchased. So these sources of renewable energy can displace fossil fuel generation even when the EVs aren’t charging.”

This effect is amplified when the vehicle can sell energy back to the grid or displace home electricity consumption through vehicle-to-grid technology at times of day when electricity is most expensive. This process coupled with cost-minimizing charging can substantially reduce total power system emissions. 

Moving forward, the researchers are looking to expand this study beyond PJM to other parts of the country; understand the effects of other kinds of load, such as AI data centers; and assess policy interventions. 

“The exciting new finding in this study is that smart charging your EV can actually result in negative charging emissions, even in a coal-heavy region like PJM,” Michalek said.