A new study by Carnegie Mellon University claims that charging an electric car at night may actually be worse for the environment that charging as soon as the owner gets home. How can that be? It depends on where the local utility gets its electricity from.
Jeremy Michalek, a professor of engineering and public policy and mechanical engineering at CMU, and his colleagues modeled the grid that supplies Washington, Philadelphia, Pittsburgh, Cincinnati and Chicago. “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,” Michalek explained. “We found that charging [an electric 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.”
The reason is that utility companies use mostly coal fired generating plants to provide the electricity for overnight use. If those coal plants need to ramp up because there is a higher demand for electricity, they spew more pollutants into the atmosphere than they would if they are just idling along.
“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.”
The research highlights once again why some people insist on saying that an electric car has “zero tailpipe emissions” rather than calling it a “zero emissions” car. Of course where the electricity comes from to recharge an electric car will always be an important consideration. In the long run, eliminating coal fired plants is the best way of lowering total carbon emissions, a concept that is generally accepted by all parties except congressional Republicans.