Although an electric bus smells a heck of a lot better than a diesel bus, it’s obviously important to remember that electric doesn’t really mean zero emissions unless your electric grid is fossil-fuel-free. You may have seen calculations showing that the equivalent miles per gallon of an electric vehicle varies in different parts of the US depending on the local mix of power plants. But a new study focused on Beijing shows that the devil is in even smaller details.

To help improve its incredible air pollution problem, China is pushing electric vehicles at the same time it increases the share of its electricity produced by renewables rather than coal, which has dominated. To see what effect this change would have, a group of researchers led by Harvard’s Xinyu Chen modeled Beijing’s transportation and electrical systems in a variety of scenarios for the year 2020.

When do you charge?

The base scenario proposed by the researchers included no electric vehicles, but they also examined cases in which the percentage of Beijing’s electricity produced by wind ranged from zero to 40 percent. That equates to as much as a 29-percent cut in CO 2 emissions and a 14-percent reduction in nitrogen oxide air pollution. The interesting thing is how the various electric vehicle scenarios change those numbers.

5.6 million personal gasoline vehicles will account for about 10 percent of Beijing's total CO 2 emissions in 2020, with coal power plants contributing about 85 percent. So the CO 2 benefit of converting those vehicles to electric depends almost entirely on how it affects the electric grid. That effect, in turn, depends on the type of charging those vehicles use.

The researchers use reasonable charging schedules to simulate hour-by-hour loads on the electric grid. In one scenario, these vehicles utilize slow charging overnight. In the other scenario, fast charging creates a peak in demand at the end of the work day as cars return home and plug in.

By spreading out the demand, slow-charging vehicles actually help utilize wind power in the grid. One issue with wind energy in Northern China has been the fact that its coal plants also produce heat piped into buildings in the winter. That means that, on windy days, coal plants can’t wind down or they'd leave people without heat—wind turbines are turned off instead. With a reliable demand for electricity, slow-charging vehicles can use that extra wind energy and keep turbines spinning longer.

Fast-charging vehicles, on the other hand, concentrate the demand into daily spikes that would likely have to be covered by ramping up coal plants. And the higher the output from coal plants, the more wind power gets curtailed as excess.

Buses help

The result is that slow-charging vehicles preserve the CO 2 emissions reduction from increasing wind power, but fast-charging vehicles eat into it. If wind turbines produced 40 percent of Beijing’s electricity, for example, slow-charging cars would bump the emissions savings from 29 to 30 percent, while fast-charging cars would limit the benefit to just 19 percent.

The story is similar for nitrogen oxide air pollution. Raising wind power to 40 percent reduces pollution by 14 percent—going up to 16 percent with slow-charging vehicles and down to 11 percent with fast-charging ones.

Electrifying the city’s buses and taxis would have a more straightforward benefit. With only 30,000 buses and 66,000 taxis, the demand for electricity is much less significant, and charging time is pretty much irrelevant to the function of the grid. That also means the benefit for total CO 2 emissions is small, but diesel buses are outsized sources of local air pollution. The 14-percent air quality benefit from expanded wind power jumps all the way to 24 percent with electric buses and taxis.

All this leads the researchers to the general conclusion that, at least in this case, “Whether an EV reduces or increases total CO 2 emissions depends largely on how and when the vehicle is charged.” While that fact has been overlooked, the researchers point out that there are certainly ways to manage charging patterns. Time-of-day pricing for electricity is an obvious example—make it cheaper to charge your vehicle at non-peak hours, and people will. Different charging rates could also be priced differently, encouraging people who don’t actually need their vehicle recharged in a couple of hours to opt for the slower electrons.

Obviously, the specifics of Northern China’s electric grid drive these results—and replacing more of the dirty coal generation would change the story—but other regions can also learn from this study. If you want to see more electric vehicles and fewer emissions, you had better pay attention to the details.

Nature Energy, 2018. DOI: 10.1038/s41560-018-0133-0 (About DOIs).