Researchers from Harvard University and Tsinghua University in Beijing have found that charging electric cars during off-peak hours could assist CO² reduction in China.

The study, published in the journal Nature Energy, showed that charging cars slowly during off-peak hours ensures more effective use of wind-generated power.

Charging vehicles in fast mode, however, typically occurs during peak power demand, triggering additional coal generators to come online and negate the environmental benefits of electrically powered cars.

“This research offers a more nuanced strategy for reducing CO² emissions and improving air quality in China,” said Michael B McElroy, professor of Environmental Studies at the Harvard John A. Paulson School of Engineering and Applied Sciences (SEAS) and the Department of Earth and Planetary Sciences and co-author of the research.

The researchers developed a comprehensive model of the energy system using real-time power demand data and driving patterns for Beijing and its suburbs. They found that how electric vehicles are charged—whether in the low-energy slow mode or high-energy fast mode—plays a significant role in the integration of wind energy.

“If people were incentivised to wait until evening and charge their vehicles in the slow-charge mode, which takes hours, the power load could take advantage of wind energy available during off-peak hours,” said Harvard China Project executive director and co-author of the study, Chris P Nielsen.

In addition to pollution caused by coal-powered generators, the study showed that buses and taxis make a significant contribution to NOX emissions, which are a major precursor for air pollution. The researchers found Beijing’s 30,000 buses and 66,000 taxis are responsible for nearly 20% of total NOX emissions, equivalent to the contribution of 8.2 million private vehicles.

According to the Global Carbon Project, China emits about 10,357 million metric tons of CO² per year, more than any other country.

The team suggests that incentives are needed to ensure private vehicles are charged in slow mode at off-peak hours and that public vehicles should be powered electrically and follow the same off-peak strategy.

“Electrifying the public fleet and introducing incentives to charge personal electric vehicles at off-peak times would be the most effective strategy to reduce NOX and CO² emissions in Beijing,” said McElroy. “This strategy could also be applied to cities across the world that have a significant source of electricity from coal.”