While China is trying to boost the take-up of electric cars on its roads to improve local air quality and reduce CO2 emissions, new research has shown that this strategy will only achieve its intended aims under certain conditions and could even worsen the issues.

Electric vehicles play a key role in China’s emission plans, but with the majority of the country’s electricity still coming from coal-fired power plants, the strategy could prove to be counterproductive, researchers from Harvard University and Tsinghua University in Beijing have found.

They said that private electric vehicles in China can have a positive effect on CO2 reduction if owners can be incentivised to slowly charge vehicles during off-peak hours, allowing for more effective use of wind-generated power.

On the other hand, quickly charging vehicles in the higher-energy ‘fast mode’ can be counterproductive.

The country is currently imposing high quotas on automakers to produce electric and hybrid cars with the hope that they will make up at least a fifth of Chinese vehicle sales by 2025.

While the plans have been criticised by the automobile industry, which called for the quotas to be lowered, China now sells more vehicles of this type than the rest of the world combined.

Meanwhile, buses and taxis in China make a significant contribution to NOX emissions, a major precursor for air pollution. The research team determined that electrifying the bus and taxi fleet offers the most effective option for improving air quality.

“It is critically important that electric vehicle charging is managed properly to maximise the benefits of renewables,” said Xinyu Chen, co-author of the study.

The researchers used real-time power demand data and driving patterns for Beijing and its suburbs to develop a comprehensive model of the energy system.

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.

Typically, vehicle charging in the 30-minutes or less fast mode occurs during hours of peak power demand. As a result, peak power demand is increased, triggering additional coal generators to come online. With these generators operational at night, the opportunity to take advantage of available wind power is reduced.

“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 Chris P. Nielsen, who also worked on the study.

In terms of lowering air pollution, the researchers found that the gas or diesel-fuelled fleet of public vehicles - about 30,000 buses and 66,000 taxis - is responsible for nearly 20 per cent of total NOX emissions, equivalent to the contribution from 8.2 million private vehicles.

Electrifying the fleet of buses and taxis in Beijing would significantly reduce total NOX emissions and increase air quality.