Cambridge, Massachusetts – U.S. researchers have found that ozone, a known pollutant in the Earth’s atmosphere, can be reduced on average when electric vehicles are charged at night, a time when prices and reliability are generally better as well.

The reduced pollution was found in models that averaged across four cities and over four representative modelling days.

Plug-in hybrid electric vehicles (PHEVs) have the capability to run off battery power and gasoline. When they run on their batteries they emit no pollutants from their exhaust, but the electricity generating units (EGUs) that provide the electricity to charge the batteries do give off pollutants. Researchers at the Massachusetts Institute of Technology and the University of Texas modelled the effect of replacing 20 per cent of the vehicle miles travelled by gasoline-powered cars with PHEVs, using three different electric car charging scenarios to study the emission of pollutants, specifically ozone, in Texas.

The scenarios were charging the car at off-peak times in the night; charging to maximize battery life by charging just before use and only the amount of charge needed to complete the trip; and charging the battery when convenient for the driver, typically just after vehicle use. The study results showed that the overall levels of pollution, resulting from EGU emissions associated with charging, were lower than the level of pollution resulting from the emissions associated with 20 per cent of vehicle miles travelled by a gasoline car.

Although nighttime charging was shown to yield the highest amount of nitrogen oxides, this led to the least amount of ozone, on average, across all the cities and hours modelled, as there is no sunlight with which the emissions can react. By the time morning comes, the pollutants are dispersed and diluted by other processes, such as wind.

“The results, in general, show positive air quality results due to the use of PHEVs, regardless of charging scenario, with the nighttime charging scenario showing the best results on average by a small margin,” said Dr. Tammy Thompson, lead author of the study. “This further supports efforts to develop regulation to encourage nighttime charging. An example would be variable electricity pricing. As more of the fleet switches over to PHEVs and a larger demand is placed on the electricity grid, it will become more important that we design and implement policy that will encourage charging behaviours that are positive for both air quality and grid reliability.”

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