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Charging Electric Cars At Night Better For The Ozone

April 19, 2011

The harmful effects of ozone that exist at low levels in the earth’s atmosphere can be reduced when charging is done at night for electric vehicles, a new study has found.

In the stratosphere, ozone becomes a protective layer of film that filters out ultraviolet rays that can cause skin cancer and DNA mutations in plants, reports AFP.

However, ozone that is formed from the reaction between hydrocarbons and nitrogen oxide with sunlight can irritate the airways of people with cardiac or respiratory, in addition to harming sensitive plants.

The popularity of Plug-in Hybrid Electric Vehicles (PHEVs) resulted from high gasoline fuel costs, increased inefficiency, and positive impact on the environment due to the lack of exhaust fumes.

It is already known that charging PHEVs at night is much more cost-effective and reliable, and research has also found that nighttime charging can lead to lower levels of pollution on average.

Researchers at the Massachusetts Institute of Technology (MIT) and the University of Texas published their study in IOP Publishing’s journal Environmental Research Letters. They modeled the effects of replacing 20% of the vehicle miles travelled (VMT) by gasoline-run cars with PHEVs.

In the study, their computer model predicted the emissions of nitrogen oxides for 2018 in three different scenarios and in four major Texas cities: Dallas/Ft. Worth, Houston, Austin and San Antonio. Nitrogen oxide is the basic ingredient for ground-level ozone.

AFP reports that the power generation for this grid in 2009 was provided by 46% gas, 35% coal, 13% nuclear and 4.5% wind.

The first scenario involved charging the car at off-peak time in the night. The second scenario was based on charging to maximize battery life, which means charging just before use and only the amount of charge needed to complete the trip); and the third scenario was charging the battery when it was a convenient time for the driver, which is usually just after vehicle use.

As a result, the study showed that the overall levels of pollution were lower from the electricity generating unit emissions associated with charging than the level of pollution that resulted from emissions associated with 20% of gasoline VMT.

Although nighttime charging yielded the highest amount of nitrogen oxides, it produced the least amount of ozone on average across all the cities because there was no sunlight to react with the emissions, the study showed.

By morning, pollutants are dispersed and diluted by wind and other processes.

“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,” says lead author Dr. Tammy Thompson of MIT.

“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 behaviors that are positive for both air quality and grid reliability.”

The researchers hope that the study’s findings will guide policies on how to encourage cleaner cars.

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