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Finding the Greenest Car: Essential Answer

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By Hillary Hull

There is a great deal of movement toward electric cars. There are also efforts to create cars that burn natural gas (or use it to power fuel cells). Is it better from a GHG or energy efficiency standpoint to use gas to make electricity to power a car? Or is it best to just burn the gas in the car directly?

Asked by Roy Johnson, MBA '86, Portola Valley, Calif.


I must have looked at a hundred cars before deciding on the right hybrid for me. I thought, as an environmentalist with some knowledge of cars, I would be able to choose easily. I certainly didn’t expect the barrage of options available—many of which met my standards for a “clean” vehicle. As of a few years ago, passenger vehicle selection was essentially limited to gasoline combustion vehicles. Now, we also have access to hybrid vehicles, electric vehicles and even vehicles that utilize fuel cells and natural gas.

Making a vehicle choice is not as simple as the direct emissions released when driving the car. As Dr. Sven Beiker, executive director of the Center for Automotive Research (CARS) at Stanford, told me, “you need to look at the upstream and downstream” emissions, too. In other words, how much energy did it take to make the vehicle? And what will happen to its parts when its useful life is over? As the landscape of the vehicle fleet shifts, the overall emissions from different vehicle types becomes increasingly important.

As an example, if the generation of electricity to power a car creates more emissions than burning gasoline would have, then the electric car can’t honestly be considered “emissions free” or “low emissions.” If the electricity is generated in a manner that reduces overall emissions—such as with hydropower, a wind turbine or solar panels—then the benefits are significant. As Beiker notes, “How clean and essentially how sustainable battery electric vehicles are really depends on where the electricity comes from.” According to my calculations (see the Nitty Gritty answer for more on that) an electric vehicle charged by electricity from an old coal steam plant only has the equivalent of about 27 miles/gallon.

Let’s compare a car powered by electricity, the Chevy Volt, with one powered directly by natural gas, the Honda Civic CNG. The Volt, a plug-in hybrid that can use both electricity and gasoline, is responsible for different emissions based on where its electricity comes from. However, the Honda Civic CNG, which runs on compressed natural gas, typically emits more greenhouse gases than the Volt. The exception is when the Volt is running on electricity that has high associated emissions. This occurs, for example, when the Volt is charged with the “typical” electricity supply of Wyoming, and other states. In Wyoming, as in much of the eastern United States, coal is used to produce a majority of the electricity. Coal is a “dirty” energy source, in that using it to produce electricity emits more than other typical sources, such as natural gas. You can use the graphs provided in the Nitty Gritty answer to find the electricity sources where you live, as well as the emissions associated with the electricity sources you’d use to charge an electric vehicle.

Analyzing the overall emissions of vehicles is complex, so choosing the cleanest vehicle is not clear-cut. But in most cases, the cleanest option is an electric vehicle powered by “green” electricity, from renewable sources, such as solar or wind, or “cleaner” electricity, such as that generated in the Western states from a mix of hydropower and natural gas. If you live in states like California, Oregon or Washington, you are better off with an electric car. But if you live in coal-intensive states, like Wyoming, Ohio, or Pennsylvania, stick with the CNG vehicle. Also, consider that a gasoline-powered Honda Civic emits more than either the Volt or the Civic CNG—whether you opt for an electric vehicle, hybrid or CNG, your car will emit less than a comparable gasoline vehicle. 

And don’t forget, the surest way to minimize your vehicle emissions is not to drive one! Taking the bus, train or biking to your destination guarantees you a lower emissions trip without the hassle of deciding which car is best for you.

Want more details on how to find the greenest car for where you live? Read on to the Nitty-gritty.
HILLARY HULL is a graduate student in civil and environmental engineering.

Comments (1)

  • Arman Syam, AIA

    Nice info

    Posted by Arman Syam, AIA on Feb 28, 2012 2:08 PM


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