Plug-In Hybrids More Than Hype

20 July 2007 (All day)

Argonne National Laboratory

Plug-in hybrids could significantly reduce carbon dioxide emissions from cars--assuming power plants improve.

If car makers and the driving public embrace hybrid cars whose batteries can be charge from a wall socket, then by 2050 such "plug-in hybrids" will reduce the amount of carbon dioxide emitted by vehicles by more than 450 million tons per year--a third of today's vehicle emissions. That's the bottom line from a study conducted by the Electric Power Research Institute (EPRI) and the Natural Resources Defense Council (NRDC), the first to examine in detail the potential benefits of plug-in hybrids. However, the savings depend on improvements in power-plant technologies.

Previous studies have generally concentrated on standard hybrids. Those cars charge up their batteries with electricity generated when the car brakes and then used that "recycled" energy to power their electric motors, which take over from the vehicle's gasoline engine at low speeds and in stop-and-go traffic. Plug-ins can also charge their batteries straight from the wall socket, which means they can travel 20 kilometers or more before the gasoline engine ever needs to fire up. Plug-ins use about a third of the fuel that a standard hybrid does, which means that the cars themselves emit less carbon dioxide and other pollutants.

But there's a catch: The electricity that comes out of the socket is typically generated in power plants that burn coal or natural gas and emit plenty of CO2. So to find out if plug-ins really do reduce overall emissions of CO2, researchers at NRDC and EPRI used a computer model to project the overall emissions from the cars and the power plants. They analyzed the evolution of plug-ins from 2010, when they are first expected to enter the market, to 2050. The model accounted for factors such as the number of cars that will be in use, the type of plants producing power, and the improvement of power grids.

In a conservative scenario--only 20% of drivers drive a plug-in--the cars would reduce emission by at least 163 million metric tons per year. If roughly 80% of drivers opt for a plug-in, however, then the vehicles could reduce the amount of CO2 emissions released into the air by up to 612 million metric tons per year in 2050. Both numbers account for the extra emissions from power plants. "There is going to be a definite improvement in air quality," says Mark Duvall, a mechanical engineer from EPRI.

The emissions savings assume that future power plants will generate significantly less carbon dioxide per kilowatt than today's plants, however. In fact, if one of today's plug-ins draws its juice from a current coal-burning power plant, then overall it will cause slightly more carbon dioxide to be released into the air than a standard hybrid.

"This is an important look at the evolution of the electric sector and car sector," says anthropologist Tom Turrentine, director of plug-in research at the University of California, Davis. "It's very difficult foreseeing how those two interact and this study tackles that" issue. Still, Turrentine says he would like to see further analysis of regional differences. He also notes that the report may underestimate emissions reductions from improved conventional cars, biofuels, and other technologies.

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