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Electric Car Owners All Plug In at Once

Data from the world's most concentrated neighborhood of Chevy Volts reveals when owners are actually charging their cars



Courtesy of General Motors Corp.

Environmentalists and renewable energy enthusiasts have been extolling electric cars for years, and now consumers are actually buying them. Electric utilities favor them as well, because plugging in the cars to recharge batteries adds to electricity sales. The utilities are worried, however, that if too many people in a community charge their vehicles at the same time of day, that synchrony would create spikes in power demand that could force the power suppliers to turn on expensive "peak generators" such as gas turbines—costs the utilities would rather avoid, and would ultimately pass on to consumers.

So would car owners all plug in at similar times? Early statistics from a concentration of electric car owners in Texas say, yes, they would.

The data come from a new, 280-hectare neighborhood in Austin, Tex., known as Mueller. The community is designed to maximize sustainable living, exploiting green building construction, intelligent appliances, smart electric meters and home management systems, as well as integrated residential, work, shopping and entertainment spaces. When complete, it is supposed to offer homes for 13,000 residents and jobs for about as many individuals.

Although energy planners for Mueller and many places worldwide had made assumptions about when and where electric car owners would charge their vehicles, real-world studies of actual consumer behavior were lacking. So Pecan Street, Inc., a research consortium based at the University of Texas at Austin, heavily instrumented homes throughout Mueller to take data every 15 seconds that would show what is really happening. The consumers have not been encouraged to use their cars in any particular way, and electricity rates do not change with the time of day, so costs do not influence their decisions.

Brewster McCracken, executive director of Pecan Street, and Chris Holcomb, data scientist at the organization, have just analyzed the first set of data, taken from 10 households over two months. "What we assumed turned out to be true," Holcomb says. "People come home at the end of a day and turn everything on." That means the television, computer, air conditioning, other appliances—along with plugging in their electric car for a recharge. The pattern held up across the two months—even on weekends, when it might be easy to plug in during the morning hours, and even though dozens of charging stations have been installed at convenient locations in and around Mueller.

The resulting demand on the power grid can be significant. The number of plugged-in cars begins to rise at 3 P.M. and continues through 8 P.M.—the same time period when home air conditioners are being turned on or up. Charging a depleted battery pack takes about four hours using a 240-volt line (similar to that for a clothes dryer), so the cumulative electric vehicle load peaks toward midnight. Most owners ended up charging their cars every other day or so, but no pattern emerged for preferred days of the week.

McCracken acknowledges that the data is only a start. As of mid-August about 40 Mueller residents were driving electric cars—mostly Chevy Volts, with some Nissan Leafs. (General Motors says Mueller has the highest concentration of Volts worldwide.) McCracken expects about 75 electric cars by the end of the year, and Pecan Street will expand its tracking. "There's much more to come," McCracken says.

Once 15 to 20 percent of residents in a neighborhood own electric cars, and if they behave like the 10 in Mueller, utilities could have real trouble meeting peak demand, McCracken says. The cars in Mueller draw about as much power as a home's central air-conditioning unit, and the two loads dwarf that from any other appliances. Pecan Street will be asking owners about when and where they use and charge their vehicles, and why they choose the options they do. The feedback might reveal ways utilities and regulators can adapt, instead of conjuring up incentives or rules that would "force people to behave in ways they don't want to, which would just put up a roadblock to using electric vehicles," McCracken says.

Pecan Street, which is developing and testing technologies and business models for smart grids and advanced energy management systems, is outfitting the grid in Mueller with more analysis equipment. It is now taking a variety of usage data on 35 homes and will be studying many more there in the future. It will also work with other sustainable neighborhood projects to brainstorm creative solutions. For example, McCracken notes, although time-of-day rates could indeed influence homeowners' recharging patterns, other solutions might be possible. San Diego Gas & Electric is finding that customers who own electric cars are much more likely to install rooftop solar panels. Those panels often reach peak output during the late afternoon on hot, sunny days—just when residents in Mueller tend to plug in their cars. So perhaps special solar-power incentives for homeowners who drive electric vehicles would be a fruitful way to lessen peak demand as the cars proliferate.

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