Girding the power grid for an electric fleet

Proponents of the Chevrolet Volt and other plug-in electric cars assert that owners will charge their cars overnight, when demand on the power grid is lowest.

But earlier studies of how people charge their electric cars, including General Motors' EV1, found conclusively that people will insist on recharging during the middle of the day as well.

Can the grid handle a large fleet of plug-ins?

The grid's capacity to charge plug-in vehicles at almost any time is more than ample, says Mark Duvall, program manager of electric transportation at the Electric Power Research Institute in Palo Alto, Calif.

All over the map

Duvall predicts that the utility industry aggressively will encourage off-peak charging.

During the six hottest months of the year, peak demand for most utilities comes on weekday afternoons. For example, Pacific Gas & Electric's residential rate peak period is 2 to 9 p.m., Monday through Friday, from May 1 to Oct. 31. The peak rate is about 29 cents per kilowatt-hour.

Everything else is either partial peak, with typical rates of about 10.5 cents per kilowatt-hour, or off-peak at 5 to 6 cents.

Every utility is different and many do not have time-of-use rates, but this will change over time, Duvall says.

But even if utilities say nighttime recharging might be less expensive, ultimately the time of day that people recharge will "fall all over the map," Duvall admits.

Some charging will occur during the peak-demand period, too--which is fine, he says. For example, it's no problem if someone wants to drive to work and plug in for a morning recharge. But the industry would prefer to see a minimal amount of charging during peak hours.

To discourage peak-period charging, utilities must revisit financial incentives to consumers, says Stan Hadley, an analyst in the power and energy systems group of the energy and transportation science division of Oak Ridge National Laboratory in Tennessee. Otherwise, Hadley says, power companies will have to build additional capacity to meet the rise in peak demands.

One way or another, electricity prices will have to vary by time, he says. For example, real-time prices, which show the cost at each point in time, can help promote night use of electricity by owners of plug-in hybrids.

Other options include time-of-use pricing, which sets prices for different parts of the day, and critical peak pricing, with a few hours of high prices when supply is most constrained.

"Each has its advantages and disadvantages, but all need a revamping of a utility's metering infrastructure," Hadley says.

Smart meters

It's not enough to make varying rates available, he says. There also must be a way to measure time-sensitive customer demands, communicate the prices to customers and automate the deferral of plug-in charging to low-demand periods.

While some utilities are developing and installing smart meters that, among other things, allow suppliers to charge different rates for consumption based on time of day and season, power suppliers must put in place other equipment and procedures as well, Hadley says. Even then, many customers likely will conduct some portion of their overall charging during peak times, he says.

Even at peak prices, electricity usually is cheaper than gasoline. So regardless of the time, it would make economic sense to immediately recharge a battery that is low on juice, Hadley says.

It's difficult to be completely confident in the ability of the grid to handle a plug-in fleet, in part because the size of that fleet remains unknown. In addition, forecasters don't know where, not just when, owners of plug-in hybrids will recharge their vehicles, says Fred Standish, a Nissan North America spokesman.

A joint study by Nissan, the state of Tennessee and the Tennessee Valley Authority aims to answer that "where." Their research will include investigating whether installing recharging stations along interstate highways in Tennessee would make sense. The study aims to determine where recharging stations should be situated.

"Is it at home, work or, perhaps, a shopping mall?" says Standish. "We don't know right now."

(Source: Automotive News)