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Plugging In

Can the grid handle the coming electric vehicle load?

Fortnightly Magazine - June 2010

to increase minimum loads at night—making use of surplus wind-energy generation—and to offer fast and accurate response to high variability in wind net load, as needed by the system operator.” 10 However, most analysts regard widespread adoption of such vehicle-to-grid (V2G) technology as many years more distant, recognizing the technological and regulatory barriers that must be overcome before it can be deployed widely.

Although the impact of likely PEV penetration rates on overall system demand is modest, if PEV adoption is highly localized ( e.g., in affluent neighborhoods), the additional charging load potentially could add stresses on the local distribution system. The expected modest PEV numbers, coupled with the fact that easily observable PEV sales give advance warning of aggregate fleet penetration, might help utilities to foresee and deal with these issues. However, this is an area that would benefit from additional research and real-world experience, and active utility intervention ultimately might be needed to prevent or address localized distribution issues.

Long Tailpipe?

From an environmental perspective, switching from gasoline to electricity as a transport fuel means trading the emissions of a gasoline-powered internal combustion engine for those of the marginal electric generator at the time the PEV is charged. 11 In addition to moving those emissions geographically from the tailpipe of the vehicle to the generator’s smokestack, the quantities emitted will differ. The CO 2 emitted from a gasoline vehicle is similar to that released by charging a PEV with power from a coal-fired plant. Coal has about the highest CO 2 emissions among common generation types, so in many regions and circumstances ( i.e., when some other resource that emits less CO 2, such as a gas plant, is on the margin), a PEV will emit less CO 2 than a gasoline vehicle. This generally is the case in New England, where gas-fired units are on the margin in most hours, emitting CO 2 at about half the rate of coal-fired power.

The annual CO 2 emissions of a conventional gasoline vehicle (at 25 to 45 mpg) can be compared with those of a PEV under several alternative charging circumstances ( see Figure 3 ). For a more direct comparison, the PEV is assumed to be all-electric rather than a plug-in hybrid. If the PEV is charged when a coal plant is on the margin, its CO 2 emissions will be similar to those of a gasoline vehicle. Note that the range of PEV emissions when charging with coal corresponds to a range of coal plant efficiency—a less efficient coal plant emits more CO 2. If a gas plant is on the margin, the PEV emits much less CO 2. On most systems, the marginal emissions rate differs over the hours of the day, so the PEV’s CO 2 emissions might depend on the particular charging pattern, and would be an average of the marginal emission rates during the charging period. The right-most bar on Figure 3 illustrates the CO 2 emissions of a PEV charged on the simulated 2020 New England power system; the