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Plugging In

Can the grid handle the coming electric vehicle load?

Fortnightly Magazine - June 2010

range reflects the emissions of different charging profiles, accounting for the marginal unit’s emissions in each charging hour. Because the New England power system has gas on the margin in the large majority of hours, a New England PEV would emit significantly less CO 2 than a gasoline vehicle, and its emissions don’t depend greatly on the charging profile.

This might not be true in all regions, particularly those dominated by coal. In some regions such as the Midwest, where coal-fired units are often marginal in off-peak hours, off-peak charging may forego much of the potential CO 2 reductions. Even in such regions, however, to the extent the power grid is decarbonized over time through the addition of less carbon-intensive resources ( e.g., gas, renewables, hydro, nuclear) and reduced reliance on coal, a PEV’s emissions can fall over time.

The effect of PEVs on the emissions of other pollutants, such as nitrogen oxides (NOx) and sulfur dioxide (SO 2), depends heavily on the emissions controls in place—the vehicle’s own emission controls for a gasoline vehicle, and the emission controls on the marginal generator for the PEV. As with CO 2, the power system’s marginal emission rates of other pollutants can vary considerably with time—and certainly from one electric system to another—depending on which unit is on the margin. In the New England analysis, NO x emissions of a PEV generally were similar or slightly lower than those of a gasoline vehicle. SO 2 emissions were a bit higher, because gasoline contains little sulfur and some generator types, particularly coal and oil-fired units, do emit SO 2 and are sometimes on the margin. More generally, electric generating units might be farther from the urban areas that are most sensitive to air quality, both geographically and temporally, compared to the internal combustion engines which emissions they would displace, though again this may depend on the particular circumstances.

Time to Plan

PEV penetration of the vehicle fleet is likely to be relatively limited for the next decade and probably beyond, in part because the high initial cost of batteries likely will continue to outweigh the potential fuel-cost savings. Due to the natural lag in replacing the existing fleet, fleet penetration by PEVs will be gradual even if they achieve a relatively high share of new vehicle sales. Further, if PEV sales are high, this will be observable before PEVs take over a large share of the fleet, giving the industry some opportunity to plan accordingly.

The energy demand that PEVs might place on the power system isn’t particularly great even at higher fleet penetration levels, and will be quite modest at more likely penetration levels. The potential effect of PEVs on electric peak loads might be another matter. Even at plausible penetration levels, PEVs could affect system peak loads if many of them are charged quickly at or near peak load times, though natural diversity in users’ schedules and habits might mitigate some of this concern. This might make charging patterns and the timing of PEV load a more important factor for