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Plugging In

Can the grid handle the coming electric vehicle load?

Fortnightly Magazine - June 2010

PEV load coincide more directly with existing system peaks, though it also might help to diversify the charging load across time. Further, it might create additional stresses on urban distribution systems by concentrating additional loads in areas and times that already have high loads. Night-time charging seems to be the best complement to current system conditions, though if charging loads were concentrated sufficiently, they could conceivably create a secondary daily peak.

A tangible example considers the potential impact of these different charging patterns on simulated New England electricity demand in 2020. The PEV demand is overlaid on both summer and winter peak day load shapes (assuming 5-percent New England fleet penetration by PEVs that get half their energy from electricity, on average) ( see Figure 2 ). If charging is heavily concentrated at or near system peak on a system like New England’s ( e.g., the evening-concentrated charging pattern), even relatively modest PEV penetration might increase system capacity needs by several percent. Other charging patterns that involve charging later or spreading it over longer periods—some of which may occur naturally—greatly reduce or eliminate the impact on system capacity needs. The increased-work-access charging pattern adds slightly to capacity needs because it adds load at system peak times, though the increment is small because the load is distributed across many hours. The other patterns have no effect on peak load at all.

The seasonality of the daily load pattern also can be important. In New England, system load usually peaks in summer in mid-afternoon hours. This is several hours before much of the likely PEV load, if drivers charge at home after work. But the winter peak occurs later in the evening, more coincident with the likely PEV load. Since New England is a summer-peaking region overall, the coincidence of PEV loads with winter peak might not be a major concern. But on a winter-peaking system, an unmanaged PEV charging profile could increase overall system capacity requirements even at low PEV penetration, giving an incentive to more aggressively manage PEV loads.

To encourage charging during off-peak hours likely would involve technological and pricing solutions. Technological solutions include smart-charging systems that can consider electrical system preferences while meeting drivers’ needs. Pricing solutions, such as dynamic pricing of electricity, could enhance consumers’ incentives for off-peak charging by offering different prices for peak and off-peak electricity. Both approaches should help to steer drivers toward better use of the electric system, though it isn’t clear how PEV owners would respond. Depending on how prices are structured, the potential savings associated with dynamic pricing might be relatively small and could be ignored, and technological solutions might be bypassed if they don’t meet drivers’ needs.

The potential to integrate PEVs into the future smart grid so that they can respond dynamically to help balance the grid’s needs, becoming a controllable resource as well as a new customer and perhaps even compensating for the variability of renewable generation sources, is a particularly attractive long-term prospect. In fact, a recent article on variable renewable energy integration states: “Plug-in electric vehicles promise