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Plugging In

Can the grid handle the coming electric vehicle load?

Fortnightly Magazine - June 2010

all PEVs would charge exclusively during off-peak periods. In fact, their demand on the regional power infrastructure depends greatly on when, and how quickly, drivers charge their vehicles. A 2008 Oak Ridge National Laboratory study estimated that the increase in energy demand would be about 1 to 2 percent in 2020, and about 2 to 5 percent in 2030, based on a very aggressive assumed fleet penetration of 10 percent by 2020, and 25 percent by 2030. 9 That report also found that faster charging, if concentrated in the evening hours, could increase peak electricity demand substantially. In some extreme scenarios examined, where all vehicles utilize rapid charging coincident with system peak, peak demand could increase as much as 10 percent in 2020, and more than 25 percent in 2030. However, this study also found that if PEVs are charged at times less coincident with the existing system peak ( i.e., charged later or more slowly, or not all at the same time), they would have a much more modest effect on peak load, and potentially no effect at all if charging occurs entirely off-peak.

Although it’s difficult to predict charging patterns confidently in the absence of actual customer experience with PEVs, natural diversity in drivers’ schedules and habits might make either of these extremes—all on-peak charging or all off-peak charging—unlikely.

Access to charging spots is an important factor shaping drivers’ charging patterns. Most PEVs likely will be charged at home in evening and night-time hours. The availability of public charging spots ( e.g., workplaces, public garages, street charging) will increase the diversity of charge times and might increase the total amount of electricity used; charging twice daily means using more electricity and less gasoline in a PHEV, though it also will encourage more charging during high-load daytime hours.

Take for example several hypothetical charging profiles that illustrate how the incremental PEV load might be distributed across the hours of the day, with the area under each summing to 100 percent over the day ( see Figure 1 ). The “evening concentrated” profile assumes that all drivers begin charging their batteries more or less simultaneously at 5 to 6 p.m. using rapid chargers that give a full charge in two hours. “Evening diversified” assumes that some drivers begin charging at 5 to 6 p.m. and some start several hours later, using slower chargers that spread the load across eight hours. In “increased work access,” half of the charging starts during morning at 8 to 9 a.m., and the other half starts at 5 to 6 p.m. In “off-peak,” charging occurs at night, starting from 10 to 11 p.m. and continuing through the early morning. These are purely hypothetical demonstrations of the potential incremental load shape; PEVs’ actual charging profiles will be driven by drivers’ information, habits, convenience and electric pricing schemes.

With substantial PEV penetration, the evening-concentrated profile might create concern for an electric system, since it concentrates a large new energy demand at or near system peak. Increased daytime charging access ( e.g., at work) might make some