A Smart Grid Challenge: EV Charging Stations

A new energy load is being added to the power grid—battery chargers for plug-in hybrid electric vehicles (PHEVs) and pure electric vehicles (EVs). While most chargers operate during the evening and into the night, an ever-increasing number will operate all day long at public charging stations.

The load that an EV charger presents to the grid varies widely, depending on the rate at which the charger refills the batteries in the vehicle. Typical chargers, used for nickel metal hydride and lithium ion batteries, will appear as significant loads, comparable to household water heaters or air conditioners, and power companies worry about their proliferation—and what it means for maintaining grid stability. A March 2007 report by the National Renewable Energy Laboratory (NREL) projected that, based on existing electricity demand and driving patterns, a 50% penetration of PHEVs would increase per-capita electricity demand by 5% to 10%.

To assist with this increased demand, charging station manufacturers are working to provide a remote, utility-controlled function for networked charging stations, allowing monitoring and control options for three levels of stakeholders: subscribers who use the stations; host businesses that have the stations on their property as a revenue source; and utility companies that wish to monitor and manage these loads.

While the 2007 NREL report acknowledges increasing electricity loads from PHEVs, it also suggests the possibility of PHEVs passing energy onto the grid. If made practical, this could help prevent auxiliary power sources from being brought on line during peak daytime periods. This would require the cooperation of the auto makers and the expertise of power engineers to interface PHEVs with synchronous inverters in parallel with the onboard chargers, as well as all of the control and communications intelligence.

Vehicle owners would have to understand and agree to this feature too, since only they would know what battery capacity is needed to accomplish the day’s errands. Imagine returning to your vehicle only to find that the batteries have been drained, limiting your range of needed travel. In addition, there is the problem that vehicle owners might be charged for the period of time it took to have their batteries drained—and then have to pay again to restore the charge.