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NREL Analyzes Impact of Climate Control in Electric Drive Vehicles

January 19, 2011

A technical paper recently released by the U.S. Department of Energy’s National Renewable Energy Laboratory (NREL) reveals the impact of climate control loads in electric drive vehicles.

In plug-in hybrid electric vehicles (PHEVs) and electric vehicles (EVs), the stored electrical energy in the battery is used to drive the electric drive motor. But using the battery for immediate air conditioning or heat reduces the vehicle’s charge-depleting (CD) range—the distance over which the vehicle operates on battery power alone—and causes additional battery wear.

NREL engineer Robb Barnitt led the effort to quantify the impact of thermally preconditioning the vehicle cabin and battery pack of PHEVs and EVs using off-board power supplied by the utility grid. His team simulated vehicle performance with and without thermal preconditioning over various drive cycles and ambient temperature scenarios.

“We knew that climate control loads would have an impact on CD range, but were surprised by the magnitude” Barnitt said. “We found that climate control loads can reduce CD range by 35%, but that thermal preconditioning can partially restore CD range by up to 19%.”

Preconditioning the battery also has a positive impact. When a vehicle sits outside on a hot summer day, the cabin isn’t the only part of the car that gets hot.  When the car is started, the battery is at least as warm as the outside air, and during driving the battery gets even hotter.  Time at elevated temperatures reduces battery life, and batteries aren’t cheap.

Pre-cooling the battery in conjunction with cabin cooling represents a partial solution. When the battery is cooler at the start of a trip, it fails to reach higher operating temperatures.

“We used a lithium-ion battery-life model to characterize battery wear,” Barnitt added. “We found that battery-capacity loss over time is driven by ambient temperatures rather than climate-control loads, but that pre-cooling the battery can reduce capacity loss by 7%.”