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Crosscutting NREL Research Advances Plug-In Hybrid Electric Vehicles

August 1, 2006

Photo of a Toyota Prius converted to a plug-in hybrid electric vehicle.

EnergyCS converted this Toyota Prius to a plug-in hybrid electric vehicle. Plugging the vehicle's cord into a standard 110-volt outlet, recharges its electric battery. A computer inside the vehicle determines when it's most efficient to fuel the vehicle with electricity or liquid fuel.

In the 1990s, NREL worked to advance hybrid electric vehicles, particularly propulsion systems, building the technology's viability in the marketplace. Today, NREL is working with industry and other national labs to take this concept one step further with plug-in hybrid electric vehicles. Plug-in hybrids have the potential to cut fuel consumption by more than 50 percent. But what will it take to advance plug-in hybrid technology for market acceptance?

"We basically need to extend the life and lower the cost of a plug-in hybrid battery," said Andrew Simpson, a post-doctoral researcher with the NREL Vehicle Systems Analysis Team. "That's one of the major technical barriers."

Plug-in hybrids have a larger battery pack than standard hybrid electric vehicles. This allows plug-in hybrids to operate predominantly on electricity for short trips. For longer trips, a plug-in hybrid draws liquid fuel from its onboard tank, which provides comparable driving range and performance to a conventional petroleum-fueled vehicle. The vehicle's onboard computer chooses when to use which fuel most efficiently. The plug-in hybrid battery can be recharged using a standard electrical 110-volt outlet at home or even at the workplace.

"I expect that home refueling will be one of the biggest selling points for plug-in hybrids," said Tony Markel, a senior engineer with the NREL Vehicle Systems Analysis Team."Since many drivers travel less than 30 miles per day, home refueling with electricity means they probably won't visit the gas station as often."

But plug-in hybrids don't necessarily require petroleum; they can use biofuels, such as ethanol and biodiesel. For a light-duty vehicle fleet, NREL researchers estimate that plug-in hybrids could reduce the per-vehicle demand for liquid fuel by more than 50 percent, making it practical to use domestically produced E85 (85 percent ethanol, 15 percent petroleum) on a national scale. Also, the fuel cost savings with plug-in hybrids could amount to more than $500 per vehicle per year.

The fuel flexibility of a plug-in hybrid extends beyond the liquid fuel it requires. The electricity used to recharge its battery can be generated by a renewable energy source, such as solar, wind, or biomass. This would provide even lower emissions and greater environmental benefits. NREL is researching how the introduction of plug-in hybrid technology may enable more widespread use of renewable energy technologies.

"With their fuel flexibility, plug-in hybrids could help reduce our dependence on imported petroleum and transition us to a renewable energy economy," Markel said.

Ultimately, the transition to plug-in hybrid technology involves the integration of many technologies under research and development at NREL including advanced vehicle systems, biodiesel, biomass/ethanol, buildings, solar and wind.

Visit the Vehicle Systems Analysis Web site for more information about plug-in hybrid research.

Illustration of a plug-in hybrid vehicle showing the fuel-powered engine and electric motor under the hood, the electric battery pack behind the rear seats and beneath the trunk, the fuel tank in the rear undercarriage, and the battery recharge plug extending from the rear of the vehicle.

Standard hybrid electric vehicles contain a small- to medium-sized battery pack and electric motor. These devices help the engine operate more efficiently and enable normally wasted braking energy to be recaptured. While hybridization helps improve the fuel efficiency of hybrid vehicles, all of the energy used still comes from the fuel tank. Even the energy stored in the battery was once fuel.

In contrast, plug-in hybrid electric vehicles have the ability to recharge their batteries with electricity from the utility grid. They typically have larger battery packs and will use the stored electric energy instead of gasoline whenever possible. Under some conditions, a plug-in hybrid may even operate on electric power only. When needed, the engine and liquid fuel will be used to extend driving range and enhance performance. An onboard computer decides when to use which fuel.

Hence its name, a plug-in hybrid features a plug, which can be plugged into a standard 110-volt outlet for recharging the batteries.

— Theresa von Kuegelgen