—€¦Or Perhaps a Hybrid You Can Plug Into a Power Outlet?
August / September 2007
Volume 5 Number 4
The Japanese and American hybrid gasoline-electric vehicles that have been gaining in popularity over the past couple of years weren't a surprise to researchers at the National Renewable Energy Laboratory who had been working on such propulsion systems since the 1990s. Today, however, they're taking the concept one step further by working on plug-in hybrid vehicles electric, or as they are known in the lab, PHEVs.
PHEVs—€”pronounced fevs—€”have characteristics of both conventional hybrid gasoline-electric vehicles and battery-electric vehicles with the potential to cut fuel consumption by more than 50 percent. But what will it take to advance this technology to market acceptance?
Answer: a better battery. "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 PHEVs to operate predominantly on electricity for short trips. For longer trips, a liquid fuel-powered internal combustion engine kicks in to provide driving range and performance comparable to a conventional car.
The vehicle's onboard computer chooses when to use electricity only or when to add power from the internal combustion engine. The plug-in hybrid battery can be recharged using a standard 110-volt outlet at home or at work.
"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 team. "Since many drivers travel fewer than 30 miles per day, home refueling with electricity means they probably won't visit the gas station as often."
But PHEVs don't necessarily require gasoline; they can use biofuels, such as ethanol or biodiesel. For a light-duty vehicle fleet, NREL researchers estimate plug-in hybrids could reduce the per-vehicle demand for liquid fuel by more than half, making it practical to use domestically produced E85 (85 percent ethanol, 15 percent gasoline) on a national scale. 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 could 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 the laboratory. NREL's vehicle systems analysts employ a variety of models and software tools to evaluate advanced vehicles—€”from their components and design to market penetration.
The technical targets tool determines pathways for maximizing the potential national impact of plug-in hybrid electric vehicles. The tool focuses on sizing vehicle components to take advantage of the vehicle's unique characteristics and make the vehicle as competitive as possible. This includes consideration of how consumers will value the new vehicle technology based on attributes such as acceleration, fuel economy and consumption, cargo capacity and cost. This information is ultimately used to estimate market penetration and project the impact on vehicle fuel use.
ADVISOR—„ (Advanced Vehicle Simulator), developed in 1994, is used to simulate and analyze conventional, advanced, light and heavy vehicles, including hybrid electric and fuel cell vehicles. It tests the effect of changes in vehicle components (such as motors, batteries, catalytic converters, climate control systems and alternative fuels) and other modifications that might affect fuel economy or performance.
In 2003, NREL transferred a license to commercialize ADVISOR to AVL Powertrain Engineering, Inc. AVL received rights to manufacture, market and sell commercial versions. A Cooperative Research and Development Agreement between NREL and AVL provides for future collaboration to develop integrated vehicle simulation tools for DOE and the private sector.
"We're moving from software development to real applications such as studying the impacts of ambient conditions on fuel cell system operations," Markel said. "I see ADVISOR as just one of the tools in our toolbox that will help our team address the technical barriers to hybrid electric vehicle design and introduction."
Probably the biggest challenge plug-in hybrids have is the cost and weight of batteries. NREL is extensively researching thermal management, modeling and systems solutions for energy storage technology. Researchers are working closely with DOE, industry and automotive manufacturers to improve energy storage devices, such as battery modules and ultracapacitors, by enhancing their thermal performance and life-cycle cost.
NREL researchers also are seeking to carry the plug-in hybrid concept a couple of steps farther by making the plug-in reversible. Called a "vehicle-to-grid" or V2G, such a two-way plug would allow the home and vehicle owner and local utility to take advantage of the extra electrical storage capacity in the vehicle batteries to meet peak demand, provide grid support services or respond to power outages. In addition, utilities pay premium rates for peak and backup power and might pay commuters to plug in their vehicles while at work to ensure their employer has high-quality power throughout the day. NREL transportation analysts are researching the potential value of such systems.
"Our design analysis helps provide vehicle developers with information on technical challenges," Markel said. "This helps generate technical targets for advancing components ideally suited for plug-in hybrid electric vehicle applications. Also, we use simulation tools to examine alternative control strategies and to optimize hybrid control based on fuel economy and vehicle lifetime cost."
Sarah Barba is in the public relations office of the National Renewable Energy Laboratory.
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How a Plug-in Hybrid Works
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 can 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.
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