Light-duty vehicles play a crucial role in two of the key challenges facing the United States: energy security and climate change. Transportation is responsible for more than two-thirds of U.S. oil consumption, and about 60 percent of the oil we use must be imported. Dependence on imported oil leads to concerns over vulnerability to disruptions, especially if world oil production peaks. Burning that oil in vehicles also accounts for one-third of U.S. emissions of carbon dioxide (CO2), the main greenhouse gas linked to global climate change. The U.S. government is seeking to reduce the use of oil to help meet both challenges. This report assesses the contributions that plug-in hybrid electric vehicles (PHEVs) can make to this effort.
The National Research Council (NRC) report Transitions to Alternative Transportation Technologies—A Focus on Hydrogen (2008) analyzed the potential for hydrogen-fueled fuel cell vehicles to penetrate the market and estimated the reductions in oil consumption and CO2 emissions that might result. The report also compared these benefits to those that might be achieved by two alternatives: vehicles operating on biofuels and vehicles with advanced internal combustion engines. The latter included hybrid electric vehicles (HEVs) but not plug-in hybrid electric vehicles (PHEVs) or all-electric vehicles (EVs).
In 2009 the U.S. Department of Energy asked the NRC to extend its analysis to PHEVs, putting them on the same basis as the other alternatives to fuel cell vehicles. This report is the result of that additional task. The statement of task is in Appendix D.
PHEVs have recently been the focus of much attention, in large part because of rapidly improving battery technology. Several manufacturers intend to introduce PHEVs over the next few years. PHEVs are similar to today’s HEVs, but they have larger batteries that can be charged from the electric grid and can supply sufficient energy to propel the vehicle for many miles. When PHEV batteries are discharged, the gasoline engine takes over, by either recharging the battery or directly providing power for propulsion. Short trips could avoid the use of gasoline altogether, and long trips are possible without the risk of being stranded, which is a concern for all-electric vehicles. PHEVs promise to reduce the use of gasoline without necessitating the major infrastructural changes that would be required for hydrogen, thus allowing an evolutionary transition from conventional vehicles. In addition, electric utilities may promote PHEVs because nighttime charging would help smooth out demand.
This report first evaluates battery and vehicle technologies to predict how costs might drop as technology improves and economies of scale increase. It considers PHEVs that can travel 10 (PHEV-10) and 40 (PHEV-40) miles on electric power as representative of all the PHEVs that may be available. Next it examines the ability of the electric grid to supply power for a growing PHEV fleet. Then it analyzes two potential market-penetration rates for PHEVs: (1) a Maximum Practical scenario, which makes optimistic assumptions about the evolution of PHEV technology, especially batteries, and about the barriers to market penetration and (2) a Probable scenario based on more likely assumptions. Because initially PHEVs will be considerably more expensive than equivalent conventional vehicles, the committee used the model developed in the hydrogen study to estimate the costs involved in supporting a transition to PHEVs. It also estimated the reduction of petroleum consumption and CO2 emissions that could result from these two scenarios. Finally, the report discusses the committee’s conclusions. The appendixes provide additional information on this study and the scenario analysis, plus a glossary of the many acronyms used in the report.
The committee was not able to analyze one potential barrier—the changes that PHEVs will require of drivers. The vehicles analyzed in the hydrogen report, even fuel cell vehicles, are functionally similar to current vehicles because they have the same range and refueling patterns. PHEVs, however, will require drivers to plug in their vehicles essentially every day. That will require a place where they can plug it in, preferably a garage or at least a car port, and the willingness to take the time to do it. While many people have the place, their willingness is a great uncertainty.