Subscriber access provided by GEORGE WASHINGTON UNIVEnvironmental Science & Technology is published by the American ChemicalSociety. 1155 Sixteenth Street N.W., Washington, DC 20036Policy AnalysisLife Cycle Assessment of Greenhouse Gas Emissionsfrom Plug-in Hybrid Vehicles: Implications for PolicyConstantine Samaras, and Kyle MeisterlingEnviron. Sci. Technol., 2008, 42 (9), 3170-3176 • DOI: 10.1021/es702178s • Publication Date (Web): 05 April 2008Downloaded from http://pubs.acs.org on January 21, 2009More About This ArticleAdditional resources and features associated with this article are available within the HTML version:• Supporting Information• Access to high resolution figures• Links to articles and content related to this article• Copyright permission to reproduce figures and/or text from this articleLife Cycle Assessment ofGreenhouse Gas Emissions fromPlug-in Hybrid Vehicles: Implicationsfor PolicyC O N S T A N T I N E S A M A R A S *,†,‡ANDK Y L E M E I S T E R L I N G†Department of Engineering and Public Policy, andDepartment of Civil and Environmental Engineering,Carnegie Mellon University, 5000 Forbes Avenue,Pittsburgh, Pennsylvania 15213-3890Received August 29, 2007. Revised manuscript receivedJanuary 15, 2008. Accepted February 4, 2008.Plug-in hybrid electric vehicles (PHEVs), which use electricityfrom the grid to power a portion of travel, could play a rolein reducing greenhouse gas (GHG) emissions from the transportsector. However, meaningful GHG emissions reductions withPHEVs are conditional on low-carbon electricity sources. Weassess life cycle GHG emissions from PHEVs and find that theyreduce GHG emissions by 32% compared to conventionalvehicles, but have small reductions compared to traditionalhybrids. Batteries are an important component of PHEVs, andGHGs associated with lithium-ion battery materials andproduction account for 2–5% of life cycle emissions fromPHEVs. We consider cellulosic ethanol use and various carbonintensities of electricity. The reduced liquid fuel requirementsof PHEVs could leverage limited cellulosic ethanol resources.Electricity generation infrastructure is long-lived, and technologydecisions within the next decade about electricity supplies inthe power sector will affect the potential for large GHG emissionsreductions with PHEVs for several decades.IntroductionReducing greenhouse gas (GHG) emissions from motorvehicles is a major challenge for climate policy. Modestincreases in vehicle efficiency have been offset by increasedtotal travel, and transportation has accounted for about 40%of the growth in carbon dioxide (CO2) emissions from allenergy-using sectors since1990 (1). OneapproachtoreducingGHGs from vehicles is improving fuel economy, e.g., thehybrid electric vehicle (HEV) (2). A second approach is alow-carbon fuel, such as c e l l u l o s i c ethanol (3–5). A thirdapproach is a plug-in hybrid (PHEV), which substituteselectricity for a portion of the petroleum used to power thevehicle. We estimate and compare life cycle GHG emissionsfrom PHEVs, an HEV, and a conventional gasoline vehicle(CV). Since emissions from PHEVs largely depend on thesources of electricity used, we consider various electricitygeneration options with varying carbon intensities as well asthe effects of using cellulosic ethanol liquid fuel.A transition to plug-in hybrids would begin to couple thetransportation and electric power generation sectors. Com-bustion emissions from U.S. (United States) automobiles andlight-duty trucks accounted for approximately 60% of GHGemissions from the U.S. transport sector, or 17% of total U.S.GHG emissions (1). Powering transport with electricity wouldshift GHG emissions and criteria pollutants from distributedvehicle tailpipes to largely centralized power plants. Col-lectively, burning fossil fuels in the transport and powersectors accounted for about 59% of GHG emissions in theUnited States in 2004 (26.2% and 32.4%, respectively) (1).The scale of the U.S. transport sector dictates that the GHGimpacts from widespread PHEV adoption will materiallyaffect U.S. GHG emissions.A plug-in hybrid in a parallel configuration can use anon-board battery to travel on electricity from the grid, andit can operate as a traditional HEV, burning liquid fuel (6, 7).PHEVs provide electric-powered travel, but have rangescomparable with conventional vehicles because they canoperate as HEVs. The vehicle’s battery can be recharged atelectrical outlets, hence PHEVs substitute electricity forgasoline to supply a portion of the power needed for travel.Vehicles that travel fewer than 50 km per day are responsiblefor more than 60% of daily passenger vehicle km traveled inthe United States (8). Thus, plug-in hybrids may be able topower a substantial portion of daily travel with electricity,and could displace a large fraction of gasoline use. In additionto concerns about climate change, dependence on importedoil supplies is seen as a threat to U.S. national security (9)and a passenger transport system partially powered byelectricity could reduce oil dependence.The life cycle GHG emissions benefits of PHEVs dependon the vehicle and battery characteristics, and on the GHGintensity of the electricity and liquid fuel used to power thevehicle. A review of PHEV design considerat ions andenvironmental assessments has been completed by Bradleyand Frank (7). Previous studies investigating GHG impactsfrom PHEVs focus solely on the impacts of electricity andgasoline for PHEV propulsion. The Electric Power ResearchInstitute (EPRI) has conducted a series of PHEV analyses.Their preliminary reports (10, 11) analyzed PHEVs chargedwith electricity produced from natural gas combined cyclepower plants. Other studies have shown larger regional GHGreductions in areas with less GHG-intensive generationportfolios (12, 13, 50). Previous estimates have found that34–73% of the existing light-duty vehicle fleet could besupported as PHEVs from the existing power supply infra-structure (12, 50). Kempton et al. estimated potential largeGHGreductions usingoffshorewind to powerplug-in vehicles(14).ArecentEPRI analysis (15) modeled the electricitysystemand PHEV adoption scenarios and found GHG reductionscompared to CVs and HEVs. The electricity charging PHEVsin that analysis was 33–84% less carbon intensive than thecurrent U.S. generation portfolio.This analysis contributes to the PHEV literature byincluding several aspects omitted by previous work. First,energy