Navigant Research Blog

EV Emissions Reconsidered

Scott Shepard — July 2, 2014

Quantifying the degree to which plug-in electric vehicles (PEVs) improve ambient air quality conditions over conventional gas or diesel-powered vehicles is an important, but difficult, question to answer.  An interview with Electric Power Research Institute’s (EPRI’s) Marcus Alexander, who will discuss the preliminary findings of a study seeking to clarify how PEVs affect environmental conditions at EPRI’s Plug-In 2014 Conference in San Jose, demonstrates the complexity of this subject.

Much of the calculation has to do with where the PEV is driven, as this dictates the carbon intensity of the electric grid used to power the vehicle.  However, in most locations throughout the United States and the globe, the operating emissions of a PEV versus a conventional vehicle on a per-mile basis lean either substantially or marginally toward conventional vehicles.

However, there are nuances to this equation beyond simple pounds of pollutant emitted per unit of energy consumed.  For instance, when a conventional vehicle consumes a gallon of gasoline or diesel, the pollutant emissions calculation is fairly straightforward.

Net Zero

Additionally, the pound of pollutant emitted varies considerably from the mobile source (vehicle) to the stationary source (power plant).  For example, Alexander states that carbon monoxide and volatile organic compounds are more tightly linked to vehicles than power plants, while sulfur dioxide emissions are associated with fossil fuel combustion at power plants.  Supplanting gas or diesel miles driven with electric miles driven can therefore reduce emissions of particular pollutants while increasing others.

However, when a PEV consumes a kilowatt-hour (kWh) of electricity, it may have a net zero impact on pollutant emissions, depending on a complex interaction of emissions regulations and available generation capacity.  Growth of wind generation over the last decade has created excess capacity, often at night when the wind blows strongest and demand is lowest.  Data from the U.S. Energy Information Administration (EIA) indicates that in 2012, net generation exceeded net load by around 2.3%.  Navigant Research estimates in the report Electric Vehicle Market Forecasts that nearly 300,000 PEVs will be in use in the United States in 2014.  Assuming an average annual PEV mileage of 12,000 and the EIA’s projections on electricity energy demand in the United States, PEVs would represent less than 0.03% of total U.S. electricity demand.

New Sources

Further, while the emissions profile of burning 1 gallon of gasoline will stay relatively consistent over time, the emissions profile of consuming 1 kWh of electricity from the grid will change as new generation assets are added to the grid and old assets retired.  In the last 2 years, nearly 15,000 MW of coal generation has been retired, with a little over 5,000 MW added.  Over the same period, 22,000 MW of renewables generation were added.  If U.S. electricity demand stays on the plateau of the last decade, the replacement of aging high-emissions assets in favor of renewables will be much easier, and the grid’s emissions profile is likely to change quickly.

EPRI’s study seeks to quantify these factors and others (such as energy consumption from lithium ion battery development) to provide the most accurate analysis of how existing PEV technologies will influence environmental conditions.  Alexander clarifies that this study, while quite comprehensive, does not investigate potential opportunities presented by PEVs, such as utilizing them for grid energy storage or ancillary services, that have yet to become market realities.  Findings from the study will be fundamental to defining the efficacy of PEVs in attaining a number of U.S. goals for air quality standards and carbon emissions reductions.

One response to “EV Emissions Reconsidered”

  1. douglas prince says:

    I always find it amusing when these type of articles pop up, talking about the “overall” net effect of PEVs, ie: taking into account the source of the electricity (a power plant). Yet, for some reason, the same argument doesn’t seem to apply to ICE engines. Why not talk of the “overall” net effect of ICEs, ie: the oil-pumping well, the refinery, the ship burning oil as it transports the oil from one part of the world to another, the gas station, etc?

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