Transportation's Role in Reducing U.S. Greenhouse Gas Emissions ...

Transportation’s Role in Reducing U.S. Greenhouse Gas Emissions: Volume 2
can match a vehicle technology with a variety of different fuel options. In fact,
GREET includes more than 100 fuel production pathways and more than 70
vehicle systems. The GREET model is based solely on fuel type and vehicle
technology, whereas other transportation models (such as TAFV, MiniCam, and
NEMS) project transportation emissions based on an expected economic
situation and/or likely consumer behavior.
This analysis uses the default values found in GREET in order to assess the
lifecycle impacts of alternative fuels relative to conventional gasoline and diesel.
The grams per vehicle mile traveled values in Table A.1 account for key GHGs,
converted to CO 2 equivalents using a 100 year (default) global warming
potential (GWP). Results from the model vary significantly depending on
various assumptions related to fuel production technology and end-use. In
addition, GREET does not account for indirect land use changes or other
indirect effects that may significantly impact final results for biofuels in
particular. Other key limitations of GREET’s “direct attributional approach”
include (U.S. EPA, 2009a):
• The economic impact of biofuel coproducts on other sectors, such as
livestock feed markets and associated indirect GHG impacts, were not
adequately assessed;
• Specific policy measures could not be evaluated in detail as a function of
different production targets, especially with regard to agricultural sector
impacts;
• Fuel replacement impacts were assessed on an average gallon of fuel basis,
rather than evaluating the specific alternative fuel production and
conventional fuel replacement emissions at the margin; and
• The RFS1 evaluation methodology did not account for the effects of falling
U.S. petroleum demand on world fuel prices, and subsequent increases in
international petroleum consumption.
Depending upon the nature of the analysis, GREET grams per mile outputs
were frequently converted to equivalent grams per gallon values, in order to
compare the potential GHG impacts of alternative fuels compared with
conventional gasoline and diesel. In this way the impact of changing fuel
efficiencies over time could be accounted for without having to re-run the
GREET model for each time period of interest. In addition, in most cases this
analysis assumes that alternative fuel vehicles accrue fuel efficiency
improvements at the same rate over time as the comparable conventional
baseline vehicle. This simplifying assumption is not made for fuel cell and
battery electric vehicles, as the source of motive power and drive train
configurations can be fundamentally different than conventional internal
combustion engine vehicles.
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