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

Transportation’s Role in Reducing U.S. Greenhouse Gas Emissions: Volume 2
renewable sources such as hydropower, geothermal, wind, solar, and other biomassderived
sources (9 percent). 36
Net GHG reductions, as well as fuel cost savings, are largely a function of BEV efficiency,
typically expressed in kWh/mi rather than mpg. PHEV batteries range from roughly 0.2
to 0.4 kWhr/mi, with a medium- to long-term average of 0.26 kWhr/mi (see Section 3.2.4).
For low-drag vehicles it is assumed that BEV battery efficiency in kWhr/mi will not vary
significantly for ranges between 100 and 200 miles, with increased weight penalties largely
offset by larger battery capacities and the more effective use of regenerative braking that is
permitted. For example, Kromer and Heywood (2007) estimate only a mild increase in
kWh/mi when moving from a 100- to 200-mile BEV range (0.22 to 0.24). However, high
drag vehicles such as traditional SUV designs are likely to incur a substantial efficiency
penalty in order to obtain a 200-mile operating range. For example, Bandivadekar et al.,
2008, p. 300 estimates such an advanced, extended range BEV to have an efficiency level of
up to 1.38 kWh/mi, for light trucks.
Due to the dramatic efficiency penalties and corresponding increases in battery size and
cost associated with longer range, vehicle design is likely to have a great impact on the
future BEV market. In order for BEVs to obtain reasonable ranges (upwards of 200 miles)
at competitive costs, the BEV market will likely remain focused on advanced, low-drag
designs. Accordingly, this analysis assumes the same efficiency value for BEVs as for
PHEVs in the long run, at 0.26 kWhr/mi, for 100 to 200 miles of operating range. The
extent to which these vehicles penetrate the market may be largely determined by
consumer willingness to alter their traditional purchasing decisions in favor of such
advanced designs and body styles.
BEV emissions are entirely dependent upon the mix of electricity sources employed and
the efficiency of electricity transmission, charging, and end use. Using the current national
electricity grid mix assumed in the GREET model, and assuming 0.4 kWh/mi, current
BEVs are estimated to provide a 33 percent decrease in life-cycle GHG emissions per VMT
compared to conventional gasoline vehicles. Fuel-specific life-cycle emission estimates
include a six percent increase in GHGs (relative to conventional gasoline LDVs) for coalfired
utilities, a 45 percent decrease for natural gas, and a 99 percent reduction for nuclear
generation.
Although overall electricity generation is projected to increase by 24 percent from 2008 to
2030, the fuel mix distribution is not projected to shift significantly under baseline
projections (U.S. DOE, 2009a). The largest increase is predicted in renewables, growing
from 9 to 14 percent of total generation, while coal, natural gas, and nuclear lose one to
two percent each. The future electricity generation mix may be affected, however, by
potential climate change policy actions at the Federal and/or multistate level, such as the
adoption of a national cap-and-trade system, which would likely reduce the contribution
36 AEO 2009, Table 8.
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