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Study Release, Summary of Findings –
Feb 26, 2009
More is Not Always Better for
Plug-in Vehicle Batteries
PITTSBURGH—
Carnegie Mellon University
professor Jeremy J. Michalek and researchers Dr. Constantine Samaras and
C.-S. Norman Shiau report in a new study that plug-in hybrid electric
vehicles with small battery packs may be the best bet for saving drivers
money while addressing U.S. dependency on foreign oil and global warming.
In an article to appear
in the journal Energy Policy, the authors find that urban drivers who can
charge their vehicles frequently (every 20 miles or less) can
simultaneously reduce petroleum consumption, greenhouse gas emissions, and
expenses with a plug-in hybrid vehicle whose battery pack is sized for
about 7 miles of electric travel per charge. In contrast, plug-in hybrid
vehicles with large battery packs – sized for 40 or more miles of electric
travel – are too expensive for fuel savings to compensate, even in
optimistic scenarios.
Plug-in hybrid vehicles
use charged batteries to propel the vehicle partly using electricity
instead of gasoline, which gives them potential to reduce petroleum
consumption and greenhouse gas emissions. “Larger battery packs allow
drivers to go longer distances on electric power. But batteries are heavy
and expensive,” says Michalek. “We accounted for the effects of additional
batteries on vehicle cost, weight and efficiency in order to understand the
net implications on petroleum consumption, cost, and greenhouse gas
emissions. Over a range of scenarios -- including fluctuating gas prices,
new battery technologies or high taxes on carbon dioxide emissions --
plug-ins with small battery packs are economically competitive with
ordinary hybrid and conventional vehicles for drivers who charge
frequently.”
The Obama
administration has set a target of putting 1 million plug-in hybrid cars on
the road by 2015, and the American Recovery and Reinvestment Act of 2009
includes a range of incentives for plug-in vehicles as well as funds for
research. “Plug-in vehicles can reduce greenhouse gas emissions in general,
but getting the most bang for the taxpayer buck means targeting the right
vehicles to the right drivers,” says Michalek. “In fact, for drivers who
charge their vehicles frequently, plug-in vehicles with small battery packs
create fewer greenhouse gas emissions than plug-ins with large battery
packs because carrying extra weight makes vehicles less efficient. Plug-ins
with large battery packs can still reduce greenhouse gas emissions for
drivers who charge less frequently as well as help shift air pollution away
from population centers, but they won’t save drivers money unless batteries
get very cheap.”
The
study, which was funded by the National Science Foundation and the Teresa
Heinz Scholars for Environmental Research Program, points out that
targeting drivers with the potential to charge frequently would not limit
plug-ins to a boutique market: nearly 50% of U.S. passenger vehicle miles
are traveled by vehicles driving less than 20 miles per day. “If gas prices
go up or battery prices come down, plug-in vehicles will be more
competitive across the board, but the small battery packs remain best on
cost, and new charging infrastructure could increase the number of drivers
who can benefit.”
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Shiau, C.-S., C.
Samaras, R. Hauffe and J.J. Michalek (2009) “Impact of battery weight and
charging patterns on the economic and environmental benefits of plug-in
hybrid vehicles,” to appear, Energy
Policy.
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