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

Transportations Role in Reducing U.S. Greenhouse Gas Emissions: Volume 2
compared to most other alternatives, with a total unit cost of over $500,000 for a full-size
heavy-duty hybrid transit bus, approximately $200,000 more than a conventional diesel
bus of similar design. Operating costs of hybrid-electric buses also tend to be higher than
other alternatives currently, due to the added expense of battery maintenance and
replacement (Clark et al., 2007). Battery technology is advancing rapidly, however, which
will likely drive down both the capital and operating costs of hybrid-electric buses while
increasing their performance and durability. One study reported cost-effectiveness values
for GHG reduction from hybrid transit buses at less than $140/tonne CO2e (McKinsey,
2009). There were estimated to be approximately 1,100 hybrid buses in operation in the
United States in 2006 (CCAP, no date).
Fuel Cells
Like batteries, fuel cells convert chemicals directly into electricity, the key difference being
that a fuel cell will continue operating as long as it is supplied with fuel and air (to
provide hydrogen and oxygen, respectively), while batteries will operate (on one charge)
only as long as it takes to consume their self-contained reactants. Also like batteries, fuel
cells are efficient, quiet and have no moving parts. Typically fuel cells are “hybridized” by
combining them with batteries. Compared to buses powered by batteries only, vehicles
equipped with fuel cells have longer driving range.
Fuel cell systems can be powered directly by hydrogen, which is the approach being
pursued by most fuel cell bus developers today. Hydrogen can obtained from a central
fueling source, or alternatively by using a reformer on the bus to separate hydrogen from
a variety of fuels including natural gas, alcohol, or gasoline, although on-board reforming
is not currently favored. Heat and water are the only direct emissions from a hydrogenpowered
fuel cell, although the energy consumption and emissions from generating the
hydrogen off-board the bus must be considered when comparing the merits of different
fuel options. Hydrogen fuel cells, while viewed by many as the propulsion system of the
future, will not likely penetrate into vehicle fleets in the numbers and time horizon called
for in the Energy Policy Act of 2005 (U.S. DOE, 2009b). While the basic technology
behind hydrogen fuel cells is well understood, many technological, institutional, and cost
barriers are hindering widespread adoption of hydrogen fuel cells in transit (see
Section 2.8 for a discussion of similar barriers to use in light-duty vehicles). On the other
hand, the unique characteristics of transit vehicles (such as their once- or twice-daily starts
in many cities) may make some fuel cell technologies viable in buses that would not be
viable in automobiles. Because fuel cells have not yet become commercially viable, buses
employing fuel cell technology are still heavily subsidized (IEA, 2002).
Biofuels
The use of biofuels in bus transit is growing, especially in municipalities that require that a
percentage of diesel come from biomass sources (Biodiesel Magazine, 2008). Because the
primary difference between buses operating with fossil diesel and biodiesel is simply the
fuel used, there is little difference in capital or infrastructure costs. However, fuel costs
can be higher for buses using B20, since the cost of B20 has historically been greater than
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