Issues include fuel cell type, hydrogen storage, special factors affecting fuel cell rail, and the question of which rail applications are appropriate for hybrid powertrains.. Carbon dio
Trang 1Rail Vehicles: Fuel Cells
AR Miller,Vehicle Projects Inc and, Supersonic Tube Vehicle LLC, Golden, CO, USA
& 2009 Elsevier B.V All rights reserved.
Introduction
This article concerns the rationale, history, principal
issues, and potential of fuel cell-powered rail vehicles
Issues include fuel cell type, hydrogen storage, special
factors affecting fuel cell rail, and the question of which
rail applications are appropriate for hybrid powertrains
It concludes with a brief discussion of a supersonic
concept vehicle, a cross between a train and an airplane
that operates in a hydrogen-filled tube and levitates on a
gas film, thereby overcoming an inherent efficiency
limitation of aircraft
Why Fuel Cell Rail?
Carbon dioxide emissions and energy security are related
issues affecting the rail industry and transportation sector
as a whole They are related by the fact that in many
nations nearly 100% of the energy for the transport
sector is based on oil, and oil is an insecure primary
energy and the principal source of carbon dioxide
emissions World oil reserves are diminishing, prices have
recently reached unprecedented heights and volatility,
and political instability threatens supply disruptions A
consensus has been reached that the burning of fossil
fuels and consequent atmospheric release of waste carbon
dioxide is a significant factor in global climate change
The greenhouse gas effect is the likely cause of melting of
the polar ice caps and the increased severity of storms
Catenary-electric and diesel-electric are the two
dominant, conventional types of locomotive, and the
for-mer superficially appears to be a solution to both
prob-lems However, a factor potentially affecting both energy
security and carbon dioxide emissions is energy efficiency
(traction work divided by chemical energy of the fuel),
because a more efficient locomotive uses less energy and,
for most locomotives, burns less oil When viewed as only
one component of a distributed machine that includes an
electricity-generating plant, possibly coal- or oil-fired,
catenary-electrics are the least energy-efficient
loco-motive type Diesel-electric locoloco-motives, although
col-lectively worse air polluters than an equal number of
catenary-electric locomotives driven by coal-fired power
plants, are more energy-efficient overall Moreover, a
ca-tenary-electric is much more costly than an equivalent
diesel-electric locomotive because of the higher
infra-structure costs (US$6–8 million per mile) Relatively low
infrastructure cost is the reason that diesel-electrics are
almost universally used on large landmasses with dis-persed population centers, such as the USA
The lower energy efficiency of the catenary-electric locomotive is most accurately shown in a ‘well-to-wheels’ analysis A complete analysis would include the energy consumption of the ‘well’, for example, the energy to pump and refine oil or the energy to mine and process coal Moreover, the efficiencies depend on the specifics of the application, in particular, the duty cycle To make a meaningful comparison by using a common primary energy, consider using a diesel engine as the prime mover
in the two types of locomotives undergoing the same duty cycle For a catenary-electric, the following are the midpoints of the typical range of efficiencies for the various processes involved in taking the energy of diesel fuel to traction power in the locomotive: Mitsubishi
8 MW diesel engine-alternator for an electricity-gener-ating plant (43.5%), voltage conversion (97%), copper transmission from power plant to locomotive (80%), and onboard conversion to traction power (85%) The product of these estimates gives the estimated overall efficiency of a catenary-electric locomotive as 29% Coal-fired steam-generating plants have similar, but probably lower, efficiencies compared to the diesel plant For a diesel-electric with the prime mover onboard, the midpoint efficiencies are as follows: 3 MW onboard diesel engine (37.5%), engine ancillaries (94%), alternator (96.5%), and onboard conversion to traction work (90%) Estimated overall efficiency for a diesel-electric loco-motive is therefore 31% While the efficiencies of the two conventional types of locomotives are similar, this an-alysis dispels any misconception that a catenary-electric locomotive is a high-efficiency vehicle
However, compared to other common forms of transport, either type of conventional locomotive pulling
a train is much more energy efficient: rail freight is 3–4 times more efficient on a tonne–km basis than rubber-tired road trucks and 50 times more efficient than air-freight The poor efficiency of airfreight is due primarily
to the power required to overcome induced drag, the drag caused by the wings diverting the incoming air to downwash and thereby providing lift to hold the vehicle aloft The equation for induced drag is as follows:
Di¼ Ciw
2
where Diis the induced drag (force), Ciis the coefficient
of induced drag specific to an airplane, w is the airplane
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