SwRI is integrating fuel
c ells into heavy-duty tractor-trailers
in a program sponsored by the US. Army
and SunLine Services Group to improve
vehicle efficiency. This multi-year p rogram is called
the "Phased Introduction of Fuel Cells into a Class 8
Tractor " and begins with the installation of a 3- to 20kilowatt fuel cell auxiliary power unit fueled with
compressed hydrogen. The program:<; ultimate goal is
to develop a complete tractor propulsion system consisting
of a diesel reformer/fuel cell hybrid electric drivetrain.
By Alan F. Montemayor
S
outhwest Research Institute's modeling and simulation program has put SwRI in the vanguard of cleaner, more efficient
fuel cell technology for transportation applications.
The application of fuel cell technology to heavy-duty trucks is
a timely issue in vehicle research and development. Total u.s.
truck fuel consumption continues to increase and has surpassed
fuel u sage by automobiles in the past decade. ApplyIng fuel efficient technology to trucks offers a higher rate of return than applying similar technology to automobiles. Thus, the transportation
industry must give careful consideration to improvements in overall fuel efficiency that can result in lower operating costs - even if
it means a small increase in capital costs.
SunLine Services Group, under contract with the U.S. Army
Tank-Automotive and Armaments Command (TACOM), has subcontracted with SwRJ to develop a heavy-duty truck powered by a
fuel cell using hydrogen derived from reformed diesel fuel. SwRJ's
ability to predict performance efficiency and emissions is one of the
key reasons SwRI has been awarded its largest vehicle integration
contract to date.
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Modeling and simulation work at SwRI grew from development sponsored by internal research. Subsequent sponsorship has been provided by the u.s. Defense Advanced Research
Projects Agency (DARPA); U.S. Council for Automotive
Research (a consortium comprised of Ford, GM and DaimlerChrysler) under the title of Partnership for a New Generation of
Vehicles Systems Analysis Toolkit; the u.s. Army National
Automotive Center in support of the Army's transition to fuel
cell power; and, most recently, SwRI has developed a new generation of simulation software, which has been funded in part
by Daimler-Chrysler.
The vehicle powertrain simulation program will be available commercially and will model and simulate various vehicle
Technology Today . Fall 2001
Electrons
powertrain configurations, including hybrid series, parallel, combination
series and parallel, fuel cells and conventional automotive powertrains.
Background
Why haven' t fuel cells in stationary or transportation
applications - with the promise of thermodynamic
efficiencies of 50 percent - already obtained widespread market penetration? The answer, according
to engineers at SwRI, is largely due to system
complexity and fuels storage efficiency, which
affect cost, long-term reliability and size.
However, the belief that fuel cells can have a positive impact in many power-producing applications is driving ever-increasing research and
development in this area.
With the help of a catalyst, fuel cells combine
hydrogen and oxygen to prod uce direct current
electricity without combustion. Cells can be combined into mod ular units that are extremely efficient, theoretically capable of converting up to 70
percent of the fuel's energy into useable electricity
under ideal circumstances. The cells require li ttle maintenance and can
produce electricity without harmful byproducts, making them suitable
for use in residential and environmentally sensitive areas.
Although British scientist Sir William Grove discovered the physical
principles behind power generation from fuel cells in 1839, it was not
until the space race of the 1960s th at investment into research and development paid off with the selection of a fuel cell to power U.S. spacecraft.
Since that time, research and development has continued, wi th the result
th at the transportation industry is beginning to accept fuel cells as a
power production
alternative to the
internal combustion engine.
The most
popular fuel cell
in development is
the proton
exchange membrane or polymer
electrolyte membrane (PEM) fuel
cell. Another candidate fuel cell
receiving considerable development is the solid
Carbon-Supported
Platinum Catalyst
PEM fuel cells feed in hydrogen and air and produce electricity directly Water vapor and heat are the only other
byproducts. The hydrogen must be pure because impurities such as sulfur and carbon monoxide can damage the
catalyst in the cell. In this diagram, protons (yef/ow arrows)
are produced by the interaction of hydrogen and the catalyst and pass through the permeable membrane (blue).
The protons combine with oxygen in the presence of a catalyst and the reaction produces water. Electrons are conducted through the catalyst support structure and ffow to
an external load.
oxide fuel cell (SOFC). The PEM fu el cell's advantage
is that it operates at a lower temperature than the
SOFe, which results in a faste r response to braking
and acceleration. However, the PEM fuel cell is intolerant of carbon monoxide and sulfur in its gaseous
hydrogen fuel supply. The choice of fuel cell - and
there are many other types being developed - is constrained by the system into whi ch it is to be integrated.
Systems in tegration - modeling and
simulation-based activity
Primary objectives in developing fuel cells for the
transportation industry are to improve efficiency and
to reduce emissions. Fuel cells operating on hydrogen
produce electricity, water vapor and heat as end prod-
Alan F Montemayor is a principal engineer working in fuel cell
systems within the advanced vehicle technology section of the Engine and
Vehicle Research Division. His current projects include introducing fuel cells
into a tractor-trailer that will run in Palm Springs, Calif., and procuring,
installing and running three fuel celfs at Brooks Air Force Base in San Antonio.
Technology Today . Fall 2001
3
7.0 ,-- - - - - -- - - In the United States, trucks consume more energy than
automobiles. This trend will continue according to data
from the US. Department of Energy Increasing the efficiency of trucks can significantly reduce overall energy
usage and help reduce air pollution.
6.0
Fuel reformation
1.0
o
1970
1980
1990
2010
2000
2020
Year
ucts. If the hydrogen has been "reformed" from other, more
complex fuels, the resulting emissions theoretically contain
only carbon dioxide and fuel-borne substances such as sulfur
compounds.
Because pure oxygen is used only in spacecraft as the oxidation agent and air is used in all other applications, some
oxides of nitrogen and other pollutants are produced. This
level of oxides of nitrogen is considerably lower than a diesel
engine might produce because the maximum temperature produced by the reaction is considerably lower for a fuel cell than
for a diesel engine.
A significant problem with using fuel cells in the transportation industry is providing a continuous stream of hydrogen to the fuel celL Hydrogen must either be stored on the
vehicle in a tank or produced through reformation of
petroleum-based fuels.
SwRl is working with a variety of companjes to develop a
reformation process that is commercially viable for use in
heavy trucks and other vehicles to preclude the need for a large
hydrogen storage tank on vehicles powered by fuel cells.
-- - ---------------------------1
Vaporizer
1
:I
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I
I
I
I
De-Sulfur
Hi-Temp
Reformer
I
I
Air Injection
I
l
Burner?
l ____
liqUid)
I
I
FUEL PROCESSOR
....
Low-Temp
Reformer
.....
I
I
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I
I I
SelOx
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Providing hydrogen to fuel cells th rough reformation is
not an insignificant task.
Natural gas is usually the fuel of choice for stationary
reforming applications because it consists of at least 90 percent
methane, which produces four hydrogen atoms per carbon
atom and because no initial va porization of the fuel is required
before reformation.
Transportation applications, however, require liquid
hyd rocarbon fuels to be processed before they are provided to
the fuel cell. Fuel processing typically begins with vaporizing
the fuel and removing sulfur that might otherwise poiso n the
reformation process and fuel cell catalysts. The nex t step is to
produce hydrogen through one of two fuel reformation
techniques.
Steam reformation reacts the hydrocarbon fuel wi th water.
If the steam reformer is run with excess water, a water-gas shift
produces some additional hydrogen in a secondary reaction.
Alternatively, partial oxidation (POX) reformation ca n be
accomplished either with a burner or a catalyst. Partial oxidation reacts fuel with a limited amount of oxygen. Diesel fuels
under high temperature and pressure yield hydrogen and carbon monoxide. Steam reformation of diesel fuel can produce
approximately twice as man y molecu les of hydrogen per molecule of fuel as POX (about ha lf of the hydrogen comes from the
steam), with the same amount of carbon monoxide produced
in either case.
Steam reformation, however, is a heatabsorbing (endothermic) process, while POX is
a heat-producing (exothermic) process.
Therefore, steam reformation requires a conFuel
stant supply of additional heat, whi ch must
come from the fuel supply, thus reducing the
overall efficiency of the steam reformation reaction toward or below that of the POX reaction.
A third promising technique for producing
Bleed
hydrogen, often referred to as au tothermal
Air
_
Fuel Ce1l
Stack
~ ----- - ----- - -- --- ------~
Depleted Tailgas
Combustion Air
4
Technology Today. Fall 2001
Liquid fuels must be "reformed" into a
hydrogen-rich gas used in fuel celfs.
There are three processes to accomplish
the reforming: steam reforming, partial
oxidation reforming and autothermal
reforming (a combination of steam and
partial oxidation). This idealized diagram
shows some of the steps that may occur
in a steam reforming process (steam
injection is not shown on the diagram).
Iteration
reforming (ATR), combines POX and steam reformation, w ith the parti al oxidation reaction providing hea t for steam . SwRI and other industry
researchers are evaluating the efficiency of
these systems for the heavy-duty fuel cell truck
application.
Mutation
Evaluation,
Ranking, &
Generation N
Fuel cell optimization
Development of an optimal design for a fuel cell to power
trucks requires an analysis of multiple subsystem configurations
with the aim of optimizing the performance of the tru ck over its
expected life cycle.
The powertrain of an optimized system will include fuel ceils,
one or more electric motors, power converters and possibly batteries or other electrical energy storage devices. Power converters w ill
be needed to convert the fuel cell direct current to the altern ating
current used by man y traction drive motors. Additionally, batteries
likely w ill be used to buffer the output of the fuel cell to optimize
its response to braking and acceleration.
Engineers in SwRI's Vehicle Systems Research Depar tment use
genetic algorithms to perform an optimization search of all of the
powertra in co mponents and component sizes that could be used in
the fuel cell-powered truck.
From a purely statistical viewpoint, the problem invol ves
many variables that must be adjusted until an optimum configuration is fo und. Whereas traditional optimization techniques require
closed form equations describing the system with continuous functions for derivation analyses, the use of genetic algorithm s ca n be
likened to a structured approach of trial and error. In this
approach, the parameters used to describe the system (such as fu el
cell maxi mum power, battery pack maximum current, electric
motor m aximum speed and torque, etc.) are defined and their values are converted to a binary string of Is and Os. These parameters
are joined to form a so-called "cluomosome." Each chromosome
offers a possible solu tion to the overall optimization problem.
Initial Design
•
0=-
o
Battery
Motor
Fuel Cell
Crossover
Selection
01100.. .
010 .. .
01 100 .. .
Generation N+ 1
Genetic algorithms mimic the process of evolution in that each new
generation (iterative computer simulation) contains mutations that may
or may not be beneficial. This powerful technique can find true optimal
solutions and avoid locaf minima or maxima.
Multiple cluomosomes are generated either at random
or by expert insight. Each possible solution is used in modeling the fuel cell truck performance and evaluated against
the performance criteria that must be established for the system, with each cluomosome ranked according to its performance. Next, combinations of chromosomes are "mated" to
define the next generation of possible solution sets and their
performance is modeled and ranked .
Additionally, each generati on of cluomosomes receives
some specific number of random mutations. It is the injection of random mutations that ensures th at the optimum
solu tion of the entire function is found and not merely a
local opti mum as may result with trad itional optimization
techniques. SwRl already has applied thi s technique to optimization involving convention al vehi cle powertrain optimization, control system optimization and engine design.
Conclusion
Fuel cells are likely to be applied to the transportation
industry in the near to midterm future. They likely will be
applied as an auxiliary power source for
heavy trucks, w hich idle to provide power
for drivers using their tru cks as a temporary dwelling. Fuel cells likely will be a
viable alternative power source on automobiles, trucks and buses early in this cennuy.
The nation's major automakers already
have developed prototype vehicles powered by fuel cells. Analysis work performed
at SwRl- which has yielded three patents
on fuel cell designs and applications for
four more patents on fuel cell designs and
sensors -likely will lead to more efficient
500 fuel cell designs and better con trol of eurrent systems. No matter how the fuel cell is
adapted for future use, SwRi will contribute to its development. .:.
Commellts about this article? Contact
MOlllemayor al (210) 522-6940 or
Genetic algorithms aid in the optimization process to size components for a hybrid electric
vehicle. Computer simulations predict the performance of the vehicle and, with the aid of
genetic algorithms, iteratively converge on an optimal configuration.
Technology Today . Fall 2001
amolltellmyor@swri.org.
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