15-02-2013, 04:23 PM
Simulating Fuel Cell Hybrid Bus Technology at the University of Delaware
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ABSTRACT
The fuel cell hybrid bus (FCHB) that shuttles University of Delaware
students and faculty across campus on the 6-mile Express Route serves as
a highly visible demonstration of the power and benefits of hydrogen fuel
cell technology. The bus is zero-emission, and much quieter than its diesel
counterparts; it can be refueled and maintained at a single location, reducing
infrastructure costs; and its series-hybrid design makes it particularly effective
for the start-and-stop driving and relatively low speeds on urban bus routes.
Simulating Fuel Cell Hybrid Bus Technology at
the University of Delaware
INTRODUCTION
Using MATLAB® and Simulink®, University
of Delaware researchers modeled the
FCHB, analyzed data from its many onboard
sensors, improved its power management
strategy, and gained key insights into
fuel cell bus design optimization.
Simulink enables us to answer design
questions that would be extraordinarily
expensive to answer via trial-and-error
hardware iterations. For example, we run
simulations that show us how the bus will
perform if we double or triple the size of
the fuel cell stack. Other simulations show
what would happen if we reduced the
4000 pounds of batteries on board to 2000
pounds. We simulate changes to the chassis
and the weight distribution of the entire
vehicle. By analyzing the results of these
simulations, we can see if a certain design is
more efficient, if it consumes less hydrogen
per mile traveled, or if it reduces stress on
the batteries.
A Multiyear, Multibus
Research Effort
Funded by the Federal Transit Administration,
the FCHB program was started in
2005 to develop and demonstrate fuel cell
buses and hydrogen refueling stations in
Delaware. The first fuel cell bus went into
operation in 2007; the second, in 2009
(Figure 1). The program will ultimately
include four buses, each incorporating
design improvements and lessons learned
from its predecessor.
What sets the University of Delaware
FCHB apart from other fuel cell buses being
tested around the world is its relatively
low cost. Our buses cost about 50% less
than comparably sized fuel cell buses currently
in operation, in large part because
we use a hard series-hybrid design.
Improving the Power
Management Strategy
In a hybrid vehicle, the power management
strategy determines which onboard power
generators are active at any given time and
at what rate they are producing power.
Our first bus came with a fairly rudimentary
power management strategy. When
the battery state-of-charge dropped below
65%, the system activated the fuel cell to
begin recharging the battery. If the system
requested maximum power immediately,
the fuel cell delivered, but it did so inefficiently
because it was outside its optimal
operating region. One of our first goals was
to improve upon this strategy.
Our Third Bus and Beyond
The third bus incorporates significant design
changes. Like the second bus, it will
have a 40 kW fuel cell stack, but we are
switching from nickel-cadmium batteries
to lithium-ion, which will reduce the
weight of the battery package from 4000
pounds to about 1500 pounds. Lithiumion
batteries have an excellent lifespan and
can be charged and discharged faster and
more frequently, which will enable us to
explore new power management strategies
in Simulink. With the increased performance,
the powertrain will be able to drive
a larger bus at higher sustained speeds;
buses three and four will be 30 feet long
and will carry up to 10 more passengers
than the 22-foot buses.
As part of the ongoing research, one of
our graduate students recently completed
a thesis in which he explored the use of
supercapacitors in a blended storage system
with fuel cells and batteries. He modeled
the supercapacitors in Simulink and
incorporated them into our FCHB model
to simulate and test advanced power management
strategies that exploit the supercapacitor’s
rapid charge and discharge
characteristics. This research is an excellent
example of the value of Simulink in
this field; it would be prohibitively expensive
and time-consuming to explore these
kinds of ideas in real hardware.