Note: Descriptions are shown in the official language in which they were submitted.
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FUEL CELL SYSTEMS AND METHODS OF OPERATING THE SAME
BACKGROUND OF THE INVENTION
Field of the Invention
The present invention relates to electrochemical fuel cell systems and
methods of operating the same.
Description of the Related Art
Electrochemical fuel cells convert fuel and oxidant into electricity. Solid
polymer electrochemical fuel cells generally employ a membrane electrode
assembly
which includes an ion exchange membrane or solid polymer electrolyte disposed
between two electrodes typically comprising a layer of porous, electrically
conductive
sheet material, such as carbon fiber paper or carbon cloth. The membrane
electrode
assembly typically comprises a layer of catalyst, usually in the form of
finely
comminuted platinum that may be supported on a support material, such as
carbon or
graphite, or unsupported, at each membrane electrode interface to induce the
desired
electrochemical reaction. In operation, the electrodes are electrically
coupled for
conducting electrons between the electrodes through an external circuit.
Typically, a
number of membrane electrode assemblies are electrically coupled in series to
form a
fuel cell stack having a desired power output.
The membrane electrode assembly is typically interposed between two
electrically conductive flow field plates, or separator plates, to form a fuel
cell. Such
flow field plates comprise flow fields to direct the flow of the fuel and
oxidant reactant
fluids to the anode and cathode electrodes of the membrane electrode
assemblies,
respectively, and to remove excess reactant fluids and reaction products, such
as water
formed during fuel cell operation.
It is well-known in the art that with uncontrolled shutdown procedures,
air permeates into the anode flow fields. Upon startup, hydrogen is supplied
to the
anode flow fields, thus forming a hydrogen/air front that moves across the
anodes
through the anode flow fields and displaces the air in front if it, which is
pushed out of
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the cell. This phenomenon, coupled with air existing in the cathodes, results
in elevated
cathode potentials and rapid corrosion of the carbonaceous materials in the
fuel cell
stack, such as the gas diffusion layers and the catalyst support material.
One method of mitigating this problem is described in U.S. Patent No.
6,887,599, which discloses sufficient fast purging of the anode flow field
with hydrogen
prior to connecting the cells to the load. It is preferred to displace the air
within the
anode flow field with fuel in less than 1.0 seconds, and preferably less than
0.2 seconds.
One method to enable a fast anode purge on startup is to open the fuel flow
valve to
allow a flow of pressurized hydrogen from the fuel source into the anode flow
field.
The hydrogen flow pushes the air out of the anode flow field. When
substantially all
the air has been displaced from the anode flow field, the auxiliary load
switch is
opened, the air flow valve is opened, and the air blower is turned on.
Most fuel cell systems contain a fuel pressure regulator or similar device
to regulate the pressure of the fuel from the fuel supply, which is typically
pressurized
to very high pressures, so that the fuel is supplied to the fuel cell stack at
the optimum
pressure. However, such fuel pressure regulators typically restrict the
volumetric flow
of the fuel, which limits air displacement from the anode flow fields on
startup quickly
enough to prevent rapid corrosion of the carbonaceous materials.
As a result, there remains a need to develop improved fuel cell systems
and methods of operating the same to prevent elevated potentials from
occurring in the
fuel cell stack on startup. The present invention addresses these issues and
provides
further related advantages.
BRIEF SUMMARY OF THE INVENTION
Briefly, the present invention relates to electrochemical fuel cell
systems, in particular, to fuel cell systems comprising a fuel accumulator
upstream of
the fuel cell stack, and to methods of operating the same.
According to one embodiment of the present invention, the fuel cell
system is disclosed including a fuel cell stack comprising an anode flow field
and a
cathode flow field; a fuel supply line for supplying a hydrogen-containing
fuel to the
anode flow field; a fuel inlet valve in the fuel supply line upstream of the
fuel cell stack;
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and an accumulator comprising an upstream inlet and a downstream outlet, the
inlet
fluidly connected to the fuel supply line and the outlet fluidly connected to
the fuel inlet
valve.
According to another embodiment of the present invention, a method of
commencing operation of a fuel cell system is disclosed, the fuel cell system
including
a fuel cell stack, the fuel cell stack comprising an anode flow field and a
cathode flow
field, wherein at least a portion of the anode flow field comprises air; a
fuel supply line
for supplying a fuel to the anode flow field; an accumulator comprising an
upstream
inlet fluidly connected to the fuel supply line, and a downstream outlet; and
a fuel inlet
valve in the fuel supply line downstream of the downstream outlet in a closed
position
to substantially isolate the fuel cell stack from the accumulator, wherein the
fuel inlet
valve is upstream of the fuel cell stack and downstream of the downstream
outlet; the
method comprising the steps of: supplying a fuel to the accumulator when the
fuel inlet
valve is closed; at least partially opening the fuel inlet valve to fluidly
connect the fuel
cell stack to the accumulator when the accumulator is at least partially
filled with fuel;
and supplying the fuel from the accumulator to the anode flow field.
These and other aspects of the invention will be evident upon review of
the following disclosure and attached figures.
BRIEF DESCRIPTION OF THE DRAWINGS
In the figures, identical reference numbers identify similar elements or
acts. The sizes and relative positions of elements in the figures are not
necessarily
drawn to scale. For example, the shapes of various elements and angles are not
drawn
to scale, and some of these elements are arbitrarily enlarged and positioned
to improve
figure legibility. Further, the particular shapes of the elements, as drawn,
are not
intended to convey any information regarding the actual shape of the
particular
elements, and have been solely selected for ease of recognition in the
figures.
Figure 1 shows a schematic of a fuel cell system configuration according
to one embodiment of the present invention.
Figure 2 shows a schematic of an alternative fuel cell system
configuration of the fuel cell system configuration in Figure 1.
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Figure 3 shows a schematic of another alternative fuel cell system
configuration of the fuel cell system configuration in Figure 1.
DETAILED DESCRIPTION OF THE INVENTION
In the following description, certain specific details are set forth in order
to provide a thorough understanding of various embodiments of the invention.
However, one skilled in the art will understand that the invention may be
practiced
without these details. In other instances, well-known structures associated
with fuel
cells, fuel cell stacks, and fuel cell systems have not been shown or
described in detail
to avoid unnecessarily obscuring descriptions of the embodiments of the
invention.
Unless the context requires otherwise, throughout the specification and
claims which follow, the word "comprise" and variations thereof, such as
"comprises"
and "comprising" are to be construed in an open, inclusive sense, that is as
"including,
but not limited to".
Reference throughout this specification to "one embodiment" or "an
embodiment" means that a particular feature, structure or characteristic
described in
connection with the embodiment is included in at least one embodiment of the
present
invention. Thus, the appearances of the phrases "in one embodiment" or "in an
embodiment" in various places throughout this specification are not
necessarily all
referring to the same embodiment. Furthermore, the particular features,
structures, or
characteristics may be combined in any suitable manner in one or more
embodiments.
Figure 1 shows an exemplary fuel cell system 8 comprising a fuel cell
stack 10, which comprises a plurality of fuel cells. Each fuel cell typically
comprises an
anode flow field and a cathode flow field (not shown). Fuel cell system 8
further
comprises a fuel supply line 12 fluidly connected to fuel supply 14 for
delivering a fuel,
such as a hydrogen-containing fuel, to fuel cell stack 10; a fuel pressure
regulator 16 in
fuel supply line 12 downstream of fuel supply 14; a fuel inlet valve 18, such
as a
solenoid valve, a pneumatically-driven valve, a pilot operated valve, or a
motor driven
valve, in fuel supply line 12 upstream of fuel cell stack 10; an oxidant
supply 20 for
supplying oxidant, such as air, to fuel cell stack 10; a delivery device 22,
such as a
compressor, blower, fan or the like, for delivering oxidant to fuel cell stack
10, and a
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controller 24 for controlling at least one operating parameter of the fuel
cell stack or
fuel cell system. In addition, fuel cell system 8 comprises a coolant loop 26
for
allowing the flow of a coolant, such as water, glycol, or mixtures thereof,
through fuel
cell stack 10 to remove heat from the reactant and product fluids in the anode
and
cathode flow fields, thereby maintaining fuel cell stack 10 at an optimum
temperature
during fuel cell operation and preventing damage to the fuel cell components.
Furthermore, fuel cell system 8 includes humidification devices 28 and 30 for
humidifying the anode and cathode reactant streams, respectively.
Fuel cell system 8 further comprises a fuel purge valve 25 for
periodically removing inerts and contaminants from the fuel cell stack. The
fuel purge
may be time-based (i.e., once every minute) and/or triggered by any detectable
fuel cell
stack operating parameter detected by sensors in the fuel cell stack or system
(not
shown), such as (but not limited to) the hydrogen concentration in the anode
flow fields,
the voltage of at least a portion of the fuel cell stack, and/or the
resistance of at least a
portion of the fuel cell stack. In some embodiments, the fuel purge valve is a
pulse
width modulated valve, such as that described in U.S. Provisional Application
No.
60/864,722, filed November 7, 2006 and entitled "SYSTEM AND METHOD OF
PURGING FUEL CELL STACKS".
Fuel pressure regulator 16 is used to regulate the pressure of the fuel
from fuel supply 14 so that the fuel is supplied to fuel cell stack 10 at the
optimum
pressure. As mentioned earlier, fuel is typically pressurized to high
pressures at the fuel
supply, such as about 3000 PSIG (approximately 200 barg) when the fuel is
supplied
from a hydrogen tank. However, since the oxidant is typically supplied at much
lower
pressures, it is not desirable to supply fuel at such high pressures because
it creates a
high pressure differential across the fuel cell and may damage the fuel cell
components.
Thus, fuel pressure regulators are used to decrease the pressure of the fuel
from the fuel
supply to a more desirable pressure.
In addition, fuel cell system 8 comprises an accumulator 32 in fuel
supply line 12 that is fluidly connected to fuel supply 14 through upstream
inlet 34, and
fluidly connected to fuel cell stack 10 through downstream outlet 36, to allow
for a
greater volumetric flow of fuel through the anode flow fields of fuel cell
stack 10. For
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example, during shutdown, fuel inlet valve 18 is closed and accumulator 32 is
at least
partially filled with pressurized hydrogen supplied through regulator 16 from
fuel
supply 14. When fuel cell system 8 is started up, fuel inlet valve 18 is
opened so that
the large volume of fuel in accumulator 32 can be pushed through the anode
flow fields
of fuel cell stack 10 as quickly as possible by the pressurized fuel supplied
through
regulator 16 from fuel supply 14. Since accumulator 32 is downstream of
regulator 16,
accumulator 32 allows for a greater volumetric flow rate of fuel to be pushed
through
the anode flow fields of fuel cell stack 10 than existing fuel cell systems
that do not use
an accumulator upstream of the fuel cell stack. In some embodiments,
accumulator 32
and regulator 16 are configured such that they allow the replacement of at
least one
volume of the total volume of the anode flow fields in fuel cell stack 10 in
less than or
equal to about 1.0 second, for example, in less than or equal to about 0.2
second. The
volume of accumulator 32 may be any suitable volume and may depend on the
operating conditions of the fuel cell stack. For example, the volume of
accumulator 32
may be at least the same as the total volume of the anode flow fields in fuel
cell stack
10 and, in some cases, may be at least double the total volume of the anode
flow fields
in fuel cell stack 10. In addition, accumulator 32 may be any shape, for
example, in the
shape of a cube or cylinder.
In some embodiments, fuel cell system 8 may further comprise a fuel
recirculation loop 38 for recirculating at least a portion of the exhausted
fuel from fuel
cell stack outlet 40, such as that shown in Figure 2. The exhausted fuel,
exhausted
through fuel purge valve 25, typically contains a small amount of unused fuel,
balance
inerts and water vapour. Recirculating at least a portion of the exhausted
fuel allows for
humidification of the incoming fuel and, thus, may eliminate the use of
humidification
device 28 for the fuel stream. Recirculation loop 38 may comprise a
recirculating
device 42, such as a pump, blower, ejector, or the like, to help recirculate
the fluids
therein. Again, accumulator 32 in fuel supply line 12 is fluidly connected to
fuel supply
14 through upstream inlet 34 and fluidly connected to fuel cell stack 10
through
downstream outlet 36.
In other embodiments, fuel cell system 8 is an air-cooled, low-pressure
fuel cell system, such as that shown in Figure 3. Fuel cell stack 10 comprises
a
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plurality of fuel cells that utilize combined oxidant-coolant flow fields to
allow for
relatively high stoichiometries of air flow (e.g., 100) at ambient pressure
through the
fuel cells, thereby eliminating the need for additional coolant flow fields in
the fuel
cells or compressors to compress the air oxidant to an elevated pressure.
Since the air is
supplied at ambient pressure, oxidant supply 20 may be the ambient
environment. In
addition, the fuel cells may utilize relatively dense gas diffusion layers so
that no
additional humidification device is necessary to humidify the fuel or air
reactants.
Examples of such fuel cells are described in U.S. Patent No. 6,451,470 and
published
U.S. Patent Appl. No. 2004/0253504. In this embodiment, fuel cell stack 10 can
be
operated in a dead-ended mode of operation to enhance fuel utilization and
efficiency.
For example, fuel purge valve 25 may be closed during operation, and opened
periodically to purge any inerts and excess water and water vapour that build
up in the
anode flow fields. In this fuel cell system configuration, the accumulator
allows a
greater volumetric flow of fuel through the anode flow fields of the fuel cell
stack on
startup, thereby minimizing elevated potentials in the fuel cell stack.
One method of starting up a fuel cell stack and system comprising an
accumulator upstream of the fuel cell stack is described herein. With
reference to
Figure 1, during shutdown, fuel inlet valve 18 is closed and fuel pressure
regulator 16 is
opened so that accumulator 32 is at least partially filled with pressurized
hydrogen. On
shutdown, at least a portion of the anode flow fields of fuel cell stack 12
will typically
contain air that migrates from the cathode flow fields to the anode flow
fields or
through minor leaks in the fuel cell system. When controller 24 receives a
signal that
fuel cell operation is initiated, fuel inlet valve 18 is opened and hydrogen
in
accumulator 32 is pushed through the anode flow fields of fuel cell stack 10
to remove
any air in the anode flow fields as quickly as possible, for example, in less
than or equal
to about 1.0 seconds, and more preferably, in less than or equal to about 0.2
seconds. In
fuel cell systems that contain a fuel purge valve downstream of the stack, the
fuel purge
valve may be at least partially open or fully open to allow for a faster purge
through the
anode flow fields.
Accumulator 32 may also help with transient conditions where a sudden
increase in fuel flow is required, such as during a periodic fuel purge in a
dead-ended
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mode of operation. During such a fuel purge, a high fuel flow rate is
desirable to
increase the efficiency of the purge by increasing the pressure drop across
the anode
flow fields, thereby removing inerts and water more quickly as reducing the
amount of
excess fuel purged. The required flow rate of fuel is typically higher than
that required
during regular operation and, in some cases, the desired flow rate may be
about 10
times greater than the flow rate during regular operation. In this situation,
the extra fuel
in the accumulator increases the pressure drop in the anode flow fields for a
longer
period of time to minimize the purge duration. As a result, when the fuel
purge valve is
actuated, the accumulator can supply fuel while maintaining a higher pressure
drop in
the anode flow fields until the fuel supply catches up to the increased demand
or until
the stack has returned to normal operation (i.e., returned to a dead-ended
mode of
operation). The purge duration will depend on the size of the accumulator and
the size
of the purge valve.
In some embodiments, the accumulator may be placed upstream of the
pressure regulator if the fuel pressure regulator is large enough to allow for
a sufficient
volumetric flow therethrough without significantly restricting fuel flow. In
this case,
the fuel pressure regulator may replace the fuel inlet valve, thereby
eliminating a
component and simplifying the fuel cell system configuration. In other
embodiments,
accumulator 26 may act as a dampener for the fuel supply to filter out
pressure spikes
and fluctuations that can potentially damage the stack.
The following examples are provided for the purpose of illustration, not
limitation.
EXAMPLES
Fuel Cell Stack Configuration:
Two 10-cell combined air-coolant fuel cell stacks were assembled with
low porosity gas diffusion layers as described in published Canadian Patent
No.
2,489,043.
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Fuel Cell System Configuration:
Two fuel cell systems were assembled with the fuel cell stacks described
above. The first fuel cell system was configured as described in Figure 3. The
second
fuel cell system was similarly configured, with a 0.42-litre volume
accumulator
between the fuel regulator and the fuel inlet valve. In both fuel cell
systems, a fan was
used to deliver air as the oxidant to the fuel cell stack. The operating
temperature of the
fuel cell stack was maintained by adjusting the speed of the fan that
delivered cooling
air to the fuel cell stack.
Test Procedure:
Each of the fuel cell systems were operated under the following on-off
cycling procedure:
Startup: 1) Start flowing air at ambient pressure at 30 degrees Celsius
through the
cathode flow fields at 200 slpm for 10 seconds.
2) Apply fuel to the anode flow fields at 350 mbar and immediately open
purge valve for 3 seconds.
Operation: 3) Operate the fuel cell stack in a dead-ended mode of operation
for 30
minutes at 350 mA/cm2 at 65 degrees Celsius while purging fuel
periodically.
Shutdown: 4) Remove load from stack.
5) Turn off fuel supply and oxidant supply.
6) Keep stack shut down for 30 minutes before beginning next cycle
(fuel purge valve and fuel inlet valve are closed throughout shut down
period).
The above on-off cycling procedure was repeated until the average cell voltage
decreased to less than about 580 mV at 350 mA/cma, or when the stack leakage
exceeded 20 cc/min.
The first fuel cell stack accumulated 210 cycles before the cycling test
was stopped due to the average cell voltage dropping to below 580 mV at 350
mA/cm2,
and had an average stack voltage degradation rate of about 345 V/cycle. The
integrated current collector showed that the hydrogen/air front purge duration
was
approximately 1.0 second (i.e., 1.0 second to push all the air out of the
anode flow
fields on startup).
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The second fuel cell stack accumulated 563 cycles before the cycling test
was stopped due to the stack leakage rate exceeding 20 cc/min, and had an
average
stack voltage degradation rate of about 150 mV/cycle. The integrated current
collector
showed that the hydrogen/air front purge duration was approximately 0.2 second
(i.e.,
0.2 second to push all the air out of the anode flow fields). As a result, the
second fuel
cell stack (having an accumulator upstream of the fuel cell stack)
demonstrated a
significant improvement in both the number of on-off cycles and voltage
degradation
rate over the first fuel cell stack, accumulated more than double the number
of cycles of
the first fuel cell stack, and exhibited less than half the voltage
degradation rate of the
first fuel cell stack.
All of the above U.S. patents, U.S. patent application publications, U.S.
patent applications, foreign patents, foreign patent applications and non-
patent
publications referred to in this specification and/or listed in the
Application Data Sheet,
are incorporated herein by reference, in their entirety.
While particular elements, embodiments, and applications of the present
invention have been shown and described, it will be understood that the
invention is not
limited thereto since modifications may be made by those skilled in the art
without
departing from the spirit and scope of the present disclosure, particularly in
light of the
foregoing teachings.
