U.S. patent application number 10/851396 was filed with the patent office on 2005-11-24 for fluid flow pulsing for increased stability in pem fuel cell.
Invention is credited to Chapman, Ivan Daryl, MacKintosh, Charles, Rapaport, Pinkhas A..
Application Number | 20050260463 10/851396 |
Document ID | / |
Family ID | 35375523 |
Filed Date | 2005-11-24 |
United States Patent
Application |
20050260463 |
Kind Code |
A1 |
Chapman, Ivan Daryl ; et
al. |
November 24, 2005 |
Fluid flow pulsing for increased stability in PEM fuel cell
Abstract
A fuel cell system includes a fuel cell stack and a flow control
device that controls a supply of a first phase fluid flowing
through the fuel cell stack. A controller monitors at least one
parameter of the fuel cell stack and controls the supply to
generate pulses of reactant when the at least one parameter crosses
a threshold to flush a second phase fluid from said fuel cell
stack.
Inventors: |
Chapman, Ivan Daryl;
(Victor, NY) ; MacKintosh, Charles; (Victor,
NY) ; Rapaport, Pinkhas A.; (Fairport, NY) |
Correspondence
Address: |
HARNESS, DICKEY & PIERCE, P.L.C.
P.O. BOX 828
BLOOMFIELD HILLS
MI
48303
US
|
Family ID: |
35375523 |
Appl. No.: |
10/851396 |
Filed: |
May 21, 2004 |
Current U.S.
Class: |
429/432 ;
429/428; 429/434; 429/442 |
Current CPC
Class: |
H01M 8/04619 20130101;
H01M 8/04753 20130101; H01M 8/04641 20130101; H01M 8/04402
20130101; H01M 8/04358 20130101; Y02E 60/50 20130101; H01M 8/04559
20130101; H01M 8/0441 20130101; H01M 8/04231 20130101; H01M 8/04432
20130101; H01M 8/04395 20130101; H01M 8/04179 20130101; H01M
8/04768 20130101; H01M 8/04552 20130101; H01M 8/04417 20130101;
H01M 8/04388 20130101 |
Class at
Publication: |
429/013 ;
429/022; 429/023; 429/025; 429/024 |
International
Class: |
H01M 008/04 |
Claims
What is claimed is:
1. A fuel cell system, comprising: a fuel cell stack; a flow
control device that controls a supply of a first phase fluid
flowing through said fuel cell stack; and a controller that
monitors at least one parameter of said fuel cell stack and
controls said supply to generate pulses of fluid when said at least
one parameter crosses a threshold to flush a second phase fluid
from said fuel cell stack.
2. The fuel cell system of claim 1 wherein said pulses are defined
by at least one pulse feature selected from pulse duration,
interval between pulses and pulse shape, as determined by said
controller.
3. The fuel cell system of claim 2 wherein said at least one pulse
feature is based on an extent by which said at least one parameter
varies from said threshold.
4. The fuel cell system of claim 1 further comprising at least one
voltage sensor sensing voltage of a respective cell of said fuel
cell stack and provides a signal to said controller.
5. The fuel cell system of claim 4 wherein said parameter includes
a cell voltage and said threshold is a minimum cell voltage
threshold.
6. The fuel cell system of claim 4 wherein said threshold includes
a fuel cell voltage fluctuation defined by at least one of a
magnitude and a frequency of the individual fuel cell voltage
fluctuations.
7. The fuel cell system of claim 1 further comprising a voltage
sensor that monitors a stack voltage and provides a signal to said
controller based on said parameter, which is said stack
voltage.
8. The fuel cell system of claim 7 wherein said threshold includes
a minimum stack voltage.
9. The fuel cell system of claim 7 wherein said threshold includes
an average voltage fluctuation defined by at least one of a
magnitude and a frequency of stack voltage fluctuations.
10. The fuel cell system of claim 1 further comprising a pressure
sensing system that senses a fluid pressure drop across said fuel
cell stack and provides a signal to said controller based on said
parameter which is said pressure drop.
11. The fuel cell system of claim 10 wherein said threshold value
includes a maximum pressure drop.
12. The fuel cell system of claim 10 wherein said threshold
includes an average pressure drop fluctuation defined by at least
one of a magnitude and a frequency of stack pressure drop
fluctuations.
13. The fuel cell system of claim 1 further comprising a
temperature sensing system that senses a fluid temperature change
across said fuel cell stack and provides a signal to said
controller based on said parameter which is said temperature
change.
14. The fuel cell system of claim 13 wherein said threshold value
includes a maximum temperature change.
15. The fuel cell system of claim 13 wherein said threshold
includes an average temperature change fluctuation defined by at
least one of a magnitude and a frequency of stack temperature
change fluctuations.
16. The fuel cell system of claim 1 wherein said parameter is a
resistance of at least one fuel cell.
17. The fuel cell system of claim 1 wherein said parameter is stack
load and said threshold value is a minimum stack load.
18. The fuel cell system of claim 1 wherein said flow control
device is a flow regulator.
19. The fuel cell system of claim 1 wherein said controller forms a
part of said flow control device.
20. The fuel cell system of claim 1 wherein said first phase fluid
is a reactant.
21. The fuel cell system of claim 1 wherein said first phase fluid
is a heat transfer fluid.
22. A method of operating a fuel cell stack, comprising: supplying
flow of a first phase fluid through said fuel cell stack;
monitoring at least one parameter of said fuel cell stack; and
pulsing said flow when said parameter crosses a threshold to flush
a second phase fluid from said fuel cell stack.
23. The method of claim 22 wherein said pulsing is conducted by
varying at least one pulse feature selected from pulse duration,
interval between pulses and pulse shape.
24. The method of claim 22 wherein said parameter comprises at
least one selected from the group consisting of: a voltage of one
or more individual cells of said fuel cell stack; a stack voltage;
a resistance of one or more individual cells of said fuel cell
stack; a stack resistance; a pressure drop across said fuel cell
stack; a load on said fuel cell stack; a stack voltage fluctuation;
a stack pressure fluctuation; a temperature change across said fuel
cell stack; and a stack temperature fluctuation.
25. The method of claim 24 wherein said threshold comprises at
least one selected from the group consisting of: a minimum cell
voltage; a minimum stack voltage; minimum cell resistance; a
maximum pressure drop; a maximum pressure drop fluctuation; a
minimum load; a maximum stack voltage fluctuation; a maximum
pressure fluctuation; a maximum temperature change; and a maximum
temperature fluctuation.
26. The method of claim 22 wherein said monitoring includes
comparing said parameter to said threshold and said pulsing is
conducted by varying at least one pulse feature selected from pulse
duration, interval between pulses and pulse shape, based on said
parameter and said threshold.
Description
FIELD OF THE INVENTION
[0001] The present invention relates to fuel cells, and more
particularly to regulating reactant and coolant flows to increase
stability of a fuel cell.
BACKGROUND OF THE INVENTION
[0002] Fuel cells produce electricity through electrochemical
reaction and have been used as power sources in many applications.
Fuel cells can offer significant benefits over other sources of
electrical energy, such as improved efficiency, reliability,
durability, cost and environmental benefits. Fuel cells may,
eventually be used in automobiles and trucks. Fuel cells may also
power homes and businesses.
[0003] There are several different types of fuel cells, each having
advantages that may make them particularly suited to given
applications. One type is a proton exchange membrane (PEM) fuel
cell, which has a membrane sandwiched between an anode and a
cathode. The membrane and respective electrodes together in an
assembly are referred to as a membrane electrode assembly (MEA). To
produce electricity through an electrochemical reaction, hydrogen
(H.sub.2) or hydrogen containing gas is supplied to the anode side
half-cell via an anode flow field and air or oxygen (O.sub.2) is
supplied to the cathode side half-cell via a cathode flow
field.
[0004] In a first half-cell reaction, dissociation of the hydrogen
(H.sub.2) at the anode generates hydrogen protons (H.sup.+) and
electrons (e.sup.-). Because the membrane is proton conductive, the
protons are transported through the membrane. The electrons flow
through an electrical load that is connected across the electrodes.
In a second half-cell reaction, oxygen (O.sub.2) at the cathode
reacts with protons (H.sup.+) and electrons (e.sup.-) are taken up
to form water (H.sub.2O). Parasitic heat is generated by the
reactions and must be regulated to provide efficient operation of
the fuel cell stack.
[0005] Fuel cell stacks include reactant flow fields and coolant
flow fields. The reactant flow fields distribute anode and cathode
reactant fluids across the individual cells of the fuel cell stack.
The coolant flow fields distribute a heat transfer (coolant) fluid
to regulate the operating temperature of the fuel cell stack. In
the case of the reactant flow fields, the anode and cathode
reactant fluids are distributed as gas phase fluids. In the case of
the coolant flow fields, the heat transfer fluid is distributed as
a liquid phase fluid.
[0006] Under certain operating conditions, a liquid phase fluid can
form in the reactant flow fields. The liquid phase fluid impedes
flow of the gas phase reactant fluids, which can result in unstable
operation of the fuel cell stack. Such a situation typically occurs
when consistently operating the fuel cell stack at a low load
level. Traditionally, the stability issues are mitigated by
applying higher anode and/or cathode stoichiometry to avoid
unstable operation. This presents a significant disadvantage,
however, in that a high percentage of the reactant is wasted. As a
result, system efficiency is decreased.
[0007] Gas and/or vapor phase fluids can also form in the coolant
flow fields. The gas phase fluid impedes flow of the liquid phase
heat transfer fluid, which can result in localized temperature
increases or hot spots within the fuel cell stack. These hot spots,
reduce the durability of the fuel cell stack and can result in
localized damage to the fuel cell stack. The presence of such hot
spots in the stack or in a particular cell can be identified by
some increase in temperature and/or in cell resistance, which leads
to some decrease in cell voltage.
SUMMARY OF THE INVENTION
[0008] Accordingly, the present invention provides a fuel cell
system including a fuel cell stack and a flow control device that
controls a supply of a first phase fluid flowing through the fuel
cell stack. A controller monitors at least one parameter of the
fuel cell stack and controls the supply to generate pulses of
reactant when the at least one parameter crosses a threshold value
to flush a second phase fluid from said fuel cell stack.
[0009] In other features, the pulses are defined by at least one
pulse feature selected from a pulse duration, interval between
pulses, a pulse shape, as determined by the controller. The at
least one pulse feature is based on an extent by which the at least
one parameter varies from the threshold value.
[0010] In other features, the fuel cell system further includes at
least one voltage sensor that senses a voltage of a respective
individual cell of the fuel cell stack and provides a signal to the
controller. The parameter includes a cell voltage and the threshold
is a minimum cell voltage threshold. Alternatively, the threshold
includes an average voltage fluctuation defined by at least one of
a magnitude and a frequency of individual cell voltage
fluctuations.
[0011] In other features, the fuel cell system further includes a
voltage sensor that monitors a stack voltage and provides a signal
to the controller based on the parameter, which is the stack
voltage. The threshold includes a minimum stack voltage.
Alternatively, the threshold includes an average voltage
fluctuation defined by at least one of a magnitude and a frequency
of stack voltage fluctuations.
[0012] In other features, the fuel cell system further includes a
pressure sensing system that senses a fluid pressure drop across
the fuel cell stack and provides a signal to the controller based
on the parameter, which is the pressure drop. The threshold
includes a maximum pressure drop. Alternatively, the threshold
includes an average pressure drop fluctuation defined by at least
one of a magnitude and a frequency of stack pressure drop
fluctuations.
[0013] In another feature, the parameter is a resistance of at
least one fuel cell.
[0014] In other features, the fuel cell system further includes a
temperature sensing system that senses a fluid temperature increase
across the fuel cell stack and provides a signal to the controller
based on the parameter, which is the temperature increase. The
threshold includes a maximum temperature increase. Alternatively,
the threshold includes an average temperature increase fluctuation
defined by at least one of a magnitude and a frequency of stack
temperature increase fluctuations.
[0015] In still another feature, the parameter is stack load and
the threshold value is a minimum stack load.
[0016] In yet another feature, the flow control device is at least
one flow regulator.
[0017] Further areas of applicability of the present invention will
become apparent from the detailed description provided hereinafter.
It should be understood that the detailed description and specific
examples, while indicating the preferred embodiment of the
invention, are intended for purposes of illustration only and are
not intended to limit the scope of the invention.
BRIEF DESCRIPTION OF THE DRAWINGS
[0018] The present invention will become more fully understood from
the detailed description and the accompanying drawings,
wherein:
[0019] FIG. 1 is a schematic illustration of a fuel cell system
according to the present invention;
[0020] FIG. 2 is a graph illustrating an exemplary pulse-wave
according to the present invention;
[0021] FIG. 3 is a graph illustrating an alternative exemplary
pulse-wave according to the present invention;
[0022] FIG. 4 is a graph illustrating a flow pulsing strategy;
[0023] FIG. 5 is a graph illustrating an alternative flow pulsing
strategy; and
[0024] FIG. 6 is a flowchart illustrating the reactant and/or
coolant flow control according to the present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0025] The following description of the preferred embodiments is
merely exemplary in nature and is in no way intended to limit the
invention, its application, or uses.
[0026] Referring now to FIG. 1, an exemplary fuel cell system 10 is
illustrated. The fuel cell system 10 includes a fuel cell stack 12
that process reactants to generate electricity. More specifically,
the fuel cell stack 12 includes an anode side through which an
anode reactant flows and a cathode side through which a cathode
reactant flows. The anode reactant is supplied by an anode reactant
source 14 and generally includes hydrogen or a hydrogen-rich
reformate. The cathode reactant is generally provided as
oxygen-rich air supplied by a compressor 16. An anode control
device (FCD) 18, such as a regulator, regulates anode reactant flow
to the fuel cell stack 12. A cathode FCD 20, such as a regulator,
regulates cathode reactant flow to the fuel cell stack 12. The
anode and cathode reactant fluids flow through the fuel cell stack
12 in gas phase form.
[0027] The fuel cell system 10 further includes a coolant system 19
that circulates a heat transfer fluid through the fuel cell stack
12. Although not illustrated, the coolant system 19 generally
includes a pump, a reservoir and a heat exchanger. The heat
transfer fluid flows through the fuel cell stack 12 in liquid phase
form to regulate the operating temperature of the fuel cell stack
12. The heat transfer fluid can be used to cool the fuel cell stack
12 or heat the fuel cell stack 12 as environmental operating
conditions so require. The coolant flow is controlled by regulating
the speed (RPM) of the pump.
[0028] A controller 22 regulates operation of the fuel cell stack
12 and coolant system 19. The controller 22 is in communication
with the anode and cathode FCD's 18,20, compressor 16 and the
coolant system 19. More specifically, the controller 22 generates
control signals to regulate operation of the fuel cell system
components based on a desired load input. The desired load input
indicates a desired power output for the fuel cell system. For
example, in the case of a vehicle, the desired load input is
generated by a throttle.
[0029] A cell sensor system 24 monitors cell voltages of individual
fuel cells or groups of individual fuel cells in the fuel cell
stack 12. The cell sensor system 24 generates cell voltage signals
that are received by the controller 22. The cell sensor system 24
can include multiple cell voltage sensors (not shown) or a single
voltage sensor (not shown) that is electrically switched to measure
the voltage of individual cells. A voltage sensor 26 monitors the
total voltage of the fuel cell stack 12. The voltage sensor 26
generates a stack voltage signal that is received by the controller
22. Inlet and outlet pressure sensors 28,30, respectively, are
provided on the cathode side of the fuel cell stack 12. Similar
pressure sensors 50, 52 are applied to the respective inlet and
outlet of the anode side of the fuel cell stack 12. The inlet
pressure sensor 28 generates an inlet pressure signal and the
outlet pressure sensor 30 generates an outlet pressure signal. The
inlet and outlet pressure signals are received by the controller
22. All other pressure sensors as well as the temperature sensors
act in a similar way, and their output signals can be received by
controller 22. The resistance of individual cells, groups of
individual cells and/or the total fuel cell stack resistance can be
measured by applying a high frequency resistance (HFR) measurement
technique. The HFR measurement technique for individual fuel cells
is described in detail in commonly assigned U.S. Pat. No.
6,376,111, issued on Apr. 23, 2002, the disclosure of which is
expressly incorporated herein by reference. The HFR measurement
technique for groups of fuel cells is described in detail in
commonly assigned U.S. Pat. No. 6,406,806, issued on Jun. 18, 2002,
the disclosure of which is expressly incorporated herein by
reference. The pressure differential between the anode and cathode
sides of the MEA is monitored and controlled so as to not exceed a
value suitable for the membrane, MEA and the stack assembly
encompassing the membrane.
[0030] Inlet temperature and pressure sensors 32,34, respectively,
are provided at the heat transfer fluid inlet to the fuel cell
stack 12. The inlet temperature sensor 32 generates an inlet
temperature (T.sub.IN) signal and the inlet pressure sensor 34
generates an inlet pressure (P.sub.IN) signal. Outlet temperature
and pressure sensors 36,38, respectively, are provided at the heat
transfer fluid outlet of the fuel cell stack 12. The outlet
temperature sensor 36 generates an outlet temperature (T.sub.OUT)
signal and the outlet pressure sensor 38 generates an outlet
pressure (P.sub.OUT) signal.
[0031] The fuel cell control of the present invention regulates
reactant flow through the fuel cell stack 12 based on a monitored
operating parameter. More specifically, the fuel cell control
generates reactant flow pulses to purge the reactant flow channels
of the fuel cell stack 12 of any potential obstructions including
liquid phase fluids. For example, under particular operating
conditions, precipitate accumulates within the reactant flow
channels. This accumulation can lead to the formation of
precipitate slugs that flow through the reactant flow channels.
This results in inefficient fuel cell operation and possible damage
to the fuel cell stack 12. As used herein, a precipitate designates
accumulation of a substance, such as condensed water forming the
slugs, which impede reactant flow. Unstable stack operation results
from the accumulation of precipitate within the reactant flow
channels. More particularly, a thin precipitate film forms on the
reactant flow channel walls. The precipitate slugs interrupt stable
operation of the fuel cell stack.
[0032] The operating parameters include, but are not limited to,
load on the fuel cell stack 12, individual cell voltage, individual
cell resistance, pressure drop across a reactant side, stack
voltage fluctuation and pressure drop fluctuation across a reactant
side, pressure and temperature change and fluctuation across the
coolant side of the fuel cell stack. When the particular operating
parameter crosses a corresponding threshold, the fuel cell control
generates reactant or coolant flow pulses. Although the reactant
flow pulses can be generated in either reactant side, the cathode
side is more susceptible to precipitate accumulation. Generation of
the reactant flow pulses is controlled to inhibit significant
pressure differentials across the polymer electrolyte membrane
(PEM) (i.e., between the anode and cathode sides). It is
anticipated that the controller 22 monitors the above-identified
operating parameters either alone or in combination. That is to
say, the controller 22 can monitor a single operating parameter,
multiple operating parameters or all of the operating parameters to
trigger reactant or coolant flow pulsing.
[0033] A pressure differential between the anode side and the
cathode side is monitored. More specifically, the reactant flow
pulsing is regulated to maintain the pressure differential below a
threshold value. The threshold value is determined based on the
particular fuel cell stack design. For example, the threshold may
be higher for a more robust fuel cell stack than for a less robust
fuel cell stack. If the pressure differential is too great, damage
to the fuel cells of the fuel cell stack 12 can occur. Therefore,
if the pressure differential exceeds the threshold value, the
reactant flow pulsing can cease or be limited to reduce the
pressure differential below the threshold.
[0034] With particular reference to FIGS. 2 and 3, the reactant
flow pulses are generated to have a particular waveform based on
the conditions instigating reactant flow pulsing. As illustrated in
FIG. 2, an exemplary waveform includes a step-wave having an
amplitude (A.sub.STEP) and a frequency (f.sub.STEP). As illustrated
in FIG. 3, an alternative exemplary waveform includes a sine-wave
having an amplitude (A.sub.SINE) and a frequency (f.sub.SINE). It
is appreciated that the waveforms illustrated herein are exemplary
in nature and other waveforms, such as saw-tooth and ramp formed
can be used in the method of the present invention. The duration of
the reactant pulse can vary. For example, it is anticipated that
the duration is for a preset period. Upon expiration of the period,
the reactant flow pulsing ceases. It is also anticipated that the
reactant flow pulsing continues until the condition that
precipitated the reactant flow pulsing is improved or resolved
completely.
[0035] Referring now to FIGS. 4 and 5, a pulse differentiates from
a typical nominal continuous flow rate conventionally used to
supply reactants without rapid modulation. The pulses of the
present invention may occur under any of a variety of conditions,
such as: termination of a nominal flow rate and initiation of a
pulsed flow at a pressure and/or flow rate greater than or less
than that of the nominal rate (see FIG. 4), continued propagation
of the nominal rate with intermittently added reactant to cause a
pressure and/or flow pulse (see FIG. 5) and other variations
consistent with the present invention. Although the nominal flow
rate may itself be considered a pulse of an extended duration and
at a relatively constant value, the present invention
differentiates from this and is directed to a more rapid modulation
of flow via pulses and the pulse features as described herein. Such
pulse features are selected from pulse duration, interval between
pulses (i.e., frequency) and pulse shape. Preferably, flow pulsing
occurs while varying at least one of the pulse features. The
invention also contemplates the continuing of a nominal flow rate
while a pulse is imposed on such flow (for example, by means of
intermittent operation of an injector).
[0036] With particular regard to fuel cell stack load, precipitate
in the flow-field channels is more likely to form during periods of
low load. At low load, less cathode and anode reactants flow
through the fuel cell stack 12. As a result, precipitate within the
fuel cell stack 12 is less likely to be flushed by the reactant
flow. Therefore, the fuel cell control implements reactant flow
pulsing to flush potential precipitate during operation at low
load. In one example, the controller 22 monitors the load of the
fuel cell stack 12. If the load falls below a load threshold, the
fuel cell control initiates reactant flow pulsing. The reactant
flow pulsing continues until the load is above the load threshold.
In another example, the reactant flow pulsing is inversely
proportional to load (i.e., the intensity of reactant flow pulsing
decreases as load increases and increases as load decreases). In
this example, however, it is preferred that there is no reactant
flow pulsing at higher load levels.
[0037] In the case of minimal cell voltage, the voltages of the
individual fuel cells in the fuel cell stack 12 are monitored, and
these voltages are compared to a cell voltage threshold. If the
lowest of the cell voltages falls below a cell voltage threshold,
the fuel cell control initiates reactant flow pulsing. In one
example, the cell voltage threshold is a preset minimum cell
voltage. In another example, the cell voltage threshold is the
difference between the average cell voltage and a preset tolerance
(e.g., 200 mV). For example, if the average cell voltage is 800 mV
and the present tolerance is 200 mV, the fuel cell control
implements reactant flow pulsing if the lowest of the cell voltages
falls below 600 mV.
[0038] Referring now to individual cell resistance, the resistances
of the individual fuel cells in the fuel cell stack 12 are
monitored and these resistances are compared to a cell resistance
threshold. If the lowest cell resistance falls below a cell
resistance threshold, the fuel cell control initiates reactant flow
pulsing. In one example, the cell resistance threshold is a preset
minimum cell voltage. In another example, the cell resistance
threshold is the difference between the average cell resistance and
a preset tolerance (e.g., 0.04 Ohm*cm.sup.2). For example, if the
average cell resistance is 0.11 Ohm*cm.sup.2 and the present
tolerance is 0.04 Ohm*cm.sup.2, the fuel cell control implements
reactant flow pulsing if the lowest cell resistance falls below
0.07 Ohm*cm.sup.2 or exceeds 0.150 Ohm*cm.sup.2.
[0039] In the case of pressure drop across a reactant side, the
fuel cell control triggers reactant flow pulsing if the pressure
difference between the inlet and outlet of the particular reactant
side (e.g., cathode side) exceeds a pressure drop threshold (e.g.,
25 kPa). For example, in the case of the cathode side, the pressure
at the cathode side inlet is detected by the inlet pressure sensor
28, and the pressure at the cathode side outlet is detected by the
outlet pressure sensor 30. The controller 22 calculates the
pressure drop as the difference between the pressure sensor signals
and compares the pressure drop to the preset threshold. The
reactant side pressure drop operating parameter is an average fuel
cell stack characteristic. Therefore, monitoring of the individual
fuel cells of the fuel cell stack 12 is not required.
[0040] With regard to average stack voltage fluctuation, the fuel
cell control monitors the fuel cell stack voltage using the voltage
sensor. The fuel cell stack voltage is the total voltage of all of
the fuel cells in the fuel cell stack 12. More particularly, the
magnitude and frequency of the stack voltage fluctuations are
monitored. If the average magnitude and/or average frequency of
fuel cell stack voltage fluctuations exceed respective thresholds,
reactant flow pulsing is triggered. The fuel cell stack voltage
fluctuation operating parameter is an average fuel cell stack
characteristic. Therefore, monitoring of the individual fuel cells
of the fuel cell stack 12 is not required.
[0041] Referring now to reactant side pressure drop fluctuation,
the fuel cell control monitors the pressure drop fluctuations using
the inlet and outlet pressure sensors 28,30. More particularly, the
magnitude and frequency of the pressure drop fluctuations are
monitored. If the average magnitude and/or average frequency of
pressure drop fluctuations exceed respective thresholds, reactant
flow pulsing is triggered. The pressure drop fluctuation operating
parameter is an average fuel cell stack characteristic. Therefore,
monitoring of the individual fuel cells of the fuel cell stack 12
is not required.
[0042] Referring now to FIG. 6, the fuel cell control of the
present invention will be described in further detail. In step 100
control measures a particular operating parameter. The operating
parameter can include, but is not limited to, load on the fuel cell
stack 12, individual cell voltage, individual cell resistance,
pressure drop across a reactant side, average stack voltage
fluctuation and pressure drop fluctuation across a reactant side.
Control determines whether the operating parameter has crossed its
corresponding threshold in step 102. If the operating parameter has
crossed its corresponding threshold, control continues in step 103.
If the operating parameter has not crossed its corresponding
threshold, control continues in step 106.
[0043] Control initiates a timer in step 103. In step 104, control
implements reactant flow pulsing. Control determines whether the
operating parameter has returned to its normal operating range
(i.e., crossed back over its corresponding threshold) in step 108.
If the operating parameter has returned to its normal operating
range, control implements normal flow control in step 106 and
control ends. If the operating parameter has not returned to its
normal operating range, control continues in step 110.
[0044] In step 110, control determines whether the timer has
achieved a timer threshold. If the timer has not achieved the timer
threshold, control increments the timer in step 112 and loops back
to step 104. If the timer has achieved the timer threshold, control
initiates an alert in step 114 and control ends. The alert informs
an operator that the fuel cell system 10 is performing at an
undesirable level and can include a visual alert and/or an audible
alert. The alert can also include information regarding the
monitored operating parameter. It is further anticipated that the
alert can include a fuel cell system shut down action to prevent
damage to the fuel cell stack 12. In this manner, the fuel cell
control attempts to correct an undesirable operating condition
using reactant flow pulsing. If the condition does not subside
after the threshold time period has expired, the fuel cell control
alerts an operator and/or terminates fuel cell system operation to
prevent damage to the fuel cell stack 12.
[0045] It is also anticipated that an additional timing loop can be
implemented to monitor the operating parameter for a threshold
period after returning to the normal range. In this manner, the
fuel cell control ensures that the operating parameter stays within
the normal range before ceasing reactant flow pulsing.
[0046] Although the foregoing discussion describes implementation
of the flow pulsing for the reactant flow fields, it is also
anticipated that flow pulsing can be implemented for the coolant
flow fields. More particularly, a gas phase fluid (e.g., steam
bubbles) can form in the coolant flow field and impede flow of the
liquid phase heat transfer fluid. Coolant flow pulsing dislodges
and flushes the gas phase fluid from the coolant flow field to
prevent localized heating across the fuel cell stack.
[0047] The heat transfer fluid flow pulsing is regulated based on a
temperature differential (.DELTA.T) and/or a pressure differential
(.DELTA.P) of the heat transfer fluid across the fuel cell stack
12. .DELTA.T is determined as the difference between T.sub.IN and
T.sub.OUT and .DELTA.P is determined as the difference between
P.sub.IN and P.sub.OUT. .DELTA.T is compared to a threshold
temperature differential. If .DELTA.T is greater than the threshold
temperature differential, the heat transfer fluid flow is pulsed as
similarly described above for the reactant fluids. .DELTA.P is
compared to a threshold pressure differential. If .DELTA.P is
greater than the threshold pressure differential, the heat transfer
fluid flow is pulsed. The pulse flow concepts of the present
invention were tested as compared to a comparative conventional
stack flow at low load. In a conventional stack run at low load,
the cathode stoichiometry is increased from 1.6 to about 1.8
increasing compressor power by about 12% in order to achieve stable
operation. In contrast, running at low load and pulsing with about
50% flow increase, for up to about a minute, at up to about 10
minute intervals resulted in stable operation while increasing
compressor power only about 2 to 3%. Furthermore, the voltage drop
in the comparative case was dramatic, dropping from 0.8V to 0.3V or
less in 1 to 2 hours. In the pulsed case, the voltage drop was
maintained at about 0.8V for 10 hours or more. The same results
applied to coolant pulsing, where periodic pulsing virtually
eliminated gas bubbles and hot spots while pump power increased
only about 6%.
[0048] The description of the invention is merely exemplary in
nature and, thus, variations that do not depart from the gist of
the invention are intended to be within the scope of the invention.
Such variations are not to be regarded as a departure from the
spirit and scope of the invention.
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