U.S. patent application number 09/819506 was filed with the patent office on 2002-01-24 for methods and apparatus for improving the cold starting capability of a fuel cell.
Invention is credited to Jia, Neng You, St-Pierre, Jean, Van der Geest, Marian E..
Application Number | 20020009623 09/819506 |
Document ID | / |
Family ID | 23607450 |
Filed Date | 2002-01-24 |
United States Patent
Application |
20020009623 |
Kind Code |
A1 |
St-Pierre, Jean ; et
al. |
January 24, 2002 |
Methods and apparatus for improving the cold starting capability of
a fuel cell
Abstract
Apparatus and methods of ceasing operation of an electric power
generating system improve the cold starting capability of the
system. The system comprises a fuel cell stack connectable to an
external circuit for supplying electric current to the external
circuit. The stack comprises at least one solid polymer fuel cell,
and the system further comprises a fuel passage for directing a
fuel stream through the stack and an oxidant passage for directing
an oxidant stream through the stack, a sensor assembly connected to
the stack for monitoring a parameter indicative of stack
performance, a controller for controlling at least one operating
parameter of the stack, and a control system communicative with the
sensor assembly and operating parameter controller. The method
comprises adjusting at least one fuel cell stack operating
parameter to cause the stack to operate under a drying condition
that causes a net outflux of water from the stack, operating the
stack under the drying condition until the water content in the
stack has been reduced, and interrupting supply of electric current
from the stack to the external circuit.
Inventors: |
St-Pierre, Jean; (Vancouver,
CA) ; Jia, Neng You; (Richmond, CA) ; Van der
Geest, Marian E.; (Vancouver, CA) |
Correspondence
Address: |
Robert W. Fleseler
McAndrews, Held & Malloy, Ltd
500 West Madison Street, 34th Floor
Chicago
IL
60661
US
|
Family ID: |
23607450 |
Appl. No.: |
09/819506 |
Filed: |
March 28, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
09819506 |
Mar 28, 2001 |
|
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09406318 |
Sep 27, 1999 |
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Current U.S.
Class: |
429/414 ;
429/429; 429/442; 429/444; 429/450; 429/465; 429/492 |
Current CPC
Class: |
H01M 8/0267 20130101;
H01M 8/241 20130101; H01M 8/04119 20130101; H01M 8/04225 20160201;
H01M 8/04156 20130101; H01M 2008/1095 20130101; H01M 8/04268
20130101; H01M 8/2457 20160201; H01M 8/04029 20130101; H01M 8/04228
20160201; H01M 8/04303 20160201; H01M 8/04007 20130101; H01M
8/04253 20130101; Y02E 60/50 20130101; H01M 2300/0082 20130101 |
Class at
Publication: |
429/13 ; 429/23;
429/22; 429/24 |
International
Class: |
H01M 008/04 |
Claims
What is claimed is:
1. A method of ceasing operation of an electric power generating
system comprising a fuel cell stack connectable to an external
circuit for supplying electric current to said external circuit,
said stack comprising at least one solid polymer fuel cell, said
system further comprising a fuel passage for directing a fuel
stream through said stack and an oxidant passage for directing an
oxidant stream through said stack, said method comprising in
sequential order: (a) adjusting at least one fuel cell stack
operating parameter to cause said stack to operate under a drying
condition that causes a net outflux of water from said stack; (b)
operating said stack under said drying condition until the water
content in said stack has been reduced; and (c) interrupting supply
of electric current from said stack to said external circuit.
2. The method of claim 1 wherein said at least one operating
parameter is selected from the group consisting of stack
temperature, oxidant and fuel stream relative humidities, oxidant
and fuel stoichiometries, and oxidant and fuel stream
pressures.
3. The method of claim 2 wherein prior to cessation of system
operation at least one of said oxidant and fuel streams directed to
said stack is humidified, and wherein in step (a) the degree of
humidification of said at least one of said oxidant and fuel
streams is reduced.
4. The method of claim 3 wherein said humidification reduction is
performed by directing at least some of said at least one of said
oxidant and fuel streams to said stack without humidification.
5. The method of claim 4 wherein said at least one of said oxidant
and fuel relative humidities is reduced by directing at least some
of said oxidant and fuel streams to said stack in fluid isolation
from said humidifier.
6. The method of claim 2 wherein prior to cessation of stack
operation, a coolant is circulated through said stack to maintain
the temperature of said stack within a desired nominal operating
range, and wherein in (a), said stack temperature is increased by
stopping coolant circulation through said stack.
7. The method of claim 2 wherein in step (a), at least one of said
oxidant and fuel stream pressures is decreased.
8. The method of claim 2, wherein in step (a), at least one of said
oxidant and fuel stoichiometries is increased.
9. The method of claim 1 wherein in step (b), a parameter
indicative of stack performance is monitored and said operation of
said stack under said drying condition is stopped when said stack
performance falls to a threshold value.
10. The method of claim 9 wherein the resistance of said stack is
monitored, and said operation of said stack under said drying
condition is stopped when said resistance exceeds a threshold
value.
11. The method of claim 10, wherein said resistance threshold value
corresponds to or above a critical membrane moisture level.
12. The method of claim 9 wherein the voltage of said stack is
monitored, and said operation of said stack under said drying
condition is stopped when said voltage falls to or below a
threshold value.
13. The method of claim 12, wherein said threshold voltage value
corresponds to or above a critical membrane moisture level.
14. A method of ceasing operation of an electric power generating
system comprising a fuel cell stack connectable to an external
circuit for supplying electric current to said external circuit,
and said stack comprising at least one solid polymer fuel cell,
said system further comprising a fuel passage for directing a fuel
stream through said stack and an oxidant passage for directing an
oxidant stream through said stack, said method comprising in
sequential order: (a) interrupting the supply of electric current
from said fuel cell stack to said external circuit; (b) adjusting
at least one of stack temperature, oxidant or fuel stream flow
rate, or oxidant or fuel stream pressure to cause a drying
condition with a net outflux of water from said stack; and (c)
flowing at least one of fuel or oxidant streams through said stack
under said drying condition until the water content in said stack
has been reduced.
15. The method of claim 14 wherein prior to cessation of stack
operation, a coolant is circulated through said stack to maintain
the temperature of said stack within a desired nominal operating
range, and wherein in step (b), said coolant circulation through
said stack is stopped.
16. The method of claim 14 wherein in step (b), at least one of
said oxidant and fuel stream pressures is decreased.
17. The method of claim 14, wherein in step (b), at least one of
said oxidant and fuel stream flow rates is increased.
18. The method of claim 14 wherein in step (c), a parameter
indicative of stack performance is monitored and said flow of
oxidant or fuel streams or both through said stack under said
drying condition is stopped when said stack performance falls to a
threshold value.
19. The method of claim 18 wherein the resistance of said stack is
monitored, and said flow of oxidant or fuel streams or both through
said stack under said drying condition is stopped when said
resistance exceeds a threshold value.
20. The method of claim 18, wherein said threshold resistance
corresponds to or above a critical membrane moisture level.
21. The method of claim 18 wherein the voltage of said stack is
monitored, and said flow of oxidant or fuel streams or both through
said stack under said drying condition is stopped when said voltage
falls below a threshold value.
22. The method of claim 21, wherein said threshold voltage
corresponds to or above a critical membrane moisture level.
23. An electric power generation system comprising: (a) a fuel cell
stack connectable to an external circuit for supplying electric
current to said external circuit, said stack comprising at least
one solid polymer fuel cell and fluid flow passages through said
stack; (b) a sensor assembly connected to said stack for monitoring
at least one parameter indicative of stack performance; (c) a
controller for controlling at least one operating parameter of said
stack; and (d) a control system communicative with said sensor
assembly and said operating parameter controller, such that upon
receipt of a shut down instruction by said control system, said
operating parameter controller is operable to adjust at least one
stack operating parameter such that said stack operates in a drying
condition that causes a net outflux of water from said stack, until
the water content in said stack has been reduced.
24. The electric power generation system of claim 23 wherein said
sensor assembly comprises a sensor for monitoring stack
resistance.
25. The electric power generation system of claim 23 wherein said
sensor assembly comprises a sensor for monitoring stack
voltage.
26. The electric power generation system of claim 23 wherein said
sensor assembly comprises at least one sensor for monitoring said
at least one operating parameter.
27. The electric power generation system of claim 26 wherein said
sensor assembly comprises a sensor for monitoring fuel stream
pressure.
28. The electric power generation system of claim 26 wherein said
sensor assembly comprises a sensor for monitoring oxidant stream
pressure.
29. The electric power generation system of claim 26 wherein said
sensor assembly comprises a sensor for monitoring fuel stream
relative humidity.
30. The electric power generation system of claim 26 wherein said
sensor assembly comprises a sensor for monitoring oxidant stream
relative humidity.
31. The electric power generation system of claim 26 wherein said
sensor assembly comprises a sensor for monitoring stack
temperature.
32. The electric power generation system of claim 31 wherein said
sensor assembly comprises temperature sensors for monitoring
coolant inlet and outlet temperatures.
33. The electric power generation system of claim 23 wherein said
operating parameter controller comprises apparatus to control fuel
and oxidant relative humidities, stoichiometries, pressures, and
stack temperature.
34. The electric power generation system of claim 33 wherein said
control system comprises a microcontroller.
35. The electric power generation system of claim 33 further
comprising a humidifier for humidifying at least one of a fuel or
oxidant stream supplied to said stack during normal operation, and
wherein said relative humidity control apparatus comprises a
humidifier bypass system having at least one bypass conduit for
directing one of fuel or oxidant to said stack in fluid isolation
from said humidifier.
36. The electric power generation system of claim 35 wherein upon
receipt of a shut down instruction by said control system, said
humidifier bypass system directs at least some of said oxidant or
fuel streams through an associated said bypass conduit during a
system shut down procedure, and to discontinue transmission through
said bypass conduit when the water content in said stack has been
reduced.
37. The electric power generation system of claim 36 wherein said
bypass conduit comprises an inlet end connected to one of said
reactant stream passages at a location upstream of said humidifier,
and an outlet end connected to the same reactant stream passage at
a location downstream of said humidifier.
38. The electric power generation system of claim 36 wherein said
bypass conduit comprises an inlet end connectable to a reactant
supply, and an outlet end connected to one of said reactant stream
inlet passages at a location downstream of said humidifier.
39. The electric power generation system of claim 36 wherein said
humidifier bypass system comprises a bypass inlet valve connected
to one of said reactant passages at a location upstream of said
humidifier, and a bypass outlet valve connected to the same
reactant passage at a location downstream of said humidifier, and
wherein said bypass conduit is connected to said bypass inlet and
outlet valves.
40. The electric power generation system of claim 33 wherein said
oxidant stoichiometry control apparatus comprises a compressor
connected to said oxidant inlet passage.
41. The electric power generation system of claim 33 wherein said
pressure control apparatus is a pressure regulator on at least one
of an oxidant and fuel passage.
42. The electric power generation system of claim 33 wherein said
stack temperature control apparatus is a coolant system having a
coolant passage through said stack and a coolant pump communicative
with said control system.
43. The electric power generation system of claim 23 wherein said
control system is operable such that said drying operation is
discontinued after said operating parameter indicative of stack
performance measured by said sensor assembly reaches a threshold
value.
Description
CROSS-REFERENCE TO RELATED APPLICATION(S)
[0001] This application a continuation-in-part of U.S. patent
application Ser. No. 09/406,318, entitled "Methods for Improving
the Cold Starting Capability of an Electrochemical Fuel Cell". The
'318 application is, in turn, a continuation-in-part of U.S. patent
application Ser. No. 09/138,625 filed Aug. 24, 1998, entitled
"Method and Apparatus for Commencing Operation of a Fuel Cell
Electric Power Generation System Below the Freezing Temperature of
Water". The '625 application is, in turn, a continuation of U.S.
patent application Ser. No. 08/659,921 filed Jun. 7, 1996, now U.S.
Pat. No. 5,798,186 issued Aug. 25, 1998, also entitled "Method and
Apparatus for Commencing Operation of a Fuel Cell Electric Power
Generation System Below the Freezing Temperature of Water". The
'318, '625 and '921 applications are each hereby incorporated by
reference in their entirety.
FIELD OF THE INVENTION
[0002] The present invention relates to techniques to improve the
cold starting capabilities of an electric power generating system
comprising a solid polymer fuel cell, and in particular relates to
methods and apparatus for reducing water content in the fuel cell
when the stack is shut down.
BACKGROUND OF THE INVENTION
[0003] Electrochemical fuel cells convert fuel and oxidant to
electricity and reaction product. Solid polymer electrochemical
fuel cells generally employ a membrane electrode assembly ("MEA")
which comprises 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 MEA contains a layer of
catalyst, typically in the form of finely comminuted platinum, at
each membrane/electrode interface to induce the desired
electrochemical reaction. In operation the electrodes are
electrically coupled to provide a circuit for conducting electrons
between the electrodes through an external circuit.
[0004] At the anode, the fuel stream moves through the porous anode
substrate and is oxidized at the anode electrocatalyst layer. At
the cathode, the oxidant stream moves through the porous cathode
substrate and is reduced at the cathode electrocatalyst layer to
form a reaction product.
[0005] In fuel cells employing hydrogen as the fuel and
oxygen-containing air (or substantially pure oxygen) as the
oxidant, the catalyzed reaction at the anode produces hydrogen
cations (protons) from the fuel supply. The ion exchange membrane
facilitates the migration of protons from the anode to the cathode.
In addition to conducting protons, the membrane isolates the
hydrogen-containing fuel stream from the oxygen-containing oxidant
stream. At the cathode electrocatalyst layer, oxygen reacts with
the protons that have crossed the membrane to form water as the
reaction product. The anode and cathode reactions in
hydrogen/oxygen fuel cells are shown in the following
equations:
Anode reaction: H.sub.2.fwdarw.2H.sup.++2e.sup.-
Cathode reaction: 1/2O.sub.2+2H.sup.++2e.sup.-.fwdarw.H.sub.2O
[0006] In typical fuel cells, the MEA is disposed between two
electrically conductive fluid flow field plates or separator
plates. Fluid flow field plates have at least one flow passage
formed in at least one of the major planar surfaces thereof. The
flow passages direct the fuel and oxidant to the respective
electrodes, namely, the anode on the fuel side and the cathode on
the oxidant side. The fluid flow field plates act as current
collectors, provide support for the electrodes, provide access
channels for the fuel and oxidant to the respective anode and
cathode surfaces, and provide channels for the removal of reaction
products, such as water, formed during operation of the cell.
Separator plates typically do not have flow passages formed in the
surfaces thereof, but are used in combination with an adjacent
layer of material which provides access passages for the fuel and
oxidant to the respective anode and cathode electrocatalyst, and
provides passages for the removal of reaction products. The
preferred operating temperature range for solid polymer fuel cells
is typically 50.degree. C. to 120.degree. C., most typically about
75.degree. C. to 85.degree. C.
[0007] Two or more fuel cells can be electrically connected
together in series to increase the overall power output of the
assembly. In series arrangements, one side of a given fluid flow
field or separator plate can serve as an anode plate for one cell
and the other side of the fluid flow field or separator plate can
serve as the cathode plate for the adjacent cell. Such a multiple
fuel cell arrangement is referred to as a fuel cell stack, and is
usually held together in its assembled state by tie rods and end
plates. The stack typically includes inlet ports and manifolds for
directing the fluid fuel stream (such as substantially pure
hydrogen, methanol reformate or natural gas reformate, or a
methanol-containing stream in a direct methanol fuel cell) and the
fluid oxidant stream (such as substantially pure oxygen,
oxygen-containing air or oxygen in a carrier gas such as nitrogen)
to the individual fuel cell reactant flow passages. The stack also
commonly includes an inlet port and manifold for directing a
coolant fluid stream, typically water, to interior passages within
the stack to absorb heat generated by the fuel cell during
operation. The stack also generally includes exhaust manifolds and
outlet ports for expelling the depleted reactant streams and the
reaction products such as water, as well as an exhaust manifold and
outlet port for the coolant stream exiting the stack. In a power
generation system various fuel, oxidant and coolant conduits carry
these fluid streams to and from the fuel cell stack.
[0008] When an electrical load (comprising one or more load
elements) is placed in an electrical circuit connecting the stack
terminals, fuel and oxidant are consumed in direct proportion to
the electrical current drawn by the load, which will vary with the
ohmic resistance of the load.
[0009] Solid polymer fuel cells generally employ perfluorosulfonic
ion exchange membranes, such as those sold by DuPont under its
NAFION.RTM. trade designation. When employing such membranes, the
fuel and oxidant reactant streams are typically humidified before
they are introduced to solid polymer fuel cells so as to facilitate
proton transport through the ion exchange membrane and to avoid
drying (and damaging) the membrane separating the anode and cathode
of each cell.
[0010] Each reactant stream exiting the fuel cell stack generally
contains water. The outlet fuel stream from the anodes generally
contains water from the incoming fuel stream plus any product water
drawn across the membrane from the cathode. The outlet oxidant
stream from the cathodes generally contains water added to humidify
the incoming oxidant stream plus product water formed at the
cathode.
[0011] In some fuel cell applications, such as, for example, motive
applications, it may be necessary or desirable to commence
operation of a solid polymer electrolyte fuel cell stack when the
stack core temperature is below the freezing temperature of water.
As used herein, the freezing temperature of water means the
freezing temperature of free water, that is, 0.degree. C. at 1
atmosphere. It may also be necessary or desirable when ceasing
operation of the solid polymer fuel cell stack to improve the cold
start capability and freeze tolerance of the stack by reducing the
amount of water remaining within the fuel, oxidant and coolant
passages of the stack. Upon freezing, water remaining within stack
passages will expand and potentially damage structures within the
stack such as, for example, the membrane/electrocatalyst interface,
the reactant passageways, conduits and seals, as well as the porous
electrode substrate material.
[0012] If there is an expectation that a solid polymer fuel cell
stack will be subjected to cold temperatures, especially
temperatures below the freezing temperature of water, one or more
special start-up and shutdown techniques and associated apparatus
may be used. These techniques may improve the cold start capability
and freeze tolerance of the stack, and improve the subsequent fuel
cell performance. A measure of electrochemical fuel cell
performance is the voltage output from the cell for a given current
density. Higher performance is associated with a higher voltage
output for a given current density or higher current density for a
given voltage output.
SUMMARY OF THE INVENTION
[0013] Water may be introduced into a fuel cell through either or
both of the oxidant and fuel supply streams to the fuel cell. Water
is produced in fuel cell by the electrochemical reaction at the
cathode. Water may escape the fuel cell via one or both of the
oxidant and fuel exhaust streams leaving the fuel cell. If the
theoretical maximum water flux exiting the fuel cell in vapour form
(for example, via the outlet reactant streams) is greater than the
water flux introduced and produced, then the fuel cell will operate
under a drying condition that causes the fuel cell to dehydrate. In
this case there is a "net outflux" of water. Conversely, if the
amount of water introduced and produced exceeds the theoretical
maximum amount of water exiting the fuel cell in vapour form, a
wetting condition exists that causes water to temporarily
accumulate in the cell (a net influx of water) until a steady state
is achieved. A water balance exists when the net influx and
theoretical outflux of water in vapour form is zero.
[0014] According to one aspect of the invention, the freeze
tolerance and cold start-up capability of an electric power
generating system is improved by removing at least some of the
excess water from fuel cells in the system before the system falls
below the freezing temperature of water. Water removal is carried
out during a system shutdown procedure. The system includes a fuel
cell stack having at least one solid polymer fuel cell. Each fuel
cell has a membrane electrode assembly (MEA) comprising an anode, a
cathode, and an ion exchange membrane interposed between the anode
and the cathode. The stack is connectable to supply electric
current to an external circuit. The system also includes fuel and
oxidant passages that direct respective fuel and oxidant streams
through the stack, a controller for controlling certain operating
parameters of the stack, a sensor assembly for monitoring at least
one parameter indicative of the cell performance of the stack, and
a control system that is communicative with the sensor assembly and
the operating parameter controller.
[0015] Upon receipt of a shut down instruction by the control
system, the operating parameter controller is operable to adjust at
least one stack operating parameter such that the stack operates
under a drying condition that causes a net outflux of water from
the fuel cells. The control system continues the drying operation
until the water content in the stack has been reduced.
[0016] The sensor assembly may include sensors for measuring stack
voltage, stack resistance, stack temperature, fuel and oxidant
pressures, fuel and oxidant relative humidities, and coolant inlet
and outlet temperatures. The controller may include apparatus to
control fuel and oxidant relative humidities, fuel and oxidant
stoichiometries, fuel and oxidant pressures, and stack
temperatures. An example of the apparatus to control oxidant
stoichiometry is a compressor that is connected to the oxidant
inlet passage. An example of the pressure control apparatus are
pressure regulators on each of the fuel and oxidant passages. An
example of the stack temperature control apparatus is a coolant
system having a coolant passage through the stack and a coolant
pump that is communicative with the control system. The control
system may be a microcontroller or like device.
[0017] If the fuel and/or oxidant supply streams are humidified
during normal operation, their relative humidities can be decreased
by reducing or stopping the transfer of water to the reactant
supply streams. For example, a humidifier bypass system comprises a
bypass conduit that allows a reactant supply stream to be directed
to the stack in fluid isolation from a humidifier that is used
during normal operation to humidify the reactant supply streams.
Upon receipt of a shut down instruction by the control system, the
humidifier bypass system directs at least some of the oxidant or
fuel streams through an associated bypass conduit during the shut
down procedure, and discontinues transmission through the bypass
conduit when the water content in the stack has been reduced.
[0018] The bypass conduit includes an inlet end that is connected
to one of the reactant stream passages at a location upstream of
the humidifier, and an outlet end that is connected to the same
reactant stream passage at a location downstream of the humidifier.
Alternatively, the bypass conduit inlet end can be connected
directly to the reactant supply. The humidifier bypass system may
include a bypass inlet valve on one of the reactant passages at a
location upstream of the humidifier, and a bypass outlet valve on
the same reactant passage at a location downstream of the
humidifier; the bypass conduit connects the bypass inlet valve to
the bypass outlet valve such that reactant fluid can be directed to
the stack in fluid isolation from the humidifier.
[0019] According to another aspect of the invention, a method is
provided for ceasing operation of an electric power generating
system that comprises a fuel cell stack connectable to an external
circuit for supplying electric current to said external circuit,
and at least one solid polymer fuel cell. The system further
comprises a fuel passage for directing a fuel stream through said
stack and an oxidant passage for directing an oxidant stream
through said stack. The method comprises in sequential order:
[0020] (a) after receiving instructions to shut down the system, at
least one fuel cell stack operating parameter is adjusted so that
the stack operates under a drying condition that causes a net
outflux of water from the stack;
[0021] (b) the stack is then operated under the drying condition
until the water content in the stack has been reduced; and
[0022] (c) the supply of electric current from the stack to the
external circuit is interrupted.
[0023] The amount of water removed should be enough to remove at
least some of the excess water from the fuel cell, but should not
be so much as to dry out the membrane. If the membrane water level
falls below its critical moisture level (the minimum amount of
water needed for the membrane to be adequately ionically
conductive), a drop in fuel cell performance will occur, which is
observable as a drop in cell output voltage, or an increase in cell
resistance. The stack voltage or resistance is monitored during the
drying operation by stack voltage and resistance sensors. When the
stack performance falls below a selected threshold level (that is,
the voltage drops below a threshold value, or the resistance
increases to a threshold value), the drying operation is stopped.
The threshold values correspond to or above the critical membrane
moisture level.
[0024] The water flux of each fuel cell can be controlled by
controlling certain operating parameters of the system. These
parameters include the fuel and oxidant relative humidities, fuel
and oxidant stoichiometries, fuel and oxidant pressures, and the
stack temperature. One or more of these parameters can be adjusted
so that the stack operates under a drying condition.
[0025] Where one or both of the oxidant and fuel supply streams are
humidified during normal operation, the oxidant and fuel relative
humidities can be reduced during shutdown by reducing the degree of
humidification to the oxidant and/or fuel supply streams. For
certain humidifiers, the amount of water transferred to the supply
stream passing through the stack can be reduced or stopped
altogether. Alternatively, the humidification reduction can be
performed by directing at least some of the oxidant and/or fuel
streams to the stack to the stack in fluid isolation from the
humidifier.
[0026] One or more additional operating parameters may be adjusted
to operate the stack under a drying condition. For example, the
stack temperature can be increased during the shut down procedure
by stopping coolant circulation through the stack. Alternatively or
in addition, the fuel and/or oxidant supply pressures can be
decreased during the shut down procedure. Alternatively or in
addition, the fuel and/or oxidant stoichiometries can be increased
during the shut down procedure. Sensors can be provided to measure
the fuel pressure, oxidant pressure, fuel relative humidity,
oxidant relative humidity, coolant inlet and outlet temperatures,
and stack temperature.
[0027] The stack can be disconnected from the external circuit
before or after the drying operation. If the latter case, then
power to various components in the system (for example, compressor,
sensors) are provided by an auxiliary power source such as a
battery. As the electrochemical reaction stops after the stack is
disconnected, the operating parameters further include air and
oxidant flow rates and excludes air and oxidant
stoichiometries.
BRIEF DESCRIPTION OF THE DRAWINGS
[0028] FIG. 1 is an exploded side view of a typical solid polymer
electrochemical fuel cell with a membrane electrode assembly
interposed between two fluid flow field plates.
[0029] FIG. 2 is a perspective cut-away view of an electrochemical
fuel cell stack.
[0030] FIG. 3 is a dimensionless representation of the net water
flux in a fuel cell obtained under different operating
conditions.
[0031] FIG. 4 is a graph showing the change in cell voltage and
resistance over time for a fuel cell operated under different
conditions.
[0032] FIG. 5 is a graph showing the change of resistance over time
in a fuel cell stack operated under a drying condition.
[0033] FIG. 6 is a schematic diagram of a fuel cell electric power
generation system incorporating a humidifier bypass purge system,
actuators, and sensors that can cooperate to perform a controlled
fuel cell drying operation at shutdown.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENT(S)
[0034] FIG. 1 illustrates a typical fuel cell 10. Fuel cell 10
includes a membrane electrode assembly 12 interposed between anode
flow field plate 14 and cathode flow field plate 16. Membrane
electrode assembly (MEA) 12 comprises an ion exchange membrane 20
interposed between two electrodes, namely, anode 21 and cathode 22.
In conventional fuel cells, anode 21 and cathode 22 comprise a
substrate of porous electrically conductive sheet material 23 and
24, respectively, for example, carbon fiber paper or carbon cloth.
Each substrate has a thin layer of electrocatalyst 25 and 26,
respectively, disposed on one surface thereof at the interface with
membrane 20 to render each electrode electrochemically active.
[0035] As further shown in FIG. 1, anode flow field plate 14 has at
least one fuel flow channel 14a formed in its surface facing anode
21. Similarly, cathode separator plate 16 has at least one oxidant
flow channel 16a formed in its surface facing cathode 22. When
assembled against the cooperating surfaces of electrodes 21 and 22,
channels 14a and 16a form the reactant flow field passages for the
fuel and oxidant, respectively. The flow field plates 14, 16 are
electrically conductive. Coolant channels (not shown) may also be
formed on the flow field plate 14, 16 (typically on the other side
of the surface having the reactant flow channels) to provide
passages for flow of a coolant therethrough.
[0036] Turning now to FIG. 2, a fuel cell stack 100 includes a
plurality of fuel cell assemblies, a series of which is designated
as 111 in FIG. 2. Each of the fuel cell assemblies includes a
membrane electrode assembly 112 interposed between a pair of fluid
flow field plates 114, 116. Fuel cell stack 100 also includes a
first end plate 130 and a second end plate 140.
[0037] Plate 130 includes fluid inlet ports 132, 134, 136 for
introducing fluid fuel, oxidant and coolant streams, respectively,
to the stack 100. Plate 140 includes fluid outlet ports 142, 144,
146 for exhausting fluid fuel, oxidant and coolant streams,
respectively, from the stack 100. The fluid outlet ports 142, 144,
146 are fluidly connected to the corresponding fluid inlet ports
132, 134, 136 via passages within the stack 100.
[0038] The fuel cell assemblies have a series of openings formed
therein, which cooperate with corresponding openings in adjacent
assemblies to form fluid manifolds 152, 154, 156, 162, 164, 166
within the stack 100. The fluid manifolds are each circumscribed by
a sealant material or gasket. In addition, a peripheral seal at the
exterior perimeter of each fuel cell fluidly isolates the interior,
electrochemically active portion of the fuel cell from the external
environment.
[0039] A fuel stream entering the stack 100 via fuel inlet port 132
is directed to the individual fuel flow field plates via manifold
152. After passing through the fuel flow field plate channels, the
fuel stream is collected in manifold 162 and exhausted from the
stack via fuel outlet port 142. Similarly, an oxidant stream
entering the stack 100 via oxidant inlet port 134 is directed to
individual oxidant flow field plates via manifold 154. After
passing through the oxidant flow field plate channels, the oxidant
stream is collected in manifold 164 and exhausted from the stack
via oxidant outlet port 144. A fluid coolant (typically water)
introduced via coolant inlet port 136 is directed to coolant
channels (not shown) in each flow field plate, or to coolant plate
assemblies (not shown) in the stack 100 via manifold 156. The
coolant stream is collected in manifold 166 and exhausted from
stack 100 via coolant outlet port 146. Coolant manifolds 156, 166
may be fitted with a compliant mechanism (not shown), such as tube
cushions or inserts made of closed cell foam, to accommodate the
expansion of freezing water. Tie rods 170 extend between end plates
130 and 140 to compress and secure stack 100 in its assembled state
with fastening nuts 172 disposed at opposite ends of each tie rod
170, and disc springs 174 interposed between the fastening nuts 172
and end plates 130, 140.
[0040] Each fuel cell 10 in stack 100 can operate satisfactorily
only when sufficient water is provided to keep membrane 20 wet and
ionically conductive. Water may be introduced in the reactant
streams and is produced in the electrochemical reaction at the
cathode 22. If the theoretical maximum quantity of water escaping
from fuel cell 10 in vapor form via the outlet reactant streams
(assuming exhaust gases are saturated with water vapor) is greater
than the water quantity introduced and produced, MEA dehydration
will tend to occur. During operation, it is important to provide
adequate humidification to the MEA, so as to avoid dehydrating the
membrane. Mathematically this condition is expressed as:
N.sub.w,o,in+N.sub.w,f,in+N.sub.w,p.gtoreq.N.sub.w,o,out+N.sub.w,f,out
(1)
[0041] wherein
[0042] N.sub.w,o,in is the inlet oxidant molar water flow rate;
[0043] N.sub.w,f,in is the inlet fuel molar water flow rate;
[0044] N.sub.w,p is the produced water molar flow rate;
[0045] N.sub.w,o,out is outlet oxidant molar water flow rate;
[0046] N.sub.w,f,out is the outlet fuel molar water flow rate.
[0047] Equation 1 terms are given by: 1 N w , o , in = v o o iA 4 F
RH o , in p s , i , in ( p o , in - RH o , in p s , o , in ) ( 2 )
N w , f , in = v f f iA 2 F RH f , in p s , f , in ( p f , in - RH
f , in p s , f , in ) ( 3 ) N w , p = iA 2 F ( 4 ) N w , o , out =
( v o o iA 4 F - iA 4 F ) RH o , out p s , o , out ( p o , out - RH
o , out p s , o , out ) ( 5 ) N w , f , out = ( v f f iA 2 F - iA 2
F ) RH f , out p s , f , out ( p f , out - RH f , out p s , f , out
) ( 6 )
[0048] wherein,
[0049] A is the geometric active surface area;
[0050] i is the current density;
[0051] F is the Faraday constant;
[0052] p.sub.f,in is the inlet fuel pressure;
[0053] p.sub.f,out is the outlet fuel pressure;
[0054] p.sub.o,in is the inlet oxidant pressure;
[0055] p.sub.o,out is the outlet oxidant pressure;
[0056] p.sub.s,f,in is the inlet fuel water vapor saturation
pressure;
[0057] p.sub.s,f,out is the outlet fuel water vapor saturation
pressure;
[0058] p.sub.s,o,in is the inlet oxidant water vapor saturation
pressure;
[0059] p.sub.s,o,out is the outlet oxidant water vapor saturation
pressure;
[0060] RH.sub.f,in is the inlet fuel relative humidity;
[0061] RH.sub.f,out is the outlet fuel relative humidity;
[0062] RH.sub.o,in is the inlet oxidant relative humidity;
[0063] RH.sub.o,out is the outlet oxidant relative humidity;
[0064] .nu..sub.f is the fuel stoichiometry;
[0065] .nu..sub.o is the oxidant stoichiometry;
[0066] .phi..sub.f is the hydrogen volume fraction in the dry
fuel;
[0067] .phi..sub.o is the oxygen volume fraction in the dry
oxidant.
[0068] Equations 5 and 6 can be somewhat simplified when it is
realized that for outlet relative humidities lower than 100%, the
MEA will be subjected to dehydrating conditions. Therefore, outlet
relative humidities of 100% represent a limiting case defining a
boundary between drying and wetting conditions (assuming that the
exhaust gases are saturated with water vapor). By introducing
equations (2) to (6) in equation (1) and simplifying with
RH.sub.o,out=1 and RH.sub.f,out=1, the following equation is
obtained: 2 v o 2 o RH o , in p s , o , in ( p o , in - RH o , in p
s , o , in ) + v f f RH f , in p s , f , in ( p f , in - RH f , in
p s , f , in ) + 1 1 2 ( v o o - 1 ) p s , o , out ( p o , out - p
s , o , out ) + ( v f f - 1 ) p s , f , out ( p f , out - p s , f ,
out ) ( 7 )
[0069] The water vapor saturation pressure is computed using a
temperature dependent empirical equation:
log
p.sub.s=-2.1794+0.02953T-9.1837.times.10.sup.-5T.sup.2+1.4454.times.10-
.sup.-7T.sup.3 (8)
[0070] The water vapor saturation pressure at each of the fuel and
oxidant inlets and outlets can thus be determined by measuring the
temperature at each location T.sub.f,in, T.sub.f,out, T.sub.o,in,
T.sub.o,out. Generally, these temperatures are closely related to
the inlet and outlet coolant temperatures (T.sub.c,in, T.sub.c,out)
which in practice are easier to accurately measure due to the
larger heat capacity of the coolant.
[0071] Each of the variables in equation (7) represent an operating
parameter of fuel cell stack 100. As written, equation (7) defines
an operating condition that produces a wetting condition (net
influx of water into cell) or water balance (equality in equation
(7)). A "water balance" is defined as a balance between water
influx and water outflux when the outflux is calculated with the
assumption that the exhaust is saturated with water vapor. If the
equation was rewritten so that the left side is less than the right
side, the equation defines an operating condition that produces a
drying condition (net outflux of water).
[0072] While it is generally desirable to operate fuel cell 10
under a wetting condition such that membrane 20 is properly
hydrated at all times, excess accumulated water in MEA 12 is not
desired if the stack 100 is to be cold started at or below
0.degree. C. after the stack has been exposed to freezing
conditions for an extended period of time. "Excess water" is hereby
defined as the amount of water exceeding the minimum required to
keep the membrane adequately ionically conductive ("critical
membrane moisture level"). While some water in MEA 12 is needed to
keep membrane 20 moist, excess water in MEA 12 will accumulate in
pores of substrates 23, 24 and in flow channels 14a, 16a and will
eventually freeze when the stack is exposed to temperatures below
0.degree. C. for prolonged periods. If the stack is started before
the MEA 12 has a chance to thaw, ice in pores of substrates pores
23, 24 may block or impede the flow of reactant that must pass
through substrate 23, 24 and to membrane 20 in order for the
electrochemical reaction to proceed. Furthermore, ice accumulation
may cause mechanical stresses inside fuel cell 10 that can cause
damage to stack 100. It is theorized that reducing the quantity of
excess water accumulated in flow channels 14a, 16a and in the
substrate pores of MEA 12 before stack 100 freezes, will reduce
reactant flow blockage caused by ice, and thus reduce the time
required for stack 100 to reach a nominal operating state after a
cold start-up from below 0.degree. C. or improve cell performance
at sub 0.degree. C. temperatures. This can be achieved by operating
each fuel cell 10 in stack 100 under a drying condition for a
period of time that is sufficient to remove at least some excess
water from MEA 12 but not excessively dry out membrane 20.
[0073] As shown in equation (7) a number of operating parameters
can be adjusted to change the operating condition of fuel cell 10,
including, oxidant and fuel stoichiometries, compositions (that is,
volume fraction in reactant stream), relative humidities,
pressures, temperatures, and relative flow configurations (for
example, concurrent and counter-flow operation). One or more of
these parameters can be adjusted so that fuel cell operation is
changed from a wetting condition to a drying condition or to a
water balance.
[0074] A series of tests were performed to verify the MEA water
flux equations (1) through (7) set out above. All tests were
performed using a Ballard Mk 513 single cell having a catalyst
loading of 0.3 mg Pt/cm.sup.2, an N112 Nafion.RTM. membrane, and
Toray CFP TGP-H-90 electrode substrates, and under the following
common operating parameters: 80.degree. C. coolant outlet, a
temperature gradient of +10.degree. C. (temperature difference
between inlet and outlet coolant temperatures) at a current density
of 1 A/cm.sup.2, air/methanol reformate (63.5% H.sub.2), 4% air
bleed, 2.5 bara fuel pressure and 100% fuel inlet relative humidity
(RH). The air inlet pressure, oxidant/fuel stoichiometries, and
nominal current densities differed between each test. In each test,
the fuel cell was first operated under a wetting condition for a
period of time sufficient for the fuel cell to produce a steady
state voltage. Then, the air inlet relative humidity of the fuel
cell was reduced from 100% to 0% and the performance of the fuel
cell was monitored by measuring the fuel cell resistance and
voltage.
[0075] FIG. 3 is a dimensionless representation of equation (7).
The x and y axes represent the right hand and left hand sides of
equation (7) respectively and the dashed line indicates an equality
in equation (7) (water balance). The dashed line therefore
separates the graph into a wetting region (y>x), and a drying
region (x>y). The operating conditions of the fuel cell in each
of the three test runs as theoretically derived from equations (1)
through (7) are plotted in FIG. 3 as triangles, squares and
circles, respectively. Filled symbols indicate a test being run at
100% air inlet relative humidity and unfilled symbols indicate a
test being run at 0% air inlet relative humidity. It can be seen
that the change in relative humidity for each of the three test
runs shifted the fuel cell operation from the wetting region to
either another location in the wetting region closer to water
balance (test 1), or into the drying region (tests 2 and 3). The
fuel cell was then operated under each of the three test
conditions; in each test, the air inlet relative humidity was
switched from 100% to 0% at Time=0. The cell responses to the
change in relative humidity were recorded by measuring both MEA
voltage and resistance and are plotted in FIG. 4. Cell performance
is expected to drop as the membrane dries out; an increase in cell
resistance and a decrease in cell voltage likely indicates that the
fuel cell is being dehydrated.
[0076] The first test (illustrated as triangles in FIGS. 3 and 4)
was conducted at 0.542 A/cm.sup.2 current density, 2.6 bara oxidant
pressure, and 1.5/1.3 oxidant/fuel stoichiometry. The first test
was designed to shift the fuel cell from a point in the wetting
region to another point in the wetting region that is closer to the
water balance (dashed line in FIG. 3) upon the change in inlet RH
(at Time=0). A small change in cell voltage and resistance (0.06
m.OMEGA. and 15 mV respectively) was found after the oxidant
relative humidity was reduced from 100% to 0% at T=0. Since the
cell was in theory still operating within the wetting region after
the RH was changed, no significant reduction in cell performance
was expected due to membrane drying. However, the operating points
plotted in FIG. 3 have uncertainties attached to them, and it is
possible that the first test run was in fact operating just within
the drying region or that the cell performance was affected by mass
transfer or other effects.
[0077] The second test (illustrated by squares in FIGS. 3 and 4)
was conducted at 0.312 A/cm.sup.2 current density, 1.5 bara oxidant
pressure, and 1.5/1.3 oxidant/fuel stoichiometry. The second test
was designed to shift the fuel cell from the wetting region to just
inside the drying region. As expected, the MEA resistance
significantly increased and the MEA voltage significantly decreased
(0.25 m.OMEGA. and 68 mV respectively) then appeared to reach a
steady state after about 4 hours. According to the water balance
equations, this steady state is predicted to be only apparent, and
eventually, the MEA should continue to dehydrate and eventually
fail.
[0078] The third test (illustrated by circles in FIGS. 4 and 5) was
conducted at 0.021 A/cm.sup.2 current density, 1.1 bara oxidant
pressure, and 5/2 oxidant/fuel stoichiometry. As shown in FIG. 3,
the operating condition of the fuel cell after the relative
humidity was reduced to 0% is deeper inside the drying region than
the first two test cases, and thus, a greater drying was expected.
This expectation was confirmed, as cell performance was found to
drop faster and by a greater magnitude than in the first two
tests.
[0079] If the fuel cell is operated under a drying condition to
remove excess water therein, the drying operation should be stopped
before the membrane water level falls below its critical moisture
level. As the membrane dries, and especially after the membrane
water level falls below its critical moisture level, the internal
fuel cell resistance increases and the voltage output decreases
significantly. To ensure that the drying operation does not cause
the membrane to fall below the critical moisture level, the fuel
cell resistance and voltage are preferably monitored during the
drying operation. The drying operation is preferably stopped once
the resistance has increased to or above a threshold level (or the
voltage has decreased below a threshold level).
[0080] This threshold level can be determined empirically as
follows. First, a fuel cell (or stack) is operated normally (under
a wetting condition) and then under a drying condition and its
resistance (and/or voltage)/time curve is determined. The fuel cell
or stack is then frozen and restarted at a sub 0.degree. C.
temperature under a normal (wetting) operating condition, and the
initial performance (before the stack temperature exceeds 0.degree.
C.) of the fuel cell or stack is measured. If there is a
degradation in initial performance, it can be concluded that the
membrane was dried beyond its critical moisture level, and that the
drying time has to be shortened (or the rate of drying reduced).
Progressively shorter periods of drying times can be tested until a
drying time (and corresponding resistance) is found that does not
dry out the membrane such that the initial cold start-up
performance is degraded. With enough empirical testing, a database
can be compiled for appropriate drying times and rates for various
operating conditions.
[0081] An example of a resistance/time curve is shown in FIG. 5.
The resistance of a Ballard fuel cell stack (10 cell) was monitored
during a drying operation. The stack was initially operated at
steady state producing 300 A with an air/fuel stoichiometry ratio
of 1.8/1.2 and at a stack temperature of 70.degree. C. The inlet
oxidant and fuel streams were humidified by passing same through a
humidifier upstream of the stack. At time=0, humidification of the
oxidant and fuel streams was stopped and the external load was
disconnected from the stack. At about 70.degree. C., a drying
operation was then carried out in which the unhumidified oxidant
and fuel streams continued to flow through the stacks at 89/25 slpm
at 0.6 barg for 120 seconds. A relatively linear but small increase
in resistance from about 3 m.OMEGA. to about 5 m.OMEGA. was
observed after 60 seconds; a steeper increase in slope was observed
at around 90 seconds and continued in a generally linear fashion
until the drying operation was stopped; the resistance measured at
the end of 120 seconds was 12 m.OMEGA..
[0082] A series of shutdown and cold start tests was also performed
on the stack, the resistance of the stack after each drying
operation was measured. The stack was initially operated at steady
state producing 300 A with an air/fuel stoichiometry ratio of
1.8/1.2 and at a stack temperature of 70.degree. C. The inlet
oxidant and fuel streams were humidified by passing same through a
humidifier upstream of the stack. At time=0, humidification of the
oxidant and fuel streams was stopped and the external load was
disconnected from the stack. At about 70.degree. C., a drying
operation was then carried out in which the unhumidified oxidant
and fuel streams continued to flow through the stacks at a fuel/air
rate of 25/89 slpm (for 10 cells) at 0.6 barg. A drying operation
was applied for each test run for different time lengths and the
corresponding stack resistance was measured at the end of the
drying operation, as follows: 12 m.OMEGA. (test 1), 7.2 m.OMEGA.
(test 2), 6.23 m.OMEGA. (test 3), 5.2 m.OMEGA. (test 4), and 5.99
m.OMEGA. (test 5). The stack was then allowed to cool to about
20.degree. C. and was subjected to a second drying operation of
unhumidified fuel and air flow at a fuel/air rate of 25/89 slpm (10
cell) and at 0.6 barg for about 1 minute.
[0083] The stack was then cooled to about -10.degree. C. and held
at that temperature. Thereafter, the stack was started at about
-10.degree. C. and the resistance was measured for each test run as
follows: 16 m.OMEGA. (test 1), 10 m.OMEGA. (test 2), 9.23 m.OMEGA.
(test 3), 6.2 m.OMEGA. (test 4), and 7.89 m.OMEGA. (test 5).
Current was varied in steps of 5 A between a range of 5 and 50 A
for about 10 seconds per step and the cell voltage at each current
was measured. It was observed that the higher the measured stack
resistance (both at shutdown and at start-up), the lower the
measured cell voltage, that is, the worse the initial cold start
performance, that is, the performance of the cell below 0.degree.
C. It is theorized that the performance losses correlate with the
degree of MEA dryness prior to freezing, which is dependent on the
parameters of the drying operation during shut down.
[0084] FIG. 6 is a schematic diagram of a fuel cell electric power
generation system 200 comprising a fuel cell stack 210 according to
one embodiment of the invention. Fuel cell stack 210 includes
negative and positive bus plates 212, 214, respectively, to which
an external circuit comprising a variable load 216 is electrically
connectable by closing switch 218. System 200 includes a fuel
(hydrogen) circuit, an oxidant (air) circuit, and a coolant water
circuit. The reactant and coolant streams are circulated in the
system in various conduits illustrated schematically in FIG. 6.
[0085] During normal operation, a hydrogen supply 220 is humidified
in humidifier 270 then delivered to stack 210 via hydrogen conduit
261. Flow through conduit 261 is controlled by hydrogen pressure
regulator 221. Hydrogen delivery pressure is measured by pressure
sensor 271. If humidification of the hydrogen stream is not
desired, hydrogen flow may be bypassed around humidifier 270
through three-way valve 272 connected to conduit 261 upstream of
humidifier 270, through hydrogen bypass conduit 274 connected to
valve 272, and through a three-way bypass valve 276 connected to
conduit 261 downstream of humidifier 270. Flow through bypass
conduit 274 is controlled by hydrogen pressure regulator 278.
Alternatively, and for certain types of humidifiers, the humidifier
may be bypassed by reducing or stopping the transfer of water to a
reactant stream passing through the humidifier.
[0086] Water in the hydrogen exhaust stream exiting stack 210 is
accumulated in a knock drum 222, which can be drained by opening
valve 223. Unreacted hydrogen is recirculated to stack 210 by a
pump 224 in recirculation loop 225. The relative humidity of the
hydrogen exhaust stream is measurable by relative humidity sensor
280.
[0087] During normal operation, air (oxidant) is humidified in
humidifier 270 then delivered to stack 210 via oxidant
humidification conduit 262. Conduit 262 has an inlet end
connectable to a compressor 230 and an outlet end connected to fuel
cell stack 210. Flow through humidification conduit 262 is
controlled by oxidant pressure regulator 231. Oxidant flow rate is
measured by mass flow sensor 282 and oxidant pressure is measured
by pressure sensor 284. If humidification of the oxidant stream is
not desired, oxidant flow may be bypassed around humidifier 270
through a three-way valve 288 connected to conduit 262 upstream of
humidifier 270, through oxidant bypass conduit 286 connected to
valve 288, and through a three-way bypass valve 266 connected to
conduit 286 downstream of humidifier 270. Flow through bypass
conduit 286 is controlled by oxidant pressure regulator 290.
[0088] Water in the oxidant exhaust stream exiting stack 210 is
accumulated in reservoir 232, which can be drained by opening valve
233, and the air stream is vented from the system via valve 234.
The relative humidity of the air exhaust stream is measured by
relative humidity sensor 291.
[0089] In coolant water loop 240, water is pumped from reservoir
232 and circulated through stack 210 by pump 241. The temperature
of the water is adjusted in a heat exchanger 242. The coolant inlet
and outlet temperatures are measured by temperature sensors 292,
294.
[0090] The cold start capability and freeze tolerance of the system
200 can be improved by reducing the amount of water remaining
within the flow channels 14a and 16a, and in the electrodes of the
MEA of each fuel cell in the stack 210 upon cessation of operation
and reduction of the stack core temperature to near or below the
freezing temperature of water. As used herein, "freeze tolerance"
refers to the ability of a fuel cell or fuel cell stack to maintain
substantially the same performance after one or more
freeze/thaw/cold start cycles, where the stack after being shut off
is exposed to sub 0.degree. C. temperatures for an extended period
of time then is cold started below 0.degree. C. or is thawed above
0.degree. C. then started.
[0091] On shutdown, the operating parameters of fuel cell stack 210
are selected so that stack 210 operates under a drying condition
until the voltage drops below (or resistance increases above) a
threshold level. A number of different operating parameters may be
adjusted to change the operation of stack 210 from a wetting
condition to a drying condition, such as air or fuel
stoichiometries, temperatures, pressures, compositions, and
relative humidities. A suggested shutdown sequence comprising a
drying operation is as follows:
[0092] (a) receive shutdown instructions;
[0093] (b) turn off coolant pump 241 so that coolant flow is
stopped (increases stack operating temperature);
[0094] (c) actuate bypass valves 272, 276, 288 and 266 so that
reactant supply to stack bypasses humidifier 270 (reduces the
reactant inlet relative humidities);
[0095] (d) adjust compressor operation to decrease oxidant supply
pressure;
[0096] (e) adjust fuel pressure regulator 278 to decrease fuel
inlet pressure;
[0097] (f) once stack resistance has exceeded (or the voltage has
decreased below) a predetermined threshold value, shut off
compressor 230 and close valves 221, 231, 278 and 290 (shuts off
the fuel and oxidant supplies to the stack);
[0098] (g) shut off hydrogen recirculation pump 224; and
[0099] (h) open switch 218 (disconnects the stack from the external
circuit).
[0100] Steps (b) to (h) should be completed before the stack 210
overheats. Empirical testing can be performed to determine the
maximum period of time for performing these steps before
overheating occurs. Alternatively or in addition, the stack
temperature can be monitored during the shut down operation; if the
stack gets too hot, the coolant pump 241 can be reactivated.
[0101] System 200 illustrated in FIG. 6 has a number of sensors to
monitor various operating parameters during stack operation,
including relative humidity sensors 280 and 291 located in the
exhaust conduits downstream of the stack, reactant supply pressure
sensors 271, 284, and inlet and outlet coolant temperature sensors
292 and 294. While these sensors are sufficient to carry out the
drying operation (b) to (h) described above, additional sensors
(not shown) are required if data for all the variables specified in
the water flux equations (1) to (8) are desired, for example, mass
flow sensors for the oxidant and fuel supplies, fuel and oxidant
relative humidity sensors upstream of stack 100, fuel and oxidant
pressure sensors downstream of the stack, stack current sensor, and
oxygen and hydrogen concentration sensors. These sensors may be
useful to precisely monitor the water flux in and out of the cell,
so that a water management program can be carried out during stack
operation to prevent excess water from building up in the stack. By
carrying out such a water management program, the amount of water
remaining at shutdown can be reduced, thereby reducing the need for
drying operation. Such a water management program may also improve
system performance and efficiency during operation, as the mass
transport limitations associated with excess water accumulated in
the fuel cell will be reduced.
[0102] System 200 as illustrated in FIG. 6 is supplied with air
from the compressor 230 and with pure hydrogen from a pressurized
hydrogen tank 220. For greater output voltages, it is advantageous
to supply fuel cells with more concentrated reactant streams and
preferably with pure reactant streams (for example, pure hydrogen
and oxygen reactants). This is an advantage because the presence of
relatively large amounts of non-reactive components in the reactant
streams can significantly increase kinetic and mass transport
losses in the fuel cells. However, in certain applications it may
be impractical to store and provide the desired reactants in pure
form. In this connection, hydrogen may be supplied to system 200 by
reforming a supply of methanol, natural gas, or the like on site or
on board (not shown).
[0103] The reformed hydrogen stream tends to contain some carbon
dioxide generated as a result of the reforming operation. Air
typically has a oxygen concentration of about 21%; the major
component in the dilute oxidant air stream is nitrogen. Known
approaches may be implemented in system 200 to increase the
concentration of the reactant in the reformed fuel and/or air
streams, that is, enrichment, to improve the performance of system
200. Such known approaches typically involve separating out a
component from the reactant stream, including cryogenic, membrane,
and pressure swing adsorption methods. In a cryogenic method,
component separation is achieved by preferentially condensing a
component out of a gaseous stream. In a membrane method, component
separation is achieved by passing the stream over the surface of a
membrane that is selectively permeable to a component in the
stream. In a pressure swing adsorption (PSA) method, a gas
component is separated from a gas stream by preferential adsorption
onto a suitable adsorbent under pressure. A PSA apparatus (not
shown) may be installed on the fuel supply conduit 261 between the
fuel supply 220 and the stack 210 to provide an enriched fuel
stream to stack 210. The PSA apparatus may also be installed on the
oxidant supply conduit 262 between the air compressor 230 and the
stack 210 to provide an enriched oxidant stream to stack 210. By
controlling the degree of enrichment provided by the PSA apparatus,
the fuel and oxidant concentrations can be controlled (.phi..sub.f,
.phi..sub.o) to encourage the stack to operate under a drying
condition during shut down.
[0104] System 200 shown in FIG. 6 may be configured so that the
oxidant and fuel stream pass through stack 210 in a concurrent flow
arrangement. According to another embodiment of the invention, one
of the fuel and oxidant streams may be reversed so that the oxidant
and fuel streams pass through the stack 210 in a counter-flow
arrangement (not shown). Such counter-flow arrangement will affect
the temperature gradients in the stack 210. Temperature sensors
(not shown) can be installed in the oxidant and fuel passages to
measure the inlet and outlet oxidant and fuel stream temperatures,
so that the effect of the temperatures on the water balance
formulas can be determined. It may be desirable in embodiments of
the present method, to intermittently reverse the reactant flow
directions during operation or shut down. An example of apparatus
and methods for reversing the relative flow directions of oxidant
and fuel through a fuel cell stack is described in U.S. Pat. No.
5,935,726.
[0105] It should be noted that the stack can be disconnected from
the external circuit prior to starting a drying operation (for
example, between step (a) and (b). In such case, an auxiliary power
source such as a battery (not shown) is provided to power the
various components in the system 200 (for example, air compressor,
pumps, actuators, sensors). After the external circuit has been
disconnected, N.sub.w,p becomes 0 in equation (7) as the
electrochemical reaction for all intents stops and no water is
produced. Substituting dry oxidant and fuel flow rates N.sub.o,in,
N.sub.f,in for oxidant and fuel stoichiometries, and equations (2)
to (6) in equation (1), the following water flux equation is
derived (wetting condition or water balance): 3 N o , in o RH o ,
in p s , o , in ( p o , in - RH o , in p s , o , in ) + N f , in f
RH f , in p s , f , in ( p f , in - RH f , in p s , f , in ) N o ,
in o p s , o , out ( p o , out - p s , o , out ) + N f , in f p s ,
f , out ( p f , out - p s , f , out ) ( 9 )
[0106] Note that the primary difference between equations (7) and
(9) is that the net water influx is reduced by elimination of the
water production term N.sub.w,p and that the reactant flow rates
cannot be defined in terms of stoichiometries, since current is 0.
Using equation (9), operating parameters can be determined that
will cause the stack to operate under a drying condition; equation
(9) can be verified by empirical testing using the same test
methods that were applied to test equation (7).
[0107] In a shutdown procedure where the external circuit is
disconnected before a drying operation is performed, the voltage
measured will be the open circuit voltage (or open circuit
resistance if resistance is measured). Empirical testing can be
performed to determine at what voltage drop (or resistance
increase) should the drying operation be stopped.
[0108] While particular elements, embodiments and applications of
the present invention have been shown and described, it will be
understood, of course, that the invention is not limited thereto
since modifications may be made by those skilled in the art without
departing from the scope of the present disclosure, particularly in
light of the foregoing teachings.
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