U.S. patent application number 12/636343 was filed with the patent office on 2011-06-16 for fuel cell operational methods for oxygen depletion at shutdown.
This patent application is currently assigned to GM GLOBAL TECHNOLOGY OPERATIONS, INC.. Invention is credited to Abdullah B. Alp, Steven G. Goebel, Balasubramanian Lakshmanan, Joseph Nicholas Lovria, Gary M. Robb, Thomas W. Tighe.
Application Number | 20110143241 12/636343 |
Document ID | / |
Family ID | 44143318 |
Filed Date | 2011-06-16 |
United States Patent
Application |
20110143241 |
Kind Code |
A1 |
Tighe; Thomas W. ; et
al. |
June 16, 2011 |
FUEL CELL OPERATIONAL METHODS FOR OXYGEN DEPLETION AT SHUTDOWN
Abstract
A method for creating an oxygen depleted gas in a fuel cell
system, including operating a fuel cell stack at a desired cathode
stoichiometry at fuel cell system shutdown to displace a cathode
exhaust gas with an oxygen depleted gas. The method further
includes closing a cathode flow valve and turning off a compressor
to stop the flow of cathode air.
Inventors: |
Tighe; Thomas W.;
(Bloomfield, NY) ; Goebel; Steven G.; (Victor,
NY) ; Robb; Gary M.; (Honeoye Falls, NY) ;
Alp; Abdullah B.; (West Henrietta, NY) ; Lakshmanan;
Balasubramanian; (Pittsford, NY) ; Lovria; Joseph
Nicholas; (Honeoye Falls, NY) |
Assignee: |
GM GLOBAL TECHNOLOGY OPERATIONS,
INC.
Detroit
MI
|
Family ID: |
44143318 |
Appl. No.: |
12/636343 |
Filed: |
December 11, 2009 |
Current U.S.
Class: |
429/428 ;
429/452 |
Current CPC
Class: |
H01M 8/04303 20160201;
H01M 8/04753 20130101; H01M 8/04223 20130101; Y02E 60/50 20130101;
H01M 8/04089 20130101; H01M 8/04798 20130101; H01M 8/04126
20130101; H01M 8/04701 20130101; H01M 8/04231 20130101; H01M
8/04302 20160201; H01M 8/04559 20130101; H01M 8/04955 20130101;
H01M 8/04395 20130101; Y02T 90/40 20130101; H01M 8/04455 20130101;
H01M 8/04746 20130101; H01M 2250/20 20130101 |
Class at
Publication: |
429/428 ;
429/452 |
International
Class: |
H01M 8/00 20060101
H01M008/00 |
Claims
1. A method for creating an oxygen depleted gas in a fuel cell
system, said method comprising: determining that the fuel cell
system has been shutdown; setting a cathode air flow; applying a
load to consume power generated by a fuel cell stack; providing a
desired cathode stoichiometry by adjusting stack current or the
load to use stack voltage as an indication of the cathode
stoichiometry so as to cause oxygen to be depleted from the cathode
air in the fuel cell stack; operating the fuel cell system at the
desired voltage and the desired cathode stoichiometry so as to
create a volume of oxygen depleted gas in at least the cathode side
of the fuel cell stack and in the cathode exhaust; and closing a
cathode exhaust valve and shutting off the compressor.
2. The method according to claim 1 wherein operating the fuel cell
system at the desired cathode stoichiometry includes operating the
fuel cell stack down to a cathode stoichiometry of approximately 1
so as to create a volume of oxygen depleted gas in the fuel cell
stack and the cathode exhaust.
3. The method according to claim 1 further comprising measuring the
cathode flow in the stack to calculate the volume of oxygen
depleted gas created while operating the fuel cell system at the
desired cathode stoichiometry.
4. The method according to claim 1 wherein achieving the desired
cathode stoichiometry includes determining the concentration of
oxygen in the cathode exhaust.
5. The method according to claim 1 further comprising adding
hydrogen to a flow of cathode air so as to control the cathode
stoichiometry.
6. The method according to claim 1 wherein creating a volume of
oxygen depleted gas in the cathode side of the stack and in the
cathode exhaust further includes feeding the oxygen depleted gas to
the anode side of the stack.
7. The method according to claim 1 further comprising providing a
higher resolution secondary controller to fix the stack load.
8. A method for creating an oxygen depleted gas in a fuel cell
system, said method comprising: determining that the fuel cell
system can be shutdown; cooling a fuel cell stack; adjusting a
cathode exhaust valve and a compressor to increase the pressure on
a cathode side of the fuel cell stack and applying a shutdown load
to achieve a desired voltage and a desired cathode stoichiometry;
operating the fuel cell system at the desired voltage and the
desired cathode stoichiometry so as to create a volume of oxygen
depleted gas in the cathode side of the fuel cell system, a water
vapor transfer unit and a water vapor transfer unit by-pass line;
closing the cathode exhaust valve and water vapor transfer unit
by-pass valves and shutting off the compressor; and closing a
cathode inlet valve when the pressure of the oxygen depleted gas on
the cathode side drops to approximately ambient pressure.
9. The method according to claim 8 further comprising feeding the
oxygen depleted gas created in the cathode side to the anode side
of the fuel cell stack.
10. The method according to claim 8 wherein the desired cathode
stoichiometry is approximately 1.
11. The method according to claim 8 further comprising calculating
the volume of oxygen depleted gas created while operating the fuel
cell system at the desired voltage.
12. The method according to claim 8 further comprising adding
hydrogen after the stack is cooled to maximize the hydrogen
available in the fuel cell stack.
13. A method for creating an oxygen depleted gas in a fuel cell
system, said method comprising: operating a fuel cell stack at a
cathode stoichiometry of approximately 1 at fuel cell system
shutdown to displace a cathode exhaust gas with an oxygen depleted
gas; and closing a cathode flow valve and turning a compressor off
to stop the flow of cathode air.
14. The method according to claim 13 wherein the cathode
stoichiometry of approximately 1 is achieved by adjusting a load
and maintaining cathode air flow at a fixed flow.
15. The method according to claim 14 wherein the initial load is
higher to consume adsorbed oxygen.
16. The method according to claim 13 wherein the cathode
stoichiometry of approximately 1 is achieved by adjusting cathode
air flow and maintaining a fixed load.
17. The method according to claim 16 wherein the cathode air flow
is adjusted by adjusting a cathode backpressure valve.
18. The method according to claim 16 wherein the cathode air flow
on a net oxygen basis is adjusted by adjusting the amount of
hydrogen flowing into a cathode input line.
19. The method according to claim 16 wherein the cathode air flow
is adjusted by adjusting the speed of the compressor.
20. The method according to claim 13 wherein fuel cell stack
voltage, the voltage of one or more fuel cells, or a stack current
sensor is used as a feedback to control cathode stoichiometry.
21. The method according to claim 13 wherein cathode gas
concentration or a cathode flow meter is used as a feedback to
control cathode stoichiometry.
22. The method according to claim 13 further comprising displacing
the oxygen in the cathode exhaust gas at elevated pressure and
closing a backpressure valve upon completion of the displacement of
the cathode exhaust gas with oxygen depleted gas.
23. The method according to claim 13 further comprising injecting
hydrogen into the cathode exhaust after the displacement of the
oxygen in the cathode exhaust gas so as to cause oxygen depleted
gas to back-flow into the fuel cell stack and upstream volumes.
24. The method according to claim 13 further comprising closing a
cathode inlet valve after the cathode exhaust is displaced with
oxygen depleted gas and the compressor is turned off.
25. The method according to claim 13 further comprising introducing
hydrogen to a cathode side of the fuel cell stack while displacing
the cathode exhaust gas with oxygen depleted gas.
26. The method according to claim 13 further comprising injecting
hydrogen into an anode or a cathode side of the fuel cell stack
after displacing the cathode exhaust gas with oxygen depleted gas
to impede oxygen from entering the stack.
27. The method according to claim 13 further comprising closing
cathode inlet and cathode outlet valves after displacing the
cathode exhaust gas with oxygen depleted gas to impede oxygen from
entering the stack.
28. The method according to claim 13 further comprising applying a
brief load after closing the cathode flow valve and turning the
compressor off so as to consume any oxygen remaining in the fuel
cell stack.
29. The method according to claim 13 further comprising flushing an
anode side of the fuel cell stack with oxygen depleted gas.
30. The method according to claim 13 further comprising cooling the
fuel cell stack prior to shutting off the compressor to limit gas
contraction and water vapor condensation.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] This invention relates generally to a system and method for
depleting the oxygen in a fuel cell stack and, more particularly,
to a system and method for creating a volume of oxygen depleted gas
throughout as much of the cathode sub-system as possible.
[0003] 2. Discussion of the Related Art
[0004] Hydrogen is a very attractive fuel because it is clean and
can be used to efficiently produce electricity in a fuel cell. A
hydrogen fuel cell is an electro-chemical device that includes an
anode and a cathode with an electrolyte therebetween. The anode
receives hydrogen gas and the cathode receives oxygen or air. The
hydrogen gas is dissociated at the anode catalyst to generate free
protons and electrons. The protons pass through the electrolyte to
the cathode. The protons react with the oxygen and the electrons at
the cathode catalyst to generate water. The electrons from the
anode cannot pass through the electrolyte, and thus are directed
through a load to perform work before being sent to the
cathode.
[0005] Proton exchange membrane fuel cells (PEMFC) are a popular
fuel cell for vehicles. The PEMFC generally includes a solid
polymer electrolyte proton conducting membrane, such as a
perfluorosulfonic acid membrane. The anode and cathode electrodes,
or catalyst layers, typically include finely divided catalytic
particles, usually platinum (Pt), supported on carbon particles and
mixed with an ionomer. The catalytic mixture is deposited on
opposing sides of the membrane. The combination of the anode
catalytic mixture, the cathode catalytic mixture and the membrane
define a membrane electrode assembly (MEA). Each MEA is usually
sandwiched between two sheets of porous material, the gas diffusion
layer (GDL), that protects the mechanical integrity of the membrane
and also helps in uniform reactant humidity diffusion. MEAs are
relatively expensive to manufacture and require certain conditions
for effective operation.
[0006] Several fuel cells are typically combined in a fuel cell
stack to generate the desired power. For example, a typical fuel
cell stack for a vehicle may have two hundred or more stacked fuel
cells. The fuel cell stack receives a cathode input gas, typically
a flow of air forced through the stack by a compressor. The fuel
cell stack also receives an anode hydrogen input gas that flows
into the anode side of the stack. Not all of the oxygen is consumed
by the stack and some of the air is output as a cathode exhaust gas
that may include water as a by-product of the chemical reaction
taking place in the stack.
[0007] The fuel cell stack includes a series of bipolar plates
positioned between the several MEAs in the stack, where the bipolar
plates and the MEAs are positioned between two end plates. The
bipolar plates include anode side and cathode side flow
distributors, or flow fields, for adjacent fuel cells in the stack.
Anode gas flow channels are provided on the anode side of the
bipolar plates that allow the anode reactant gas to flow to the
respective MEA. Cathode gas flow channels are provided on the
cathode side of the bipolar plates that allow the cathode reactant
gas to flow to the respective MEA. One end plate includes anode gas
flow channels, and the other end plate includes cathode gas flow
channels. The bipolar plates and end plates are made of a
conductive material, such as stainless steel or a conductive
composite. The end plates conduct the electricity generated by the
fuel cells out of the stack. The bipolar plates also include flow
channels through which a cooling fluid flows.
[0008] Water is generated as a by-product of the stack operation,
therefore, the cathode exhaust gas from the stack will typically
include water vapor and liquid water. It is known in the art to use
a water vapor transfer (WVT) unit to capture some of the water in
the cathode exhaust gas, and use the water to humidify the cathode
input airflow. Water in the cathode exhaust gas at one side of the
water transfer elements, such as membranes, is absorbed by the
water transfer elements and transferred to the cathode air stream
at the other side of the water transfer elements.
[0009] When a fuel cell system is shut down, unreacted hydrogen gas
remains in the anode side of the fuel cell stack. This hydrogen gas
is able to diffuse through or cross over the membrane and react
with the oxygen in the cathode side. As the hydrogen gas diffuses
to the cathode side, the total pressure on the anode side of the
stack is reduced, where it is possible to reduce the pressure below
ambient pressure. This pressure differential can draw air from
ambient into the anode side of the stack. It is also possible for
air to enter the anode by diffusion from the cathode. When the air
enters the anode side of the stack it can generate air/hydrogen
fronts that creates a short circuit in the anode side, resulting in
a lateral flow of hydrogen ions from the hydrogen flooded portion
of the anode side to the air-flooded portion of the anode side.
This current combined with the high lateral ionic resistance of the
membrane produces a significant lateral potential drop (.about.0.5
V) across the membrane. This produces a local high potential
between the cathode side opposite the air-filled portion of the
anode side and adjacent to the electrolyte membrane that drives
rapid carbon corrosion, and causes the electrode carbon layer to
get thinner. This decreases the support for the catalyst particles,
which decreases the performance of the fuel cell.
[0010] In automotive applications, there are a large number of
start and stop cycles over the life of the vehicle and the life of
the fuel cell system each of which may generate an air/hydrogen
front as described above. An average vehicle can experience 40,000
startup/shutdown cycles over its useful life. Start and stop cycles
are damaging to the fuel cell system due to the potential which may
be generated by an air/hydrogen front, and the best demonstrated
mitigation of damage still causes approximately 2 to 5 .mu.V of
degradation per start and stop cycle. Thus, the total degradation
over the 40,000 start and stop cycle events can exceed 100 mV.
However, by not allowing air to enter the fuel cell stack while the
fuel cell system is shutdown, damage during subsequent restarts may
be reduced or prevented.
[0011] It is known in the art to purge the hydrogen gas out of the
anode side of the fuel cell stack at system shutdown by forcing air
from the compressor into the anode side at high pressure. However,
the air purge creates the air/hydrogen front discussed above that
causes at least some corrosion of the carbon support structure.
[0012] Another known method in the art is to provide a cathode
re-circulation to reduce cathode corrosion at system shutdown, as
described in the commonly owned U.S. non-provisional patent
application titled, "Method for Mitigating Cell Degradation Due to
Startup and Shutdown Via Cathode Re-Circulation Combined with
Electrical Shorting of Stack," U.S. Ser. No. 11/463,622, filed Aug.
10, 2006, which is incorporated herein by reference. Particularly,
it is known to pump a mixture of air and a small amount of hydrogen
through the cathode side of the stack at system shutdown so that
the hydrogen and oxygen combine in the cathode side to reduce the
amount of oxygen, and thus the potential that causes carbon
corrosion.
[0013] It is also known to stop the cathode air flow while
maintaining positive anode side hydrogen pressure at shutdown, and
then to apply a load to the stack to allow the oxygen to be
consumed by hydrogen, followed by closing the inlet and outlet
valves of the anode and cathode sides, as described in the commonly
owned U.S. non-provisional patent application titled, "Method of
Mitigating Fuel Cell Degradation Due to Startup and Shutdown Via
Hydrogen/Nitrogen Storage," U.S. Ser. No. 11/612,120, filed Dec.
18, 2006, which is incorporated herein by reference. While it has
been shown that these techniques do help to mitigate corrosion of
the carbon support, these techniques may not remove all of the
oxygen, especially from the volumes beyond the stack, or add the
complexity of a cathode recycle system. Therefore, there is a need
in the art for an improved or simplified way to prevent oxygen rich
air from being present at start-up of a fuel cell system.
SUMMARY OF THE INVENTION
[0014] In accordance with the teachings of the present invention, a
method for creating an oxygen depleted gas in a fuel cell system is
disclosed. The method includes operating a fuel cell stack at a
desired cathode stoichiometry at fuel cell system shutdown to
displace a cathode exhaust gas with an oxygen depleted gas. The
method further includes closing a cathode flow valve and turning
off a compressor to stop the flow of cathode air.
[0015] Additional features of the present invention will become
apparent from the following description and appended claims, taken
in conjunction with the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0016] FIG. 1 is a schematic block diagram of a fuel cell system;
and
[0017] FIG. 2 is a flow diagram illustrating a non-limiting
embodiment showing an oxygen depletion procedure at fuel cell
system shut-down.
DETAILED DESCRIPTION OF THE EMBODIMENTS
[0018] The following discussion of the embodiments of the invention
directed to a system and method for depleting oxygen in a fuel cell
stack is merely exemplary in nature, and is in no way intended to
limit the invention or its applications or uses.
[0019] FIG. 1 is a schematic block diagram of a fuel cell system 10
including a fuel cell stack 12 having an anode side and a cathode
side. An injector 20 injects hydrogen into the fuel cell stack 12
from a hydrogen source 14 on an anode input line 16. The injector
20 can be any injector, injector/ejector, or bank of injectors
suitable for the purposes described herein. An anode purge valve 22
is provided on the anode side of the stack 12 to purge the anode
with fresh hydrogen, and to receive an oxygen depleted gas, as will
be described in more detail below. In an alternate embodiment, a
blocking valve or a stationary recirculation pump could be used to
restrict the recirculation path.
[0020] In this embodiment, the fuel cell system 10 employs anode
recirculation where an anode recirculation gas is output from the
stack 12 and is recirculated back to the anode input by an anode
recirculation line 54 through the injector 20 to reduce the amount
of hydrogen gas being discharged from the stack 12. Water is
removed from the recirculated anode gas by a water separation
device 56 provided in the anode recirculation line 54. The water
separation device 56 collects and holds water in a manner well
understood to those skilled in the art. A bleed/drain valve 24 is
provided in an anode exhaust gas line 18 and is periodically opened
to drain water from a holding tank within the water separation
device 56, and is also periodically opened to remove nitrogen from
the anode side of the stack 12 based on a schedule well understood
to those skilled in the art. In an alternate embodiment, a separate
bleed valve and drain valve could be used without departing from
the scope of the present invention.
[0021] A compressor 30 provides an air flow to the cathode side of
the fuel cell stack 12 on cathode input line 32 through a water
vapor transfer (WVT) unit 34 that humidifies the cathode input air.
A cathode exhaust gas line 40 directs the cathode exhaust to the
WVT unit 34 to provide the humidity to humidify the cathode input
air. A by-pass line 36 is provided around the WVT unit 34 and a
by-pass valve 38 is provided in the by-pass line 36 and is
controlled to selectively redirect the cathode input air through or
around the WVT unit 34 to provide the desired amount of humidity to
the cathode input air. Alternatively, the cathode by-pass line 36
may be provided around the WVT unit 34 on the cathode exhaust line
40, although not shown in this embodiment. A cathode by-pass line
46 is provided to connect the cathode input line 32 and the cathode
exhaust line 40 to allow air from the compressor 30 to by-pass the
stack 12. A cathode by-pass valve 48 is provided to selectively
control the amount of air flow through the cathode by-pass line 46,
as is described in more detail below. Alternatively, a compressor
recirculation path and valve may be utilized to recirculate air
flow from the compressor outlet back to the compressor inlet,
thereby allowing oxygen to be removed in the cathode input line 32,
which is described in more detail below.
[0022] A connector line 28 is provided to connect the anode input
line 16 and the cathode input line 32 to provide hydrogen to the
cathode side of the stack 12 by selectively controlling hydrogen to
a cathode valve 58. An anode purge line 44 is provided to connect
the cathode input line 32 to the anode purge valve 22 to provide a
path for an oxygen depleted gas to fill the anode side of the stack
12, which is discussed in more detail below.
[0023] A cathode input valve 26 is provided on the cathode input
line 32 to control the flow of air into the stack 12, and a cathode
back-pressure valve 42 is provided in the cathode exhaust gas line
40 to selectively control the flow of cathode exhaust, to increase
the pressure in the cathode side of the stack 12 and to provide a
diffusion limitation during fuel cell system off time, as discussed
in more detail below.
[0024] A variable shutdown load 50 is electrically coupled to the
fuel cell stack 12 to cause oxygen to be consumed by providing a
load across the stack 12 and causing the voltage to reach a
predetermined level, which is discussed in more detail below. A
controller 52 is capable of controlling the injector 20, the anode
purge valve 22, the bleed/drain valve 24, the compressor 30, the
cathode input valve 26, the cathode back-pressure valve 42, the
cathode by-pass valve 48 and the by-pass valve 38. The controller
52 is also capable of calculating or estimating the cathode
stoichiometry and the amount of oxygen depleted gas passing through
the cathode exhaust gas line 40, which is discussed in more detail
below.
[0025] As discussed above, there is a need in the art for removing
oxygen within a fuel cell stack upon shutdown to prevent the
occurrence of air/hydrogen fronts during system stop and start
cycles. FIG. 2 is a flow diagram 60 showing a method for depleting
the oxygen in the cathode side upon shutdown of the fuel cell
system 10 for the purpose of preventing the occurrence of
air/hydrogen fronts within the fuel cell stack 12. This method
starts after the controller 52 determines that the fuel cell system
10 has requested a shutdown at box 62. In one non-limiting
embodiment, the controller 52 may determine shutdown has been
requested when a vehicle or the fuel cell system 10 that is part of
a vehicle has been turned off. However, those having skill in the
art will readily recognize that a variety of triggers could
indicate that the fuel cell system 10 has requested a shutdown.
[0026] Upon shutdown of the fuel cell system 10, the fuel cell
stack 12 is operated down to a low cathode stoichiometry and the
pressure on the cathode side is optionally increased at box 64 to
begin to generate a volume of oxygen depleted gas within the stack
12 and the cathode exhaust line 40. As is discussed in more detail
below, stack voltage is a function of cathode stoichiometry,
therefore, a low cathode stoichiometry can be achieved by adjusting
the variable shutdown load 50 value of the stack 12 and/or the
cathode flow by adjusting the cathode back-pressure valve 42, the
cathode by-pass valve 48 and the compressor 30, as is discussed in
more detail below. Oxygen is depleted when the cathode
stoichiometry is low, e.g., a cathode stoichiometry of
approximately 1, because during low cathode stoichiometry the
actual flow rate of the air or oxygen is approximately the same as
the rate of the consumption of oxygen by the fuel cell stack 12 to
generate the desired current. The optimum cathode stoichiometry may
be determined by monitoring the entire stack voltage and adjusting
the load 50 or cathode flow to achieve a predetermined stack
voltage which correlates to a low cathode stoichiometry. The
desired stack voltage to achieve a low cathode stoichiometry will
vary depending on the fuel cell system used and in particular the
number of cells in the stack. In one non-limiting embodiment,
approximately 50 V provides the desired low cathode stoichiometry
when the air flow rate is fixed by setting the speed of the
compressor 30 and adjusting the variable shutdown load 50 value of
the stack 12 and/or by adjusting the cathode back-pressure valve
42, as is discussed in more detail below.
[0027] To generate a volume of oxygen depleted gas, the fuel cell
stack 12 is operated at a low cathode stoichiometry and the
pressure is optionally increased on the cathode side at box 64 as
is described in more detail below. Throughout the method as
described herein, the pressure of the anode reactant side of the
stack is typically maintained high relative to the pressure in the
cathode side of the stack 12. This is done to ensure that the air
does not flow into the anode when the anode bleed valve 24 is
opened, as is apparent to those skilled in the art.
[0028] The cathode stoichiometry may be slowly lowered to avoid a
situation where the fuel cell stack 12 needs more oxygen than is
supplied, i.e., an under stoichiometry condition. The cathode
stoichiometry should not be operated at an under stoichiometry
condition because excess hydrogen will be pumped into the cathode
side of the fuel cell stack 12. More particularly, excess hydrogen
will be pumped into the individual fuel cells of the fuel cell
stack 12 which have a cathode stoichiometry below 1 due to air flow
maldistribution, meaning less air is pumped into certain fuel cells
of the stack 12. When not enough air flow is provided to certain
cells in the stack 12, an under stoichiometry condition exists, and
the protons and electrons driven by the stack current will not have
enough oxygen for reaction to form product water in that cell so
will recombine as hydrogen gas (the above mentioned hydrogen
pumping) which will exit through the cathode exhaust gas line 40.
Excess hydrogen present in the cathode exhaust 40 may cause
hydrogen emission constraints to be exceeded. In addition, the
initial shutdown load 50 at the beginning of operating the fuel
cell stack 12 down to a low cathode stoichiometry at the box 64 may
be higher than average so as to more rapidly discharge the adsorbed
hydrogen and oxygen within the fuel cell stack 12 to shorten the
duration of the oxygen depletion process.
[0029] Either the measured or estimated voltage of the fuel cells
in the fuel cell stack 12 or measured or estimated hydrogen
emissions can be used by the controller 52 to estimate the cathode
stoichiometry while operating the stack 12 down to a low cathode
stoichiometry at the box 64. Measuring the voltage of the stack 12
is typically preferred as the components necessary are available in
almost all fuel cell systems. Operating the fuel cell stack 12 at a
low cathode stoichiometry may be achieved by using the measured or
estimated voltage of the fuel cells in the stack 12 as a feedback
to the controller 52 because stack voltage indicates the average
cathode stoichiometry of the stack 12. An air flow meter may not
provide sufficient resolution at low air flow, therefore, an air
flow meter may not be available in the system to fine tune the
cathode stoichiometry while operating the stack 12 down to a low
cathode stoichiometry at low current densities. Air flow signals
from a flow meter can have oscillations, therefore, using fixed
compressor and valve command signals may be preferred over running
a closed loop air flow control set point in this mode.
[0030] As is discussed above, the cathode stoichiometry may be
adjusted by changing air flow, stack load, or a combination
thereof. When using a fixed shutdown load 50 and adjusting air flow
by adjusting the cathode input valve 26, and/or the cathode
back-pressure valve 42, the cathode bypass valve 48 and the speed
of the compressor 30, a flow meter may be utilized to adjust the
flow of air from the compressor 30 before fixing the position of
the cathode input valve 26 and/or the cathode back-pressure valve
42, the cathode bypass valve 48 and the speed of the compressor 30.
For example, the speed of the compressor 30 may be increased and
the cathode back-pressure valve 42 may be slightly closed to
increase the pressure on the cathode side of the stack 12. An
increased cathode side pressure may optionally be used to allow a
backflow of oxygen depleted gas into the upstream portions of the
cathode after the compressor is turned off. Using an anode pressure
strategy specific to the shut down process also helps optimize the
amount of hydrogen resulting in the cathode during off time, as is
readily apparent to those having skill in the art. In addition, the
nitrogen content in the anode side of the fuel cell stack 12 may be
controlled during the shutdown process to optimize the hydrogen
partial pressure.
[0031] The cathode stoichiometry is reduced at the box 64 by
monitoring the voltage of the fuel cells in the stack 12 using the
controller 52, the variable shutdown load 50 and adjusting the
cathode back-pressure valve 42 so as to achieve the voltage which
correlates to a low cathode stoichiometry, such as a cathode
stoichiometry of 1. If the cathode stoichiometry drops below 1, a
noticeable drop in the voltage of the fuel cells in the stack 12
will be observed because, as discussed above, stack voltage is an
indication of the overall stoichiometry of the fuel cell stack 12.
The individual cells can provide information on how many cells are
operating in an over-stoichiometry or an under-stoichiometry state,
and the controller 52 may limit the number of cells operating in an
under-stoichiometry state to control the possibility of hydrogen
pumping by adjusting the cathode back-pressure valve 42 accordingly
to increase the cathode stoichiometry to 1. In addition, the
controller 52 may limit the number of cells operating in an
over-stoichiometry state to control excess oxygen residuals by
adjusting the cathode back-pressure valve 42 accordingly to
decrease the cathode stoichiometry to 1.
[0032] When using the shutdown load 50 a higher resolution
secondary controller may be used to fine tune the shutdown load 50
to achieve a cathode stoichiometry of 1. Alternatively, and as
mentioned above, a hydrogen sensor in the cathode exhaust line 40
could also be used to determine the level of hydrogen emitted from
the fuel cell system and the cathode back-pressure valve 42 may be
adjusted by the controller to achieve the desired level of hydrogen
emitted, which also can indicate cathode stoichiometry. Since low
cathode stoichiometry may not be calculated accurately, a stack
cathode stoichiometry of 1 may be assumed when the stack voltage
decreases toward a predetermined value, such as 200 V. Other stack
voltages may be used for the predetermined value without departing
from the scope of this invention.
[0033] In another non-limiting embodiment, a low flow of hydrogen
may be added to the cathode input air by opening the hydrogen to a
cathode valve 58 on the connector line 28 to finely control the
cathode stoichiometry of the fuel cell stack 12 by consuming small
amounts of oxygen. Hydrogen addition to the cathode, if well mixed,
may also be used to achieve lower oxygen concentration levels in
the cathode exhaust, thereby limiting the variation in oxygen
levels from cell to cell in the fuel cell stack 12 by reducing the
variation of oxygen concentration in the cathode flow from cell to
cell.
[0034] Driving loads are not used during the shutdown procedure
described herein. Therefore, auxiliary loads including the
compressor 30 and coolant pumps, heaters, and battery charging may
be used to provide the shutdown load 50 during the oxygen depletion
process. For example, the stack 12 may be provided with a shutdown
load 50 that includes a varying coolant heater load with a base end
cell heater and coolant pump load.
[0035] Because the voltage of the fuel cell stack 12 is lower
during the depletion step at the box 64, the voltage may not be
adequate to power high voltage components, such as the compressor
30. Thus, the fuel cell stack 12 may need to be maintained at
higher voltages suitable for compressor operation, or the
compressor 30 may be supplied with voltage from a battery. Since
battery operation capability is typically required for the start-up
of the fuel cell system 10, a power supply from a battery is
typically available. In addition, electrical architectures such as
a boost converter may be available, thus the low voltage of the
fuel cell stack 12 during this shutdown procedure may not prevent
operation of high voltage components, as is readily apparent to
those skilled in the art.
[0036] By controlling the stack average stoichiometry down to a low
cathode stoichiometry that does not allow excessive hydrogen
pumping into the cathode, an oxygen depleted air mixture is created
without an excessive amount of hydrogen in the cathode side. As
stated above, reducing the cathode stoichiometry to approximately 1
is desired at the box 64, and may be achieved through voltage
feedback from the measured voltage of the fuel cells in the stack
12 by adjusting the cathode back-pressure valve 42 and stack load
50, and maintaining the speed of the compressor 30 constant. For
example, stack voltage limitation control may be modified by
altering the voltage set point and the gains for the controller 52
that performs the current limitation based on the error between the
voltage set point and the actual stack voltage feedback.
[0037] Once the desired cathode stoichiometry is achieved, the
oxygen in the cathode side is displaced at box 66. As discussed
above, oxygen is depleted when the cathode stoichiometry is
approximately 1 because the actual flow rate of the air or oxygen
is approximately the same as the rate of the consumption of the
oxygen or air by the fuel cell stack 12. By depleting the oxygen in
the cathode side of the stack 12 with the compressor 30 operating,
an oxygen depleted gas is created in the stack and flows out of the
stack 12 and into the cathode exhaust line 40. Enough oxygen
depleted gas is generated by this method to displace the cathode
outlet volume including any by-pass plumbing such as the WVT unit
by-pass plumbing directing cathode exhaust gas to the WVT unit 34,
although not shown in the embodiment of FIG. 1 because the by-pass
line 36 is provided in the cathode input line 22. The amount of
time needed to displace the oxygen in the cathode exhaust line 40
will vary depending on the fuel cell system 10 used, but would
approximately be the cathode outlet volume divided by the cathode
volume flow rate. A non-limiting example of the amount of time
needed is several seconds.
[0038] At the end of the generation of the oxygen depleted gas at
the box 66, the cathode back-pressure valve 42 is closed at box 68.
Closing the cathode back-pressure valve 42 reduces the amount of
air flowing through the cathode side of the stack 12. Any cathode
bypass valves in the system 10, such as the cathode by-pass valve
48 in the cathode by-pass line 46 may be opened during this step to
prevent a surge in compressor operation or an increase in pressure
that may otherwise occur during the time period after the back
pressure valve is closed and before the compressor is shut off. As
is readily apparent to those skilled in the art, if the compressor
30 is a positive displacement compressor, pressure build up would
be relieved through a pressure relief valve. The compressor 30 is
also shut off at the box 68, and due to the drop in cathode air
flow that occurs upon compressor shutdown, oxygen depleted gas will
expand into the cathode input line 32 and will displace the
oxygen-containing air therein. Stack main contactors are also
opened during this stage to isolate the stack high voltage
side.
[0039] If the WVT unit 34 and the by-pass line 36 are provided in
the fuel cell system 10, the oxygen depleted gas may expand through
the WVT unit 34 by closing the by-pass valve 38, as well as through
both the cathode input line 32 and the by-pass line 36 by partially
opening the by-pass line valve 38, or through the WVT unit 34 and
then the by-pass line 36 by keeping the by-pass valve 38 closed
initially upon shutdown of the compressor 30, then opening the
by-pass valve 38. In addition, the cathode inlet valve 26 is closed
after the back flow of oxygen depleted gas has expanded into the
cathode input line 32 sufficiently, and is done at box 70 once the
cathode side pressure is at or near ambient. Any remaining air flow
due to the compressor 30 spinning down may be by-passed around the
stack 12 using the cathode by-pass line 46 by opening the cathode
by-pass valve 48 or by recirculating flow through the compressor
30.
[0040] Any remaining oxygen within the cathode side of the fuel
cell stack 12 may be consumed by applying the shutdown load 50.
Alternatively, a small amount of oxygen may be left in the cathode
side of the fuel cell stack 12 to consume hydrogen that may be
hydrogen pumped when the bleed resistor engages or crosses over to
the cathode side, to ensure that the amount of hydrogen in the
cathode exhaust does not exceed hydrogen emission constraints. In
addition, the anode pressure at shut down may be used to ensure
that the amount of hydrogen in the cathode exhaust does not exceed
hydrogen emission constraints.
[0041] To limit the amount of air pulled into the stack 12 after
shutdown due to gas contraction and water vapor condensation, the
stack 12 may be cooled prior to and during the oxygen depletion
step at the box 64. Without a pressurized depletion followed by a
back-flow of the oxygen depleted gas, the oxygen depleted gas may
be pulled back into the upstream volumes by closing the cathode
inlet valve 26 and cooling the stack 12 to draw the oxygen depleted
gas from the exhaust into the fuel cell stack 12 due to gas
contraction and water vapor condensation. Furthermore, a final
hydrogen addition may be provided after the fuel cell stack 12 has
cooled to maximize the available hydrogen.
[0042] To achieve reliable freeze starts, a method of drying the
stack 12 to clear excess water within the stack 12 followed by a
brief hydration step may be included to improve membrane
conductivity upon fuel cell system restart. However, the cathode
depletion step may accommodate the rehydration step required for
ensuring reliable freeze starts, as operation of the fuel cell
stack 12 at a cathode stoichiometry of approximately 1 is typically
a wet operation, i.e., a high relative humidity operation.
Alternatively, the WVT unit 34 may be controlled to achieve a
desired relative humidity to provide adequate rehydration in the
time frame needed to create the oxygen depleted gas.
[0043] Leaving too much hydrogen in the fuel cell stack 12 upon
shutdown may lead to concerns regarding excessive hydrogen
emissions upon restart of the fuel cell system 10. Thus, the anode
side of the fuel cell stack 12 may be flushed with oxygen depleted
gas which has been generated at elevated pressure according to the
method described above. For example, by opening the anode purge
valve 22 in the anode purge input line 44, closing the hydrogen
flow into the anode input line 16 and opening the bleed/drain valve
24 a flow path is created to allow the oxygen depleted gas to flow
up the cathode inlet line 32 through the anode purge input line 44
and into the anode side of the fuel cell stack 12 through the anode
purge valve 22. In addition, the cathode inlet valve 26 may be
closed upon flushing the anode side with oxygen depleted gas to
prevent the high pressure gas from escaping through the cathode
input line 32. If an anode recycle is provided, a recycle blocking
valve or a recycle pump (not shown) is necessary to force the
oxygen depleted gas through the stack 12 instead of bypassing
through the anode recycle plumbing. This recycle blocking valve or
pump would be located in the anode recirculation line 54 between
the anode inlet of the stack 12 and the anode exhaust gas line 18
and is preferably located between the injector 20 (typically a jet
pump driven recirculation device) and the water separation device
56. Once the anode side has been filled with the oxygen depleted
gas, the cathode inlet valve 26 may be opened to discharge any
remaining pressure and to displace air in the cathode inlet line 32
that is upstream of the cathode inlet valve 26.
[0044] The anode purge input line 44 may also provide an anode
purge using the anode purge valve 22 for restart to remove the
oxygen depleted gas in the anode side of the stack 12. Additionally
the connector line 28 may also be used to supply hydrogen to the
cathode side during freeze starts. A better quality valve may be
used for the cathode inlet valve 26 than used for the back-pressure
valve 42 to encourage preferential leakage on the cathode exhaust
line 40 where there is a greater volume of oxygen depleted gas.
[0045] The foregoing discussion discloses and describes merely
exemplary embodiments of the present invention. One skilled in the
art will readily recognize from such discussion and from the
accompanying drawings and claims that various changes,
modifications and variations can be made therein without departing
from the spirit and scope of the invention as defined in the
following claims.
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