U.S. patent application number 10/746307 was filed with the patent office on 2005-06-30 for start up cascaded fuel cell stack.
Invention is credited to Balliet, Ryan J., Fuller, Thomas F..
Application Number | 20050142407 10/746307 |
Document ID | / |
Family ID | 34700626 |
Filed Date | 2005-06-30 |
United States Patent
Application |
20050142407 |
Kind Code |
A1 |
Fuller, Thomas F. ; et
al. |
June 30, 2005 |
Start up cascaded fuel cell stack
Abstract
A cascaded fuel cell stack (9a) includes a plurality of groups
(10-12) of fuel cells (13) connected electrically in series by
means of conductive separator plates (58, 59) and current
collecting pressure plates (56, 57). Each group has an inlet fuel
distributing fuel inlet manifold (17a, 19c, 20c), a fuel exit
manifold (19a, 20a) of each group except the last feeding the inlet
manifold of each subsequent group. A microcontroller responds to
signals from a plurality of voltage sensing devices (48a-48c) to
cause corresponding switches (50a-50c) (a) to connect each group,
and all preceding groups in the sequence, to a voltage limiting
device (VLD) (45), or (b) to connect each group to its own (VLD
(45a-45c), in response to sensing a predetermined average cell
voltage across the corresponding group. When normal operation
occurs, the microcontroller connects the main load and disconnects
the voltage limiting device (53) (25).
Inventors: |
Fuller, Thomas F.;
(Glastonbury, CT) ; Balliet, Ryan J.; (West
Hartford, CT) |
Correspondence
Address: |
M. P. Williams
210 Main Street
Manchester
CT
06040
US
|
Family ID: |
34700626 |
Appl. No.: |
10/746307 |
Filed: |
December 26, 2003 |
Current U.S.
Class: |
429/432 ;
429/429; 429/458; 429/467; 429/514; 429/517 |
Current CPC
Class: |
H01M 8/04228 20160201;
H01M 8/04302 20160201; H01M 8/0488 20130101; H01M 8/249 20130101;
H01M 8/04888 20130101; H01M 8/04559 20130101; H01M 8/04225
20160201; H01M 8/04303 20160201; H01M 8/04223 20130101; Y02E 60/50
20130101 |
Class at
Publication: |
429/023 ;
429/037; 429/038 |
International
Class: |
H01M 008/04; H01M
008/24 |
Claims
We claim:
1. A cascaded fuel cell stack operable to provide electrical
output, comprising: at least two groups of fuel cells, arranged in
a sequence having a first group and a last group, in serial fuel
relationship so that fuel flows from each group except said last
group into the next successive group in said sequence, and so that
each group except said first group receives fuel from the next
preceding group in said sequence, said groups of fuel cells
connected electrically in series between a pair of current
collecting pressure plates, one pressure plate connected to said
first group and another pressure plate connected to said last
group; a controller; a main load; a switch operable by said
controller to connect said main load to said pressure plates so
that the electrical output of said stack will drive said load; and
characterized by: one or more voltage limiting devices, said
voltage limiting devices selected from (a) an auxiliary resistive
load and (b) an energy storage system; characterized by: each of
said groups of fuel cells having a fuel exit manifold and having an
inlet fuel distributing fuel inlet manifold, the fuel exit manifold
of each group except said last group connected to the inlet fuel
distributing fuel inlet manifold of the next successive group in
said sequence, the fuel distributing fuel inlet manifold of each
group except said first group connected to the fuel exit manifold
of the next preceding group in said sequence; a conductive
separator plate disposed between adjacent ones of said groups; a
plurality of voltage sensing devices, each connected to either (c)
two of said separator plates or (d) one of said separator plates
and one of said pressure plates, each providing a signal to said
controller in response to the presence of a predetermined average
cell voltage across a corresponding one of said groups; and a
plurality of switches, one for each of said groups, each operable
by said controller in response to said signal from a corresponding
one of said voltage sensing devices for electrically connecting a
corresponding one of said groups to one of said voltage limiting
devices.
2. A cascaded fuel cell stack operable to provide electrical
output, comprising: at least two groups of fuel cells, arranged in
a sequence having a first group and a last group, in serial fuel
relationship so that fuel flows from each group except said last
group into the next successive group in said sequence, and so that
each group except said first group receives fuel from the next
preceding group in said sequence, said groups of fuel cells
connected electrically in series between a pair of current
collecting pressure plates, one pressure plate connected to said
first group and another pressure plate connected to said last
group; a controller; a main load; characterized by: each of said
groups of fuel cells having a fuel exit manifold and having an
inlet fuel distributing fuel inlet manifold, the fuel exit manifold
of each group except said last group connected to the inlet fuel
distributing fuel inlet manifold of the next successive group in
said sequence, the fuel distributing, composite fuel inlet manifold
of each group except said first group connected to the fuel exit
manifold of the next preceding group in said sequence.
3. A cascaded fuel cell stack operable to provide electrical
output, comprising: at least two groups of fuel cells, arranged in
a sequence having a first group and a last group, in serial fuel
relationship so that fuel flows from each group except said last
group into the next successive group in said sequence, and so that
each group except said first group receives fuel from the next
preceding group in said sequence, said groups of fuel cells
connected electrically in series between a pair of current
collecting pressure plates, one pressure plate connected to said
first group and another pressure plate connected to said last
group; a controller; a main load; a switch operable by said
controller to connect said main load to said pressure plates so
that the electrical output of said stack will drive said load; and
one or more voltage limiting devices, said voltage limiting devices
selected from (a) an auxiliary resistive load and (b) an energy
storage system; characterized by: a conductive separator plate
disposed between adjacent ones of said groups; a plurality of
voltage sensing devices, each connected to either (c) two of said
separator plates or (d) one of said separator plates and one of
said pressure plates, each providing a signal to said controller in
response to the presence of a predetermined average cell voltage
across a corresponding one of said groups; and a plurality of
switches, one for each of said groups, each operable by said
controller in response to said signal from a corresponding one of
said voltage sensing devices for electrically connecting a
corresponding one of said groups to one of said voltage limiting
devices.
4. A fuel cell stack according to claim 3 wherein: there is one
voltage limiting device, a first side of which is electrically
connected to the one of said pressure plates connected to said
first group; and each of said switches is operable by said
corresponding signal to electrically connect a corresponding one of
said groups and all of said groups which precede said one group in
said sequence to a second side of said one voltage limiting
device.
5. A fuel cell stack according to claim 3 wherein: there is a
separate one of said voltage limiting devices for each of said
groups; and each of said switches is operable by said corresponding
signal to electrically connect a corresponding one of said groups
across a corresponding one of said voltage limiting devices.
Description
TECHNICAL FIELD
[0001] This invention relates to cascaded control of fuel flow and
voltage of cascaded fuel cell stacks, such as during startup and
shutdown.
BACKGROUND ART
[0002] To achieve very high fuel utilizations, about 98% or 99%,
when operating a fuel cell stack 9 on pure hydrogen, a cascade fuel
flow field, illustrated in FIG. 1, comprises a plurality of groups
10-12 of fuel cells 13 arranged in flow-series relationship so that
fuel from a source (not shown) passing through a fuel inlet valve
16 enters a fuel inlet manifold 17, flows through a first group 10
of fuel cells 13, then enters a first turn-around manifold 19, then
flows through the second group 11 of fuel cells 13, thence through
a second turn-around manifold 20 and through the third group 12 of
fuel cells 13, to an exit manifold 22.
[0003] For a typical 40 kilowatt fuel cell stack, the first group
10 has a large number of cells 13, which may be on the order of
about 200 cells, the second group 11 has a lesser number of cells
13, which may be on the order of about 70 cells, and the third
group 12 may have on the order of about 25 cells. As is known, this
assures that all of the cells get adequate hydrogen even with high
hydrogen utilization, provided that the last group of cells 12 get
adequate hydrogen.
[0004] Referring to FIG. 1, during the production of electricity in
normal fuel cell operation mode, a microcontroller 25 provides a
signal on a line 26 to cause a fuel inlet valve 16 to be open, to
provide fuel to the inlet manifold 17. The processor 25 also
provides a signal on a line 27 to cause a normal fuel outlet valve
23 to be open. Under this condition, the fuel enters the inlet
manifold 17, passes through the group 10 of cells 13, into the
first turn-around manifold 19, through the group 11 of cells 13,
through the second turn-around manifold 20, through the group 12 of
cells 13, through the exit manifold 22, through the outlet valve
23, and to the exhaust 30.
[0005] The fuel cell stack may include a recycle loop 38 driven by
a pump 39, all in a conventional fashion; however, the use of a
recycle loop is optional.
[0006] In commonly owned, copending U.S. patent application Ser.
No. 09/742,481, filed Dec. 20, 2000, it is shown that the more
rapidly the fresh hydrogen-containing fuel flows through the anode
flow field upon start-up, to displace the air therein, the quicker
the hydrogen/air interface moves through the anode flow field, and
the less time there is for the occurrence of corrosion of the
platinum catalyst and catalyst support.
[0007] In copending U.S. patent application Ser. No. 09/921,809,
filed Aug. 3, 2001, a cascade reactant flow field of a fuel cell
stack has additional fuel inlet valves to provide inlet fuel
directly to each cascade of the stack and at least one additional
exhaust valve to remove fuel directly from each cascade of the
stack. This may be used for rapid deployment of fuel into the fuel
flow field during start-up.
[0008] Although rapid purging reduces startup problems referred to
hereinbefore, performance decay of cascaded fuel cell stacks is
still unacceptable.
[0009] It is also known in the art, as illustrated in copending
U.S. patent application Ser. No. 10/305,301, filed Nov. 26, 2002 to
connect a voltage limiting device 45 across the main electrical
output terminals 46, 47 as soon as average voltage per cell of
about 0.2 volts is detected by a voltage sensing device 48, which
causes the microcontroller 25 to close a switch 50. The voltage
limiting device, in the aforementioned application, is simply an
auxiliary load resistor.
[0010] When it is determined, either by the passage of time or by
sensing parameters of the fuel cell stack, that normal operation
can be achieved, the micro controller 25 will close a switch 52 to
connect the main load 53 across the fuel cell electrical output
terminals 46, 47, and open the switch 50.
[0011] If the voltage limiting device is not connected across the
stack while the groups of cells 10 are being fed hydrogen, then
these cells will have excessive voltage, and resulting carbon
corrosion and ultimate performance decay. On the other hand, if the
voltage limiting device 45 is connected across the stack prior to
hydrogen reaching the second and third groups 11, 12 of cells, the
anodes in the cells in the second and third groups are driven to an
elevated potential that results in corrosion of the carbonaceous
catalyst support and other components of the cells.
[0012] The foregoing problems have resulted in the conclusion that
fuel cell stacks with cascade fuel feed are impractical, due to the
certainty of early performance decay.
DISCLOSURE OF INVENTION
[0013] Objects of the invention include: a fuel cell stack having
the high-fuel-utilization advantage of a cascade fuel flow field
without the startup problems, such as reverse cell voltage, carbon
corrosion and performance decay, heretofore associated therewith;
and an improved fuel cell stack utilizing cascade flow fields which
has no unusual degradation of catalysts and other parts as a
consequence of frequent shut-down and start-ups.
[0014] This invention is predicated in part on our discovery that
fuel distribution in the diverse groups of fuel cells within a
cascaded fuel cell stack is operationally ineffective, and in part
on the fact that voltage control during startup and shutdown of a
fuel cell stack must be accomplished in conjunction with each group
of fuel cells receiving fuel.
[0015] According to the present invention, a cascade of groups of
fuel cells arranged in serial fuel flow arrangement includes an
inlet fuel distributing fuel inlet manifold for each group of the
series, thereby providing operationally adequate fuel distribution
in each group of fuel cells. According further to the invention,
voltage control during startup of a fuel cell stack in a serial
fuel flow, cascaded fuel cell stack, is accomplished one group of
fuel cells at a time, thereby responding directly to the
introduction of fuel to each group.
[0016] The invention allows startup of each group of a cascaded
group of fuel cells in a stack to be started up, one group at a
time, each group having advantageous startup conditions as would be
the case of individual, non-cascaded fuel cell stacks known to the
prior art.
[0017] Other objects, features and advantages of the present
invention will become more apparent in the light of the following
detailed description of exemplary embodiments thereof, as
illustrated in the accompanying drawing.
BRIEF DESCRIPTION OF THE DRAWINGS
[0018] FIG. 1 is a schematic diagram of a three-group cascade fuel
cell stack known to the prior art.
[0019] FIG. 2 is a schematic diagram of an exemplary three-group
cascade fuel cell stack, in accordance with the invention.
[0020] FIGS. 3-5 are partial, simplified schematic diagrams of
various voltage limiting devices.
MODE(S) FOR CARRYING OUT THE INVENTION
[0021] Referring to FIG. 2, a cascaded fuel cell stack 9a according
to the invention includes an inlet fuel distributing fuel inlet
manifold 17a at the inlet to the fuel flow fields of the fuel cells
13 in the first group 10. The fuel distributing manifold 17a may be
a cascade fuel inlet manifold as described in copending U.S. patent
application Ser. No. 10/269,654, filed Oct. 10, 2002. The device
disclosed therein divides the fuel in half a number of times, such
as four times, so that it is evenly distributed across the entire
stack of fuel cells, whereby each fuel cell fuel flow field
receives a uniform amount of fuel, simultaneously with the fuel
flow fields of the other fuel cells. The fuel distributing manifold
17a may also take the form of a permeable baffle inlet fuel gas
distributor, as disclosed in copending U.S. patent application
Serial No. (Docket No. C-2950), filed Dec. 15, 2003, entitled
"Permeable Inlet Fuel Gas Distributor for Fuel Cells", in which
fuel is evenly distributed by being forced through a permeable
baffle, which may be porous, have orifices, be in the form of
screening, mesh or other materials. Or, other inlet fuel
distributing, composite fuel inlet manifolds may be used.
[0022] The fuel passing through the fuel flow fields of the first
group 10 of fuel cells 13 will reach a fuel exit manifold 19a,
which is not itself in direct communication with the fuel cells 13
of the group 11. Instead, the fuel exit manifold 19a feeds a fuel
conduit 19b which in turn feeds another fuel distributing fuel
inlet manifold 19c. Although the manifolds 17a, 20a, and 20c and
fuel conduit 20b are shown as separate elements, they may be
integrated into a single structure; similarly, the manifolds 19a,
19c, and 22 may be integrated into a single structure along with
fuel conduit 19b. This manifold, similar to the manifold 17a, will
distribute the fuel evenly throughout the group 11 of fuel cells
13. This is in contrast to the manner in which fuel is distributed,
in the prior art shown in FIG. 1, in which there is an inherent
tendency for the fuel cells 13 of the group 11 which are closest to
the group 10 to receive the fuel first, and to receive more fuel
than those fuel cells 13 which are disposed closer to the group 12.
This is an important aspect of the present invention.
[0023] Instead of a plain turnaround manifold between the group 11
and the group 12, the group 11 has a fuel exit manifold 20a which
feeds a fuel conduit 20b which carries the fuel to another fuel
distributing fuel inlet manifold 20c, similar to the other
manifolds 17a, 19c. This in turn causes a uniform, simultaneous
flow of fuel entering the fuel flow fields of the fuel cells 13 in
the group 12. This is in contrast to the tendency, shown in the
prior art of FIG. 1, for the fuel to enter those fuel cells 13 of
the group 12 which are closer to the group 11, causing less fuel
and a delay in a fuel arrival for the fuel cells 13 which are
farther away from the group 11. The group 12 of fuel cells 13 all
communicate with a fuel exit manifold 22, which in turn may feed
the fuel recycle apparatus 38, 39 illustrated in FIG. 1, if
desired.
[0024] Thus, a first aspect of the present invention includes
distributing the fuel evenly to each group in turn so that all of
the fuel cells in a given group receive a uniform amount of fuel
simultaneously with the other fuel cells of that same group.
[0025] A second aspect of the present invention includes substack
voltage monitors 48a-48c which in turn cause the microcontroller 25
to operate separate voltage limiting device (VLD) switches 50a-50c.
The substack voltage monitors 48a-48c are connected either to two
conductive separator plates 58, 59 or to one end plate 56, 57 and
one separator plate 58, 59.
[0026] When the fuel cell is started up, the microcontroller will
open the valve 16 by means of a signal on the line 26 and hydrogen
will begin to flow through the fuel distributing fuel inlet
manifold 17a. The fuel will enter the fuel cells simultaneously and
this will cause voltage to develop between the end plate 57 and the
separator plate 59. When the sub stack voltage monitor 48a
determines a suitable average cell voltage, which might be on the
order of 0.2 volts per cell, the microcontroller 25 will close the
switch 50a connecting the voltage limiting device across the group
10 of fuel cells 13. The two other groups of fuel cells have not
yet received fuel and are not yet connected to the voltage limiting
device. Once fuel begins entering the group 11 of fuel cells 13,
uniformly dispersed by the fuel distributing fuel inlet manifold
19c, the cell voltage will begin to build up between the separator
plate 58 and the separator plate 59. When the sub stack voltage
monitor 48b senses an adequate voltage, which may be on the order
of 0.2 volts per cell, the microcontroller 25 will cause the switch
50b to close and the switch 50a to thereafter open, thereby
connecting the group 11 in series with the group 10 across the
voltage limiting device 45 through the switch 50b. Thus, voltage is
limited at the appropriate time when fuel is building up in the
fuel flow fields of the fuel cells 13.
[0027] Eventually fuel will reach the fuel distributing fuel inlet
manifold 20c and voltage will begin to build up between the end
plate 56 and the separator plate 58. When the sub stack voltage
monitor 48c indicates an average voltage, which might be on the
order of 0.2 volts/cell, the microcontroller 25 will close the
switch 50c and thereafter open the switch 50b, thereby causing all
three groups 10-12 to be connected across the voltage limiting
device 45 through the switch 50c. When the microcontroller
determines that all three groups are operating normally, it will
cause the switch 52 to close thereby connecting the main load 53
across the stack 9a by connecting it to the pressure plates 56, 57,
and the microcontroller will immediately open the switch 50c.
[0028] Upon shutdown, as is known, the air to the cathode is turned
off after which the microcontroller will close the switch 50c, so
that the voltage limiting device 45 consumes the energy as the
oxygen in the cathode oxidant channels of all three groups 10-12 is
depleted. Thereafter, the microcontroller 25 will shut off the fuel
by means of the valve 16; the switch 50c typically will remain
closed until the next time that the fuel cell stack is to be
operated.
[0029] The voltage limiting device 45 may be a simple resistive
auxiliary load 45a (FIG. 4) as in the aforementioned application
Ser. No. 10/305,301, or it may be a resistive load 45b (FIG. 5)
which is connected and disconnected, repetitively, in a pulse width
modulation fashion by a control 45c and a switch 45d as disclosed
in copending U.S. patent application Ser. No. 10/681,493 filed Oct.
7, 2003. The voltage limiting device 45 may be an energy recovery
and storage apparatus as disclosed in copending U.S. patent
application Ser. No. 10/669,273 filed Sep. 23, 2003, which includes
batteries 45e (FIG. 6) and capacitors as well as buck and boost
controls 45f.
[0030] FIG. 3 illustrates that instead of a single voltage limiting
device 45 for the entire stack, each group may have its own voltage
limiting device 45a-45c; in such case, each switch 50a-50c, once
closed, will remain closed until the microcontroller closes switch
52.
[0031] Although disclosed in a three group configuration herein,
the invention may be used in cascaded fuel cell stacks having only
two groups, or having three, four or more groups. The voltage
limiting aspects of the invention are preferably used with the
inlet fuel distributing aspects of the invention, but the various
aspects of the invention may be used separately.
[0032] The aforementioned patent applications are incorporated
herein by reference.
[0033] Thus, although the invention has been shown and described
with respect to exemplary embodiments thereof, it should be
understood by those skilled in the art that the foregoing and
various other changes, omissions and additions may be made therein
and thereto, without departing from the spirit and scope of the
invention.
* * * * *