U.S. patent application number 10/356403 was filed with the patent office on 2004-08-05 for flow restrictors in fuel cell flow-field.
Invention is credited to Rock, Jeffrey Allan.
Application Number | 20040151960 10/356403 |
Document ID | / |
Family ID | 32770795 |
Filed Date | 2004-08-05 |
United States Patent
Application |
20040151960 |
Kind Code |
A1 |
Rock, Jeffrey Allan |
August 5, 2004 |
Flow restrictors in fuel cell flow-field
Abstract
Flow-field for a PEM fuel cell having a plurality of
flow-channels including flow-restrictors strategically located
throughout to achieve desired pressure differentials between fuel
and oxidant supply and exhaust manifolds, and between adjacent
flow-channels. A preferred flow-restrictor comprises a constriction
in the flow channel that has a cross-sectional area that is less
than the cross-sectional area of the flow-channel.
Inventors: |
Rock, Jeffrey Allan;
(Rochester, NY) |
Correspondence
Address: |
CARY W. BROOKS
General Motors Corporation
Legal Staff, Mail Code 482-C23-B21
P.O. Box 300
Detroit
MI
48265-3000
US
|
Family ID: |
32770795 |
Appl. No.: |
10/356403 |
Filed: |
January 31, 2003 |
Current U.S.
Class: |
429/444 ;
429/482; 429/513; 429/517 |
Current CPC
Class: |
Y02E 60/50 20130101;
H01M 8/0247 20130101; H01M 2008/1095 20130101 |
Class at
Publication: |
429/030 ;
429/038; 429/025 |
International
Class: |
H01M 008/10; H01M
008/04; H01M 008/02 |
Claims
1. A PEM fuel cell comprising (1) a proton exchange membrane having
opposing cathode and anode faces on opposite sides of said
membrane, (2) a gas-permeable electrically-conductive current
collector engaging at least one of said faces, and (3) a
current-collecting plate engaging said gas-permeable current
collector and defining a gas flow-field confronting said
gas-permeable current collector, said flow-field comprising a
plurality of lands engaging said, gas-permeable current collector
and defining a plurality of gas flow-channels, each of said
flow-channels having (a) an inlet end communicating with a supply
manifold that supplies a reactant gas at a first pressure to all of
said flow-channels, and (b) an exit end communicating with an
exhaust manifold that receives said gas from said flow-channels, a
first flow-restrictor in a first flow-channel to reduce said first
pressure to a second pressure downstream of said first
flow-restrictor that is less than said first pressure, and a second
flow-restrictor in a second flow-channel next adjacent said first
flow-channel for maintaining a third pressure in said second
flow-channel upstream of said second flow-restrictor sufficiently
above said second pressure to drive said gas from second
flow-channels into said first flow-channel through said
gas-permeable current collector.
2. A PEM fuel cell according to claim 1 wherein said flow-channels
each have a first cross-sectional area transverse the direction of
gas flow through said flow-channel, and at least one of said
flow-restrictors comprises a constriction in said flow-channel
having a second cross-sectional area less said than said first
cross-sectional area.
3. A PEM fuel cell according to claim 1 wherein at least one of
said flow-restrictors comprise a tortuous segment of said
flow-channel.
4. A PEM fuel cell according to claim 1 including a plurality of
ports each communicating a said manifold with a said flow-channel,
and at least one of said flow-restrictors is a said port sized to
provide said second and/or said third pressures.
5. A PEM fuel cell comprising (1) a proton exchange membrane having
opposing cathode and anode faces on opposite sides of said
membrane, (2) a gas-permeable, electrically-conductive current
collector engaging at least one of said faces, (3) a
current-collecting plate engaging said gas-permeable current
collector and defining, a gas flow field confronting said
gas-permeable current collector, said flow-field comprising a
plurality of lands engaging said gas-permeable current collector
and defining a plurality of gas non-serpentine flow-channels, each
of said flow-channels having (a) an inlet leg communicating with a
supply manifold that supplies a reactant gas at a first pressure to
all said flow-channels, (b) an exit leg communicating with an
exhaust manifold that receives said gas from said flow-channels,
and (c) at least one medial leg intermediate said inlet and exit
legs, a first flow-restrictor in the inlet leg of a first of said
flow channels for producing a second pressure downstream of said
first flow-restrictor that is less than said first pressure, and a
second flow-restrictor in the exit leg of a second said
flow-channel next adjacent said first flow-channel for maintaining
a third pressure in said second flow-channel upstream of said
second flow-restrictor sufficient to drive said gas between said
first and second flow-channels through said gas permeable current
collector.
6. A PEM fuel cell according to claim 5 wherein each said
flow-channel is branched ed so as to provide a medial leg having at
least first and second branches, each having a first end
communicating with said inlet leg and a second end communicating
with said exhaust leg.
7. A PEM fuel cell according to claim 6 wherein said flow channel
is bifurcated and said first branch has a third flow-restrictor
proximate said first end that reduces the pressure in said first
branch down stream of said third flow-restrictor to a fourth
pressure that is below said second pressure, and said second branch
has a fourth flow-restrictor proximate said exit leg for
maintaining a fifth pressure in said second branch upstream of said
fourth flow-restrictor sufficient to drive said gas between first
and second branches through said gas permeable current
collector.
8. A PEM fuel cell comprising (1) a proton exchange membrane having
opposing cathode and anode faces on opposite sides of said
membrane, (2) a gas-permeable electrically-conductive current
collector engaging at least one of said faces, (3) a
current-collecting plate engaging said gas-permeable current
collector and defining a gas flow-field confronting said gas
permeable current collector, said flow-field comprising a plurality
of lands engaging said gas-permeable current collector and defining
a plurality of non-serpentine gas flow-channels,, each of said
flow-channels having (a) an inlet leg for receiving gas at a first
pressure from a supply manifold common to all said flow channels,
(b) an exit leg for discharging said gas into an exhaust manifold
common to all said flow-channels, and (c) first and second medial
legs intermediate said inlet and exit legs, said medial legs each
having a first end communicating with said inlet leg and a second
end communicating with said exit leg, said first medial leg having
a first flow-restrictor proximate said first inlet leg that reduces
the pressure in said first medial leg down stream of said first
flow-restrictor to a second pressure that is below said first
pressure, and said second medial leg has a second flow-restrictor
proximate said exit leg for maintaining a third pressure in said
second medial leg upstream of said second flow-restrictor
sufficient to drive said gas between first and second medial legs
through said gas-permeable current collector.
Description
TECHNICAL FIELD
[0001] This invention relates to PEM fuel cells and more
particularly to the reactant flow fields therefor.
BACKGROUND OF THE INVENTION
[0002] Fuel cells have been proposed as a power source for many
applications. One such fuel cell is the PEM (i.e., Proton Exchange
Membrane) fuel cell. PEM fuel cells are well known in the art and
include in each cell thereof a so-called
"membrane-electrode-assembly" (hereafter MEA) comprising a thin
(i.e., ca. 0.0015-0.007 inch), proton-conductive, polymeric,
membrane-electrolyte having an anode electrode film (i.e., ca.
0.002 inch) formed on one face thereof, and a cathode electrode
film (i.e., ca. 0.002 inch) formed on the opposite face thereof.
Such membrane-electrolytes are well known in the art and are
described in such U.S. Pat. Nos. 5,272,017 and 3,134,697, as well
as in the Journal of Power Sources, Volume 29 (1990) pages 367-387,
inter alia. In general, such membrane-electrolytes are made from
ion-exchange resins, and typically comprise a perfluoronated
sulfonic acid polymer such as NAFION.TM. available from the E.I.
DuPont de Nemours & Co. The anode and cathode films, on the
other hand, typically comprise (1) finely divided carbon particles,
very finely divided catalytic particles supported on the internal
and external surfaces of the carbon particles, and proton,
conductive material. (e.g., NAFION.TM.) intermingled with the
catalytic and carbon particles, or (2) catalytic particles, sans
carbon, dispersed throughout a polytetrafluoroethylene (PTFE)
binder. One such MEA and fuel cell is described in U.S. Pat. No.
5,272,017 issued Dec. 21, 1993, and assigned to the assignee of the
present invention.
[0003] The MEA is sandwiched between sheets of porous,
gas-permeable, conductive material, known as a "diffusion layer",
which press against the anode and cathode faces of the MEA and
serve as (1) the primary current collectors for the anode and
cathode, and (2) mechanical support for the MEA. Suitable such
primary current collector sheets comprise carbon or graphite paper
or cloth, fine mesh noble metal screen, and the like, through which
the gas can diffuse, or be driven, to contact the MEA underlying
the lands, as is well known in the art.
[0004] The thusly formed sandwich is pressed between a pair of
electrically conductive plates which serve as secondary current
collectors for collecting the current from the primary current
collectors, and for conducting current between adjacent cells
internally of the stack (i.e., in the case of bipolar plates), and
externally of the stack (in the case of monopolar plates at the
ends of the stack). The secondary current collecting plates each
contain at least one active region including a so-called
"flow-field" that distributes the fuel cell's gaseous reactants
(e.g., H.sub.2 or O.sub.2/air) over the surfaces of the anode and
cathode. The flow-field includes a plurality of lands which engage
the primary current collector and define therebetween a plurality
of grooves or flow-channels through which the gaseous reactants
flow between a supply manifold in a header region of the plate at
one end of the channel and an exhaust manifold in a header region
of the plate at the other end of the channel.
[0005] The pressure differentials (1) between the supply manifold
and the exhaust manifold, and (2) between adjacent flow channels or
segments of the same flow channel, are, of considerable importance
in designing a fuel cell. Serpentine channels have been used to
achieve desired manifold-to-manifold pressure differentials as well
as inter-channel pressure differentials. Serpentine flow-channels
have an odd number of legs extending, in switchback style, between
the supply and exhaust manifolds of the stack. Serpentine flow
channels use various widths, depths and lengths to vary the
pressure differentials between the supply and exhaust manifolds,
and may be designed to drive some reactant gas trans-land between
adjacent channels or between adjacent segments of the same channel
via the current collecting diffusion layer in order to expose the
MEA confronting the land separating the legs to reactant. For
example, some gas can flow from an upstream leg of a channel (i.e.
where pressure is higher) to a parallel downstream leg of the same
channel (i.e. where the pressure is lower) by moving through the
diffusion layer engaging the land that separates the upstream leg
from the parallel downstream leg. Non-serpentine flow-channels have
been proposed that extend more or less directly between the supply
and exhaust manifolds, i.e. without any hairpin/switchback-type
turns therein, and hence in shorter lengths than the serpentine
flow-channels. Pressure differential management is more difficult
with non-serpentine flow-channels than with serpentine
flow-channels.
[0006] The present invention is directed to a PEM fuel cell
flow-field that offers significant design flexibility in achieving
desired pressure differentials between the supply and exhaust
manifolds, and between adjacent flow-channels. The invention
utilizes flow-restrictions strategically located throughout the
flow-field to achieve the desired pressure differentials, and is
particularly useful with non-serpentine flow-channels.
SUMMARY OF THE INVENTION
[0007] The present invention relates to a PEM fuel cell of the type
that has (1) a proton exchange membrane having opposing cathode and
anode faces, (2) a gas-permeable, electrically-conductive current
collector engaging at least one of the faces, and, (3) a
current-collecting plate engaging the gas-permeable current
collector, which current-collecting plate has a gas flow-field
thereon that confronts the gas-permeable current collector. The
flow-field comprises a plurality of lands that engage the
gas-permeable current collector, and define a plurality of gas
flow-channels through which the gaseous reactants (i.e. H.sub.2 and
O.sub.2) flow. The flow-channels each have (a) an inlet end
communicating with a supply manifold that supplies a reactant gas
to the flow-channels at a first pressure, and (b) an exit end
communicating with an exhaust manifold that receives the reactant
gas from the flow-channels. In accordance with the present
invention, there is provided: (1) a first flow-restrictor in a
first flow-channel for reducing the first pressure to a second
pressure downstream of the first flow-restrictor that is less than
the first pressure; and (2) a second flow-restrictor in a second
flow-channel, next adjacent the first flow-channel, for maintaining
a third pressure in the second flow-channel upstream of the second
flow-restrictor sufficiently above the second pressure that it
drives some of the gas from the second flow-channel into the first
flow-channel through the gas-permeable current collector that
engages the land that separates the two flow-channels. The
flow-restrictor will preferably comprise a constriction in the flow
channel that has a smaller cross-sectional area than the
flow-channel itself. Alternatively, the flow-restrictor could be a
tortuous segment of flow-channel, or ports at the entrance to and
exits from the flow-channels that are smaller than the
flow-channels themselves. The flow-restrictors will preferably be
located proximate the inlet and exit ends of the flow-channels
where they can impact the upstream and downstream pressures over
the longest lengths of flow-channel.
[0008] According to a preferred embodiment of the invention, a
non-serpentine flow-field has a plurality of flow-channels each of
which has (a) an inlet leg communicating with the supply manifold,
(b) an exit leg communicating with the exhaust manifold, (c) at
least one medial leg intermediate the inlet and exit legs, (d) a
first flow-restrictor in the inlet leg of a first flow-channel for
producing a second pressure downstream of the first flow-restrictor
that is less than a first pressure in the supply manifold, and (e)
a second flow-restrictor in the exit leg of a second flow-channel
next adjacent the first flow-channel for maintaining a third
pressure in the second flow-channel upstream of the second
flow-restrictor that is sufficient to drive the gas between the
first and second flow-channels through the gas permeable current
collector that engages the land that separates the two
flow-channels. Most preferably, each flow channel has a branched
midsection so as to provide a medial leg that has at least first
and second branches, each of which has a first end communicating
with the inlet leg of the flow-channel, and a second end
communicating with the exhaust leg of the flow channel. In this
context (i.e. a flow-field having branched midsection): (i) one
embodiment of the invention has the flow-restrictors located only
in the inlet and outlet legs of the flow-channels; (ii) another
embodiment has the flow-restrictors located only in the branches of
the bifurcated midsection; and (iii) in still another embodiment,
the flow-restrictors are located in both the inlet/outlet legs and
in the branches of the furcated midsection.
BRIEF DESCRIPTION OF THE DRAWINGS
[0009] The invention will be better understood when considered in
the light of the following detailed description of certain specific
embodiments thereof which is given hereafter in conjunction with
the several figures in which:
[0010] FIG. 1 is a schematic, exploded, isometric, illustration of
a PEM fuel cell stack (only two cells shown);
[0011] FIG. 2 is an isometric, exploded, view of an MEA and bipolar
plate of a PEM fuel cell stack;
[0012] FIG. 3 is an enlargement of a portion of the bipolar plate
of FIG. 2 where indicated thereon;
[0013] FIG. 4 is a plan view of the bipolar plate of FIG. 2;
[0014] FIG. 5 is an enlarged, isometric view of one embodiment of a
flow-restrictor (i.e. a short constriction) in accordance with the
present invention;
[0015] FIG. 6 is an isometric view of another embodiment of a
flow-restrictor (i.e. an elongated constriction) in accordance with
the present invention;
[0016] FIG. 7 is an enlarged, isometric view of still another
embodiment of a flow-restrictor (i.e. tortuous-path) in accordance
with the present invention;
[0017] FIG. 8 schematically depicts one layout of a flow-field in
accordance with the present invention, but showing only the
centerlines of each of the flow-channels and the locations of the
flow-restrictors;
[0018] FIG. 9 schematically depicts another layout of a flow-field
in accordance with the present invention, but showing only the
centerlines of each of the flow-channels and the locations of the
flow-restrictors;
[0019] FIG. 10 schematically depicts still, another layout of a
flow-field in accordance with the present invention, but showing
only the centerlines of each of the flow-channels and the locations
of the flow-restrictors.
DESCRIPTION OF THE PREFERRED EMBODIMENT
[0020] For simplicity, only a two-cell stack (i.e. one bipolar
plate) is illustrated and described hereafter, it being understood
that a typical stack will have many more such cells and bipolar
plates. FIG. 1 depicts a two-cell, bipolar PEM fuel cell stack
having a pair of membrane-electrode-assemblies (MEAs) 4 and 6
separated from each other by an electrically conductive,
liquid-cooled, bipolar plate 8. The MEAs 4 and 6, and bipolar plate
8, are stacked together between stainless steel clamping plates 10
and 12, and monopolar end plates 14 and 16. The clamping plates 10,
12 are electrically insulated front the end plates 14, 16 by a
gasket or dielectric coating (not shown). The monopolar end plates
14 and 16, as well as both working faces of the bipolar plate 8,
contain a plurality of grooves or channels 18, 20, 22, and 24
defining a so-called "flow field" for distributing fuel and oxidant
gases (i.e., H.sub.2 & O.sub.2) over the faces of the MEAs 4
and 6. Nonconductive gaskets 26, 28, 30, and 32 provide seals and
electrical insulation between the several components of the fuel
cell stack. Gas-permeable carbon/graphite diffusion papers 34, 36,
38 and 40 press up against the electrode faces of the MEAs 4 and 6.
The end plates 14 and 16 press up against the carbon/graphite
papers 34 and 40 respectively, while the bipolar plate 8 presses up
against the carbon/graphite paper 36 on the anode face of MEA 4,
and against carbon/graphite paper 38 on the cathode face of MEA
6.
[0021] The bipolar plates 8 may comprise graphite, graphite-filled
polymer, or metal. Preferably, the bipolar plates will comprise two
separate metal sheets/panels bonded together so as to provide a
coolant flow passage therebetween. Bonding may, for example, be
accomplished by brazing, diffusion bonding, or gluing with a
conductive adhesive, as is well known in the art.
[0022] FIG. 2 is an isometric, exploded view of a bipolar plate 8,
first primary porous current collector 42, MEA 43 and second
primary porous current collector 44 as they are stacked together in
a fuel cell. A second bipolar plate (not shown) would underlie the
second primary current collector 44 to form one complete cell.
Similarly, another set of primary current collectors and, MEA (not
shown) will overlie the upper sheet 58. The bipolar plate 8
comprises a first exterior metal sheet 58, a second exterior metal
sheet 60, and an optional, perforated, interior metal sheet 62
which is brazed interjacent the first metal sheet 58 and the second
metal sheet 60. The metal sheets 58, 60 and 62 are made as thin as
possible (e.g., about 0.002-0.02 inches thick), and may be formed
by stamping, by photo etching (i.e., through a photolithographic
mask) or any other conventional process for shaping sheet metal.
The external sheet 58 is formed so as to provide a reactant gas
flow field characterized by a plurality of lands 64 which define
therebetween a plurality of non-serpentine gas flow channels 66
through which one of the fuel cell's reactant gases (i.e. H.sub.2)
flows from near one edge 68 of the bipolar plate to near the
opposite edge 70 thereof. When the fuel cell is fully assembled,
the lands 64 press against the primary current collectors lying
above it (not shown) which, in turn, presses against the MEA with
which it is associated (not shown). In operation, current flows
from the primary current collector through the lands 64 and thence
through the stack. The H.sub.2 gas is supplied to flow-channels 66
from a header or supply manifold formed by aligned openings 72 in
the several plates, gaskets, etc., and exits the channels 66 via an
exhaust manifold formed by aligned openings 74 in the several
plates, gaskets, etc. O.sub.2/air is supplied to the flow-channels
on the underside of plate 60 from a header or supply manifold
formed by aligned openings 76 in the several plates, gaskets, etc.,
and exhausted through an exhaust manifold formed by aligned
openings 78 in the several plates, gaskets, etc.. Coolant passes
between the sheets 58 and 60 from an inlet manifold formed by
aligned openings 75 in the several plates, gaskets, etc. to an
outlet manifold formed by openings 77 in the several plates,
gaskets, etc.. In this regard, the bipolar plate 8 (e.g. see FIG.
2) has a central active region "A" that engages the primary current
collector, and is bordered by inactive header regions "B" and "C".
The active region A has a working face having an anode flow field
20 comprising a plurality of flow-channels 66 for distributing
hydrogen over the face of the MEA 4 that it confronts. A similar
working face 22 on the opposite (i.e. cathode) side (not shown) of
the bipolar plate 8 serves to distribute air over the face of the
MEA 6 that it confronts. The active region: A of the bipolar plate
8 is flanked by two inactive header regions, or border portions, B
and C that contain the several openings 72, 74, 75, 76, 77 and 78
therethrough. When the plates are stacked together, the openings in
one bipolar plate are aligned with like openings in the other
bipolar plates. Other components of the stack such as gaskets 26,
28, 30 and 32, as well as the membrane of the MEAs 4 and 6 and the
end plates 14, 16 have corresponding openings (see FIG. 1) that
align with the openings 72, 74, 75, 76, 77 and 78 in the bipolar
plates in the stack, and together therewith form the aforesaid
manifolds for supplying and exhausting gaseous reactants and liquid
coolant to/from the stack. Referring to FIG. 1, oxygen/air is
supplied to the air supply manifold 76 of the stack via appropriate
supply plumbing 80, while hydrogen is supplied to the hydrogen
supply manifold 72 via supply plumbing 82. Exhaust plumbing for
both the H.sub.2 (84) and O.sub.2/air (86) are also provided for
the H.sub.2 and air exhaust manifolds. Additional plumbing 88 and
90 is provided for respectively supplying liquid coolant to, and
removing coolant from, the coolant inlet 75 and outlet 77
manifolds.
[0023] Metal sheet 60 is similar to sheet 58. Like sheet 58, the
underside of the sheet 60 has a working face 22 that engages the
first current collector 42. An optional, perforated, interior,
metal sheet 62 may be used interjacent the exterior sheets 58 and
60, and, includes a plurality of apertures 92 that cause turbulent
flow of the coolant for more effective heat exchange with the
exterior sheets 58, and 60 respectively. The several sheets 58, 60
and 62 are preferably brazed together.
[0024] FIGS. 3 and 4 are, respectively, an enlarged, isometric view
of the cornerof plate 58 where indicated on FIG. 2, and a plan view
of plate 58 more clearly showing: several flow-restrictors 94 in
the inlet legs 96 of the fow-channels 66; the several
flow-restrictors 98 in the exit legs 100 of flow-channels 66; and
the several flow-restrictors 102 in the branches/medial legs 104
and 106 of bifurcated flow-channels 66. In this regard, each flow
channel has an inlet leg 96 communicating with the supply manifold
72, an exit leg 100 communicating with the exhaust manifold 74; and
medial legs/branches 104 and 106, in the midsections of the
flow-channels, communicating with the inlet and exit legs 96 and
100 as more fully described in copending U.S. patent application
Ser. No. (Attorney's docket no. GP-303028), that is filed
concurrently herewith and is intended to be incorporated herein by
reference. The inlet legs 96 communicate with the supply manifold
72 via a plurality of openings 108 and a slot 110 that communicates
with the manifold 72 via a passageway (not shown) that underlies
section 112 of the plate 60. Similarly, the exit legs 100
communicate with the exhaust manifold 74 via a plurality of
openings 114 which in turn communicate with the exhaust manifold 74
via a slot 116 that communicates with the manifold 74 via a
passageway (not shown) that underlies section 118 of the plate 60.
The flow-restrictors are strategically positioned/located
throughout the flow-field, as needed, to achieve desired pressure
differentials therein. Several, but not all, such
positionings/locations, are discussed hereinafter in conjunction
with FIGS. 8-10.
[0025] The flow restrictors 94, 98, 102 will preferably comprise
constrictions in the flow channels. In this regard, each flow
channel 66 has a first cross-sectional area (i.e. transverse the
direction of gas flow therein) that predominates throughout most of
the length of the flow channel 66, and the constrictions 94, 96,
102 will have a second cross-sectional area that is less than the
first cross-sectional area. Ideally, the several constrictions are
sized to result in the same flow rate in all of the medial legs
104, 106 of the flow-channels 66, and the same flow rate in the
inlet 96 and exit 100 legs of the flow-channels 66. In some
circumstances it may be necessary for one or more of the
flow-restrictors to have a different pressure drop than the other
flow-restrictors. Hence one constriction may have a different
cross-sectional area, than the other constrictions. For example,
differences between the inlet and outlet flow rates may necessitate
making the downstream constrictions more severe (i.e. smaller) than
the upstream constrictions to achieve the same total pressure
drop.
[0026] FIGS. 5-7 depict alternative types of flow-restrictors. FIG.
5 depicts a preferred embodiment of a flow-restrictor in accordance
with the present invention, and shows a short constriction 120 in
the flow-channel 66. The constriction 122 of FIG. 6 is similar to
FIG. 5 except that it is elongated to achieve a somewhat greater
pressure drop thereacross for the same cross-sectional are as FIG.
5. FIG. 7 depicts a flow restrictor 124 that is a tortuous segment
of the flow-channel 66 that utilizes extra flow-channel length and
multiple hairpin turns 125 to provide a desired pressure drop in a
short segment of flow-channel 66. Another alternative for the inlet
96 and exit 100 legs of the flow-channels 66 is to make the
entrance and exit ports 109 and 115 (see FIG. 4) to/from the
flow-channels 66 smaller than the channel itself.
[0027] FIG. 8 is a simplified representation of a flow-field
showing only (a) the supply and exhaust manifolds, (b) the
centerlines of each flow-channel, and (c) one embodiment of the
placement of flow restrictors in accordance with the present
invention. More specifically, FIG. 8 shows a supply manifold 126,
an exhaust manifold 128, and a plurality of flow channels 130 (i.e.
only the centerlines thereof shown) extending therebetween. Each
flow-channel 130 has an inlet end 132 that communicates with the
supply manifold 126, and an exit end 134 that communicates with the
exhaust manifold 128. A plurality of flow-restrictors 136, 138 are
strategically positioned in the flow-channels 130 to achieve
desired pressure differentials throughout the flow-field. More
specifically yet, a flow restrictor 136 is positioned near the
inlet end 132 of every other flow channel 130 (e.g. the odd
numbered flow-channels). Similarly, a flow restrictor 138 is placed
near the exit end 134 of all the other flow-channels 130 (e.g. the
even numbered flow-channels). Hence, a first flow-channel 130(a)
has a flow restrictor 136(a) near its inlet end 132, while a next
adjacent second flow-channel 130(b) has a flow restrictor 138(a)
near its exit end 134. A reactant gas is supplied to the
flow-channels from the supply manifold 126 at a first pressure. The
flow-restrictor 136a in the first flow-channel serves to
immediately drop the pressure in the first flow-channel 130(a)
downstream of the flow-restrictor 136a while the pressure in the
second flow-channel 130b remains essentially the same as in the
supply manifold 126 (i.e. less any losses attributable to the
length of the second flow-channel) which is greater than that in
the first flow-channel 130a downstream of flow restrictor 136a.
Proper sizing of the flow-restrictors results in: a sufficient
pressure differential between the first and second flow-channels
130a, 130b to drive gas therebetween through the intervening
gas-permeable current collector; and an equal pressure drop between
the inlet 132, and exit 134 ends of the first and second
flow-channels. The same principles apply to the remaining sets of
adjacent flow-channels of the flow field.
[0028] Like FIG. 8, FIG. 9 is a simplified representation of a
flow-field showing only (a) the supply and exhaust manifolds, (b)
the centerlines of each flow-channel, and (c) another embodiment of
the placement of the flow restrictors in accordance with the
present invention. More specifically, FIG. 9 shows a supply
manifold 140, an exhaust manifold 142, and a plurality of flow
channels 144 extending therebetween. Each flow-channel 144 has: an
inlet leg 143 having an inlet end 148 that communicates with the
supply manifold 140; an exit leg 150 having an exit end 152 that
communicates with the exhaust manifold 142; and at least one medial
leg 146. In the embodiment shown, each flow-channel 144 is
bifurcated at its midsection so as to provide two branches or
medial legs 146(a) and 146(b) for each flow channel 144. The medial
legs/branches 146(a) and 146(b) each communicate with the inlet and
exit legs 143 and 150 for receiving and exhausting a reactant gas
from and to the supply 140 and exhaust 142 manifolds, respectively.
In this embodiment, flow restrictors 154 are positioned in one of
the branches/medial legs 146(a) near the inlet leg 143 and flow
restrictors 156 are positioned in another, next adjacent branch
146b near the exit leg 150. Proper sizing of the flow-restrictors
154, 156 establishes a pressure differential between adjacent
branches 146a, 146b of the same bifurcated flow-channel 144
sufficient to drive reactant gas therebetween through the
intervening gas-permeable current collector. The same principles
apply to the remaining bifurcated flow-channels of the flow
field.
[0029] FIG. 10 is a simplified representation of a flow-field
showing only (a) the supply and exhaust manifolds, (b) the
centerlines of each flow-channel, and (c) still another, and
preferred, embodiment of the placement of the flow restrictors in
accordance with the present invention. More specifically, FIG. 10
depicts a combination of the flow-restrictor placements of the
embodiments shown in FIGS. 8 and 9. In this regard,
flow-restrictors 158 and 160 are positioned in the inlet and exit
legs 162 and 164, respectively, and flow restrictors 166 and 168
are positioned at the beginning of one medial leg 146(a), and at
the end of another medial leg 146(b) of the same bifurcated
flow-channel 144.
[0030] When using flow-restrictors 154, 156 only in the branches
146a, 146b of the bifurcated midsection (see FIG. 9) of
flow-channels 144, none of the inlet 143 and exit 150 legs would
have pressure differentials. When using flow-restrictors only in
the inlet and exit legs, but not in the branches of a bifurcated
flow-channel, half of the branches would have no pressure
differential with their neighbor. When using flow-restrictors 158,
160, 166, 168 in both the inlet/exit legs and in the branches of
the bifurcated midsection (see FIG. 10), the inlet 162 and exit 164
legs would have uniform pressure differentials, and half of the
bifurcation branches 146a, 146b would have more pressure
differential than the rest. This is considered to be the preferred
condition since the criteria for pressure differential is that it
should drive enough flow to provide better stack performance than
achievable only by diffusion through the gas-permeable current
collector yet not so much flow that it causes the membranes to dry
out.
[0031] Virtually unlimited placement possibilities exist for the
location of the several flow-restrictors depending on the pressure
differential profile sought to be achieved by the flow-field
designer. Hence, the invention is not limited to the specific
embodiments set forth above, but rather only to the extent set
forth hereafter in the claims which follow.
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