U.S. patent application number 10/100672 was filed with the patent office on 2007-01-18 for lightweight direct methanol fuel cell and supporting systems.
This patent application is currently assigned to Creare Inc.. Invention is credited to Wayde H. Affleck, Christopher J. Crowley, Michael G. Izenson.
Application Number | 20070015035 10/100672 |
Document ID | / |
Family ID | 23056147 |
Filed Date | 2007-01-18 |
United States Patent
Application |
20070015035 |
Kind Code |
A1 |
Izenson; Michael G. ; et
al. |
January 18, 2007 |
LIGHTWEIGHT DIRECT METHANOL FUEL CELL AND SUPPORTING SYSTEMS
Abstract
A lightweight direct methanol fuel cell unit (20) comprising a
fuel cell stack (24) enclosed within a housing (22). In one
embodiment the fuel cell stack includes a plurality of polymer
electrolyte membrane electrode assemblies (52) stacked
alternatingly with a plurality of bipolar plates (48). Each bipolar
plate includes a cathode flow field (78) defined by a porous
cathode flow field structure (56) and an anode flow field (62)
defined by an upper plate (58) and a lower plate (60) separated
from one another by a plurality of spacers (64). The anode flow
fields are manifolded with one another via manifold embossments
(118, 120) that are hermetically sealed with one another with a
gasket (126). The fuel cell stack and housing are shaped so as to
form four manifold regions (36) in the spaces between the fuel cell
stack and housing. The fuel cell stack is compressed within the
housing by compression members (94) located between the fuel cell
stack and housing so as to place the sidewall (30) of the housing
into tension. Supporting systems for the fuel cell unit include a
fuel handling system (502), an oxidant handling system (600), and a
liquid inventory control system (700).
Inventors: |
Izenson; Michael G.;
(Hanover, NH) ; Crowley; Christopher J.; (Lyme,
NH) ; Affleck; Wayde H.; (Enfield, NH) |
Correspondence
Address: |
DOWNS RACHLIN MARTIN PLLC
199 MAIN STREET
P O BOX 190
BURLINGTON
VT
05402-0190
US
|
Assignee: |
Creare Inc.
Hanover
NH
|
Family ID: |
23056147 |
Appl. No.: |
10/100672 |
Filed: |
March 18, 2002 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
60276314 |
Mar 16, 2001 |
|
|
|
Current U.S.
Class: |
429/444 ;
429/458; 429/469; 429/481; 429/506 |
Current CPC
Class: |
H01M 8/04171 20130101;
H01M 8/247 20130101; H01M 8/2475 20130101; H01M 8/04007 20130101;
Y02E 60/50 20130101; H01M 8/246 20130101; H01M 8/0234 20130101;
H01M 8/0276 20130101; H01M 8/0206 20130101; H01M 8/04291 20130101;
H01M 8/241 20130101; H01M 8/04119 20130101; H01M 8/04208 20130101;
H01M 8/248 20130101; H01M 8/0271 20130101; H01M 8/1009 20130101;
H01M 8/1011 20130101; H01M 8/0232 20130101; H01M 8/0267 20130101;
H01M 8/2484 20160201; H01M 8/0297 20130101; H01M 8/04097 20130101;
Y02E 60/523 20130101; H01M 8/025 20130101; H01M 8/04186 20130101;
H01M 8/2483 20160201; H01M 8/0228 20130101; H01M 8/0278 20130101;
H01M 8/0286 20130101; H01M 8/0258 20130101; H01M 8/0662
20130101 |
Class at
Publication: |
429/038 ;
429/030; 429/022; 429/024; 429/032; 429/013; 429/025 |
International
Class: |
H01M 8/02 20060101
H01M008/02; H01M 8/10 20060101 H01M008/10; H01M 8/04 20060101
H01M008/04 |
Goverment Interests
STATEMENT OF GOVERNMENT RIGHTS
[0002] This invention was made with Government support under
contract MDA972-01-C-0070 awarded by the Defense Advanced Research
Project Agency. The Government has certain rights in the invention.
Claims
1. A fuel cell unit, comprising: a first flow field structure
having a first electrical type that is either an anode-type or a
cathode-type; a membrane electrode assembly located adjacent said
first flow field structure, said membrane electrode assembly
including first and second diffusion layers and an ion conducting
membrane located therebetween; a flow field wick having a second
electrical type that is either said anode-type or said
cathode-type, said second electrical type being opposite said first
electrical type of said first flow field structure, said flow field
wick located adjacent said membrane electrode assembly opposite
said first flow field structure and comprising a layer that
includes: a first face located adjacent said membrane electrode
assembly; passageways for wicking a liquid to said membrane
electrode assembly during use of the fuel cell unit; and at least
one channel for conducting the liquid in a direction substantially
parallel to said first face during use of the fuel cell unit; and a
second flow field structure of said first electrical type, said
second flow field structure located adjacent said flow field wick
opposite said membrane electrode assembly without an intermediate
anode flow field or cathode flow field located therebetween.
2. A fuel cell unit according to claim 1, wherein said first and
second flow field structures are of the anode-type.
3. A fuel cell unit according to claim 1, wherein said flow field
wick includes a second face spaced from said first face and said at
least one channel comprises at least one groove formed in said
second face.
4. A fuel cell unit according to claim 3, wherein said first face
directly confronts said membrane electrode assembly.
5. A fuel cell unit according to claim 1, wherein said flow field
wick comprises porous pyrolytic graphite.
6. A fuel cell unit according to claim 1, wherein each of said
first and second flow field structures comprises a stamped metal
plate.
7. A fuel cell unit according to claim 1, wherein each of said
first and second flow field structures comprises first and second
plates that together define a corresponding flow field.
8. A fuel cell unit according to claim 7, wherein said first and
second plates are made of metal.
9. A fuel cell unit according to claim 7, wherein said first plate
includes a plurality of integrally formed spacers defining a space
between said first plate and said second plate for said
corresponding flow field.
10. A fuel cell unit according to claim 9, wherein said plurality
of spacers comprise embossments formed on said first plate.
11. A fuel cell unit according to claim 7, wherein said second
plate contains a plurality of apertures in fluid communication with
said membrane electrode assembly.
12. A fuel cell unit according to claim 7, wherein said first plate
comprises a plurality of integrally formed spacers defining a space
between said first plate and said second plate for said
corresponding flow field and said second plate contains a plurality
of apertures in fluid communication with said membrane electrode
assembly.
13. A fuel cell unit according to claim 7, wherein said first and
second plates each includes a peripheral region and said first and
second plates are fluidly sealed with one another at said
peripheral regions so as to partially define said corresponding
flow field.
14. A fuel cell unit according to claim 13, wherein said peripheral
region of said first plate and said peripheral region of said
second plate are joined to one another and sealed by a crimped
joint.
15. A fuel cell unit according to claim 1, wherein the fuel cell
unit is a direct methanol fuel cell unit.
16. A fuel cell unit according to claim 15, wherein said flow field
wick has an outer periphery and said first and second flow fields
structures are manifolded radially outward of said outer
periphery.
17-23. (canceled)
24. A fuel cell unit according to claim 7, wherein each of said
first plates includes a first manifold aperture and each of said
second plates includes a second manifold aperture coaxial with said
first manifold aperture.
25. A fuel cell unit according to claim 24, wherein said
corresponding flow field is substantially planar and at least one
of said first and second manifold apertures is formed in a manifold
embossment having a central axis substantially perpendicular to
said corresponding flow field.
26. A fuel cell unit according to claim 25, wherein said manifold
embossment includes a thin-walled cylindrical portion.
27-29. (canceled)
30. A fuel cell unit according to claim 1, wherein each of said
first and second flow field structures includes a first manifold
outlet and a second manifold outlet, wherein said first and second
flow field structures are in fluid communication with one another
via said first and second manifold outlets.
31. A fuel cell unit according to claim 30, further comprising a
manifold sealing member located between each of corresponding ones
of said first and second manifold outlets of said first and second
flow field structures.
32. A fuel cell unit according to claim 30, wherein said
corresponding flow field is substantially planar and each of said
first and second manifold outlets is formed in a manifold
embossment having a central axis substantially perpendicular to
said corresponding flow field.
33. A fuel cell unit according to claim 32, wherein each of said
manifold embossments includes a thin-walled cylindrical
portion.
34. A fuel cell unit according to claim 33, wherein said
cylindrical portions of said manifold embossments of said first and
second flow field structures engage one another so that one
surrounds the other.
35. A fuel cell unit according to claim 34, wherein sealing member
is positioned between said corresponding engaged ones of said
cylindrical portions.
36-44. (canceled)
45. A fuel cell unit according to claim 1, wherein said flow field
wick has an outer periphery and each of said first and second flow
field structures has a portion extending beyond said outer
periphery of said flow field wick, the fuel cell unit further
comprising a sealing member located radially outward of said outer
periphery of said flow field wick and confronting said portions of
said first and second flow field structures extending beyond said
outer periphery of said flow field wick.
46. A fuel cell unit according to claim 45, wherein said sealing
member is an O-ring.
47. A fuel cell unit according to claim 45, wherein one of said
first and second flow field structures includes a plurality of
depressions formed therein to provide a plurality of passageways
that circumvent said sealing member.
48. A fuel cell unit according to claim 47, wherein each depression
is coated with a hydrophobic coating.
49. A fuel cell unit according to claim 45, wherein said membrane
electrode assembly has a portion captured between a corresponding
sealing member and one of said first and second flow field
structures.
50-82. (canceled)
83. A fuel cell unit, comprising: a first anode flow field
structure; a membrane electrode assembly located adjacent said
first anode flow field structure; a cathode flow field wick located
adjacent said membrane electrode assembly opposite said first anode
flow field structure and comprising: a first face located adjacent
said membrane electrode assembly; at least one channel for
conducting an oxidant in a direction substantially parallel to said
first face during use of the fuel cell unit; and passageways for
wicking liquid to said membrane electrode assembly from said at
least one channel during use of the fuel cell unit; and a second
anode flow field structure located adjacent said cathode flow field
wick opposite said membrane electrode assembly without an
intermediate cathode flow field located therebetween.
84. A fuel cell unit according to claim 83, wherein said membrane
electrode assembly includes first and second diffusion layers and a
proton exchange membrane located therebetween.
85. A fuel cell unit according to claim 83, wherein each of said
first and second anode flow field structures comprises a pair of
plates defining a corresponding anode flow field.
86. A fuel cell unit according to claim 85, wherein said pair of
plates are sealed with one another at a peripheral seal.
87. A fuel cell unit according to claim 86, wherein said peripheral
seal comprises a crimped seal.
88. A fuel cell unit according to claim 83, wherein said cathode
flow field wick includes a second face spaced from said first face
and said at least one channel comprises at least one groove formed
in said second face.
89. A fuel cell unit according to claim 83, wherein said first
anode flow field is located immediately adjacent said membrane
electrode assembly.
90. A fuel cell unit according to claim 83, wherein said cathode
flow field wick is located immediately adjacent said membrane
electrode assembly.
91. A fuel cell unit according to claim 83, wherein said cathode
flow field wick comprises a porous pyrolytic graphite layer, said
at least one channel formed within said porous pyrolytic graphite
layer.
92. A fuel cell unit, comprising: a first anode flow field
structure; a membrane electrode assembly located adjacent said
first flow field structure; a cathode flow field wick located
adjacent said membrane electrode assembly opposite said first anode
flow field structure and comprising: a first face confronting said
membrane electrode assembly; a second face spaced from said first
face, said second face containing at least one groove for
conducting an oxidant in a direction substantially parallel to said
first face during use of the fuel cell unit; and pores extending
from said first face for wicking liquid from said membrane
electrode assembly; and a second anode flow field structure located
adjacent said cathode flow field wick opposite said membrane
electrode assembly without an intermediate cathode flow field
located therebetween.
93. A fuel cell unit according to claim 92, wherein said membrane
electrode assembly includes first and second diffusion layers and a
proton exchange membrane located therebetween.
94. A fuel cell unit according to claim 92, wherein each of said
first and second anode flow field structures comprises a pair of
plates defining a corresponding anode flow field.
95. A fuel cell unit according to claim 94, wherein said pair of
plates are sealed with one another at a peripheral seal.
96. A fuel cell unit according to claim 95, wherein said peripheral
seal comprises a crimped seal.
97. A fuel cell unit according to claim 92, wherein said first
anode flow field is located immediately adjacent said membrane
electrode assembly.
98. A fuel cell unit according to claim 92, wherein said cathode
flow field wick is located immediately adjacent said membrane
electrode assembly.
99. A fuel cell unit according to claim 92, wherein said cathode
flow field wick comprises a porous pyrolytic graphite layer, said
at least one channel formed within said porous pyrolytic graphite
layer.
Description
RELATED APPLICATION DATA
[0001] This application claims the benefit of priority of U.S.
Provisional Patent Application No. 60/276,314, filed Mar. 16, 2001,
entitled "Compact, Lightweight Direct Methanol Fuel Cell."
FIELD OF THE INVENTION
[0003] The present invention is generally related to the field of
fuel cells. More particularly, the present invention is directed to
a lightweight fuel cell stack and supporting systems therefor.
BACKGROUND OF THE INVENTION
[0004] A fuel cell electrochemically converts a fuel and an oxidant
into direct current electricity that may be used to power any of a
variety of electrical devices, such as electromechanical equipment,
e.g., motors and actuators, digital and analog circuits, e.g.,
microprocessors and radio transmitters, and other electrical
equipment, e.g., heaters and sensors, among others. Fuel cells are
generally categorized by the type of fuel, e.g., methanol or
hydrogen, and the type of electrolyte, e.g., solid polymer, solid
oxide, molten carbonate and phosphoric acid, used to effect the
electrochemical process within the fuel cell.
[0005] One type of fuel cell that has emerged as a popular variant
is the proton exchange membrane (PEM) (also known as "polymer
electrolyte membrane") type fuel cell. The PEM is a thin sheet of
polymer that allows hydrogen ions (protons) to pass through it.
When used in a fuel cell, the side of the PEM in contact with the
fuel is in electrical contact with an anode electrode and the side
of the PEM in contact with the oxidant is in electrical contact
with a cathode electrode. Hydrogen from the fuel side of the cell
ionizes and passes through the PEM to combine with oxygen on the
oxidant side of the cell. As each hydrogen ion enters the anode
electrode, an electron is split from the hydrogen atom. These freed
electrons then become the source of electric current that can power
an external load.
[0006] During operation, a hydrogen-rich fuel is provided to the
anode side of the PEM as the source of hydrogen atoms that provide
the ions and electrons during the electrochemical process that
splits the electrons and ions from one another. An oxidant,
typically oxygen via air, is provided to the cathode side of the
PEM. When the hydrogen ions passing through the PEM reach the
cathode side of the PEM, they combine with oxygen to produce
water.
[0007] A popular type of PEM fuel cell utilizes methanol as the
source of hydrogen atoms for the electrochemical reaction with the
PEM. Methanol/PEM fuel cells are desirable due to their relatively
low operating temperatures, generally innocuous byproducts, e.g.,
carbon dioxide and water, and ease of storing the methanol fuel
under standard conditions. At standard conditions, i.e., standard
temperature and pressure, methanol is liquid. Thus, the methanol
fuel is typically stored in conventional liquid-type fuel tanks. In
contrast, other types of fuel cells, e.g., hydrogen fuel cells,
typically require their fuels to be stored under non-standard
conditions. For example, hydrogen fuel may be stored as a cryogenic
liquid or a pressurized gas. Liquefying hydrogen at cryogenic
temperatures is an expensive process, and storing liquefied
hydrogen requires bulky insulated containers that vent and lose
hydrogen due to heat leaks. Similarly, compressing and storing
hydrogen gas is relatively costly, and storing this highly
flammable gas is more problematic than storing liquid methanol.
[0008] Early methanol fuel cell systems included a reformer, e.g.,
a steam reformer, that stripped from the methanol molecules the
hydrogen necessary for the electrochemical reaction with the
electrolyte that produced the electricity. The present focus of
methanol fuel cells, however, is on direct methanol fuel cells in
which the liquid methanol fuel is circulated into direct contact
with the anode, rather than just the hydrogen atoms split from the
methanol molecules. In lieu of the reformer, a methanol break-down
catalyst is typically provided adjacent the PEM to remove the
hydrogen atoms from the methanol molecules. Direct methanol fuel
cells have the advantages of, among other things, lighter weight,
reduced complexity, and lower cost due to the elimination of the
reformer.
[0009] In general, to provide a usable amount of electricity all
direct methanol fuel cells require a PEM having a relatively large
surface area. This is typically accomplished by providing a
plurality of PEMs, a plurality of fuel (anode) flow fields, and a
plurality of oxidant (cathode) flow fields stacked alternately with
one another to form a generally compact fuel cell stack, which is
typically enclosed within a housing. The anode and cathode flow
fields are typically provided by plates made from various materials
and having channels or other flow regions formed therein. Since
parallel anode flow fields, and parallel cathode flow fields, are
spaced from one another, manifolds must be provided to distribute
the fuel and oxidant to all of the corresponding flow fields.
Depending upon a particular design of a direct methanol fuel cell
system, the fuel cell stack must be supported by a variety of
supporting systems, which may include a fuel storage and delivery
system, a fuel recirculation system, a carbon dioxide removal
system, an oxidant delivery system, a cathode exhaust system, a
water circulation system, and/or a cooling system, among
others.
[0010] Fuel cell system designers are continually striving to
reduce the complexity of fuel cell systems for a number of reasons
including lower cost, manufacturing efficiency, and reduced
maintenance. In addition, since important applications for fuel
cells include, among other things, manned and unmanned spacecraft,
terrestrial vehicles, and portable electronic equipment, such as
computers and cell phones and similar devices, designers are also
continually striving to decrease the weight and size of fuel cell
stacks, housings, and supporting systems.
SUMMARY OF INVENTION
[0011] In one aspect, the present invention is directed to a fuel
cell unit. The fuel cell unit comprises at least one flow field
structure that includes a first plate having a plurality of
apertures. A second plate confronts the first plate so as to form a
flow field between the first plate and the second plate. A membrane
electrode assembly confronts the first plate.
[0012] In another aspect, the present invention is directed to a
fuel cell unit comprising a fuel cell stack. The fuel stack
comprises a plurality of first flow field structures each having an
outer periphery. A plurality of second flow field structures are
located alternatingly with the plurality of first flow field
structures. Each of the plurality of second flow field structures
defines a cavity for receiving a fluid. Each cavity has first
portion extending radially outward of the outer periphery of each
immediately adjacent second flow field structure. Each of the first
portions fluidly communicating with at least one adjacent first
portion radially outward of the outer periphery of each immediately
adjacent second flow field structure.
[0013] In a further aspect, the present invention is directed to a
fuel cell unit comprising a fuel cell stack that comprises a
plurality of first flow field structures each defining a flow field
having an outer periphery. A plurality of second flow field
structures are located alternatingly with respect to the plurality
of first flow field structures. Each of the plurality of second
flow field structures has a portion extending beyond the outer
periphery of each immediately adjacent first flow field structure.
A plurality of sealing members are each located radially outward of
the outer periphery of a corresponding one of the plurality of
first flow fields structures and confront two of the plurality of
second flow field structures.
[0014] In yet another aspect, the present invention is directed to
a fuel cell system that has a fluid inventory and utilizes a
recirculated fluid having a temperature. The fuel cell system
comprises at least one anode flow field for conducting the
recirculated fluid and a fluid inventory control system. The fluid
inventory control system comprises a sensor for measuring the fluid
inventory. The sensor generates a first control signal. A first
flowpath extends from the at least one anode flow field to the
sensor. At least one second flowpath is in fluid communication with
the first flowpath and extends from the sensor to the first anode
flow field. A device is operatively coupled to the sensor for
controlling the temperature of the recirculated fluid in response
to the first signal.
BRIEF DESCRIPTION OF THE DRAWINGS
[0015] For the purpose of illustrating the invention, the drawings
show a form of the invention that is presently preferred. However,
it should be understood that the present invention is not limited
to the precise arrangements and instrumentalities shown in the
drawings, wherein:
[0016] FIG. 1 is a perspective view of a fuel cell stack and
housing according to the present invention;
[0017] FIG. 2 is a cross-sectional view of the fuel cell stack and
housing as taken along line 2-2 of FIG. 1;
[0018] FIG. 3 is a partially exploded perspective view of a portion
of the fuel cell stack of the fuel cell unit of FIG. 1;
[0019] FIG. 4 is an enlarged partial cross-sectional view of the
fuel cell stack as taken along line 4-4 of FIG. 2 showing a system
for sealing anode flow fields and cathode flow fields from one
another;
[0020] FIG. 5 is a partial cross-sectional view of the fuel cell
unit as taken along line 5-5 of FIG. 1 showing compression members
for compressing the fuel cell stack;
[0021] FIG. 6 is an enlarged partial cross-sectional view of the
fuel cell stack as taken along line 6-6 of FIG. 2 showing oxidant
inlets in conjunction with the sealing system of FIG. 5;
[0022] FIG. 7 is an enlarged cross-sectional view of the fuel cell
stack as taken along line 7-7 of FIG. 2 showing an alternative
system for sealing anode flow fields and cathode flow fields from
one another;
[0023] FIG. 8 is a partial cross-sectional view of the fuel cell
stack as taken along line 8-8 of FIG. 7 showing oxidant inlets in
conjunction with the sealing system of FIG. 7;
[0024] FIG. 9 is a transverse cross-sectional view of an
alternative fuel cell unit of the present invention;
[0025] FIG. 10 is an elevational view of one of the bipolar plate
assemblies of the fuel cell unit of FIG. 9;
[0026] FIG. 11 is a plan view of the cathode flow field structure
of the bipolar plate assembly of FIG. 10;
[0027] FIG. 12 is a plan view of the anode flow field structure of
the bipolar plate assembly of FIG. 10;
[0028] FIG. 13 is a schematic diagram of a fuel handling system of
the present invention;
[0029] FIG. 14 is a schematic diagram of an oxidant handling system
of the present invention; and
[0030] FIG. 15 is a schematic diagram of a liquid inventory control
system of the present invention.
DETAILED DESCRIPTION OF THE INVENTION
[0031] Referring now to the drawings, wherein like numerals
indicate like elements, FIG. 1 illustrates in accordance with the
present invention a fuel cell unit, which is generally denoted by
the numeral 20. As will become apparent from the accompanying
drawings and corresponding description, fuel cell unit 20, alone
and in conjunction with the various supporting systems described
below, contains a number of innovative features that can result in
reduced size, weight, cost, and complexity of the fuel cell unit
and the supporting systems when compared to conventional fuel cell
systems having comparable power outputs. Such reductions are
important in developing practical fuel cell units and supporting
systems. These reductions are a result of a number of factors,
including innovative choice of materials for, and design of, the
various components of fuel cell 20 unit and supporting systems and
innovative approaches to supporting the operation of the fuel cell
unit.
[0032] As shown in FIG. 1, fuel cell unit 20 may comprise a housing
22 and a fuel cell stack 24 contained within the housing. Housing
22 may include an upper wall 26, a lower wall 28, and a sidewall 30
extending between the upper wall and lower wall, typically but not
necessarily, in a direction parallel to the central axis 32 of fuel
cell unit 20. As used herein, the terms "upper" and "lower" are
used for convenience to express the relative position of the
corresponding elements as they appear in the appurtenant drawings.
Elements modified by these terms, of course, can be located at
orientations other than those shown in the drawings.
[0033] As shown more particularly in FIG. 2, sidewall 30 of housing
22 may form a cylinder having an inside diameter D.
Correspondingly, fuel cell stack 24 may have a rectangular
transverse cross-sectional shape generally having four vertices 34
so as to generally define four manifold regions 36, e.g., an anode
inlet manifold region 38, an anode outlet manifold region 40, a
cathode inlet manifold region 42, and a cathode outlet manifold
region 44, between fuel cell stack 24 and housing 22. The distance
C between diagonally opposing vertices 34 may be slightly less that
inside diameter D of housing 22 so as to form an interference fit
between fuel cell stack 24 and sidewall 30 that may aid sealing
adjacent manifold regions 36 from one another. To further improve
the seals between adjacent manifold regions 36, a sealing member
46, such as a formed gasket or caulk bead, may be provided. Sealing
member 46 may be made of any suitable material, such as silicone or
Viton.RTM. rubber (Viton.RTM. is a registered trademark of E.I.
DuPont de Nemours and Company Corporation, Wilmington, Del.). Those
skilled in the art will appreciate that distance C may be equal to
or less than inside diameter D and that the sealing detail between
vertices 34 of fuel cell stack 24 and housing 22 may need to be
modified accordingly.
[0034] Although sidewall 30 is shown as being circular in shape and
fuel cell stack 24 is shown as being rectangular in shape so as to
form four generally circular-segment manifold regions 36 with
housing 22, the shapes of the sidewall and the fuel cell stack may
be any desired. For example, fuel cell stack 24 may be circular and
sidewall 30 may be square so as to form generally triangular
manifold regions adjacent the vertices of the square sidewall.
Other examples include a rectangular fuel cell stack contained in
an elliptical sidewall or a quadrilateral fuel cell stack contained
in a quadrilateral sidewall, wherein the fuel cell stack is smaller
that the housing and is rotated relative to the housing so as to
form four triangular manifold regions. Those skilled in the art
will appreciate the variety of shape combinations that may be used
to form manifold regions 36 between fuel cell stack 24 and housing
22. In addition, fuel cell stack 24 may be shaped and sized to
conformally and snugly fit within housing 22 so as not to form any
manifold regions. In such an embodiment, manifolds for fuel cell
stack 24 may be provided either external to housing 22 or within
the fuel cell stack. However, those skilled in the art will
appreciate that creating manifold regions 36 between fuel cell
stack 24 and housing 22 may have certain advantages over fuel cell
units manifolded external to its housing or within its fuel cell
stack.
[0035] Referring to FIGS. 3 and 4, fuel cell stack 24 may comprise
a plurality of bipolar plates 48 stacked with one another along a
stacking axis 50 and generally separated from one another by
membrane electrode assemblies 52. Each bipolar plate 48 generally
includes an anode flow field structure 54 and a cathode flow field
structure 56. Each anode flow field structure 54 may comprise an
upper plate 58 and a lower plate 60 that together define an anode
flow field 62 for receiving a fuel. Upper and lower plates 58, 60
may be made of any conductive material, such as metal or pyrolytic
graphite sheets, among others. In the embodiment shown, upper and
lower plates 58, 60 are made of stainless steel coated with a
relatively thin layer of gold to improve electrical conductivity
and prevent corrosion. Upper and lower plates 58, 60 may be any
thickness desired. However, in the present embodiment, upper and
lower plates may have a thickness of about 0.025 mm to about 0.250
mm.
[0036] Upper plate 58 may contain a plurality of spacers 64 that
contact lower plate 60 so as to form anode flow field 62 and
provide structures for preventing anode flow field structure 54
from deforming significantly when fuel cell stack 24 is compressed
with housing 22 (FIG. 1), as discussed below. Spacers also provide
many points of electrical contact between upper plate 58 and lower
plate 60. As those skilled in the art will readily appreciate,
spacers 64 may be either formed integrally with upper plate 58 or
lower plate 60 or may be formed separate from both the upper and
lower plates and secured between the upper and lower plates in any
manner known in the art. However, in the present embodiment,
spacers 64 are formed integrally with upper plate 58 using a
stamping, or embossing, process. Such embossing processes are known
in the art and, therefore, need not be described in detail herein.
For example, upper and lower plates 58, 60 may be fabricated using
the techniques described in U.S. Pat. No. 6,170,568 to Valenzuela
entitled "Radial Flow Heat Exchanger," which is incorporated by
reference herein.
[0037] Lower plate 60 may contain a plurality of apertures 66 that
allow anode flow field 62 to fluidly communicate with membrane
electrode assembly 52. As those skilled in the art will readily
appreciate, apertures 66 may be any shape and size, and arranged in
any pattern, desired to suit a particular design. For example, as
shown, apertures 66 are elongated slots having a length of about 4
mm and a width of about 2 mm and are arranged in a staggered
pattern. The arrangement of spacers 64 and apertures 66 may be
coordinated with one another so that the spacers contact the upper
surface of lower plate 60 when upper plate 58 and the lower plate
are in their proper position with respect to one another.
[0038] Upper plate 58 and lower plate 60 may be secured to one
another at their respective peripheral regions 68 to create a fluid
seal between anode flow field 62 and regions located radially
outward from anode flow field structure 54. For example, as shown
in FIG. 5, peripheral regions 68 may be secured to one another with
a folded and crimped joint 70 similar to the crimped joint between
the sidewall and top of a conventional soda can. Crimped joint 70
may be augmented with a sealant (not shown), such as silicone, if
needed. Alternatively to crimped joint 70, peripheral regions 68
may be secured to one another by other means, such as diffusion
bonding, sealing with epoxy, or welding, among others.
[0039] Each cathode flow field structure 56 may be a plate having
an upper side 72, a lower side 74 and a plurality of channels 76
formed in at least one of the upper and lower sides to provide a
cathode flow field 78. Cathode flow field structure 56 may be made
of a microporous material, such as porous pyrolytic graphite (PPG)
or conductive porous metal/ceramic composite materials, among
others. As described below, a microporous material, such as PPG,
may be desirable from a liquid-management perspective. As shown,
cathode flow field structure 56 may contain a plurality of straight
channels 76 adjacent to membrane electrode assembly 52 extending
from one end of the cathode flow field structure to an apposing
end. Those skilled in the art, however, will appreciate that
cathode flow field structure 54 may include any configuration of
channel(s) 76, including a single serpentine channel, required to
suit a particular design.
[0040] Membrane electrode assembly 52 may comprise any type of
electrolyte suitable for the type of fuel cell unit that fuel cell
unit 20 is designed to be. In the present embodiment, fuel cell
unit 20 may be a direct methanol type fuel cell unit. Accordingly,
membrane electrode assembly 52 may comprise a polymer electrolyte
membrane (PEM) (also known as a "proton exchange membrane"), such
as a Nafion.RTM. membrane available from E.I. DuPont de Nemours and
Company Corporation, Wilmington, Del. Those skilled in the art will
appreciate that the membrane electrode assembly 52 will comprise
another material, such as a solid oxide, molten carbonate or
phosphoric acid, among others. Membrane electrode assembly 52 also
comprises an anode electrode 80 located adjacent anode flow field
structure 54 and a cathode electrode 81 located adjacent cathode
flow field structure 56. Depending upon the type of fuel cell,
membrane electrode assembly 52 may include catalyst layers (not
shown) on one or both of its electrodes 80, 81 that react with the
methanol fuel to free the hydrogen molecules therefrom. For
example, in a direct methanol fuel cell utilizing a PEM
electrolyte, the PEM may be joined to sheets of carbon paper that
make up anode and cathode electrodes 80, 81. Anode and cathode
electrodes 80, 81 can be coated with a relatively thin layer of
platinum, or other catalyst. Alternatively, a catalyst such as
platinum black can be applied directly to the PEM.
[0041] Anode flow field 62 and cathode flow field 78 may be fluidly
sealed from one another with a sealing member 82 located between
adjacent anode flow field structures 54. Sealing member 82 may be a
gasket, e.g., an "O-ring" having a shape corresponding generally to
the shape of the outer periphery 84 of cathode flow field structure
56. Of course, sealing member 82 may be another shape, if desired.
Sealing member 82 may be made of a suitable compressible material,
such as silicone or Viton.RTM. rubber. In the embodiment shown,
membrane electrode assembly 52 extends radially beyond sealing
member 82 generally to simplify the fluid seal between anode and
cathode flow fields 62, 78.
[0042] Referring again to FIG. 1, and also to FIG. 5, fuel cell
unit 20 may further include a terminal 86 located at the upper end
of fuel cell stack 24. Terminal 86 may include a plate 88 located
within housing 22 and a contact 90 that extends through a
corresponding aperture 92 in upper end wall 26 of the housing. A
mentioned, it may be desirable to maintain fuel cell stack 24 in
compression within housing 22. This is generally so to maintain
good electrical contact throughout stack 24. Axial compression also
increases the effectiveness of sealing members 82 (FIGS. 3 and 4)
to prevent cross-leakage between anode and cathode flow fields 62,
78 and between the anode and cathode flow fields and manifold
regions 36 where appropriate.
[0043] Fuel cell stack 24 may be maintained in compression in any
of a number of different ways. For example, as shown in FIG. 5, one
or more compression members 94 may be placed between housing 22 and
terminal plate 88. Each compression member 94 may be a
Belleville-type disc spring, a leaf spring, a coil spring, a
silicone gasket, or any other resilient compressible body.
Alternatively, the height along stacking axis of uncompressed fuel
cell stack 24 and any relatively rigid bodies stacked therewith,
e.g., plate 88 of terminal 86, may be made somewhat smaller than
the distance between upper end wall 26 and lower end wall 28 of
housing 22. Thus, when fuel cell stack 24 is inserted into housing
22 it must be compressed accordingly for the housing to be properly
sealed. When fuel cell stack 24 is compressed within housing 22 in
either of the above-described manners, sidewall 30 of the housing
is placed into tension to counteract the compressive force induced
into the fuel cell stack. This allows housing 22 to provide at
least two functions, that of a housing and a tensile member, that
in conventional fuel cell units are typically provided by separate
structures. Accordingly, this consolidation of functions can result
in a weight savings with respect to conventional fuel cell units.
Those skilled in the art will understand that other means exist for
maintaining fuel cell stack in compression, including conventional
means, such as tension rods extending through the fuel cell stack,
among others.
[0044] Referring now to FIGS. 1 and 2, housing 22 may further
include an oxidant inlet 96 and an oxidant outlet 98 in fluid
communication with, respectively, cathode inlet and outlet manifold
regions 42, 44. As described below, oxidant inlet 96 may be in
fluid communication with an oxidant handling system and oxidant
outlet 98 may be in fluid communication with an exhaust system, a
liquid inventory system, and/or a fuel handling system, among other
systems. FIG. 6 shows a pair of adjacent anode flow field
structures 54 and corresponding intermediate cathode flow field
structure 56 and membrane electrode assembly 52 to illustrate a
manner in which cathode inlet manifold region 42 (FIG. 2) may be in
fluid communication with cathode flow field 78 via a plurality of
passageways 100 to allow an oxidant to flow therebetween.
[0045] Passageways 100 may be defined by the space between sealing
member 82 and upper plate 58 at each depression 102 in the upper
plate located immediately adjacent the sealing member. Each
depression 102 may correspond to a spacer, e.g., embossed spacer 64
(FIGS. 3 and 4), or may be a depression formed particularly for the
purpose of creating passageways 100. Sealing member 82 should be
radially spaced from the outer periphery 84 of cathode flow field
structure 56 so that passageways 100, which are located distal from
a plane containing membrane electrode assembly 52, are in fluid
communication with channels 76 of the cathode flow field structure
located proximate to the plane of the membrane electrode assembly.
Depressions 102 may be coated with a material, such as a
fluorocarbon, that makes passageways hydrophobic to prevent fluid
from blocking the flow of oxidant through the passageways. Those
skilled in the art will appreciate that other means may be provided
to allow cathode inlet manifold region 42 (FIG. 2) to fluidly
communicate with cathode flow field 78. For example, in lieu of, or
in addition to, depressions 102 in upper plate 58, sealing member
82 may be provided with transverse grooves and/or apertures (not
shown) extending between cathode inlet manifold region 42 and
cathode flow field 78. It is noted that passageways 100 may be
provided between cathode flow field 78 and cathode outlet manifold
44 (FIG. 2) in a similar manner.
[0046] Referring again to FIGS. 2 and 4, each anode flow field
structure 54 may contain an inlet manifold region 104 and an outlet
manifold region 106. In the embodiment shown, the flow direction of
the fuel is generally straight through anode flow field 62.
Accordingly, inlet and outlet manifold regions 104, 106 are located
at opposing ends of anode flow field structure 54. Other
configurations of anode flow field 62 having different fuel flow
paths may have inlet and outlet manifold regions 104, 106 at
locations other than opposing ends. Each inlet and outlet manifold
region 104, 106 may contain an upper manifold aperture 108 in upper
plate 58 and a lower manifold aperture 110 in lower plate 60. As
those skilled in the art will readily appreciate, if
sub-manifolding is desired, each inlet and outlet manifold region
104, 106 may contain more than one each of upper and lower manifold
apertures 108, 110. As shown in FIGS. 1 and 5, housing 22 may
include a fuel inlet 112 and a fuel outlet 114 in fluid
communication with, respectively, inlet and outlet manifold regions
104, 106 of anode flow field structures 54 of fuel cell stack 24.
Although fuel inlet 112 and fuel outlet 114 are shown entering
housing 22 through upper end wall 26, each may extend through the
housing elsewhere, such as through lower end wall 28 or sidewall
30.
[0047] FIG. 4 shows a joint 116 between outlet manifold regions 106
of adjacent anode flow field structures 54 that allows
corresponding anode flow fields 62 to fluidly communicate with one
another. Inlet manifold regions 104 may be joined in the same, or
other, manner. Each lower manifold aperture 110 may be formed in a
lower embossment 118 of lower plate 60, and each upper manifold
aperture 108 may be formed in an upper embossment 120 of upper
plate 58. Correspondingly, joint 116 may be comprised of portions
of lower and upper embossments 118, 120 and one or more sealing
means, if required. For example, each lower embossment 118 may
include a cylindrical portion 122 that extends into corresponding
upper manifold aperture 108 defined by a similar cylindrical
portion 124. A gasket 126, or other sealing means, may be used to
seal any space that may be present between the respective
cylindrical portions 122, 124 of lower and upper embossments 118,
120.
[0048] Gasket 126 shown is generally circular and has a U-shaped
cross-sectional shape that allows it to engage cylindrical portion
124 of upper embossment 120. Gasket 126 may allow cylindrical
portion 122 of lower embossment 118 to slide relative to upper
embossment 120 to allow for any movement that may occur when fuel
cell stack 24 is compressed within housing 22 (FIG. 1). Gasket 126
should be made of an insulating material to prevent short
circuiting of fuel cell unit 20. Accordingly, gasket 126 may be
made of any suitable material, such as silicone or Viton.RTM.
rubber. Those skilled in the art will appreciate the variety of
joints 116 that may be formed between adjacent anode flow
structures 54. For example, in contradistinction to joint of FIG.
5, cylindrical portion 124 of upper embossment 120 may be smaller
in diameter that cylindrical portion 122 of lower embossment 118 so
that it fits within the cylindrical portion of the lower
embossment. In addition, cylindrical portions 122, 124 of lower and
upper embossments 118, 120 may have the same diameter so that they
generally form a butt joint with one another, with or without an
intermediate gasket (not shown) or other seal. Moreover, one or
both cylindrical portions 122, 124 and/or one or both lower and
upper embossments 118, 120 may be eliminated. In this case, a
suitable gasket (not shown), such as a compressible toroid-shaped
gasket, may surround corresponding adjacent upper and lower
manifold apertures 108, 110 to provide a sealing function, if
required.
[0049] FIG. 7 shows an alternative means for sealing anode flow
field 62' and cathode flow field 78' (see FIG. 8) from one another
within fuel cell stack 24' comprising anode flow field structures
54' and cathode flow field structures 56' similar to the
corresponding structures of fuel cell unit 20 of FIGS. 1-6. In this
embodiment, however, channels 76' of cathode flow field structure
56' are oriented parallel to the primary direction of fuel flow 128
in anode flow field so that oxidant flow 130 is parallel to the
fuel flow and, in this embodiment, in the same direction, too.
Accordingly, manifold region 36' may provide at least two
functions, that of containing outlet manifold regions 106' of anode
flow field structures 54' and providing a cathode outlet manifold
132.
[0050] Cathode flow field structure 56' may be the same as cathode
flow field structure 56 of FIG. 3, except for a rabbet 134 formed
therein adjacent and surrounding its outer periphery 84'. Rabbet
134 may contain a support frame 136 made of a substantially
impervious material, e.g., graphite or plastic, among others. Lower
plate 60' of anode flow field structure 54' may include a sealing
member 138 that extends along the length of support frame 136 and
effects a fluid seal between anode flow field region 62' and
cathode flow field region 78' when membrane electrode assembly 52'
is compressed between the sealing member and support frame. Sealing
member 138 may be formed integrally with lower plate 60', e.g., by
stamping, embossing, or otherwise forming the lower plate.
Alternatively, sealing member 138 may be formed separately from
lower plate 60'. For example, sealing member 138 may be a gasket,
which may or may not be compressible. As shown in FIG. 8, frame
member 136 may contain grooves 140 and/or apertures that allow
cathode flow field 78' to fluidly communicate with manifold region
36', i.e., cathode outlet manifold 132. The sealing detail shown in
FIGS. 7 and 8 may be used at the opposing side, i.e., inlet side,
of fuel cell stack 24'. The sides of fuel cell stack 24' parallel
to the direction of flow in anode and cathode flow fields 62', 78'
may be completely sealed, e.g., by coating these sides with
silicone, or other suitable sealing material. This is so because no
fuel or oxidant flow occurs through these sides.
[0051] FIGS. 9-12 show an alternative embodiment of a fuel cell
unit 320 of the present invention. As shown, housing 322 is
substantially similar to housing 22 of FIG. 1. That is, housing 322
includes a circular sidewall 330 and a lower end wall 328 that
includes oxidant inlet 396 and oxidant outlet 398 that fluidly
communicate with a corresponding cathode inlet and outlet manifold
regions 342, 344, which serve as the cathode inlet and outlet
manifolds themselves. Housing 322 and fuel cell stack 324 also
define anode inlet and outlet manifolds 338, 340, which similarly
serve as the anode inlet and outlet manifolds. Not shown are a fuel
inlet and a fuel outlet that communicate with anode inlet and
outlet manifold regions 338, 340 via an upper end wall, also not
shown. Those skilled in the art will appreciate that the fuel inlet
396 and outlet 398 and oxidant inlet and outlet need not be located
as indicated, but rather may be located at any suitable
locations.
[0052] Among the differences between fuel cell unit 320 and fuel
cell unit 20 of FIGS. 1-6 is the use of a single plate for anode
flow field structure 354 rather than the substantially sealed
two-plate anode flow field structure 54 of FIGS. 1-6. Depending
upon the type of fuel cell unit and the overall design of fuel cell
unit 320, this difference can result in a reduction in the weight
of the fuel cell unit. However, if fuel cell unit 320 is of the
liquid fuel type, e.g., a direct methanol fuel cell, and the
dimensions of fuel cell stacks 324, 24 and corresponding housings
322, 22 are substantially the same, the weight savings of single
plate anode flow field structures 354 may be more than offset by
the weight of additional fuel needed to fill entire anode inlet and
outlet manifold regions 338, 340 rather than the self-contained
inlet and outlet manifold regions 104, 106 of two-plate anode flow
field structures 54.
[0053] As shown particularly in FIG. 10, each bipolar plate 348
generally comprises cathode flow field structure 356 and anode flow
field structure 354. Referring to FIGS. 10 and 11, cathode flow
field structure 356 may be made of a porous material similar to
cathode flow field structure 56 described above in connection with
FIGS. 1-6. Cathode flow field structure 356 may include a first
side containing a plurality of channels 376 and a second side that
is substantially planar. Channels 376 define the predominant
direction of oxidant flow 430 through cathode flow field structure
356. Referring to FIGS. 10 and 12, anode flow field structure 354
of each bipolar plate 348 may be a plate similar to upper plates 58
of anode flow field structure 54 of the embodiment of FIGS. 1-6.
Similarly, anode flow field structure 354 may contain a plurality
of spacers 364 that may be either formed integrally with the anode
flow field structure 354 by stamping, embossing, or another
process, or formed separately and secured to the anode flow field
structure by, e.g., welding, bonding or other means.
[0054] Anode flow field structure 354 may confront cathode flow
field structure 356 so that spacers 364 extend in a direction away
from the cathode flow field structure. To form fuel cell stack 324
using a plurality of bipolar plates 348, the bipolar plates may be
stacked alternatingly with a membrane electrode assembly (not
shown) such that channels 376 confront the cathodes in the membrane
electrode assemblies and spacers 364 confront the anodes. The
membrane electrode assembly may comprise any suitable electrolyte.
However, if fuel cell unit is a direct methanol fuel cell unit, the
membrane electrode assembly may include a PEM similar to PEM
described above in connection with FIGS. 1-6. Each membrane
electrode assembly may optionally be located adjacent to one or
more porous layers, e.g., carbon fiber paper, similar to fuel cell
stack of FIGS. 1-6.
[0055] Spacers 364 generally define a criss-cross pattern of
channels that generally define anode flow field 362. However, the
primary direction of fuel flow 428 through these channels is
perpendicular to the longitudinal direction of channels 376 of
cathode flow field structure 356. This is necessarily so due to the
particular arrangement of anode and cathode manifold regions 338,
340, 342, 344 present in fuel cell unit 320 of FIG. 9. Those
skilled in the art will recognize that alternative shapes of
housing 322 and/or fuel cell stack 324 and/or alternative
arrangements of manifold regions 336 may be used and that the flow
patterns within anode and cathode flow fields 362, 378 may change
accordingly.
[0056] FIG. 13 shows a fuel cell system 500 that includes a fuel
handling system 502. Fuel handling system 502 is particularly
described in connection with a direct methanol type fuel cell
system that utilizes liquid methanol as the fuel. However, those
skilled in the art will understand that fuel handling system 502
may be used with any hydrocarbon fuel that can be used in the form
of a water solution. Fuel cell system 500 may include a fuel cell
unit 504, which is represented in FIG. 13 by a partial fuel cell
stack 506 comprising several bipolar plates 508 separated from one
another by corresponding membrane electrode assemblies 510. Bipolar
plates 508 and membrane electrode assemblies 510 may, but need not,
be as described in connection with fuel cell units 20, 320
described above in connection with FIGS. 1-12. Each bipolar plate
508 generally includes an anode side having an anode flow field 512
and a cathode side having a cathode flow field 514. Of course, fuel
cell unit 504 may comprise a sole anode flow field 512 and a sole
cathode flow field 514 separated from one another by one membrane
electrode assembly 510 in lieu of the plurality of bipolar plates
508 shown.
[0057] Fuel handling system 502 generally includes a fuel supply
conduit 516 for supplying a water/methanol fuel solution to anode
flow fields 512 of fuel cell stack 506 and a fuel return conduit
518 for recirculating the water/methanol solution through the fuel
handling system. As used herein, and in the claims appended hereto,
the term "conduit" includes any piping, tubing, manifolding, or
other fluid-carrying structure. Fuel handling system 502 may
include at least one recirculating pump 52 for circulating the
water/methanol solution through fuel supply conduit 516, anode flow
fields 512, and fuel return conduit 58, preferably at a relatively
high flow rate. Recirculating pump 520 may generally be located at
any point within fuel handling system 502 and may be any suitable
pump, such as a centrifugal pump, a lobe pump, a screw pump, or a
displacement pump, among others.
[0058] Fuel handling system 502 may also include a fuel injection
system 522 that includes at least one fuel supply 524 for providing
fuel handling system 502 with concentrated methanol in a
concentration higher than the methanol concentration in the
methanol/water solution supplied to fuel cell stack 506. Fuel
supply 524 may be a tank or other storage reservoir. When fuel cell
unit 504 is operating, a portion of the methanol (CH.sub.3OH) in
the methanol/water solution within anode flow fields 512 is
stripped of its hydrogen atoms, e.g., by a catalyst (not shown)
within each bipolar plate 508. The freed hydrogen atoms are then
stripped of their electrons as they pass through each corresponding
membrane electrode assembly 510 to create an electrical potential.
Some of the remaining carbon and oxygen atoms combine to form
carbon dioxide gas (CO.sub.2), which remains in the methanol/water
solution. Thus, some of the methanol is depleted from the
recirculating methanol/water solution with each pass of the
recirculating methanol/water solution through anode flow fields
512. Since it is desirable to provide fuel cell stack 506 with a
predetermined optimal methanol concentration, additional methanol
should be provided to the recirculating methanol/water solution to
make up for the methanol used in the fuel cell stack.
[0059] Accordingly, fuel injection system 522 may include a control
system 526 in communication with a methanol concentration sensor
528 and a regulating valve 530 located between fuel supply 524 and
fuel supply conduit 516 for controlling the amount of concentrated
methanol provided to the recirculating methanol/water solution.
Control system 526 may monitor a signal from methanol concentration
sensor 528 and actuate regulating valve 530 to control the flow
rate of methanol into the recirculating methanol/water solution.
Control system 526 may be any suitable digital or analog control
system. Such control systems are well known in the art and,
therefore, need not be described in detail. Methanol concentration
sensor 528 may be any type of concentration sensor, such as the
voltage-type sensor described in U.S. Pat. No. 4,810,597 to Kumagai
et al., which is incorporated herein by reference. Regulating valve
530 may be any suitable type of controllable valve, such as a
rotary valve or a gate valve, among others.
[0060] Fuel handling system 502 may further include a carbon
dioxide separator 532, or gas scrubber, for separating the carbon
dioxide from the methanol/water solution exiting fuel cell stack
506. Carbon dioxide separator 532 may be located upstream of
recirculating pump 520 so that the carbon dioxide gas does not
interfere with the operation of the pump. Carbon dioxide separator
532 may include a housing 534 having an upstream chamber 536, a
downstream chamber 538, and a carbon dioxide vent 540 in fluid
communication with the upstream chamber. Downstream chamber 538 may
be separated from upstream chamber by microporous element 542
having a bubble point greater than the suction head of
recirculating pump 520. Accordingly, as recirulating pump 520 draws
the methanol/water solution through microporous element 542, the
carbon dioxide gas is trapped in upstream chamber 536, where it is
vented through carbon dioxide vent 540. It is noted that the carbon
dioxide gas vented via carbon dioxide vent 540 may include methanol
vapor. This methanol vapor may be handled as described below in
connection with the oxidant handling system of FIG. 14. Housing 534
may be made of any suitable material, such as Nuryl or
polycarbonate, among others. Microporous element 542 may also be
any shape, such as tubular, and may be made of any suitable porous
material that is wettable by the methanol/water solution. Although
fuel handling system 502 is shown with carbon dioxide separator
having microporous element 542, the carbon dioxide separator may be
any suitable type known in the art.
[0061] In addition, fuel handling system 502 may include water
recycling system 544 for recycling water from cathode flow fields
514 produced by the oxidation of the hydrogen ions that pass
through membrane electrolyte assemblies 510 and water that crosses
the membrane electrolyte assemblies due to electro-osmotic drag.
Water recycling system 544 may include a water recycling conduit
546 in fluid communication with cathode flow fields 514 at its
upstream end and recirculating pump 520 at its downstream end.
Recirculating pump 520 generally provides the suction that draws
the water through water recycling system 546.
[0062] FIG. 14 shows fuel cell system 500 in conjunction with an
oxidant handling system 600. Like fuel handling system 502, oxidant
handling system 600 is particularly described in connection with a
direct methanol type fuel cell system that utilizes air as the
oxidant. However, those skilled in the art will understand that
oxidant handling system 600 may be used with other types of
oxidants, such as pure oxygen, among others, that may be supplied
from a pressurized tank.
[0063] Oxidant handling system 600 generally includes an oxidant
supply conduit 602 for supplying oxidant gas to cathode flow fields
514 of fuel cell stack 506 and an exhaust gas conduit 604 for
exhausting air and gaseous products of the electrochemical process
from the cathode flow fields. Oxidant handling system 600 may also
include a methanol cleanup system 606 in fluid communication with
carbon dioxide vent 540 (FIG. 13) of fuel handling system 502. As
mentioned above, the carbon dioxide vented from fuel handling
system 502 may contain methanol vapor. Accordingly, oxidant
handling system 600 may be provided with methanol cleanup system
606 to remove this methanol vapor from fuel cell system 500. For
various reasons, it may be desirable to prevent, or otherwise limit
the amount of, methanol vapor exhausted into the environment
surrounding fuel cell system. Methanol cleanup system 606 may
include a conduit 608 for conducting the carbon dioxide and
methanol vapor from carbon dioxide vent 540 (FIG. 13) and a
catalyst 610, e.g., a precious metal, such as platinum, that causes
the methanol vapor to react with oxygen to form carbon dioxide and
water, which are then exhausted from oxidant handling system along
with the gases from cathode flow fields 514. Oxidant handling
system 600 may further include a blower 612 to force air through
cathode flow fields exhaust gas conduit and methanol cleanup
system. Blower may be any type of blower such as an inline fan or a
centrifugal fan. Of course, if the oxidant is pure oxygen supplied
from a pressurized tank, blower 612 may be eliminated.
[0064] FIG. 15 shows fuel cell system 500 that includes a liquid
inventory system 700 for regulating the balance of water in fuel
cell unit. Liquid inventory system 700 can play important role in
reducing the overall weight of fuel cell system 500. This is so
because water lost from fuel cell system 500, e.g., by evaporation
and exhaustion from oxidant system 600 (FIG. 14) would otherwise
have to be replaced with water from a relatively massive,
carry-along water supply. This approach would be detrimental,
particularly for fuel cell systems that must be as lightweight as
possible. With respect to a direct methanol fuel cell system,
maintaining a constant water inventory is possible, despite
inevitable water transport from the fuel cell stack, because one
byproduct of the reactions within a direct methanol fuel cell is
water. A net two moles of water is produced for every mole of
methanol consumed. Achieving a water-balanced operation generally
requires a liquid inventory control system 700 to recover water
from cathode flow fields 514 of fuel cell stack 506 and carefully
controlling the operating conditions, e.g., methanol concentration
in the methanol/water solution and temperature, of the fuel cell
stack. Importantly, liquid inventory control system 700 should
collect water from cathode flow fields 514 that cross membrane
electrode assemblies due to electro-osmotic drag.
[0065] Referring to FIG. 15, and also to FIG. 13, liquid inventory
control system 700 may comprise recirculating pump 520, fuel return
conduit 518, and water recycle conduit 546. As discussed above,
recirculating pump 520 provides the methanol/water solution to the
inlet side of anode flow fields 512, preferably at a relatively
high flow rate, and recirculates the methanol/water solution
through fuel handling system 502. Recirculating pump 520 also
provides a suction to outlet side of cathode flow fields 514 to
prevent backflow of the methanol/water solution into the cathode
flow fields. When cathode flow field 514 of each bipolar plate 508
is defined by a cathode flow field structure 548, made of a porous
material, such as PPG discussed above in connection with FIGS.
1-12, the suction head of recirculating pump 520 is preferably less
than the bubble point of the porous material. In this manner, the
porous cathode flow field structures 548 remain wetted and
recirculating pump 520 will draw only excess water from cathode
flow field 514. Porous cathode flow field structures 548 should be
hydrophilic to allow them to wick water away from membrane
electrode assemblies 510 and remain wetted. Thus, a porous cathode
flow field structure 548 of this type may be considered a wick.
Passageways (not shown) similar to passageways 100 (FIG. 6) may be
used to allow access for wicks around sealing members 82 (FIG. 4)
to contact the porous cathode flow field structure 548, if such a
sealing detail is provided.
[0066] Liquid inventory control system 700 may also include a
sensor 701 to measure the liquid inventory. Sensor 701 can consist
of any mechanical or electronic device that can produce a control
signal in response to the amount of liquid, e.g., methanol/water
solution, in fuel cell system 500. Sensor 701 may comprise an
accumulator 702 for accumulating excess methanol/water solution in
fuel handling system 502. Accumulator 702 may be located downstream
of fuel return conduit 518 and water recycling conduit 546, e.g.,
in fuel supply conduit 516 of FIG. 13 downstream of, and in fluid
communication with, recirculating pump 520. It is noted that FIG.
13 shows only a sole line representing fuel supply conduit 516.
However, a portion of fuel supply conduit 516 may include two or
more pipes, tubes, or other conduits, such as a warm fuel delivery
conduit 704 and a cool fuel conduit 706, extending between
accumulator 702 and inlet side of anode flow fields 512. In this
manner, liquid inventory control system 700 may be used to control
not only the balance of water in fuel cell system 500 but also
control the temperature of fuel cell stack 506.
[0067] In this connection, fuel cell unit 504 may include thermal
insulation 708 surrounding fuel cell stack 506 to create an
insulated region 710. Accordingly, a relatively large portion of
warm fuel supply conduit 704 may be located within insulated region
710 to keep the methanol/water solution carried therein relatively
warm. In contrast, a relatively large portion of cool fuel supply
conduit 706 may be located outside insulated region 710 to allow
the methanol/water solution carried therein to become relatively
cool. Cool fuel supply conduit 706 may be augmented with one or
more cooling devices 712, such as a radiator or fins, to further
effect the cooling of the methanol/water solution carried by the
cool fuel supply conduit. The methanol/water solution carried by
cool fuel supply conduit 706 may be cooled as much as required for
a particular design, down to about the temperature of the ambient
environment outside of insulated region 710.
[0068] The temperature of methanol/water solution delivered to
anode flow fields 512 may be controlled in a number of ways, such
as by controlling one or more control valves 714 located in warm
and cool fuel supply conduits 704, 706 that extend between
accumulator 702 and the anode flow fields 512. Each control valve
714 may be any type of valve, such as the various types described
above in connection with valve of fuel injection system.
[0069] A control system 716 may be used to actuate one or more of
control valves 714 in response to the methanol/water solution
inventory needs of fuel cell stack 506. In one embodiment, wherein
accumulator 702 is a variable volume reservoir, e.g., bellows 718,
control system 716 may be the bellows in conjunction with an
actuator link 720 extending between the movable end 722 of the
bellows and at least one of control valves 714, e.g., the control
valve for cool fuel supply conduit 706. If the temperature of fuel
cell stack 506 is too low, then bellows 718 will begin to
accumulate methanol/water solution therein and begin to expand such
that movable end moves away from control valves 714, which remain
relatively fixed. In response to the movement of movable end 722 of
bellows 718, actuator link 720, or other motion sensor, actuates
control valve 714 of cool fuel supply conduit 706 so that less
methanol/water solution flows through the cool fuel supply conduit
706. In response, fuel cell stack 506 will warm up so that more
water will evaporate and be exhausted from fuel cell system 500.
Likewise, if fuel cell stack 506 is too hot, control system 716
will cause more methanol/water solution to flow through cool fuel
supply conduit 706 so that the fuel cell stack 506 cools and
evaporates less water. Those skilled in the art will appreciate
that control system 716 may actuate control valve 714 of warm fuel
supply conduit 704, alone or in combination with control valve 714
of cool fuel supply conduit 706, in a similar, albeit opposite,
manner to achieve similar results.
[0070] In an alterative embodiment, sensor 701 may send via control
system 716 a control signal to a cooling fan that blows air across
cool fuel supply conduit 706, or alternatively, sole fuel supply
conduit 516 (FIG. 13), and any cooling devices 712, such as cooling
fins, that may be present on the fuel supply conduit(s). Thus, if
the methanol/water solution is accumulating in fuel cell system
500, then control system 716 would decrease the fan speed. This
would allow fuel cell system 500 to warm up and evaporate water at
a higher rate.
[0071] As mentioned, actuator link 720 may be replaced by a
suitable sensor to detect the accumulation of methanol/water
solution in accumulator 702. In conjunction, a separate control
valve actuator (not shown) may also be provided. If accumulator 702
is bellows 718, a suitable sensor may be a motion sensor or
displacement sensor, among others. Those skilled in the art will
understand the variety of different types of sensors that may be
used with different types of accumulators 702. For example, if
accumulator 702 is a fixed reservoir, e.g., a rigid-walled tank,
the sensor may be a sensor that detects the relative level of the
surface of the methanol/water solution within the reservoir.
Depending upon the type of control valve(s) 714, corresponding
control valve actuator(s) may be of the rotary or linear type among
others. Although liquid inventory system 700 is shown in
conjunction with recirculating a fuel solution through anode flow
fields 512, certain aspects of the fluid inventory control system
may be used to cool fuel cell stack 506 by circulating water or
other coolant through cathode flow field 514 or a coolant flow
field (not shown) in a similar manner to regulate the operating
conditions of the stack. Those skilled in the art will appreciate
the changes necessary to implement such systems.
[0072] As mentioned, the operating conditions of fuel cell stack
506 affect the net gain or loss of water from fuel cell system 500.
The main loss mechanism for water is its evaporation from cathode
flow fields 514 and the transport of the resulting water vapor out
of fuel cell stack 506 along with the other gases exhausted from
the cathode flow fields (see FIG. 14 and accompanying description).
The exhaust gas is typically saturated with water, so that the
temperature of fuel cell stack 506 and the flow rate of gas through
cathode flow fields 516 are critical parameters for maintaining
water-balanced operation. In addition to liquid inventory system
described above in connection with FIG. 15, additional measures may
be taken to conserve water, if needed. For example, exhaust gases
can be at least partially recirculated through cathode flow fields
514 to maintain a relatively high humidity within the cathode flow
fields without a large rate of water vapor loss. In addition, a
condenser (not shown) may be installed in exhaust gas conduit 604
(FIG. 14) to recover water from the exhaust gas. The recovered
water could then be returned to the suction side of recirculating
pump 520, e.g., using a wick or other device that prevents gases
from entering fuel handling system 502 (FIG. 13).
[0073] While the present invention has been described in connection
with a preferred embodiment, it will be understood that it is not
so limited. On the contrary, it is intended to cover all
alternatives, modifications and equivalents as may be included
within the spirit and scope of the invention as defined above and
in the claims appended hereto.
* * * * *