U.S. patent application number 11/829932 was filed with the patent office on 2008-02-14 for efficient micro fuel cell systems and methods.
This patent application is currently assigned to ULTRACELL CORPORATION. Invention is credited to Ian W. Kaye.
Application Number | 20080038601 11/829932 |
Document ID | / |
Family ID | 33556427 |
Filed Date | 2008-02-14 |
United States Patent
Application |
20080038601 |
Kind Code |
A1 |
Kaye; Ian W. |
February 14, 2008 |
EFFICIENT MICRO FUEL CELL SYSTEMS AND METHODS
Abstract
Described herein are fuel cell systems and methods of using fuel
cell systems. The systems include a fuel cell that generates
electrical energy using hydrogen and a fuel processor that produces
hydrogen from a fuel source. The fuel processor includes a reformer
and a burner that heats the reformer.
Inventors: |
Kaye; Ian W.; (Livermore,
CA) |
Correspondence
Address: |
BEYER WEAVER LLP
P.O. BOX 70250
OAKLAND
CA
94612-0250
US
|
Assignee: |
ULTRACELL CORPORATION
399 Lindbergh Avenue
Livermore
CA
94551
|
Family ID: |
33556427 |
Appl. No.: |
11/829932 |
Filed: |
July 29, 2007 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10877771 |
Jun 25, 2004 |
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11829932 |
Jul 29, 2007 |
|
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60482996 |
Jun 27, 2003 |
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60483416 |
Jun 27, 2003 |
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60482981 |
Jun 27, 2003 |
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Current U.S.
Class: |
429/423 ;
429/441; 429/457; 429/518 |
Current CPC
Class: |
B01J 2219/00835
20130101; H01M 8/0631 20130101; C01B 2203/0827 20130101; H01M
8/0247 20130101; H01M 8/04037 20130101; B01J 2208/00407 20130101;
H01M 8/04619 20130101; H01M 8/04731 20130101; B01J 2219/2467
20130101; H01M 8/04388 20130101; B01J 2208/00415 20130101; B01J
2208/00504 20130101; B01J 2208/00646 20130101; B01J 2208/00716
20130101; C01B 2203/0805 20130101; C01B 2203/142 20130101; Y02P
70/50 20151101; C01B 2203/066 20130101; H01M 8/04067 20130101; B01J
8/0484 20130101; B01J 2219/247 20130101; H01M 8/04268 20130101;
H01M 8/04425 20130101; B01J 19/249 20130101; C01B 2203/1247
20130101; C01B 2203/1223 20130101; B01J 2219/2465 20130101; B01J
8/0285 20130101; B01J 8/0438 20130101; C01B 2203/0261 20130101;
H01M 8/0258 20130101; H01M 8/04373 20130101; H01M 8/04559 20130101;
H01M 8/04738 20130101; H01M 2008/1095 20130101; B01J 2219/2481
20130101; H01M 8/04022 20130101; B01J 2219/00907 20130101; B01J
2208/00398 20130101; H01M 8/04626 20130101; H01M 8/04225 20160201;
H01M 8/04589 20130101; B01J 2219/2482 20130101; C01B 2203/0244
20130101; C01B 2203/0811 20130101; H01M 8/04007 20130101; B01J
8/0214 20130101; H01M 8/04955 20130101; B01J 2219/2479 20130101;
B01J 19/0093 20130101; C01B 3/38 20130101; C01B 2203/0822 20130101;
H01M 8/04776 20130101; Y02E 60/50 20130101; C01B 2203/0233
20130101; C01B 2203/0866 20130101; H01M 8/04298 20130101; H01M
8/04365 20130101; B01J 8/0221 20130101; H01M 8/0267 20130101; B01J
2208/00061 20130101; B01J 2219/2475 20130101; B01J 2219/2458
20130101; C01B 2203/0844 20130101; H01M 8/04223 20130101; B01J
2208/00309 20130101; B01J 2219/00873 20130101; H01M 8/0228
20130101; B01J 2219/00783 20130101; B01J 2219/2459 20130101; C01B
2203/82 20130101; H01M 8/04604 20130101; B01J 8/0242 20130101; B01J
2219/2453 20130101; H01M 8/0662 20130101; C01B 3/323 20130101; H01M
8/0435 20130101; H01M 8/04753 20130101; B01J 2219/2462 20130101;
H01M 8/04768 20130101; H01M 8/0618 20130101; Y02P 20/10
20151101 |
Class at
Publication: |
429/020 |
International
Class: |
H01M 8/06 20060101
H01M008/06 |
Claims
1. A fuel cell system for producing electrical energy, the fuel
cell system comprising: a fuel processor that includes a reformer
configured to receive a fuel source, configured to output hydrogen,
and including reformer chamber that contains a catalyst which
facilitates the production of hydrogen, wherein the reformer
chamber has a volume greater than about 0.1 cubic centimeters and
less than about 50 cubic centimeters; a burner configured to
provide heat to the reformer; and a fuel cell including a fuel cell
stack that is configured to produce electrical energy using
hydrogen output by the fuel processor, the fuel cell stack includes
a set of bi-polar plates and a set of membrane electrode assembly
layers, where each bi-polar plate in the stack comprises i) a
substrate, ii) a first channel field including channels formed as
troughs into the substrate for a first surface of the bi-polar
plate, ii) a second channel field including channels formed as
troughs into the substrate for a second surface of the bi-polar
plate, iii) a heat transfer appendage in material continuity with
the substrate and at least partially arranged external to the fuel
cell stack; and plumbing configured to transport a heating medium
from the fuel processor to the heat transfer appendage.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation of and claims priority to
U.S. patent application Ser. No. 10/877,771, filed Jun. 25, 2004
and entitled, "EFFICIENT MICRO FUEL CELL SYSTEMS AND METHODS",
which claims priority under 35 U.S.C. .sctn.119(e) from a) U.S.
Provisional Patent Application No. 60/482,996 filed Jun. 27, 2003,
b) U.S. Provisional Patent Application No. 60/483,416 filed Jun.
27, 2003, c) U.S. Provisional Patent Application No. 60/482,981
filed Jun. 27, 2003; each of these patent applications is
incorporated by reference herein in its entirety for all
purposes.
BACKGROUND OF THE INVENTION
[0002] The present invention relates to fuel cell technology. In
particular, the invention relates to systems for improving fuel
cell system efficiency.
[0003] A fuel cell electrochemically combines hydrogen and oxygen
to produce electrical energy. The ambient air readily supplies
oxygen. Hydrogen provision, however, calls for a working supply.
Gaseous hydrogen has a low energy density that reduces its
practicality as a portable fuel. Liquid hydrogen, which has a
suitable energy density, must be stored at extremely low
temperatures and high pressures, making storing and transporting
liquid hydrogen burdensome.
[0004] A reformed hydrogen supply processes a fuel source to
produce hydrogen. The fuel source acts as a hydrogen carrier.
Currently available hydrocarbon fuel sources include methanol,
ethanol, gasoline, propane and natural gas. Liquid hydrocarbon fuel
sources offer high energy densities and the ability to be readily
stored and transported. A fuel processor reforms the hydrocarbon
fuel source to produce hydrogen.
[0005] Fuel cell evolution so far has concentrated on large-scale
applications such as industrial size generators for electrical
power back-up. Consumer electronics devices and other portable
electrical power applications currently rely on lithium ion and
similar battery technologies. Fuel cell systems that generate
electrical energy for portable applications such as electronics
would be desirable but are not yet commercially available. In
addition, techniques that increase fuel cell system efficiency
would be beneficial.
SUMMARY OF THE INVENTION
[0006] The present invention relates to fuel cell systems and
methods of using fuel cell systems. The systems include a fuel cell
that generates electrical energy using hydrogen and a fuel
processor that produces hydrogen from a fuel source. The fuel
processor includes a reformer and a burner that heats the
reformer.
[0007] A heat efficient fuel cell system described herein heats
internal portions of a fuel cell using a heating medium from a fuel
processor. The heating medium may comprise gases exhausted at high
temperatures from the fuel processor, which are then transported to
the fuel cell. The heating medium may also include a gas that
reacts catalytically in the fuel cell to produce heat. When the
gases include methanol for example, a catalyst in the fuel cell
facilitates production of heat using the methanol. Similarly, a
catalyst in the fuel cell may be employed to facilitate heat
production using hydrogen output from the reformer and provided to
the fuel cell. Heating a fuel cell in this manner allows fuel cell
operating temperatures to be reached sooner during fuel cell
warm-up periods, and permits elevated operating temperatures to be
maintained when electrical energy is not being generated by the
fuel cell.
[0008] Systems and methods for expediting fuel cell system start up
are provided. The systems use electrical heat to expedite start up
time before operating temperatures are reached.
[0009] Methods for shutting down a fuel cell system are also
described that reduce the amount of moisture and gases in the
reformer and in one or more fuel cell components at shut down.
[0010] One hydrogen efficient fuel cell system described herein
transports hydrogen to an inlet of a burner. The hydrogen may
comprise unused hydrogen from a fuel cell and/or hydrogen produced
in a reformer. The burner comprises a catalyst that facilitates
production of heat in the presence of the hydrogen.
[0011] In one aspect, the present invention relates to a fuel cell
system for producing electrical energy. The fuel cell system
comprises a fuel processor and a fuel cell. The fuel processor
includes a reformer configured to receive a fuel source, configured
to output hydrogen, and including a catalyst that facilitates the
production of hydrogen. The fuel processor also includes a burner
configured to provide heat to the reformer. The fuel cell comprises
a fuel cell stack configured to produce electrical energy using
hydrogen output by the fuel processor. The fuel cell also comprises
a heat transfer appendage that a) includes a portion arranged
external to the fuel cell stack and b) is in conductive thermal
communication with an internal portion of the fuel cell stack. The
fuel cell system also comprises plumbing configured to transport a
heating medium from the fuel processor to the fuel cell.
[0012] In another aspect, the present invention relates to a method
for generating electrical energy in a fuel cell that receives
hydrogen from a fuel processor. The fuel processor is configured to
process a fuel source to produce the hydrogen. The method comprises
providing the fuel source to the fuel processor. The method also
comprises transporting a heating medium from the fuel processor to
the fuel cell when electrical energy output by the fuel cell
includes less than an electrical threshold or when temperature of a
component in the fuel cell is less than a temperature threshold.
The method further comprises heating a portion of the fuel cell.
The method additionally comprises transporting hydrogen from the
fuel processor to the fuel cell. The method also comprises
detecting temperature of the component or electrical output of the
fuel cell. The method further comprises generating electrical
energy in the fuel cell when the temperature of the component is
about equal to or greater than the threshold temperature or when
electrical energy output by the fuel cell is about equal to or
greater than an electrical threshold.
[0013] In yet another aspect, the present invention relates to a
method for shutting down a fuel cell system comprising a fuel cell
that received hydrogen from a fuel processor. The fuel processor
includes a reformer and a burner that provided heat to the
reformer. The method comprises stopping electrical energy
generation in the fuel cell. The method also comprises
discontinuing a supply of a fuel source to the reformer, which is
configured to receive the fuel source and output hydrogen. The
method further comprises generating heat in the burner to heat to
the reformer after discontinuing the supply of the fuel source to
the reformer. The method additionally comprises discontinuing heat
generation in the burner. The method also comprises flushing the
burner with air.
[0014] In still another aspect, the present invention relates to a
fuel cell system for producing electrical energy. The fuel cell
system comprises a fuel processor and a burner. The fuel processor
includes a reformer configured to receive a fuel source, configured
to output hydrogen, and including a catalyst that facilitates the
production of hydrogen. The fuel processor also includes a burner
configured to provide heat to the reformer. The fuel cell is
configured to receive hydrogen produced in the reformer and
configured to produce electrical energy using the hydrogen. The
fuel cell system also comprises plumbing configured to transport
hydrogen to the burner.
[0015] In another aspect, the present invention relates to a fuel
cell system for producing electrical energy. The fuel cell system
comprises a fuel processor and a burner. The fuel processor
includes a reformer configured to receive a fuel source, configured
to output hydrogen, and including a catalyst that facilitates the
production of hydrogen. The fuel processor also includes a burner
configured to provide heat to the reformer. The fuel cell is
configured to receive hydrogen produced in the reformer and
configured to produce electrical energy using the hydrogen. The
fuel cell system also comprises plumbing configured to transport
oxygen from the fuel cell to the fuel processor.
[0016] In yet another aspect, the present invention relates to a
method for starting up a fuel processor including a reformer and a
burner that provides heat to the reformer. The method comprises
generating heat using an electrical heater that is configured to
heat the burner or a fuel source provided to the burner. The method
also comprises supplying the fuel source to the burner. The method
further comprises catalytically generating heat in the burner to
heat the reformer. The method additionally comprises supplying the
fuel source to the reformer. The method also comprises generating
hydrogen in the reformer.
[0017] In still another aspect, the present invention relates to a
system for heating a fuel source before catalytic heat generation
within a burner included in a fuel processor. The system comprises
a reformer configured to receive the fuel source, configured to
output hydrogen, and including a catalyst that facilitates the
production of hydrogen. The system also comprises a burner
configured to provide heat to the reformer. The system comprises an
electric heater configured to heat the burner or the fuel source
provided to the burner.
[0018] In another aspect, the present invention relates to a fuel
cell system for producing electrical energy. The fuel cell system
comprises a fuel processor. The fuel processor includes a reformer
configured to receive a fuel source, configured to output hydrogen,
and including a catalyst that facilitates the production of
hydrogen. The fuel processor also includes a burner configured to
provide heat to the reformer. The fuel cell comprises a fuel cell
stack configured to produce electrical energy using hydrogen output
by the fuel processor. The fuel cell also comprises a heat transfer
appendage that a) includes a portion arranged external to the fuel
cell stack and b) is in conductive thermal communication with an
internal portion of the fuel cell stack. The fuel cell system also
comprises plumbing configured to transport a heating medium or a
cooling medium between the fuel processor and the fuel cell.
[0019] In still another aspect, the present invention relates to a
fuel cell system for producing electrical energy. The fuel cell
system comprises a fuel processor. The fuel processor includes a
reformer configured to receive a fuel source, configured to output
hydrogen, and including a catalyst that facilitates the production
of hydrogen. The fuel processor also includes a burner configured
to provide heat to the reformer. The fuel cell comprises a fuel
cell stack configured to produce electrical energy using hydrogen
output by the fuel processor. The fuel cell also comprises a heat
transfer appendage that a) includes a portion arranged external to
the fuel cell stack and b) is in conductive thermal communication
with an internal portion of the fuel cell stack. The fuel cell
system also comprises control logic configured to regulate heat
transfer or temperature for one or more components within the fuel
cell system.
[0020] These and other features and advantages of the present
invention will be described in the following description of the
invention and associated figures.
BRIEF DESCRIPTION OF THE DRAWINGS
[0021] FIG. 1A illustrates a fuel cell system for producing
electrical energy in accordance with one embodiment of the present
invention.
[0022] FIG. 1B illustrates schematic operation for the fuel cell
system of FIG. 1A in accordance with a specific embodiment of the
present invention.
[0023] FIG. 2A illustrates a cross sectional view of a fuel cell
stack for use in the fuel cell of FIG. 1A in accordance with one
embodiment of the present invention.
[0024] FIG. 2B illustrates an outer top perspective view of a fuel
cell stack and fuel cell in accordance with another embodiment of
the present invention.
[0025] FIG. 2C illustrates a ion conductive membrane fuel cell
(PEMFC) architecture for the fuel cell of FIG. 1A in accordance
with one embodiment of the present invention.
[0026] FIG. 2D illustrates a top perspective view of bi-polar
plates in accordance with one embodiment of the present
invention.
[0027] FIG. 2E illustrates a widely used and conventional bi-polar
plate that comprises a plate/cooling layer/plate architecture.
[0028] FIG. 2F illustrates a cross sectional view of a fuel cell
stack for use in the fuel cell of FIG. 1A in accordance with
another embodiment of the present invention.
[0029] FIG. 3A illustrates a cross-sectional side view of a fuel
processor used in the fuel cell system of FIG. 1A in accordance
with one embodiment of the present invention.
[0030] FIG. 3B illustrates a cross-sectional front view of the fuel
processor used in the fuel cell system of FIG. 1A taken through a
mid-plane of fuel processor.
[0031] FIG. 4A illustrates a heat efficient fuel cell system in
accordance with one embodiment of the present invention.
[0032] FIG. 4B illustrates a heat efficient fuel cell system in
accordance with another embodiment of the present invention.
[0033] FIG. 5 illustrates a process flow for generating electrical
energy in a fuel cell that receives hydrogen from a fuel processor
in accordance with one embodiment of the present invention.
[0034] FIG. 6 illustrates an embodiment of the fuel cell system of
FIG. 1A that routes hydrogen from an anode exhaust of the fuel cell
back to a burner in the fuel processor.
[0035] FIG. 7 illustrates a process flow for shutting down a fuel
cell system comprising a fuel cell that received hydrogen from a
fuel processor in accordance with one embodiment of the present
invention.
[0036] FIG. 8 illustrates a schematic operation for a fuel cell
system in accordance with another specific embodiment of the
present invention.
[0037] FIG. 9 illustrates of a system for producing electrical
energy for a portable electronics device in accordance with one
embodiment of the present invention.
[0038] FIG. 10A illustrates a system for heating a fuel source
before catalytic heat generation within burner 30 in accordance
with one embodiment of the present invention.
[0039] FIG. 10B illustrates a process flow for starting up a fuel
processor in accordance with one embodiment of the present
invention.
[0040] FIG. 10C illustrates a system for electrically heating a
reformer in accordance with one embodiment of the present
invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0041] The present invention is described in detail with reference
to a few preferred embodiments as illustrated in the accompanying
drawings. In the following description, numerous specific details
are set forth in order to provide a thorough understanding of the
present invention. It will be apparent, however, to one skilled in
the art, that the present invention may be practiced without some
or all of these specific details. In other instances, well known
process steps and/or structures have not been described in detail
in order to not unnecessarily obscure the present invention.
Fuel Cell System
[0042] FIG. 1A illustrates a fuel cell system 10 for producing
electrical energy in accordance with one embodiment of the present
invention. Fuel cell system 10 comprises a hydrogen fuel supply 12
and a fuel cell 20.
[0043] Hydrogen supply 12 provides hydrogen to fuel cell 20. As
shown, supply 12 includes a `reformed` hydrogen supply that
processes a fuel source to produce hydrogen. Fuel source 17 acts as
a carrier for hydrogen and can be processed to separate hydrogen.
Fuel source 17 may include any hydrogen bearing fuel stream,
hydrocarbon fuel or other hydrogen fuel source such as ammonia.
Currently available hydrocarbon fuel sources 17 suitable for use
with the present invention include methanol, ethanol, gasoline,
propane, butane and natural gas, for example. Several hydrocarbon
and ammonia products may also produce a suitable fuel source 17.
Liquid fuel sources 17 offer high energy densities and the ability
to be readily stored and shipped. Storage device 16 may contain a
fuel mixture. When the fuel processor 15 comprises a steam
reformer, storage device 16 may contain a fuel mixture of a
hydrocarbon fuel source and water. Hydrocarbon fuel source/water
fuel mixtures are frequently represented as a percentage fuel
source in water. In one embodiment, fuel source 17 comprises
methanol or ethanol concentrations in water in the range of
1%-99.9%. Other liquid fuels such as butane, propane, gasoline,
military grade "JP8" etc. may also be contained in storage device
16 with concentrations in water from 5-100%. In a specific
embodiment, fuel source 17 comprises 67% methanol by volume.
[0044] As shown, the reformed hydrogen supply comprises a fuel
processor 15 and a fuel source storage device 16. Storage device 16
stores fuel source 17, and may comprise a portable and/or
disposable fuel cartridge. A disposable cartridge offers a user
instant recharging. In one embodiment, the cartridge includes a
collapsible bladder within a hard plastic case. A separate fuel
pump typically controls fuel source 17 flow from storage device 16.
If system 10 is load following, then a control system meters fuel
source 17 to deliver fuel source 17 to processor 15 at a flow rate
determined by the required power level output of fuel cell 20.
[0045] Fuel processor 15 processes the hydrocarbon fuel source 17
and outputs hydrogen. A hydrocarbon fuel processor 15 heats and
processes a hydrocarbon fuel source 17 in the presence of a
catalyst to produce hydrogen. Fuel processor 15 comprises a
reformer, which is a catalytic device that converts a liquid or
gaseous hydrocarbon fuel source 17 into hydrogen and carbon
dioxide. As the term is used herein, reforming refers to the
process of producing hydrogen from a fuel source. Fuel processor 15
is described in further detail below.
[0046] Fuel cell 20 electrochemically converts hydrogen and oxygen
to water, generating electrical energy and heat in the process.
Ambient air commonly supplies oxygen for fuel cell 20. A pure or
direct oxygen source may also be used for oxygen supply. The water
often forms as a vapor, depending on the temperature of fuel cell
20 components. The electrochemical reaction also produces carbon
dioxide as a byproduct for many fuel cells.
[0047] In one embodiment, fuel cell 20 is a low volume ion
conductive membrane (PEM) fuel cell suitable for use with portable
applications such as consumer electronics. A ion conductive
membrane fuel cell comprises a membrane electrode assembly that
carries out the electrical energy generating electrochemical
reaction. The membrane electrode assembly includes a hydrogen
catalyst, an oxygen catalyst and a ion conductive membrane that a)
selectively conducts protons and b) electrically isolates the
hydrogen catalyst from the oxygen catalyst. A hydrogen gas
distribution layer contains the hydrogen catalyst and allows the
diffusion of hydrogen therethrough. An oxygen gas distribution
layer contains the oxygen catalyst and allows the diffusion of
oxygen and hydrogen protons therethrough. The ion conductive
membrane separates the hydrogen and oxygen gas distribution layers.
In chemical terms, the anode comprises the hydrogen gas
distribution layer and hydrogen catalyst, while the cathode
comprises the oxygen gas distribution layer and oxygen
catalyst.
[0048] A PEM fuel cell often includes a fuel cell stack having a
set of bi-polar plates. A membrane electrode assembly is disposed
between two bi-polar plates. Hydrogen distribution 43 occurs via a
channel field on one plate while oxygen distribution 45 occurs via
a channel field on a second facing plate. Specifically, a first
channel field distributes hydrogen to the hydrogen gas distribution
layer, while a second channel field distributes oxygen to the
oxygen gas distribution layer. The `term `bi-polar` refers
electrically to a bi-polar plate (whether comprised of one plate or
two plates) sandwiched between two membrane electrode assembly
layers. In this case, the bi-polar plate acts as both a negative
terminal for one adjacent membrane electrode assembly and a
positive terminal for a second adjacent membrane electrode assembly
arranged on the opposite face of the bi-polar plate.
[0049] In electrical terms, the anode includes the hydrogen gas
distribution layer, hydrogen catalyst and bi-polar plate. The anode
acts as the negative electrode for fuel cell 20 and conducts
electrons that are freed from hydrogen molecules so that they can
be used externally, e.g., to power an external circuit. In a fuel
cell stack, the bi-polar plates are connected in series to add the
potential gained in each layer of the stack. In electrical terms,
the cathode includes the oxygen gas distribution layer, oxygen
catalyst and bi-polar plate. The cathode represents the positive
electrode for fuel cell 20 and conducts the electrons back from the
external electrical circuit to the oxygen catalyst, where they can
recombine with hydrogen ions and oxygen to form water.
[0050] The hydrogen catalyst separates the hydrogen into protons
and electrons. The ion conductive membrane blocks the electrons,
and electrically isolates the chemical anode (hydrogen gas
distribution layer and hydrogen catalyst) from the chemical
cathode. The ion conductive membrane also selectively conducts
positively charged ions. Electrically, the anode conducts electrons
to a load (electrical energy is produced) or battery (energy is
stored). Meanwhile, protons move through the ion conductive
membrane. The protons and used electrons subsequently meet on the
cathode side, and combine with oxygen to form water. The oxygen
catalyst in the oxygen gas distribution layer facilitates this
reaction. One common oxygen catalyst comprises platinum powder very
thinly coated onto a carbon paper or cloth. Many designs employ a
rough and porous catalyst to increase surface area of the platinum
exposed to the hydrogen and oxygen.
[0051] In one embodiment, fuel cell 20 comprises a set of bi-polar
plates formed from a single plate. Each plate includes channel
fields on opposite faces of the plate. The single bi-polar plate
thus dually distributes hydrogen and oxygen: one channel field
distributes hydrogen while a channel field on the opposite face
distributes oxygen. Multiple bi-polar plates can be stacked to
produce a `fuel cell stack` in which a membrane electrode assembly
is disposed between each pair of adjacent bi-polar plates.
[0052] Since the electrical generation process in fuel cell 20 is
exothermic, fuel cell 20 may implement a thermal management system
to dissipate heat from the fuel cell. Fuel cell 20 may also employ
a number of humidification plates (HP) to manage moisture levels in
the fuel cell. Further description of a fuel cell suitable for use
with the present invention is included in commonly owned co-pending
patent application entitled "Micro Fuel Cell Architecture" naming
Ian Kaye as inventor and filed on the same day as this patent
application, which is incorporated by reference for all
purposes.
[0053] While the present invention will mainly be discussed with
respect to PEM fuel cells, it is understood that the present
invention may be practiced with other fuel cell architectures. The
main difference between fuel cell architectures is the type of ion
conductive membrane used. In one embodiment, fuel cell 20 is
phosphoric acid fuel cell that employs liquid phosphoric acid for
ion exchange. Solid oxide fuel cells employ a hard, non-porous
ceramic compound for ion exchange and may be suitable for use with
the present invention. Generally, any fuel cell architecture may
benefit from one or more systems and controls improvements
described herein. Other such fuel cell architectures include direct
methanol, alkaline and molten carbonate fuel cells.
[0054] Fuel cell 20 generates dc voltage that may be used in a wide
variety of applications. For example, electrical energy generated
by fuel cell 20 may be used to power a motor or light. In one
embodiment, the present invention provides `small` fuel cells that
are configured to output less than 200 watts of power (net or
total). Fuel cells of this size are commonly referred to as `micro
fuel cells` and are well suited for use with portable electronics
devices. In one embodiment, fuel cell 20 is configured to generate
from about 1 milliwatt to about 200 watts. In another embodiment,
fuel cell 20 generates from about 3 W to about 20 W. Fuel cell 20
may also be a stand-alone fuel cell, which is a single unit that
produces power as long as it has an a) oxygen and b) hydrogen or a
hydrocarbon fuel supply. A stand-alone fuel cell 20 that outputs
from about 40 W to about 100 W is well suited for use in a laptop
computer.
[0055] In one embodiment, fuel processor 15 is a steam reformer
that only needs steam to produce hydrogen. Several types of
reformers suitable for use in fuel cell system 10 include steam
reformers, auto thermal reformers (ATR) or catalytic partial
oxidizers (CPOX). ATR and CPOX reformers mix air with the fuel and
steam mix. ATR and CPOX systems reform fuels such as methanol,
diesel, regular unleaded gasoline and other hydrocarbons. In a
specific embodiment, storage device 16 provides methanol 17 to fuel
processor 15, which reforms the methanol at about 250.degree. C. or
less and allows fuel cell system 10 use in applications where
temperature is to be minimized.
[0056] FIG. 1B illustrates schematic operation for fuel cell system
10 in accordance with a specific embodiment of the present
invention. As shown, fuel cell system 10 comprises fuel container
16, hydrogen fuel source 17, fuel processor 15, fuel cell 20,
multiple pumps 21 and fans 35, fuel lines and gas lines, and one or
more valves 23.
[0057] Fuel container 16 stores methanol as a hydrogen fuel source
17. An outlet 26 of fuel container 16 provides methanol 17 into
hydrogen fuel source line 25. As shown, line 25 divides into two
lines: a first line 27 that transports methanol 17 to a burner 30
for fuel processor 15 and a second line 29 that transports methanol
17 to reformer 32 in fuel processor 15. Lines 25, 27 and 29 may
comprise plastic tubing, for example. Separate pumps 21a and 21b
are provided for lines 27 and 29, respectively, to pressurize the
lines and transmit the fuel source at independent rates if desired.
A model P625 pump as provided by Instech of Plymouth Meeting, Pa.
is suitable to transmit liquid methanol for system 10 is suitable
in this embodiment. A flow sensor or valve 23 situated on line 29
between storage device 16 and fuel processor 15 detects and
communicates the amount of methanol 17 transfer between storage
device 16 and reformer 32. In conjunction with the sensor or valve
23 and suitable control, such as digital control applied by a
processor that implements instructions from stored software, pump
21b regulates methanol 17 provision from storage device 16 to
reformer 32.
[0058] Fan 35a delivers oxygen and air from the ambient room
through line 31 to dewar 150 of fuel processor 15. Fan 35b delivers
oxygen and air from the ambient room through line 33 to oxygen
distribution 45 in fuel cell 20. In this embodiment, a model
AD2005DX-K70 fan as provided by Adda USA of California is suitable
to transmit oxygen and air for fuel cell system 10. A fan 37 blows
cooling air over fuel cell 20 and its heat transfer appendages
46.
[0059] Fuel processor 15 receives methanol 17 from storage device
16 and outputs hydrogen. Fuel processor 15 comprises burner 30,
reformer 32, boiler 34 and dewar 150. Burner 30 includes an inlet
that receives methanol 17 from line 27 and a catalyst that
generates heat with methanol presence. Boiler 34 includes an inlet
that receives methanol 17 from line 29. The structure of boiler 34
permits heat produced in burner 30 to heat methanol 17 in boiler 34
before reformer 32 receives the methanol 17. Boiler 34 includes an
outlet that provides heated methanol 17 to reformer 32. Reformer 32
includes an inlet that receives heated methanol 17 from boiler 34.
A catalyst in reformer 32 reacts with the methanol 17 and produces
hydrogen and carbon dioxide. This reaction is slightly endothermic
and draws heat from burner 30. A hydrogen outlet of reformer 32
outputs hydrogen to line 39. In one embodiment, fuel processor 15
also includes a preferential oxidizer that intercepts reformer 32
hydrogen exhaust and decreases the amount of carbon monoxide in the
exhaust. The preferential oxidizer employs oxygen from an air inlet
to the preferential oxidizer and a catalyst, such as ruthenium or
platinum, that is preferential to carbon monoxide over carbon
dioxide.
[0060] Dewar 150 pre-heats air before the air enters burner 30.
Dewar 150 also reduces heat loss from fuel cell 20 by heating the
incoming air before it escapes fuel processor 15. In one sense,
dewar 150 acts as a regenerator that uses waste heat in fuel
processor 15 to increase thermal management and thermal efficiency
of the fuel processor. Specifically, waste heat from burner 30 may
be used to pre-heat incoming air provided to burner 30 to reduce
heat transfer to the air in the burner so more heat transfers to
reformer 32.
[0061] Line 39 transports hydrogen from fuel processor 15 to fuel
cell 20. Gaseous delivery lines 31, 33 and 39 may comprise plastic
tubing, for example. A hydrogen flow sensor (not shown) may also be
added on line 39 to detect and communicate the amount of hydrogen
being delivered to fuel cell 20. In conjunction with the hydrogen
flow sensor and suitable control, such as digital control applied
by a processor that implements instructions from stored software,
fuel processor 15 regulates hydrogen gas provision to fuel cell
20.
[0062] Fuel cell 20 includes an hydrogen inlet port that receives
hydrogen from line 39 and delivers it to a hydrogen intake manifold
for delivery to one or more bi-polar plates and their hydrogen
distribution channels. An oxygen inlet port of fuel cell 20
receives oxygen from line 33 and delivers it to an oxygen intake
manifold for delivery to one or more bi-polar plates and their
oxygen distribution channels. An anode exhaust manifold collects
gases from the hydrogen distribution channels and delivers them to
an anode exhaust port, which outlets the exhaust gases into the
ambient room. A cathode exhaust manifold collects gases from the
oxygen distribution channels and delivers them to a cathode exhaust
port.
[0063] In addition to the components shown in shown in FIG. 1B,
system 10 may also include other elements such as electronic
controls, additional pumps and valves, added system sensors,
manifolds, heat exchangers and electrical interconnects useful for
carrying out functionality of a fuel cell system 10 that are known
to one of skill in the art and omitted herein for sake of
brevity.
2. Fuel Cell
[0064] FIG. 2A illustrates a cross sectional view of a fuel cell
stack 60 for use in fuel cell 20 in accordance with one embodiment
of the present invention. FIG. 2B illustrates an outer top
perspective view of a fuel cell stack 60 and fuel cell 20 in
accordance with another embodiment of the present invention.
[0065] Referring initially to FIG. 2A, fuel cell stack 60 is a
bi-polar plate stack that comprises a set of bi-polar plates 44 and
a set of membrane electrode assembly (MEA) layers 62. Two MEA
layers 62 neighbor each bi-polar plate 44. With the exception of
topmost and bottommost membrane electrode assembly layers 62a and
62b, each MEA 62 is disposed between two adjacent bi-polar plates
44. For MEAs 62a and 62b, top and bottom end plates 64a and 64b
include a channel field 72 on the face neighboring an MEA 62.
[0066] The bi-polar plates 44 in stack 60 also each include two
heat transfer appendages 46. More specifically, each bi-polar plate
44 includes a heat transfer appendage 46a on one side of the plate
and a heat transfer appendage 46b on the opposite side. Heat
transfer appendages 46 are discussed in further detail below.
[0067] As shown, stack 60 includes twelve membrane electrode
assembly layers 62, eleven bi-polar plates 44 and two end plates
64. The number of bi-polar plates 44 and MEA layers 62 in each set
may vary with design of fuel cell stack 60. Stacking parallel
layers in fuel cell stack 60 permits efficient use of space and
increased power density for fuel cell 20. In one embodiment, each
membrane electrode assembly 62 produces 0.7 V and the number of MEA
layers 62 is selected to achieve a desired voltage. Alternatively,
the number of MEA layers 62 and bi-polar plates 44 may be
determined by the allowable thickness in an electronics device. A
fuel cell stack 60 having from one MEA 62 to several hundred MEAs
62 is suitable for many applications. A stack 60 having from about
three MEAs 62 to about twenty MEAs 62 is also suitable for numerous
applications. Fuel cell 20 size and layout may also be tailored and
configured to output a given power.
[0068] Referring to FIG. 2B, top and bottom end plates 64a and 64b
provide mechanical protection for stack 60. End plates 64 also hold
the bi-polar plates 44 and MEA layers 62 together, and apply
pressure across the planar area of each bi-polar plate 44 and each
MEA 62. Bolts 82a and 82b connect and secure top and bottom end
plates 64a and 64b together.
[0069] Fuel cell 20 includes two anode ports that open to the
outside of fuel cell stack 60: an inlet anode port or inlet
hydrogen port 84, and an outlet anode port or outlet hydrogen port
86. Inlet hydrogen port 84 is disposed on top end plate 64a,
couples with an inlet line to receive hydrogen gas, and opens to an
inlet hydrogen manifold 102 (see FIG. 2D) that is configured to
deliver inlet hydrogen gas to a channel field 72 on each bi-polar
plate 44 in stack 60. Outlet port 86 receives outlet gases from an
anode exhaust manifold 104 (see FIG. 2D) that is configured to
collect waste products from the anode channel fields 72 of each
bi-polar plate 44. Outlet port 86 may provide the exhaust gases to
the ambient space about the fuel cell output the gases to a line
that couples to port 86 for transportation of the anode exhaust
gases as described below.
[0070] Fuel cell 20 includes two cathode ports: an inlet cathode
port or inlet oxygen port (not shown), and an outlet cathode port
or outlet water/vapor port 90. Inlet oxygen port is disposed on
bottom end plate 64b, couples with an inlet line to receive ambient
air, and opens to an oxygen manifold 106 that is configured to
deliver inlet oxygen and ambient air to a channel field 72 on each
bi-polar plate 44 in stack 60. Outlet water/vapor port 90 receives
outlet gases from a cathode exhaust manifold 108 (see FIG. 2D) that
is configured to collect water (typically as a vapor) from the
cathode channel fields 72 on each bi-polar plate 44.
[0071] FIG. 2C illustrates a ion conductive membrane fuel cell
(PEMFC) architecture 120 for use in fuel cell 20 in accordance with
one embodiment of the present invention. As shown, PEMFC
architecture 120 comprises two bi-polar plates 44 and a membrane
electrode assembly layer (or MEA) 62 sandwiched between the two
bi-polar plates 44. The MEA 62 electrochemically converts hydrogen
and oxygen to water, generating electrical energy and heat in the
process. Membrane electrode assembly 62 includes an anode gas
diffusion layer 122, a cathode gas diffusion layer 124, a hydrogen
catalyst 126, ion conductive membrane 128, anode electrode 130,
cathode electrode 132, and oxygen catalyst 134.
[0072] Pressurized hydrogen gas (H.sub.2) enters fuel cell 20 via
hydrogen port 84, proceeds through inlet hydrogen manifold 102 and
through hydrogen channels 74 of a hydrogen channel field 72a
disposed on the anode face 75 of bi-polar plate 44a. The hydrogen
channels 74 open to anode gas diffusion layer 122, which is
disposed between the anode face 75 of bi-polar plate 44a and ion
conductive membrane 128. The pressure forces hydrogen gas into the
hydrogen-permeable anode gas diffusion layer 122 and across the
hydrogen catalyst 126, which is disposed in the anode gas diffusion
layer 122. When an H.sub.2 molecule contacts hydrogen catalyst 126,
it splits into two H+ ions (protons) and two electrons (e-). The
protons move through the ion conductive membrane 128 to combine
with oxygen in cathode gas diffusion layer 124. The electrons
conduct through the anode electrode 130, where they build potential
for use in an external circuit (e.g., a power supply of a laptop
computer) After external use, the electrons flow to the cathode
electrode 132 of PEMFC architecture 120.
[0073] Hydrogen catalyst 126 breaks hydrogen into protons and
electrons. Suitable catalysts 126 include platinum, ruthenium, and
platinum black or platinum carbon, and/or platinum on carbon
nanotubes, for example. Anode gas diffusion layer 122 comprises any
material that allows the diffusion of hydrogen therethrough and is
capable of holding the hydrogen catalyst 126 to allow interaction
between the catalyst and hydrogen molecules. One such suitable
layer comprises a woven or non-woven carbon paper. Other suitable
gas diffusion layer 122 materials may comprise a silicon carbide
matrix and a mixture of a woven or non-woven carbon paper and
Teflon.
[0074] On the cathode side of PEMFC architecture 120, pressurized
air carrying oxygen gas (O.sub.2) enters fuel cell 20 via oxygen
port 88, proceeds through inlet oxygen manifold 106, and through
oxygen channels 76 of an oxygen channel field 72b disposed on the
cathode face 77 of bi-polar plate 44b. The oxygen channels 76 open
to cathode gas diffusion layer 124, which is disposed between the
cathode face 77 of bi-polar plate 44b and ion conductive membrane
128. The pressure forces oxygen into cathode gas diffusion layer
124 and across the oxygen catalyst 134 disposed in the cathode gas
diffusion layer 124. When an O.sub.2 molecule contacts oxygen
catalyst 134, it splits into two oxygen atoms. Two H+ ions that
have traveled through the ion selective ion conductive membrane 128
and an oxygen atom combine with two electrons returning from the
external circuit to form a water molecule (H.sub.2O). Cathode
channels 76 exhaust the water, which usually forms as a vapor. This
reaction in a single MEA layer 62 produces about 0.7 volts.
[0075] Cathode gas diffusion layer 124 comprises a material that
permits diffusion of oxygen and hydrogen protons therethrough and
is capable of holding the oxygen catalyst 134 to allow interaction
between the catalyst 134 with oxygen and hydrogen. Suitable gas
diffusion layers 124 may comprise carbon paper or cloth, for
example. Other suitable gas diffusion layer 124 materials may
comprise a silicon carbide matrix and a mixture of a woven or
non-woven carbon paper and Teflon. Oxygen catalyst 134 facilitates
the reaction of oxygen and hydrogen to form water. One common
catalyst 134 comprises platinum. Many designs employ a rough and
porous catalyst 134 to increase surface area of catalyst 134
exposed to the hydrogen or oxygen. For example, the platinum may
reside as a powder very thinly coated onto a carbon paper or cloth
cathode gas diffusion layer 124.
[0076] Ion conductive membrane 128 electrically isolates the anode
from the cathode by blocking electrons from passing through
membrane 128. Thus, membrane 128 prevents the passage of electrons
between gas diffusion layer 122 and gas diffusion layer 124. Ion
conductive membrane 128 also selectively conducts positively
charged ions, e.g., hydrogen protons from gas diffusion layer 122
to gas diffusion layer 124. For fuel cell 20, protons move through
membrane 128 and electrons are conducted away to an electrical load
or battery. In one embodiment, ion conductive membrane 128
comprises an electrolyte. One electrolyte suitable for use with
fuel cell 20 is Celtec 1000 from PEMEAS USA AG of Murray Hill, N.J.
(www.pemeas.com). Fuel cells 20 including this electrolyte are
generally more carbon monoxide tolerant and may not require
humidification. Ion conductive membrane 128 may also employ a
phosphoric acid matrix that includes a porous separator impregnated
with phosphoric acid. Alternative ion conductive membranes 128
suitable for use with fuel cell 20 are widely available from
companies such as United technologies, DuPont, 3M, and other
manufacturers known to those of skill in the art. For example, WL
Gore Associates of Elkton, Md. produces the primea Series 58, which
is a low temperature MEA suitable for use with the present
invention.
[0077] In one embodiment, fuel cell 20 requires no external
humidifier or heat exchanger and the stack 60 only needs hydrogen
and air to produce electrical power. Alternatively, fuel cell 20
may employ humidification of the cathode to fuel cell 20 improve
performance. For some fuel cell stack 60 designs, humidifying the
cathode increases the power and operating life of fuel cell 20.
[0078] FIG. 2D illustrates a top perspective view of bi-polar
plates 44p and 44q in accordance with one embodiment of the present
invention. Bi-polar plate 44 is a single plate 44 with a first
channel fields 72 disposed on opposite faces 75 of the plate
44.
[0079] Functionally, bi-polar plate 44 a) delivers and distributes
reactant gasses to the gas diffusion layers 122 and 124 and their
respective catalysts, b) maintains separation of the reactant
gasses from one another between MEA layers 62 in stack 60, c)
exhausts electrochemical reaction byproducts from MEA layers 62, d)
facilitates heat transfer to and/or from MEA layers 62 and fuel
cell stack 60, and e) includes gas intake and gas exhaust manifolds
for gas delivery to other bi-polar plates 44 in the fuel stack
60.
[0080] Structurally, bi-polar plate 44 has a relatively flat
profile and includes opposing top and bottom faces 75a and 75b
(only top face 75a is shown) and a number of sides 78. Faces 75 are
substantially planar with the exception of channels 76 formed as
troughs into substrate 89. Sides 78 comprise portions of bi-polar
plate 44 proximate to edges of bi-polar plate 44 between the two
faces 75. As shown, bi-polar plate 44 is roughly quadrilateral with
features for the intake manifolds, exhaust manifolds and heat
transfer appendage 46 that provide deviation from the quadrilateral
shape.
[0081] The manifold on each plate 44 is configured to deliver a gas
to a channel field on a face of the plate 44 or receive a gas from
the channel field 72. The manifolds for bi-polar plate 44 include
apertures or holes in substrate 89 that, when combined with
manifolds of other plates 44 in a stack 60, form an inter-plate 44
gaseous communication manifold (such as 102, 104, 106 and 108).
Thus, when plates 44 are stacked and their manifolds substantially
align, the manifolds permit gaseous delivery to and from each plate
44.
[0082] Bi-polar plate 44 includes a channel field 72 or "flow
field" on each face of plate 44. Each channel field 72 includes one
or more channels 76 formed into the substrate 89 of plate 44 such
that the channel rests below the surface of plate 44. Each channel
field 72 distributes one or more reactant gasses to an active area
for the fuel cell stack 60. Bi-polar plate 44 includes a first
channel field 72a on the anode face 75a of bi-polar plate 44 that
distributes hydrogen to an anode (FIG. 2C), while a second channel
field on opposite cathode face 75b distributes oxygen to a cathode.
Specifically, channel field 72a includes multiple channels 76 that
permit oxygen and air flow to anode gas diffusion layer 122, while
channel field 72b includes multiple channels 76 that permit oxygen
and air flow to cathode gas diffusion layer 124. For fuel cell
stack 60, each channel field 72 is configured to receive a reactant
gas from an intake manifold 102 or 106 and configured to distribute
the reactant gas to a gas diffusion layer 122 or 124. Each channel
field 72 also collects reaction byproducts for exhaust from fuel
cell 20. When bi-polar plates 44 are stacked together in fuel cell
60, adjacent plates 44 sandwich an MEA layer 62 such that the anode
face 75a from one bi-polar plate 44 neighbors a cathode face 75b of
an adjacent bi-polar plate 44 on an opposite side of the MEA layer
62.
[0083] Bi-polar plate 44 may include one or more heat transfer
appendages 46. Heat transfer appendage 46 permits external thermal
management of internal portions of fuel cell stack 60. More
specifically, appendage 46 may be used to heat or cool internal
portions of fuel cell stack 60 such as internal portions of each
attached bi-polar plate 44 and any neighboring MEA layers 62, for
example. Heat transfer appendage 46 is laterally arranged outside
channel field 72. In one embodiment, appendage 46 is disposed on an
external portion of bi-polar plate 44. External portions of
bi-polar plate 44 include any portions of plate 44 proximate to a
side or edge of the substrate included in plate 44. External
portions of bi-polar plate 44 typically do not include a channel
field 72. For the embodiment shown, heat transfer appendage 46
substantially spans a side of plate 44 that does not include intake
and output manifolds 102-108. For the embodiment shown in FIG. 2A,
plate 44 includes two heat transfer appendages 46 that
substantially span both sides of plate 44 that do not include a gas
manifold.
[0084] Peripherally disposing heat transfer appendage 46 allows
heat transfer between inner portions of plate 44 and the externally
disposed appendage 46 via the plate substrate 89. Conductive
thermal communication refers to heat transfer between bodies that
are in contact or that are integrally formed. Thus, lateral
conduction of heat between external portions of plate 44 (where the
heat transfer appendage 46 attaches) and central portions of
bi-polar plate 44 occurs via conductive thermal communication
through substrate 89. In one embodiment, heat transfer appendage 46
is integral with substrate material 89 in plate 44. Integral in
this sense refers to material continuity between appendage 46 and
plate 44. An integrally formed appendage 46 may be formed with
plate 44 in a single molding, stamping, machining or MEMs process
of a single metal sheet, for example. Integrally forming appendage
46 and plate 44 permits conductive thermal communication and heat
transfer between inner portions of plate 44 and the heat transfer
appendage 46 via substrate 89. In another embodiment, appendage 46
comprises a material other than that used in substrate 89 that is
attached onto plate 44 and conductive thermal communication and
heat transfer occurs at the junction of attachment between the two
attached materials.
[0085] Heat may travel to or form the heat transfer appendage 46.
In other words, appendage 46 may be employed as a heat sink or
source. Thus, heat transfer appendage 46 may be used as a heat sink
to cool internal portions of bi-polar plate 44 or an MEA 62. Fuel
cell 20 employs a cooling medium to remove heat from appendage 46.
Alternatively, heat transfer appendage 46 may be employed as a heat
source to provide heat to internal portions of bi-polar plate 44 or
an MEA 62. In this case, a catalyst may be disposed on appendage 46
to generate heat in response to the presence of a heating
medium.
[0086] For cooling, heat transfer appendage 46 permits integral
conductive heat transfer from inner portions of plate 44 to the
externally disposed appendage 46. During hydrogen consumption and
electrical energy production, the electrochemical reaction
generates heat in each MEA 62. Since internal portions of bi-polar
plate 44 are in contact with the MEA 62, a heat transfer appendage
46 on a bi-polar plate 44 thus cools an MEA 62 adjacent to the
plate via a) conductive heat transfer from MEA 62 to bi-polar plate
44 and b) lateral thermal communication and conductive heat
transfer from central portions of the bi-polar plate 44 in contact
with the MEA 62 to the external portions of plate 44 that include
appendage 46. In this case, heat transfer appendage 46 sinks heat
from substrate 89 between a first channel field 72 on one face 75
of plate 44 and a second channel field 72 on the opposite face of
plate 44 to heat transfer appendage 46 in a direction parallel to a
face 75 of plate 44. When a fuel cell stack 60 includes multiple
MEA layers 62, lateral thermal communication through each bi-polar
plate 44 in this manner provides interlayer cooling of multiple MEA
layers 62 in stack 60--including those layers in central portions
of stack 60.
[0087] Fuel cell 20 may employ a cooling medium that passes over
heat transfer appendage 46. The cooling medium receives heat from
appendage 46 and removes the heat from fuel cell 20. Heat generated
internal to stack 60 thus conducts through bi-polar plate 44, to
appendage 46, and heats the cooling medium via convective heat
transfer between the appendage 46 and cooling medium. Air is
suitable for use as the cooling medium.
[0088] As shown, heat transfer appendage 46 may be configured with
a thickness that is less than the thickness between opposite faces
75 of plate 44. The reduced thickness of appendages 46 on adjacent
bi-polar plates 44 in the fuel cell stack 60 forms a channel 190
between adjacent appendages. Multiple adjacent bi-polar plates 44
and appendages 46 in stack form numerous channels 190. Each channel
190 permits a cooling medium or heating medium to pass therethrough
and across heat transfer appendages 46. In one embodiment, fuel
cell stack 60 includes a mechanical housing that encloses and
protects stack 60. Walls of the housing also provide additional
ducting for the cooling or heating medium by forming ducts between
adjacent appendages 46 and the walls.
[0089] The cooling medium may be a gas or liquid. Heat transfer
advantages gained by high conductance bi-polar plates 44 allow air
to be used as a cooling medium to cool heat transfer appendages 46
and stack 60. For example, a dc-fan may be attached to an external
surface of the mechanical housing. The fan moves air through a hole
in the mechanical housing, through channels 190 to cool heat
transfer appendages 46 and fuel cell stack 60, and out an exhaust
hole or port in the mechanical housing. Fuel cell system 10 may
then include active thermal controls. Increasing or decreasing
coolant fan speed regulates the amount of heat removal from stack
60 and the operating temperature for stack 60. In one embodiment of
an air-cooled stack 60, the coolant fan speed increases or
decreases as a function of the actual cathode exit temperature,
relative to a desired temperature set-point.
[0090] For heating, heat transfer appendage 46 allows integral heat
transfer from the externally disposed appendage 46 to inner
portions of plate 44 and any components and portions of fuel cell
20 in thermal communication with inner portions of plate 44. A
heating medium passed over the heat transfer appendage 46 provides
heat to the appendage. Heat convected onto the appendage 46 then
conducts through the substrate 89 and into internal portions of
plate 44 and stack 60, such as portions of MEA 62 and its
constituent components.
[0091] In one embodiment, the heating medium comprises a heated gas
having a temperature greater than that of appendage 46. As will be
described below, exhaust gases from burner 30 or reformer 32 of
fuel processor 15 may each include elevated temperatures that are
suitable for heating one or more appendages 46.
[0092] In another embodiment, fuel cell comprises a catalyst 192
disposed in contact with, or in proximity to, a heat transfer
appendage 46. The catalyst 192 generates heat when the heating
medium passes over it. The heating medium in this case may comprise
any gas or fluid that reacts with catalyst 192 to generate heat.
Typically, catalyst 192 and the heating medium employ an exothermic
chemical reaction to generate the heat. Heat transfer appendage 46
and plate 44 then transfer heat into the fuel cell stack 60, e.g.
to heat internal MEA layers 62. For example, catalyst 192 may
comprise platinum and the heating medium includes the hydrocarbon
fuel source 17 supplied to fuel processor 15 (FIGS. 2A and 2F). The
fuel source 17 may be heated to a gaseous state before it enters
fuel cell 20. This allows gaseous transportation of the heating
medium and gaseous interaction between the fuel source 17 and
catalyst 192 to generate heat. Similar to the cooling medium
described above, a fan disposed on one of the walls 199 then moves
the gaseous heating medium within fuel cell 20.
[0093] In a specific embodiment, the hydrocarbon fuel source 17
used to react with catalyst 192 comes from a reformer exhaust or
burner exhaust in fuel processor 15. This advantageously pre-heats
the fuel source 17 before receipt within fuel cell 20 and also
efficiently uses or burns any fuel remaining in the reformer or
burner exhaust after processing by fuel processor 15.
Alternatively, fuel cell 20 includes a separate hydrocarbon fuel
source 17 feed that directly supplies hydrocarbon fuel source 17 to
fuel cell 20 for heating and reaction with catalyst 192. In this
case, catalyst 192 may comprise platinum. Other suitable catalysts
192 include palladium, a platinum/palladium mix, iron, ruthenium,
and combinations thereof. Each of these will react with a
hydrocarbon fuel source 17 to generate heat. Other suitable heating
medium include hydrogen or any heated gases emitted from fuel
processor 15, for example.
[0094] When hydrogen is used as the heating medium, catalyst 192
comprises a material that generates heat in the presence of
hydrogen, such as palladium or platinum. As will be described in
further detail below, the hydrogen may include hydrogen supplied
from the reformer 32 in fuel processor 15.
[0095] As shown in FIGS. 2A and 2F, catalyst 192 is arranged on,
and in contact with, each heat transfer appendage 46. In this case,
the heating medium passes over each appendage 46 and reacts with
catalyst 192. This generates heat, which is absorbed via conductive
thermal communication by the cooler appendage 46. Wash coating may
be employed to dispose catalyst 192 on each appendage 46. A ceramic
support may also be used to bond catalyst 192 on an appendage
46.
[0096] FIG. 2F illustrates two examples in which a thermal catalyst
192 is disposed in proximity to heat transfer appendage 46.
Proximity in this case refers to being arranged relative to heat
transfer appendage 46 such that heat generated by catalyst 192
transfers to appendage 46, either by conduction, convection and/or
radiation. As shown in FIG. 2F, fuel cell 20 comprises a bulkhead
195 that contains catalyst 192. Bulkhead 195 attaches to a heat
transfer appendage 46 and either a) forms walls with the appendage
46 that contain catalyst 192 or b) includes its own set of wall
that contain catalyst 192. Catalyst pellets 192 are then disposed
in bulkhead 195. The bulkhead 195 allows the heating medium to pass
over and interact with catalyst 192.
[0097] As shown in FIG. 2F, the fuel cell 20 includes a mechanical
housing 197 that encloses and protects stack 60. Walls 199 of
housing 197 and appendages 46 combine to form ducting 193. The
inter-appendage ducting 193 permits a) catalyst 192 to be packed
into the ducting 193 and b) permits the heating medium to pass
through ducting 193 and over catalyst 192. In this case, catalyst
192 is packed in ducting 193 with a packing density loose enough to
permit a gas to pass therethrough without encountering excessive
resistance. A fan is then used to provide the heating medium into
ducting 193.
[0098] For catalyst-based heating, heat then a) transfers from
catalyst 192 to appendage 46, b) moves laterally though bi-polar
plate 44 via conductive heat transfer from lateral portions of the
plate that include heat transfer appendage 46 to central portions
of bi-polar plate 44 in contact with the MEA layers 62, and c)
conducts from bi-polar plate 44 to MEA layer 62. When a fuel cell
stack 60 includes multiple MEA layers 62, lateral heating through
each bi-polar plate 44 provides interlayer heating of multiple MEA
layers 62 in stack 60, which expedites fuel cell 20 warm up.
[0099] Bi-polar plates 44 of FIG. 2A include heat transfer
appendages 46 on each side. In this case, one set of heat transfer
appendages 46a is used for cooling while the other set of heat
transfer appendages 46b is used for heating. Although heat transfer
appendages 46 of FIG. 2F are illustrated with two different types
of heating via catalyst 192 (namely, by packing into ducting 193
and storage in bulkheads 195), it is understood that fuel cell 20
need not include multiple methods of heating appendages 46 and may
only include one the aforementioned techniques. In addition, while
bi-polar plates 44 illustrated in FIGS. 2A and 2D show plates 44
with two heat transfer appendages 46 disposed on sides of stack 60,
appendage 46 arrangements can be varied to affect and improve heat
dissipation and thermal management of fuel cell stack 60 according
to other specific designs. For example, one or more than two heat
transfer appendages 46 may be employed on a single plate 44 to
increase heat transfer between internal and external portions of
plate 44. In addition, appendages 46 need not span a side of plate
44 as shown and may be tailored based on how the heating fluid is
channeled through the housing 197.
[0100] Although the present invention provides a bi-polar plate 44
having channel fields 72 that distribute hydrogen and oxygen on
opposing sides of a single plate 44, many embodiments described
herein are suitable for use with conventional bi-polar plate
assemblies that employ two separate plates for distribution of
hydrogen and oxygen. FIG. 2E illustrates a widely used and
conventional bi-polar plate 300 that comprises a plate/cooling
layer/plate architecture.
[0101] Bi-polar plate 300 includes two plates 302a and 302b that
sandwich a cooling layer 304. Top plate 302a includes a channel
field 306a on its top face 308 that distributes oxygen. Bottom
plate 302b includes a channel field 306b on its bottom face 308
that distributes hydrogen (or oxygen when top plate 302a
distributes hydrogen). Cooling layer 304 runs a cooling medium such
as de-ionized water through cooling channels 310. The cooling
medium actively cools each plate 302. The cooling medium may be
routed such that the temperature increase occurs in the same
direction as reducing oxygen partial pressure in the cathode.
Similar to bi-polar plate 44, bi-polar plate 300 is referred to as
a `bi-polar plate` since it acts electrically as a cathode for one
MEA and as an anode for another MEA. Bi-polar plate 300 serves
similar functions for a fuel cell as those described above for
bi-polar plate 44. Top and bottom plates 302a and 302b may each
comprise silicon with channels etched in their faces to provide
channel fields 306.
[0102] While the present invention has mainly been discussed so far
with respect to a reformed methanol fuel cell (RMFC), the present
invention may also apply to other types of fuel cells, such as a
solid oxide fuel cell (SOFC), a phosphoric acid fuel cell (PAFC), a
direct methanol fuel cell (DMFC), or a direct ethanol fuel cell
(DEFC). In this case, fuel cell 20 includes components specific to
these architectures, as one of skill in the art will appreciate. A
DMFC or DEFC receives and processes a fuel. More specifically, a
DMFC or DEFC receives liquid methanol or ethanol, respectively,
channels the fuel into the fuel cell stack 60 and processes the
liquid fuel to separate hydrogen for electrical energy generation.
For a DMFC, channel fields 72 in the bi-polar plates 44 distribute
liquid methanol instead of hydrogen. Hydrogen catalyst 126
described above would then comprise a suitable anode catalyst for
separating hydrogen from methanol. Oxygen catalyst 134 would
comprise a suitable cathode catalyst for processing oxygen or
another suitable oxidant used in the DMFC, such as peroxide. In
general, hydrogen catalyst 126 is also commonly referred to as an
anode catalyst in other fuel cell architectures and may comprise
any suitable catalyst that removes hydrogen for electrical energy
generation in a fuel cell, such as directly from the fuel as in a
DMFC. In general, oxygen catalyst 134 may include any catalyst that
processes an oxidant in used in fuel cell 20. The oxidant may
include any liquid or gas that oxidizes the fuel and is not limited
to oxygen gas as described above. An SOFC, PAFC or MCFC may also
benefit from inventions described herein, for example. In this
case, fuel cell 20 comprises an anode catalyst 126, cathode
catalyst 134, anode fuel and oxidant according to a specific SOFC,
PAFC or MCFC design.
3. Fuel Processor
[0103] FIG. 3A illustrates a cross-sectional side view of fuel
processor 15 in accordance with one embodiment of the present
invention. FIG. 3B illustrates a cross-sectional front view of fuel
processor 15 taken through a mid-plane of processor 15 that also
shows features of end plate 182. Fuel processor 15 reforms methanol
to produce hydrogen. Fuel processor 15 comprises monolithic
structure 100, end plates 182 and 184, reformer 32, burner 30,
boiler 34, boiler 108, dewar 150 and housing 152. Although the
present invention will now be described with respect to methanol
consumption for hydrogen production, it is understood that fuel
processors of the present invention may consume another fuel
source, as one of skill in the art will appreciate.
[0104] As the term is used herein, `monolithic` refers to a single
and integrated structure that includes at least portions multiple
components used in fuel processor 15. As shown, monolithic
structure 100 includes reformer 32, burner 30, boiler 34 and boiler
108. Monolithic structure 100 may also include associated plumbing
inlets and outlets for reformer 32, burner 30 and boiler 34.
Monolithic structure 100 comprises a common material 141 that
constitutes the structure. The monolithic structure 100 and common
material 141 simplify manufacture of fuel processor 15. For
example, using a metal for common material 141 allows monolithic
structure 100 to be formed by extrusion. In some cases, monolithic
structure 100 is consistent in cross sectional dimensions between
end plates 182 and 184 and solely comprises copper formed in a
single extrusion.
[0105] Housing 152 provides mechanical protection for internal
components of fuel processor 15 such as burner 30 and reformer 32.
Housing 152 also provides separation from the environment external
to processor 15 and includes inlet and outlet ports for gaseous and
liquid communication in and out of fuel processor 15. Housing 152
includes a set of housing walls 161 that at least partially contain
a dewar 150 and provide external mechanical protection for
components in fuel processor 15. Walls 161 may comprises a suitably
stiff material such as a metal or a rigid polymer, for example.
Dewar 150 improves thermal heat management for fuel processor 15 by
a) allowing incoming air to be pre-heated before entering burner
30, b) dissipating heat generated by burner 32 into the incoming
air before the heat reaches the outside of housing 152.
[0106] Referring to FIG. 3B, boiler 34 heats methanol before
reformer 32 receives the methanol. Boiler 34 receives methanol via
fuel source inlet 81, which couples to methanol supply line 27 of
FIG. 1B. Since methanol reforming and hydrogen production via a
catalyst 102 in reformer 32 often requires elevated methanol
temperatures, fuel processor 15 pre-heats the methanol before
receipt by reformer 32 via boiler 34. Boiler 34 is disposed in
proximity to burner 30 to receive heat generated in burner 30. The
heat transfers via conduction through monolithic structure from
burner 30 to boiler 34 and via convection from boiler 34 walls to
the methanol passing therethrough. In one embodiment, boiler 34 is
configured to vaporize liquid methanol. Boiler 34 then passes the
gaseous methanol to reformer 32 for gaseous interaction with
catalyst 102.
[0107] Reformer 32 is configured to receive methanol from boiler
34. Walls 111 in monolithic structure 100 and end walls 113 on end
plates 182 and 184 define dimensions for a reformer chamber 103. In
one embodiment, end plate 182 and/or end plate 184 includes also
channels 95 that route heated methanol exhausted from boiler 34
into reformer 32. The heated methanol then enters the reformer
chamber 103 at one end of monolithic structure 100 and passes to
the other end where the reformer exhaust is disposed. In another
embodiment, a hole disposed in a reformer 32 wall receives inlet
heated methanol from a line or other supply. The inlet hole or port
may be disposed on a suitable wall 111 or 113 of reformer 32.
[0108] Reformer 32 includes a catalyst 102 that facilitates the
production of hydrogen. Catalyst 102 reacts with methanol 17 and
produces hydrogen gas and carbon dioxide. In one embodiment,
catalyst 102 comprises pellets packed to form a porous bed or
otherwise suitably filled into the volume of reformer chamber 103.
Pellet diameters ranging from about 50 microns to about 1.5
millimeters are suitable for many applications. Pellet diameters
ranging from about 500 microns to about 1 millimeter are suitable
for use with reformer chamber 103. Pellet sizes may be varied
relative to the cross sectional size of reformer chamber 103, e.g.,
as reformer chamber 103 increases in size so do catalyst 102 pellet
diameters. Pellet sizes and packing may also be varied to control
the pressure drop that occurs through reformer chamber 103. In one
embodiment, pressure drops from about 0.2 to about 2 psi gauge are
suitable between the inlet and outlet of reformer chamber 103. One
suitable catalyst 102 may include CuZn coated onto alumina pellets
when methanol is used as a hydrocarbon fuel source 17. Other
materials suitable for catalyst 102 include platinum, palladium, a
platinum/palladium mix, nickel, and other precious metal catalysts
for example. Catalyst 102 pellets are commercially available from a
number of vendors known to those of skill in the art. Catalyst 102
may also comprise catalyst materials listed above coated onto a
metal sponge or metal foam. A wash coat of the desired metal
catalyst material onto the walls of reformer chamber 103 may also
be used for reformer 32.
[0109] Reformer 32 is configured to output hydrogen and includes an
outlet port 87 that communicates hydrogen formed in reformer 32
outside of fuel processor 15. For example, a line 39 may
communicate to an anode of fuel cell 20 for electrical energy
generation or to a heating port for heating of a catalyst proximate
to one or more heat transfer appendages. Port 87 is disposed on a
wall of end plate 82 and includes a hole that passes through the
wall (see FIG. 3B). The outlet hole port may be disposed on any
suitable wall 111 or 113.
[0110] Hydrogen production in reformer 32 is slightly endothermic
and draws heat from burner 30. Burner 30 generates heat and is
configured to provide heat to reformer 32. As shown in FIG. 3B,
burner 30 comprises two burners 30a and 30b and their respective
burner chambers 105a and 105b that surround reformer 32. In one
embodiment, burner 30 uses electrical resistance and electrical
energy to produce heat.
[0111] In the embodiment shown, burner 30 employs catalytic
combustion to produce heat. A catalyst 104 disposed in each burner
chamber 105 helps a burner fuel passed through the chamber generate
heat. Burner 30 includes an inlet that receives methanol 17 from
boiler 108 via a channel in one of end plates 182 or 184. In one
embodiment, methanol produces heat in burner 30 and catalyst 104
facilitates the methanol production of heat. In another embodiment,
waste hydrogen from fuel cell 20 produces heat in the presence of
catalyst 104. Suitable burner catalysts 104 may include platinum or
palladium coated onto alumina pellets for example. Other materials
suitable for catalyst 104 include iron, tin oxide, other
noble-metal catalysts, reducible oxides, and mixtures thereof. The
catalyst 104 is commercially available from a number of vendors
known to those of skill in the art as small pellets. The pellets
that may be packed into burner chamber 105 to form a porous bed or
otherwise suitably filled into the burner chamber volume. Catalyst
104 pellet sizes may be varied relative to the cross sectional size
of burner chamber 105. Catalyst 104 may also comprise catalyst
materials listed above coated onto a metal sponge or metal foam or
wash coated onto the walls of burner chamber 105. A burner outlet
port 89 (FIG. 3A) communicates exhaust formed in burner 30 outside
of fuel processor 15.
[0112] Some fuel sources generate additional heat in burner 30, or
generate heat more efficiently, with elevated temperatures. Fuel
processor 15 includes a boiler 108 that heats methanol before
burner 30 receives the fuel source. In this case, boiler 108
receives the methanol via fuel source inlet 85. Boiler 108 is
disposed in proximity to burner 30 to receive heat generated in
burner 30. The heat transfers via conduction through monolithic
structure from burner 30 to boiler 108 and via convection from
boiler 108 walls to the methanol passing therethrough.
[0113] Air including oxygen enters fuel processor 15 via air inlet
port 91. Burner 30 uses the oxygen for catalytic combustion of
methanol. Dewar 150 is configured such that air passing through
dewar chamber 156 receives heat generated in burner 30. Dewar 150
offers thus two functions for fuel processor 15: a) it permits
active cooling of components within fuel processor 15 before the
heat reaches an outer portion of the fuel processor, and b) it
pre-heats the air going to burner 30. Air first passes along the
outside of dewar 150 before passing through apertures in the dewar
and along the inside of dewar 150. This heats the air before
receipt by air inlet port 93 of burner 30. A burner 30 in fuel
processor 15 generates heat and typically operates at an elevated
temperature. In one embodiment, fuel processor 15 comprises a dewar
150 to improve thermal management for fuel processor 15. Dewar 150
at least partially thermally isolates components internal to
housing 152--such as burner 30--and contains heat within fuel
processor 15.
[0114] Although the present invention will primarily be described
with respect to the annular reformer and burner shown in FIGS. 3A
and 3B, it is anticipated that fuel cell systems described herein
are also applicable to other fuel processor designs. Many
architectures employ a planar reformer disposed on top or below to
a planar burner. Micro-channel designs fabricated in silicon
commonly employ such stacked planar architectures and would benefit
from fuel cell systems described herein. Further description of
planar fuel processors suitable for use with the present invention
are included in commonly owned co-pending patent application
entitled "Planar Micro Fuel Processor" naming Ian Kaye as inventor
and filed on the same day as this patent application, which is
incorporated by reference for all purposes.
4. Efficient Fuel Cell Systems
[0115] Fuel processor components operate at elevated temperatures.
Burner 30 temperatures from about 200 degrees Celsius to about 800
degrees Celsius are common. Many fuel cells 20 require elevated
temperatures for electrical energy production. More specifically,
the electrochemical reaction responsible for hydrogen consumption
and electrical energy generation typically requires an elevated
temperature. Start temperatures in the MEA layer 62 and its
constituent parts greater than 150 degrees Celsius are common.
[0116] One embodiment of the present invention heats internal
portions of fuel cell 20 using heat generated in fuel processor 15
or gases exhausted from fuel processor 15 that can be used for
catalytic combustion in fuel cell 20. FIG. 4A illustrates a heat
efficient fuel cell system 400 in accordance with one embodiment of
the present invention. System 400 runs gases exhausted from burner
30 to fuel cell 20 to provide heat to fuel cell 20.
[0117] System 400 comprises components of system 10 described with
respect to FIG. 1B and also comprises plumbing configured to
transport a heating medium from fuel processor 15 to fuel cell 20.
As the term is used herein, plumbing may comprise any tubing,
piping and/or channeling that communicates a gas or liquid from one
location to a second location. The plumbing may also comprise one
or more valves, gates or other devices to facilitate and control
flow. A fan or pump may also be included to pressurize a line and
move the heating medium. Plumbing between burner 30 and fuel cell
20 includes an outlet 402 on fuel processor 15 that exhausts heated
gases from burner 30 to a line 41, which transmits the heated gases
to fuel cell 20. A `line` refers to tubing, piping and/or
channeling that is dedicated for fluid or gas communication between
two locations.
[0118] In a specific embodiment, line 41 transports the heated
gases to fan 37, which moves the heated gases within fuel cell 20
and across the fuel cell stack and heat transfer appendages.
Alternatively, the plumbing may be configured to transport the
heating medium from burner 30 to one or more heat transfer
appendages. In this case, line 41 may continue through the fuel
cell housing and open in the proximity of the heat transfer
appendages. A hole in the fuel cell housing then allows line 41 to
pass therethrough or connect to a port that communicates the gases
to plumbing inside the fuel cell for delivery to the fuel cell
stack and heat transfer appendage. For catalytic heat generation in
fuel cell 20, the plumbing may also transport the heating medium to
facilitate gaseous interaction with the catalyst, such as plumbing
delivery to one or more bulkheads.
[0119] In one embodiment, the heating medium comprises heated gases
exhausted from burner 30. A catalytic burner or electrical
resistance burner operates at elevated temperatures. Cooling air
exhausted from an electric burner or product gases exhausted from a
catalytic burner are often greater than about 100 degrees Celsius
when the gases leaves the fuel processor. For many catalytic
burners, depending on the fuel source employed, the heating medium
is commonly greater than about 200 degrees Celsius when the heating
medium leaves the fuel processor. These heated gases are
transported to the fuel cell for convective heat transfer in the
fuel cell, such as passing the heated gases over one or more heat
transfer appendages 46 for convective heat transfer from the warmer
gases into the cooler heat transfer appendages.
[0120] In another embodiment, burner 30 is a catalytic burner and
the heating medium comprises the fuel source. Catalytic combustion
in burner 30 is often incomplete and the burner exhaust gases
include unused and gaseous methanol. Fuel cell 20 then comprises a
thermal catalyst that facilitates production of heat in the fuel
cell in the presence of methanol. The fuel source is typically
vaporized prior to reaching the burner to facilitate catalytic
combustion. In this case, line 41 transports the gaseous and unused
methanol to the thermal catalyst in fuel cell 20. Several suitable
thermal catalyst arrangements for transferring heat into heat
transfer appendages 46 were described above (see FIGS. 2A and 2F).
Suitable methanol catalysts, such as platinum or palladium coated
onto alumina pellets, were also described above with respect to
catalyst 104 in burner 30.
[0121] System 400 also comprises one or more sensors to help
regulate thermal management in system 400. A temperature sensor 404
detects temperature for a component in fuel processor 15. Sensor
406 may be arranged within burner 30 for detecting the temperatures
within the burner. Other components in fuel processor 15 whose
temperature may be monitored by sensor 404 include: reformer 32,
boiler 34, boiler 108 and gases at the inlet at outlet ports of
each of these components. A temperature sensor 406 detects
temperature for a component in fuel cell 20. For example, sensor
406 may be arranged in contact with the substrate 89 of one or more
bi-polar plates 44 for detecting the temperature of the plate.
Other component in fuel cell 20 whose temperature may be monitored
by sensor 406 include: MEA layer 62 and gases in an inlet or outlet
manifold. Suitable temperature sensors for use with the present
invention are widely commercially available from numerous sources
known to those of skill in the art.
[0122] FIG. 4B illustrates a heat efficient fuel cell system 420 in
accordance with another embodiment of the present invention. System
420 comprises plumbing configured to transport a heating medium
from reformer 32 to fuel cell 20 to provide heat to fuel cell 20.
As shown, line 422 transports reformer exhaust from an outlet port
of reformer 32 to heat transfer appendage 46. Line 422 may also
transport the reformer exhaust to fan 37, which moves the heated
gases within fuel cell 20 and across the heat transfer appendages.
In another embodiment, reformer 32 includes a single output that
splits into line 422 for reformer exhaust heating in fuel cell 20
and into line 39 for hydrogen delivery to the anode. A valve may be
employed to control flow between the two lines.
[0123] In one embodiment, the reformer exhaust is at an elevated
temperature corresponding to the temperature in reformer 32.
Reformer exhausts above 100 degrees Celsius are common. Heat in the
reformer exhaust then convects onto the heat transfer appendage to
heat the fuel stack and its internal components. In another
embodiment, hydrogen production in reformer 32 is often incomplete
and the reformer exhaust gases include unused and gaseous methanol.
Fuel cell 20 then comprises a thermal catalyst that facilitates
production of heat in the fuel cell in the presence of methanol.
Boiler 34 vaporizes the methanol prior to reaching reformer 32. In
this case, line 422 transports the gaseous and unused methanol to
the thermal catalyst in fuel cell 20. Suitable methanol catalysts,
such as platinum or palladium coated onto alumina pellets, were
also described above with respect to catalyst 104 in burner 30.
Alternatively, fuel cell 20 may include a thermal catalyst that
facilitates production of heat in the fuel cell in the presence of
hydrogen. In this case, the heating medium comprises hydrogen
produced in reformer 32. Suitable hydrogen catalysts that help
hydrogen produce heat include platinum or palladium, for example.
Several suitable thermal catalyst arrangements for transferring
heat into heat transfer appendages 46 were described above (see
FIGS. 2A and 2F).
[0124] FIGS. 4A and 4B illustrate two systems 400 and 420 that
transport a heating medium from the fuel processor to the fuel
cell. The present invention may flexibly employ heating from fuel
processor 15 to fuel cell 20 for improving efficiency of the fuel
cell system. For example, a heating medium may be passed over one
or more appendages 46 during start up of fuel cell 20, or during
periods of electrical generation inactivity when fuel cell 20 is
cooling from elevated temperatures associated with operation.
[0125] FIG. 5 illustrates a process flow 500 for generating
electrical energy in a fuel cell that receives hydrogen from a fuel
processor in accordance with one embodiment of the present
invention. The fuel processor is configured to process a fuel
source to produce the hydrogen and comprises a burner and a
reformer.
[0126] Process flow 500 begins by providing the fuel source to the
fuel processor (502). Supply from fuel storage 16 is described
above with respect to FIGS. 1A and 1B. When the fuel processor
comprises a catalytic burner, the fuel source is supplied to both
the reformer and burner and is used in the burner to generate heat.
A catalyst in the reformer receives heat from the burner and
reforms the fuel source to produce hydrogen.
[0127] Process flow 500 then transports a heating medium from the
fuel processor to the fuel cell when a component in the fuel cell
has a temperature that is less than a threshold temperature or when
electrical energy output by the fuel cell includes less than an
electrical threshold (504). Electrical energy output by the fuel
cell is typically dc power characterized by a dc voltage and
current. The electrical threshold then refers to a desired
electrical output for the fuel cell. For example, if the fuel cell
electrical output drops below 0.54 Volts per cell, then load from
the fuel cell may be turned off and the heating medium transported
from the fuel processor to the fuel cell. The electrical threshold
may be represented as a desired output according to a polarization
curve for the fuel cell, which is stored in software available to
the controller of the fuel cell system. The polarization curve
represents acceptable electrical energy output for the fuel
cell.
[0128] For temperature monitoring, the fuel cell component refers
to a portion of the fuel cell whose temperature affects fuel cell
performance. The threshold temperature refers to a desired
temperature to begin external heating of the component. For a
functional component of the fuel cell, the threshold temperature
may relate to a required temperature for the component to
contribute to electrical energy generation. Operating temperatures
in the MEA 62 and its constituent parts greater than 190 degrees
Celsius are common. An MEA 62 comprising a Celtec 1000 electrolyte
from PEMEAS USA AG mentioned above may require operating
temperatures greater than 100 degrees Celsius. The component may
alternatively comprise a bi-polar plate, a gas diffusion layer, a
membrane, or any other component mentioned above with respect to
fuel cell 20. The threshold temperature may also vary based on the
component being sensed. Detecting temperature of a bi-polar plate
on an external surface of the fuel cell stack allows the
temperature sensor to remain outside the stack. In this case, the
threshold temperature may accommodate the difference in temperature
between an outside portion of the plate where the sensor attaches
and an inside portion proximate to the MEA. Temperature variation
in a single plate may range from 2 to 5 degrees Celsius for
example. In general, the threshold temperature may vary between
about 50 degrees Celsius and about 1000 degrees Celsius for some
fuel cell components.
[0129] In one embodiment, the heating medium is transported to the
fuel cell during a start-up period before the fuel cell begins
generating electrical energy, e.g., in response to a request for
electrical energy. Heating a fuel cell in this manner allows fuel
cell component operating temperatures to be reached sooner and
expedites warm-up time needed when initially turning on fuel cell
20.
[0130] In another embodiment, the heating medium is transported
from the fuel processor to the fuel cell during a period of
non-activity in which the fuel cell does not produce electrical
energy and the component cools (502). Since many fuel cells require
elevated temperatures for operation and the electrical energy
generating process is exothermic, the fuel cell usually does not
require external heating during electrical energy generation.
However, when electrical energy generation ceases for an extended
time and the component drops below a threshold operating
temperature, the heating medium may then be transported from the
fuel processor to regain the operating temperature and resume
electrical energy generation. This permits operating temperatures
in a fuel cell to be maintained when electrical energy is not being
generated by the fuel cell.
[0131] The heating medium then heats a portion of the fuel cell
(506). The heating medium may comprise heated exhaust (including
air, combustion products and unused methanol) from a burner or
heated exhaust from a reformer (including air, reforming reactants
and reforming products such as hydrogen). Heating may also employ
catalytic combustion in the fuel cell. During fuel cell system
start-up, the reformer may not yet have reached its operating
temperature and the exhaust/heating medium may comprise a high
concentration of CO and un-processed fuel (also referred to as
`dirty hydrogen`) that is unsuitable for use in a fuel cell. The
hydrogen may be catalytically combusted to generate heat in a
portion of the fuel cell responsible for heat generation. Suitable
examples of heat transfer appendage techniques to heat a fuel cell
and its internal components with the heating medium are described
above.
[0132] Process flow 500 detects electrical output of the fuel cell
and/or temperature for the component (510) before transportation of
the heating medium begins, and afterwards. Logic implemented by a
controller then compares the detected parameter with stored values
for either threshold. For detecting current and voltage, the
controller logic compares the measured amount with a stored
polarization curve. If the measured voltage or current is output by
the fuel cell is about equal to or greater than an electrical
threshold, or a shift in the polarization curve has occurred, then
flow of the heating medium may begin, and in some cases, electrical
energy generation may cease. The component temperature may
similarly be read and compared with stored values.
[0133] When the component temperature drops below the threshold
temperature or the electrical output returns to an unacceptable
condition, then flow of the heating medium may stop, e.g., using a
valve between the fuel processor and fuel cell. Similarly, if the
component temperature subsequently rises above the threshold
temperature, then flow of the heating medium may again resume.
Thus, when needed, process flow 500 transports hydrogen from the
fuel processor to the fuel cell (508). Electrical energy is
generated (512) in the fuel cell when the temperature of the
component is about equal to or greater than the threshold
temperature or when electrical energy output by the fuel cell is
about equal to or greater than an electrical threshold.
[0134] Efficient systems and methods of the present invention may
also transport hydrogen in the fuel cell system to burner 30 in
fuel processor 15. A catalyst in the burner then reacts with the
hydrogen to produce heat in the burner. The hydrogen may come from
the anode exhaust of fuel cell 20 and/or from the reformer 32
exhaust.
[0135] FIG. 6 illustrates a fuel system 440 that routes unused
hydrogen from fuel cell 20 back to burner 30 in accordance with one
embodiment of the present invention. Burner 30 includes a thermal
catalyst that reacts with the unused hydrogen to produce heat.
[0136] Fuel system 440 comprises plumbing that is configured to
transport hydrogen to burner 30. Line 51 is configured to transmit
unused hydrogen from fuel cell 20 to burner 30 of fuel processor
15. For system 440, burner 30 includes two inlets: an inlet 55
configured to receive the hydrogen fuel source 17 and an inlet 53
configured to receive the hydrogen from line 51. Anode gas
collection channels, which distribute hydrogen provided by fuel
processor 15 to each membrane electrode assembly layer, collect and
exhaust the unused hydrogen to a hydrogen exhaust manifold (see
FIGS. 2A-2F), which delivers the hydrogen to line 51. In one
embodiment, gaseous delivery in line 51 back to fuel processor 15
relies on pressure at the exhaust of the anode gas distribution
channels, e.g., in the anode exhaust manifold. In another
embodiment, an anode recycling pump or fan is added to line 51 to
pressurize line 51 and return unused hydrogen back to fuel
processor 15. A fan may also pressurize line 39 to deliver the
hydrogen from an outlet of fuel processor 15 to an anode inlet of
fuel cell 20, which also pressurizes flow of hydrogen in line
51.
[0137] Since hydrogen consumption within fuel cell 20 is often
incomplete and the anode exhaust often includes unused hydrogen,
re-routing the anode exhaust to burner 30 allows fuel cell system
10 to capitalize on unused hydrogen in fuel cell 20 and increase
hydrogen usage and efficiency in system 10.
[0138] Line 442 is configured to transmit hydrogen output by
reformer 32 to burner 30 of fuel processor 15. Before a reformer
reaches its operating temperature upon system start from a cool
temperature or rest state, imperfect hydrogen generation at low
temperatures may lead to reformer output that is unsuitable for use
in a fuel cell. In situations where the reformer output is
unsuitable, fuel system 440 re-routes hydrogen and reformer exhaust
to burner 30 via line 442. Burner 30 catalytically uses the
hydrogen to produce heat. The heat may be provided to the reformer
to expedite warm up time for fuel processor 15 and fuel cell system
440.
[0139] Fuel cell system 440 provides flexibility to use different
fuels in a catalytic burner 30. For example, if fuel cell 20 can
reliably and efficiently consume over 90% of the hydrogen in the
anode stream, then there may not be sufficient hydrogen to maintain
reformer and boiler operating temperatures in fuel processor 15.
Under this circumstance, methanol supply is increased to produce
additional heat to maintain the reformer and boiler
temperatures.
[0140] FIG. 8 illustrates a schematic operation for a fuel cell
system 460 in accordance with another specific embodiment of the
present invention. Burner 30 is configured to receive oxygen from
an oxygen exhaust included in fuel cell 20. Cathode gas collection
channels, which distribute oxygen and air from the ambient room to
each membrane electrode assembly layer, collect and exhaust any
unused oxygen in fuel cell 20. Line 466 receives unused oxygen from
an exhaust manifold, which collects oxygen from each cathode gas
collection channel. Line 466 transports the oxygen to an inlet 464
of burner 30. Since oxygen consumption within fuel cell 20 is often
incomplete and the cathode exhaust includes unused oxygen,
re-routing the cathode exhaust to burner 30 allows fuel cell system
10 to capitalize on unused oxygen in fuel cell 20 and increase
oxygen usage and efficiency in system 10. Fuel cell 20 also heats
the oxygen before burner 30 receives the oxygen. Oxygen in the air
provided to burner 30 is consumed as part of the combustion
process. Heat generated in the burner 30 will heat cool incoming
air, depending on the temperature of the air. This heat loss to
incoming cool air reduces the heating efficiency of burner 30, and
typically results in a greater consumption of methanol. To increase
the heating efficiency of burner 30, the present invention heats
the incoming air in fuel cell 20 so less heat generated in the
burner passes into the incoming air. In other words, fuel cell 20
allows pre-heats air before reaching the burner, thus increasing
efficiency of system 460.
[0141] Fuel cell system 460 also transports unused hydrogen from
the anode of fuel cell 20 back to the burner of fuel processor 15
for catalytic combustion and generation of heat. Fuel cell system
460 also employs an electric heater 462 for heating reformer 32
with electrical energy.
5. Electronics Device Implementation
[0142] FIG. 9 shows a schematic illustration of a system 200 for
producing electrical energy in a portable electronics device 202 in
accordance with one embodiment of the present invention. System 200
comprises fuel processor 15 and fuel cell 20 included within an
electronics device 202 and a hydrogen fuel source storage device 16
coupled to electronics device 202 via connector 104 and mating
connector 140.
[0143] In one embodiment, fuel processor 15 and fuel cell 20 are
incorporated into electronics device 202 (within its volume and
outer housing) as an integral module, and storage device 16 is a
removable device. Fuel cell powered laptop computers 202 may
comprise slightly modified existing products, with fuel processor
15 and fuel cell 20 and related system components fitted generally
into the space provided for a battery pack. Mating connector 140 is
included in this allocated space for connection to a removable
storage device 16. Storage device 16 mechanically interfaces with
electronics device 202. In one embodiment, connectors 104 and 140
provide sufficient mechanical force to maintain position between
the storage device 16 and electronics device 202. In another
embodiment, electronics device 202 includes a mechanical slot that
storage device 16 fits and slides into.
[0144] When connector 104 and mating connector 140 interface, fuel
cell system controller 214 digitally communicates with memory 206
using link 217 for bi-directional communication therebetween. In
another embodiment, controller 214 uses a wireless interrogator to
communicate with an RFID antennae and memory 206 included in
storage device 16. Controller 214 may read any information stored
in memory 206 such as a fuel type stored in the storage device 16,
a model number for storage device 16, a volume capacity for bladder
205 or storage device 16, a number of refills provided to storage
device 16, the last refill date, the refilling service provider,
and a current volume for the storage device. Controller 214
estimates the remaining power in storage device 16 by comparing the
fuel source 17 level since last use or refill against a consumption
rate for a particular laptop computer. Controller 214 may also
write transient information to memory 206, such as an updated
volume for the storage device. The controller 214 communicates with
a main controller 210 for computer 202 and computer memory 216 via
communications bus 212. Computer memory 216 may store instructions
for the control of fuel system 10 such as read and write protocol
and instructions for communication with a digital memory 206.
[0145] Power management 219 controls power provision by fuel cell
system 10 and electrochemical battery 222. Thus, power management
219 may inform controller 214 how much power is needed for laptop
computer 22 operation and controller 214 responds by sending
signals to fuel cell 20, fuel processor 15 and a pump that draws
fuel from storage device 16 to alter fuel cell power production
accordingly. If fuel cell system 10 runs out of fuel source 17,
then power management 219 switches to electrical power provision
from battery 222.
[0146] System 200 may also be configured for `hot swappable`
capability. Hot swapping of storage device 16 refers to removing
storage device 16 from a fuel processor or electronics device 202
it provides hydrogen fuel source 17 to, without shutting down the
receiving device or without compromising hydrogen fuel source
provision to the receiving device for a limited time. A hot
swappable system implies fuel source provision when connector 104
and mating connector 140 are separated. Further description of hot
swappable fuel cell systems suitable for use with the present
invention are described in commonly owned co-pending patent
application entitled "Portable Fuel Cartridge for Fuel Cells"
naming Ian Kaye as inventor and filed on the same day as this
patent application, which is incorporated by reference for all
purposes.
[0147] Main controller 210 is preferably a commercially available
microprocessor such as one of the Intel (including Pentium.TM.) or
Motorola family of chips, a reduced instruction set computer (RISC)
chip such as the PowerPC.TM. microprocessor available from
Motorola, Inc, or any other suitable processor. Memory 216 may
comprise some form of mass storage but can be eliminated by
providing a sufficient amount of RAM to store user application
programs and data. Memory 216 may also contain the basic operating
system for the computer system. It is generally desirable to have
some type of long term mass storage such as a commercially
available hard disk drive, nonvolatile memory such as flash memory,
battery backed RAM, PC-data cards, or the like. Regardless of
computer system configuration, it may employ one or more memories
or memory modules configured to store program instructions for
controlling fuel cell and thermal systems described herein. Such
memory or memories may also be configured to store data structures,
control programs, or other specific non-program information
described herein.
[0148] In addition, although the present invention is primarily
described with respect to fuel cell systems and methods operating
on a fuel cell system, many of the methods and techniques described
constitute system controls and will comprise digital control
applied by control logic that implements instructions from stored
software. The control logic includes any combination of hardware
and software needed for control within system 10. For example, the
control logic may include instructions stored in memory 216 that
are executed by main controller 210. The stored instructions may
correspond to any methods or elements explained in the process
flows described herein. Input/output logic may be employed to
facilitate communication between main controller 210 and components
of fuel system 10. In one embodiment, the control logic is
configured to regulate heat transfer or temperature in system 10 by
controlling the routing of liquids and gases between fuel cell 20,
fuel processor 15 and electronics device 202. In a specific
embodiment, the control logic is configured to start fuel system
10. This includes control logic configured to start a fuel
processor including a reformer and a burner that provides heat to
the reformer. In another specific embodiment, the control logic is
configured to shut down a fuel cell system comprising a fuel cell
that received hydrogen from a fuel processor including a reformer
and a burner that provided heat to the reformer. In another
specific embodiment, the control logic is configured to regulate
the transport of a heating medium from the fuel processor to the
fuel cell when electrical energy output by the fuel cell includes
less than an electrical threshold or when temperature of a
component in the fuel cell is less than a temperature threshold. In
this case, memory 216 may include one or more polarization curves
that help determine when to transport of the heating medium.
6. Fuel System Start Up
[0149] Another aspect of the present invention relates to methods
for improving fuel cell system start up. Fuel cell system
components often require elevated temperatures before electrical
energy production occurs. Techniques described herein expedite the
time needed for fuel cell system start up.
[0150] Many fuel processors avoid providing a liquid fuel source to
a burner or reformer catalyst. During normal operation, a boiler
vaporizes the fuel source before receipt by the burner or reformer.
However, a boiler may not have sufficient heat during start-up to
heat the fuel source. In this case, the present invention heats and
vaporizes a fuel source using electrically generated heat that may
be readily turned on during fuel processor and system start-up.
[0151] FIG. 10A illustrates a system for heating a fuel source
before catalytic heat generation within burner 30 in accordance
with one embodiment of the present invention. As shown, the system
includes a fuel processor 15 and an electric heater 806. The fuel
processor 15 comprises a reformer 32, burner 30 and boiler 34,
which were described above with respect to FIG. 1A. Reformer 32
comprises a catalyst 804 that facilitates the production of
hydrogen. Catalyst 804 reacts with methanol 17 and facilitates the
production of hydrogen gas. In one embodiment, catalyst 804
comprises pellets packed to form a porous bed or otherwise suitably
filled into the volume of the reformer chamber.
[0152] Burner 30 comprises a catalyst bed 808 that helps a burner
fuel passed through the burner chamber generate heat. In one
embodiment, methanol produces heat in burner 30 and catalyst 808
facilitates methanol-based production of heat. In another
embodiment, waste hydrogen from fuel cell 20 produces heat in the
presence of catalyst 808. Suitable burner catalysts 808 may include
platinum or palladium coated onto alumina pellets for example.
[0153] Electric heater 806 is configured to heat burner 30 or the
fuel source 17 provided to burner 30. As shown, electrical heater
806 is disposed within the burner chamber 810 and intercepts the
fuel source 17 before the fuel source passes over catalyst bed 808.
In this case, a portion of chamber 810 is reserved for electrical
heater 806 and heating of fuel source 17. A small gap 816 is left
between heater 806 and catalyst 808 to allow room for the fuel
source 17 to heat, vaporize and spread within the burner chamber.
Gap 816 sizes from about 2 millimeters to about 5 millimeters are
suitable for many small fuel processors.
[0154] Electrical heaters suitable for use in fuel processor 15 may
employ a resistive heating element. A rechargeable battery,
capacitor or other electrical power supply 820 provides electrical
energy to heater 806. In one embodiment, a fuel cell that receives
hydrogen from fuel processor 15 outputs electrical energy to
recharge the capacitor 820. Electric heater 806 may comprise a
thin-film platinum, gold, graphite, nickel, chromium, aluminum,
alloy or other base metal that may be deposited and used for a
resistive heater. A model P/N CSS-01110 cartridge heater as
provided by Omega of Stamford, Conn. is suitable for use as
electrical heater 806 in some embodiments.
[0155] Electric heater 806 may also comprise a catalyst disposed on
an outside surface of heater 806 that generates heat in the
presence of fuel source 17. Platinum, for example, may be coated
onto the external surface of the heater to interact with fuel
source 17 to generate heat in the presence of the fuel source. The
catalytic heat then increases heat generation and warming of the
incoming fuel source 17.
[0156] In another embodiment, electrical heater 806 is embedded in
the burner catalyst bed 808 and heats the catalyst bed 808. In this
case, an insulating cover is used to electrically isolate heater
806 from catalyst 808. The insulating cover includes a high
temperature, electrically insulating material, such as a ceramic
tube. Alternatively, electrical heater 806 may be disposed external
to burner 30 and in thermal communication with fuel source supply
27 to heat fuel source 17 before it enters burner 30. A heat
exchanger may be employed to facilitate heat transfer between
supply 27 line and heater 806.
[0157] Burner 30 shows a common inlet 812 that receives both fuel
source 17 and air that have been mixed prior to entry into burner
chamber 810. In another embodiment, burner 30 includes two separate
and dedicated inlets for fuel source 17 and air, respectively. The
dedicated fuel source inlet may also comprise an atomizing nozzle
to facilitate vaporization of the fuel source.
[0158] Thermal communication between burner 30 and reformer 32 also
allows heat generated by electrical heater 806 to heat fuel source
17 entering reformer 32. In another embodiment, the present
invention employ electrical heat to warm a reformer or reformer
catalyst during start-up. FIG. 10C illustrates a system for
electrically heating a reformer 32 in accordance with one
embodiment of the present invention.
[0159] Reformer comprises an inlet 31a that receives fuel source
17, catalyst 804 and an outlet 31b that outputs hydrogen gas. In
one embodiment, reformer 32 is sized for portable applications and
comprises a reformer chamber having a volume greater than about 0.1
cubic centimeters and less than about 50 cubic centimeters.
Reformer 32 volumes between about 0.5 cubic centimeters and about 2
cubic centimeters are suitable for laptop computer applications.
Further description of annular fuel processors suitable for use
with the present invention are included in commonly owned
co-pending patent application entitled "Annular Fuel Processor and
Methods" naming Ian Kaye as inventor and filed on the same day as
this patent application, which is incorporated by reference for all
purposes.
[0160] Electric heater 806 is configured to heat reformer 32. As
shown, electric heater 806 is embedded in the reformer 32 catalyst
804 bed. An insulating cover, such as a ceramic tube, electrically
isolates heater 806 from catalyst 804. In this case, the electric
heater 806 applies heat directly into the reforming catalyst 804 so
that the whole fuel processor 15 is not heated at startup. This
brings the fuel cell online (even at reduced power) faster. In
addition, disposing the electric heater 806 in thermal contact with
the reformer 32 catalyst minimizes external power requirements from
a battery or capacitor.
[0161] Heater 806 heats reformer 32 or catalyst 804 during fuel
processor start-up. A temperature sensor detects the temperature of
the reformer catalyst bed or a wall of reformer 32. When the
catalyst 804 reaches a desired temperature for example, heater 806
is turned off. Burner 30 comprises an inlet 33 that receives the
fuel source 17. A catalyst bed 808 in burner 30 heats reformer 32
after electric heater 806 is turned off. Electric heater 806
readily thus heats up reformer 32 and allows it to reach operating
temperature quickly, with assistance from burner 30 if needed and a
minimal amount of electrical heat input.
[0162] An electric heater may also be employed when the fuel
processor is constructed using MEMS technology. In one MEMs design,
a reformer comprises three separate chips: a heater/boiler chip, a
reformer/heater chip and a preferential oxidizer chip. All three
chips may have a glass cover and share common manifolds that direct
process gasses to the correct chip. The heater/boiler chip has a
thin film heater deposited in the flow channels, suitable for use
during startup. The heater/reformer and preferential oxidizer chips
include MEMS deposited temperature sensors either on the flow
channels, or on the glass cover. Deposition of thin film heaters
and sensors is well understood to those of skill in MEMS
technology.
[0163] FIG. 10B illustrates a process flow 820 for starting up a
fuel processor in accordance with one embodiment of the present
invention. The fuel processor includes a reformer and a burner that
provides heat to the reformer. A fuel cell receives hydrogen
produced by the fuel processor.
[0164] Process flow 820 begins by generating heat using an
electrical heater that is configured to heat the burner or a fuel
source provided to the burner (822). The electrical heater may also
be configured to heat the reformer or a fuel source provided to the
reformer. The heat may vaporize the fuel source. The electrical
heater generates heat for a set duration or until a particular
operating condition is reached. In one embodiment, the electrical
heater generates heat for at least ten seconds before the fuel
source is supplied to the burner. Some fuel processors may be
heated for 30 seconds, up to a minute, or even longer. A threshold
start temperature may also be used to determine the heating
duration. A temperature sensitive catalyst, burner or reformer may
require that the electrical heater generate heat until the
catalyst, burner or reformer reaches a threshold start temperature.
Some burner catalysts include a threshold start temperature above
60 degrees Celsius. Some reformer catalysts include a threshold
start temperature above 100 degrees Celsius. Alternatively, the
electrical heater generates heat until the reformer walls reach 150
degrees Celsius or some other operating temperature. In one
embodiment, air and the fuel source mix before the fuel source
reaches the burner and electrical heater. In this case, the
electrical heater may be disposed outside the burner to pre-heat
the fuel source before entering the burner.
[0165] Process flow 820 then supplies the fuel source to the burner
(824). Typically, a pump moves the fuel source and turns on via a
system controller. The controller may also turn on a fan that
provides air to the burner. A catalyst in the burner then
catalytically generates heat in the burner to heat the reformer
(826). The fuel source enters the burner before the reformer
reaches its operating temperature. If the burner catalyst requires
a lower operating temperature than the reformer catalyst, catalytic
heat generation in the burner may be used to continue heating the
reformer--and the electric heater is turned off after the fuel
source is supplied to the burner. If the reformer has not yet
reached its operating temperature, the reformer exhaust may
comprise a high concentration of CO and un-processed fuel (`dirty
hydrogen`) that is unsuitable for use in a fuel cell. As described
above, the hydrogen may be routed from a reformer outlet to a
burner inlet to react with a thermal catalyst in the burner and
generate additional heat in the burner to expedite the time needed
for the reformer to reach operating temperature.
[0166] The fuel source is then supplying to the reformer (828). A
catalyst in the reformer then catalytically generates hydrogen
(830). Plumbing transports the hydrogen to a fuel cell that
generates electricity using the hydrogen. In one embodiment, the
electrical heater receives energy from a capacitor that is
recharged by the fuel cell after the fuel cell system gains a
steady operating status. The capacitor or rechargeable battery may
also be recharged during system start-up when then fuel cell is at
limited capacity, e.g., about 5-15% rated power. At this point, the
fuel cell power is enough to operate the electric heater, and the
startup capacitor or rechargeable battery can be turned off or
recharged.
7. Fuel System Shutdown
[0167] The present invention also includes methods for shutting
down a fuel cell system. FIG. 7 illustrates a process flow 600 for
shutting down a fuel cell system comprising a fuel cell that
received hydrogen from a fuel processor in accordance with one
embodiment of the present invention. The fuel processor includes a
reformer and a burner that provided heat to the reformer. Process
flow 600 is particularly useful to expunge any liquids in the fuel
cell system, including those present when the system is initially
shut down and those that accumulate via condensation as the system
cools.
[0168] Process flow 600 begins by stopping electrical energy
generation in the fuel cell (602). This may occur electrically by
varying the charge in the anode and/or cathode. Hydrogen supply to
the fuel cell may also be ceased, e.g., using a valve on a line
that transports hydrogen to the fuel cell.
[0169] Process flow 600 then discontinues a supply of the fuel
source to the reformer (604). For system 10 of FIG. 1B, valve 23
disposed on line 29 between fuel tank 14 and reformer 32 cuts fuel
source provision to the reformer. Cutting power to a pump may also
be used to discontinue the fuel supply. Heat is then generated in
the burner to heat to the reformer after discontinuing the supply
of the fuel source to the reformer (606). In some cases, heat
generation in the burner may continue for greater than about 30
seconds after discontinuing the supply of the fuel source to the
reformer. Alternatively, heat generation in the burner may continue
unless the fuel cell load has dropped below 10% rated power for a
few minutes or unless a polarization curve for the fuel cell has
lowered, e.g., the voltage of the fuel cell has reduced
significantly for a given current.
[0170] Heat generation in the burner is then discontinued (608).
For an electric burner, this may be done via an electrical switch
or digital control. For a catalytic burner such as that used in
system 10 of FIG. 1B, the burner then includes an inlet to receive
the fuel source from a supply of the fuel source and the burner
catalytically generates heat using the fuel source. A valve
disposed on a line 27 between a fuel tank 14 and reformer 32 may
cut fuel supply to the reformer. The burner is then flushed with
air (510). Air supply may continue for greater than about 60
seconds after discontinuing the supply of the fuel source to the
reformer. Alternatively, air supply may continue until the burner
temperature reaches a threshold cooling temperature, such as 80
degrees Celsius The above steps sufficiently shut down the reformer
and ensure no fuel is left in the reformer chamber or burner.
[0171] The present invention may also shut down the fuel cell. To
do so, air is provided to a cathode gas distribution system in the
fuel cell after discontinuing hydrogen supply to the fuel cell.
Powering a fan that pressurizes air supply to the cathode gas
distribution system may do this. The fuel cell may also be cooled.
For system 10 of FIG. 1B, fan 37 may be turned on to move cooling
air across the heat transfer appendages 46 until a desired
shut-down temperature is reached. Both fans may run until the fuel
cell is cooled to a desired temperature and moisture has been
removed by the air supply from the cathode gas distribution
system.
8. CONCLUSION
[0172] While this invention has been described in terms of several
preferred embodiments, there are alterations, permutations, and
equivalents that fall within the scope of this invention which have
been omitted for brevity's sake. For example, although the present
invention has been described with respect to methods operating on a
fuel cell system, many of the methods and techniques described
constitute system controls and will comprise digital control
applied by a processor that implements instructions from stored
software. While not described in detail, such digital control of a
mechanical system is well known to one of skill in the art and the
present invention may thus relate to instructions stored in
software capable of carrying out methods described herein. It is
therefore intended that the scope of the invention should be
determined with reference to the appended claims.
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