WO2000002271A2 - Internal cooling arrangement for fuel cell stack - Google Patents

Internal cooling arrangement for fuel cell stack Download PDF

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Publication number
WO2000002271A2
WO2000002271A2 PCT/GB1999/002074 GB9902074W WO0002271A2 WO 2000002271 A2 WO2000002271 A2 WO 2000002271A2 GB 9902074 W GB9902074 W GB 9902074W WO 0002271 A2 WO0002271 A2 WO 0002271A2
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WO
WIPO (PCT)
Prior art keywords
fuel cell
coolant
stack
cooling
reactant gas
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Application number
PCT/GB1999/002074
Other languages
French (fr)
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WO2000002271A3 (en
Inventor
Gerard Francis Mclean
Original Assignee
Ballard Power Systems Inc.
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Publication date
Application filed by Ballard Power Systems Inc. filed Critical Ballard Power Systems Inc.
Priority to AU45264/99A priority Critical patent/AU4526499A/en
Publication of WO2000002271A2 publication Critical patent/WO2000002271A2/en
Publication of WO2000002271A3 publication Critical patent/WO2000002271A3/en

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    • HELECTRICITY
    • H01ELECTRIC ELEMENTS
    • H01MPROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M8/00Fuel cells; Manufacture thereof
    • H01M8/02Details
    • H01M8/0202Collectors; Separators, e.g. bipolar separators; Interconnectors
    • H01M8/0204Non-porous and characterised by the material
    • H01M8/0221Organic resins; Organic polymers
    • HELECTRICITY
    • H01ELECTRIC ELEMENTS
    • H01MPROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M8/00Fuel cells; Manufacture thereof
    • H01M8/02Details
    • H01M8/0202Collectors; Separators, e.g. bipolar separators; Interconnectors
    • H01M8/0204Non-porous and characterised by the material
    • H01M8/0223Composites
    • H01M8/0228Composites in the form of layered or coated products
    • HELECTRICITY
    • H01ELECTRIC ELEMENTS
    • H01MPROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M8/00Fuel cells; Manufacture thereof
    • H01M8/02Details
    • H01M8/0202Collectors; Separators, e.g. bipolar separators; Interconnectors
    • H01M8/0247Collectors; Separators, e.g. bipolar separators; Interconnectors characterised by the form
    • H01M8/0254Collectors; Separators, e.g. bipolar separators; Interconnectors characterised by the form corrugated or undulated
    • HELECTRICITY
    • H01ELECTRIC ELEMENTS
    • H01MPROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M8/00Fuel cells; Manufacture thereof
    • H01M8/02Details
    • H01M8/0202Collectors; Separators, e.g. bipolar separators; Interconnectors
    • H01M8/0258Collectors; Separators, e.g. bipolar separators; Interconnectors characterised by the configuration of channels, e.g. by the flow field of the reactant or coolant
    • HELECTRICITY
    • H01ELECTRIC ELEMENTS
    • H01MPROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M8/00Fuel cells; Manufacture thereof
    • H01M8/02Details
    • H01M8/0202Collectors; Separators, e.g. bipolar separators; Interconnectors
    • H01M8/0258Collectors; Separators, e.g. bipolar separators; Interconnectors characterised by the configuration of channels, e.g. by the flow field of the reactant or coolant
    • H01M8/0263Collectors; Separators, e.g. bipolar separators; Interconnectors characterised by the configuration of channels, e.g. by the flow field of the reactant or coolant having meandering or serpentine paths
    • HELECTRICITY
    • H01ELECTRIC ELEMENTS
    • H01MPROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M8/00Fuel cells; Manufacture thereof
    • H01M8/02Details
    • H01M8/0202Collectors; Separators, e.g. bipolar separators; Interconnectors
    • H01M8/0267Collectors; Separators, e.g. bipolar separators; Interconnectors having heating or cooling means, e.g. heaters or coolant flow channels
    • HELECTRICITY
    • H01ELECTRIC ELEMENTS
    • H01MPROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M8/00Fuel cells; Manufacture thereof
    • H01M8/04Auxiliary arrangements, e.g. for control of pressure or for circulation of fluids
    • H01M8/04007Auxiliary arrangements, e.g. for control of pressure or for circulation of fluids related to heat exchange
    • H01M8/04067Heat exchange or temperature measuring elements, thermal insulation, e.g. heat pipes, heat pumps, fins
    • H01M8/04074Heat exchange unit structures specially adapted for fuel cell
    • HELECTRICITY
    • H01ELECTRIC ELEMENTS
    • H01MPROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M8/00Fuel cells; Manufacture thereof
    • H01M8/24Grouping of fuel cells, e.g. stacking of fuel cells
    • H01M8/241Grouping of fuel cells, e.g. stacking of fuel cells with solid or matrix-supported electrolytes
    • HELECTRICITY
    • H01ELECTRIC ELEMENTS
    • H01MPROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M8/00Fuel cells; Manufacture thereof
    • H01M8/24Grouping of fuel cells, e.g. stacking of fuel cells
    • H01M8/2457Grouping of fuel cells, e.g. stacking of fuel cells with both reactants being gaseous or vaporised
    • HELECTRICITY
    • H01ELECTRIC ELEMENTS
    • H01MPROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M8/00Fuel cells; Manufacture thereof
    • H01M8/24Grouping of fuel cells, e.g. stacking of fuel cells
    • H01M8/2465Details of groupings of fuel cells
    • HELECTRICITY
    • H01ELECTRIC ELEMENTS
    • H01MPROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M8/00Fuel cells; Manufacture thereof
    • H01M8/24Grouping of fuel cells, e.g. stacking of fuel cells
    • H01M8/2465Details of groupings of fuel cells
    • H01M8/2483Details of groupings of fuel cells characterised by internal manifolds
    • HELECTRICITY
    • H01ELECTRIC ELEMENTS
    • H01MPROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M2300/00Electrolytes
    • H01M2300/0017Non-aqueous electrolytes
    • H01M2300/0065Solid electrolytes
    • H01M2300/0082Organic polymers
    • HELECTRICITY
    • H01ELECTRIC ELEMENTS
    • H01MPROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M8/00Fuel cells; Manufacture thereof
    • H01M8/04Auxiliary arrangements, e.g. for control of pressure or for circulation of fluids
    • H01M8/04007Auxiliary arrangements, e.g. for control of pressure or for circulation of fluids related to heat exchange
    • YGENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02EREDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
    • Y02E60/00Enabling technologies; Technologies with a potential or indirect contribution to GHG emissions mitigation
    • Y02E60/30Hydrogen technology
    • Y02E60/50Fuel cells

Definitions

  • This invention relates to the cooling of fuel cell stacks, particularly proton exchange membrane (PEM)- type fuel cell stacks.
  • PEM proton exchange membrane
  • typically heat-conductive cooling plates are interspersed with fuel cell strata in order to conduct heat from the stack to a coolant, such as water, flowing therethrough.
  • the coolant plates have a coolant flow field extending over at least one of their surfaces; the flow field entrance and exit ports are connected to supply and sink plena for the coolant.
  • the fuel cell strata in the stack are provided with aligned apertures that, in the stack dimension, constitute plena for the supply of coolant fluid to the stack and exhaust of heated effluent coolant from the stack. Heat is removed from the fuel cells to the cooling plates and thence to the coolant as it flows past the cooling plates, thereby cooling the stack.
  • Ballard U.S. Patent No. 5,230,966 (Voss) issued 27 July 1993 illustrates a typical cooling arrangement for a PEM-type fuel cell stack.
  • the cooling plates are interspersed in the stack in the ratio of approximately one cooling plate for every three to five fuel cells in the stack.
  • This inclusion of a cooling plate in the stack contributes appreciably to the stack dimensions, increasing the overall stack volume and weight by an amount depending upon the size and mass of the cooling plate.
  • fuel cell stacks of this type are intended for use in vehicles and other mobile applications, it is desirable to keep bulk and weight of fuel cell stacks to a minimum. Since the use of such coolant plates contributes to both bulk and weight of the fuel cell stack in an undesirable manner, it would be desirable to eliminate the coolant plates.
  • the coolant is manifolded in a manner similar to the manner of manifolding reactant gases to the stack.
  • the requirement for coolant flow in plena extending in the stack dimension through the stack necessitates that the fuel cell membrane electrode assembly (MEA) layers in the stack be punctured to provide the necessary aperture for the flow of coolant therethrough. Puncturing of the MEA of the stack is inherently undesirable; for example, it complicates the manufacturing process and risks damage to the MEA layers.
  • Breault U.S. Patent 4,233,369, issued 11 November 1980, describes a cooler assembly for use between consecutive cells in a fuel cell stack that comprises a fibrous gas porous holder layer sandwiched between and bonded to a pair of gas impervious graphite plates.
  • the fibrous gas porous holder layer has coolant tubes passing therethrough within channels in the layer.
  • the channels have substantially the same depth as the holder layer thickness and as the outer diameter of the tubes.
  • Breault ' s design does not appreciably reduce the volume required by the cooling components of the stack, and thus does not appreciably contribute to the efficient use of space within the fuel cell stack.
  • Grevstad U.S. Patent 3,969,145, issued 13 July 1976, describes a cooler comprising a number of cooler tubes located in the separator plates between fuel cells in a stack. In the Grevstad configuration, a separate sealed piping structure is inserted into grooves cut into separator plates.
  • the Grevstad design is similar to that of Voss, mentioned previously, except that instead of forming coolant flow channels directly within separator plates as Voss discloses, Grevstad instead inserts a separately formed coolant conduit into the separator plate grooves. As best seen in Figure 2 of the Grevstad patent, this prior design does not alter the manner in which the reactant gas flow channel walls contact the MEA layers relative to conventional structures. Indeed, Grevstad pays little or no attention to the resulting coolant flow field deriving from his cooling arrangement.
  • McElroy U.S. Patent 4,678,724, issued 7 July 1987, discloses internally cooled bipolar separator plates within a fuel cell stack.
  • the cooling structure proposed by McElroy is in addition to the reactant gas flow path arrangements and does not attempt, in conjunction with the flow paths, to minimize the overall volume required.
  • Reiser U.S. Patent 3,964,930, issued 22 June 1976, discloses tubes for carrying coolant through a fuel cell stack.
  • the tubes are located in tubes or passageways formed in the separator plates.
  • Reiser's coolant conduit structure does not occupy its own plane within the fuel cell stack, but is coplanar with one of the reactant gas flow fields. As illustrated in Figure 3 of the Reiser patent, the cooling conduits lie in direct contact with the adjacent MEA layer.
  • Reiser's coolant tubes are electrically insulated from the rest of the fuel cell structure, and thus are not useful to carry electric current between adjacent fuel cells.
  • a fuel cell stack to which the present invention relates comprises a series of fuel cells in which reactant flow fields are provided between an MEA layer and separator layers of each fuel cell.
  • the flow field comprises, for each reactant gas, a number of flow channels interconnected in a flow field.
  • internal coolant conduits are provided that lie next to and in the plane of or nearly in the plane of at least some of the separator layers (enough for cooling of the stack) .
  • the coolant conduits extend adjacent and parallel to the reactant gas flow channels. Cooling of the stack is obtained by flow of coolant through the internal coolant conduits thus provided, and without the need of any special cooling plates.
  • Such coolant conduits may be provided in association with one or both of the reactant gas flow fields for each fuel cell, depending upon the configuration and the cooling needs of the stack. It is contemplated that at least one set of such coolant conduits will be provided for each fuel cell, although depending upon stack geometry, some stacks may be adequately cooled with one set of coolant conduits for each successive pair of fuel cells. It is doubtful that for fuel cells of moderate power rating, any fewer sets of coolant conduits than one for each pair of fuel cells will be satisfactory.
  • coolant conduits may be more favourably located than are conventional coolant conduits, and may be conveniently and economically located within or between other layers in a fuel cell stack so as to dispense with the need for separate cooling plates .
  • the coolant conduits preferably follow reactant gas flowpaths and may be located within existing reactant gas flowpath channels where space permits.
  • the coolant conduits may be located within separator plates in or near the lands or channel walls formed in such plates.
  • the coolant conduits may be formed between adjacent successive strata in the fuel cell stack.
  • auxiliary cooling function is implemented either by making available space for cooling that is less than optimally utilized for the primary function of the stratum, or by having a stratum matingly engage a neighbouring stratum to provide suitable spaces for coolant conduits between the neighbouring strata.
  • the inventor has recognized that in conventional designs, the reactant gas flow channels formed in fuel cell strata within a fuel cell stack are not optimally utilized throughout the cross-section of the flow channels for their primary purpose, viz to supply reactant gas to the adjacent porous electrode layers of the MEA strata in the stack.
  • the coolant conduits When the coolant conduits are provided within previously formed reactant flow channels to occupy space within the channels that otherwise would be available for reactant gas flow, the coolant conduits can be formed in or fixed to the separator layers.
  • the coolant conduits are preferably positioned in a space that would, if it were available for reactant gas flow, comprise a relatively inefficient portion of the space occupied by the reactant gas flow channels. This implies that the coolant conduits should generally be sufficiently remote from the MEA surfaces exposed to the reactant gas flow through the channels that these exposed MEA surfaces may continue to receive an adequate transfer of reactant gas through the adjacent porous electrode layer to the associated catalytic electrode and polymer electrolyte without undue obstruction by the coolant conduits.
  • coolant conduits remote from the MEA surfaces preserves the electrochemical efficiency of the MEA layer in contra distinction to the proposal in the prior Reiser U.S. Patent 3,964,930, which proposes that the coolant conduits be located in direct contact with the electrolyte layer.
  • the inventor has further recognized that given the possibility of reducing flow-channel cross-section
  • Such coolant conduits may be formed integrally with reactant gas distribution (flow-field) plates in the stack, or may be formed by contact of adjacent successive such distribution plates (or other layers) within the fuel cell stack.
  • the fuel cells may each provide (or share in the provision of) integrated internal cooling arrangements according to the invention, the provision in the stack of separate cooling plates can be completely eliminated, and fuel cell stack dimensions and weight can be correspondingly reduced, thereby improving the overall power density of the fuel cell stack.
  • the inventor has further recognized that the geometry of separator plates (flow plates) within conventional PEM-type fuel cell stacks and their interaction with adjacent MEA strata, including associated boundary effects, render non-uniform the electric current density across the area of contact of flow-plate lands with MEA layers; accordingly, it is possible to provide cooling conduits within the lands and possibly within other portions of the flow-plate structure itself, locating such cooling conduits away from the areas of peak current density.
  • tubular and undulate fuel cell stack configurations comprise extended parallel reactant gas conduits that are space-efficient and economical to manufacture.
  • Such configurations lend themselves to end connection or coupling to supply and exhaust plena for the reactant gases.
  • Coolant conduits according to the present invention could be coupled to inlet and outlet coolant plena respectively located at the ends of the coolant conduits. With such configurations, the puncturing of MEA layers can be avoided, thus avoiding the risk of damage or contamination and the special sealing problems associated with conventional PEM-type fuel cell stacks.
  • cooling conduits of the present invention are easily isolated from the electrochemical activity of the fuel cell stack (and indeed may be made from electrically insulating material of adequately high thermal conductivity) , and consequently may carry readily ionizable cooling fluids such as antifreeze, technically and economically superior cooling is possible without any appreciable risk of contamination of, or electrochemical interaction with, the vital active components of the fuel cell stack.
  • present conventional designs frequently require the use of such fluids as de-ionized water for use as the coolant in order to avoid creating unwanted conductive paths within the stack, and so as to avoid damage to membranes and catalysts.
  • Figure 1 is a schematic isometric view of a portion of a fuel cell stack of conventional design illustrating coolant plates interspersed with fuel cells in the stack.
  • FIG 2 is a schematic fragmentary section view of a portion of a conventional fuel cell arrangement showing a membrane electrode assembly (MEA) layer sandwiched between fuel and oxidant distribution/separator plates.
  • Figure 3 is a schematic plan view of a representative cooling conduit for installation in a conventional reactant distribution plate in accordance with the principles of the present invention.
  • MEA membrane electrode assembly
  • Figure 4 is a schematic plan view of a conventional fuel cell reactant distribution plate showing the cooling conduit of Figure 3 installed therein, in accordance with the principles of the present invention.
  • Figure 5 is a schematic fragmentary section view of a portion of a fuel cell reactant distributor plate illustrating a coolant conduit installed therein, in accordance with the present invention, the view being taken perpendicular to the flow dimension of the reactant distribution plate.
  • Figure 6 is a schematic fragmentary section view of a portion of a fuel cell reactant distributor plate illustrating an alternative coolant conduit installed therein, in accordance with the present invention, the view being taken perpendicular to the flow dimension of the reactant distribution plate.
  • Figure 7 is a schematic fragmentary section view of an undulate MEA layer sandwiched between consecutive separator plates in a stackable fuel cell arrangement and including cooling conduits within the troughs of the undulate MEA layer, constructed in accordance with the principles of the invention.
  • Figure 8 is a schematic fragmentary section view of a stackable fuel cell arrangement wherein opposed separator plates and electrode layers form cylinders suitable for passing reactant gas flow therethrough, and including conduits constructed in accordance with the principles of the present invention.
  • Figure 9 is a schematic fragmentary section view of a variant of a fuel cell of the general sort illustrated in Figure 7, having undulate MEA layers sandwiched between separator plates in a stacked series, but in which the cooling conduits are formed by mating planar and oblique portions of contacting consecutive separator layers in the stack, constructed in accordance with the present invention.
  • Figure 10 is a schematic fragmentary exploded section view of the structure of Figure 9.
  • Figure 11 is a schematic fragmentary section view of a portion of an alternative undulate MEA layer/separator layer configuration of the general type illustrated in Figure 9 and including cooling conduits in accordance with the invention, but in which the cooling conduits are smaller and the spacing between consecutive layers of the fuel cell stack is smaller than what appears in Figure 9, permitting a higher power density than is available from the Figure 9 arrangement.
  • Figure 12 is a schematic exploded section view of the structure of Figure 11.
  • Figure 13 is a schematic fragmentary section view of a portion of a reactant distribution plate positioned adjacent a planar MEA layer, illustrating the regions of the reactant gas channel that are relatively effectively utilized for transmission of reactant to the MEA layer, and those regions of the channel that are relatively poorly thus utilized.
  • Figure 14 is a schematic fragmentary section view of an undulate MEA layer positioned in contact with an adjacent separator layer, and showing the relatively effectively utilized portion of the reactant flow channel and the relatively poorly utilized portion of the reactant flow channel.
  • the stack dimension is the dimension in which fuel cell layers and other layers are stacked; it is perpendicular to the broad surfaces of the layers within the stack.
  • the flow dimension is the dimension parallel to the major flow channels comprising the reactant gases flow fields. (In a stack, depending upon configuration, it is conceivable that the fuel gas flow channels might be designed to lie perpendicular to the oxidant gas flow channels. However, relative to any one flow field, the flow dimension is free from ambiguity. )
  • the transverse dimension is that dimension that, for any given flow field, is perpendicular to both the flow and stack dimensions.
  • cooling plates 14 made of coated metal or other suitable material that functions as an effective heat conductor while avoiding contamination of the fuel cells or reactant gases therein.
  • the fuel cell stack 10 illustrated in Figure 1 is partial only; one might expect as many as 100 or more fuel cells to be mounted in a stack.
  • the lowermost portion of the stack 10 is illustrated, resting upon terminal base plate 15.
  • Cooling plates 14 are illustrated in Figure 1 as constituting every fourth layer of the fuel cell stack, three fuel cells 12 being interposed between each consecutive pair of cooling plates 14. It follows that fuel cell 12b, relatively remote from the nearest cooling plate 14, will operate at a higher temperature than fuel cells 12a and 12c that are in contact with an adjacent cooling plate 14.
  • the cooling plates 14 and the fuel cells 12 are provided with apertures 16 so as to provide, in the stack dimension of stack 10, plenum chambers for incoming and effluent fuel and oxidant gases.
  • Coolant plenum apertures 18 are provided for inlet and effluent coolant flow within the stack. Heat flow is predominantly from the coolant plates 14 (which receive heat from the fuel cells in the stack) into the conductive coolant that flows therethrough from and into respective supply and exhaust plenum chambers formed by the sequence of aligned apertures 18 throughout the stack 10.
  • Figure 2 illustrates a fragment of an individual fuel cell layer within the stack 10.
  • a membrane electrode assembly (MEA) layer 22 is shown sandwiched between a fuel gas distribution and separator plate 24 and an oxidant gas distribution and separator plate 26.
  • Each of the plates 24, 26 is provided with a serpentine flowpath (not apparent from Figure 2, but conventional) which, in cross-section, appears as a series of spaced flow channels 28. Electrical conductivity through the stack is maintained by physical and electrical contact between the solid abutting portions 27 of the distribution plates 24, 26, and the sandwiched MEA layer 22 kept under compression between the abutting solid portions 27 of the distribution plates 24, 26. It is to be observed that although fuel and oxidant gases flow through the entirety of the flow channels 28 forming the respective fuel and oxidant flowpaths in the distribution plates 24, 26, nevertheless, primarily that portion of the gas flow immediately adjacent the exposed MEA layer 22 is able effectively to reach the fuel cell electrodes and contribute to the electrochemical performance of the fuel cell.
  • cooling conduits in the vicinity of areas of the lands in which electric current flow is minimal; current flow tends to peak in those portions of the lands 27 that border on the flow channels 28, and tends to be minimal generally centrally of each land 27. Consequently, coolant conduits could be formed within the lands or in the areas of conjunction of the lands 27 with the planar portions of the separator plates 26 (say), each such cooling conduit being located generally equidistantly from the two nearest flow channel neighbours 28.
  • cooling conduits within the separator plates 24 or 26 are expected to exceed the cost of placing discrete or integral cooling conduits within "waste space" of the flow channels 28. Accordingly, most of the discussion in this specification will focus on cooling conduits formed within or immediately adjacent flow channels.
  • coolant conduit 40 is arranged to lie immediately adjacent the portion of flow channel 28 in distribution plate 24 (for example - one could have chosen plate 26 instead of 24), so that the conduit 40 is not positioned to interfere with the effective reactant gas flow within flow channel 28 that is immediately adjacent the exposed surface of MEA layer 22.
  • the coolant conduits 40 are bonded to the adjacent portions of distribution plates 24. If the distribution plates 24 and the coolant conduits 40 are made of metal, then such bonding may be by welding or soldering. If they are not made of metal or are provided with non-metallic surfaces, the use of a cement such as an high-temperature-resistant epoxy resin may be suitable. It is common knowledge that metals can easily contaminate the reactant gases and be led into contact with the catalytic electrode layers of MEA strata, thereby contaminating the catalyst and rendering the fuel cell inefficient or inoperative.
  • any exposed reactive metal surfaces may be coated with some suitable inert material such as a noble metal or carbon or plastics coating.
  • suitable inert material such as a noble metal or carbon or plastics coating.
  • Materials other than metals can be used for constructing the conduits 40; suitable materials that have adequate heat transfer capability and yet are sufficiently inert that they can be used in direct contact with the reactant gases include solid and woven graphite materials.
  • An alternative inert material with satisfactory heat conductivity that is expected to be useful is polytetrafluoroethylene (PTFE) such as that sold under the trademark TEFLON.
  • Boundary walls 41 for cooling conduit 40 may be made of either conductive material or non-conductive material in the designer's preference. Non-conducting walls for the cooling conduits would have the advantage of allowing conductive coolant fluids to be used without disturbing the internal electrical balance within the fuel cell, since the conductive coolant fluids would be electrically isolated from the electrochemically active portions of the fuel cell by the non-conductive walls of the cooling conduit. On the other hand, if the walls 41 are made of conductive material, then the conduit walls themselves may, depending upon the configuration chosen, serve a useful function in current collection and transmission.
  • Figure 5 does not lend itself to such electrically conductive function of the conduit walls 41, but other configurations can easily be imagined in which the coolant conduit is shaped, dimensioned and located in such a position that electrical conductivity of the walls might be advantageous for collection and transmission of fuel cell current.
  • the walls 41 or other coolant conduit walls to be described below are selected to be conductive or non-conductive, it is of course important that the coolant within the coolant conduits 40 be isolated physically from the reactant gases and MEA layers; it is important to prevent migration of the coolant materials into the electrolytic membrane layer.
  • FIG. 6 illustrates an alternative exemplary design that may, if the designer wishes, be substituted for that illustrated in Figure 5.
  • the cooling conduit walls 45 are triangular in cross-sectional configuration; the inverted base of the triangle is dimensioned to permit the base to fit snugly within the reactant gas flow channel 28.
  • the triangular wall configuration 45 perforce circumscribes a triangular interior conduit space 43. Note that the Figure 6 embodiment tends to promote stability of the cooling conduit 43 within the flow channel 28 while presenting minimum interference with reactant gas flow within flow channel 28; further, the Figure 6 embodiment would be expected to be relatively economical to manufacture.
  • Figure 7 illustrates a waved, or undulate, MEA layer 70 sandwiched between separator layers 72 that make electrical connection with the apices 74 of the MEA layer 70.
  • the separator layers 72 together with the sandwiched undulate MEA layer 70 form flow channels 78 for the reactant gases.
  • Such an arrangement is described in British patent application [B&M 60222] (McLean) filed on [concurrently herewith] .
  • the uppermost "troughs" of the undulate MEA layer 70 constitute fuel flow channels 76 and the lower complementary channels 78 serve to carry the oxidant gas, or vice versa - the selection is arbitrary, as is the orientation shown in the illustration.
  • coolant flow conduits 79 may be provided immediately adjacent the separator layers 72 and spaced away from the MEA layer 70 so as to leave ample flow within channels 76, 78 sufficient to feed the exposed surfaces of MEA layer 70.
  • coolant conduits 79 are shown both in the oxidant flow channels 78 and in the fuel flow channels 76.
  • each of the coolant conduits 79 would be connected between inlet and outlet coolant plenum chambers (not shown) to complete the coolant cycling system.
  • FIG. 8 illustrates a tubular construction for a fuel cell stack of the type described in applicant's British patent application Serial No. [B&M 60222] (McLean) filed on [concurrently herewith] .
  • transversely consecutive semi-cylindrical separator elements 80 combine to form, for each layer, a separator continuum 82.
  • Oppositely disposed the separator layers 82 are transversely consecutive semi- cylindrical porous electrode elements forming a mating porous electrode layer 84.
  • Sandwiched between each pair of opposed electrode layers 84 is an MEA layer 86.
  • Located between opposed separator layers 82 and porous electrode layers 84 are planar separator plates 88.
  • semicylindrical conduits 85 next to MEA layers 86 will carry the reactant gases while semicylindrical conduits 89 next to separator layers 82 will carry the coolant.
  • the separator plates 88 serve as coolant plates, but they are not merely coolant plates. All layers within a fuel cell stack that are capable of heat transfer, function to at least some limited extent as coolant layers or plates.
  • FIG 7 separate coolant conduits 79 are illustrated that are inserted in the flow path arrangement, and attached to the adjacent separator layer 72. It is possible, however, to configure separator plates to provide coolant channels without requiring a separate manufacturing operation to form and attach the coolant channels.
  • Figure 9 illustrates this possibility in the case of a fuel cell stack 93 a portion of which is schematically illustrated, each stratum 91 in the stack 93 including an undulate MEA layer 70, essentially similar to the layer 70 illustrated in Figure 7.
  • the separator layers 90 are not formed as planar layers, but rather as having triangular troughs 92 that periodically interrupt the planar portions 94 of the separator layers 90.
  • Sequential separator layers 90 in the stack are transversely offset from one another so that the triangular trough 92 of any given separator layer 90 lies immediately adjacent a planar portion 94 of the next adjacent separator layer 90.
  • This structural arrangement is more obviously apparent from an inspection of the schematic exploded view of Figure 10.
  • the triangular trough portions 92 combine with facing and mating planar portions 94 to form conduits 96 of triangular cross- section that are suitable for coolant flow. Note that it is not essential that there be a sealing contact between consecutive separator layers 90; no harm is done if there are small leaks between neighbouring triangular conduits 96.
  • Figure 11 is an exploded schematic view of what is seen in Figure 11.
  • each coolant conduit is located in contact with (or formed by) the separator layers so as to lie in a region that is most remote from the active surfaces of the nearby MEA layer.
  • FIGs 13 and 14 Referring first to Figure 13, a conventional reactant gas distribution plate 27 is shown in contact with an adjacent MEA layer 22 (compare Figure 2) .
  • that region 131 of the flow ⁇ channel 28 carries reactant gas that is able effectively to make contact with, and to penetrate, the exposed surface 133 of the MEA layer 22.
  • region 135 remote from the exposed surface 133 of the MEA layer 22 is relatively ineffective to promote the electrochemical reaction; reactant gas in region 135 has difficulty in reaching the MEA layer 22. Accordingly, if a portion of the reactant flow channel 28 were to be reallocated to the space required for a coolant conduit, it would follow that such conduit should be located within the relatively inefficient region 135. As a practical matter, a coolant conduit having a configuration resembling that of the region 135 would be expensive to manufacture. Further, it is desired to encourage the penetration of reactant gas into the MEA layer at the limits of the exposed portion 133 of the MEA layer 22. Consequently, it is not desired to locate a coolant conduit in that part of the reactant flow path 28 at the limits of the exposed portion 133 of the MEA layer 22.
  • FIG 14 it is logical to excise from flow channel 75, shown bounded by separator layer 72 and MEA layer 70 in that illustration, the inefficient portion of the flow channel 75 and to make the inefficient portion available to accommodate a coolant conduit 79 (compare Figure 7) .
  • a coolant conduit 79 By installing a coolant conduit 79 in an inefficient portion of the previously available flow channel space 75, the residual space 76 left for reactant flow gas is considerably diminished, yet, because the region 76 is the region that is immediately adjacent the MEA surface 70, the overall efficiency of the fuel cell can be maintained (with such adjustments to pressures and flow rates as are needed to maintain operating efficiency of each fuel cell) .
  • the cooling conduit 79 is placed adjacent and preferably fixed to the separator plate 72 so that it remains well out of contact with the MEA layer 70.
  • Figure 13 has taken, by way of example, a conventional reactant flow distribution plate of the sort illustrated in Figure 2
  • Figure 14 has taken, by way of example, an undulate MEA layer arrangement such as that illustrated in Figure 7, the principles derived from a review of Figures 13 and 14 can be applied to most, if not all, other reactant channel configurations.
  • the accompanying drawings are schematic; engineering details of any given design implementation of the invention may vary according to designers' preferences that meet objectives that may or may not directly relate to cooling.
  • the designer may wish to increase reactant gas turbulence to improve transfer of reactant gas to the electrochemically active portions of the fuel cell.
  • the designer may choose rough or irregular surfaces for the walls enclosing reactant gas flowpaths. It may be convenient for the designer to select rough or irregular coolant conduit external walls (that will be in contact with reactant gases) to help achieve such objective.
  • the cross- sectional shape and dimensions of the coolant channel can be selected not only with cooling in mind but also for optimization of the reactant gas flowpath cross- section.
  • An integrated design approach should facilitate the achievement of such technical objectives while permitting the designer to aim for manufacturing ⁇ simplicity and relatively low cost of manufacture.

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Abstract

In a PEM-type fuel cell, coolant conduits (40) are provided that follow the reactant gas flowpaths (28) on either side of the MEA layer, either within a space that otherwise would have been part of the reactant gas flowpaths, or nearby. The positioning of the coolant conduits is selected to permit them to remove heat effectively from the fuel cell while lying in relatively space-inefficient portion of the fuel cell, preferably a space that would have, were it not for the presence of the coolant conduits, have been occupied by a relatively space-inefficient portion of the flowpaths. These coolant conduits can eliminate the need for coolant conduits that pass in the stack dimension through the MEA layers.

Description

Internal Cooling Arrangement for Fuel Cell Stack
Field of Invention
This invention relates to the cooling of fuel cell stacks, particularly proton exchange membrane (PEM)- type fuel cell stacks.
Background
In PEM-type fuel cell stacks of the type heretofore manufactured, typically heat-conductive cooling plates are interspersed with fuel cell strata in order to conduct heat from the stack to a coolant, such as water, flowing therethrough. Typically, the coolant plates have a coolant flow field extending over at least one of their surfaces; the flow field entrance and exit ports are connected to supply and sink plena for the coolant. Typically, the fuel cell strata in the stack are provided with aligned apertures that, in the stack dimension, constitute plena for the supply of coolant fluid to the stack and exhaust of heated effluent coolant from the stack. Heat is removed from the fuel cells to the cooling plates and thence to the coolant as it flows past the cooling plates, thereby cooling the stack. Ballard U.S. Patent No. 5,230,966 (Voss) issued 27 July 1993 illustrates a typical cooling arrangement for a PEM-type fuel cell stack.
Typically in conventional PEM-type fuel cell stacks, the cooling plates are interspersed in the stack in the ratio of approximately one cooling plate for every three to five fuel cells in the stack. This inclusion of a cooling plate in the stack of course contributes appreciably to the stack dimensions, increasing the overall stack volume and weight by an amount depending upon the size and mass of the cooling plate. As fuel cell stacks of this type are intended for use in vehicles and other mobile applications, it is desirable to keep bulk and weight of fuel cell stacks to a minimum. Since the use of such coolant plates contributes to both bulk and weight of the fuel cell stack in an undesirable manner, it would be desirable to eliminate the coolant plates.
Further, since heat is generated approximately equally in each of the fuel cells within the stack, yet the proximity of any given fuel cell to the nearest cooling plate may vary due to the overall configuration, it follows that some fuel cells nearer the cooling plates can operate at a lower temperature than those fuel cells that are more remote from the nearest cooling plate, creating temperature gradients within the fuel cell stack. Since there is normally an optimum operating temperature for each fuel cell, it follows that at least some fuel cells in the stack will not operate at optimum temperature, with the consequence that overall performance of the fuel cell stack is reduced relative to the theoretical maximum available from the fuel cells in the stack.
As mentioned briefly above, when cooling plates are interspersed with the fuel cell layers in the stack, typically the coolant is manifolded in a manner similar to the manner of manifolding reactant gases to the stack. Typically, this implies that a common plenum configuration is required for the coolant flow, as would also be the case for supply and exhaust of the reactant gases. The requirement for coolant flow in plena extending in the stack dimension through the stack necessitates that the fuel cell membrane electrode assembly (MEA) layers in the stack be punctured to provide the necessary aperture for the flow of coolant therethrough. Puncturing of the MEA of the stack is inherently undesirable; for example, it complicates the manufacturing process and risks damage to the MEA layers.
A number of prior proposals for alternative cooling arrangements have been made. Breault, U.S. Patent 4,233,369, issued 11 November 1980, describes a cooler assembly for use between consecutive cells in a fuel cell stack that comprises a fibrous gas porous holder layer sandwiched between and bonded to a pair of gas impervious graphite plates. The fibrous gas porous holder layer has coolant tubes passing therethrough within channels in the layer. The channels have substantially the same depth as the holder layer thickness and as the outer diameter of the tubes. Breault ' s design does not appreciably reduce the volume required by the cooling components of the stack, and thus does not appreciably contribute to the efficient use of space within the fuel cell stack.
Grevstad, U.S. Patent 3,969,145, issued 13 July 1976, describes a cooler comprising a number of cooler tubes located in the separator plates between fuel cells in a stack. In the Grevstad configuration, a separate sealed piping structure is inserted into grooves cut into separator plates. The Grevstad design is similar to that of Voss, mentioned previously, except that instead of forming coolant flow channels directly within separator plates as Voss discloses, Grevstad instead inserts a separately formed coolant conduit into the separator plate grooves. As best seen in Figure 2 of the Grevstad patent, this prior design does not alter the manner in which the reactant gas flow channel walls contact the MEA layers relative to conventional structures. Indeed, Grevstad pays little or no attention to the resulting coolant flow field deriving from his cooling arrangement.
McElroy, U.S. Patent 4,678,724, issued 7 July 1987, discloses internally cooled bipolar separator plates within a fuel cell stack. The cooling structure proposed by McElroy is in addition to the reactant gas flow path arrangements and does not attempt, in conjunction with the flow paths, to minimize the overall volume required.
Reiser, U.S. Patent 3,964,930, issued 22 June 1976, discloses tubes for carrying coolant through a fuel cell stack. The tubes are located in tubes or passageways formed in the separator plates. Reiser's coolant conduit structure does not occupy its own plane within the fuel cell stack, but is coplanar with one of the reactant gas flow fields. As illustrated in Figure 3 of the Reiser patent, the cooling conduits lie in direct contact with the adjacent MEA layer. Reiser's coolant tubes are electrically insulated from the rest of the fuel cell structure, and thus are not useful to carry electric current between adjacent fuel cells. Reiser does not mention the potential decrease in fuel cell performance that may result from insertion of nonconducting cooling components immediately next to the electrolyte layer - there will be significant portions of Reiser's electrolyte layer that are in contact with gas impermeable electrically insulating components, and thus fuel cell efficiency necessarily must decrease. Reiser does not disclose the utility of this design for PEM-type fuel cells.
Externally manifolded coolant layers have also been described in the prior literature. Kamoshita, U.S. Patent 5,041,344, issued 20 August 1991, is representative. Kamoshita continues to require separate cooling plates to achieve cooling of the stack.
Summary of the Invention
A fuel cell stack to which the present invention relates comprises a series of fuel cells in which reactant flow fields are provided between an MEA layer and separator layers of each fuel cell. The flow field comprises, for each reactant gas, a number of flow channels interconnected in a flow field. According to the invention, internal coolant conduits are provided that lie next to and in the plane of or nearly in the plane of at least some of the separator layers (enough for cooling of the stack) . The coolant conduits extend adjacent and parallel to the reactant gas flow channels. Cooling of the stack is obtained by flow of coolant through the internal coolant conduits thus provided, and without the need of any special cooling plates. Such coolant conduits may be provided in association with one or both of the reactant gas flow fields for each fuel cell, depending upon the configuration and the cooling needs of the stack. It is contemplated that at least one set of such coolant conduits will be provided for each fuel cell, although depending upon stack geometry, some stacks may be adequately cooled with one set of coolant conduits for each successive pair of fuel cells. It is doubtful that for fuel cells of moderate power rating, any fewer sets of coolant conduits than one for each pair of fuel cells will be satisfactory.
The present inventor has recognized that coolant conduits may be more favourably located than are conventional coolant conduits, and may be conveniently and economically located within or between other layers in a fuel cell stack so as to dispense with the need for separate cooling plates . In one embodiment of the invention, the coolant conduits preferably follow reactant gas flowpaths and may be located within existing reactant gas flowpath channels where space permits. In another embodiment of the invention, the coolant conduits may be located within separator plates in or near the lands or channel walls formed in such plates. In yet another embodiment of the invention, the coolant conduits may be formed between adjacent successive strata in the fuel cell stack.
In each of the foregoing embodiments of the invention, there is a commonality of design approach to the extent that strata in the fuel cell stack are designed to perform not only their primary function but also an auxiliary cooling function. The auxiliary cooling function is implemented either by making available space for cooling that is less than optimally utilized for the primary function of the stratum, or by having a stratum matingly engage a neighbouring stratum to provide suitable spaces for coolant conduits between the neighbouring strata. The inventor has recognized that in conventional designs, the reactant gas flow channels formed in fuel cell strata within a fuel cell stack are not optimally utilized throughout the cross-section of the flow channels for their primary purpose, viz to supply reactant gas to the adjacent porous electrode layers of the MEA strata in the stack. Those portions of the flow-channel cross-section that are nearest to the porous electrode layer are most effective in enabling reactant gas to reach and penetrate the porous electrode layer. On the other hand, those portions of the flow channel more remote from the MEA layer are relatively inefficient for transferring reactant gas to the porous electrode layer. From this insight, the inventor has recognized that such inefficient portion of the flow channel could be utilized instead for the provision of a coolant conduit that would follow at least a portion of such flow channel, and would provide individual cooling for the fuel cell in which it is located. (Given the same insight, another design solution might be to reduce reactant gas flow channel dimensions, which would necessitate locating the coolant conduits somewhere else. However, given also that the MEA layer is to some extent deformable, especially under the clamping forces typically encountered in a fuel cell stack, the designer cannot as a practical matter reduce the gross dimensions of the flow channels too much as compared with conventional designs, because of the risk that the MEA layer material might migrate to an unacceptable extent into the reactant gas flowpaths, choking off gas flow. )
When the coolant conduits are provided within previously formed reactant flow channels to occupy space within the channels that otherwise would be available for reactant gas flow, the coolant conduits can be formed in or fixed to the separator layers. The coolant conduits are preferably positioned in a space that would, if it were available for reactant gas flow, comprise a relatively inefficient portion of the space occupied by the reactant gas flow channels. This implies that the coolant conduits should generally be sufficiently remote from the MEA surfaces exposed to the reactant gas flow through the channels that these exposed MEA surfaces may continue to receive an adequate transfer of reactant gas through the adjacent porous electrode layer to the associated catalytic electrode and polymer electrolyte without undue obstruction by the coolant conduits. Note that the location of coolant conduits remote from the MEA surfaces preserves the electrochemical efficiency of the MEA layer in contra distinction to the proposal in the prior Reiser U.S. Patent 3,964,930, which proposes that the coolant conduits be located in direct contact with the electrolyte layer.
The inventor has further recognized that given the possibility of reducing flow-channel cross-section
(given flow channels of conventional size) without unduly impeding reactant transfer to the porous electrode layer adjacent, it is possible to reconfigure the internal structure of fuel cells so as to provide suitable coolant conduits in convenient spaces within or between fuel cell strata. Such design solutions continue to involve the placing of suitable coolant conduits adjacent flow channels, but need not involve any encroachment on the flow channels themselves. Such coolant conduits may be formed integrally with reactant gas distribution (flow-field) plates in the stack, or may be formed by contact of adjacent successive such distribution plates (or other layers) within the fuel cell stack. Further, because the fuel cells may each provide (or share in the provision of) integrated internal cooling arrangements according to the invention, the provision in the stack of separate cooling plates can be completely eliminated, and fuel cell stack dimensions and weight can be correspondingly reduced, thereby improving the overall power density of the fuel cell stack.
The inventor has further recognized that the geometry of separator plates (flow plates) within conventional PEM-type fuel cell stacks and their interaction with adjacent MEA strata, including associated boundary effects, render non-uniform the electric current density across the area of contact of flow-plate lands with MEA layers; accordingly, it is possible to provide cooling conduits within the lands and possibly within other portions of the flow-plate structure itself, locating such cooling conduits away from the areas of peak current density. However, as a practical matter, it may be more difficult to design such cooling conduits to be located within the flow-plate material and to manufacture them economically than to design them into the "waste space" of the flow channels, or between adjacent strata in a fuel cell stack.
The design of a fuel cell stack in accordance with the principles of the invention is intended primarily for use with PEM-type fuel cells, but the principles may be applied to other types of fuel cells in which internal coolant conduits may conveniently follow reactant gas flow channels (or may be positioned in close proximity thereto, preferably with end-to-end orientation generally following the flow-path pattern) .
The principles of the present invention can be applied not only to fuel cell stacks comprising conventional fuel cells previously known, but also to tubular fuel cell stack configurations and undulate tube cell stack configurations of the sort described and claimed in the applicant's British patent application Serial No. [B&M
60222] (McLean) filed on [concurrently herewith] , and also to stamped, corrugated and similar non- conventional fuel cell layer architecture. Such tubular and undulate fuel cell stack configurations comprise extended parallel reactant gas conduits that are space-efficient and economical to manufacture. Such configurations lend themselves to end connection or coupling to supply and exhaust plena for the reactant gases. Coolant conduits according to the present invention could be coupled to inlet and outlet coolant plena respectively located at the ends of the coolant conduits. With such configurations, the puncturing of MEA layers can be avoided, thus avoiding the risk of damage or contamination and the special sealing problems associated with conventional PEM-type fuel cell stacks. Because the cooling conduits of the present invention are easily isolated from the electrochemical activity of the fuel cell stack (and indeed may be made from electrically insulating material of adequately high thermal conductivity) , and consequently may carry readily ionizable cooling fluids such as antifreeze, technically and economically superior cooling is possible without any appreciable risk of contamination of, or electrochemical interaction with, the vital active components of the fuel cell stack. By contrast, present conventional designs frequently require the use of such fluids as de-ionized water for use as the coolant in order to avoid creating unwanted conductive paths within the stack, and so as to avoid damage to membranes and catalysts.
Summary of the Drawings
Figure 1 is a schematic isometric view of a portion of a fuel cell stack of conventional design illustrating coolant plates interspersed with fuel cells in the stack.
Figure 2 is a schematic fragmentary section view of a portion of a conventional fuel cell arrangement showing a membrane electrode assembly (MEA) layer sandwiched between fuel and oxidant distribution/separator plates. Figure 3 is a schematic plan view of a representative cooling conduit for installation in a conventional reactant distribution plate in accordance with the principles of the present invention.
Figure 4 is a schematic plan view of a conventional fuel cell reactant distribution plate showing the cooling conduit of Figure 3 installed therein, in accordance with the principles of the present invention.
Figure 5 is a schematic fragmentary section view of a portion of a fuel cell reactant distributor plate illustrating a coolant conduit installed therein, in accordance with the present invention, the view being taken perpendicular to the flow dimension of the reactant distribution plate.
Figure 6 is a schematic fragmentary section view of a portion of a fuel cell reactant distributor plate illustrating an alternative coolant conduit installed therein, in accordance with the present invention, the view being taken perpendicular to the flow dimension of the reactant distribution plate. Figure 7 is a schematic fragmentary section view of an undulate MEA layer sandwiched between consecutive separator plates in a stackable fuel cell arrangement and including cooling conduits within the troughs of the undulate MEA layer, constructed in accordance with the principles of the invention.
Figure 8 is a schematic fragmentary section view of a stackable fuel cell arrangement wherein opposed separator plates and electrode layers form cylinders suitable for passing reactant gas flow therethrough, and including conduits constructed in accordance with the principles of the present invention.
Figure 9 is a schematic fragmentary section view of a variant of a fuel cell of the general sort illustrated in Figure 7, having undulate MEA layers sandwiched between separator plates in a stacked series, but in which the cooling conduits are formed by mating planar and oblique portions of contacting consecutive separator layers in the stack, constructed in accordance with the present invention.
Figure 10 is a schematic fragmentary exploded section view of the structure of Figure 9. Figure 11 is a schematic fragmentary section view of a portion of an alternative undulate MEA layer/separator layer configuration of the general type illustrated in Figure 9 and including cooling conduits in accordance with the invention, but in which the cooling conduits are smaller and the spacing between consecutive layers of the fuel cell stack is smaller than what appears in Figure 9, permitting a higher power density than is available from the Figure 9 arrangement.
Figure 12 is a schematic exploded section view of the structure of Figure 11.
Figure 13 (prior art) is a schematic fragmentary section view of a portion of a reactant distribution plate positioned adjacent a planar MEA layer, illustrating the regions of the reactant gas channel that are relatively effectively utilized for transmission of reactant to the MEA layer, and those regions of the channel that are relatively poorly thus utilized.
Figure 14 is a schematic fragmentary section view of an undulate MEA layer positioned in contact with an adjacent separator layer, and showing the relatively effectively utilized portion of the reactant flow channel and the relatively poorly utilized portion of the reactant flow channel.
Detailed Description
In this specification, three dimensional referents are used. The stack dimension is the dimension in which fuel cell layers and other layers are stacked; it is perpendicular to the broad surfaces of the layers within the stack. The flow dimension is the dimension parallel to the major flow channels comprising the reactant gases flow fields. (In a stack, depending upon configuration, it is conceivable that the fuel gas flow channels might be designed to lie perpendicular to the oxidant gas flow channels. However, relative to any one flow field, the flow dimension is free from ambiguity. ) The transverse dimension is that dimension that, for any given flow field, is perpendicular to both the flow and stack dimensions.
Referring to Figure 1, in a conventional fuel cell stack 10, interspersed among fuel cell layers 12 are cooling plates 14 made of coated metal or other suitable material that functions as an effective heat conductor while avoiding contamination of the fuel cells or reactant gases therein. The fuel cell stack 10 illustrated in Figure 1 is partial only; one might expect as many as 100 or more fuel cells to be mounted in a stack. In the case of the exemplary schematic arrangement of Figure 1, the lowermost portion of the stack 10 is illustrated, resting upon terminal base plate 15. Cooling plates 14 are illustrated in Figure 1 as constituting every fourth layer of the fuel cell stack, three fuel cells 12 being interposed between each consecutive pair of cooling plates 14. It follows that fuel cell 12b, relatively remote from the nearest cooling plate 14, will operate at a higher temperature than fuel cells 12a and 12c that are in contact with an adjacent cooling plate 14.
The cooling plates 14 and the fuel cells 12 are provided with apertures 16 so as to provide, in the stack dimension of stack 10, plenum chambers for incoming and effluent fuel and oxidant gases. Coolant plenum apertures 18 are provided for inlet and effluent coolant flow within the stack. Heat flow is predominantly from the coolant plates 14 (which receive heat from the fuel cells in the stack) into the conductive coolant that flows therethrough from and into respective supply and exhaust plenum chambers formed by the sequence of aligned apertures 18 throughout the stack 10. Figure 2 illustrates a fragment of an individual fuel cell layer within the stack 10. A membrane electrode assembly (MEA) layer 22 is shown sandwiched between a fuel gas distribution and separator plate 24 and an oxidant gas distribution and separator plate 26. Each of the plates 24, 26 is provided with a serpentine flowpath (not apparent from Figure 2, but conventional) which, in cross-section, appears as a series of spaced flow channels 28. Electrical conductivity through the stack is maintained by physical and electrical contact between the solid abutting portions 27 of the distribution plates 24, 26, and the sandwiched MEA layer 22 kept under compression between the abutting solid portions 27 of the distribution plates 24, 26. It is to be observed that although fuel and oxidant gases flow through the entirety of the flow channels 28 forming the respective fuel and oxidant flowpaths in the distribution plates 24, 26, nevertheless, primarily that portion of the gas flow immediately adjacent the exposed MEA layer 22 is able effectively to reach the fuel cell electrodes and contribute to the electrochemical performance of the fuel cell.
Recognizing that the portions of channels 28 most remote from the MEA layer 22 do not directly contribute to electrochemical activity, but simply carry surplus reactant gases, the present inventor has recognized that this under-utilized flow channel space would be available for the insertion of a cooling conduit. As mentioned above, it is also possible to form the cooling conduits in the vicinity of areas of the lands in which electric current flow is minimal; current flow tends to peak in those portions of the lands 27 that border on the flow channels 28, and tends to be minimal generally centrally of each land 27. Consequently, coolant conduits could be formed within the lands or in the areas of conjunction of the lands 27 with the planar portions of the separator plates 26 (say), each such cooling conduit being located generally equidistantly from the two nearest flow channel neighbours 28. However, the cost of fabrication of cooling conduits within the separator plates 24 or 26 is expected to exceed the cost of placing discrete or integral cooling conduits within "waste space" of the flow channels 28. Accordingly, most of the discussion in this specification will focus on cooling conduits formed within or immediately adjacent flow channels.
Looking at a simplified reactant gas flow-field or distribution plate 32 in plan view (Figure 4) , one can observe a conventional serpentine flowpath 34 running from a reactant supply plenum 36 to a reactant exhaust plenum 38. Given the pattern of the flowpath 34, it is a simple matter to devise a mating serpentine coolant conduit 40 connecting inlet coolant plenum 42 with outlet coolant plenum 44.
In section (Figure 5) it can be seen that the coolant conduit 40 is arranged to lie immediately adjacent the portion of flow channel 28 in distribution plate 24 (for example - one could have chosen plate 26 instead of 24), so that the conduit 40 is not positioned to interfere with the effective reactant gas flow within flow channel 28 that is immediately adjacent the exposed surface of MEA layer 22.
Preferably the coolant conduits 40 are bonded to the adjacent portions of distribution plates 24. If the distribution plates 24 and the coolant conduits 40 are made of metal, then such bonding may be by welding or soldering. If they are not made of metal or are provided with non-metallic surfaces, the use of a cement such as an high-temperature-resistant epoxy resin may be suitable. It is common knowledge that metals can easily contaminate the reactant gases and be led into contact with the catalytic electrode layers of MEA strata, thereby contaminating the catalyst and rendering the fuel cell inefficient or inoperative. Accordingly, steps must be taken to avoid metal contamination of the catalyst - for example, any exposed reactive metal surfaces (if metal is to be used at all in the construction of the conduits 40 or the distribution plates 24) may be coated with some suitable inert material such as a noble metal or carbon or plastics coating. Materials other than metals can be used for constructing the conduits 40; suitable materials that have adequate heat transfer capability and yet are sufficiently inert that they can be used in direct contact with the reactant gases include solid and woven graphite materials. An alternative inert material with satisfactory heat conductivity that is expected to be useful is polytetrafluoroethylene (PTFE) such as that sold under the trademark TEFLON.
Boundary walls 41 for cooling conduit 40 (and boundary walls of other cooling conduit designs and arrangements to be described below) may be made of either conductive material or non-conductive material in the designer's preference. Non-conducting walls for the cooling conduits would have the advantage of allowing conductive coolant fluids to be used without disturbing the internal electrical balance within the fuel cell, since the conductive coolant fluids would be electrically isolated from the electrochemically active portions of the fuel cell by the non-conductive walls of the cooling conduit. On the other hand, if the walls 41 are made of conductive material, then the conduit walls themselves may, depending upon the configuration chosen, serve a useful function in current collection and transmission. The specific configuration of Figure 5 does not lend itself to such electrically conductive function of the conduit walls 41, but other configurations can easily be imagined in which the coolant conduit is shaped, dimensioned and located in such a position that electrical conductivity of the walls might be advantageous for collection and transmission of fuel cell current. Whether or not the walls 41 or other coolant conduit walls to be described below are selected to be conductive or non-conductive, it is of course important that the coolant within the coolant conduits 40 be isolated physically from the reactant gases and MEA layers; it is important to prevent migration of the coolant materials into the electrolytic membrane layer.
The shape and dimensions of the cooling conduit, and its orientation within flow channel 28, are in the discretion of the designer. Figure 6 illustrates an alternative exemplary design that may, if the designer wishes, be substituted for that illustrated in Figure 5. The cooling conduit walls 45 are triangular in cross-sectional configuration; the inverted base of the triangle is dimensioned to permit the base to fit snugly within the reactant gas flow channel 28. The triangular wall configuration 45 perforce circumscribes a triangular interior conduit space 43. Note that the Figure 6 embodiment tends to promote stability of the cooling conduit 43 within the flow channel 28 while presenting minimum interference with reactant gas flow within flow channel 28; further, the Figure 6 embodiment would be expected to be relatively economical to manufacture.
Figure 7 illustrates a waved, or undulate, MEA layer 70 sandwiched between separator layers 72 that make electrical connection with the apices 74 of the MEA layer 70. The separator layers 72 together with the sandwiched undulate MEA layer 70 form flow channels 78 for the reactant gases. Such an arrangement is described in British patent application [B&M 60222] (McLean) filed on [concurrently herewith] . The uppermost "troughs" of the undulate MEA layer 70 constitute fuel flow channels 76 and the lower complementary channels 78 serve to carry the oxidant gas, or vice versa - the selection is arbitrary, as is the orientation shown in the illustration. Again, because the reactant flow channels 76, 78 operate with maximum electrochemical efficiency immediately adjacent the MEA layer 70, coolant flow conduits 79 may be provided immediately adjacent the separator layers 72 and spaced away from the MEA layer 70 so as to leave ample flow within channels 76, 78 sufficient to feed the exposed surfaces of MEA layer 70. In Figure 7, such coolant conduits 79 are shown both in the oxidant flow channels 78 and in the fuel flow channels 76. Depending upon the cooling requirements of the fuel cell, the dimensions of the structures, the materials used, and other parameters, it may not be necessary to provide such coolant conduits in all of the flow path channels. Again, each of the coolant conduits 79 would be connected between inlet and outlet coolant plenum chambers (not shown) to complete the coolant cycling system.
Figure 8 illustrates a tubular construction for a fuel cell stack of the type described in applicant's British patent application Serial No. [B&M 60222] (McLean) filed on [concurrently herewith] . In this arrangement, transversely consecutive semi-cylindrical separator elements 80 combine to form, for each layer, a separator continuum 82. Oppositely disposed the separator layers 82 are transversely consecutive semi- cylindrical porous electrode elements forming a mating porous electrode layer 84. Sandwiched between each pair of opposed electrode layers 84 is an MEA layer 86. Located between opposed separator layers 82 and porous electrode layers 84 are planar separator plates 88.
Two coolant possibilities are available from the Figure 8 configuration. Depending upon fuel cell structural and operational parameters, it may be sufficient to provide cooling conduits 87 tucked in the available spaces lying between each set of three adjoining semi- cylindrical separator elements 80. In that case, planar separator plates 88 could be omitted and reactant gas can be supplied within the entire tubular space of full cylindrical conduits 83. Or, if more cooling is needed, and sufficient reactant gas is available to serve the fuel cells by supplying the reactant gas in only half of the available tubular space 83 that would be present if planar separator plates 88 are absent, then these planar separator plates 88 are provided. In such latter case, semicylindrical conduits 85 next to MEA layers 86 will carry the reactant gases while semicylindrical conduits 89 next to separator layers 82 will carry the coolant. One could alternatively design undulate plates (not shown) in substitution for planar separator plates 88 that would augment the volume of the tubular space 83 available for reactant gas and reduce the volume available for coolant flow.
In a sense, the separator plates 88 serve as coolant plates, but they are not merely coolant plates. All layers within a fuel cell stack that are capable of heat transfer, function to at least some limited extent as coolant layers or plates. By stating in this specification that the need for special coolant plates is eliminated by the present invention, we do not wish to imply that plates and layers within the fuel cell stack are not capable of some degree of heat conductivity and transfer.
In Figure 7, separate coolant conduits 79 are illustrated that are inserted in the flow path arrangement, and attached to the adjacent separator layer 72. It is possible, however, to configure separator plates to provide coolant channels without requiring a separate manufacturing operation to form and attach the coolant channels. Figure 9 illustrates this possibility in the case of a fuel cell stack 93 a portion of which is schematically illustrated, each stratum 91 in the stack 93 including an undulate MEA layer 70, essentially similar to the layer 70 illustrated in Figure 7. In the case of Figure 9, however, the separator layers 90 are not formed as planar layers, but rather as having triangular troughs 92 that periodically interrupt the planar portions 94 of the separator layers 90. Sequential separator layers 90 in the stack are transversely offset from one another so that the triangular trough 92 of any given separator layer 90 lies immediately adjacent a planar portion 94 of the next adjacent separator layer 90. This structural arrangement is more obviously apparent from an inspection of the schematic exploded view of Figure 10. When consecutive separator layers 90 come into contact with one another, the triangular trough portions 92 combine with facing and mating planar portions 94 to form conduits 96 of triangular cross- section that are suitable for coolant flow. Note that it is not essential that there be a sealing contact between consecutive separator layers 90; no harm is done if there are small leaks between neighbouring triangular conduits 96. Note also that triangular configuration of the coolant conduit cross-sections is not essential; trapezoidal or continuously curved shapes would work; triangular configuration however is simple and likely to be relatively inexpensive to manufacture. In the choice of cross-section design, the designer will take into account not only manufacturing cost but other pertinent factors such as heat transfer requirements, flow characteristics (especially the desideratum of inducing turbulent flow within the cooling conduits) , structural strength and integrity, etc. The configuration of Figure 9 accordingly provides a relatively inexpensive solution to provision of coolant conduits within a fuel cell stack. The coolant conduits 96 are effective and cheaply manufactured, yet the Figure 9 design avoids the creation of any undue impediment to gas flow within reactant flow channels 98 formed between the MEA layers 70 and adjacent separator layers 90.
It is possible to increase the power density of the fuel cell stack relative to the power density available from the Figure 9 arrangement by reducing the cross- sectional area of the coolant conduits 96 thereby permitting a closer stacking of the MEA layers 70 and separator layers 90. Such higher power-density arrangement is illustrated in Figure 11. In this case, the separator layers 110 are provided with wider and deeper triangular troughs 112 and narrower planar portions 114 than are respectively found in the counterpart separator layers 90 of Figure 9. Accordingly, when consecutive separator layers 110 are offset transversely from one another and placed in contact with one another, the change in relative dimensions of the troughs 112 and planar portions 114 results in the creation of much smaller coolant conduits 116 than coolant conduits 96 that result from the Figure 9 construction. Note also that there is a narrowing of the available space for reactant gas to reach those portions of MEA layer 70 that lie closest to the separator layers 110 as compared with the counterpart configuration of Figure 9. For this reason, the Figure 11 arrangement would probably operate more successfully relative to the Figure 9 arrangement if the reactant gases were under a somewhat higher pressure, so that reactant gases would penetrate these narrow spaces at the limits of the reactant gas conduits 118 illustrated in Figure 11. Again, for an aid to understanding, Figure 12 is an exploded schematic view of what is seen in Figure 11.
It will be noted from an inspection of the illustrations that for each embodiment, each coolant conduit is located in contact with (or formed by) the separator layers so as to lie in a region that is most remote from the active surfaces of the nearby MEA layer. The reason for this can be better understood by referring to Figures 13 and 14. Referring first to Figure 13, a conventional reactant gas distribution plate 27 is shown in contact with an adjacent MEA layer 22 (compare Figure 2) . Within the reactant flow channel 28, that region 131 of the flow ~ channel 28 carries reactant gas that is able effectively to make contact with, and to penetrate, the exposed surface 133 of the MEA layer 22. By contrast, that region 135 remote from the exposed surface 133 of the MEA layer 22 is relatively ineffective to promote the electrochemical reaction; reactant gas in region 135 has difficulty in reaching the MEA layer 22. Accordingly, if a portion of the reactant flow channel 28 were to be reallocated to the space required for a coolant conduit, it would follow that such conduit should be located within the relatively inefficient region 135. As a practical matter, a coolant conduit having a configuration resembling that of the region 135 would be expensive to manufacture. Further, it is desired to encourage the penetration of reactant gas into the MEA layer at the limits of the exposed portion 133 of the MEA layer 22. Consequently, it is not desired to locate a coolant conduit in that part of the reactant flow path 28 at the limits of the exposed portion 133 of the MEA layer 22.
While mention has been made above of the possibility of forming cooling conduits within lands of separator plates instead of within or adjacent reactant gas flow channels, it is to be noted that for some of the design variants discussed in this specification, the separator " plates are constructed without discrete broad lands that would be suitable for such purpose. Such design variants tend inherently to be more space-efficient than the conventional designs that include separator plates having broad lands. In such efficient design variants, the only available space for suitable location of coolant channels may be within or next to the flow channels.
Accordingly, referring now to Figure 14, it is logical to excise from flow channel 75, shown bounded by separator layer 72 and MEA layer 70 in that illustration, the inefficient portion of the flow channel 75 and to make the inefficient portion available to accommodate a coolant conduit 79 (compare Figure 7) . By installing a coolant conduit 79 in an inefficient portion of the previously available flow channel space 75, the residual space 76 left for reactant flow gas is considerably diminished, yet, because the region 76 is the region that is immediately adjacent the MEA surface 70, the overall efficiency of the fuel cell can be maintained (with such adjustments to pressures and flow rates as are needed to maintain operating efficiency of each fuel cell) . The cooling conduit 79 is placed adjacent and preferably fixed to the separator plate 72 so that it remains well out of contact with the MEA layer 70.
While Figure 13 has taken, by way of example, a conventional reactant flow distribution plate of the sort illustrated in Figure 2, while Figure 14 has taken, by way of example, an undulate MEA layer arrangement such as that illustrated in Figure 7, the principles derived from a review of Figures 13 and 14 can be applied to most, if not all, other reactant channel configurations.
Some trade-offs may have to be made. For example, let us suppose that despite adjustment of other parameters, the inclusion of a coolant channel 79 within what would otherwise be available reactant gas flow path space 75 in Figure 14 diminishes the overall performance of the fuel cell to some extent. Notwithstanding this hypothetical diminution in operational performance, we have, by reason by the present invention, eliminated cooling plates 14 (Figure 1) from the fuel cell stack. The result is expected to be an overall improvement in power density for the fuel cell stack, even if, contrary to expectations, there is some diminution in individual fuel cell performance that is more than trivial. Note in particular that with optimized geometry such as that illustrated in Figure 11, overall fuel cell stack power density can be appreciably raised relative to the Figure 1 design.
Some empirical testing and selection of preferred positioning of coolant conduits within individual fuel cell layers may well be appropriate, because a number of factors have to be taken into account in deciding how to shape and size such coolant conduits. Persons skilled in the technology know that it is desirable, within a reactant gas flow channel, to have a pressure drop from the inlet to the outlet side of the channel, and to have a channel shape that facilitates turbulence within the channel, thereby improving the ability of the gas to reach the exposed surface of the MEA layer and to penetrate same. The fuel cell reaction product
(water) must also be satisfactorily removed, and the flow channel design must meet this objective. The present description does not take into account such empirical approaches that may be desirable to optimize reaction product removal, pressure drop and turbulent flow in the design of the residual flow channel configuration for any given fuel cell stack type. Manufacturing cost will also have a significant role to play; that parameter, together with the power density parameter previously discussed, may favour such designs as that of Figure 11. However, individual designers will have their own respective preferences and priorities, and will be able to choose appropriate designs while relying upon the general principles apparent from the foregoing description.
For simplicity, the accompanying drawings are schematic; engineering details of any given design implementation of the invention may vary according to designers' preferences that meet objectives that may or may not directly relate to cooling. For example, the designer may wish to increase reactant gas turbulence to improve transfer of reactant gas to the electrochemically active portions of the fuel cell. To this end, the designer may choose rough or irregular surfaces for the walls enclosing reactant gas flowpaths. It may be convenient for the designer to select rough or irregular coolant conduit external walls (that will be in contact with reactant gases) to help achieve such objective. Further, the cross- sectional shape and dimensions of the coolant channel can be selected not only with cooling in mind but also for optimization of the reactant gas flowpath cross- section. An integrated design approach should facilitate the achievement of such technical objectives while permitting the designer to aim for manufacturing ~ simplicity and relatively low cost of manufacture.
While particular elements, embodiments and applications of the present invention have been shown and described, it will be understood, of course, that the invention is not limited thereto, since modifications may be made by those skilled in the applicable technologies, particularly in light of the foregoing description. The appended claims include within their ambit such modifications and variants of the exemplary embodiments of the invention described herein as would be apparent to those skilled in the applicable technologies.

Claims

1. A PEM-type fuel cell for incorporation into a fuel cell stack, having an MEA layer and reactant gas flow channels forming at least one flowpath on either side of the MEA layer and suitable means for segregating supplied reactant gases from one another, characterized in that a coolant conduit is provided for flow of cooling fluid so as to remove heat from the fuel cell, the cooling conduit being located in a relatively space-inefficient portion of the fuel cell, and the cooling conduit lying generally parallel to at least one said reactant gas flow channel in such fuel cell.
2. A fuel cell as defined in claim 1, wherein the flow channels extend substantially parallel to one another, and those lying on either side of the MEA layer are coupled to selected ones of one another in the vicinity of their respective ends to form at least one said reactant gas flowpath.
3. A fuel cell is defined in claim 1 or 2, wherein the cooling conduit follows a selected one of the reactant gas flowpaths .
3. A fuel cell is defined in claim 3, wherein the coolant conduit is located within what would otherwise be a relatively inefficient space of the associated reactant gas flow path.
4. A fuel cell as defined in claim 3 or 4, wherein a plurality of said coolant conduits are provided to follow substantially the entirety of the flowpaths on the surface of the side of the MEA layer on which said selected one of the reactant gas flowpaths lies, thereby to provide substantially uniform cooling over said last-mentioned surface of the fuel cell.
4. A fuel cell according to claim 3 or 4, wherein at least two said coolant conduits are provided within the fuel cell, one following one of the two reactant gas flowpaths, and the other following the other of the two reactant gas flowpaths.
6. A fuel cell as defined in claim 6, wherein a plurality of said coolant conduits are provided to follow substantially the entirety of the flowpaths, thereby to provide substantially uniform cooling over the surfaces of the fuel cell.
7. A fuel cell is defined in any of claims 3 to
7, wherein the boundary walls of each said coolant conduit are dimensioned, shaped and positioned to avoid close proximity to the neighbouring MEA layer.
8. A fuel cell is defined in any of claims 3 to
8. wherein each said coolant conduit is formed between contacting surfaces of separator layers of consecutive fuel cells in the fuel cell stack.
9. A fuel cell is defined in any of claims 3 to 8, wherein the fuel cell is bounded by separator layers that mate with separator layers of adjacent consecutive fuel cells in the stack to form the coolant conduits.
10. A fuel cell as defined in claim 10, wherein the outer surfaces of said separator layers are formed as a regular alternating sequence of surface portions parallel to the MEA layer and surface portions oblique to the MEA layer, the surface pattern of one of said surfaces being offset from the surface pattern of the other of said surfaces, such that adjacent mating offset surfaces of consecutive fuel cell separator layers form the coolant conduits.
11. A fuel cell as defined in claim 11, wherein the coolant conduits are of triangular cross-section.
12. A fuel cell stack having fuel cells in which a reactant gas flow field is provided between an MEA layer and a separator layer, the flow field comprising one or more reactant gas flow channels, characterized in that coolant conduits interspersed throughout the stack are provided to lie next to at least some of the separator layers and to extend adjacent and parallel to neighbouring flow channels, whereby cooling of the stack is obtained by flow of coolant through the coolant conduits without benefit of special-purpose cooling plates.
13. A fuel cell stack as defined in claim 13, wherein the coolant conduits are fixed to the separator layers .
14. A fuel cell stack as defined in claim 13, wherein the coolant conduits are formed in the interface between adjacent contacting separator layers of consecutive fuel cells in the stack.
15. A fuel cell stack as defined in any of claims 13 to 15, wherein each coolant conduit is positioned in a space that, were it instead part of a reactant flow channel, would comprise a relatively inefficient portion of the flow channel lying relatively remote from the neighbouring MEA layer.
PCT/GB1999/002074 1998-07-01 1999-07-01 Internal cooling arrangement for fuel cell stack WO2000002271A2 (en)

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GBGB9814120.3A GB9814120D0 (en) 1998-07-01 1998-07-01 Cooling of fuel cell stacks
GB9814120.3 1998-07-01

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GB9814120D0 (en) 1998-08-26
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WO2000002267A3 (en) 2000-09-14
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GB2339067A (en) 2000-01-12
AU4525399A (en) 2000-01-24

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