Abstract:
A method and device for operating a fuel cell system. The device includes a flowfield plate that includes a header section and a channel section. The header section includes inlet flowpaths and outlet flowpaths, where the inlets formed in the header section are fluidly decoupled from one another, as are the outlets. The channel section is divided into multiple circuits, each dedicated to a corresponding inlet and outlet. The circuits may be of different flow capacities, and may be operated independently of one another, making the device particularly adapted to both full and part-power operation.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     The present invention relates generally to operating a fuel cell system, and more particularly to staging the flow of fuel cell reactants during various operating power levels.  
         [0002]     In a typical fuel cell configuration, an electrolyte is sandwiched between electrodes (specifically, an anode and a cathode) such that positive ions generated at the anode flow through the electrolyte and react with ions generated at the cathode, while current generated by the flow of free electrons produced at the anode during the oxidation of the anode reactant (for example, hydrogen) and consumed at the cathode during the reduction of the cathode reactant (for example, oxygen) can be used to power one or more external devices. In one type of fuel cell, called a proton exchange membrane (PEM) fuel cell, the electrolyte makes up part of a core called a membrane electrode assembly (MEA). The MEA includes a hydrated ion exchange membrane and two gas diffusion layers disposed between electrically-conducting cathode and anode flowfield plates, where one of the diffusion layers is between the first catalyst and the anode flowfield plate and the other is between the second catalyst and the cathode flowfield plate. Continuous channels in the plates allow for the reactants to pass over the corresponding diffusion layer, catalyst and electrode, where an electrochemically active catalyst (such as platinum) is typically disposed between each face of the membrane and the respective diffusion layers. Cooling channels may also be employed to keep temperature buildup to a minimum; these cooling channels may be formed in the anode or cathode flowfield plates (for example, on the opposite side of the plate from the respective anode or cathode circuit), or may define their own flowfield plate.  
         [0003]     By stacking individual fuel cells relative to one another, more powerful systems can be built, where each individual fuel cell is electrically connected (typically in series to increase overall stack voltage). In a stack configuration, both faces of the flowfield plates can be used (in what is termed a bipolar plate), where one side of a plate promotes the anode reaction, while the opposite side of the plate promotes the cathode reaction. An example of a single plate being used as both an anode and cathode collector can be found in U.S. Pat. No. 6,503,653, owned by the assignee of the present invention and herein incorporated by reference. In such a fuel cell stack, common fluid delivery conduit can be used to efficiently bring the reactants (as well as coolant or hydrating fluid) to each plate via header (also referred to as a manifold) that is formed by the aligned stacking of apertures within each plate.  
         [0004]     Reactants and reaction products are transported to and from the fuel cell membrane assembly through the channels formed in the flowfield plates. These passages typically extend from an inlet header to an outlet header through a path configured to maximize contact area and consequent reaction within each cell. The channels are often overdesigned, in that they are sized to provide adequate flow capability for the system&#39;s full-power setting. Thus, even in low-power situations, the fluid continues to traverse all of the channels, causing a reduction in fluid velocity and a concomitant reduction in the motive force used to remove excess water that builds up at the cathode as a result of the fuel-oxygen reaction. In these conditions of inadequate fluid throughflow, which is particularly acute at low power settings, the water can become trapped within the channel and diffusion layer, impeding subsequent flow and causing portions of the cell to stop producing electricity, resulting in unstable system performance.  
         [0005]     Prior approaches to stabilizing the fuel cell at low power settings have focused on utilizing multiple stacks with extensive valving and control systems. Such approaches add complexity, weight, volume and cost to the system. Accordingly, there exists a need for an improved fuel cell system that can be operated at various power levels without reductions in system performance or durability or increases in system complexity. There also exists a need for improvements in fuel cell systems operating at various power levels such that the dynamic range of stable operation is enhanced.  
       BRIEF SUMMARY OF THE INVENTION  
       [0006]     These needs are met by the present invention, wherein a fuel cell system and a method of operating the system in such a way as to avoid the operational instabilities of the prior art is disclosed. In accordance with a first aspect of the present invention, a device including a flowfield plate is disclosed. The flowfield plate includes a header section and a channel section. The header section includes an inlet region and an outlet region, where within each region, a plurality of flowpaths are defined that are fluidly decoupled from one another. The channel section has numerous circuits, each configured to convey a fluid across a portion of the flowfield plate&#39;s surface. In the present context, the term “fluid” includes compounds, mixtures, elements or the like in either their gaseous or liquid state. By way of example, the inlet region is divided such that a first inlet flowpath is exclusively fluidly coupled to a first circuit, while a second inlet flowpath is exclusively fluidly coupled to a second circuit, and a third inlet flowpath is exclusively fluidly coupled to a third circuit. The outlet region can be similarly divided. In the present context, a channel is an individual, dedicated path that operates to convey the fluid (such as a reactant used in a fuel cell) between the inlet and outlet flowpaths in a substantially autonomous manner, such that (absent a fluid interconnection between them) there is little or no cross-talk among the various channels during the period the fluid is traversing the path of each channel. In the present context, the term “substantially” is utilized to represent the inherent degree of uncertainty that may be attributed to any quantitative comparison, value, measurement, or other representation. As such, it refers to an arrangement of elements or features that, while in theory would be expected to exhibit exact correspondence or behavior, may in practice embody something slightly less than exact. The term also represents the degree by which a quantitative representation may vary from a stated reference without resulting in a change in the basic function of the subject matter at issue.  
         [0007]     Optionally, the numerous circuits are of different flow capacity such that depending on the power requirements of the fuel cell, one or more of the circuits can be used simultaneously. In a preferred embodiment, the channel section has at least three circuits. In such configuration, a first of the circuits has a flow capacity of up to approximately sixty percent of the flow capacity of the channel section, while second and third the circuits has a flow capacity of up to approximately twenty five and fifteen percent, respectively. In a more particular form, smaller secondary circuits have fewer channels than a primary, or main, circuit. This helps make the smaller circuits more responsive to operational transients and part-power (i.e., low power) operating conditions. The channels making up the circuits may define a change of direction angle that is small relative to traditional serpentine layouts with angular and hairpin (i.e., U-shaped) bends that involve a reversal of flow direction, thereby reducing the impact of pressure drops within the channels. For example, bend angles in individual flow channels in excess of ninety degrees will out of necessity involve at least some component of flow reversal, where U-shaped bends amount to a one hundred and eighty degree change in direction, amounting to a complete reversal of flow. In other configurations, the channels may exhibit significant (or even complete) flow reversal. The device may further include a coupling section to fluidly connect the header and channel sections. The coupling section may be made up of numerous manifolds, each capable of feeding fluid to and receiving fluid from numerous flow channels. Each of the flowpaths is made up of an aperture formed into the header section; this allows each aperture to be in fluid communication with the coupling section. The flowfield plate can be used to transport reactant fluid, cooling fluid or both for a fuel cell. Accordingly, in one form, the flowfield plate can be a cathode flowfield plate. In one embodiment, the coolant flowpaths are fluidly decoupled from the cathode flowpaths to provide for some measure of cooling, while in another embodiment they could be directly coupled to the flowpath to provide additional cooling in the active areas. Multiple circuits can be utilized on one or all of the anode, cathode and cooling layers, and in various combinations to achieve better environmental conditions (including temperature, humidity and reactant concentrations) for the reactions. These can motivated by both performance and durability considerations.  
         [0008]     In another option, an MEA can be disposed between anode and cathode flowfield plates. The MEA is made up of an ion exchange membrane with an anode side and a cathode side, a diffusion layer in fluid communication with each membrane side, and a catalyst cooperative with the diffusion layer and the ion exchange membrane. In this way, when reactants (such as hydrogen and oxygen) are introduced to the respective anode and cathode sides of the ion exchange membrane, an electrochemical reaction occurs, producing electricity. Additionally, the device may include one or more valves placed in fluid communication with one or both of the cathode flowfield header sections. The valve can be used to preferentially route at least a portion of the second reactant to at least one of the first and second circuits. A controller can be incorporated to regulate the opening and closing of the one or more valves, while a sensor can also be included to detect conditions upon which the controller will open or close the valve. A control mechanism can be configured to affect distribution of fluid between the various circuits as a function of a power demand. For example, when the power level is low, the reactant or coolant may be routed through one or more of the smaller circuits, while when the power level is high, the larger circuit can be used, possibly in conjunction with one or more of the smaller circuits.  
         [0009]     The device may further include a power conversion mechanism configured to take the electricity generated by the electrochemical reaction and convert it to motive power, where the motive power can be used to propel a vehicular platform. The vehicular platform (an example of which can be a car, truck, motorcycle, rail craft, aircraft or watercraft) may house the power-production device and the power conversion mechanism. In one form, the vehicular platform may include a passenger compartment, wheels, a directional control mechanism (such as a steering wheel) cooperative with the wheels, and a braking mechanism configured to retard the effects of the propulsive force on the platform.  
         [0010]     According to yet another aspect of the invention, a fuel cell stack includes a plurality of fuel cells. Each of the cells in the stack includes an anode flowfield plate, a cathode flowfield plate and an MEA, the last made up of an anode, cathode and membrane. The plurality of flowpaths include a first flowpath and a second flowpath fluidly decoupled from one another, and a control mechanism to affect distribution of fluid between the first and second flowpaths as a function of a power demand on the fuel cell. Thus, during a first power demand level placed on the fuel cell stack, the first flowpath conveys a portion of the second reactant, while during a second power demand level placed on the fuel cell stack, at least the second flowpath conveys a portion of the second reactant. In this mode of operation, the second power demand level is considered greater than the first power demand level. Thus, during a low power setting, the smaller first flowpath can be used to transport reactant, and may (although not necessarily) remain on during higher power settings. By contrast, the second larger flowpath does not permit flow of the reactant until a minimum need threshold (determined by, for example, a fuel cell power setting) is met. In addition, the control mechanism can be used to control the actuation of one or more valves, where a control signal can come from one or more sensors in signal communication with the controller, or from manual input.  
         [0011]     In one optional embodiment, the cathode flowfield plate includes three circuits, while in another, the anode flowfield plate includes at least two circuits. An additional option includes a plurality of cooling channels placed in a heat exchange relationship with at least one of the cathode or anode flowfield plates. These cooling channels can be formed on a separate plate, or may be formed on one side of the cathode or anode flowfield plate. In another embodiment, the cooling channels may be interspersed on the same side of the same plate as the anode or cathode channels.  
         [0012]     According to still another aspect of the invention, a method of operating a fuel cell system is disclosed. The method includes introducing a first reactant and second reactant to the fuel cell, and distributing at least one of the reactants within a flowfield plate as a function of fuel cell power. As previously described, the system may include an anode flowfield plate, a cathode flowfield plate and an MEA. Optionally, the method includes preferentially routing most or all of an oxygen-bearing reactant to one or more of the smaller circuits at least during part-power operating conditions of the system, for example, when the fuel cell power falls below a predetermined threshold. In the present context, part-power operating conditions are those associated with low power operation, where the fuel cell power output is below (preferably well below) its rated capacity. The system may also include a control mechanism to affect distribution of fluid between the various channels as a function of fuel cell power, and may include one or more valves, as well as be responsive to signals from one or more sensors. This way, upon receipt of a signal from the sensor, the controller can manipulate the valve to achieve fluid routing commensurate with the fuel cell operating conditions. As before, the smaller circuit (or circuits) is characterized by a lower flow capacity than the primary (main) circuit. In one form, the channel section comprises three circuits, the first with about sixty percent of the overall channel flow capacity, the second with about twenty five percent and the third about fifteen percent. It will of course be appreciated by those skilled in the art that other ratios between the circuits may be employed, and that such ratios are within the scope of the present invention. 
     
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS  
       [0013]     The following detailed description of the present invention can be best understood when read in conjunction with the following drawings, where like structure is indicated with like reference numerals and in which:  
         [0014]      FIG. 1  shows a block diagram of a fuel cell system configured for vehicular application;  
         [0015]      FIG. 2  shows a cutaway view of a PEM fuel cell;  
         [0016]      FIG. 3A  shows an obverse side of a cathode flowfield plate incorporating multiple circuits therein, as well as a divided header section for separating fluid flow into and out of those circuits;  
         [0017]      FIG. 3B  shows a reverse side of the flowfield plate of  FIG. 3A , highlighting cooling channels;  
         [0018]      FIG. 3C  shows a detail of one of the individual flow channels of the flowfield plate of  FIG. 3B , highlighting the bifurcation;  
         [0019]      FIG. 4  shows a fuel cell stack with conduit for reactant delivery to and removal from the stack;  
         [0020]      FIG. 5  shows an exploded view of a fuel cell stack with conduit for reactant delivery to and removal from the stack;  
         [0021]      FIG. 6  shows a vehicle employing the fuel cell system of the present invention; and  
         [0022]      FIG. 7  shows an obverse side of an alternate embodiment of the cathode flowfield plate. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0023]     Referring initially to  FIG. 1 , a block diagram highlights the major components of one configuration of a mobile fuel cell system  1 . The system includes a fuel delivery system  100  (made up of fuel source  100 A and oxygen source  100 B), fuel processing system  200 , fuel cell  300 , one or more energy storage devices  400 , a drivetrain  500  and one or more motive devices  600 , shown notionally as a wheel. While the present system  1  is shown for mobile (such as vehicular) applications, it will be appreciated by those skilled in the art that the use of the fuel cell  300  and its ancillary equipment is equally applicable to stationary applications, such as for electric power generators. It will also be appreciated by those skilled in the art that other fuel delivery and fuel processing systems are available. For example, there could be, in addition to a fuel source  100 A and oxygen source  100 B, a water source (not shown). Likewise, in some variants where substantially purified fuel is already available, the fuel processing system  200  may not be required. The energy storage devices  400  can be in the form of one or more batteries, capacitors, electricity converters, or even a motor to convert the electric current coming from the fuel cell  300  into mechanical power such as rotating shaft power that can be used to operate drivetrain  500  and one or more motive devices  600 . The fuel processing system  200  may be incorporated to convert a raw fuel, such as methanol into hydrogen or hydrogen-rich fuel for use in fuel cell  300 ; otherwise, in configurations where the fuel source  100 A is already supplying substantially pure hydrogen, the fuel processing system  200  may not be required. Although only a single fuel cell  300  is shown, it will be appreciated by those skilled in the art that fuel cell system  1  (especially those for vehicular and related applications) may be made from a stack of such cells serially connected. Thus, while the term “fuel cell” is generally indicative of a single fuel cell within a larger stack of such cells, it may also be used to define the entire stack. Such usage will be clear, based on the context.  
         [0024]     Referring next to  FIG. 2 , one cell  300  of a fuel cell stack includes an anode flowfield plate  310 , cathode flowfield plate  330 , and MEA  320  disposed between anode flowfield plate  310  and cathode flowfield plate  330 . Channels  311  carry fluid (such a first reactant, typically a fuel such as gaseous hydrogen) to enable the fluid to contact the respective diffusion layer  326 A and catalyst  324 A (the latter typically in the form of finely-divided particles of a noble metal, such as platinum) of the anode side of the MEA  320 . Similarly, channels  331  carry fluid (such as a second reactant, typically an oxidant such as gaseous oxygen) to enable the fluid to contact the respective diffusion layer  326 C and catalyst  324 C of the cathode side of the MEA  320 . The ion exchange membrane  322  is placed between each of the anode flowfield plate  310  and cathode flowfield plate  330  to allow the ionized fuel produced at the anode side of the MEA  320  to flow through the membrane while inhibiting the passage of electrical current, which instead is routed through the conductive anode and cathode flowfield plates  310 ,  330  to a load (not shown) such that a motor or related current-responsive device may be operated. Upon introduction of fuel into the anode and oxidant into the cathode and subsequent reaction with the MEA  320 , electricity is generated, producing heat, water and water vapor. This water formation is especially prevalent on the cathode flowfield plate  330  of the fuel cell  300 , as ionized hydrogen can combine with ionized oxygen to form water droplets.  
         [0025]     Referring next to FIGS.  3 -A through  3 C, obverse and reverse sides respectively are shown for a cathode flowfield plate  330  according to an embodiment of the present invention. Plate  330  includes a channel section  331  and a header section  332  disposed around the periphery of the channel section  331 . Within the header section  332 , numerous passages define flowpaths for anode and cathode reactants, as well as coolant. These flowpaths make up an inlet header  332 A and an outlet header  332 B. It will be appreciated that within the confines of the cathode flowfield plate  330 , although both headers  332 A and  332 B accommodate both inlet and outlet flows, the designation “inlet” and “outlet” is purely arbitrary, pertaining to the direction of flow of the oxygen-bearing fluid. A similar convention could be adopted to refer to any headers formed on the anode flowfield plate. The inlet header  332 A includes inlet flowpaths  333 A,  333 B and  333 C for the flow of reactant, flowpaths  343 A and  343 B for the flow of coolant and flowpaths  313 A and  313 B for the flow of anode reactant (the latter of which will be described in more detail below). The inlet header  332 A is divided such that a septum S (only one of which is labeled) keeps fluid flowing through the various flowpaths separate from each other.  
         [0026]     Referring with particularity to  FIG. 3A , channel section  331  is divided up into a first, larger circuit  331 A and two smaller circuits  331 B,  331 C. These circuits are aligned with and connect the inlet flowpaths  333 A,  333 B and  333 C of the inlet header  332 A to their respective outlet flowpaths  334 A,  334 B and  334 C of outlet header  332 B through numerous individual flow channels  337 . The presence of the divided flowpath ensures that fluid flow through the stacked header section  332  remains dedicated to the corresponding circuits within channel section  331 . Each of circuits  331 A,  331 B and  331 C include groupings of the individual flow channels  337 , where connection of a grouping is effected through a manifold  335 . One or more manifolds are in turn grouped into dedicated flow with one of the circuits  331 A,  331 B and  331 C. First circuit  331 A defines the majority of the channel flow capacity (about sixty percent, as shown). Second and third circuits  331 B,  331 C connect the smaller inlet flowpaths  333 B,  333 C to the respective smaller outlet flowpaths  334 B in a manner generally similar to that of the first circuit  331 A, capable of transporting approximately twenty five and fifteen percent respectively of flow capacity. Instead of having numerous hairpin (U-shaped) and related serpentine bends, the circuits employ small-angle bends, thereby minimizing pressure loss for fluid flowing through the channels while still providing a long flowpath. It will be appreciated that flowfield plate  330  can be configured to have the cathode flowfield formed on one side, while the cooling flowfield can be formed on the other, as shown with particularity in  FIGS. 3B and 3C , or the flowfields can be formed on separate plates.  
         [0027]     Referring with particularity to  FIGS. 3B and 3C , the distribution of coolant is shown. Channel section  341  is divided up into a first, larger circuit  341 A and a smaller circuit  341 B. Header section has a pair of inlet flowpaths  343 A,  343 B fluidly connected to outlet flowpaths  344 A,  344 B, employing numerous individual flow channels  347 , which can form up to a ninety degree bend therein to allow coolant coverage over the substantial entirety of the surface of channel section  341 . Each of the individual flow channels  347  is bifurcated at the first ninety degree bend, splitting into parallel branches  347 A and  347 B. This approach allows the channels to cover the same amount of channel section  341  surface area without having to resort to the U-turn serpentine of prior art designs.  
         [0028]     By maintaining separate flowpaths and corresponding circuits (discussed below), the adaptability of flowfield plate  330  to varying fluid flow levels, such fluid flow levels often commensurate with power levels generated by the fuel cell  300 , is enhanced. For example, under low power operating conditions, it may not be desirable to keep all of the flow channels  337  of channel section  331  open for fluid passage, as under such low power conditions, the spreading of the driving force and amount of flow across the substantial entirety of the channels may not provide enough motive force within each channel to remove water that forms within the channels. Such a situation could, if not resolved, lead to a gradual buildup of water droplets, which could in turn lead to unstable operation (especially at lower power settings) and possible shutdown of one or more channels, leading to a loss in power output.  
         [0029]     Referring next to  FIG. 4 , details of the anode flowfield plate  310  include many of the same features of the cathode flowfield plate  330 , including attributes of both the obverse side (for reactant) and the reverse side (for coolant). Channel section  311  is surrounded by header section  312 , which is made up of an inlet header  312 A and an outlet header  312 B. Channel section  311  is made up of circuits  311 A and  311 B, where individual flow channels  317  make up the circuits. As before, the various inlet flowpaths  313 A,  313 B,  331 A,  331 B,  331 C,  343 A and  343 B are defined by apertures formed through the surface of the header section  312  so that upon aligned stacking of numerous plates, a built-up conduit is formed through which reactant or coolant can be conveyed.  
         [0030]     Referring next to  FIG. 7 , an obverse side shown of an alternate embodiment cathode flowfield plate  1330  is shown. Whereas the embodiment depicted in  FIGS. 3A, 3B  and  4  included divided flow circuits amongst each of the anode, coolant and cathode circuits (with the last having a three-way split), the cathode flowfield plate  1330  includes a single coolant circuit with inlet flowpath  1343  and outlet flowpath  1344 , as well as single anode circuit with inlet flowpath  1313  and outlet flowpath  1314 , and a dual cathode circuit with divided flowpaths. The first of the divided cathode flowpaths extends from inlet flowpath  1333 A to outlet flowpath  1334 A, while the second extends from inlet flowpath  1333 B to outlet flowpath  1334 B. As can be seen from the figure, there are more individual flow channels  1337  making up the first of the divided cathode flowpaths than there are making up the second. In a preferred mode of operation, the smaller second cathode flowpath is used for low power conditions. For example, where a fuel cell stack  3000  is used for a vehicular or related mobile applications and the vehicle is not consuming significant amounts of power (such as at an idle condition), the flow of reactant fluid to the cathode  330  can be limited to the second circuit. Also unlike the embodiment depicted in  FIGS. 3A, 3B  and  4 , the present embodiment does include bends in the individual flow channels  1337  that can involve significant changes in flow direction. For example, in addition to the ninety degree bends  1337 A (which are similar to the bends shown in  FIGS. 3B, 3C  and  4 ), the present embodiment also includes U-shaped bends  1337 B.  
         [0031]     Referring next to  FIG. 5 , a fuel cell stack  3000  made up of numerous individual fuel cells  300  that are fed by fluid conduits  3100  is shown. Stack  3000  includes numerous plates or layers aligned together in a generally laminated fashion, terminating in end plates  350  and  360 . End plate  350  includes subcomponents, including a wet end baseplate  350 A, wet end insulator  350 B and wet end terminal  350 C, which includes a diffusion media  350 D. Seal  353  is placed between the end plate  350  and anode flowfield plate  310 . Cathode flowfield plate  330 , with inlet header  332 A and outlet header  332 B, can be placed adjacent the anode flowfield plate  310 . Although not shown, it will be appreciated by those skilled in the art that the anode and cathode flowfield plates  310 ,  330  could be formed on opposing sides of the same plate in a bipolar fashion. Electrolyte layer  320  and another seal  355  are also included.  
         [0032]     Fluid conduits  3100  are used to transport the reactants and coolant to and from the stack  3000 . A first conduit  3110  (which has one or more valves or related flow-regulating devices disposed therein) can be fluidly coupled to inlet header  332 A. This header feeds an oxygen-bearing reactant to inlet flowpaths  333 A,  333 B and  333 C and coolant to the coolant inlet flowpaths  343 A,  343 B, while receiving a hydrogen-bearing fluid leaving the anode flowpaths  313 A,  313 B. A second conduit  3130  can be fluidly coupled to outlet header  332 B. This header can be used to remove the products of the electrochemical reaction reactant from the outlet flowpaths  334 A,  334 B and  334 C, as well as remove the coolant from coolant outlet  344 A,  344 B that has passed through the stack  3000 . Similarly, second conduit  3130  can be used to supply the hydrogen-bearing reactant to inlet flowpaths  314 A and  314 B of the outlet header  332 B.  
         [0033]     Referring lastly to  FIG. 6 , a vehicle incorporating a fuel cell system according to the present invention is shown. Fuel cell  300  is fluidly coupled to a fuel supply  100 A. While the vehicle is shown notionally as a car, it will be appreciated by those skilled in the art that the use of fuel cell systems in other vehicular forms is also within the scope of the present invention.  
         [0034]     While certain representative embodiments and details have been shown for purposes of illustrating the invention, it will be apparent to those skilled in the art that various changes may be made without departing from the scope of the invention, which is defined in the appended claims.