Abstract:
A fuel cell system ( 1 ) including at least one fuel cell ( 2 ) having a cathode area ( 3 ) and an anode area ( 4 ) is disclosed. The cathode area ( 3 ) and the anode area ( 4 ) have feed conduits ( 31, 41 ) and discharge conduits ( 32, 42 ). Downstream of the anode area ( 4 ) and the cathode area ( 3 ), a junction ( 12 ) of the discharge conduits ( 32, 42 ) is provided. The junction ( 12 ) fluidically communicates with an area ( 13 ) which includes a material that is catalytically active with respect to a reaction of a fuel for the fuel cell ( 2 ) with an oxidant for the fuel cell ( 2 ). The feed conduit ( 31 ) leading to the cathode area ( 3 ) is configured in such a manner that it fluidically communicates with the cathode area ( 3 ) in at least two different sites ( 19, 20 ) in each of the fuel cells ( 2 ). A humidifying device ( 16 ) is provided in the feed conduit ( 41 ) leading to the anode area ( 4 ).

Description:
The present invention relates to a fuel cell system having at least one fuel cell, which includes a cathode region and an anode region, the cathode region and the anode region each having inflow and outflow lines. 
     BACKGROUND 
     From the U.S. Pat. No. 6,124,054, a fuel cell system is known, in which the waste gases emerging from the anode and the cathode region are made to converge and are fed to a catalytic conversion process. The underlying difficulty regarding humidification is only discussed in detail in the U.S. Pat. No. 5,503,944 A1 referenced therein. It describes carrying out the humidification process from an ultrapure-water (cooling) circuit through the cathode flow fields having a porous design. Thus, the gases are supplied in a dry process. However, it is difficult to protect such an ultrapure-water circuit from frost, so that it has considerable disadvantages with regard to its suitability for temperatures below the freezing point. Such humidification processes are certainly not conceivable in mobile applications, such as in vehicles. 
     A similar type of design is also known from the German Patent Application DE 100 24 570 A1. It provides for additionally directing the waste gases from the catalytic combustion to an expander in order to utilize the residual energy contained in the same. It also describes an anode design where the active surface area of the anode is reduced by at least one step in the direction of fuel flow, so that the fuel is utilized at a higher proportion in the fuel cell. A comparable approach providing this type of cascading anode design is also derived from the German Patent Application DE 197 21 817 A1. 
     Neither of the two last-mentioned publications allow for the underlying difficulty regarding humidification which, however, is essential for the operation of a PEM fuel cell. Namely, if a gas stream is directed to a PEM fuel cell which has not been humidified, thus which is able to absorb water from the PEM (proton-conducting membrane or polymer electrolyte membrane), it dries out the PEM. It is effectively damaged as a result. 
     For that reason, the International Patent Application WO 01/11216 A2 describes humidifying the gas streams using membrane humidifiers. However, these devices are very large and costly, in particular when air is used as an oxidant stream, due to the comparatively large volume to be humidified that also includes inert components. This constitutes a significant disadvantage for low cost, compact fuel cell systems, as are required in vehicles, for example, for purposes of energy supply and propulsion. 
     In addition, from the German Examined Application DE 100 55 253 B4, as well as the non-prepublished German Applications DE 103 46 594 and DE 10 2004 058 117 of the Applicant, supplementary cathode components are known, which make it possible to reduce the outlay required for humidifying the oxidant by providing appropriate flow guidance therefor. These supplementary components of the oxidant distributor structures, denoted as “injection flow field,” allow the oxidant to flow into the cathode in each of the fuel cells at least two different locations. This reduces the volumetric flow at each of the inflow locations, thereby minimizing the risk inherent to the particular inflow region of the oxidant, of the PEM drying out at certain spots. Finally, the components allow water retention in the region of the cathode, thereby reducing the demands placed on the humidification process. 
     SUMMARY OF THE INVENTION 
     It is, therefore, an object of the present invention to further simplify a fuel cell system having at least one fuel cell, which includes a cathode region and an anode region, in a way that will provide a more compact, simpler, and less costly design and, at the same time, a most effective possible energy balance. 
     By employing the injection flow fields described above, the fuel cell system according to the present invention succeeds in retaining a comparatively large quantity of water in the cathode region in or at the PEM. Thus, in conjunction with the fuel humidified in accordance with the present invention, the need for humidifying the oxidant, i.e., typically air, may be altogether eliminated. The fuel humidifier may be designed to be a very small unit, which is more compact and less costly than would be required for a cathode air humidifier. Moreover, a small humidifier of this kind may be more readily accommodated in a system that is packed closely together in a narrow space, as is required, in particular, in vehicles for purposes of energy supply and propulsion. 
     This effect is also enhanced by the fact that the fuel, for example, nearly pure hydrogen from a storage tank used as a hydrogen source, is not circulated through an anode circuit as is frequently the case. Instead, the unreacted residual fuel exits the anode region directly. This residual fuel is then fed, together with the residual oxidant from the cathode region, for example oxygen-enriched air, to a region having the catalytically active material. On the one hand, this prevents unreacted hydrogen from being released from the system into the ambient environment; on the other hand, the residual energy contained in the waste gas is at least thermally utilized. This significantly reduces the volumetric flow of the fuel as compared to an anode circuit circulation. This, in turn, is associated with a very advantageous reduction in the required humidifier size. 
     Thus, the fuel cell system according to the present invention may be designed to be very simple, efficient, compact and cost-optimized. 
     Another embodiment of the fuel cell system according to the present invention provides that the region having the catalytically active material be in fluid communication with a fluid flow machine. 
     Typically, a catalytic combustion of the residual fuel with the residual oxidant takes place in the region having the catalytically active material. Thermal energy, primarily in the form of hot waste gases, is produced in the process. These hot waste gases are then able to expand in the fluid flow machine. The thus derived mechanical energy may be utilized either directly, for example, to at least assist in driving a pump-type device for the oxidant and/or indirectly to generate electrical energy via a generator. 
     Another embodiment of the present invention provides that the active surface of the anode region be reduced from the inflow line in the direction of the outflow line. 
     Due to this continuous or stepped reduction or cascading of the anode surface in the direction of fuel flow, the same active surface (membrane, electrocatalyst, electrode) is always available for the (still) present fuel volume for each volume fraction in the anode region. The fuel used may nevertheless be substantially reacted when working with an anode region that is minimized with respect to size or surface area. 
     Typically, unreacted residual fuel on the order of less than 5% of the original fuel volume is realistic when working with two to four steps of the cascading configuration. When a volumetric fuel flow is optimized in this manner, a humidifier of the above described type, which is smaller by a factor of ten than an air-side humidifier, then suffices for the fuel. 
     Another advantageous embodiment of the fuel cell system according to the present invention may also provide for the humidifier to have inflow ports and outflow ports for two mass flows separated by a membrane that is essentially only permeable to water or water vapor. 
     Membranes of this kind, which are only permeable to water, typically in the form of water vapor, and which prevent the passage of hydrogen, oxygen, nitrogen, carbon dioxide, etc. therethrough, are known, for example, from the International Patent Application WO 01/11216 mentioned above. They may also be designed as flat membranes, hollow fibers or the like. 
     Besides likewise conceivable, cyclical absorption or adsorption humidifiers, in which the gas stream to be humidified and the gas stream supplying the humidity alternately traverse two spaces provided with absorbers or adsorbers, or humidifiers in which the gas flows through liquid water, the membrane humidifiers constitute a variant of humidifiers that are very compact, simple in terms of interconnection and efficient. 
     One beneficial refinement derived therefrom may provide for the humidifier to be configured in a way that allows it to be traversed, on the one hand, by the already present moist waste gas from the cathode region and, on the other hand, by the fuel to be humidified. In this context, the waste gas flows from the cathode region, then first through the humidifier, before arriving in the region having the catalytically active material. 
     Here the advantage is derived that a potential membrane leakiness to the fuel is not critical (in the case of hydrogen, low leakage rates are certainly not to be completely ruled out due to the small molecular sizes), since the fuel diffusing through to the air side is subsequently still directed to the region having the catalytically active material, and is coreacted there. Thus, the fuel is neither released into the ambient environment, nor does its energy content remain unused. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Other advantageous embodiments of the present invention are described below with reference to the drawing, whose figures show: 
         FIG. 1 : a schematic representation of a first specific embodiment of the fuel cell system according to the present invention; 
         FIG. 2 : a schematic representation of one possible embodiment of an oxidant distributor structure in the individual cells of the fuel cell; 
         FIG. 3 : a schematic representation of the anode-side interconnection of the fuel cell; and 
         FIG. 4 : a schematic representation of an alternative specific embodiment of the fuel cell system according to the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     A fuel cell system  1  is shown schematically in  FIG. 1 . It includes a fuel cell  2 , which, in this case, is to be assembled from a plurality of individual cells to form a PEM fuel cell stack. Fuel cell  2  includes a cathode region  3  and an anode region  4 , which are separated from one another by a PEM  5 . Both cathode region  3 , as well as anode region  4  have an inflow line  31 ,  41 , as well as an outflow line  32 ,  42 , respectively. 
     During operation of the fuel cell, a fuel, for example nearly pure hydrogen, is supplied via inflow line  41  to anode region  4 . In the exemplary embodiment shown here, this fuel originates from a fuel source  6 , for example a hydrogen pressure tank. The volumetric flow and the pressure of the fuel arriving in anode region  4  are adjusted via suitable valve devices  7  and pressure-reduction devices  8 . Via a pump-type device  9 , air is fed as oxidant to cathode region  3  through its inflow line  31 . This air originates from the ambient environment of fuel cell system  1  and is typically drawn in by pump-type device  9  via an air filter  10 , as well as possibly via a sonic silencer (not shown). Moreover, a heat exchanger  11  may be expediently provided as an intercooler between pump-type device  9  and cathode region  3 . 
     The air is then reacted together with the fuel in fuel cell  2  in the usual manner known per se to generate electric power. The waste gases from this reaction exit cathode region and, respectively, anode region  3 ,  4  through respective outflow lines  32 ,  42 . The waste gases are then mixed together in the region of a junction  12  and fed to a region  13  having a catalytically active material where the residual fuel reacts to completion with the residual oxidant. Hot waste gases are produced by this reaction in region  13 , referred to, in short, as catalyst in the following. Downstream of catalyst  13 , these hot waste gases are directed into a fluid flow machine  14  where they expand. The mechanical energy thus derived from the hot waste gas is supplied either to a generator  15  and/or to pump-type device  9 . In this context, it is particularly advantageous when the unit made up of fluid flow machine or expander  14 , pump-type device  9  and generator or electrical machine  15 , is designed as an electric turbocharger in such a way that, depending on how much energy expander  14  supplies, pump-type device  9  is operated either solely by the expander or with the assistance of electrical machine  15  in motor operation. When the expander supplies more energy than required by pump-type device  9 , then electrical energy may also be directly produced by electrical machine  15  in generator mode. 
     Moreover, the combination of anode region  4  traversed by the fuel, and catalyst  13  has the positive effect that no unburnt fuel is released into the ambient environment. In addition, as a result of the reaction in catalyst  13  including downstream fluid flow machine  14 , such a high waste gas temperature is able to be reached, that no liquid water escapes from the system. This is very beneficial in mobile applications, in particular, since liquid water is not released into the components of the exhaust-system branch, nor into the ambient environment. Otherwise, safety problems could arise in road traffic, since the water could freeze, for example, and/or damage the road pavement. 
     As is generally known, drying effects occur on PEM  5  during the operation of fuel cell  2 . These effects are countered very effectively in two different ways in the case of fuel cell system  1  illustrated here. On the one hand, the supplied fuel is humidified in a humidifier  16 . In this context, in the exemplary embodiment presented here, humidifier  16  is designed as a membrane humidifier. Humidifier  16  has a schematically indicated membrane  17 , as well as two inflow ports  161 ,  163  and two outflow ports  162 ,  164 . Membrane  17  is made of a material that is essentially only permeable to water or water vapor. Materials of this kind, such as hydrophilic membranes or the like, are known in principle from the International Patent Application WO 01/11216 A2 mentioned at the outset. At this point, the fuel flows from fuel source  6  through inflow port  161  into humidifier  16 . There, it flows along membrane  17  and exits humidifier  16  through outflow port  162  to then flow into anode region  4  of the fuel cell. At the same time, for example in counterflow thereto, moist waste gas flows out of the cathode region through inflow ports  163  into humidifier  16 . In the process, the moist waste gas releases water or water vapor through membrane  17  to the fuel, thereby humidifying the same before traversing outflow port  164  to reach junction  12 . 
     Contrary to the customary humidifiers used for the substantially greater inflow of air to cathode region  3 , humidifier  16  used for the fuel, as descrilbed here, may be designed to be much smaller and more compact. 
     It is especially beneficial that humidifier  16  is interconnected in a way that allows the outflowing waste gas of cathode region  3  to arrive in catalyst  13  while imparting moisture thereto. When hydrogen is used as fuel, a certain percentage of the fuel (up to 5% of the same) will, namely, always diffuse to the other side of the membrane, which essentially is only permeable to water or water vapor. Thus, at least a small quantity of hydrogen always reaches the dehumidified waste gas from cathode region  3 . However, since this is first fed to catalyst  13  downstream of humidifier  16 , this fuel may also be coreacted there, so that its energy content is utilized, thereby ensuring that no unburnt fuel is released into the ambient environment. 
     In order to entirely eliminate the need for the much more costly process of humidifying the air flowing to cathode region  3  in conjunction with the fuel humidification, the design of cathode region  3  is such the air flows as oxidant into cathode region  3  in each individual cell at a plurality of different locations. An oxidant distributor structure  18  designed in this manner, i.e., a so-called flow field, for distributing the air over the cathode-side surface of PEM  5  is shown exemplarily in  FIG. 2 . One or more gas-carrying channels  19  configured in a meander shape branch off from inflow line  31  in the usual manner. In addition, following a certain path length, each of gas-carrying channels  19  features another fluid connection  20  between gas-carrying channel  19  and inflow line  31 , through which fresh, unspent oxidant is able to arrive in gas-carrying channels  19 . Thus, the air flows as oxidant into cathode region  3  at at least two different locations in each of gas-carrying channels  19 . This reduces the volumetric flow at each of the inflow locations, thereby minimizing the risk inherent to the particular inflow region of the oxidant, of PEM  5  drying out at certain spots. Ultimately, this allows water to be retained in the region of PEM  5 . Other supplementary components of this kind are derived from the German Patent Applications DE 100 55 253 B4, DE 103 46 594 and DE 10 2004 058 117 of the Applicant, as already mentioned at the outset. 
     In conjunction with the humidification of the fuel in humidifier  16 , fuel cell  2  may be operated safely and reliably. Thus, by combining humidifier  16  for the fuel and appropriately designed oxidant distributor structure  18 , the need is eliminated for the other humidification measures commonly used in known methods heretofore. 
     The configuration whereby anode region  4  is interconnected to fuel inflow and outflow lines  41 ,  42  is much simpler, more compact and thus less costly and more efficient than, for example, a so-called anode loop, where excess fuel is supplied and is then recirculated via a return line from outflow line  42  into inflow line  41  using costly blowers. However, in the case of the “open” interconnection of anode region  4  described here, a certain quantity of unspent fuel is always discharged from fuel cell  2  into catalyst  13 . However, since the energy yield in fuel cell  2  is typically higher than in the design composed of catalyst  13  and fluid flow machine  14 , the goal must be to keep this quantity as low as possible. 
     To accomplish this and, nevertheless, ensure a good functionality of fuel cell  2 , a cascading of anode region  4  is implemented for fuel cell system  1 . A design of this kind is indicated schematically in  FIG. 3 . In the cascading arrangement, individual sections  43 ,  44 ,  45 ,  46 ,  47 ,  48  of anode region  4 , which are equal in area, are configured in such a way that the fuel, coming from inflow line  41 , first flows in parallel through a first number of sections  43 ,  44 ,  45 . It subsequently flows, likewise in parallel, through another smaller number of sections  46 ,  47 , as well as, if indicated, through still another even smaller number of sections  48 . Due to this cascading of individual sections  43 ,  44 ,  45 ,  46 ,  47 ,  48  of anode region  4  of fuel cell  2 , given an excellent performance of fuel cell  2 , it may be achieved that only a minimum of total excess fuel is needed, since the active surface of anode region  4  per volume fraction of fuel remains approximately the same over the entire anode region  4 . Thus, a sufficient amount of excess fuel is available for reaction to each of sections  43 ,  44 ,  45 ,  46 ,  47 ,  48 . Nevertheless, the total excess and thus the quantity of unreacted fuel is minimized. Based on initial calculations and trials, values on the order of less than 5% of the fuel drawn from fuel source  6  appear to be realistic for a residual quantity of unreacted fuel. 
     One alternative specific embodiment of fuel cell system  1  is shown in  FIG. 4 . It differs from the described specific embodiment merely in that the heat originating from heat exchanger  11  is likewise supplied to fluid flow machine  14 . To that end, prior to entering into catalyst  13 , the waste-gas stream is utilized to cool the compressed air in heat exchanger  11 . Thus, higher waste gas temperatures are attainable downstream of catalyst  13 , thereby permitting a higher energy yield for the fluid flow machine. Besides the design illustrated here, merely one of the waste gas streams, for example the waste gas stream having a significantly greater volume from cathode region  3 , may be utilized for cooling the compressed air. All other embodiments and advantages are analogous to those of fuel cell system  1  described in  FIG. 1 .