Abstract:
A hydrogen feed system for a fuel cell arrangement in which the hydrogen can be taken at a higher pressure from a supply tank standing under pressure or under pressure from a reforming device and can be fed after relaxation to a lower pressure into the fuel cell arrangement, with a return flow loop being provided in the hydrogen circuit, so that a part of the non-consumed hydrogen emerging from the fuel cell arrangement can be fed back into the latter, is characterized in that a pump is provided which brings about the recirculation of the hydrogen and which can be driven from the pressure energy of the hydrogen taken from the tank or coming from a reforming unit.

Description:
RELATED APPLICATION  
       [0001]    This is a continuation-in-part of U.S. patent application Ser. No. 09/997,397 filed on Nov. 29, 2001, and assigned to the assignee of the present invention. 
     
    
     
       TECHNICAL FIELD  
         [0002]    The present invention relates to a hydrogen feed system, in particular but not exclusively to a hydrogen feed system for a fuel cell arrangement in which the hydrogen can be taken at higher pressure from a supply tank standing under pressure or under pressure from a reforming device and can be fed after relaxation to a lower pressure into the fuel cell arrangement, with a return flow loop being provided in the hydrogen circuit, so that a part of the non-consumed hydrogen emerging from the fuel cell arrangement can be fed back into the latter.  
         BACKGROUND OF THE INVENTION  
         [0003]    Fuel cells are known in diverse forms. The present invention is, however, only concerned with fuel cells which operate with hydrogen as a fuel. Such fuel cells are known in the form of so-called PEM fuel cells (Proton Exchange Membrane fuel cells). A fuel cell arrangement based on PEM fuel cells can admittedly consist of a single fuel cell, but normally consists of a stack of fuel cells arranged above one another or alongside one another, which together form a so-called stack. Each fuel cell has a proton permeable membrane with electrodes on both sides, and indeed a cathode and an anode, which both have a catalytic coating. Hydrogen is supplied to the stack at the anode side at a certain overpressure, i.e. pressure above atmospheric pressure. At the cathode side air is likewise supplied to the stack with a suitable overpressure. In the operation of the fuel cell protons, which are delivered by the hydrogen, diffuse through the membrane and react at the cathode side of the membrane with the air that is supplied. In this way, water vapor is formed on the one hand which is led away as an exhaust gas at the cathode side and, on the other hand, current is produced, which can, for example, be used to drive a vehicle in which the fuel cell arrangement is incorporated.  
           [0004]    At the anode side of the fuel cell arrangement excess hydrogen, i.e. not yet consumed hydrogen, leaves the stack and is either burned in order to produce heat or is at least partly returned to the stack via a return loop, that is to say recirculated. A procedure of this kind has certain advantages. It is on the one hand more economical and, on the other hand, the return of the hydrogen ensures that adequate moisture is always present, so that the membranes remain moist. This is an important precondition for the disturbance-free operation of a fuel cell.  
           [0005]    It is thus known to return at least a part of the non-consumed hydrogen emerging from the fuel cells to the fuel cell arrangement again. In order to achieve this, a pressure increase must, however, take place, because the pressure at the inlet side of the fuel cell arrangement is higher than at the outlet side. This pressure increase, however, proves to be problematic. Since hydrogen molecules are small, the pumping of hydrogen is difficult and the danger of leakages is very great. Motor-operated pumps, in which the motor is arranged outside of the hydrogen circuit, are problematic in practice because it is extremely difficult to adequately seal the rotating drive shaft of the pump. Hydrogen leakages are, however, extremely dangerous, particularly when a vehicle is stationary, because they can lead to an ignitable gas mixture.  
           [0006]    In order to avoid the use of such pumps, it has already been proposed, in the international application with the publication no. WO 99/05741, to use so-called eductors. These consist of a nozzle with a convergent section and a divergent section and hydrogen is injected at a higher pressure into the narrow throat between the convergent section and the divergent section, whereby a suction action takes place with an increase in pressure, so that hydrogen is sucked in at a lower pressure at the convergent side and emerges with a pressure increase at the divergent side. An eductor has the advantage that it can be relatively easily sealed, since there are no moving parts. It is, however, problematic that the eductor first functions correctly with a certain throughput, so that a second eductor is necessary in order to maintain the hydrogen circulation at low flow rates. In the second eductor water is injected into the narrow throat between the convergent and the divergent section for which a water supply and a water pump are required and it is clear that the duration of the operating time is restricted because the available quantity of water is restricted. Through the different components that are required a system of this kind also proves relatively complex and leakages must also be feared here.  
           [0007]    The object of the present invention is to provide an apparatus which makes it possible to obtain the pressure increase required for a hydrogen recirculation at a relatively favorable cost without having to fear leakages.  
         SUMMARY OF THE INVENTION  
         [0008]    In order to satisfy this object, provision is made in accordance with the invention, in a hydrogen feed system of the initially named kind, for a pump to be provided which brings about the recirculation of the hydrogen and which can be driven by the pressure energy of the hydrogen taken from the tank or coming from a reforming unit.  
           [0009]    Viewed differently, the solution of the invention consists in the provision of a pump which is connected to the fuel cell arrangement in order to carry out the hydrogen recirculation; in the hydrogen coming from the supply tank or from a reforming unit being supplied to the pump to drive the pump, and in the hydrogen which is relaxed by the driving of the pump being supplied together with the recirculated hydrogen to the fuel cell arrangement.  
           [0010]    Since the recirculation of the hydrogen takes place by a pump operated by the hydrogen itself, in particular in the form of a displacement pump, this pump can be fully accommodated within the hydrogen circuit, so that the pump is fully integrated into the line system and no shaft passes through the wall of the line system, so that leakages at shaft lead-throughs cannot occur. All line connections must admittedly be sealed against hydrogen now as previously, this problem can, however, be solved substantially more simply, because no movable parts are present at the line connections in operation. The pump is, so to say, hermetically sealed off relative to the environment.  
           [0011]    Since the drive energy is obtained from the hydrogen pressure which is in any event present, through the relaxation of the hydrogen pressure, no additional energy need be supplied, so that the power yield from the fuel cells is not reduced by the energy required for the driving of the pump.  
           [0012]    Preferred embodiments of the invention can be found in the subordinate claims and also in the further description and in the accompanying drawings. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]    [0013]FIG. 1 is a schematic representation of a first embodiment of a hydrogen feed system in accordance with the invention using a piston pump;  
         [0014]    [0014]FIG. 2 is a schematic representation of a membrane pump in accordance with the invention during a first half of a pumping cycle; and  
         [0015]    [0015]FIG. 3 is a schematic representation of the membrane pump of FIG. 2 during a second half of a pumping cycle.  
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0016]    [0016]FIG. 1 shows a double-acting piston pump  10  which delivers hydrogen from a supply tank  35  or from a reforming unit  37  to the anode side of a fuel cell arrangement  11  via a schematically illustrated line  13 A and supplies a part of the excess hydrogen emerging from the fuel cells at the anode side back to the fuel cells anew via a return loop. The return loop comprises in this example schematically indicated lines  13 B and  13 C which lead from the fuel cell arrangement  11  to connections  38  and  42  at the end faces  30  and  32  of the pump  10  and lines  13 D and  13 E which lead from the connections  40  and  44  at the ends  30  and  32  of the pump  10  to the fuel cell arrangement. In a practical embodiment the lines  13 B and  13 C and the lines  13 A,  13 D and  13 E can each be realized as a line which lead via internal passages of the pump housing to the respective connections or to the corresponding openings of the pump. The pump is thus incorporated into the return loop. The return loop  13 B,  13 C,  13 D,  13 E together with the line  13 A and the flow paths within the pump  10  and at the anode side of the fuel cell arrangement  11  form the hydrogen circuit.  
         [0017]    In a manner known per se, the fuel cell arrangement  11  also has a cathode side to which air is supplied via the line  15 A. Protons, which are delivered by the hydrogen at the anode side, are transported by means of the membranes contained in the fuel cells to the cathode side and react there with oxygen from the air that is supplied to form water, which leaves the cathode side of the fuel cells via the line  15 B in the form of water vapor together with excess air or air components, such as nitrogen, which do not participate in the reaction.  
         [0018]    The action of the pump  10  will be explained in more detail in the following. It has a double piston  12  which, depending on the prevailing pressure conditions, can be moved to the right in accordance with the arrow  14  and to the left in accordance with the arrow  16  within a cylinder housing  18 . The double piston  12  has two piston heads  20  and  22  which are connected together via a hollow connection part  24 . A valve flap  26  is tiltably mounted at the cylinder housing  18  via a pivot axle  28 , so that it can be tilted between the inclined position shown in continuous lines and a likewise inclined second position  26 ′ which is schematically illustrated in broken lines. The pivot axle  28  can be so designed that it either does not pass through the cylinder housing at all or is realized by stub axles which are stationary in operation and are completely sealed off relative to the cylinder housing.  
         [0019]    The tilting or rocking motion of the valve flap  26  takes place here through a to and fro movement of the double piston  12 . When the piston is moved further to the right in the direction of the arrow  14 , the valve flap  26  is tilted by an abutment  27  present on the connecting part  24  into the broken line position  26 ′, whereas, with a subsequent movement of the double piston  12  to the left in accordance with the arrow  16 , the valve flap  26  is moved back into the position shown in continuous lines in FIG. 1 by the further abutment  29  provided on the connecting part.  
         [0020]    In the illustrated arrangement four pressure chambers I, II, III and IV are present, with the pressure chamber I being provided between the valve flap  26  and the piston head  22 , the pressure chamber II between the valve flap  26  and the piston head  20 , the pressure chamber III between the piston head  20  and the left-hand end  30  of the cylinder  18  and the pressure chamber IV between the piston head  22  and the right-hand end  32  of the cylinder  18 .  
         [0021]    Between the valve flap  26  and the connecting part and between the valve flap  26  and the cylinder housing  18  there are seals (not shown) in order to seal off the chamber I relative to the chamber II in both positions of the valve flap  26 . Between the piston head  20  and the cylinder housing  18  and between the piston head  22  and the cylinder housing  18  there are further seals (not shown) which seal off the chamber II relative to the chamber III and the chamber I relative to the chamber IV respectively.  
         [0022]    The double piston pump  10  has in this example six connections  34 ,  36 ,  38 ,  40 ,  42  and  44 . The connection  34  is connected to a supply tank  35  which contains hydrogen H 2  and delivers this at a constant drive pressure to the connection  34 . In order to achieve this constant drive pressure, a pressure regulating valve  33  is provided between the supply tank  35  and the connection  34 . In the position shown in FIG. 1, hydrogen flows at the delivery pressure P delivery  into the pressure chamber I and flows via a restrictor  46  adjacent to the piston head  22  into the hollow connecting part  24 . The hydrogen leaves the hollow connecting part  24  again via a second restrictor  48  and is then located in the pressure chamber II.  
         [0023]    The restrictors  46  and  48  need not necessarily be provided in a hollow connection part  24 , they can, for example, be provided in the valve flap  26  or in a suitable flow passage in the cylinder housing  18 . In other respects it is not essential to provide two restrictors, a single restrictor is sufficient.  
         [0024]    In the position shown in FIG. 1, hydrogen is supplied with reduced pressure in accordance with the arrow  13 A and  13 E to the fuel cells  11 . This pressure is termed here P feed . One can thus write a first equation, namely:  
           P   feed   =P   delivery   −ΔP   46,48    1)    
         [0025]    where ΔP 46,48  represents the pressure loss via the restrictors  46  and  48 .  
         [0026]    In the fuel cells a part of the H 2  supplied to the anode side migrates in the form of protons through the membrane that is present there, so that at the outlet of the fuel cells at the anode side the remainder of the supplied hydrogen is present as H 2 -containing exhaust gases (the exhaust gases also include water vapor which originates from the anode side of the membrane). These exhaust gases have a pressure which is lower than the pressure P feed  and a part of these exhaust gases is supplied with a pressure which is designated here by P return  to the connection  38  and to the connection  42 .  
         [0027]    A further part of the H 2 -containing exhaust gases is removed from the hydrogen circuit via a pressure regulating valve  39  as exhaust gases and is further used in a manner known per se, it is, for example, burned for heat recovery or for the sake of safety.  
         [0028]    It is now assumed that the double piston  12  moves in accordance with the arrow  14 , the corresponding pressure build-up in the chamber IV leads to a non-return valve  52  adjacent the connection  42  being closed and to a non-return valve  54  adjacent to the connection  44  being opened. The pressure in chamber II also increases. The exhaust gases present in the chamber IV which previously had the pressure P return  are now compressed and leave the connection  54  at an elevated pressure P feed , so that they can be supplied with the gases from the connection  36  to the fuel cells.  
         [0029]    At the end of the movement in accordance with the arrow  14 , the valve flap  26  swings by contact with the abutment  27  into the broken line position  26 ′ and the pressure relationships ensure, as will later be explained in more detail, that the double piston  12  moves to the left in accordance with the arrow  16 . During this the gases which are present in the chamber III with the initial pressure P return  are forced, as a result of the movement of the double piston, at an elevated pressure P feed  out of the non-return valve  56  arranged adjacent to the connection  40 . The gases present in the chamber I are also forced out of the chamber I by this movement of the double piston. At the same time, the non-return valve  58  which is arranged adjacent to the connection  38  closes and prevents further H 2 -containing exhaust gases from flowing into the chamber III and prevents compressed H 2 -containing exhaust gases being forced out of the connection  38  again. The exhaust gases which leave the cylinder  18  at the connection  40  are then supplied again to the fuel cells together with the H 2  coming from the connection  36 .  
         [0030]    During the movement of the double piston  12  to the left in accordance with the arrow  16  a depression arises in the chamber IV, whereby the non-return valve  54  closes and the non-return valve  52  opens in order to allow H 2 -containing exhaust gases from the fuel cells at the pressure P return  to enter into the chamber IV. When the double piston  12  has reached its extreme left-hand position, the valve flap  26  swings over again on contact with the abutment  29  and the piston now starts to move to the right again under the prevailing pressure conditions in accordance with the arrow  14 , whereby the working cycle repeats. The pressure conditions are so selected by the design of the non-return valves  52 ,  54 ,  56  and  58  and by the design of the restrictors  46 ,  48  that this manner of operation continuously repeats and the hydrogen gas is always supplied at the pressure P feed  to the fuel cells  11 . A part of these gases consisting of H 2  thus consist, so to say, of fresh hydrogen which comes from the tank (or from a reforming unit), whereas a further part of the hydrogen supplied to the fuel cells consists of exhaust gases of the fuel cells consisting of H 2  and water vapor.  
         [0031]    Important in this arrangement is that the pump is operated solely by the drive pressure P delivery  and is fully contained in the hydrogen circuit or in the return loop. There is thus no motor positioned outside of the hydrogen circuit, i.e. outside of the wall of the line system conducting the hydrogen, which has to drive anything in the H 2′  circuit via a shaft, so that the entire H 2  circuit can be executed in closed manner. There is thus no danger that H 2  can escape from leaky positions because, as stated, no shaft is present which leads from the outside into the pump system, so that a leakage at such a shaft cannot occur.  
         [0032]    In order to explain the pressure forces which lead to the movement of the double piston  12  and which are responsible for the realization of the working cycle in more detail, a mathematical treatment of the sequence of movement will now be given.  
         [0033]    It is first assumed that the double piston  12  moves to the right in accordance with the arrow  14  and that the valve flap  26  has the position shown in continuous lines in FIG. 1. It is moreover assumed that the piston heads  20  and  22  have an effective area A on both sides. As a result, a force K IR  directed to the right acts in the chamber I on the piston  12  of:  
         
       K 
       IR 
       =P 
       delivery 
       ×A  
     
         [0034]    In the chamber II a force acts in the opposite direction, i.e. to the left, on the piston  12  of:  
         
       K 
       IIL 
       =P 
       feed 
       ×A.  
     
         [0035]    In the chamber III a pressure P return  prevails and this acts on the piston  12  to the right with a force of:  
         
       K 
       IIIR 
       =P 
       return 
       ×A  
     
         [0036]    In the chamber IV the feed pressure P feed  again prevails, so that in the chamber IV a force acts on the double piston  12  which is directed to the left of:  
         
       K 
       IVL 
       =P 
       feed 
       ×A  
     
         [0037]    Now all the forces which act on the double piston  12  in the direction to the right are summed up, that is to say that the total force to the right K GR  results from the force K IR  which originates from the chamber I plus the force K IIIR  which results from the chamber III, i.e.:  
           K   GR   =K   IR   +K   IIIR   =P   delivery   ×A+P   return   ×A   2)  
         [0038]    The total force to the left amounts to K GL :  
           K   GL   =K   IIL   +K   IVL   =P   feed   ×A+P   feed   ×A= 2 P   feed   ×A   3)  
         Furthermore  P   return   =P   feed   −ΔP   Br   4)  
         [0039]    applies where ΔP Br  is the pressure loss of the hydrogen at the anode side of the fuel cell system (measured between the inlet and the outlet) with this pressure or pressure difference also being determined by the pressure regulating valve  39  and/or by the consumed hydrogen (since the valve  39  may also be an on/off valve which is only discontinuously opened for venting purposes to vent, e.g. nitrogen which has accumulated in the anode circuit) which ultimately ensures that the pressure at the outlet of the anode side of the fuel cells lies at P return .  
         [0040]    As a consequence, the total force K G  which acts on the piston  12  to the right is given by the following equation:  
           K   G   =K   GR   −K   GL =( P   delivery   ×A+P   return   ×A )−2 P   feed   ×A   5)  
         [0041]    Taking account of the equation 4) one can write:  
                     K   G     =     A        (       P   delivery     +     (       P   feed     -     Δ                   P   Br         )     -     2        P   feed         )                   =     A        (       P   delivery     -     P   feed     -     Δ                   P   Br         )                     6   )                               
 
         [0042]    Taking account of the equation 1) one can simplify this equation as follows:  
                     K   G     =     A        (       (       P   feed     +     Δ                   P     46   ,   48           )     -     P   feed     -     Δ                   P   Br         )                   =     A        (       Δ                   P     46   ,   48         -     Δ                   P   Br         )                     7   )                               
 
         [0043]    Since A is constant, one can see that the condition for a positive net force directed to the right on the piston  12  is:  
         Δ P   46,48   &gt;ΔP   Br    
         [0044]    i.e., the total pressure loss at the restrictors  46 ,  48  must be greater than the pressure loss between the inlet and the outlet of the hydrogen circuit in the fuel cell arrangement.  
         [0045]    This equation is admittedly not entirely correct, because the effective area of the piston heads  20 ,  22  in the chambers I, II are somewhat smaller than A, because the connection part  24  has a finite cross-sectional area. Nevertheless, the correction that is required is relatively small so that the condition set forth applies at least approximately.  
         [0046]    The above treatment applies for the movement of the double piston  12  to the right. At the end of this movement, the valve flap  26  is changed over or swung over and then the same relationship applies for the movement of the double piston to the left. Thus, a continuous to and fro movement of the double piston is achieved and indeed irrespective of the actual hydrogen requirement for the fuel cell system so that the pumping action is effective under all load conditions and thus also functions at low loads or with a low power yield of the fuel cell system.  
         [0047]    It has been found that the restrictors  46  and  48  are not actually necessary and that a restrictor can indeed be omitted altogether with advantage. In this case the connecting part  24  of the double piston  12  does not need to be hollow, since flow no longer takes place through the hollow connecting rod between the chambers I and II. The possibility of omitting the restrictors and avoiding flow through the connecting rod might at first seem rather surprising in view of the equations given above. However, if the restrictors are omitted, then this is equivalent to setting ΔP 46,48  equal to infinity and the equation  
         Δ P   46,48   &gt;ΔP   Br    
         [0048]    is always satisfied since with flow through the fuel cell system ΔP Br  will always be finite.  
         [0049]    The equations set out above in any case cover the case in which flow takes place through the fuel cell system and are to some extent an oversimplification because they ignore the fact that hydrogen is also consumed in the fuel cells, i.e. leaves the anode circuit via the permeable cell membranes and reacts with oxygen at the cathode side. In fact equation 6) indicates that if no hydrogen is consumed this leads to ΔP Br =0 and P feed =P delivery  which means that the force K G =0. K G &gt;0 means that  
           P   delivery   −P   feed   −ΔP   Br &gt;0  
         [0050]    Thus  
           P   delivery   &gt;P   feed   +ΔP   Br   8)  
         [0051]    Since equation 4) shows that P return =P feed   −ΔP   Br    
         Δ P   Br   =P   feed   −P   return   9)  
         [0052]    and equation 8) can be changed to  
         
       P 
       delivery 
       &gt;P 
       feed 
       +P 
       feed 
       −P 
       return  
     
         [0053]    i.e.  
           P   delivery &gt;2 P   feed   −P   return   10)  
         [0054]    If we set P return  to be slightly less than P feed , indicating that the fuel cell system has started working by consuming hydrogen then, equation 10) makes it clear that P delivery  is slightly greater than P feed  which shows that as soon as P feed  is reduced by consuming hydrogen the pump starts working.  
         [0055]    Thus if the pressure loss at the restrictors is infinitely high, i.e. no restrictor is provided, the pump will always work as soon as hydrogen is consumed in the fuel cell.  
         [0056]    As an alternative the piston pump  10  in the hydrogen feed system shown in FIG. 1 a membrane pump  100  can also be used in accordance with the invention as is, for example, shown in FIGS. 2 and 3.  
         [0057]    The membrane pump  100  has a first membrane chamber  102  and a second membrane chamber  104  between which a gas passage  106  is disposed. The membrane chambers  102 ,  104  have identical cross-sections which are extended in a direction parallel to the gas passage  106  and, in this example, are of symmetrical diamond shape, which is not mandatory.  
         [0058]    The first membrane chamber  102  is divided by a first membrane  108  into a first outer chamber space  110  and a first inner chamber space  112  and the second membrane chamber  104  is divided by a second membrane  114  into a second inner chamber space  116  and a second outer chamber space  118 . The membranes  108 ,  114  extend substantially parallel to one another on both sides of the gas passage  106 .  
         [0059]    A coupling element  166  couples the membranes  108 ,  114  together mechanically, with the ends  168 ,  170  of the coupling element  166  being attached to the membranes  108 ,  114  in the vicinity of the membrane centers. The coupling element  166  has a degree of freedom of movement in the direction extending perpendicular to the plane of the membranes  108 ,  114 .  
         [0060]    The outer chamber spaces  110 ,  118  are respectively provided with an inlet  120 ,  122  in an end region associated with a supply tank, with each inlet being equipped with a non-return valve  124 ,  126  and serving for the supply of unused hydrogen from the fuel cell arrangement. Furthermore, the outer chamber spaces  110 ,  118  each have an outlet  132 ,  134  respectively provided with a corresponding non-return valve  128 ,  130  in their end region associated with the fuel cells in order to feed hydrogen at an elevated pressure in a collection line  136  back to the fuel cell arrangement again.  
         [0061]    Furthermore, a passage  138 ,  140  is in each case provided which connects the end regions of the inner chamber spaces  112 ,  116  at the supply tank side to the gas passage  106 . The passage openings  142 ,  144  of the passages  138 ,  140  at the gas passage side are located in this example at the same level and at opposite sides of the gas passage  106 .  
         [0062]    In the region of the passage openings  142 ,  144  a rocker valve or flap valve  146  is provided in the gas passage  106  and it divides the gas passage  106  into a section  148  associated with the supply tank and a section  150  associated with the fuel cell arrangement.  
         [0063]    The rocker valve  146  has two sealing limbs  152 ,  154  and a tilting limb  156  which are arranged in a Y-shape and connected together. The rocker valve  146  is pivotally mounted at the point at which all three limbs adjoin one another. The sealing limbs  152 ,  154  are formed such that they can close the passage openings  142 ,  144  either with respect to the section  148  adjacent the supply tank or with respect to the section  150  of the gas passage associated with the fuel cell arrangement.  
         [0064]    The rocker valve  146  can only adopt two stable positions, i.e. switching states. In the first position (FIG. 2) the first passage opening  142  is sealed off relative to the section  148  of the gas passage  106  associated with the supply tank and is open towards the section  150  associated with the fuel cell arrangement, while the second passage opening  144  is open towards the section  148  of the gas passage associated with the supply tank and is sealed off with respect to the section  150  associated with the fuel cell arrangement. Gas flowing out of the supply tank into the gas passage  106  can thus only pass into the second inner chamber space  116  while gas from the first inner chamber space  112  can flow into the section  150  of the gas passage  106  associated with the fuel cell arrangement.  
         [0065]    In the second position, which is shown in FIG. 3, the flow relationships are directly reversed, i.e. gas flowing out of the supply tank into the gas passage  106  can only pass into the first inner chamber space  112 , whereas gas from the second inner chamber space  116  is able to flow into the section  150  of the gas passage  106  associated with the fuel cell arrangement.  
         [0066]    The rocker valve  146  is switched with the aid of its tilting limb  156 , which is actuated by a drive element  158 . The drive element  158  has a V-shaped fork section formed by first and second fork limbs  160 ,  162  into which the tilting limb  156  engages. After a displacement of the fork section in a direction perpendicular to the gas passage  106  by an amount which corresponds to the spacing of the free ends of the fork limbs  160 ,  162 , one of the fork limbs  160 ,  162  enters into contact with the tilting limb  156  and acts on this with a force which leads to the tilting valve  146  switching over and adopting its other position. The fork section is rigidly connected with the aid of the rod section  164  to the rod-like coupling element  166 . The coupling element  166  results in a synchronous deflection of the first and second membranes  108 ,  114 . An adequate deflection of the membranes  108 ,  114  leads to an actuation of the rocker valve  146 .  
         [0067]    The manner of operation of the membrane pump  100  is as follows:  
         [0068]    At the start of the first half of the pumping cycle shown in FIG. 2 the membranes  108 ,  114  have a maximum deflection to the left, i.e. the coupling element  166  and thus also the fork section and the tilting limb  156  are located in their left hand position. The rocker valve  146  adopts the position which is hereby termed the first position in which the passage  138  forms a flow connection from the first inner chamber space  112  to the section  150  associated with the fuel cell arrangement and the passage  140  forms a flow connection from the section  148  of the gas passage  106  associated with the supply tank to the second inner chamber space  116 .  
         [0069]    Hydrogen from the supply tank thus flows through the gas passage  106  and the open second passage opening  144  into the second inner chamber space  116 . This increases the pressure in the second inner chamber space  116  and leads to a deflection of the membranes  108 ,  114  to the right. In this way the volume of the second outer chamber space  118  is made smaller and the gas present therein is compressed and displaced through the non-return valve  130  and the outlet  134  into the collection line  136 .  
         [0070]    Furthermore, through the elevated pressure, the non-return valve  126  is closed so that no additional hydrogen returned from the fuel cell arrangement can flow into the second outer chamber space  118 .  
         [0071]    At the same time the volume of the first inner chamber space  112  is reduced through the deflection of the membranes  108 ,  114 . Gas present therein is urged through the open passage  138  into the section  150  of the gas passage  106  associated with the fuel cell arrangement and likewise enters into the collection line  136 .  
         [0072]    Moreover, the volume of the first outer chamber space  110  is enlarged through the movement of the membranes  108 ,  114  and as a consequence a depression is produced in the first inner chamber space  110 . Through the depression the non-return valve  128  at the outlet  132  is closed so that no hydrogen can flow from the collection line  136  back into the first outer chamber space  110 . Instead of this the depression generates a suction action which leads to opening of the non-return valve  124  and the inlet  120  of the first outer chamber space  110  and sucks in hydrogen returned from the fuel cell arrangement.  
         [0073]    The coupling element  166  and with it the drive element  158 , i.e. the fork section, moves to the right together with the membranes  108 ,  114 . If the extreme deflection of the membranes  108 ,  114  is reached then the left limb  160  of the fork of the drive element  158  moves the tilting limb  156  with it and actuates in this manner the rocker valve  146 .  
         [0074]    The rocker valve  146  then rocks over and adopts its second position. Thus the second half of the pumping cycle starts, in which the first inner chamber space  112  is filled with hydrogen which flows in through the gas passage  106  out of the supply tank. The above-described process takes place in the reverse direction  
         [0075]    The pressure (P delivery ) with which the hydrogen coming from the supply tank or from the reformer unit is thus reduced into the gas passage  106  and may for example lie at about 300 kPa overpressure (gage pressure) while the relaxed hydrogen is supplied to the fuel cells with a feed pressure (P feed ) which may be approximately 220 kPa overpressure. The H 2  containing exhaust gases which are partly returned from the fuel cells back into the membrane pump  100  have a pressure (P return ) which may be approximately 180 kPa overpressure at the inlets  120 ,  122 .  
         [0076]    In a modified embodiment the drive element  158  can be formed by a spring or lever element which interacts with the rocker valve  146  and the coupling element  166  and which adopts or defines the fine two stable positions in accordance with FIGS. 2 and 3 respectively.  
         [0077]    In the membrane pump  100  a continuous to and fro movement of the membranes  108 ,  114  is achieved which is driven solely by the gas flow, which is substantially independent of the quantity of hydrogen flowing. It thus also still functions at low loads or with low power output of the fuel cell system. In this example the flow to the fuel cells is the recycled flow plus the flow from the tank. It will be noted that the membrane pump embodiment does not involve any flow restrictors, further confirming that these are not necessary in the piston pump embodiment.