Abstract:
A fuel cell system includes at least one fuel cell having an anode chamber, a cathode chamber, a hydrogen pressure reservoir, a recirculation line connecting an outlet of the anode chamber to an inlet of the anode chamber, a recirculation conveyor with a compressor wheel in the region of the recirculation line, and a turbine for expanding the hydrogen that is under pressure before entry into the anode chamber. The recirculation conveyor is driven at least partially by the turbine. The turbine and the compressor wheel are formed in one component.

Description:
BACKGROUND AND SUMMARY OF THE INVENTION 
     Exemplary embodiments of the present invention relate to a fuel cell system. 
     German patent document DE 10 2006 003 799 B4 describes a fuel cell system in which pressure is relieved from a hydrogen pressure reservoir in a turbine. This turbine then, at least partially, drives a recirculation conveying means in the region of a recirculation line around an anode chamber of at least one fuel cell of the fuel cell system. The construction, which may also have for assistance an electric motor drive for recirculation conveying means, in such case utilizes the pressure energy in the hydrogen pressure reservoir upon the expansion of the hydrogen that is necessary anyway, in order to utilize said energy for driving the recirculation conveying means. 
     The construction in this German patent document is comparatively complex, and is costly with regard to the sealing and the mixing of the recirculated gas flow and the fresh hydrogen. The complex structural form furthermore requires a comparatively large amount of installation space, and is correspondingly heavy. This German patent document further proposes to accelerate the hydrogen to speeds above the speed of sound of the hydrogen, i.e. to speeds above Mach 1, with the aid of a Laval nozzle. Using a Laval nozzle in such case, however, is disadvantageous because the desired effect can be achieved merely at a single load point or in the region of a single load point of the fuel cell system. For only there does the correct pressure ratio, which is dependent on the length of the Laval nozzle, occur. At all the other load points or operating points, what is called a vertical compression shock will occur. After the Laval nozzle, speeds that are lower than the speed of sound then occur. 
     German patent document DE 10 2008 045 170 A1 describes the utilization of the heat sink produced upon the expansion of hydrogen from a hydrogen pressure reservoir in order to cool heat generating component parts in the fuel cell system. 
     German Utility Model DE 20 2005 017 574 U1 discloses a split cage motor for driving a recirculation conveying means for recirculating anode exhaust gas around the anode chamber of a fuel cell. The advantage of such a split cage motor is that the hydrogen carrying region together with the rotor can be sealed off hermetically with respect to the stator of the split cage motor, so that there are no complications with regard to the sealing off of hydrogen carrying regions. 
     Exemplary embodiments of the present invention are directed to a very compact and energy efficient fuel cell system that avoids the disadvantages mentioned above. 
     According to exemplary embodiments of the present invention, a compressor wheel and turbine are combined in a single component so that a simple and efficient fuel cell can be provided that has a very compact construction. Thus, in addition to the high energy efficiency due to the utilization of the pressure of the hydrogen after the pressure regulator of the compressed gas reservoir, a very compact construction of the device is also achieved. This can be made correspondingly simple, lightweight and with a low structural volume. In addition to the structural volume, this also saves on weight and costs. 
     In one particularly beneficial development of the fuel cell system according to the invention, the hydrogen flows onto the turbine via a nozzle having a nozzle needle, the throughflow of hydrogen being adjustable by a relative movement of the nozzle with respect to the nozzle needle. The throughflow of hydrogen through the nozzle and the displaceable nozzle needle—or conversely—is adjustable, and can thus be adapted to the respective load demand or the respective operating point of the fuel cell system. In order to achieve the highest possible speed at the nozzle exit, the annular area produced by the needle and nozzle should be the narrowest point of the hydrogen metering device. If at the same time a critical pressure ratio is present, then, driven by this pressure ratio, the speed of sound of the hydrogen is achieved at the narrowest point of the needle valve. The hydrogen then enters the turbine in ideal manner at Mach 1. 
     In a further very advantageous configuration of the fuel cell system according to the invention, the turbine is a Pelton turbine. Such a Pelton turbine utilizes the energy applied by a gas jet, which can be generated, for example, in accordance with the configuration set forth above via a nozzle, comparatively efficiently, so that a particularly high energy yield can be achieved with the Pelton turbine. Thus, the energy requirement that has to be applied by other drive means can be minimized, so that overall the efficiency of the fuel cell system can be increased. 
     In a further very beneficial and advantageous variant of the fuel cell system according to the invention, the component has channels connecting the region of the turbine and the region of the compressor wheel together. In addition to the pure momentum exchange between the hydrogen gas under high pressure that is expanding and the turbine, overflowing of gas out of the region of the turbine thus also takes place once it has imparted a momentum to the turbine, in the region of the compressor wheel. Thus, the volumetric flow of the recirculated exhaust that is conveyed via the compressor wheel is ideally mixed with the volumetric flow of the fresh hydrogen penetrating via the channels out of the region of the turbine into the region of the compressor wheel, so that a very well mixed and very homogeneous gas flow of fresh hydrogen and recirculated anode exhaust gas is present after the recirculation conveying means. This means that the route between the recirculation conveying means and the anode chamber of the fuel cell can be shortened, since it no longer has to be used in order to ensure the uniform distribution of the gas. Accordingly, installation space and material for lines can be saved with the fuel cell system. In addition to the saving on installation space, a saving in terms of weight and costs also occurs. 
     In a beneficial configuration of the invention, the channels are formed, starting from the region of the turbine, such that the hydrogen flowing through the channels flows into the region of the compressor wheel substantially in the same direction as the flow of the exhaust gas in the compressor wheel. In the case of a configuration of the compressor wheel as a momentum exchange machine or side channel compressor, it is thus achieved that a momentum of the hydrogen flowing through the channels due to the inflow in the same direction as the conveyed exhaust can be utilized such that said hydrogen supports the vortical movement of the exhaust gas through the clever configuration of the channels. Due to the fact that the natural vortical movement of the exhaust gas is supported by the inflowing fresh hydrogen, the efficiency of the compressor wheel can be increased. 
     In an advantageous development of the fuel cell system according to the invention, the component can be driven by means of an electric drive motor, the drive motor being formed as a pancake motor. Typically, it may be necessary for a further drive to be present in addition to the drive of the recirculation conveying means via the turbine, in order for example to be able to continue to operate the recirculation conveying means even when the fuel cell system is idling, in order thus to ensure the uniform distribution of the gases in the recirculation circuit. Furthermore, in certain operating states the energy in the region of the turbine alone is not sufficient to drive the recirculation conveying means due to the admetered amount of fresh hydrogen being very small. Then the electric drive motor can contribute to driving the component, and hence in particular the compressor wheel. The drive motor in this case should be formed as a pancake motor. Such a pancake motor is constructed similarly to a split cage motor, and has a stator and a rotor that is formed to be able to be sealed off hermetically therefrom. Unlike with a split cage motor, however, it has a flat structural form, so that the rotor is arranged as a disk-shaped component spaced apart from the stator, which substantially likewise is formed in a disk shape. The construction thus, with the necessary sealing between rotor and stator, which in the case of a hydrogen atmosphere would be practically unable to be realized by seals with rotating union, permits a very compact structural form. The arrangement of the rotor relative to the stator in a sealed-off space is also referred to as a plate seal. 
     In a particularly beneficial development of this aspect, a stator of the pancake motor can be arranged above the component, the component being provided with magnetic elements and forming a rotor of the pancake motor. This can, for example, be realized by introducing additional magnets into the component, which magnets point with their magnetic fields in the direction of the stator of the pancake motor and are sealed off therefrom by a wall which is magnetically permeable. Corresponding excitation of the stator then ensures rotation of the rotor and hence rotation of the component, so that rotation of the compressor wheel also takes place. 
     In a beneficial development of this aspect, the rotor can be arranged in a space which is sealed off relative to the stator. This construction, as is conventional with pancake motors or split cage motors, is particularly advantageous when used with the hydrogen atmosphere, since the hydrogen atmosphere can then be sealed off comparatively simply, since there are no rotating unions through seals or similar. 
     In a particularly beneficial and advantageous development thereof, the space can be the interior of a water separator in the recirculation line. The construction, and thereof in particular the component with the turbine and the compressor wheel, can therefore be integrated directly in a water separator, for example by the latter forming the cover of such a water separator. The stator of the pancake motor is then arranged above the water separator, so that overall a very compact construction is produced that integrates the recirculation conveying means into a water separator which is present anyway and manages with minimal additional installation space compared with a conventional water separator. 
     As is also known from the prior art, the bearings of the component may be formed at least partially as gas bearings or hydrogen bearings, which are operated by hydrogen from the region of the hydrogen pressure reservoir. In addition, according to an advantageous configuration of the fuel cell system according to the invention, provision is made for at least one of the axial bearings of the component to be formed by means of a bearing tip. Such a bearing by means of a bearing tip relies on minimization of the friction by a pointed or spherical shaping of a bearing element, which then runs in a corresponding shell or ideally, in the case of a spherical formation, on a corresponding counter tip with a spherical surface. Such a construction can be realized here too, since it provides a very simple and efficient axial bearing with minimal effort and minimal costs. 
     In this case, the bearing tip can be formed spherically and to run on a correspondingly spherical counter surface. One of the surfaces may in this case be formed of ceramic, and the other surface of steel. With time, at most the steel tip will wear away, but this can be replaced very simply. The bearing may, for example, be braced via spring elements or the like such that the tips or spheres always contact one another with a comparable contact pressure force in regular operation. Further, the region of greatest Hertzian stress in the region of the sphere (i.e. the region in which the frictional heat is produced) can be cooled via a volumetric hydrogen flow. 
     An exemplary method in accordance with the present invention, the electrical components and/or magnetic components and/or bearings in the region of the recirculation conveying means can be cooled by means of hydrogen. Since the fundamental part of the recirculation conveying means with bearings and other heat generating components is typically the region around the component, here there is the possibility of achieving particularly simple and efficient cooling by cooling with hydrogen, since the hydrogen from the tank is approximately at ambient temperature and thus will be significantly cooler than the hydrogen in the anode circuit. The latter can therefore cool the components without additional cooling media having to be brought into the hydrogen carrying region, with corresponding expense with regard to sealing. A further advantage of this method is that the hydrogen is heated up thereby. This results in a reduction in the condensation of water if the hydrogen meets the recirculated exhaust of the anode chamber. This reduces, or ideally avoids entirely, the introduction of liquid into the anode chamber, so that the performance of the fuel cell is not adversely affected by channels “blocked” with liquid water in the anode chamber. 
     In accordance with exemplary embodiments of the present invention, the method can involve setting the throughflow through the nozzle to such that a speed of the hydrogen upon entry into the turbine is in the range from 0.8-1.05 of the speed of sound (Mach 1) of the hydrogen. Such a flow rate of the hydrogen guarantees a high momentum exchange onto the turbine, which may in particular be formed as a Pelton turbine, and also makes it possible to effect controlled imparting of the momentum onto the turbine, since ultrasonic flow effects (compression shocks) typically do not occur here. 
     In a particularly beneficial method for driving a recirculation conveying means in a fuel cell system the electric drive motor can be designed for a constant or approximately constant operating point. This designing of the electric drive motor for a constant or approximately constant operating point and the variation of the drive power required for the compressor wheel by a corresponding variation of the hydrogen flow onto the turbine makes it possible to use a very simple and inexpensive electric drive motor with correspondingly simple and inexpensive electronics, since said motor, due to the constant or approximately constant operating point for which it is designed, can be realized correspondingly simply and efficiently and only a low mechanical drive power has to be provided. A large spread of speeds can be dispensed with, so that as a result of this, in addition to an improvement in the efficiency, a reduction in costs, installation space and weight can also be achieved. The electric drive motor as a result can preferably be designed as a low voltage motor. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING FIGURES 
       Further advantageous configurations of the fuel cell system according to the invention and method related aspects for operating such a fuel cell system will furthermore become apparent from the example of embodiment, which will be explained in greater detail below with reference to the figures. 
       Therein: 
         FIG. 1  shows a diagrammatic view of the section of a fuel cell system which is relevant for the invention; 
         FIG. 2  shows a recirculation conveying means in one possible construction according to the invention; and 
         FIG. 3  shows a possible configuration of a bearing in the recirculation conveying means. 
     
    
    
     DETAILED DESCRIPTION 
     In the illustration of  FIG. 1 , a section of a fuel cell system  1  can be recognized in very highly diagrammatic form. The system has a fuel cell  2  in the form of a PEM fuel cell. In such a PEM fuel cell, which is typically stacked as a stack consisting of a large number of individual cells, anode chambers  3  are separated from cathode chambers  5  by means of a proton conducting membrane  4 . The cathode chambers or the cathode chamber  5  in this case is/are supplied with air as oxygen supplier via an incoming air line  6 ; unspent exhaust air passes out of the fuel cell system  1  via an exhaust air line  7 . Since the region of the air supply of the cathode chamber  5  is of no further relevance for the invention presented here, it will not be discussed in greater detail. It may, however, be formed in any manner whatsoever which is known per se with corresponding conveying means, air filters, recirculation of the exhaust air, utilization of pressure energy and thermal energy in the exhaust air via a turbine or the like. 
     The anode chamber  3  of the fuel cell  2  is supplied with hydrogen from a hydrogen pressure reservoir  8 . In this hydrogen pressure reservoir  8 , the hydrogen is stored under high pressure, for example under pressures of the order of 700 bar. It then passes via a valve means  9 , for example a shutoff valve and a pressure regulator, and a hydrogen feed line  10  into the region of a turbine  11 , in which the hydrogen is expanded and in so doing gives off energy to the turbine  11 . The hydrogen then passes into the anode chamber  3  via an anode feed line  12 . The anode chamber  3  in the example of embodiment shown here is supplied with more hydrogen than can be electrochemically reacted therein. This is known from the prior art and is generally conventional. The supplying of the anode chamber  3  with more hydrogen than can be reacted therein ensures that all the regions of the anode chamber are supplied with sufficient hydrogen and the entire active surface of the membrane  4  is ideally utilized. The exhaust flowing out of the anode chamber  3  passes via a recirculation line  13  to a compressor wheel  14 , and is conveyed back by the latter into the anode feed line  12  and hence into the anode chamber  3 . The compressor wheel  14  in this case, just like the turbine  11 , is part of a recirculation conveying means  15 . The compressor wheel  14  itself in this case can be formed as any flow compressor whatsoever, preferably as a radial compressor or side channel compressor. The recirculation conveying means  15  in this case is driven via the turbine  11  and the hydrogen that is expanded therein from the hydrogen feed line  10 , and can additionally receive drive power from an electric drive motor  16  if this is necessary for conveying the exhaust gas in the recirculation line  13 . The drive motor  16  may be formed in any manner whatsoever, for example as a high voltage motor or as a low voltage motor. Low voltage motors with a voltage of less than 60 V in this case have the advantage that they can typically be designed more simply and inexpensively. Forming the drive motor  16  as a low voltage motor, in particular as a 12 volt or 24 volt motor, would therefore be preferable. In principle, it would of course also be conceivable, in the event of an excess of energy in the region of the turbine  11 , to operate the drive motor  16  regeneratively, in order thus to generate electricity. 
     Furthermore, a water separator  17  is arranged in a manner known per se and in a construction known per se in the region of the recirculation line  13 . The water separator  17  may in this case either be the sole water separator  17  in the region of the circulation around the anode chamber  3 , or a second optional water separator may additionally be arranged (not shown here) in the region of the anode feed line  12 . The water separator  17 , in addition to separating off liquid water from the exhaust in the recirculation line  13 , furthermore assumes the functionality of draining off gas from the region of the recirculation line  13 . The construction in this case corresponds to the construction described in PCT International Publication No. WO 2008/052578 A1. In the circulation around the anode chamber  3  of the fuel cell  2  that is formed by the recirculation line  13  and the anode feed line  12 , a small portion of water becomes enriched during operation, this being produced as product water partially in the anode chamber  3  of the fuel cell, even if the major part of the product water is produced in the cathode chamber  5 . Furthermore, inert gases, such as for example nitrogen, with time pass through the membranes  4  from the cathode chamber  5  into the region of the anode chamber  3 , and then, since they are not used up, are jointly circulated via the recirculation conveying means  15 . Over a longer operating period, water and inert gas thus become enriched in the circulation around the anode chamber  3 . The hydrogen concentration drops accordingly, so that the performance of the fuel cell  2  suffers. The water and the inert gas are therefore drained off from time to time. This is done such that the water separator  17  has a drain line  18  and a discharge valve  19 , what is called a drain/purge valve  19 . Once a sufficient amount of water has accumulated in the water separator  17 , opening of the drain/purge valve  19  can take place, for example dependent on water level or in time controlled manner. The water then flows away out of the water separator  17  and then a certain proportion of the gas accumulated in the circulation around the anode chamber  3  will likewise flow away with it, so that the concentration of the hydrogen subsequently increases. The water and/or the gases that is/are removed may in this case for example be introduced into the region of the incoming air line  6 , or they may also be guided out of the fuel cell system  1  and released into the environment. Since typically hydrogen is present in the gases, this should be correspondingly diluted or afterburned in order to avoid harmful emissions. 
     In the illustration of  FIG. 2 , the recirculation conveying means  15  of the fuel cell system  1  is described once again with reference to an exemplified possible highly integrated construction. The recirculation conveying means  15  in this case is embodied as a cover integrated in the water separator  17 . It consists substantially of a housing  20  and a rotationally movable component  21  arranged therein that has the turbine  11  and the compressor wheel  14 . The turbine  11  in this case can be seen in the upper part of the component  21  illustrated in  FIG. 2 . It is formed by way of example as a Pelton turbine. In the illustration, two individual blades thereof can be seen. The component  21  is borne in the housing  20  or in a housing cover  22  with a bearing means  23 , so that the component  21  can rotate about an axis of rotation  24 . The region of the turbine  11  is approached by a stream of hydrogen from the hydrogen pressure reservoir  8 , which hydrogen flows via the hydrogen feed line  10  into the region of a nozzle  25 , and from there reaches the blades of the turbine  11 . The nozzle  25  in this case is formed such that a nozzle needle  26 , as indicated by the double arrow, can be adjusted by means of an actuator  27  such that the flow through the nozzle  25  is adjustable. This means that the supplied fresh hydrogen can be metered accordingly, and at the same time a suitable speed can be set in order for the flow to approach the blades of the turbine  11  ideally at the speed of sound of the hydrogen. The nozzle needle  26 , in the embodiment shown here, is for example regulated or controlled via the actuator  27  and a restoring spring  28 . In the region of the compressed gas reservoir  8  there is typically a pressure level of approximately 700 bar. After the valve means  9 , there is then a pressure of the order of 8 to 15 bar that is reduced via the pressure regulator of the valve means. The hydrogen gas, which flows through the nozzle or the needle valve  25  at the speed of sound, thus undergoes a pressure drop of approx. 8 to 15 bar in the direction of flow ahead of the nozzle  25  and approx. 1 to 3 bar after the nozzle  25 , upon flowing into the turbine  11 . This pressure drop suffices to achieve a speed of Mach 1 in the region of the annular area between the nozzle  25  and nozzle needle  26 , which forms the narrowest cross section of the hydrogen metering means. 
     Due to the rotation of the turbine  11 , the entire component  21  is caused to rotate, so that furthermore the compressor wheel  14  rotates in addition to the turbine  11 . The compressor wheel, in the example of embodiment illustrated here, is formed as a side channel compressor (momentum exchange machine), which draws in the exhaust from the anode chamber  3  substantially in the axial direction of its axis of rotation  24  out of the water separator  17 , as is indicated by the arrow represented by A. The exhaust A that is drawn in in the axial direction is then conveyed accordingly in the region of the compressor wheel  14 , and is set in a rotary motion before flowing away in a tangential direction, as indicated by the arrow B, into the region of a diffuser  29 , which is either connected to the anode feed line  12  or forms said line itself. In addition to the compressor wheel  14  that is formed as a side channel compressor, alternative configurations, for example in the form of a radial compressor, would likewise be conceivable. 
     In order then to mix the fresh hydrogen admetered in the region to the turbine  11  with the exhaust A from the anode chamber  3 , channels  30  are provided in the component  21 . These connect the region of the turbine  11 , and here in particular the region between the individual blades of the turbine  11 , to the region of the compressor wheel  14 . The channels  30  in this case are oriented such that the accumulating hydrogen flows through the channels from the region of the turbine  11  into the region of the compressor wheel  14 . Since with this configuration as a side channel compressor a corresponding rotary motion of the exhaust A that is drawn in is produced, the orientation of the channels  30  in the example of embodiment shown here is selected such that they run slightly obliquely in the direction of the axis of rotation  24 , so that the gas flowing through the channels  30  meets the gas flow of the anode exhaust gas represented by the arrows A, B in the same direction. This achieves very good mixing and uniform distribution of the fresh hydrogen and the anode exhaust gas. In this case, further energy contained in the fresh hydrogen can be utilized for uniform distribution. In the region of the diffuser  29 , the flow is then calmed and slowed accordingly, so that the necessary pressure for flowing into the anode chamber is ensured. 
     Should the amount of hydrogen flowing through the channels  30  from the region of the turbine  11  into the region of the compressor wheel  14  not suffice to provide all the fresh hydrogen that is required for operating the fuel cell  2 , a portion of the hydrogen, if required, may furthermore flow via an optional bypass in the housing  20 , which is provided there with the reference numeral  31 , into the region of the diffuser  30 , in order thus to provide a sufficient amount of fresh hydrogen. 
     Driving of the recirculation conveying means  15  via the turbine  11  in this case is useful and possible over large sections of the operation. However, there are operating points, for example idling operation or similar, for which an insufficient amount of fresh hydrogen is admetered to drive the compressor wheel  14  in the component  21  via the turbine  11  (alone). For these cases, and for assisting driving in all other cases, therefore, the electric drive motor  16  is provided. The latter is formed as a pancake motor in the configuration of  FIG. 2 , and has a stator  32  arranged above the housing cover  22 , which stator can be cooled by the surrounding atmosphere via cooling fins  33 . The stator  32  in this case is embodied to be separated from the rotating component  21  due to the housing cover  22 . Due to the housing cover  22 , the region of the component  21  can be separated from the stator  32  and sealed off relative thereto, so that the hydrogen present in the region of the rotating component  21  cannot pass into the region of the stator  32  and out of the recirculation conveying means  15 . Through the housing cover  22 , which has to be permeable to magnetic lines of force without, however, generating significant hysteresis losses, a magnetic coupling to permanent magnets  34  that are arranged in the region of the component  21  takes place. These permanent magnets  34  cooperate with windings  35  of the stator  32  so that the component  21  directly forms a rotor of the pancake motor  16  and can be driven accordingly. 
     The rotor in the form of the component  21  in this case has hydrogen constantly flowing around it and is thereby ideally cooled. Something comparable applies to the deep groove ball bearings  36  used in the bearing means  23  in the example of embodiment illustrated here, via which bearings the component  21  is borne in the illustration of  FIG. 2 . Due to the integration of the bearing means  23  and of the component  21  as a rotor of the pancake motor  16  in the hydrogen atmosphere, the use of hydrogen as cooling gas results in the possibility of ideal cooling, without it being necessary to guide liquid cooling medium or the like through into the region of the hydrogen atmosphere and back out again. This means that sealing problems can be largely avoided. 
     The bearing means illustrated here with the two deep groove ball bearings  36  in this case is efficient and effective, but relatively cost intensive due to the use of deep groove ball bearings. Instead of the deep groove ball bearings  36 , other bearing concepts could also be realized, in particular a gas bearing means with hydrogen as bearing gas both for the axial bearing means and for the radial bearing means. 
       FIG. 3  illustrates a possible alternative construction of the bearing means  23 , in which the radial bearing means occurs by means of hydrogen bearings. In suitable recesses or pockets  37 , hydrogen from the region of the compressed gas reservoir  8  is passed via a choke  38  in order thus to bring about a gas bearing means with hydrogen for the radial bearing means of a shaft  39  of the component  21 . As already mentioned, such a gas bearing means could also be provided for the axial bearing means. As an alternative to this, it is also possible to use the variant illustrated in  FIG. 3  for one or, as illustrated here, both axial bearings. The axial bearings  40  in this case are formed substantially with bearing tips  41  that are fastened, for example, via elastic elements  42  in the region of the shaft  39  of the component  21 . The bearing tips  41  can then, for example, run in corresponding bearing shells, or, as illustrated here, in corresponding bearing tips  43  that are connected to the housing  20  or the housing cover  22 .  FIG. 3  illustrates a construction in which the bearing tips  41 ,  43  are formed in each case as spherical elements that run on one another. This produces a sliding bearing means with minimal surface contact. In the region of the greatest Hertzian stress, with these bearings heat is also correspondingly produced, so that they can optionally be cooled by hydrogen from the supply means  44  which are shown in broken lines. 
     The bearing tips  41  in this case in the ideal configuration are formed of as hard as possible a material, for example of spheres of industrial ceramic or similar. In particular, the construction in this case can be selected such that the bearing tips  41  in the region of the shaft  39  of the component  21  are formed from a harder or softer material than the bearing tips  43  in the region of the housing  20  or the housing cover  22 . By way of example, the bearing tips  41  in the region of the component  21  may be formed of hardened steel and the bearing tips  43  in the region of the housing  20  or the housing cover  22  may be formed of ceramic. With this configuration, the bearing tip  43  of hardened steel will wear faster than the bearing tip  41  of ceramic, which undergoes practically no wear. When servicing the recirculation conveying means  15 , in this case replacement of the bearing tips  41  made of hardened steel could take place, in order thus to ensure as easy as possible a rotation of the bearing over a long life of the bearing means  23 . The construction illustrated in  FIG. 3  in this case, compared with the deep groove ball bearings  36  as are illustrated in  FIG. 2 , has the crucial advantage that it is simpler and more inexpensive to realize, and that the bearing friction becomes minimal. 
     The construction of the recirculation conveying means  15  according to the example of embodiment illustrated here can be constructed very inexpensively, simply and extraordinarily compactly, and for example, as illustrated in  FIG. 2 , be integrated in a water separator  17 . This produces a very compact and space-saving construction which, due to the ideal uniform distribution of the gases, furthermore makes a very short anode feed line  12  possible. Due to the fact that a large part of the drive energy is obtained from the pressure energy of the hydrogen stored in the compressed gas reservoir  8  via the turbine  11 , the pancake motor  16  has to intervene or assist the rotation of the component  21  only in certain circumstances. It can therefore be designed to be correspondingly small and efficient, which results in a smaller required installation space and also in simpler and more inexpensive component parts for the pancake motor  16 . Furthermore, it is possible to design the pancake motor  16  merely for a single operating point that is constant or approximately constant. Thus, once again a considerable saving is obtained with regard to the engine electronics and the control electronics as well. Due to the fact that a major portion of the energy can be applied via the turbine  11  during regular operation, it is sufficient if the pancake motor  16  is designed, for example, for operation during idling. Dynamic peaks and the like can be provided via the hydrogen from the compressed gas reservoir  8 , which in these situations is typically required anyway, since the fuel cell  2  itself then likewise requires more hydrogen in order to meet dynamic requirements. 
     Since the pancake motor  16  can accordingly be designed simply, it is now possible to construct it as a low voltage motor, in particular as a 12 volt motor. Such a 12 volt motor in this case is particularly small and can accordingly be realized inexpensively. The construction thus once again permits considerable savings in costs. 
     The construction of the fuel cell system  1  in this case, as has already been described several times, is extraordinarily compact, inexpensive and lightweight, and ensures very good efficiency of the fuel cell system  1 . Its preferred application is in the field of motor vehicles, and here in particular for driving passenger cars, since here cost optimized systems with minimal installation space and maximum efficiency can display their particular advantages, in particular if dynamic or highly dynamic operation of the fuel cell system  1  is required. 
     The foregoing disclosure has been set forth merely to illustrate the invention and is not intended to be limiting. Since modifications of the disclosed embodiments incorporating the spirit and substance of the invention may occur to persons skilled in the art, the invention should be construed to include everything within the scope of the appended claims and equivalents thereof.