Abstract:
The invention regards a fuel cell system and method for recovering moisture from an outgoing oxidant stream and humidifying an incoming oxidant stream in a fuel cell. A plurality of dryers is used to recover moisture from an outgoing oxidant stream from the fuel cell and to humidify an incoming oxidant stream for the fuel cell. The fuel cell comprises an anode for receiving fuel and a cathode for receiving the incoming oxidant stream and discharging the outgoing oxidant stream, and an electrolyte between the anode and the cathode. The moisture recovery and humidification involves (i) intermittently switching each dryer in the plurality of dryers into and out of one of a first mode of operation for recovering moisture from the outgoing oxidant stream and a second mode of operation for humidifying the incoming oxidant stream such that during use at least one dryer is in the first mode of operation and at least one dryer is in the second mode of operation; (ii) directing the outgoing oxidant stream from the cathode through at least one dryer in the first mode of operation to recover moisture from the outgoing oxidant stream; and (iii) directing the incoming oxidant stream through at least one dryer in the second mode of operation to humidify the incoming oxidant stream with moisture.

Description:
RELATED APPLICATIONS  
       [0001]    This application is a continuation-in-part of U.S. patent application Ser. No. 09/941,935, filed Aug. 30, 2001, and is also a continuation-in-part of U.S. patent application Ser. No. 09/592,644, filed Jun. 13, 2000. 
     
    
     
       FIELD OF THE INVENTION  
         [0002]    This invention relates to electrochemical fuel cells. More particularly, this invention relates to electrochemical fuel cells incorporating a regenerative dryer device for recovering water and humidifying a reactant stream of the fuel cell.  
         BACKGROUND OF THE INVENTION  
         [0003]    Generally, a fuel cell is a device which converts the energy of a chemical reaction into electricity. It differs from a battery in that the fuel cell can generate power as long as the fuel and oxidant are supplied.  
           [0004]    A fuel cell produces an electromotive force by bringing the fuel and oxidant into contact with two suitable electrodes and an electrolyte. A fuel, such as hydrogen gas, for example, is introduced at a first electrode where it reacts electrochemically in the presence of the electrolyte and catalyst to produce electrons and cations in the first electrode. The electrons are circulated from the first electrode to a second electrode through an electrical circuit connected between the electrodes. Cations pass through the electrolyte to the second electrode. Simultaneously, an oxidant, typically air, oxygen enriched air or oxygen, is introduced to the second electrode where the oxidant reacts electrochemically in presence of the electrolyte and catalyst, producing anions and consuming the electrons circulated through the electrical circuit; the cations are consumed at the second electrode. The anions formed at the second electrode or cathode react with the cations to form a reaction product. The first electrode or anode may alternatively be referred to as a fuel or oxidizing electrode, and the second electrode may alternatively be referred to as an oxidant or reducing electrode. The half-cell reactions at the two electrodes are, respectively, as follows:  
           [0005]    The external electrical circuit withdraws electrical current and thus receives electrical power from the cell. The overall fuel cell reaction produces electrical energy which is the sum of the separate half-cell reactions written above. Water and heat are typical by-products of the reaction.  
           [0006]    In practice, fuel cells are not operated as single units. Rather, fuel cells are connected in series, stacked one on top of the other, or placed side by side. A series of fuel cells, referred to as a fuel cell stack, is normally enclosed in a housing. The fuel and oxidant are directed through manifolds to the electrodes, while cooling is provided either by the reactants or by a cooling medium. Also within the stack are current collectors, cell-to-cell seals and, insulation, with required piping and instrumentation provided externally of the fuel cell stack. The stack, housing, and associated hardware make up the fuel cell module.  
           [0007]    Fuel cells may be classified by the type of electrolyte, either liquid or solid. The present invention is primarily concerned with fuel cells using a solid electrolyte, such as a proton exchange membrane (PEM). The PEM has to be kept moist with water because the available membranes will not operate efficiently when dry. Consequently, the membrane requires constant humidification during the operation of the fuel cell, normally by adding water to the reactant gases, usually hydrogen and air.  
           [0008]    The proton exchange membrane used in a solid polymer fuel cell acts as the electrolyte as well as a barrier for preventing the mixing of the reactant gases. An example of a suitable membrane is a copolymeric perfluorocarbon material containing basic units of a fluorinated carbon chain and sulphonic acid groups. There may be variations in the molecular configurations of this membrane. Excellent performances are obtained using these membranes if the fuel cells are operated under fully hydrated, essentially water-saturated conditions. As such, the membrane must be continuously humidified, but at the same time the membrane must not be over humidified or flooded as this degrades performances. Furthermore, the temperature of the fuel cell stack must be kept above freezing in order to prevent freezing of the stack.  
           [0009]    Cooling, humidification and pressurization requirements increase the cost and complexity of the fuel cell, reducing its commercial appeal as an alternative energy supply in many applications. Accordingly, advances in fuel cell research are enabling fuel cells to operate without reactant conditioning, and under air-breathing, atmospheric conditions while maintaining usable power output.  
           [0010]    The current state-of-the-art in fuel cells, although increasingly focusing on simplified air-breathing, atmospheric designs, has not adequately addressed operations in sub-zero temperatures, which requires further complexity of the design. For instance, heat exchangers and thermal insulation are required, as are additional control protocols for startup, shutdown, and reactant humidifiers.  
           [0011]    Where a solid polymer proton exchange membrane (PEM) is employed, it is generally disposed between two electrodes formed of porous, electrically conductive material. The electrodes are generally impregnated or coated with a hydrophobic polymer such as polytetrafluoroethylene. A catalyst is provided at each membrane/electrode interface, to catalyze the desired electrochemical reaction, with a finely divided catalyst typically being employed. The membrane electrode assembly is mounted between two electrically conductive plates, each of which has at least one flow passage formed therein. The fluid flow conductive fuel plates are typically formed of graphite. The flow passages direct the fuel and oxidant to the respective electrodes, namely the anode on the fuel side and the cathode on the oxidant side. The electrodes are electrically coupled in an electric circuit, to provide a path for conducting electrons between the electrodes. In a manner that is conventional, electrical switching equipment and the like can be provided in the electric circuit. The fuel commonly used for such fuel cells is hydrogen, or hydrogen rich reformate from other fuels (“reformate” refers to a fuel derived by reforming a hydrocarbon fuel into a gaseous fuel comprising hydrogen and other gases). The oxidant on the cathode side can be provided from a variety of sources. For some applications, it is desirable to provide pure oxygen, in order to make a more compact fuel cell, reduce the size of flow passages, etc. However, it is common to provide air as the oxidant, as this is readily available and does not require any separate or bottled gas supply. Moreover, where space limitations are not an issue, e.g. stationary applications and the like, it is convenient to provide air at atmospheric pressure. In such cases, it is common to simply provide channels through the stack of fuel cells for flow of air as the oxidant, thereby greatly simplifying the overall structure of the fuel cell assembly. Rather than having to provide a separate circuit for oxidant, the fuel cell stack can be arranged simply to provide a vent, and possibly, some fan or the like, to enhance air flow.  
           [0012]    There are various applications for which humidification of fuel cells poses particular problems and challenges. For example, operation of fuel cells in mobile vehicles usually means that there is no readily available supply of water for humidifying incoming oxidant and fuel streams. It is usually undesirable to have to provide water to a vehicle for this purpose and also to have to carry the excess weight of the water around in the vehicle. In contrast, for stationary applications, providing a supply of water for humidification is usually quite possible.  
           [0013]    However, there also some stationary applications for which humidification is not straightforward. For example, fuel cells are often used to provide power to remote sensing equipment, in locations where water may not be readily available. Additionally, such remote use of fuel cells often occurs at locations with extreme climatic conditions. Thus, it has been known to use fuel cell stacks in the Antarctic regions and the like, for providing supply to scientific instruments. It is simply not realistic to provide a separate supply of water for humidification, because of the problems associated with preventing freezing of the water supply. Additionally, ambient air used as an oxidant is excessively dry, so that humidification is more critical than when using relatively moist air at more moderate temperatures. It will be appreciated that similar extreme conditions can be found in desert locations and the like.  
         SUMMARY OF THE INVENTION  
         [0014]    Accordingly, the present invention is based on the realization that, as a fuel cell inherently produces excess moisture or water as a waste product, this water is available for recycling to humidify incoming flows to the fuel cell.  
           [0015]    More particularly, the present inventors have realized that it is advantageous to recover water from the waste or outlet flows from a fuel cell or fuel cell stack, so as to avoid having to provide a separate water source to humidify the oxidant and/or fuel streams.  
           [0016]    It has also been recognized that, in extreme climatic conditions, it is desirable, and even in some situations essential, that the humidity of discharged fuel and/or oxidant streams be below certain levels. For example, in extremely cold conditions, if the discharged streams contain significant moisture levels, then this moisture can immediately freeze. In practice, this will form a mist or fog or fine droplets or ice pellets, which would tend to build up on the outside of the apparatus. It will be appreciated that, for a stationary installation intended to supply power to scientific instruments over a long period of time, such a possibility is highly undesirable, and could lead to blockage of vents, undesirable loading due to build-up of ice and other problems. For these reasons, it is desirable that discharged streams contain reduced levels of moisture.  
           [0017]    In accordance with a first aspect of the present invention, [there is provided a fuel cell system comprising a fuel cell, a plurality of dryers and a first switch means. The fuel cell has an anode with an anode inlet for receiving a fuel gas and an anode outlet, a cathode with a cathode inlet for receiving an incoming oxidant gas stream and a cathode outlet for discharging an outgoing oxidant gas stream, and an electrolyte between the anode and the cathode. Each dryer in the plurality of dryers has a first mode of operation for recovering moisture from the outgoing oxidant gas stream and a second mode of operation for humidifying the incoming oxidant gas stream, and is connectable to the cathode outlet in the first mode and to the cathode inlet in the second mode. The first switch means is operable to, for each dryer in the plurality of dryers, switch the dryer into and out of the second mode of operation in which the first switch means fluidly connects the dryer to the cathode inlet and obstructs fluid connection between the dryer and the cathode outlet, and to switch the dryer into and out of the first mode of operation in which the first switch means fluidly connects the dryer to the cathode outlet and obstructs fluid connection between the dryer and the cathode inlet. In use, the plurality of dryers includes at least one dryer in the first mode of operation, and at least one dryer in the second mode of operation.  
           [0018]    While the invention is applicable to a single fuel cell, it is anticipated that the invention will usually be applied to a plurality of fuel cells configured as a fuel cell stack. In such a case, a cathode inlet and outlet are connected to respective inlet and outlet manifolds connected to each of the fuel cells.  
           [0019]    A separate, co-pending application, Ser. No. 09/592,643, filed simultaneously herewith under the title “Water Recovery in the Anode Side of a Proton Exchange Membrane Fuel Cell” is directed to water recovery on the anode side of a fuel cell. Nonetheless, the present invention envisages that water or moisture recovery could be effected on both the cathode side and the anode side. In this case, the fuel cell is preferably adapted for use with hydrogen as a fuel.  
           [0020]    Another aspect of the present invention provides a method of recovering moisture from an outgoing oxidant stream and humidifying an incoming oxidant stream in a fuel cell. In accordance with this second aspect of the present invention, there is provided a method of recovering moisture from an outgoing oxidant stream from a fuel cell and humidifying an incoming oxidant stream for a fuel cell using a plurality of dryers. The fuel cell comprises an anode for receiving fuel and a cathode for receiving the incoming oxidant stream and discharging the outgoing oxidant stream, and an electrolyte between the anode and the cathode. The method comprises the steps of: (i) intermittently switching each dryer in the plurality of dryers into and out of one of a first mode of operation for recovering moisture from the outgoing oxidant stream and a second mode of operation for humidifying the incoming oxidant stream such that during use at least one dryer is in the first mode of operation and at least one dryer is in the second mode of operation; (ii) directing the outgoing oxidant stream from the cathode through at least one dryer in the first mode of operation to recover moisture from the outgoing oxidant stream; and (iii) directing the incoming oxidant stream through at least one dryer in the second mode of operation to humidify the incoming oxidant stream with moisture. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0021]    For a better understanding of the present invention and to show more clearly how it may be carried into effect, reference will now be made, by way of example, to the accompanying drawings which show preferred embodiments of the present invention and in which:  
         [0022]    [0022]FIG. 1 is a schematic view of a first embodiment of a regenerative dryer device for recovering and recycling water on the cathode side of a fuel cell stack;  
         [0023]    [0023]FIG. 2 is a schematic view of a second embodiment of a regenerative dryer device for recovering and recycling water on the cathode side of a fuel cell stack;  
         [0024]    [0024]FIG. 3, in a perspective view, shows dryers and switch means of a regenerative dryer device for recovering and recycling water on the cathode side of a fuel cell stack in accordance with a third embodiment of the invention;  
         [0025]    [0025]FIG. 4 a,  in a sectional view, shows a housing of a switch means of the regenerative dryer device of FIG. 3;  
         [0026]    [0026]FIG. 4 b,  in a perspective view, shows the housing of the switch means of FIG. 4 a;    
         [0027]    [0027]FIG. 5 a,  in a perspective view, shows a rotary member of the switch means of the regenerative dryer device of FIG. 3;  
         [0028]    [0028]FIG. 5 b,  in a sectional view, shows the rotary member of FIG. 5 a;    
         [0029]    [0029]FIG. 5 c,  in a cut away perspective view, illustrates the rotary member of FIG. 5 a;    
         [0030]    [0030]FIG. 5 d,  in a perspective view, orthogonal to the perspective view of FIG. 5 a,  shows the rotary member of FIG. 5 a;    
         [0031]    [0031]FIG. 6, in a sectional view, shows a further embodiment of a regenerative dryer device for recovering water on the cathode side of a fuel cell stack, incorporating the components of FIGS.  3 - 5 ;  
         [0032]    [0032]FIG. 7, in a cross-sectional view taken along line A-A of FIG. 6, illustrates the relationship between the size of slots in the rotary and the openings of the chambers in the end housing;  
         [0033]    [0033]FIG. 8, in a schematic view, illustrates an apparatus for recovering and recycling water on the anode side of a fuel cell stack in accordance with a further embodiment of the invention;  
         [0034]    [0034]FIG. 9, in a schematic view, illustrates a further embodiment of an apparatus for recovering and recycling water on the anode side of a fuel cell stack; and  
         [0035]    [0035]FIG. 10, in a schematic view, illustrates a further embodiment of an apparatus for recovering and recycling water on the anode side of a fuel cell stack. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0036]    Referring first to FIG. 1, there is illustrated in a schematic view, a regenerative dryer device  10  in accordance with the first embodiment of the invention. The regenerative dryer device  10  includes a fuel cell stack  12 , although it will be appreciated that the fuel cell stack  12  could comprise just a single fuel cell. In known manner, the fuel cell stack has inlets and outlets for both fuel and an oxidant. In FIG. 1, an inlet  14  and an outlet  16  are shown for the oxidant. Commonly, the oxidant is air, although for certain applications it can be pure oxygen.  
         [0037]    A first switch means  100  is provided having a first inlet port  24  fluidly connected to the inlet  14  and a first outlet port  26  fluidly connected to the outlet  16  of the fuel cell stack  12 , to introduce incoming oxidant gas stream into the fuel cell stack  12  while simultaneously discharging outgoing oxidant gas stream from the fuel cell stack  12  without mixing the two streams. A plurality of dryers  300  are provided. Each dryer  300  is fluidly connected to each of a plurality of first dryer ports  30  of the first switch means  100  and has an external port  80  for taking in or exhausting gas. A pump  20  is disposed between the first switch means  100  and the inlet  14  for supplying oxidant gas stream from the switch means  100  to the fuel cell stack  12 .  
         [0038]    The first switch means  100  operates in a manner detailed below. Generally, at any given time during operation, the first switch means  100  provides fluid communication between the inlet  14  and some of the plurality of dryers  300 , while simultaneously permitting fluid communication between the outlet  16  and the others of the plurality of dryers  300 , respectively. Hereinafter, the dryers fluidly connected to the inlet  14  are referred to as working in intake mode while the dryers fluidly connected to the outlet  16  are referred to as working in exhaust mode. The individual dryers working in each mode change with time.  
         [0039]    In more detail, at a given time, a first plurality of dryers  300  working in the intake mode are connected to the inlet  14  of the fuel cell stack  12  via the first switch means  100 . Consequently, the pump  20  draws the oxidant, such as ambient air, through the external ports  80  of the first plurality of dryers  300  into the dryers  300 . At least one of the first plurality of dryers  300  will previously have been, in exhaust mode, charged with moisture from the outgoing oxidant stream, so that incoming air picks up moisture and is humidified during passage through the at least one dryer  300 . The humidified air then passes through the first switch means  100  and through the pump  20  to the stack oxidant inlet  14 . Simultaneously, a second plurality of dryers  300  working in the exhaust mode are connected to the outlet  16  of the fuel cell stack  12  via the first switch means  100 . Consequently, warm and humidified air discharged from the oxidant outlet  16  passes through the second plurality of dryers  300 . At least one of the second plurality of other dryers  300  will previously have been, in intake mode, dehumidified by the incoming, relatively dry air. The passage of the outgoing air through the at least one other dryer dries and dehumidifies the outgoing air, and simultaneously charges the at least one other dryer  300  with moisture.  
         [0040]    After a predetermined time period, determined by the capacities of the dryers  300 , as well as the pressure and flow rate of incoming air and outgoing air, the switch means  100  switches so that the operation of at least one of the dryers switches from intake mode to exhaust mode. Thus, the said at least one dryer, which will have given up retained moisture while operating in intake mode, then has moist outgoing air from the outlet  16  passed through, to recharge said at least one dryer with moisture. Likewise, simultaneously, at least one of the other dryers previously operated in exhaust mode now operates in intake mode. Thus, incoming air passes through the said at least one other dryer to pick up moisture and dehumidifies the said at least one other dryer.  
         [0041]    The switch of operation mode is repeated for different dryers as time goes on, so that during prolonged operation of the fuel cell stack  12 , each of the plurality of dryers  300  will eventually have operated in both modes. This causes two main effects. Firstly, this ensures that the incoming air stream is humidified at a reasonably constant level. Correspondingly, the exhausted air stream is dehumidified. This is of particular advantage in cold climates. It ensures that moisture in air discharged from the external ports  80  of the dryers will not tend to immediately form frost or ice, which, over a period of time, can tend to build up and possibly block the ports in the apparatus.  
         [0042]    It can be appreciated that the first switch means  100  serves to select dryers that are to switch their operation modes. This selection can be made randomly or sequentially. Sequential selection is preferred since it allows every dryer equal chance to switch between two operation modes and avoids duplicate selection for any single dryer. Particularly, the sequential selection can be simply made by sequentially switching a first dryer port  30  from being connected to one of the first inlet port  24  and first outlet port  26  to the other of the first inlet port  24  and first outlet port  26 . Therefore, during each switch operation, one dryer switches from intake mode to exhaust mode while simultaneously another dryer switches from exhaust mode to intake mode. However, during each switch operation, more than one dryer can switch from intake mode to exhaust mode. Likewise, more than one dryer can switch from exhaust mode to intake mode. It can also be appreciated that the number of dryers switching from intake mode to exhaust mode may or may not be the same as that of the dryers switching from exhaust mode to intake mode.  
         [0043]    Hence, “sequential selection” should be construed as selection in a certain manner. It is not limited to switching operation mode of dryers according to their physical position. That is to say, physically adjacent dryers may, but need not switch operation mode one after another.  
         [0044]    Referring to FIG. 2, this shows a second embodiment of the apparatus. In this second embodiment, many components are similar to the first embodiment, and for simplicity and brevity, a description of these components is not repeated. Rather, these components are given the same reference numerals, and it will be understood that they function in the same manner as for the first embodiment.  
         [0045]    The sole additional element in this second embodiment is the provision of a second switch means  200 . The second switch means  200  has a second inlet port  34  connected to an external oxidant source, a second outlet port  36  for discharging oxidant exhaust and a plurality of second dryer ports  35  respectively connected to the external ports  80  of the plurality of dryers  300 . A pump or compressor  90  is provided for supplying oxidant from the external source into the second inlet port  36 . In this embodiment, the overall apparatus has only one inlet  34  and one outlet  36  interfacing with the environment. This provides better sealing and control of oxidant flow.  
         [0046]    In the same manner as described in the first embodiment of the present invention, the first switch means  100  determines, at any given time, whether a dryer  300  is connected to the inlet  14  or the outlet  16  of the fuel cell stack  12 . In a similar manner, the second switch means  200  determines, at any given time, whether a dryer  300  is connected to the inlet  34  or the outlet  36 . In order to ensure proper operation, the switch operation of the first switch means  100  has to be in phase with that of the second switch means  200 . In other words, the first and second switch means  100  and  200  should be synchronized so that when a dryer  300  is in fluid communication with the inlet  14  via the first switch means  100 , the second switch means  200  provides fluid communication between this dryer  300  and the second inlet port  34 . Meanwhile, for any dryer  300  in fluid communication with the outlet  16  via the first switch means  100 , the second switch means  200  permits fluid communication between said dryer and the second outlet port  36 .  
         [0047]    As mentioned above, the switch operation can be done sequentially or randomly. The switch means  100  and  200  may be a multi-way valve means. The plurality of dryers  300  may be disposed individually or adjacent to one another. Depending on the configuration of the switch means  100 ,  200  and the dryers  300 , the switch operation can be done “continuously” or gradually, as will be detailed below.  
         [0048]    Reference will now be made to FIGS.  3 - 7 , which show embodiments of dryers and switch means. As shown in FIG. 3, a plurality of dryers  300  are contained in a dryer housing  350  comprising multiple chambers  301 . For illustration only, the dryer housing  350  comprises five chambers  301 . It will be appreciated that the dryer housing  350  may comprise any number of chambers. Each chamber  301  is separated by partition walls  303  from adjacent chambers  301 , and filled with the humidity exchange media (not shown). Suitable exchange media comprises random oriented fiber based carbon paper, commercially available from E-TEK, or carbon cloth commercially available from W. L. Gore. The media can be coated with a desiccant material. When a humid gas stream passes through the chamber, humidity is retained in the media and later picked up by a dry gas stream flowing through the media. Hence, each chamber filled with exchange media operates as a dryer mentioned above.  
         [0049]    An end housing  140  is adapted to be mounted onto one end of the dryer housing  350 . The end housing  140  has a connection portion  141  and a dispersion portion  142 . The dispersion portion  142  of the end housing  140  has a plurality of compartments  150 , e.g. five compartments in this example, divided by partition walls  151 . The number and position of compartments  150  correspond to that of the chambers  301 . In other words, the compartments  150  and the chambers  301  are in alignment during operation.  
         [0050]    [0050]FIGS. 4 and 5 respectively show details of the end housing  140  and a rotary member  180  disposed therein. Referring to FIGS. 4 a  and  4   b,  the connection portion  141  of the end housing  140  has a chamber  160  for accommodating the rotary member  180 , and an associated open end. The connection portion  141  has a smaller diameter than the dispersion portion  142 . A journal  145  is provided at the center of the open end of the dispersion portion  142 . The journal  145  has an enlarged diameter portion  146 . Each compartment  150  has an opening  156  that is provided on the enlarged diameter portion  146  and fluidly communicates the chamber  160  with each compartment  150 , respectively.  
         [0051]    [0051]FIGS. 5 a - 5   d  show the detailed structure of the rotary member  180 . The rotary member  180  has a plurality of reduced diameter portions. Specifically, in this example, the rotary member  180  has a first segment  220 , a second segment  240 , and a third segment  260 , as well as a first reduced diameter portion  230  and a second reduced diameter portion  250 . The segments  220 ,  240  and  260  can have the same diameter. Likewise, the reduced diameter portions  230 ,  250  can have the same reduced diameter. Within an end surface  210  of the rotary member  180 , a slot  211  is provided. Preferably, slot  211  is arc shaped, and has a smaller radius of curvature than the end surface  210  of the first segment  220 . The slot  211  extends axially throughout the first segment  220 . On the outer wall  221  of the first segment  220  two slots can be provided, namely slots  212  and  213 . Slot  211  is in fluid communication with slot  212 .  
         [0052]    The rotary member  180  has an inner bore  214  extending axially throughout the length thereof. The inner bore  214  extends to a position adjacent to the end surface  210 , at which point it has a reduced diameter portion  215  for supporting a shaft  190  (FIG. 6) that extends therethrough after assembly. The inner bore  214  is isolated from slots  211  and  212 . Slot  213  is in fluid communication with inner bore  214 .  
         [0053]    The second reduced diameter portion  250  is provided with a plurality of holes that penetrate this portion, namely, a plurality of gas dispersion holes  251  and pinholes  253 . In a known manner, at least one of the pinholes  253  can be used to accommodate a pin (not shown) to fix the rotary member  180  to the shaft  190  so that the rotary member  180  rotates with the shaft  190  to disperse the gas streams, as will hereinafter be described.  
         [0054]    On an end surface  270  of the third segment  260 , a number of screw holes  271  are provided. These screw holes  271  are used to accommodate screws to enable the rotary member  180  to be removed from the end housing  140  during disassembly.  
         [0055]    As can be seen in FIG. 3, the journal  145  of the end housing  140  has a hub  149  used to accommodate the shaft  190 . A central bore  305  is also provided on the dryer housing  350  to accommodate the shaft  190 . Therefore, it can be appreciated that during operation, a shaft  190  can pass through the central bore  305 , hub  149  and inner bore  214  to support the dryer housing  350 , end housing  140 and the rotary member  180  respectively. However, only the rotary member  180  rotates with the shaft  190  while the end housing  140  and dryer housing  350  remain stationary. The rotary member  180  and the end housing  140  together operate as the first switch means  100  in a manner detailed below. Another end housing  140 ′ with a rotary members  180 ′ disposed within its chamber can be mounted onto the other end of the dryer housing  350  to operate as another switch means  200 , as shown in FIG. 6. As can been seen in FIG. 6, the shaft  190  also passes through the inner bore  214 ′ of the rotary member  180 ′. Likewise, the rotary member  180 ′ is also fixed to the shaft  190  in the above-described manner and rotates with the shaft  190 .  
         [0056]    The end housing  140  and the rotary member  180  are dimensioned such that each of the slots  212 ,  213  of the first segment  220  of the rotary member  180  are substantially aligned with each of a plurality of openings  156  (FIG. 4 a ) of the plurality of compartments  150  for dispersing gases when the rotary member  180  is disposed in the end housing  140 . Furthermore, at least the segment  240  of the rotary member  180  has substantially the same diameter as that of the chamber  160  of the end housing  140  such that segment  240  separates the chamber  160  into two inner spaces  440  and  441  when disposed therein. The other end housing  140 ′ and the other rotary member  180  are also dimensioned accordingly.  
         [0057]    In known manner, sealing means, such as O-rings can be provided between the rotary member, specifically, the first segment  220 , second segment  240 , and third segment  260  and the inner wall of the chamber  160 . The open ends of the end housings  140 ,  140 ′ are then closed. As mentioned above, the connection portion  141  has a smaller diameter than the dispersion portion  142 . This configuration is preferred since it reduces size of dynamic sealing, and hence the risk of leakage, between rotary members  180 ,  180 ′ and respective end housings  140 ,  140 ′.  
         [0058]    As shown in FIG. 6, during operation, an incoming oxidant stream  1  enters the apparatus from one side thereof through a gas port  143 , and flows into the inner space  440 . From here, the incoming oxidant  1  flows through the plurality of gas dispersion holes  251  located on the second reduced diameter portion  250 , into the inner bore  214 . Next, the incoming oxidant  1  flows along the length of the inner bore  214 , and exits the rotary member  180  through slot  213 . As the rotary member  180  is continuously rotating with the shaft  190 , the incoming oxidant  1  flows into one of the compartments  150  via a respective opening  156  when the rotary member  180  rotates into a position where slot  213  fluidly communicates with one of the openings  156 . As the incoming oxidant  1  is usually conveyed by a blower or compressor (not shown), the incoming oxidant  1  is forced to flow along the axial direction into the media  110  supported in at least one chamber  301  of the dryer housing  350 . Then, the incoming oxidant  1  continues to flow into the corresponding compartment  150 ′ of the other end housing  140 ′. From here, the incoming oxidant  1  flows through opening  156 ′ (not shown, but analogous to opening  156 ) and slot  213 ′ respectively, and enters the inner bore  214 ′ of the other rotary member  180 ′. Next, the incoming oxidant  1  flows along the length of the inner bore  214 ′, exits through the plurality of holes  251 ′ (not shown, but analogous to holes  251 ), passes through the inner space  440 ′, exits the apparatus through a gas port  143 ′, and passes into inlet  14  of the fuel cell stack  12 . As the incoming oxidant  1  flows across the media  110 , it picks up the heat and humidity retained in the media  110 . Since the rotary member  180  continually rotates with the shaft  190 , the incoming oxidant  1  flows through the whole cross section of the media  110  to pick up humidity from the media  110 .  
         [0059]    An outgoing oxidant stream  2  enters the apparatus through a gas port  144 ′ of the end housing  140 ′ from the outlet  16  of the fuel cell stack  12 , and flows into the inner space  441 ′. From here, the outgoing oxidant  2  passes through slots  211 ′ and  212 ′, respectively. The outgoing oxidant  2  then flows into one of compartments  150 ′ via a respective opening  156 ′ when the rotary member  180 ′ rotates into a position where slot  212 ′ fluidly communicates with one of the openings  156 ′. Next, the outgoing oxidant  2  flows through the media  110  supported in at least one chamber  301  of the dryer housing  350  to a corresponding chamber  150  of the end housing  140 . As the outgoing oxidant  2  flows along the media  110 , heat and humidity is retained in the media  110 . From here, the outgoing oxidant  2  flows through opening  156 , slots  212  and  211  respectively, and enters the inner space  441  of the end housing  140 . Next, the outgoing oxidant  2  exits the apparatus through a gas port  144  (FIG. 4 b ). Likewise, since the rotary member  180 ′ continually rotates with the shaft  190 , the outgoing oxidant  2  flows through the whole cross section of the media  110  to transfer humidity to the media  110 .  
         [0060]    As mentioned above, in order to ensure proper operation, the rotary members  180  and  180 ′ have to rotate in phase. It can be done by mounting the rotary members  180  and  180 ′ correspondingly on the shaft  190  since the two rotary members will then rotate together with the shaft  190 .  
         [0061]    [0061]FIG. 7 shows the relationship between the size of the slots  212 ,  213  and the openings  156 , in case of five compartments  150 . In this Figure, segment  212   a  and  213   a  respectively indicates a cord corresponding to the arc shaped slots  212  and  213 . Therefore, the two ends of each segment  212   a,    213   a  represent the two ends of each slot  212 ,  213 , respectively. As illustrated in FIG. 7, the rotary member  180  is rotating in a clockwise direction. The slots  212 ,  213  and the openings  156  should be sized such that at the moment the slot  213  leaves a compartment, the slot  212  does not communicate with the same compartment. In the case of compartment  150   b  when the fluid communication between the slot  213  and the opening  156   b  is cut off, the slot  212  does not fluidly communicate with the same opening  156   b.  Similarly, when the fluid communication between the slot  212  and opening  156   d  is cut off, the slot  214  does not fluidly communicate with the opening  156   d.  In other words, at any time, any one of compartment  150 , chamber  301 , and compartment  150 ′, will only contain either one of the incoming or outgoing oxidant streams. The partition walls  151  separate each compartment so that the incoming and outgoing oxidant streams will never mix. To ensure no mixing of the gas streams, the size of the slots  212 ,  213  of the rotary member  180  and the size of the openings  156  are selected depending on the actual number of chambers  301  and compartment  150 .  
         [0062]    As will be appreciated from the above description, by continuous rotating of the shaft  190 , and hence the rotary members  180  and  180 ′, the switch operation of the first and second switch means  100  and  200  can be done “gradually”. It will also be appreciated that at any give time, it is possible that not all the dryers are working, i.e. having an oxidant stream flowing therethrough. For example, at the moment shown in FIG. 7, compartment  150   b,  and hence the corresponding chamber, does not have any oxidant stream. It is considered to be in a neutral position. However, although not preferred, the slots  212 ,  213  can also be suitably sized so that such neutral positions do not exist.  
         [0063]    It is to be understood that although in the above example, the dryer housing  350 , the connection portion  142 ,  142 ′ and dispersion portion  143 ,  143 ′ of the end housing  140 ,  140 ′, and the first, second and third segments  220 ,  240 ,  260  are all described as cylindrical in shape, the actual shape may vary as will be required in particular situations. They may also have different perimetrical extents at different axial positions. Therefore, the words “diameter” and “radial” should not be understood to restrict to cylindrical shape.  
         [0064]    Reference will now be made to FIGS. 8, 9 and  10 , which show three separate embodiments of an apparatus for effecting drying of the fuel stream in a fuel cell stack. In particular, this technique is particularly intended for a fuel stream comprising hydrogen, although it will be recognized by those skilled in the art that this technique has applicability to a wide range of other fuels. An example of another fuel is a hydrogen rich reformate fuel, i.e. a fuel produced by reforming a hydrocarbon fuel, to produce a gas mixture rich in hydrogen.  
         [0065]    Referring to FIG. 8, a first embodiment of the apparatus for drying the anode flow is indicated generally by the reference  40 . It again includes a fuel cell stack indicated generally at  42 , and a fuel inlet  44  and a fuel outlet  46  are provided. A main hydrogen or fuel inlet  48  is provided immediately upstream from the stack fuel inlet  44 .  
         [0066]    The outlet  46  is connected to a water separator  50  and then to a T-connector  52 . One branch of the T-connector  52  is connected through a pump  54  back to the fuel inlet  44 .  
         [0067]    The other branch of the T-connector  52  is connected through a shut-off valve  56  and then through a dryer  58  to a vent port  60 .  
         [0068]    In a normal mode of operation, the shut-off valve  56  is closed, and the pump  54  actuated to cycle hydrogen through the stack  42 .  
         [0069]    As is known, a common problem with fuel cells is that nitrogen tends to diffuse across the membrane from the cathode side to the anode side and consequently, after a period of time, nitrogen tends to build up on the anode or hydrogen side of the stack. Additionally, there can be a problem with build-up and moisture on the membrane.  
         [0070]    For these two reasons, periodically, for example every 5 minutes, the anode side can be purged. For this purpose, a shut-off valve  56  is opened for a short period, for example 5 seconds, to vent gas through the dryer  58  to the vent port  60 . Typically, the anode side is operated at a slight positive pressure. Opening the valve  56  causes the pressure pulse to pass through the stack, which can have the effect of causing the water to “jump out of” pores of the electrodes and gas diffusion media. In any event, whatever the exact mechanism, it has been found that an abrupt and sharp purge cycle tends to promote venting of excess moisture, in addition to built up and unwanted gases.  
         [0071]    At the end of the 5 second purge cycle, the valve  56  is closed again.  
         [0072]    The dryer  58  serves to ensure that gas vented through the vent port  60  has a low level of humidity. This can be desirable in certain circumstances. In particular, in cold climates, this ensures that there is no problem with moisture and the vented gas tending to form frost and ice particles and build up on or around the apparatus.  
         [0073]    The dryer  58  can be replaced at suitable intervals, e.g. when replacing the fuel that supplies the hydrogen, where hydrogen is supplied from a cylinder. Alternatively, it may be possible to provide some variant configuration in which incoming fuel is passed through the dryer  58  to pick up moisture accumulated therein.  
         [0074]    In FIGS. 9 and 10, components common to FIG. 9 are given the same reference numerals. For the reasons given above, a description of these components is not repeated, for simplicity and brevity.  
         [0075]    Thus, in FIG. 9, a dryer  62  is provided between the separator  50  and the T-connector  52 . The shut-off valve  56  is then provided immediately above the T-connector  52  as before, but here is connected directly to a vent port  60 .  
         [0076]    [0076]FIG. 9 functions, in use, in effect, to maintain a desired humidity level within the anode side of the fuel cell stack  42 . Thus, excess moisture can be separated in the separator  50 , but it is anticipated that the dryer  62  will run in an essentially saturated condition, so as to maintain humidity at a desired level.  
         [0077]    Again, as for FIG. 9, the shut-off valve  56  can be opened periodically, e.g. every 5 minutes for purge cycle of, for example, 5 seconds. This again prevents build up of nitrogen in the anode side of the stack. To the extent that water is removed from the fuel cell from the purge cycle, this water would be either separated by the separator  50 , in the case of water droplets, or otherwise absorbed by the dryer  62 .  
         [0078]    To the extent that dryer  62  is used to maintain a constant humidity level, it should not be necessary to exchange the dryer at any time. However, it may be desirable to replace the dryer from time to time, as contaminants may tend to build up in the dryer  62 .  
         [0079]    Finally, with reference to FIG. 10, the third embodiment of the anode aspect of the invention includes all the elements of FIG. 9. It additionally includes a second hydrogen inlet  72 , a hydrogen control valve  74  and a second shut-off valve  76 .  
         [0080]    In normal use, this third embodiment functions in much the same manner as the first embodiment of FIG. 8. Thus, hydrogen is usually supplied through the main fuel inlet  48 . The pump  54  is run to cycle hydrogen continuously through the separator  50 .  
         [0081]    Theoretically, again for example every 5 minutes, a short purge cycle (again, for example 5 seconds) can be effected by opening the shut-off valve  56 . Simultaneously, the second shut-off valve  76  is opened. This again permits gas to vent from the anode side of the stack through the dryer  58  to the vent port  60 .  
         [0082]    Now, when moisture builds up in the dryer  58 , periodically the supplied hydrogen is switched from the main fuel inlet  48  to the second hydrogen inlet  72 . For this purpose, a valve (not shown) will be closed to close off the main fuel inlet  48 . Simultaneously, the hydrogen control valve  74  would be opened. The second shut-off valve  76  would remain closed and the first shut-off valve  56  opened. This permits supply of hydrogen from the second hydrogen inlet  72  through the dryer  58  towards the anode side of the stack  42 .  
         [0083]    The pump  54  would be run as before. Consequently, hydrogen will be cycled through the stack and the water separator  50 . As hydrogen is consumed, fresh hydrogen will be supplied from the inlet  72 , and this hydrogen would be humidified in the dryer  58  thereby serving to remove moisture from the dryer  58  and recharge the dryer.  
         [0084]    After a suitable period of time, the hydrogen control valve  74  will be closed and hydrogen supply would be recommenced through the main hydrogen or fuel inlet  48 . The dryer  58  would then be in a dried or recharge condition, ready to recover moisture from gas during the purge cycle.  
         [0085]    The advantage of this embodiment, as compared to that of FIG. 9, is that it recovers moisture and uses it to add humidity to incoming hydrogen. At the same time, it does not require replacement of the dryer, to effect recharging of the dryer.  
         [0086]    Where humidification is provided just on the cathode side, it is recognized that, in use, water is generated primarily on the cathode side, due to proton migration through the membrane. For this reason, water recovery from the cathode side can be optimal. Nonetheless, depending on the operating conditions, significant moisture can be generated or occur on the anode side. For example, if the oxidant side is maintained at a significantly higher pressure than the anode or fuel side, then water generated during reaction can be caused to flow back through the membrane, so that a significant quantity of water appears on the anode side and so that the exhausted anode fuel stream is significantly humidified. In such cases, recovering or controlling moisture in the exhausted fuel stream is desirable.