Abstract:
Access openings in a fuel cell assembly are isolated from their surroundings by use of a closure member that is actuated by the pressure in a fluid stream within the fuel cell assembly. An isolating apparatus prevents undesirable water loss or gain in certain fuel cell types and protects the fuel cell assembly from contamination.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a method and apparatus for isolating a fuel cell assembly from its surroundings, such as for protection against dehydration during shutdown or for other purposes, such as protection against contaminants. 
     BACKGROUND OF THE INVENTION 
     Electrochemical fuel cells convert reactants, namely, fuel and oxidant, into reaction products and in the process generate electric power. In a typical fuel cell using hydrogen gas as fuel and oxygen or compressed air as oxidant, the reaction product is water. 
     Solid polymer fuel cells typically employ a membrane electrode assembly (MEA) comprising a solid polymer electrolyte or ion exchange membrane disposed between two electrode layers. The membrane, in addition to being an ion conductive (typically proton conductive) material, also acts as a barrier for isolating the reactant fluid streams from each other. The MEA is interposed between two electrically conductive separator plates impermeable to the reactant flow streams and having flow channels forming flow fields to direct the fuel and oxidant to the respective electrode layers. 
     In a fuel cell stack, a plurality of fuel cells are connected together to increase the overall output power of the assembly. The fuel cell stack is interposed between a pair of end plates have inlet and outlet ports associated therewith for feeding and exhausting the oxidant and fuel fluid streams, respectively. The end plates may also have inlet and outlet ports associated therewith for flowing a coolant stream through the stack. 
     It is desirable that the ion-exchange membrane in the fuel cell stack be kept moist to maintain adequate ionic conductivity and to reduce structural damage that may result if the membranes are allowed to become too dry. It is known that leaks in membranes may occur near reactant stream inlet ports. Such leaks may be caused or worsened by membrane dehydration during fuel cell stack operation, thereby resulting in the formation of cracks or holes. 
     In a phosphoric acid electrolyte fuel cell stack, it is desirable to prevent water from entering the stack during shutdown periods. Additionally, it is desirable in some applications to protect the fuel cell stack from contaminants or other hazards, such as exposure to salt water, particularly in marine applications. 
     SUMMARY OF THE INVENTION 
     An improved method isolates an electrochemical fuel cell assembly from its surroundings, the fuel cell assembly having an access opening formed therein. The access opening is either of a reactant stream inlet port or a reactant stream outlet port for directing a working fluid stream (such as, for example, a coolant stream) to or from the fuel cell stack or other component in the fuel cell assembly (for example, a reactant stream humidifier). In a preferred embodiment, the access opening is an opening formed in an enclosure containing the fuel cell assembly. The improved method comprises the steps of: 
     (a) providing the access opening with a closure member that is normally biased to a closed position in which the access opening is closed, the closure member having a pressure activated actuator for urging the closure member to an open position when the actuator is exposed to pressure; and 
     (b) operatively connecting the actuator to a fluid stream of the fuel cell assembly such that the actuator is exposed to fluid pressure of the fluid stream for urging the closure member to the open position, thereby opening the access opening. 
     In preferred embodiments, the access opening is an oxidant stream inlet or an oxidant stream outlet, and the fluid stream that provides the actuating pressure is a fuel stream. 
     An improved fuel cell assembly has an access opening formed therein through which the assembly is exposed to its surroundings. The improved assembly comprises a closure member that is movable between an open position and a closed position, the closure member being operatively associated with the access opening and normally biased to the closed position such that when the access opening is closed the fuel cell assembly is isolated from its surroundings. In operation, the closure member preferably includes a pressure-activated actuator for urging the closure member to the open position when the actuator is exposed to pressure. The actuator optionally comprises a conduit fluidly connected to a fluid stream of the fuel cell assembly for exposing the actuator to the fluid stream pressure and urging the closure member to the open position, thereby opening the access opening. 
     An improved fuel cell assembly also comprises a solid polymer electrolyte fuel cell comprising first and second fluid flow channels and an isolation valve that is switchable between a closed position and an open position for, respectively, closing and opening said first fluid flow channel. The valve is normally biased to the closed position for closing the first fluid flow channel during shutdown of the fuel cell assembly. The valve further includes an actuator that is responsive to fluid pressure in the second fluid flow channel during start up of the fuel cell assembly for switching the valve to the open position. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a three-dimensional, perspective view of an end plate of a humidity exchanger used in a fuel cell assembly in which the end plate includes an isolation valve; 
     FIG. 2 is an exploded perspective view of the stack isolation valve illustrated in FIG. 1; 
     FIGS. 3 a  and  3   b  are schematic illustrations of, respectively, a fuel cell assembly comprising a humidity exchanger in which the humidity exchanger includes a pair of isolation valves, and a similar fuel cell assembly but without a humidity exchanger in which the fuel cell stack includes a pair of isolation valves; and 
     FIG. 4 shows the fuel cell assembly of FIG. 3 b  also provided with an enclosure having louvers that are capable of being operated in conjunction with the isolation valves. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Referring first to FIGS. 1 and 2, reference numeral  10  generally designates an end plate of an oxidant humidity exchanger used in a fuel cell assembly for purposes of humidifying an oxidant stream to be supplied to a solid polymer electrolyte fuel cell stack. Humidity exchanger end plate  10  has an inlet  12  for an oxidant stream and is provided with an isolation valve  14  which is housed in a pair of recesses  16  and  18  provided in the end plate  10 . Recess  16  surrounds oxidant stream inlet  12  and has a bore  13  extending through end plate  10 . 
     Isolation valve  14  comprises a rubber valve face  20  on a valve stem  22  located in recess  16 . Valve face  20  cooperates with a valve seat  24  in recess  16  for closing bore  13  to the flow of an oxidant stream therethrough. Valve stem  22  is centred in inlet  12  and bore  13  using centering guides  15   a  and  15   b , respectively. 
     Valve  14  further comprises a diaphragm plunger  26  housed in recess  18 . Diaphragm plunger  26  has a stem  28 , which is received in a bore  30  in recess  18  for guiding plunger  26 . A rocker  32  extends between plunger  26  and valve stem  22 . At one end, rocker  32  is received in a slot  34  in the stem  22  and at its other end, it is received in a slot in plunger  26 . The latter slot is not visible in the view shown in FIG.  2 . 
     Valve  14  further comprises a cover plate  36  which is mounted on the end plate  10  by means of screws (not shown) and a gasket  38  which is interposed between cover plate  36  and end plate  10  for forming an air-tight seal. 
     As further shown in FIG. 2, cover plate  36  includes an opening  40  for receiving a pilot line extending from a fuel supply line feeding the stack to which end plate  10  is attached with fuel. Oxidant stream inlet  12  is also located on cover plate  36  and includes a hose fitting  41  for connection to an oxidant fluid stream. 
     Gasket  38  includes a diaphragm  42 , which is located between the opening  40  on the one side and plunger  26  on the other side. Gasket  38  further has an opening  44 , which is located adjacent oxidant stream inlet  12 . A spring  46  urges valve face  20  against valve seat  24 , thereby closing bore  13  to the flow of oxidant therethrough. This is the case when there is little or no fuel stream flow in the fuel supply line and consequently little or no pressure in the pilot line. When the fuel supply is opened, fluid pressure is exerted on diaphragm  42  through the pilot line. The exertion of fluid pressure on diaphragm  42  through the pilot line depresses diaphragm  42  and plunger  26 , which in turn causes rocker  32  to urge valve face  20  away from valve seat  24  against the pressure of spring  46 , thereby opening valve  14  and maintaining it in an open position until the pressure in the pilot line drops when the fuel supply is shut off. 
     In the above example, isolation valve  14  is described as being provided at oxidant inlet  12  of a humidity exchanger. It will be appreciated, however, that an isolation valve  14  can also be provided at the oxidant inlet or outlet of the fuel cell stack. 
     FIGS. 3 a  and  3   b  show solid polymer electrolyte fuel cell assemblies with and without an oxidant humidity exchanger, respectively. In FIG. 3 a , a fuel cell stack  50  employs a humidity exchanger  55  having an oxidant inlet  12   a  and humidified oxidant outlet  12   b , which also serves as the oxidant inlet to fuel cell stack  50 . Further, humidity exchanger  55  has an inlet  48   a  that receives exhaust oxidant from fuel cell stack  50 . After exchanging water vapor with the oxidant supplied at oxidant inlet  12   a , the water-depleted exhaust oxidant is vented at outlet  48   b.    
     As shown in FIG. 3 a , humidity exchanger inlet  12   a  and outlet  48   b  each include an isolation valve  14 . FIG. 3 b  shows a fuel cell assembly that is similar to that of FIG. 3 a , except that no humidity exchanger is employed in the fuel cell assembly of FIG. 3 b . In the fuel cell assembly of FIG. 3 b , fuel cell stack  50  has an oxidant inlet  12  and oxidant outlet  48 , each of which has an associated an isolation valve  14 . 
     In both of FIGS. 3 a  and  3   b , reference numeral  51  designates an air filter and reference numeral  52  designates an air pump for providing compressed air (oxidant) to oxidant inlet  12   a  and  12 , respectively. Fuel (such as, for example, hydrogen gas) is supplied from a fuel container  54  via fuel valve  56  and pressure regulator  58  to fuel cell stack  50  through fuel supply line  60 . Unreacted fuel is exhausted through outlet pipe  63 . 
     As shown in FIGS. 3 a  and  3   b , each isolation valve  14  is connected to fuel supply line  60  by means of a pilot line  62 . Valves  14  are thus automatically opened when fuel valve  56  is opened to supply fuel to stack  50 . When stack  50  is shut down, valves  14  close, thereby isolating the cathode flow fields in stack  50  from the external atmosphere. Such isolation prevents the water residing within the cathode flow fields from evaporating and making stack  50  more difficult to start. 
     The foregoing principles can also be applied to isolate an entire fuel cell system from its environment. In FIG. 4, the system of FIG. 3 b  is shown schematically as including an enclosure  64  having louvers  66 , which can open and close. The opening and closing of louvers  66  is effected by means of diaphragm-actuated controllers  68 , which are connected by pilot lines  70  to fuel supply line  60 , in a manner similar to isolation valves  14 . Controllers  68  operate in the substantially the same way as valves  14  in that controllers  68  open louvers  66  when fuel valve  56  is opened (for example, by an actuator button (not shown) on the exterior of enclosure  64 ). When the fuel is shut off (and the flow of the fuel stream thereby discontinued), louvers  66  are closed and the system is isolated from the environment. The system is thus protected from contamination and can even tolerate submersion in salt water, which is particularly advantageous in the case of marine applications. 
     Isolation valves may also be provided at a fuel inlet and/or outlet or at a coolant inlet and/or outlet of a fuel cell stack. Isolation valves may also be provided at the inlet and/or outlet of any other working fluid in the fuel cell assembly. In addition, the oxidant supply line or other source of pressure may be employed instead of the fuel supply line to actuate the isolation valve. 
     While particular elements, embodiments and applications of the present invention have been shown and described, it will be understood, of course, that the invention is not limited thereto since modifications may be made by those skilled in the art without departing from the scope of the present disclosure, particularly in light of the foregoing teachings.