Abstract:
Hydrogen in a fuel cell is vented and purged with a fail-closed valve using stored energy (e.g. from a capacitor) when a diagnostic parameter is outside of an acceptable operating range. The valve closes after the stored energy has depleted. A safety switch in the relay circuit of the solenoid switch (or solenoid valve) is also grounded in computer-implemented shutdowns. Benefits from the invention include use of air-compatible catalysts, minimized parasitic losses to power output, minimized contamination of fuel cell internal surfaces after venting, minimized risk of explosive mixture buildup, and efficient operation.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates to an apparatus and to a method for shutdown and venting of a fuel cell stack in a fuel cell power system.  
       BACKGROUND OF THE INVENTION  
       [0002]     Fuel cell power systems convert a fuel and an oxidant to electricity. One fuel cell power system type of keen interest employs use of a proton exchange membrane (hereinafter “PEM”) to catalytically facilitate reaction of fuels (such as hydrogen) and oxidants (such as air/oxygen) into electricity. The PEM is a solid polymer electrolyte that facilitates transfer of protons from the anode to the cathode in each individual fuel cell of the stack of fuel cells normally deployed in a fuel cell power system.  
         [0003]     In a typical fuel cell assembly (fuel cell stack) within a fuel cell power system, individual fuel cells have flow fields with inlets to fluid manifolds; these collectively provide channels for the various reactant and cooling fluids reacted in the fuel cell stack to flow into each cell. Gas diffusion assemblies then provide a final fluid distribution to further disperse reactant fluids from the flow field space to the reactive anode and cathode.  
         [0004]     Coordinated shutdown is one factor in effective operation of a fuel cell stack or set of fuel cell stacks. In this regard, hydrogen is a substantive component of the fuel fed to the reactive anodes of the fuel cell stack, and, upon shutdown, a working inventory of hydrogen-containing fuel is present in the fuel reactant channels of the fuel cell stack. While not inherently flammable as relatively high concentration hydrogen, the residual fuel in the stack is subject to potential mixing with oxygen in air, especially if the fuel cell is inoperable for a period of time. Such mixing of oxygen has the potential to create a potentially flammable mixture within the fuel cell stack at a certain threshold composition.  
         [0005]     Two general shutdown strategies or modes represent the scope of shutdown approaches of a fuel cell system: a “normal” shutdown mode and a “rapid” shutdown mode. “Normal” shutdown proceeds through a process of (a) disconnecting the load, (b) consuming excess hydrogen, and (c) cooling the system in a manner to minimize internal stresses induced from thermal change. With a “rapid” shutdown, conditions are present such as a malfunction or error detection which require the fuel cell to be shut down in a manner which does not enable the time needed for consuming excess hydrogen. When a “rapid” shutdown is implemented, the residual fuel is handled by venting reformate or hydrogen to the atmosphere.  
         [0006]     There are typically two venting solenoid valves for fuel reactant in a conventional fuel cell stack of a fuel cell power system: a first valve at the outlet of the fuel processor generating the fuel and a second valve at the outlet of the anode manifolds of the stack. During normal operation, these venting valves are closed to prevent hydrogen leakage and/or discharge to the atmosphere. Conventional fuel cell power systems not having a fuel processor typically use at least one vent valve on the stack anode. During a “rapid” shutdown, each venting valve opens to vent hydrogen to the atmosphere. This approach rapidly removes potential and thermal energy from the system and vents residual hydrogen within the fuel cell.  
         [0007]     These vent valves have traditionally been provided as normally-open valves in which the valve is biased open via spring returns. As should be apparent, in the powered-down state, these valves are open and therefore allow fluid flow into and out of the anode manifolds. The fail-open valve is deployed so that the system vents fuel out the combustible vent even during a full loss of system power (usually the most dramatic “rapid” shutdown situation). As should also be apparent, a normally-open vent valve remains open after the system has shutdown in the continued absence of any electrical power. Such as when the system is turned off.  
         [0008]     One disadvantage, however, in using normally-open vent valves is that, in some particular shutdown instances where the vent valves remain open, some residual hydrogen remains in the fuel cell manifolds. When a long downtime ensues, it is possible for air to flow through the vent over time until a combustible mixture occurs in some part of the system. It is also possible for a combustible mixture to evolve when power failure occurs after a “normal” shutdown executed without a purge (e.g., a “normal shutdown” initiated under controlled conditions which deteriorates into a “rapid” shutdown due to power loss).  
         [0009]     Another disadvantage in using normally-open vent valves is that catalyst deactivation may ensue after shutdown of the fuel cell and loss of power. In this regard, a number of catalysts deactivate in the presence of air, especially immediately after shutdown when the catalyst is still thermally “hot”. One approach to handling this concern is to use an air tolerant catalyst; but this approach severely hampers the choice of possible catalysts. Another approach is to provide a purge mechanism whereby hydrogen is purged from the system with an inert gas such as nitrogen to provide a nitrogen blanket over the active catalyst elements. Such alternatives define a more expensive design than is achievable with a catalyst not as air tolerant.  
         [0010]     A further disadvantage in using normally-open vent valves is that, in powered-down mode, a path to the fuel processor and/or stack manifolds is available for dust and other potentially harmful elements to contaminate the internal channels and surfaces of the fuel cell. Such contamination shortens fuel cell life and also may diminish fuel cell performance in comparison to a fuel cell which is not contaminated.  
         [0011]     An additional disadvantage in using normally-open vent valves derives from controller lockup where the control computer for the fuel cell system establishes fail-last control element positioning. In this context, the normally-open vent valves may be inappropriately sustained in a closed position.  
         [0012]     Yet another disadvantage in using normally-open vent valves derives from the fact that all normally-open valves must be energized to remain shut—the primary operating state of vent valves. As such, these normally-open vent valves constitute a parasitic power burden on the fuel cell power system in normal real-time operation, inherently lowering overall operating system efficiency.  
         [0013]     What is needed is a holistic approach to fuel cell venting and purging which provides coordinated shutdown of the fuel cell at low cost, protection of the catalyst after a power failure in the fuel cell power system, a basis for appropriate shutdown of the fuel cell stack and/or fuel cell stack set when conditions collectively indicate the need for such an operational event, and optimal efficiency in fuel cell power system operation. The present invention is directed to fulfilling this need.  
       SUMMARY OF THE INVENTION  
       [0014]     The present invention provides a fuel cell with a normally-closed valve in fluid connection with the fuel reactant flow field(s) to vent the fuel reactant gases from the fuel reactant flow field(s). The normally-closed valve is operably disposed in an energizing circuit configured to open the normally-closed valve when a control signal is less than a shutdown threshold value.  
         [0015]     As a method, the invention provides a fuel cell where venting of the fuel reactant from the fuel reactant flow field(s) is done with a normally-closed valve when a control signal has a value less than a shutdown threshold value. The invention further provides for closing of the normally-closed valve after the fuel reactant vents to a safe quantity in the fuel cell.  
         [0016]     In one form, the invention provides for using a normally-closed solenoid switch in a control circuit so that the normally-closed valve vents the fuel reactant from the fuel cell when the solenoid relay circuit is powered at a voltage below a shutdown threshold voltage value. In another form, the invention provides for using a solenoid valve with the energizing circuit linked directly to the relay circuit of the valve.  
         [0017]     The invention also provides for use of a safety switch in the solenoid relay circuit. An output voltage is measured with the control computer, an operational status variable is defined using executable logic in the computer from the output voltage measurement, and the solenoid relay circuit is grounded with the safety switch when the operational status variable is defined to a shutdown value.  
         [0018]     The invention also provides for use of discrete electrical energy storage (e.g., a capacitor) of sufficient magnitude to open the normally-closed valve until the fuel reactant vents from the fuel cell. In this regard, the invention provides for storing sufficient electrical energy to sustain the normally-closed valve in an open position until the fuel reactant vents to a satisfactory level in the venting step, opening the normally-closed valve with the stored electrical energy to vent the fuel reactant, and closing the normally-closed valve after the stored electrical energy has been depleted.  
         [0019]     Benefits from the present invention include (a) cost savings from eliminating the need to use of air-compatible catalysts; (b) minimized parasitical losses to power output from the fuel cell stack venting system; (c) minimized contamination of internal fuel cell stack manifolds and surfaces after venting; and (d) an avoidance of a combustible air/fuel mixture in the anode after shutdown.  
         [0020]     Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the preferred embodiment of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0021]     The present invention will become more fully understood from the detailed description and the accompanying drawings, wherein:  
         [0022]      FIG. 1  presents a fuel cell power system block flow diagram;  
         [0023]      FIG. 2  shows a vent valve control circuit in tripped state (closed position to discharge the capacitor of the circuit) for controlling two vent valves with one normal shutdown controller interface and one energy storage capacitor;  
         [0024]      FIG. 3  shows a vent valve control circuit in tripped state (closed position to discharge the capacitor of the circuit) for controlling two vent valves with two normal shutdown controller interfaces and one energy storage capacitor; and  
         [0025]      FIG. 4  shows dual vent valve control circuits with each in tripped state (closed position to discharge the capacitor of the circuit) for controlling two vent valves with two normal shutdown controller interfaces and two energy storage capacitors. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0026]     The following description of the preferred embodiment is merely exemplary in nature and is in no way intended to limit the invention, its application, or uses.  
         [0027]     Real-time computer process control is generally implemented to control the fuel cell power system described herein. In this regard, real-time computer processing is broadly defined as a method of computer processing in which an event causes a given reaction within an actual time limit, and wherein computer actions are specifically controlled within the context of and by external conditions and actual times. As an associated clarification in the realm of process control, real-time computer controlled processing relates to the performance of associated process control logical decision, and quantitative operations intrinsic to a process control decision program functioning as part of a controlled apparatus implementing a process (such as the fuel cell benefiting from the present invention) wherein the process control decision program is periodically executed with fairly high frequency usually having a period of between 20 ms and  2  sec for tactical control.  
         [0028]     The preferred embodiments use at least one normally-closed valve to vent fuel reactant from the fuel cell stack. In this regard, a normally-closed valve is a valve, for automatic control, closed to fluid transmission when electrical power (or pneumatic pressure) is absent from the actuator of the valve. It should also be noted that a normally-closed switch is usually a spring-loaded switch which is positioned (“electrically closed”) to enable electrical current transmission when electrical power (or any counter-force to the spring) is absent from the actuating relay circuit of the (spring-loaded) switch.  
         [0029]     In one embodiment, an electrical circuit having a capacitor energizes the normally-closed vent valve from the capacitor when a solenoid relay of the circuit is in a power-off or shutdown condition so that a (first) electrical switch in mechanical linkage with the relay electrically “closes” for a period of time to enable the capacitor to discharge through the actuator of the valve. The capacitor is sized to hold the normally-closed vent valve open for a time period sufficient for venting fuel cell fuel reactant to a satisfactory level in the system.  
         [0030]     During normal (non-rapid-shutdown mode) operation in a preferred embodiment, the real-time computer controller energizes the solenoid relay to a position enabling the computer controller to control the vent valves in real-time operation through a second electrical switch of the solenoid. Note that the non-energized solenoid relay of this embodiment with two active electrical switches provides a closed circuit with respect to enabling the energy-storing capacitor to discharge even as the energized solenoid relay provides a first closed contact via the second electrical switch between the control computer and the vent valve, and a second closed contact via the first electrical switch to enable charging of the capacitor. The charging circuit of the capacitor via the first electrical switch is further stabilized by a diode.  
         [0031]     Power loss in the system below a threshold value releases the solenoid relay into a position where the first electrical switch is commensurately positioned to discharge the capacitor through the actuator of the fail-closed valve. The capacitor discharges the stored energy for a particular duration of time until it has essentially completely discharged its energy store. During this discharge of energy, the vent valve is energized and hence remains in an open position until the electromagnetic force from the solenoid is overcome by the spring force of the valve causing the valve to close. Thus, the vent valve defaults into a fluidly closed configuration since there is no longer sufficient power to sustain it in open position against its closing spring. This action reseals the fuel cell stack to prevent the anode side of the fuel cell from ongoing exposure to the atmosphere in the shutdown state after the residual fuel has been vented or purged from the fuel cell.  
         [0032]     The present invention is further understood with reference to a generic fuel cell power system. Therefore, before further describing the invention, a general overview of the power system within which the present invention operates is provided. In the system, a hydrocarbon fuel is processed in a fuel processor, for example, by reformation and partial oxidation processes, to produce a reformate gas which has a relatively high hydrogen content on a volume or molar basis. Therefore, reference is made to hydrogen-containing as having relatively high hydrogen content. The invention is hereafter described in the context of a fuel cell fueled by an H 2 -containing reformate regardless of the method by which such reformate is made. It is to be understood that the principles embodied herein are applicable to fuel cells fueled by H 2  obtained from any source, including reformable hydrocarbon and hydrogen-containing fuels such as methanol, ethanol, gasoline, alkaline, or other aliphatic or aromatic hydrocarbons or hydrogen storage-based systems.  
         [0033]     As shown in  FIG. 1 , a fuel cell power system  100  includes a fuel processor  112  for catalytically reacting a reformable hydrocarbon fuel stream  114 , and water in the form of steam from a water stream  116 . In some fuel processors, air is also used in a combination partial oxidation/steam reforming reaction. In this case, fuel processor  112  also receives an air stream  118 . The fuel processor  112  contains one or more reactors wherein the reformable hydrocarbon fuel in stream  114  undergoes dissociation in the presence of steam in stream  116  and air in stream  118  (optionally oxygen from oxygen storage tank  118 ) to produce the hydrogen-containing reformate exhausted from fuel processor  112  in reformate stream  120 . Fuel processor  112  typically also includes one or more downstream reactors, such as water-gas shift (WGS) and/or preferential oxidizer (PrOx) reactors that are used to reduce the level of carbon monoxide in reformate stream  120  to acceptable levels, for example, below 20 ppm. H 2 .containing reformate  120  or anode feed stream is fed into the anode side flow fields of fuel cell stack system  122 . Concurrently, a cathode feed stream in the form of air in stream  124  is fed into the cathode side flow fields of fuel cell stack system  122 . The hydrogen from reformate stream  120  and the oxygen from oxidant stream  124  react in fuel cell stack system  122  to produce electricity.  
         [0034]     Anode exhaust (or effluent)  126  from the anode side of fuel cell stack system  122  contains some unreacted hydrogen. Cathode exhaust (or effluent)  128  from the cathode side of fuel cell stack system  122  may contain some unreacted oxygen. These unreacted gases represent additional energy recovered in combustor  130 , in the form of thermal energy, for various heat requirements within power system  100 . Specifically, a hydrocarbon fuel  132  and/or anode effluent  126  are combusted, catalytically or thermally, in combustor  130  with oxygen provided to combustor  130  either from air in stream  134  or from cathode effluent stream  128 , depending on power system  100  operating conditions. Combustor  130  discharges exhaust stream  154  to the environment, and the heat generated thereby is directed to fuel processor  112  as needed.  
         [0035]     Vent valve  172  is operably disposed in anode feed stream  120  between the hydrogen source  112  and the fuel cell  122 . Likewise, vent valve  174  is operably disposed in anode exhaust stream  126 . Vent valves  172 ,  174  provide normally closed valves for venting fuel reactant from fuel cell stack  122  to the atmosphere when a diagnostic parameter measured by sensor  170  falls outside of an acceptable operating range thus indicating that a shutdown process should be initiated. Real-time computer  164  effect issues a control signal to control of vent valves  172 ,  174  in response to a signal from sensor  170 . The hydrogen feed to fuel cell stack system  122  is controlled in part through manipulation of vent valves  172 ,  174  by real-time computer  164  with respect to measurements from sensor  170  in enabling hydrogen-containing gas to flow to fuel cell system  122 .  
         [0036]     Sensor  170  is illustrated in  FIG. 1  as measuring a diagnostic parameter associated with or internal to fuel cell stack  122 . Exemplary stack diagnostic parameters may include: voltage of a cell, a cluster of cells or the fuel cell stack; pressure differential across the stack inlet (i.e., the anode and cathode inlets); relative humidity within the fuel cell stack; or operating temperature of the fuel cell stack. The diagnostic parameter may also be associated with other components of the fuel cell power system  100  outside of an external to the fuel cell stack  122  such as the fuel processor  112 , the fuel, air or water supply for feed streams  114 ,  116 ,  118  or the combustor  130 . An exemplary system diagnostic parameter includes operating parameters of an air compressor providing air feed stream  118  such as compressor speed or air discharge temperature. The diagnostic parameter may be determined directly (i.e., measured) from sensor  170  as indicated in  FIG. 1  such as a measured cell voltage. Alternately, the diagnostic parameter may be determined indirectly (i.e. computed based on an analytical or empirical model) such as computing the relative humidity based on the pressure, temperature and load on the fuel cell stack. Thus, one skilled in the art will recognize that the present invention has utility with a wide variety of diagnostic parameters for indicating that a shutdown process should be initiated.  
         [0037]     Controller logic  166  is provided in real-time computer  164  for execution in real-time by computer  164 . In this regard, controller logic  166  is also denoted as “software” and/or a “program” and/or an “executable program” within real-time computer  164  as a data schema holding data and/or formulae information and/or program execution instructions. Controller logic  166  is, in a preferred embodiment, machine code resident in the physical memory storage (i.e., without limitation, “RAM” “ROM” or a computer disk) of computer  164 . Controller logic  166  is preferably derived from a source language program compiled to generate the machine code. The physical memory storage is in electronic data communication with a central processing unit (CPU) of computer  164  which reads data from the physical memory, computationally modifies read data into resultant data, and writes the resultant data to the physical memory. Computer  164  also reads control signals from sensor  170  and effects control signals to valves  172  and  174  according to the provisions of controller logic  166 . In one embodiment, computer  164  and executable code for controller logic  166  are provided as an ASIC (application-specific integrated circuit).  
         [0038]     Sensor  170  is used as a feedback sensor to generate a control signal for initiating shutdown of the fuel cell stack  122 . Along with other feedback loops and control decisions (not shown), computer-implemented determination of fuel cell operating parameters via measurements from sensor  170  are used in computer-implemented control (effected in controller logic  166 ) of vent valves  172 ,  174 . Furthermore, the control signal for initiating shutdown may be based on an evaluation of one or more diagnostic parameters.  
         [0039]      FIG. 2  presents single vent valve control circuit  300  in tripped state or de-energized (S2) position for controlling two vent valves with one normal shutdown controller interface and one energy storage capacitor  330 . Control circuit  300  includes controller interfaces  302 ,  304  and a pair of switches  308  operated by relay  306 . Control circuit  300  also includes a charging circuit having a resistor  310 , diode  312  and capacitor  330 . In  FIG. 3 , vent valves  172  and  174  are represented by resistors  320  and  322 , respectively, in circuit  300 . Capacitor  330  is charged via charging resistor  310  and diode  312  when relay  306  holds switch  308  in energized (S1) position (note that switch  308  is shown in S2 position to discharge capacitor  330  in  FIG. 3 ), and capacitor  330  is discharged via diode  314  to open normally-closed valves  320  and  322  when relay  306  releases switch  308  to S2 position.  
         [0040]     Controller  304  represents control computer  164 , rapid shutdown logic in controller logic  166 , and a safety switch controlled by computer  164  for grounding the relay circuit of relay  306 . Control computer  164  measures output voltage via sensor  170 , defines an operational status variable with executable logic in controller logic  166  from the voltage as measured, and electrically grounds the solenoid relay circuit of relay  306  when the operational status variable is defined to a shutdown value.  
         [0041]     Controller  302  also represents control computer  164 , operational and normal shutdown logic in controller logic  166 , and a transistor  314  controlled by computer  164  for adjusting the positions of valves  172  and  174  via diode  316  in real-time operation when switch  308  is in S1 position. Note that diode  316  conducts electricity to resistors  320  and  322  via a second electrical switch of solenoid switch  308 . In effect, circuit  300  effectively provides three separate conditionally-operative internal electrical circuits where a first circuit is electrically active in the de-energized position of the solenoid and the other two circuits are electrically active in the complementary energized position of the solenoid. In this regard, note that a “closed” and de-energized solenoid switch  308  in S2 position provides a first electrically closed circuit with respect to enabling energy storing capacitor  330  to discharge; and an “open” and energized solenoid switch  308  in S1 position provides (a) a second electrically closed circuit between control computer  164  and vent valves  172  and  174  via resistors  320  and  322  and (b) a third electrically closed circuit between resistor  310 , diode  312 , and capacitor  330  for charging capacitor  330 .  
         [0042]     In brief reference to  FIG. 1 , control lines are depicted between computer  164  and each of vent valves  172 ,  174 . These control lines respectively summarily reference the separate controller outputs  302 ,  304  in  FIG. 2 , as well as the division between the normal shutdown controller and the rapid shutdown controller shown for each valve in  FIGS. 3 and 4 . Note that in  FIG. 1 , the power loss relay  306  and the double throw switch  308  (shown in  FIGS. 2-4 ) are internal to the controller  164  and thus not shown separately.  
         [0043]     Returning to a consideration of  FIG. 2 , when solenoid switch  308  is in S1 position, capacitor  330  is charged through charge resistor  310  and diode  312  in circuit  300 . Discharge of capacitor  330  is prevented in S1 position of switch  308  by diode  312 . Voltage drop for circuit  300  is the difference between positive voltage source  340  and negative voltage drain  342 .  
         [0044]     A simplified vent system embodiment is enabled from consideration of circuit  300  without controllers  302  and  304 , diode  316 , and resistor  322  with switch  308  providing only one electrical contact between (in S1 position) charging resistor  310 , diode  312 , and capacitor  330  and (in S2 position) between capacitor  330 , diode  314 , and resistor  320 . In such an embodiment, no real-time computer control of vent valve  172 , and no safety switch shutdown is provided in reaction to the measurement of sensor  170 ; however, such an embodiment does provide normally-closed valve  172  in fluid connection with the fuel reactant flow field(s) of fuel cell stack  122  to vent the reactant gas from the fuel reactant flow field(s) where normally-closed valve  172  is disposed in an energizing circuit configured to open normally-closed valve  172  when the output voltage has a value less than the shutdown threshold voltage value necessary to retain the spring of relay  306  to keep switch  308  in S1 position.  
         [0045]     Turning now to  FIG. 3 , an alternative embodiment shows vent valve control circuit  400  similar to control circuit  300 . Control circuit  400  is in a tripped state (S2 position to discharge capacitor  430  of circuit  400 ) for controlling vent valves  172  and  174  with two normal shutdown controller interfaces  302  and  402  and one energy storage capacitor  430 . This circuit is derived from circuit  300 , with many elements reprised from  FIG. 2  and labeled accordingly. However, controller  402  and diode  410  are configured in parallel with controller  302  and diode  316  to enable separate real-time independent manipulations of valves  172 ,  174  represented by resistors  320 ,  322 . Note that solenoid switch  404  has three electrical switches on relay  306  instead of the two electrical switches of solenoid switch  308 .  
         [0046]     With reference to  FIG. 4 , an alternative embodiment shows dual vent valve control circuits  500  similar to circuits  300  and  400 . Control circuit  500  shows each of switch  308  and switch  508  in tripped state (S2 position to discharge the capacitors of their circuits) for controlling vent valves  172  and  174  with two normal shutdown controller interfaces  302  and  402  and two energy storage capacitors  530  and  532 . Controller  304  represents control computer  164 , rapid shutdown logic in controller logic  166 , and a safety switch controlled by computer  164  for grounding the relay circuit of relay  306 . Controller  502  represents control computer  164 , rapid shutdown logic in controller logic  166 , and a safety switch controlled by computer  164  for grounding the relay circuit of relay  504 . The dual circuits of  FIG. 4  are each appreciated as essentially duplicates of circuit  300  except for the fully independent control of valves  172 ,  174  represent by resistors  320 ,  322 . This arrangement provides for custom and independent sizing of all circuit elements respective to each valve so that, for example, resistor  310  and resistor  506  are independently sized, capacitor  530  and capacitor  532  with their comparable charging resistors  310  and  506  and with their comparable diodes  312  and  512  are all independently sized, and discharge diodes  520  and  522  are independently sized to enable fully independent operation of valves  174  and  172 . Fully independent executable logical sections in executable logic  166  in controllers  502 ,  402 ,  304 , and  302  are also enabled in circuit  500 .  
         [0047]     As should be appreciated, other control elements (not shown) are also adjusted by computer  164  in providing control and shutdown of fuel cell power system  100  in alternative embodiments. In one embodiment, for example, a nitrogen purge (not shown) effects in a normal shutdown to forcefully displace residual hydrogen from the fuel cell stack  122 . In another embodiment, a block valve is closed between fuel processor  112  prior to the fluid connection between fuel cell stack  122  and valve  172  so that fuel processor  112  is isolated from fuel cell stack  122  during shutdown.  
         [0048]     Benefits of the described invention are the provision of a shutdown of a fuel cell, protection of the catalyst after a power failure in the fuel cell power system, a basis for appropriate shutdown of the fuel cell stack and/or fuel cell stack set when conditions collectively indicate the need for such an operational event, and optimal efficiency in fuel cell power system operation as parasitic power usage is minimized in the vent system. Post shutdown mixing of residual hydrogen fuel with air and contamination of the internal passages of the fuel cell stack are also prevented after the vent valves close after venting. Minimization of a “domino effect” of fuel cell component failures is also an auxiliary benefit of proper vent management upon shutdown. In some embodiments, the capacitors are designed to maintain sufficient hydrogen in the fuel cell system to enable prompt restart. The present invention also enables a design option for an air-sensitive catalyst to be deployed in the fuel processor of a fuel cell power system.  
         [0049]     The description of the invention is merely exemplary in nature and, thus, variations that do not depart from the gist of the invention are intended to be within the scope of the invention. Such variations are not to be regarded as a departure from the spirit and scope of the invention.