Abstract:
An electric power generating system is provided that comprises a fuel cell stack having at least one solid polymer fuel cell, a cooling system having a coolant flow path that directs coolant to and from the stack, a fuel regulating system having a fuel flow path and for regulating the supply of fuel from a fuel supply to the stack via the fuel flow path, and a hydrogen concentration sensor. The sensor is located in the vicinity of the fuel regulating system and in the coolant flow path at a location downstream of the stack to detect hydrogen that may have been discharged by components of the power generating system in the coolant flow path upstream of the sensor, or by the fuel regulating system. In the event the measured hydrogen concentration exceeds a threshold level, steps are taken to reduce or stop the discharge of hydrogen from the power generating system.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to fuel cells, and particularly to a fuel cell system having a sensor for detecting the concentration of hydrogen in the vicinity of the system. 
     2. Description of the Related Art 
     Electrochemical fuel cells convert fuel and oxidant to electricity. Solid polymer electrochemical fuel cells generally employ a membrane electrode assembly (“MEA”) which comprises an ion exchange membrane or solid polymer electrolyte disposed between two electrodes typically comprising a layer of porous, electrically conductive sheet material, such as carbon fiber paper or carbon cloth. The MEA contains a layer of catalyst, typically in the form of finely comminuted platinum, at each membrane/electrode interface to induce the desired electrochemical reaction. In operation the electrodes are electrically coupled to provide a circuit for conducting electrons between the electrodes through an external circuit. Typically, a number of MEAs are serially coupled electrically to form a fuel cell stack having a desired power output. 
     In typical fuel cells, the MEA is disposed between two electrically conductive fluid flow field plates or separator plates. Fluid flow field plates have at least one flow passage formed in at least one of the major planar surfaces thereof. 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 fluid flow field plates act as current collectors, provide support for the electrodes, provide access channels for the fuel and oxidant to the respective anode and cathode surfaces, and provide channels for the removal of reaction products, such as water, formed during operation of the cell. 
     Due to their zero- or low-emission nature, and ability to operate using renewable fuels, the use of fuel cells as primary and/or backup power supplies is likely to become increasingly prevalent. For example, a fuel cell stack can serve as an uninterruptible power supply for computer, medical, or refrigeration equipment in a home, office, or commercial environment. Other uses are of course possible. 
     SUMMARY OF THE INVENTION 
     Operating and environmental factors relevant to efficient fuel cell system operation may include the concentration of hydrogen in the surrounding environment, the concentration of oxygen in the surrounding environment, fuel cell stack temperature, ambient air temperature, current flow through the fuel cell stack, voltage across the fuel cell stack, and voltage across the MEAs. These factors become increasingly relevant when the fuel cell operating environment is a human habitable space with a low air flow exchange rate and/or when the space is small, such as a utility room or closet. Consequently, there is a need for improved control systems for fuel cell systems, particularly for fuel cell systems that operate in enclosed environments and/or habitable environments, and for methods of controlling such fuel cell systems. 
     According to an aspect of the invention, there is provided an electric power generating system comprising a fuel cell stack that comprises at least one solid polymer fuel cell, a fluid flow path for directing fluid to and from the stack, a fuel regulating system for regulating the supply of fuel from a fuel supply to the stack, and a hydrogen concentration sensor located in the fluid flow path at a location downstream of the stack for detecting hydrogen that is discharged or leaked from the power generating system. 
     Preferably, the hydrogen concentration sensor is also located in the vicinity of the fuel regulating system. The fuel regulating system may also be located in the fluid flow path and may comprise the following components: a fuel flow path for directing the fuel supply to the stack, a fuel supply connector for connecting the fuel supply to the fuel flow path, a pressure relief valve in the fuel flow path, and a main fuel valve also in the fuel flow path, and a fuel pressure regulator also in the fuel flow path. The fuel stream supplied to the stack comprises hydrogen. 
     The aforementioned fluid flow path is suitably a coolant flow path for directing coolant to and from the fuel cell stack, but may also be an oxidant flow path. The fuel cell stack may be air-cooled, in which case a cooling fan may be used to direct the coolant air to the stack via the coolant flow path. The coolant flow path between the cooling fan and the stack may be defined by a duct. The stack is preferably provided with coolant flow channels that enable the passage of coolant air through the stack. Coolant air exhausted from the stack is preferably directed by a portion of a power generating system housing to the hydrogen concentration sensor. 
     Preferably, the fuel regulating system is located in the coolant flow path at a location downstream of the stack. Hydrogen that may have been discharged or leaked from the stack or the fuel regulating system into the coolant air flow path will then be carried by the coolant air stream to the hydrogen concentration sensor. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the drawings, identical reference numbers identify similar elements or acts. The sizes and relative positions of elements in the drawings are not necessarily drawn to scale. For example, the shapes of various elements and angles are not drawn to scale, and some of these elements are arbitrarily enlarged and positioned to improve drawing legibility. Further, the particular shapes of the elements as drawn, are not intended to convey any information regarding the actual shape of the particular elements, have been selected solely for ease of recognition in the drawings. 
         FIG. 1  is an isometric, partially exploded, view of a fuel cell system including a fuel cell stack and controlling electronics including a fuel cell ambient environment monitoring and control system. 
         FIG. 2  is a schematic diagram representing fuel flow through a cascaded fuel cell stack of the fuel cell system of FIG.  1 . 
         FIG. 3  is a schematic diagram of the fuel cell system as partly illustrated in FIG.  1 . 
         FIG. 4  is a schematic diagram of an additional portion of the fuel cell ambient environment monitoring and control system of  FIG. 3 , including a fuel cell microcontroller selectively coupled between the fuel cell stack and a battery. 
         FIG. 5  is a top, right isometric view of a structural arrangement of various components of the fuel cell system of FIG.  1 . 
         FIG. 6  is a top, right isometric view of the structural arrangement of various components of the fuel cell system of  FIG. 5  with selected components removed from view. 
         FIG. 7  is top, left isometric view of the structural arrangement of various components of the fuel cell system of FIG.  5 . 
         FIG. 8  is a top, right exploded isometric view of a fuel regulating portion of the fuel cell system of FIG.  5 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In the following description, certain specific details are set forth in order to provide a thorough understanding of various embodiments of the invention. However, one skilled in the art will understand that the invention may be practiced without these details. In other instances, well known structures associated with fuel cells, microcontrollers, sensors, and actuators have not been described in detail to avoid unnecessarily obscuring the descriptions of the embodiments of the invention. 
     Unless the context requires otherwise, throughout the specification and claims which follow, the word “comprise” and variations thereof, such as “comprises” and “comprising” are to be construed in an open, inclusive sense, that is as “including but not limited to.” 
     Fuel Cell System Overview 
       FIG. 1  shows a portion of a fuel cell system  10 , namely, a fuel cell stack  12  and an electronic fuel cell monitoring and control system  14 . Fuel cell stack  12  includes a number of fuel cell assemblies  16  arranged between a pair of end plates  18   a ,  18   b , one of the fuel cell assemblies  16  being partially removed from fuel cell stack  12  to better illustrate the structure of fuel cell assembly  16 . Tie rods (not shown) extend between end plates  18   a ,  18   b  and cooperate with fastening nuts  17  to bias end plates  18   a ,  18   b  together by applying pressure to the various components to ensure good contact therebetween. 
     Each fuel cell assembly  16  includes a membrane electrode assembly  20  including two electrodes, the anode  22  and the cathode  24 , separated by an ion exchange membrane  26 . Electrodes  22 ,  24  can be formed from a porous, electrically conductive sheet material, such as carbon fiber paper or cloth, that is permeable to the reactants. Each of electrodes  22 ,  24  is coated on a surface adjacent the ion exchange membrane  26  with a catalyst  27 , such as a thin layer of platinum, to render each electrode electrochemically active. 
     The fuel cell assembly  16  also includes a pair of separators or flow field plates  28  sandwiching membrane electrode assembly  20 . In the illustrated embodiment, each of the flow field plates  28  includes one or more reactant channels  30  formed on a planar surface of flow field plate  28  adjacent an associated one of the electrodes  22 ,  24  for carrying fuel to anode  22  and oxidant to cathode  24 , respectively. (Reactant channel  30  on only one of flow field plates  28  is visible in  FIG. 1. ) The reactant channels  30  that carry the oxidant also carry exhaust air and product water away from cathode  24 . As will be described in more detail below, fuel stack  12  is designed to operate in a dead-ended fuel mode, thus substantially all of the hydrogen fuel supplied to it during operation is consumed, and little if any hydrogen is carried away from stack  12  in normal operation of system  10 . However, embodiments of the present invention can also be applicable to fuel cell systems operating on dilute fuels which are not dead-ended. 
     In the illustrated embodiment, each flow field plate  28  preferably includes a plurality of cooling channels  32  formed on the planar surface of the flow field plate  28  opposite the planar surface having reactant channel  30 . When the stack is assembled, the cooling channels  32  of each adjacent fuel cell assembly  16  cooperate so that closed cooling channels  32  are formed between each membrane electrode assembly  20 . The cooling channels  32  transmit coolant air through the fuel stack  12 . The cooling channels are preferably straight and parallel to each other, and traverse each plate  28  so that cooling channel inlets and outlets are located at respective edges of plate  28 . 
     While the illustrated embodiment includes two flow field plates  28  in each fuel cell assembly  16 , other embodiments can include a single bipolar flow field plate (not shown) between adjacent membrane electrode assemblies  20 . In such embodiments, a channel on one side of the bipolar plate carries fuel to the anode of one adjacent membrane electrode assembly  20 , while a channel on the other side of the plate carries oxidant to the cathode of another adjacent membrane electrode assembly  20 . In such embodiments, additional flow field plates  28  having channels for carrying coolant (e.g., liquid or gas, such as coolant air) can be spaced throughout fuel cell stack  12 , as needed to provide sufficient cooling of stack  12 . 
     End plate  18   a  includes a fuel stream inlet port (not shown) for introducing a supply fuel stream into fuel cell stack  12 . End plate  18   b  includes a fuel stream outlet port  35  for discharging an exhaust fuel stream from fuel cell stack  12  that comprises primarily water and non-reactive components and impurities, such as any introduced in the supply fuel stream or entering the fuel stream in stack  12 . Fuel stream outlet port  35  is normally closed with a valve in dead-ended operation. Although fuel cell stack  12  is designed to consume substantially all of the hydrogen fuel supplied to it during operation, traces of unreacted hydrogen may also be discharged through the fuel stream outlet port  35  during a purge of fuel cell stack  12 , effected by temporarily opening a valve at fuel stream outlet port  35 . Each fuel cell assembly  16  has openings formed therein to cooperate with corresponding openings in adjacent assemblies  16  to form internal fuel supply and exhaust manifolds (not shown) that extend the length of stack  12 . The fuel stream inlet port is fluidly connected to fluid outlet port  35  via respective reactant channels  30  that are in fluid communication with the fuel supply and exhaust manifolds, respectively. 
     The end plate  18   b  includes an oxidant stream inlet port  37  for introducing supply air (oxidant stream) into fuel cell stack  12 , and an oxidant stream outlet port  39  for discharging exhaust air from fuel cell stack  12 . Each fuel cell assembly  16  has openings  31 ,  34 , formed therein to cooperate with corresponding openings in adjacent fuel cell assemblies  16  to form oxidant supply and exhaust manifolds that extend the length of stack  12 . The oxidant inlet port  37  is fluidly connected to the oxidant outlet port  39  via respective reactant channels  30  that are in fluid communication with oxidant supply and exhaust manifolds, respectively. 
     In one embodiment, the fuel cell stack  12  includes forty-seven fuel cell assemblies  16 . ( FIGS. 1 and 2  omit a number of the fuel cell assemblies  16  to enhance drawing clarity). The fuel cell stack  12  can include a greater or lesser number of fuel cell assemblies to provide more or less power, respectively. 
     As shown in  FIG. 2 , fuel is directed through fuel cell stack  12  in a cascaded flow pattern. A first set  11  composed of the first forty-three fuel cell assemblies  16  are arranged so that fuel flows within the set in a concurrent parallel direction (represented by arrows  13 ) that is generally opposite the direction of the flow of coolant through fuel cell stack  12 . Fuel flow through a next set  15  of two fuel cell assemblies  16  is in series with respect to the flow of fuel in the first set  11 , and in a concurrent parallel direction within the set  15  (in a direction represented by arrows  17 ) that is generally concurrent with the direction of the flow of coolant through fuel cell stack  12 . Fuel flow through a final set  19  of two fuel cells assemblies  16  is in series with respect to the first and second sets  11 ,  15 , and in a concurrent parallel direction within the set  19  (in a direction represented by arrow  21 ) generally opposite the flow of coolant through the fuel cell stack  12 . The oxidant is supplied to each of the forty-seven fuel cells in parallel, in the same general direction as the flow of coolant through the fuel cell stack  12 . 
     The final set  19  of fuel cell assemblies  16  comprises the purge cell portion  36  of the fuel cell stack. The purge cell portion  36  accumulates non-reactive components which are periodically vented by opening a purge valve. 
     Each membrane electrode assembly  20  is designed to produce a nominal potential difference of about 0.6 V between anode  22  and cathode  24 . Reactant streams (hydrogen and air) are supplied to electrodes  22 ,  24  on either side of ion exchange membrane  26  through reactant channels  30 . Hydrogen is supplied to anode  22 , where platinum catalyst  27  promotes its separation into protons and electrons, which pass as useful electricity through an external circuit (not shown). On the opposite side of membrane electrode assembly  20 , air flows through reactant channels  30  to cathode  24  where oxygen in the air reacts with protons passing through the ion exchange membrane  26  to produce product water. 
     Fuel Cell System Sensors and Actuators 
     With continuing reference to  FIG. 1 , the electronic monitoring and control system  14  comprises various electrical and electronic components on a circuit board  38  and various sensors  44  and actuators  46  distributed throughout fuel cell system  10 . The circuit board  38  carries a microprocessor or microcontroller  40  that is appropriately programmed or configured to carry out fuel cell system operation. microcontroller  40  can take the form of an Atmel AVR RISC microcontroller available from Atmel Corporation of San Jose, Calif. The electronic monitoring and control system  14  also includes a persistent memory  42 , such as an EEPROM portion of microcontroller  40  or discrete nonvolatile controller-readable media. 
     Microcontroller  40  is coupled to receive input from sensors  44  and to provide output to actuators  46 . The input and/or output can take the form of either digital and/or analog signals. A rechargeable battery  47  powers the electronic monitoring and control system  14  until fuel cell stack  12  can provide sufficient power to electronic monitoring and control system  14 . Microcontroller  40  is selectively couplable between fuel cell stack  12  and battery  47  for switching power during fuel cell system operation and/or to recharge battery  47  during fuel cell operation. 
       FIG. 3  show various elements of fuel cell system  10  in further detail, and shows various other elements that were omitted from  FIG. 1  for clarity of illustration. 
     With particular reference to  FIG. 3 , fuel cell system  10  provides fuel (e.g., hydrogen) to anode  22  by way of a fuel system  50 . Fuel system  50  includes a source of fuel such as one or more fuel tanks  52 , and a fuel regulating system  54  for controlling delivery of the fuel. Fuel tanks  52  can contain hydrogen, or some other fuel such as methanol. Alternatively, fuel tanks  52  can represent a process stream from which hydrogen can be derived by reforming, such as methane or natural gas (in which case a reformer is provided in fuel cell system  10 ). 
     Fuel tanks  52  each include a fuel tank valve  56  for controlling the flow of fuel from respective fuel tank  52 . Fuel tank valves  56  may be automatically controlled by microcontroller  40 , and/or manually controlled by a human operator. Fuel tanks  52  may be refillable, or may be disposable. The fuel tanks  52  may be integral to fuel system  50  and/or fuel cell system  10 , or can take the form of discrete units. In this embodiment, fuel tanks  52  are hydride storage tanks. Fuel tanks  52  are positioned within the fuel cell system  10  such that they are heatable by exhaust coolant air warmed by heat generated by fuel cell stack  12 . Such heating facilitates the release of hydrogen from the hydride storage media. 
     Fuel system  50  includes a hydrogen concentration sensor S 5 , hydrogen heater current sensor S 6  and a hydrogen sensor check sensor S 11 . Hydrogen heater current sensor S 6  can take the form of a current sensor that is coupled to monitor a hydrogen heater element that is an integral component of hydrogen concentration sensor S 5 . Hydrogen sensor check sensor S 11  monitors voltage across a positive leg of a Wheatstone bridge in a hydrogen concentration sensor S 5 , discussed below, to determine whether hydrogen concentration sensor S 5  is functioning. 
     Fuel tanks  52  are coupled to the fuel regulating system  54  through a filter  60  that ensures that particulate impurities do not enter fuel regulating system  54 . Fuel regulating system  54  includes a pressure sensor  62  to monitor the pressure of fuel in fuel tanks  52 , which indicates how much fuel remains in fuel tanks  52 . A pressure relief valve  64  automatically operates to relieve excess pressure in fuel system  50 . Pressure relief valve  64  can take the form of a spring and ball relief valve. A main gas valve solenoid CS 5  opens and closes a main gas valve  66  in response to signals from microcontroller  40  to provide fluid communication between the fuel tanks  52  and fuel regulating system  54 . Additional controllers such as a hydride valve solenoid CS 7  controls flow through the fuel tank valves  56 . A hydrogen regulator  68  regulates the flow of hydrogen from fuel tanks  52 . Fuel is delivered to the anodes  22  of the fuel cell assemblies  16  through a hydrogen inlet conduit  69  that is connected to fuel stream inlet port of stack  12 . 
     Sensors  44  of the electronic monitoring and control system  14  monitor a number of fuel cell system operating parameters to maintain fuel cell system operation within acceptable limits. For example, a stack voltage sensor S 3  measures the gross voltage across fuel cell stack  12 . A purge cell voltage sensor S 4  monitors the voltage across purge cell portion  36  (the final set  19  of fuel cell assemblies  16  in cascaded design of FIG.  2 ). A cell voltage checker S 9  ensures that a voltage across each of the fuel cell assemblies  16  is within an acceptable limit. Each of the sensors S 3 , S 4 , S 9  provide inputs to microcontroller  40 , identified in  FIG. 3  by arrows pointing toward the blocks labeled “FCM” (i.e., fuel cell microcontroller  40 ). 
     A fuel purge valve  70  is provided at fuel stream outlet port  35  of fuel cell stack  12  and is typically in a closed position when stack  12  is operating. Fuel is thus supplied to fuel cell stack  12  only as needed to sustain the desired rate of electrochemical reaction. Because of the cascaded flow design, any impurities (e.g. nitrogen) in the supply fuel stream tend to accumulate in purge cell portion  36  during operation. A build-up of impurities in purge cell portion  36  tends to reduce the performance of purge cell portion  36 ; should the purge cell voltage sensor S 4  detect a performance drop below a threshold voltage level, microcontroller  40  may send a signal to a purge valve controller CS 4  such as a solenoid to open the purge valve  36  and discharge the impurities and other non-reactive components that may have accumulated in purge cell portion  36 . The venting of hydrogen by the purge valve  70  by the purge valve during a purge is limited to prevent the ambient environment monitoring and control systems, discussed below, from triggering a failure or fault. 
     Fuel cell system  10  provides oxygen in an air stream to the cathode side of membrane electrode assemblies  20  by way of an oxygen delivery system  72 . A source of oxygen or air  74  can take the form of an air tank or the ambient atmosphere. A filter  76  ensures that particulate impurities do not enter oxygen delivery system  72 . An air compressor controller CS 1  controls an air compressor  78  to provide the air to fuel cell stack  12  at a desired flow rate. A mass air flow sensor S 8  measures the air flow rate into fuel cell stack  12 , providing the value as an input to microcontroller  40 . A humidity exchanger  80  adds water vapor to the air to keep the ion exchange membrane  26  moist. The humidity exchanger  80  also removes water vapor which is a byproduct of the electrochemical reaction. Excess liquid water is provided to an evaporator  58 . 
     The fuel cell system  10  removes excess heat from fuel cell stack  12  and uses the excess heat to warm fuel tanks  52  by way of a cooling system  82 . Cooling system  82  includes a fuel cell temperature sensor S 1 , for example a thermister that monitors the core temperature of the fuel cell stack  12 . The temperature is provided as input to microcontroller  40 . A stack current sensor S 2 , for example a Hall sensor, measures the gross current through the fuel cell stack  12 , and provides the value of the current as an input to microcontroller  40 . A cooling fan controller CS 3  controls the operation of one or more cooling fans  84  for cooling fuel cell stack  12 . After passing through the fuel cell stack  12 , the warmed coolant air circulates around the fuel tanks  52 . The warmed coolant air then passes through the evaporator  58 . A power circuit relay controller CS 6  connects, and disconnects, fuel cell stack  12  to, and from, an external electrical circuit in response to microcontroller  40 . A power diode  59  provides one-way isolation of the fuel cell system  10  from the external load to provide protection to the fuel cell system  10  from the external load. A battery relay controller CS 8  connects, and disconnects, fuel cell monitoring and control system  14  between the fuel cell stack  12  and the battery  47 . 
     The fuel cell monitoring and control system  14  (illustrated in  FIG. 4 ) includes sensors for monitoring fuel cell system  10  surroundings and actuators for controlling fuel cell system  10  accordingly. For example, a hydrogen concentration sensor S 5  (shown in  FIG. 3 ) for monitoring the hydrogen concentration level in the ambient atmosphere surrounding fuel cell stack  12 . The hydrogen concentration sensor S 5  can take the form of a heater element with a hydrogen sensitive thermister that may be temperature compensated. An oxygen concentration sensor S 7  (illustrated in  FIG. 4 ) to monitor the oxygen concentration level in the ambient atmosphere surrounding fuel cell system  10 . An ambient temperature sensor S 10  (shown in FIG.  3 ), for example a digital sensor, to monitor the ambient air temperature surrounding fuel cell system  10 . 
     With reference to  FIG. 4 , microcontroller  40  receives the various sensor measurements such as ambient air temperature, fuel pressure, hydrogen concentration, oxygen concentration, fuel cell stack current, air mass flow, cell voltage check status, voltage across the fuel cell stack, and voltage across the purge cell portion of the fuel cell stack from various sensors described below. Microcontroller  40  provides the control signals to the various actuators, such as air compressor controller CS 1 , cooling fan controller CS 3 , purge valve controller CS 4 , main gas valve solenoid CS 5 , power circuit relay controller CS 6 , hydride tank valve solenoid CS 7 , and battery relay controller CS 8 . 
     Fuel Cell System Structural Arrangement 
       FIGS. 5-8  illustrate the structural arrangement of the components in fuel cell system  10 . For convenience, “top”, “bottom”, “above”, “below” and similar descriptors are used merely as points of reference in the description, and while corresponding to the general orientation of the illustrated fuel cell system  10  during operation, are not to be construed to limit the orientation of the fuel cell system  10  during operation or otherwise. 
     Referring to  FIGS. 5-7 , the air compressor  78  and cooling fan  84  are grouped together at one end (“air supply end”) of the fuel cell stack  12 . Fuel tanks  52  (not shown in  FIGS. 5-7 ) are mountable to the fuel cell system  10  on top of, and along the length of, fuel cell stack  12 . The components of fuel regulating system  54  upstream of fuel cell stack  12  are located generally at the end of stack  12  (“hydrogen supply end”) opposite the air supply end. 
     Air compressor  78  is housed within an insulated housing  700  that is removably attached to the fuel cell stack  12  at the air supply end. The housing  700  has an air supply aperture  702  covered by the filter  76  that allows supply air into housing  700 . The air compressor  78  is a positive displacement low pressure type compressor and is operable to transmit supply air to oxidant air supply conduit  81  at a flow rate controllable by the operator. An air supply conduit  81  passes through a conduit aperture  704  in compressor housing  700  and connects with an air supply inlet  706  of humidity exchanger  80 . Mass flow sensor S 8  is located on an inlet of air compressor  78  and preferably within the compressor housing  700 . 
     Humidity exchanger  80  may be of the type disclosed in U.S. Pat. No. 6,106,964, and is mounted to one side of fuel cell stack  12  near the air supply end. Air entering into the humidity exchanger  80  via supply conduit  81  is humidified and then exhausted from the humidity exchanger  80  and into fuel cell stack  12  (via the supply air inlet port of the end plate  18   b ). Exhaust air from the fuel cell stack  12  exits via the exhaust air outlet port in end plate  18   b  and is directed into humidity exchanger  80 , where water in the air exhaust stream is transferred to the air supply stream. The air exhaust stream then leaves the humidity exchanger  80  via the air exhaust outlet  712  and is transmitted via an air exhaust conduit (not shown) to evaporator  58  (not shown in  FIGS. 5-7 ) mountable to a cover (not shown) above fuel cell stack  12 . 
     Cooling fan  84  is housed within a fan housing  720  that is removably mounted to the air supply end of fuel cell stack  12  below compressor housing  700 . Fan housing  720  includes a duct  724  that directs coolant air from cooling fan  84  to the cooling channel openings at the bottom of fuel cell stack  12 . Coolant air is directed upwards and through fuel cell stack  12  (via the cooling channels  32 ) and is discharged from the cooling channel openings at the top of fuel cell stack  12 . During operation, heat extracted from fuel cell stack  12  by the coolant air is used to warm fuel tanks  52  that are mountable directly above and along the length of stack  12 . Some of the warmed coolant air can be redirected into air supply aperture  702  of the compressor housing  700  for use as oxidant supply air. 
     Referring particularly to  FIG. 7 , the circuit board  38  carrying microcontroller  40 , oxygen sensor S 7  and ambient temperature sensor S 10  is mounted on the side of fuel cell stack  12  opposite humidity exchanger  80  by way of a mounting bracket  730 . Positive and negative electrical power supply lines  732 ,  734  extend from each end of fuel cell stack  12  and are connectable to an external load. An electrically conductive bleed wire  736  from each of power supply lines  732 ,  734  connects to circuit board  38  at a stack power-in terminal  738  and transmits some of the electricity generated by fuel cell stack  12  to power the components on the circuit board  38 , as well as sensors  44  and actuators  46  which are electrically connected to circuit board  38  at terminal  739 . Similarly, the battery  47  (not shown in  FIGS. 5-7 ) is electrically connected to circuit board  38  at battery power in terminal  740 . Battery  47  supplies power to the circuit board components, sensors  44  and actuators  46  when fuel cell stack output has not yet reached nominal levels (e.g., at startup); once fuel cell stack  12  has reached nominal operating conditions, fuel cell stack  12  can also supply power to recharge battery  47 . 
     Referring generally to  FIGS. 5-7  and particularly to  FIG. 8 , a bracket  741  is provided at the hydrogen supply end for the mounting of a fuel tank valve connector  53 , hydrogen pressure sensor  62 , pressure relief valve  64 , main gas valve  66 , and hydrogen pressure regulator  68  above fuel cell stack  12  at the hydrogen supply end. A suitable pressure regulator may be a Type  912  pressure regulator available from Fisher Controls of Marshalltown, Iowa. A suitable pressure sensor may be a transducer supplied Texas Instruments of Dallas, Tex. A suitable pressure relief valve may be supplied by Schraeder-Bridgeport of Buffalo Grove, Ill. Pressure relief valve  64  is provided for fuel tanks  52  and may be set to open at about 350 psi. A low pressure relief valve  742  is provided for the fuel cell stack  12 . Bracket  741  also provides a mount for hydrogen concentration sensor S 5 , hydrogen heater current sensor S 6  and hydrogen sensor check sensor S 11 , which are visible in  FIG. 6  in which bracket  741  is transparently illustrated in hidden line. Fuel tanks  52  are connectable to fuel tank connector  53 . When fuel tank and main gas valves  56 ,  66  are opened, hydrogen is supplied under a controlled pressure (monitored by pressure sensor  62  and adjustable by hydrogen pressure regulator  68 ) through fuel supply conduit  69  to the fuel inlet port of end plate  18   a.    
     Purge valve  70  is located at the fuel outlet port at end plate  18   b . A purge conduit (not shown) connects purge valve  70  to an inlet in duct  724 . Purge discharge is thus directed from purge valve  70 , through the purge conduit and into the duct, wherein the purge discharge is diluted in the coolant air stream that is eventually exhausted from fuel cell system  10 . 
     The fuel cell system  10  and fuel tanks  52  are housed within a system cover (not shown) and coupled to a base (not shown) at mounting points  744 . The portion of the cover covering the stack  12  and fuel regulating system  54  is shaped so that coolant air exhausted from the top of the fuel cell stack  12  is directed by this portion of the cover, past fuel regulating system  54  and hydrogen concentration sensor S 5 , through an outlet (not shown) in the cover and out of fuel cell system  10 . 
     The fuel cell system  10  is designed so that components that are designed to discharge hydrogen or that present a risk of leaking hydrogen, are as much as practical, located in the cooling air path or have their discharge or leakage directed to the cooling air path. The cooling air path is defined by duct  724 , cooling air channels of stack  12 , and the portion of the system cover above stack  12 ; a cooling air stream passing through the cooling air path is shown by the arrows  725  in  FIGS. 5 ,  6  and  7 . Components directly in the cooling air path include fuel tanks  52 , and components of fuel regulating system  54  such as pressure relief valve  64 , main gas valve  66 , and hydrogen regulator  68 . Components not directly in the cooling air path that are fluidly connected to the cooling air path, and include purge valve  70  connected to duct  724  via purge conduit (not shown) and low pressure relief valve  742  connected to an outlet near fuel regulating system  54  via conduit  746 . When cooling air fan  84  is operational, the cooling air stream carries leaked or discharged hydrogen through duct  724 , past stack  12 , and out of system  10  in the direction of the arrows shown in  FIGS. 5 ,  6 , and  7 . Hydrogen concentration sensor S 5  is strategically placed far downstream in the cooling air stream to detect hydrogen carried in the cooling air stream. 
     Hydrogen concentration sensor S 5  is also placed in the vicinity of the components of fuel regulating system  54  to improve detection of hydrogen leaks or discharges from fuel regulating system  54 . 
     Although specific embodiments, and examples for, the invention are described herein for illustrative purposes, various equivalent modifications can be made without departing from the spirit and scope of the invention, as will be recognized by those skilled in the relevant art. For example, the hydrogen sensor may be installed in the oxidant exhaust conduit or somewhere in the oxidant stream downstream of the stack. If the sensor is placed in the oxidant exhaust stream, the oxidant exhaust stream may be positioned so that the sensor is also in the vicinity of the fuel regulating system. The teachings provided herein of the invention can be applied to other fuel cell systems, not necessarily the PEM fuel cell system described above. 
     Commonly assigned U.S. patent applications Ser. No. 09/916,241, entitled FUEL CELL AMBIENT ENVIRONMENT MONITORING AND CONTROL APPARATUS AND METHOD; Ser. No. 09/916,117, entitled FUEL CELL CONTROLLER SELF INSPECTION Ser. No. 09/916,115, entitled FUEL CELL ANOMALY DETECTION METHOD AND APPARATUS; Ser. No: 09/916,211, entitled FUEL CELL PURGING METHOD AND APPARATUS; Ser. No. 09/916,213, entitled FUEL CELL RESUSCITATION METHOD AND APPARATUS; Ser. No. 09/916,240, entitled FUEL CELL SYSTEM METHOD, APPARATUS AND SCHEDULING; Ser. No. 09/916,239, entitled FUEL CELL SYSTEM AUTOMATIC POWER SWITCHING METHOD AND APPARATUS; and Ser. No. 09/916,118, entitled PRODUCT WATER PUMP FOR FUEL CELL SYSTEM, all filed Jul. 25, 2001, are incorporated herein by reference, in their entirety. 
     The various embodiments described above and in the applications and patents incorporated herein by reference can be combined to provide further embodiments. The described methods can omit some acts and can add other acts, and can execute the acts in a different order than that illustrated, to achieve the advantages of the invention. 
     These and other changes can be made to the invention in light of the above detailed description. In general, in the following claims, the terms used should not be construed to limit the invention to the specific embodiments disclosed in the specification, but should be construed to include all fuel cell systems, controllers and processors, actuators, and sensors that operate in accordance with the claims. Accordingly, the invention is not limited by the disclosure, but instead its scope is to be determined entirely by the following claims.