Patent Publication Number: US-10763528-B2

Title: Operation of a fuel cell stack to prevent low oxygen concentrations in a surrounding enclosed space

Description:
INTRODUCTION 
     The present disclosure relates to operating a fuel cell stack (FCS) for a vehicle that may be enclosed by a surrounding structure such as a closed garage or other confined space. More specifically, the present disclosure provides methods to operate the FCS according to a conditional operating mode that assumes the vehicle is in an enclosed space when certain operating conditions of the vehicle are met. The conditional operating mode is designed so that, if the vehicle is actually contained within an enclosed space, the operation of the FCS will be modified so that the FCS does not overly deplete the available oxygen within that space beyond an acceptable level. The conditional operating mode also seeks to avoid unnecessary modifications to the operation of the FCS when the vehicle is not actually in an enclosed space but, nonetheless, the conditional operating mode is triggered. 
     A proton-exchange membrane (PEM) fuel cell is an electro-chemical device that includes a membrane-electrode-assembly having an anode catalyst layer and a cathode catalyst layer disposed on opposite sides of a proton-conducting solid polymer electrolyte. The anode catalyst layer receives hydrogen gas and the cathode catalyst layer receives oxygen or air. The hydrogen gas is dissociated at the anode catalyst layer to generate free protons and electrons. The protons migrate through the electrolyte and the electrons are directed through a load to perform work. The protons and electrode eventually reach the cathode catalyst layer where they react with oxygen to generate water. A FCS includes a plurality of similar PEM fuel cells that are separated by bipolar plates and connected to common supplies of hydrogen gas and oxygen or air. 
     The cathode catalyst layers of the FCS are supplied with oxygen from the surrounding ambient environment and consume that oxygen as needed to support the on-going operation of the FCS. If the vehicle is in an enclosed space when the FCS is running—such as when the vehicle is running in a closed garage to heat the passenger compartment—the amount of available oxygen within the enclosed space may be depleted faster than it can be replenished by the exchange of air from outside the enclosed space to inside the enclosed space. As such, under these circumstances, the oxygen concentration within the enclosed space may begin to drop. A conditional operating mode for a FCS is therefore needed that can address the issue of a falling oxygen concentration within an enclosed space whenever certain operating conditions of the vehicle suggest that the vehicle could be in an enclosed space without, of course, unnecessarily disrupting operation of the FCS when the vehicle is assumed to be, but is not actually in, such an enclosed space. 
     SUMMARY 
     A method of operating a fuel cell stack according to one embodiment may include several steps. One step involves determining when a vehicle that is powered by the fuel cell stack is in a non-moving state. In another step, an O 2  concentration of an assumed enclosed space is calculated over time while the vehicle is in the non-moving state. In yet another step, a set of O 2  concentration concern levels is established that includes a first O 2  concentration concern level being less than a standard atmospheric O 2  concentration and a second O 2  concentration concern level being less than the first O 2  concentration concern level. In still another step, the fuel cell stack is operated without restriction when the vehicle is in the non-moving state so long as the O 2  concentration of the assumed enclosed space remains greater than the first O 2  concentration concern level. 
     The method of the aforementioned embodiment may include several steps or be further defined. For example, the step of determining when the vehicle that is powered by the fuel cell stack is in a non-moving state may include determining that a speed of the vehicle is less than or equal to five kilometers/hour. As another example, the step of determining when the vehicle that is powered by a fuel cell stack is in a non-moving state may include determining that the vehicle is stationary. And, in yet another example, the step of determining when the vehicle that is powered by the fuel cell stack is in a non-moving state may include determining that the vehicle is stationary and that GPS coordinates of the vehicle indicate that the vehicle is not on a road. 
     Additionally, the step of calculating the O 2  concentration of the assumed enclosed space over time may include setting the O 2  concentration of the assumed enclosed space at standard atmospheric O 2  concentration when the vehicle is first determined to be in the non-moving state and thereafter adjusting the O 2  concentration of the assumed enclosed space based on an amount of O 2  consumed by the fuel cell stack over time and an amount of O 2  replenished to the assumed enclosed space over time. The amount of O 2  consumed by the fuel cell stack may include an amount of O 2  consumed due to generating a current of the fuel cell stack, an amount of O 2  consumed due to cathode catalytic heating (CCH), and an amount of O 2  consumed due to stack voltage recovery (SVR), and the amount of O 2  replenished to the enclosed space may be based on 0.03 air exchanges/hour. 
     Still further, the first O 2  concentration concern level may be less than 21 mol % and greater than 19 mol % and, separately, the second O 2  concentration concern level may be less than 20 mol % and greater than 18 mol %. More narrowly, the first O 2  concentration concern level may be 20 mol % and the second O 2  concentration concern level may be 19 mol %. 
     The step of operating the fuel cell stack without restriction in the aforementioned method may include operating the fuel cell stack with CCH and SVR processes enabled. Moreover, the aforementioned method may further comprise the additional step of operating the FCS in a low-power state when the O 2  concentration of the assumed enclosed space is equal to or less than the first O 2  concentration concern level yet greater than the second O 2  concentration concern level. The aforementioned method may also further comprise the step of shutting down the fuel cell stack when the O 2  concentration of the assumed enclosed space is equal to or less than the second O 2  concentration concern level. 
     A method of operating a fuel cell stack according to another embodiment may include several steps. One step involves determining when a vehicle that is powered by the fuel cell stack is in a non-moving state. In another step, an O 2  concentration of an assumed enclosed space is calculated over time while the vehicle is in the non-moving state. In yet another step, the O 2  concentration of the assumed enclosed space is compared over time with a set of O 2  concentration concern levels that includes a first O 2  concentration concern level and a second O 2  concentration concern level. The first O 2  concentration concern level is less than a standard atmospheric O 2  concentration and the second O 2  concentration concern level is less than the first O 2  concentration concern level. In still another step, the fuel cell stack is operated when the vehicle is in the non-moving state according to a conditional operating mode. The conditional operating mode includes (1) operating the fuel cell stack without restriction so long as the O 2  concentration of the assumed enclosed space remains greater than the first O 2  concentration concern level; (2) operating the fuel cell stack in a low-power state when the O 2  concentration of the assumed enclosed space is equal to or less than the first O 2  concentration concern level yet greater than the second O 2  concentration concern level; and (3) shutting down the fuel cell stack when the O 2  concentration of the assumed enclosed space is equal to or less than the second O 2  concentration concern level. 
     The method of the aforementioned embodiment may include several steps or be further defined. For example, the step of determining when the vehicle that is powered by the fuel cell stack is in a non-moving state may include determining that the vehicle is stationary. As another step, the step of determining when the vehicle that is powered by the fuel cell stack is in a non-moving state may include determining that the vehicle is stationary and that GPS coordinates of the vehicle indicate that the vehicle is not on a road. 
     Additionally, the step of calculating the O 2  concentration of the assumed enclosed space over time includes setting the O 2  concentration of the assumed enclosed space at standard atmospheric O 2  concentration when the vehicle is first determined to be in a non-moving state and thereafter adjusting the O 2  concentration of the assumed enclosed space based on an amount of O 2  consumed by the fuel cell stack over time and an amount of O 2  replenished to the assumed enclosed space over time. The amount of O 2  consumed by the fuel cell stack includes an amount of O 2  consumed due to generating a current of the fuel cell stack, an amount of O 2  consumed due to cathode catalytic heating (CCH), and an amount of O 2  consumed due to SVR, and the amount of O 2  replenished to the enclosed space is based on 0.03 air exchanges/hour 
     Still further, the first O 2  concentration concern level may be less than 21 mol % and greater than 19 mol %, and the second O 2  concentration concern level may be less than 20 mol % and greater than 18 mol %. Moreover, the step of operating the fuel cell stack in the low-power state may include disabling CCH and SVR of the fuel cell stack. 
     A method of operating a fuel cell stack according to yet another embodiment may include several steps. One step involves determining when a vehicle that is powered by a fuel cell stack is in a non-moving state. In another step, an O 2  concentration of an assumed enclosed space that encompasses the vehicle is calculated over time while the vehicle is in the non-moving state. In yet another step, the O 2  concentration of the assumed enclosed space is compared over time with a set of O 2  concentration concern levels that includes a first O 2  concentration concern level and a second O 2  concentration concern level. The first O 2  concentration concern level is less than 21 mol % and greater than 19 mol % and the second O 2  concentration concern level is less than 20 mol % and greater than 18 mol %. In another step, the fuel cell stack is operated when the vehicle is in the non-moving state according to a conditional operating mode. The conditional operating mode includes (1) operating the fuel cell stack without restriction so long as the O 2  concentration of the assumed enclosed space remains greater than the first O 2  concentration concern level; (2) idling the fuel cell stack when the O 2  concentration of the assumed enclosed space is equal to or less than the first O 2  concentration concern level yet greater than the second O 2  concentration concern level; and (3) shutting down the fuel cell stack when the O 2  concentration of the assumed enclosed space is equal to or less than the second O 2  concentration concern level. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic illustration of a fuel cell stack encompassed by an assumed enclosed space that shows various oxygen consumption processes and an oxygen replenishment process that can be used to compute a calculated O 2  concentration of the assumed enclosed space over time according to practices of the present disclosure; 
         FIG. 2  is a graph that depicts four different scenarios in which the operation of a fuel cell stack may affect the calculated O 2  concentration of the assumed enclosed space according to practices of the present disclosure, wherein the y-axis is the calculated O 2  concentration in mole percentage and the x-axis is time in seconds; 
         FIG. 3  is a flow diagram that depicts one approach for determining whether a vehicle is in a non-moving state according to practices of the present disclosure; 
         FIG. 4  is a flow diagram that depicts another approach for determining whether a vehicle is in a non-moving state according to practices of the present disclosure; 
         FIG. 5  is a graph that plots the calculated O 2  concentration within the assumed enclosed space over time for comparison of the calculated O 2  concentration against a set of established O 2  concentration concern levels so long as the vehicle remains in the non-moving state according to practices of the present disclosure, wherein the y-axis is the calculated O 2  concentration in mole percentage and the x-axis is time in seconds; 
         FIG. 6  is also a graph that plots the calculated O 2  concentration within the assumed enclosed space over time for comparison of the calculated O 2  concentration against a set of established O 2  concentration concern levels so long as the vehicle remains in the non-moving state according to practices of the present disclosure, wherein the y-axis is the calculated O 2  concentration in mole percentage and the x-axis is time in seconds; and 
         FIG. 7  is a flow diagram illustrating how the disclosed method including the conditional operating mode of the fuel cell stack may be performed by the vehicle. 
     
    
    
     DETAILED DESCRIPTION 
     Hydrogen is an attractive fuel for operating a vehicle because it is clean and can be used to efficiently produce electricity in a fuel cell. A number of individual PEM fuel cells that consume hydrogen as part of an electro-chemical reaction that produces electrical current are typically combined in a fuel cell stack (FCS) to generate the desired power for operating a vehicle. For example, a typical FCS for a vehicle may have two hundred or more stacked PEM fuel cells. The FCS typically receives a flow of hydrogen gas and a flow of an oxidant gas such as air or oxygen, and then distributes each of those reactant gasses to the anode catalyst layers and the cathode catalyst layers, respectively, of the various PEM fuel cells by way of bipolar plates that separate the PEM fuel cells. The FCS thus consumes both hydrogen and oxygen during normal operation. 
     In some types of enclosed spaces (e.g., a garage), oxygen from the air in the local environment may be consumed by the FCS faster than it is replenished. To address this issue, a method that includes a conditional operating mode of the FCS may be implemented when a vehicle  10  that is powered by a FCS  12  is in a non-moving state, as shown schematically in  FIG. 1 . Specifically, and as shown in the diagram of  FIG. 1 , the conditional operating mode calls for the vehicle  10  to be treated as if it is contained within an enclosed spaced, referred to herein as an “assumed enclosed space” and identified by reference numeral  14 , whenever the vehicle  10  is determined to be in a non-moving state. The assumed enclosed space  14  is thus a virtual enclosure having a volume that may or may not be large enough to encompass the vehicle  10 . This space  14  is used to model the amount of O 2  in the volume of the assumed enclosed space  14  based on the operation of the FCS  12  and the pressure and temperature of the assumed enclosed space  14 , which can be estimated, measured, or simply assigned. In that regard, the assumed enclosed space  14  and the calculated amount of O 2  within that space  14  is designed to conservatively model what would happen if the vehicle  10  is actually contained within an enclosed space, such as a garage, even though it may not always be, and to take responsive action based on the O 2  concentration calculated within the assumed enclosed space  14  to help ensure that the O 2  concentration in an actual enclosed space is not excessively depleted. 
       FIG. 1  schematically illustrates how oxygen may be consumed and replenished in the assumed enclosed space  14 , which may be designed to be just large enough to surround the vehicle  10  in accordance with the definition of a “small garage” as set forth in SAE J 2578, 3rd Edition (August 2014). The SAE J 2578, 3rd Edition (August 2014) (“the SAE J 2578 standard”) is incorporated herein by reference in its entirety. The size and volume of the assumed enclosed space  14  may vary depending on the size of the vehicle  10 . Other standards (e.g., ISO standard, JIS standard, local regulation, etc.) may also be used in lieu of the SAE J 2578 standard to define the size of the assumed enclosed space  14 , if desired, or the assumed enclosed space  14  may be an approximation of an actual surrounding enclosure whose dimensions can be assessed and estimated using vision systems or LIDAR. Still further, the assumed enclosed space  14  may simply be predefined. In many instances, however, regardless of how the assumed enclosed space  14  and its volume are ascertained, the assumed enclosed space  14  may have a volume that ranges from 5 m 3  to 200 m 3  or, more narrowly, from 20 m 3  to 50 m 3 . Additionally, to allow for the amount and concentration of O 2  to be modeled within the assumed enclosed space  14 , a temperature and pressure within the space  14  may be calibrated based on measured ambient atmospheric conditions, or those may be assigned. For instance, in one implementation, the temperature and pressure of the assumed enclosed space may be selected to be 60° C. and 1 atmosphere for simplification. 
     When the vehicle  10  containing the FCS  12  is in the non-moving state, the FCS  12  may consume oxygen in several ways, which can reduce the O 2  concentration in the assumed enclosed space  14 . In a first process  16 , the FCS  12  consumes O 2  from the volume of the assumed enclosed space  14  while simultaneously consuming H 2  to generate an electric current. The general cathode half-reaction for consuming O 2  and generating water, the general anode half-reaction for consuming H 2  and generating a flow of electrons, and the overall reaction of the fuel cells in the FCS  12  are depicted below:
 
2H 2 →H + +4 e   −   Anode:
 
O 2 +4H + +4 e   − →2H 2 O  Cathode:
 
2H 2 +O 2 →2H 2 O  Overall:
 
     To that end, the molar flow of O 2  ({dot over (n)} O2 ) required to support the production of a given electrical current (I) by the FCS  12  for operation of the various vehicle systems can be calculated continuously with time while the vehicle  10  is in the non-moving state through the following equation: 
     
       
         
           
             
               
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     In a second process  18 , the FCS  12  consumes O 2  from the volume of the assumed enclosed space  14  to support cathode catalytic heating (CCH) of the FCS  12 . During CCH, H 2  is routed around the polymer electrolytes and delivered to the cathode catalyst layers of the FCS  12  along with O 2  to facilitate the combustion of the H 2 . The heat generated at the cathode catalyst layers as a result of combusting H 2  is then used to heat the FCS  12  to its optimal operating temperature range and to also heat the passenger cabin of the vehicle  10 , if desired. CCH is thus useful when starting the vehicle  10 , especially at cold or freezing temperatures after the vehicle  10  has been in a non-operational state for an extended period. The molar flow rate of O 2  ({dot over (n)} O2 ) required to support CCH can be calculated continuously with time while the vehicle  10  is in the non-moving state through the following equation: 
     
       
         
           
             
               
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     In a third process  20 , the FCS  12  consumes O 2  from the volume of the assumed enclosed space  14  to support stack voltage recovery (SVR) of the FCS  12 . During SVR operation, the FCS  12  runs with a low voltage in order to remove contaminant build up and/or impurities from the catalyst layers and/or the solid polymer electrolytes within the stack (e.g., sulfate build-up). Subsequently, water washes away the contaminants. A SVR cycle is commonly implemented during vehicle start up, and the molar flow of O 2  ({dot over (n)} O2 ) required to support a SVR cycle can be calculated continuously with time while the vehicle  10  is in the non-moving using the same equation as set forth above for drawing electrical current (I). 
     Other factors may also influence the amount of O 2  present within the assumed enclosed space  14  besides the direct consumption of O 2 . For example, in some cases, the O 2  concentration in the assumed enclosed space  14  can be further reduced based on the output of unreacted H 2  from the FCS  12 , which can dilute the O 2  and lower its concentration. The effect of O 2  dilution is generally negligible when compared to the direct consumption of O 2  through stack current generation, CCH, and SVR, and for that reason it can usually be ignored when calculating the O 2  of the assumed enclosed space  14  over time, although provisions can be made to take O 2  dilution into account if desired as part of the overall method. 
     The assumed enclosed space  14  is also replenished by air at an air/exchange rate  22  from outside of the assumed enclosed space  14 . The air/exchange rate  22  may be set to any value to adjust the modeling of the O 2  concentration within the assumed enclosed space  14  as desired. In one embodiment, according to the incorporated SAE J 2578 standard, the air/exchange rate  22  may be 0.03 air exchanges/hour, meaning that 3% of the air by volume in a designated space is replenished each hour. Of course, the air/exchange rate  22  is calibratable and may be set higher or lower than 0.03 air exchanges/hour. By knowing the volume, temperature, pressure, and original O 2  concentration of the assumed enclosed space  14 , as well as the amount of O 2  consumed by processes  16 ,  18 ,  20  and replenished by the air/exchange rate  22 , the amount and concentration of O 2  in the assumed enclosed space  14  can be calculated or modeled over time during the time the vehicle  10  is in the non-moving state and the FCS  12  is operating using the Ideal Gas Law or some other appropriate equation or algorithm. This information can be used to inform the conditional operating mode of the FCS  12  and to trigger certain actions with respect to the operation of the FCS  12  when the modeled O 2  concentration of the assumed enclosed space  14  eclipses certain predetermined O 2  concentration concern levels. 
     With reference, for the moment, to  FIG. 2 , a graph is shown in which several different operating scenarios of the FCS  12  are depicted with the y-axis representing a calculated O 2  concentration in mole percentage of the assumed enclosed space  14  and the x-axis representing time in seconds. The volume of assumed enclosed space  14  is that established in the SAE J 2578 standard. Additionally, the air/exchange rate  22  is set at 0.03 air exchanges/hour, the temperature and pressure within the assumed enclosed space  14  is set to be 60° C. and 1 atmosphere, respectively, and the O 2  concentration of the assumed enclosed space  14  is set at the standard atmospheric O 2  concentration of 21 mol % at the time the vehicle  10  that includes the FCS  12  is first determined to be in the non-moving state. Each of the current generating process  16 , the CCH process  18 , and the SVR process  20  are deemed to consume O 2  from the assumed enclosed space  14  as described above. The graph here is meant to show how certain operating scenarios of the FCS  12  can deplete the modeled O 2  concentration within the assumed enclosed space  14  and how quickly such depletion can occur. 
     Four specific operating scenarios of the FCS  12  are illustrated in  FIG. 2 . In a first scenario, identified by reference numeral  30 , a 200 second warm-up cycle  32  is performed using the CCH process  18 , followed by an idle period  34  of greater than 1200 seconds without running the CCH or SVR processes  18 ,  20 . In this scenario  30 , the O 2  concentration of the assumed enclosed space  14  dropped by approximately 0.7 mol % during the warm-up cycle  32 , but then decreased only slightly by another 0.1 mol % to 0.2 mol % over the lengthier idle period  34 . In a second scenario, identified by reference numeral  36 , a 200 second warm-up cycle  38  is performed in the same way as the first scenario  30 , followed by an idle period  40  of 1200 seconds in which the CCH process  18  was used to provide 8 kW of cabin heating. In this scenario  36 , after 1400 seconds, the O 2  concentration of the assumed enclosed space  14  dropped by over 2.5 mol %. 
     In a third scenario, identified by reference numeral  42 , a 200 second warm-up cycle  44  is performed in the same way as the first and second scenarios  30 ,  36 , followed by a 30 second SVR cycle  46  and then an idle period  48  of 820 seconds in which the CCH process  18  was used to provide 8 kW of cabin heating. Here, after 1100 seconds, O 2  concentration of the assumed enclosed space  14  dropped by over 3.0 mol %. Finally, in a fourth scenario, identified by reference numeral  50 , two 30 second SVR cycles  52  are performed followed by an idle period  54  of greater than 1100 seconds without running the CCH or SVR processes  18 ,  20 . In this scenario  50 , the O 2  concentration of the assumed enclosed space  14  dropped by approximately 0.5 mol % during each SVR cycle  20 , but then decreased only slightly by another 0.1 mol % to 0.2 mol % over the lengthier idle period  54 . 
     In light of the various ways the FCS  12  may consume O 2  and actually affect the O 2  concentration within the assumed enclosed space  14  over time, the conditional operating mode relies on a set of programmed O 2  concentration concern levels to ensure that the calculated O 2  concentration of the space  14  does not drop too excessively. But first, in order to trigger the conditional operating mode, the vehicle  10  that is powered by the FCS  12  is first determined to be in a non-moving state. This can be accomplished by observing certain available parameters of the vehicle  10 , including its speed and/or location (using GPS coordinates), as well as utilizing proximity sensor(s), vision systems, and/or LIDAR to observe the surroundings of the vehicle  10 , to name a few options. After determining that the vehicle is in the non-moving state, an O 2  concentration of the assumed enclosed space  14  is calculated over time, as discussed above in connection with  FIGS. 1-2 , while the vehicle  10  is in the non-moving state. The calculated O 2  concentration of the assumed enclosed space  14  is then compared over time to a set of established O 2  concentration concern levels. Based on this ongoing comparison, the conditional operating mode may restrict certain operations of the FCS  12 , as will be further explained below. 
     Determining whether the vehicle  10  is in a non-moving state can be carried out by considering the speed of the vehicle  10 . For example, and referring now to  FIG. 3 , one approach  60  for determining when the vehicle  10  is in a non-moving state is shown in a flow diagram. The approach  60  starts in box  62 . In box  64 , the approach  60  involves determining whether the vehicle is moving at a speed indicative of a non-moving state. In one embodiment, a speed of less than or equal to 5 kilometers/hour may be programmed to be indicative of a non-moving state. In another narrower embodiment, the vehicle  10  may only be considered to be in a non-moving state when the vehicle is stationary (i.e., a speed of 0 kilometers/hour). If the speed condition for a non-moving state is not met, the approach  60  proceeds through arrow  66  to box  68  where the vehicle  10  is determined to not be a non-moving state and the conditional operating mode is therefore not initiated. If the speed condition for a non-moving state is met, the approach proceeds through arrow  70  to box  72  where the vehicle  10  is determined to be in a non-moving state and the conditional operating approach is initiated until the vehicle  10  in no longer in the non-moving state. 
     Another more robust approach  74  for determining when the vehicle  10  is in a non-moving state, which is shown in  FIG. 4  as a flow diagram, may be carried out by considering the speed and position of the vehicle  10 . This approach starts in box  76 . In box  78 , the approach involves determining whether the vehicle is moving at a speed indicative of a non-moving state in the same way as previously described in connection with  FIG. 3 . If the speed condition for a non-moving state is not met, the approach  74  proceeds through arrow  80  to box  82  where the vehicle  10  is determined to not be a non-moving state and the conditional operating mode is therefore not initiated. If, however, the speed condition for a non-moving state is met, the approach  74  proceeds through arrow  84  to box  86  where GPS coordinates of the vehicle  10  are referenced to ascertain whether the vehicle  10  is on a road (e.g., a highway). If the vehicle  10  is on a road, the approach proceeds through arrow  88  to box  90  where the vehicle  10  is determined to not be a non-moving state and the conditional operating mode is therefore not initiated. If the vehicle  10  is not on a road, the approach proceeds through arrow  92  to box  94  where the vehicle  10  is determined to be in a non-moving state and the conditional operating approach is initiated until the vehicle  10  in no longer in the non-moving state. 
     If the vehicle is determined to be in a non-moving state by either approach  60 ,  74  described above, or some other approach, the O 2  concentration of the assumed enclosed space  14  is calculated over time while the vehicle  10  remains in the non-moving state. This may entail initially setting the O 2  concentration of the assumed enclosed space  14  at the standard atmospheric O 2  concentration of 21 mol % at the time the vehicle  10  that includes the FCS  12  is first determined to be in the non-moving state and then adjusting the O 2  concentration of the assumed enclosed space  14  based on the amount of O 2  consumed by the FCS  12  over time and the amount of O 2  replenished to the assumed enclosed space  14  over time. The amount of O 2  consumed by the FCS  12  over time and an amount of O 2  replenished to the assumed enclosed space  14  over time can be easily calculated using the O 2  consumption processes  16 ,  18 ,  20  and the air/exchange rate  22  discussed above, respectively, in connection with  FIGS. 1-2 . And by additionally knowing the volume, temperature, and pressure of assumed enclosed space  14 —each of which can be specified in a variety of ways as explained above—the O 2  concentration within the assumed enclosed space  14  can be continuously calculated. 
     Referring now to  FIG. 5 , the calculated O 2  concentration within the assumed enclosed space  14  is compared over time against a set of established O 2  concentration concern levels so long as the vehicle  10  remains in the non-moving state. Here, in this figure, the calculated O 2  concentration (y-axis) is identified by reference numeral  96  and is plotted against time in seconds (x-axis). The set of O 2  concentration concern levels are also shown in this figure and includes a first O 2  concentration concern level  98  and a second O 2  concentration concern level  100 . The first O 2  concentration concern level  98  is less than the standard atmospheric O 2  concentration (i.e., less than 21%) and the second O 2  concentration concern level  100  is less than the first O 2  concentration concern level  98 . For example, the first O 2  concentration concern level  98  may be less than 21 mol % and greater than 19 mol %, and the second O 2  concentration concern level  100  may be less than 20 mol % and greater than 18 mol %. In one particular example, as shown, the first O 2  concentration concern level  98  may be set at 20 mol % and the second O 2  concentration concern level  100  may be set at 19 mol %. 
     The conditional operating mode of the FCS  12  performs its function based on the comparison of the calculated O 2  concentration  96  within the assumed enclosed space  14  against the set of established O 2  concentration concern levels. So long as the calculated O 2  concentration  96  within the assumed enclosed space  14  remains above the first O 2  concentration concern level  98 , which is the case between 0 seconds and approximately 340 seconds in  FIG. 5 , the FCS  12  can operate without restriction, meaning that all of its oxygen consuming processes  16 ,  18 ,  20  are enabled and can be operated as called-upon either individually or in combination to support the demands of the vehicle  10 . In particular, the the current generating process  16  can run supply any necessary current (I) required by the vehicle  10 , warm-up and cabin heating as enabled by the CCH process  18  can be performed, and cycles of the SVR process  20  can be conducted however desired. 
     If at some point the calculated O 2  concentration  96  reaches the first O 2  concentration concern level  98 , which occurs at approximately 340 seconds in  FIG. 5 , the FCS  12  is restricted to operating in a low-power state in which the FCS  12  outputs no more than 15% of its maximum power output in order to slow the rate at which the calculated O 2  concentration  96  is dropping. This may entail restricting the FCS  12  to a maximum power output of 8 kW. In one particular example of a low-power state, the FCS  12  may be idled. When idled, the FCS  12  supplies current (I) to power its associated prime mover, typically an electric motor that may or may not be coupled to an internal combustion engine, and its accessories without any applied loads. Essentially, the current generating process  16  is operational so that the FCS  12  can supply enough current (I) to power the prime mover while the prime mover is not coupled to the drivetrain and the foot pedal is not depressed. Each of the CCH process  18  and the SVR process  20  may also be disabled. Running the FCS  12  in the low-power state, particular in idle mode with the CCH and SVR processes  18 ,  20  disabled, results in a fairly constant and minimal reduction in the calculated O 2  concentration  96  over time and allows the calculated O 2  concentration  96  to remain above the second O 2  concentration concern level  100  for an extended period. For example, in the example of  FIG. 5 , the calculated O 2  concentration  96  will not reach the second O 2  concentration concern level  100  for approximately another 9,000 seconds (˜2.6 hours or 9,350 seconds since the vehicle entered the non-moving state) when the vehicle  10  is idled and the CCH and SVR processes  18 ,  20  are disabled. 
     Should the calculated O 2  concentration  96  reach the second O 2  concentration concern level  100 , which occurs at approximately 9,350 seconds in the example of  FIG. 5  as mentioned above, the FCS  12  is shut down to halt all of the oxygen consuming processes  16 ,  18 ,  20  and thus prevent any further decrease in the calculated O 2  concentration  96 . In that regard, since only the air/exchange rate  22  is affecting the amount O 2  in the assumed enclosed space  14 , the calculated O 2  concentration  96  should begin to rise until it eventually arrives at the standard atmospheric O 2  concentration of 21 mol %. By operating the FCS  12  in a low-power state when the calculated O 2  concentration  96  is equal to or less than the first O 2  concentration concern level  98 , yet greater than the second O 2  concentration concern level  100 , and shutting down the FCS  12  when the calculated O 2  concentration  96  is equal to or less than the second O 2  concentration concern level  100 , the conditional operating mode of the FCS  12  provides an orderly and controlled procedure for ensuring the FCS  12  will not deplete the available O 2  too extensively in an actual enclosed space such as a garage, if and when the vehicle  10  is in such a space, while at the same time not burdening the use of the FCS  12  with nuisance actions when the vehicle  10  is not in an actual enclosed space but is nonetheless in a non-moving state. 
     The capacity of the conditional operational mode of the FCS  12  to avoid nuisance actions—most notably unnecessary shut downs of the FCS  12 —is demonstrated in  FIG. 6 . Here, the calculated O 2  concentration  96  of the assumed enclosed space  14  is plotted against time in circumstances when the vehicle  10  has been determined to be in a non-moving state, although, in actuality, the vehicle  10  is not parked in an actual enclosed space but instead is stuck in a traffic jam where a shutdown of the FCS  12  is not desired. And since the program that administers the overall method including the conditional operating mode of the FCS  12  may be unable to distinguish between being parked in an actual garage or being stuck in a traffic jam, as the speed of the vehicle  10  in both instances may be 0 km/hour, the FCS  12  is subjected to the conditional operating mode by default even though the rationale behind implementing the conditional operating mode is not present. 
     As can be seen, and still referring to  FIG. 6 , the vehicle  10  is determined to be in a non-moving state at about 60 seconds due to traffic congestion, which is more likely to occur if the determination of a non-moving state is based on the speed of the vehicle  10  as described in connection with  FIG. 3 . The FCS  12  operates without restriction while stuck in traffic form approximately another 190 seconds (just over three minutes) until the calculated O 2  concentration  96  of the assumed enclosed space  14  reaches the first O 2  concentration concern level  98 , which has been set at 20 mol %. At this point, and with the vehicle  10  still in the non-moving state due to traffic congestion, the FCS  12  is restricted to operating in the low-power state, and is preferably idled with the CCH and SVR processes  18 ,  20  disabled, until such time that the vehicle  10  moves to negate its status as being in a non-moving state, which occurs after about another 400 seconds. Indeed, the idling of the FCS  12  during this 400 second period (just under 7 minutes) slowed the rate of decrease of the calculated O 2  concentration  96  to the extent that the FCS  12  was nowhere close to being shut down on account of the calculated O 2  concentration  96  reaching the second O 2  concentration concern level  100 , which has been set at 19 mol %. In fact, the FCS  12  could have idled for at least a few hours before the calculated O 2  concentration  96  approached the second O 2  concentration concern level  100 . Consequently, the use of the first O 2  concentration concern level  98  to trigger the restrictive operation of the FCS  12  in the low-power state should prevent nuisance shut down of the FCS  12  as the length of time the FCS  12  can operate in such a state before triggering a shutdown will almost assuredly outlast any amount of time the vehicle  10  may be in a non-moving state due to a traffic jam or otherwise. 
     The various operations needed to administer the overall method including the conditional operating mode of the FCS stack  12  can be programmed into a standard vehicle electronic control unit or another control system that controls the operation of the vehicle and/or the FCS  12 . Such programming, for example, may follow the flow diagram illustrated in  FIG. 7 . In the illustrated flow diagram, a program that performs the overall method starts in box  102 . In box  104 , the program calculates the volume of the assumed enclosed space  14  to be used for carrying out the method. Subsequently, the program determines in box  106  if the vehicle  10  is in a non-moving state, as discussed above, for instance, in connection with  FIGS. 3-4 . If the vehicle  10  is determined not to be in a non-moving state, the program proceeds through arrow  108  to box  110 . In box  110 , the program resets the O 2  concentration of the assumed enclosed space  14  to the standard atmospheric concentration of 21 mol %, thus avoiding the conditional operating mode, and then cycles through boxes  106  and  110  until the vehicle  10  is determined to be in a non-moving state. 
     If the vehicle  10  is determined to be in a non-moving state in box  106 , the program follows arrow  112  to box  114  where the calculated O 2  concentration  96  of the assumed enclosed space  14  is computed based on a series of sub-calculations including calculating the consumption of O 2  in the assumed enclosed space  14  due to the current generating process  16 , box  116 , calculating the consumption of O 2  in the assumed enclosed space  14  due to the CCH process  18 , box  118 , calculating the consumption of O 2  in the assumed enclosed space  14  due to the SVR process  20 , box  120 , and calculating the replenishment of O 2  in the assumed enclosed space  14  due to the air/exchange rate  22 , box  122 , all of which may be computed using a selected temperature (e.g., 60° C.) and pressure (e.g., 1 atmosphere) of the assumed enclosed space  14  and an initial O 2  concentration that equals the standard atmospheric O 2  concentration (e.g., 21 mol %). The program then follows arrow  124  to box  126  where the calculated O 2  concentration  96  of the assumed enclosed space  14  is compared against the first O 2  concentration concern level  98 . If the calculated O 2  concentration  96  of the assumed enclosed space  14  is greater than the first O 2  concentration concern level  98 , the program proceeds through arrow  128  and continues to cycle through boxes  106 ,  114 , and  126 , thus operating the FCS  12  without restriction, until the program indicates in box  124  that the calculated O 2  concentration  96  has reached the first O 2  concentration concern level  98 . 
     When the calculated O 2  concentration  96  has reached the first O 2  concentration concern level  98 , as indicated in box  124 , the program proceeds through arrow  130  to box  132 . In box  132 , the FCS  12  operates in a low-power state as described above, which may include idling the FCS  12  and disabling both the CCH process  18  and the SVR process  120 . The program then proceeds through arrow  134  to box  136  where the calculated O 2  concentration  96  is compared against the second O 2  concentration concern level  100 . If the calculated O 2  concentration  96  of the assumed enclosed space  14  is greater than the second O 2  concentration concern level  100 , the program proceeds through arrow  138  and continues to cycle through boxes  106 ,  114 ,  126 , and  136 , thus continuing to restrict operation of the FCS  12  to the low-power state, until the program indicates in box  136  that the calculated O 2  concentration  96  has reached the second O 2  concentration concern level  100 . When the calculated O 2  concentration  96  has reached the second O 2  concentration concern level  100 , as indicated in box  136 , the program proceeds through arrow  140  to box  142 . In box  142 , the FCS  12  is shut down. Of course, if the program at any time determines in box  106  that the vehicle  10  is no longer in a non-moving state and the FCS  12  has not been shut down, full unrestricted operation of the FCS  12  is restored and the calculated O 2  concentration  96  of the assumed enclosed space  14  is reset to the standard atmospheric concentration of 21 mol %. 
     The system and method(s) discussed herein provide advantages over simple timing systems that take a particular action (e.g., shut off the FCS) after a set time. Computing the calculated O 2  concentration of the assumed enclosed space  14  over time and comparing that value against the set of O 2  concentration concern levels  98 ,  100  corrective actions to be taken with respect to the operation of the FCS  12  that would not be available with a simple countdown-style, timing system. Additionally, a countdown-style, timing system can result in nuisance shutdowns of the FCS  12  that the presently disclosed method can avoid. 
     The above description of preferred exemplary embodiments and specific examples are merely descriptive in nature; they are not intended to limit the scope of the claims that follow. Each of the terms used in the appended claims should be given its ordinary and customary meaning unless specifically and unambiguously stated otherwise in the specification.