Patent Publication Number: US-2005136296-A1

Title: Controlling a fuel cell system

Description:
BACKGROUND  
      The invention generally relates to controlling a fuel cell system.  
      A fuel cell is an electrochemical device that converts chemical energy produced by a reaction directly into electrical energy. For example, one type of fuel cell includes a polymer electrolyte membrane (PEM), often called a proton exchange membrane, that permits only protons to pass between an anode and a cathode of the fuel cell. At the anode, diatomic hydrogen (a fuel) is reacted to produce hydrogen protons that pass through the PEM. The electrons produced by this reaction travel through circuitry that is external to the fuel cell to form an electrical current. At the cathode, oxygen is reduced and reacts with the hydrogen protons to form water. The anodic and cathodic reactions are described by the following equations: 
 
H 2 →2H + +2e −  at the anode of the cell, and  Equation 1 
 
O 2 +4H + +4e − →2H 2 O at the cathode of the cell.  Equation 2 
 
      A typical fuel cell has a terminal voltage near one volt DC. For purposes of producing much larger voltages, several fuel cells may be assembled together to form an arrangement called a fuel cell stack, an arrangement in which the fuel cells are electrically coupled together in series to form a larger DC voltage (a voltage near 100 volts DC, for example) and to provide more power.  
      The fuel cell stack may include flow plates (graphite composite or metal plates, as examples) that are stacked one on top of the other, and each plate may be associated with more than one fuel cell of the stack. The plates may include various surface flow channels and orifices to, as examples, route the reactants and products through the fuel cell stack. Several PEMs (each one being associated with a particular fuel cell) may be dispersed throughout the stack between the anodes and cathodes of the different fuel cells. Electrically conductive gas diffusion layers (GDLs) may be located on each side of each PEM to form the anode and cathodes of each fuel cell. In this manner, reactant gases from each side of the PEM may leave the flow channels and diffuse through the GDLs to reach the PEM.  
      A fuel cell system may include a fuel processor that converts a hydrocarbon (natural gas or propane, as examples) into a fuel flow for the fuel cell stack. For a given output power of the fuel cell stack, the fuel flow to the stack must satisfy the appropriate stoichiometric ratios governed by the equations listed above. Thus, a controller of the fuel cell system may determine the appropriate output power from the stack and based on this determination, estimate the fuel flow to satisfy the appropriate stoichiometric ratios. In this manner, the controller regulates the fuel processor to produce this flow, and in response to controller determining that the output power should change, the controller estimates a new rate of fuel flow and controls the fuel processor accordingly.  
      The fuel cell system may provide power to an external load, such as a load that is formed from residential appliances and electrical devices that may be selectively turned on and off to vary the power that is consumed by the load. Thus, the power that is consumed by the external load may not be constant, but rather, the power that is consumed by the load may vary over time and abruptly change in steps. For example, if the fuel cell system provides power to a house, different appliances/electrical devices of the house may be turned on and off at different times to cause the power that is consumed by the external load to vary in a stepwise fashion over time.  
      One type of conventional fuel cell system includes a fuel cell stack that operates at a fixed operating point. Thus, the output power from the fuel cell stack remains constant. Such a conventional fuel cell system does not adjust the total power demanded from the fuel cell stack in accordance with the power that is demanded by the external load. Rather, other loads to the stack are adjusted to keep the fuel cell stack operating at the fixed setpoint. For example, when the power demanded by the external load is relatively low, the stack may furnish more power to a power grid, parasitic loads to the stack may be increased, etc. so that the system has an overall constant output power that does not vary in accordance with the power that is demanded by the external load. Thus, this type of conventional system may be generally inefficient.  
      Therefore, it may be desirable for the fuel cell system to have a variable output power so that the total power output of the system follows the power that is demanded by the external load. In such a system, the fuel cell stack typically supplies a power that varies over a certain range to track the power that is demanded by the external load. However, at the low and high ends of this range, the fuel cell stack and possibly other components of the fuel cell system may be relatively inefficient. Furthermore, the life of the fuel cell stack may be significantly reduced when the fuel cell stack operates near the boundaries of the range.  
      Thus, there is a continuing need for an arrangement and/or technique to address one or more of the problems that are stated above. There may also be continuing need for an arrangement and/or technique to address one or more problems that are not stated above.  
     SUMMARY  
      In an embodiment of the invention, a technique that is usable with a fuel cell system includes using a stored energy source of the system to supply power to an external load, placing a fuel cell stack of the system in an inactive state during the using, returning the fuel cell stack to an active state to recharge the stored energy source and returning the fuel cell stack to the inactive state in response to the completion of the charging.  
      In another embodiment of the invention, a technique that is usable with a fuel cell system includes determining a system power demand in a fuel cell system and in response to the determination, isolating a fuel cell stack from the fuel cell system.  
      In another embodiment of the invention, a technique that is usable with a fuel cell system includes determining whether a fuel cell stack is exhibiting unstable behavior during an interval of lower power demand from the fuel cell stack and in response to the determination, isolating the fuel cell stack from the fuel cell system.  
      In yet another embodiment of the invention, a technique includes pulsing a fuel processor with an input reactant to minimize at least one of a power loss and startup time of the fuel processor.  
      Advantages and other features of the invention will become apparent from the following drawing, the description and claims.  
    
    
     BRIEF DESCRIPTION OF THE DRAWING  
       FIG. 1  is a schematic diagram of a fuel cell system according to an embodiment of the invention.  
       FIGS. 2, 3  and  4  are flow diagrams depicting operation of the fuel cell system according to embodiments of the invention. 
    
    
     DETAILED DESCRIPTION  
      Referring to  FIG. 1 , an embodiment of a fuel cell system  10  in accordance with the invention includes a fuel cell stack  20  (a PEM-type fuel cell stack, for example) that is capable of producing power that is used (as described below) to power an external load  50  (a residential load, for example) of the system  10  as well as parasitic elements (valves, fans, etc.) of the system  10  in response to fuel and oxidant flows that are provided by a fuel processor  22  and an air blower  24 , respectively. In this manner, the fuel cell system  10  controls the fuel production of the fuel processor  22  (i.e., controls the rate at which the fuel processor  22  provides reformate) to control the fuel flow that is available for electrochemical reactions inside the fuel cell stack  20 . Reactant control valves  44  of the fuel cell system  10  generally route most of this fuel flow to the stack  20 . In some embodiments of the invention, the remainder of the fuel flow may be diverted (via a conduit  55 ) to a flare, or oxidizer  38 .  
      The power that is produced by the fuel cell stack  20  is ultimately consumed by the external load  50 , parasitic elements of the fuel cell system  20  and possibly a power grid  56  (when switches  57  and  58  are closed, a scenario not assumed for purposes of simplifying the following description). If the fuel flow inside the fuel cell stack  20  is sufficient to satisfy the appropriate stoichiometric relationships (defined by Eqs. 1 and 2 above), the fuel cell stack  20  produces the appropriate level of power for the total system power demand (i.e., the total amount of power demanded by the external load  50  and the various loads of the fuel cell system  10 ). Unconsumed, or unreacted, fuel passes through the fuel cell stack  20  to the oxidizer  38 .  
      In some embodiments of the invention, the fuel cell system  10  includes a stored energy source that is used as an energy transfer mechanism for the fuel cell stack  20 . More specifically, as described below, in some embodiments of the invention, the stored energy source provides power directly to the external load  50  and serves as an energy buffer so that the fuel cell stack  20  may (when active) operate at a fixed power output while the stored energy source produces a power output that follows the power that is demanded by the external load  50 . As described in the various embodiments below, it is assumed that this stored energy source includes a bank of electrically connected battery cells (lead acid battery cells, for example), referred to as the “battery bank  41 ” herein. However, in other embodiments of the invention, another stored energy source may be used in place of the battery bank  41 .  
      In some embodiments of the invention, the fuel cell stack  20  is “inactive” when the reactant flows to the stack  20  are shut off to isolate the stack  20 , as further described below. Otherwise, the fuel cell stack  20  is “active.” In some embodiments of the invention, the fuel processor  22  is “inactive” when the fuel processor  22  is turned down to the point at which the fuel processor is just warm enough to self-ignite when the reformate flow is to be re-established to the fuel cell stack  20 , as further described below.  
      In accordance with some embodiments of the invention, the battery bank  41  is the direct source of power for the external load  50  and supplies power that follows the power that is demanded from the load  50 . Thus, the power that is produced by the battery bank  41  is variable, depending on the power that is demanded by the load  50 . Unlike conventional load following fuel cell systems, the power that is output from the fuel cell stack  20  does not generally follow the load  50 . Rather, when active, the fuel cell stack  20  operates at essentially the same operating point and thus, when active, produces a fixed output power, regardless of the power that is demanded by the external load  50 . Because the battery bank  41  is used to provide power to the external load  50  and has an output power that follows the power that is demanded from the load  50 , the fuel cell system  10  is a load following system.  
      In some embodiments of the invention, the power from the fuel cell stack  20  is used to recharge the battery bank  41  after the bank  41  is drained to a predetermined state of charge. During their active states, the fuel cell stack  20  and the fuel processor  22  operate at generally fixed points that may be the most efficient system operating points, i.e., the operating points, at which degradation of the fuel cell stack is the least or the operating points at which the fuel cell stack  20  has the highest efficiency.  
      In some embodiments of the invention, the sole function of the fuel cell stack  20  is to charge the battery bank  41 . Therefore, in these embodiments of the invention, when the battery bank  41  is not being charged, the fuel cell system  10  returns the fuel cell stack  20  and the fuel processor  22  to their inactive states of operation. Thus, essentially, in some embodiments of the invention, only the battery bank  41  directly powers the load  50 ; and the primary function of the fuel cell stack  20  is to recharge the battery bank  41 .  
      The advantages of the above-described arrangement may include one or more of the following. First, this arrangement may allow the fuel cell system  20  to operate at a fixed operating point at which less degradation occurs to the fuel cell stack  20 . In this manner, the degradation of a fuel cell stack  20  is not constant over its potential operating range. Rather, the degradation may be much greater at the low or high ends of the stack&#39;s operating range. In particular, the degradation may be much greater at low current densities. Thus, operation of the fuel cell stack  20  at low current densities may be a significant contributor to stack degradation in residential applications where fuel cell systems must supply power to the load at the lower end of the turndown range for lengthy intervals of time. However, with the arrangement set forth above, the fuel cell system  20  operates at a fixed setpoint for purposes of charging the battery bank  41 . Thus, the system  10 , in some embodiments of the invention, uses the battery bank  41  to directly supply power to the load  50  at levels that, if this power were directly supplied by the stack, would cause high degradation to the fuel cell stack  20 .  
      An additional advantage of the above-described arrangement is that the lifetime of the fuel cell stack  20  is increased. More specifically, if the power that is demanded by the load  50  is less than the fixed output power of the fuel cell stack  20 . The above-described approach can greatly reduce the amount of time the fuel cell stack  20  is in use. Furthermore, operation of the fuel cell stack at a constant current level also allows for a simplified design of an inverter  33  (see  FIG. 1 ) of the fuel cell system  10 , as compared to a design in which the inverter  33  varies the current that is provided by the fuel cell stack  20 .  
      As a more specific example of this latter point, in a hypothetical scenario, the power demanded by the external load  50  may be 500 Watts (W) AC for five hours; and for the scenario, the operating range of the fuel cell stack  20  is assumed (as an example) to be 500 W to 5000 W AC. In a traditional load following system, the stack may operate at 500 W for five hours to supply the power that is demanded by the load; and such operation of the stack may reduce the stack life by ten hours and force the stack to operate at the edge of its turndown range, at which power to the stack may be extremely inefficient. However, using the above-described technique in accordance with some embodiments of the invention, the battery bank  41  supplies the 500 W to the external load  50  for five hours. This removes 2500 W-Hr of energy from the battery bank  41 . The fuel cell stack  20  may then operate at a fixed setpoint of 2500 W and recharge the battery bank  41  in approximately 60 minutes (as compared to 5 hours), excluding any parasitic loses. This means that the stack life is only reduced by 60 minutes, and the stack may operate at a more efficient operating point (i.e., an operating point of 2500 W, as compared to operating at the lower end of its range at 500 W).  
      Another possible advantage of this arrangement is that the complexity and cost of the fuel cell system  10  is greatly decreased if the fuel processor  22  and the fuel cell stack  20  operate at fixed operating points instead of operating over an entire range. This eliminates the need for actuators and sensors that otherwise may be required to dynamically change flow rates, pressures, temperature, etc.  
      Thus, in accordance with some embodiments of the invention, the fuel cell system  10  may use a technique  100  that is depicted in  FIG. 2 . In accordance with the technique  100 , the fuel cell system  10  determines (diamond  102 ) whether it is time to charge the battery bank  41 . In some embodiments of the invention, the decision in diamond  102  may be aided via a battery monitoring circuit  43  (See  FIG. 1 ). The battery monitoring circuit  43  may be part of a battery circuit  45  that includes the battery bank  41  and monitors the power received by and furnished from the battery bank  41  (and thus, determines the net energy currently stored in the battery bank  41 ). As an example, the battery monitoring circuit  43  may monitor the incoming power to the battery bank and monitor the outgoing power from the battery bank  41  by monitoring a terminal voltage of the battery bank  41  and a current (via a current sensor  69  ( FIG. 1 )) flowing to and from the battery bank  41 .  
      Pursuant to the technique  100  ( FIG. 2 ), upon determining that it is time to charge the battery bank  41  (diamond  102 ), the fuel cell system  10  returns the fuel processor  22  and the fuel cell stack  20  to predetermined active operating points, as depicted in block  104 . As an example, if the power output range for the fuel cell system is 500 to 5000 W, the fuel processor  22  and fuel cell stack  20  may be operated so that the fuel cell stack  20  provides a constant output power of 2500 W when active.  
      Next, according to the technique  100 , the fuel cell stack  20  operates (block  106 ) at the fixed power output to charge the battery bank  41 . As described further below, in some embodiments of the invention, the fuel cell system  10  controls when the battery bank  41  charges by regulating a terminal voltage of the battery bank  41 .  
      Subsequently, pursuant to the technique  100 , the fuel cell system  10  returns (block  108 ) the fuel processor  22  and the fuel cell stack  20  to predetermined inactive operating points; and continues (block  110 ) using the battery bank  41  as the sole source of power for the external load  50  until the fuel cell system  10  determines (diamond  102 ) that it is time to charge the battery bank  41  once again.  
      In some embodiments of the invention, the fuel cell system  10  may vary the operating point of the fuel cell stack  20  so that the stack  20  generally tracks the power that is demanded by the load  50 . However, a potential challenge associated with this arrangement is that at low power operating points, the fuel cell stack and its auxiliary components may become very inefficient. Furthermore, the performance of the fuel cell stack  20  may become unstable when producing a relatively low level of power for the external load  50 . To avoid these potential problems, in accordance with some embodiments of the invention, a technique may be used to isolate the fuel cell stack  20  from the external load  50  in response to the system power demand being very low or the stack  20  exhibiting unstable behavior at a low power operating point.  
      More specifically, in accordance with some embodiments of the invention, the fuel cell system  10  may use a technique  150  that is depicted in  FIG. 3  for purposes of operating the fuel cell stack  20 . Pursuant to the technique  120 , the fuel cell system determines (diamond  122 ) whether the system power demand (the power demanded by the external load  50  and the power-consuming components of the fuel cell system  10 ) is below a predetermined threshold and determines (diamond  124 ) whether the stack  20  exhibits unstable behavior at a low operating setpoint. If either condition is true, then in accordance with the technique  120 , the fuel cell system  10  isolates (block  128 ) the fuel cell stack  20  from the rest of the system  10 .  
      As an example, this isolation may include closing the control valves  44  ( FIG. 1 ) that supply reactant flows to the fuel cell stack  20  and electrically disconnecting the load  50  from the stack  20 . The removal of reactants from the fuel cell stack  20  prevents the stack  20  from drying out while the load  50  is removed.  
      For purposes of electrically disconnecting the load  50  from the stack  20 , in some embodiments of the invention, the fuel cell system  10  may include a switch  29  that is coupled in series between the fuel cell stack  20  and the load  50  so that by opening the switch  29 , the fuel cell system  10  may disconnect the fuel cell stack  20  from the load  50 .  
      While the fuel cell stack  20  is isolated, power for the load  50  is provided by the battery bank  41 . The fuel cell system  10  subsequently considers two conditions that may cause the fuel cell system  10  to reconnect the fuel cell stack  20  to the system  10  to remove the stack  20  from isolation. First, the fuel cell system  10  determines (diamond  132 ) whether the battery bank  41  has reached a critical charge level. More specifically, with the battery bank  41  supplying the power for the load  50 , eventually, the battery bank  41  may become sufficiently discharged so that supplemental power may be required from the fuel cell stack  20 . Additionally, the fuel cell system  10  determines (diamond  134 ) whether there is a significant increase in the power that is demanded by the load  50 .  
      If either one of these conditions occur, then, in accordance with the technique  120 , the fuel cell system  10  returns (block  136 ) the fuel cell stack  20  from isolation. In other words, the fuel cell system  10  opens the control valves  44  to reestablish reactant flows to the fuel cell stack  20 , brings the fuel processor  22  out of its idle state and closes the switch  29  to electrically connect the fuel cell stack  20  to the external load  50 . Subsequently, the fuel cell stack  20  may provide the additional power needed by the load  50  and/or provide power needed to recharge the battery bank  41 . Control returns to diamond  122  in which the fuel cell system  10  once again determines whether the system power demand has decreased below the predetermined threshold, as depicted in diamond  122 .  
      Among the possible advantages for the technique  120 , the fuel cell system  10  may have an increased system efficiency for a load profile that includes low power demands (power demands less than 1.5 kW for a system in which the stack  20  follows the load, as an example). The stability of the fuel cell system  10  may be increased. The system life of the fuel cell system  10  may be increased due to the reduced use of the stack  20  and auxiliary components at low power demands. Furthermore, the stack life may be increased due to less use of low current densities, where stack degradation rates may be greater. Other and different advantages may be possible, in other embodiments of the invention.  
      In some embodiments of the invention, the fuel cell system  10  may employ the technique  120  in conjunction with operating the fuel cell stack  20  at a fixed power output (a power output of about 5 kW, for example) when active. In these embodiments of the invention, the fuel cell system  10  may deem the system power low threshold point to be the fixed operating point of the stack  20 . For example, when active, the fuel cell stack  20  may provide a constant output of about 5 kW. For this example, should the system power demand drop below 5 kW, then the fuel cell system  10  isolates the fuel cell stack  20  and proceeds in accordance with the technique  120 . Other variations are possible.  
      In some embodiments of the invention, as set forth above, the fuel processor  22  may be operated at its lowest stable operating point when the fuel cell stack  20  is isolated from the system. However, this option may be challenging, in that the fuel processor  22  may be extremely inefficient when operating at this low operating point. Another option is to simply turn off the fuel processor  22  when the fuel cell stack  20  is isolated from the system. However, this option may be undesirable due to long startup time constants that are associated with turning on the fuel processor  22 .  
      To address these challenges, in accordance with some embodiments of the invention, the fuel processor  22  is operated in a pulsed fashion when the fuel cell stack  20  is not operating. Because the need to provide reformate to the fuel cell stack  20  is not present when the fuel cell stack  20  is not operating, the fuel processor  22  is operated in a fashion that allows the fuel processor  22  to quickly start up and produce reformate quickly when the fuel cell stack  20  begins operating again.  
      More specifically, in accordance with some embodiments of the invention, the pulsing of the fuel processor  22  includes turning off incoming fuel to the processor  22  to allow the temperature inside the fuel processor  22  to slowly decrease. When the temperature decreases to a predetermined threshold, the incoming fuel to the fuel processor  22  is turned back on so that the processor  22  operates to restore its temperature to some nominal level. This technique is repeated until the need for reformate by the fuel cell stack  20  returns.  
      In addition to turning on and off the fuel flow to the fuel processor  22 , in some embodiments of the invention, an oxidant flow to the flow processor  22  is turned on when the fuel flow to the processor  22  is turned on and turned off when the fuel flow to the processor is turned off.  
      In some embodiments of the invention, the “temperature” discussed in conjunction with pulsing the fuel processor  22  may be one of three following temperatures associated with the processor: a low temperature shift (LTS) temperature, an auto thermal reformer (ATR) temperature and an anode tail gas oxidizer (ATO) temperature. Thus, in some embodiments of the invention, should any of these three temperatures decrease below respective thresholds, then fuel cell system  10  reestablishes the fuel input flow to the fuel processor  22 .  
      The thresholds for the ATO and ATR temperatures may be set based on the respective minimum temperatures that, when reached, cause associated heaters (normally off) to automatically turn on. Operation of one or both heaters may be relatively inefficient, as such operation may increase power losses and thus, decrease the system efficiency. Therefore, the low temperature thresholds for the ATO and ATR temperatures may be slightly above the minimum temperatures that cause the heaters to turn on. The threshold for the LTS temperature may be based on the desired time for the fuel processor  22  to turn back on and furnish the desired output reformate flow when the fuel flow to the fuel processor  22  is turned back on. In some embodiments of the invention, the threshold for the LTS temperature is selected so that the fuel processor  22  produces the appropriate level of reformate approximately five to ten minutes after the fuel flow to the reformer  22  is re-established.  
       FIG. 4  depicts a technique  150 , in accordance with some embodiments of the invention, for operating the fuel processor  22  in a pulsed fashion for purposes of maximizing the efficiency of the fuel processor  22  while permitting a fast turn on for the fuel processor  22  when reformate is needed by the stack  20 . In accordance with the technique  150 , the fuel cell system  10  determines (diamond  152 ) whether the fuel cell stack  20  is operating. If not, the fuel cell system  10  turns off the reactant flows (the fuel input flow and oxidant input flow) to the fuel processor  22 , as depicted in block  154 .  
      The turning off of the reactant flows to the fuel processor  22  causes the LTS, ATR and LTO temperatures inside the fuel processor  22  to decrease. Eventually, one of these temperatures reaches a point which the temperature is below its respective threshold. Thus, decreasing the temperature of the fuel processor  22  below this point may introduce significant transient times for bringing the fuel processor  22  back on line and may cause one or more heaters of the processor  22  to turn on.  
      Therefore, in accordance with the technique  150 , the fuel cell system  10  determines (diamond  156 ) whether a temperature of the fuel processor  22  is below its respective minimum temperature threshold. If the temperature is near this point, then, in accordance with the technique  150 , the fuel cell system  10  reestablishes the reactant flows to the fuel processor  22 , as depicted in block  158 . Subsequently, the fuel cell system  10  determines (diamond  160 ) whether the temperature of the fuel processor  22  has reached a predetermined nominal level. If so, then control returns to diamond  152  so that the pulsed operation may discontinue if the fuel cell stack  20  is not operating. Otherwise, the fuel cell system  10  continues with the above-described pulsed operation of the fuel processor  22 .  
      Among the potential advantages of the technique  150 , the system efficiency may be increased over a load profile that includes periods of low power demand from the load  50 . Additionally, the life of the fuel cell system  10  may be increased due to the reduced use of catalysts and auxiliary components at periods of low power demand from the load  50 . Other and different advantages may be possible in other embodiments of the invention.  
      Referring back to  FIG. 1 , among the other features of the fuel cell system  10 , the battery monitoring circuit  43  may provide a signal (called CR) that when asserted (driven high, for example) indicates that the battery bank  41  needs to be charged. The battery monitoring circuit  43  may determine when the bank  41  needs to be charged by monitoring a terminal voltage (called V DC ) of the bank  41 , a voltage that decreases below a predetermined threshold to indicate that charging is needed. Alternatively, the battery monitoring circuit  43  may monitor the V DC  voltage and a current of the bank  41  (via the current sensor  69 ) to monitor a net charge flowing out of the battery bank  41 . In this manner, when the net charge exceeds a predetermined threshold, the battery monitoring circuit  43  asserts the CR signal. The battery monitoring circuit  43  may also determine when charging is complete by monitoring the current into the battery  41  (via the current sensor  69 ). In this manner, when the current approaches a predefined minimum threshold level, the battery monitoring circuit  43  deems the charging to be complete and de-asserts (drives low, for example) the CR signal. Other variations are possible.  
      The fuel cell system  10  includes a controller  60  that, among other things, receives the request (communicated over a communication line  47 ) from the battery monitoring circuit  43  to charge the battery bank  41  and controls other components (described below) of the fuel cell system  10  to cause the fuel cell stack  20  to charge the battery bank  41 . In general, the controller  60  controls these and other components of the fuel cell system  10  to cause the fuel cell system  10  to perform one or more of the techniques  100 ,  120  and  150 , depending on the particular embodiments of the invention.  
      More specifically, in accordance with some embodiments of the invention, the controller  60  may communicate with the fuel processor  22  (via electrical communication lines  46 ) to control operation of the fuel processor  22 , adjust the setpoint of the fuel processor  22  and monitor a temperature (via a temperature sensor  23 ) of the fuel processor  22 . Furthermore, the controller  60  may operate a control valve  19  that supplies hydrocarbons to the fuel processor  22  for purposes of selectively shutting off the fuel flow to the fuel processor  22 . The controller  60  may also, in some embodiments of the invention, control (via electrical communication line  66 ) operation of the control valves  44 .  
      In some embodiments of the invention, the controller  60  monitors the power that is consumed by the external load  50  and the parasitic elements of the fuel cell system  10  by monitoring the cell voltages, the terminal stack voltage (called “V TERM ”) and an output current (called “I1”) of the fuel cell stack  20 . From these measurements, the controller  60  may determine when the fuel cell stack  20  is exhibiting unstable behavior at low power and may determine when the system power demand is below a predetermined threshold.  
      In some embodiments of the invention, the fuel cell system  10  includes a cell voltage monitoring circuit  40  to monitor the cell voltages of the fuel cell stack and the V TERM  stack voltage. Furthermore, the fuel cell system may include a sensor  49  to measure the I1 output current. The cell voltage monitoring circuit  40  communicates (via a serial bus  48 , for example) indications of the measured cell voltages to the controller  60 . The current sensor  49  is coupled in series with an output terminal  31  of the fuel cell stack  20  to provide an indication of the output current (via an electrical communication line  52 ). With the information about the power that is being demanded from the system and by the load  50  and information regarding the status of the fuel cell stack  20 , the controller  60  may then perform one or more of the techniques  100 ,  120  and  150 .  
      In some embodiments of the invention, the controller  60  may include one or more microprocessors or microcontrollers and may include a memory  63  that stores one or more programs  65 . The programs  65 , in turn, include instructions that when executed by the microprocessor(s)/microcontrollers(s) of the controller  60  cause the microprocessor(s)/microcontroller(s) to interact with the various components of the fuel cell system  10  for purposes of performing one or more of the techniques  100 ,  120  and  150 .  
      Among the other features of the fuel cell system  10 , the system  10  may include a DC-to-DC voltage regulator  30  that regulates the V TERM  stack voltage to produce the V DC  voltage that may be used to charge the bank  41  and may be converted into an AC voltage for the external load  50 . In this manner, the fuel cell system  10  may include an inverter  33  that converts the V DC  voltage into an AC voltage that appears on output terminals  32  of the inverter  33  and system  10 . Besides being controlled by the controller  60  to divert some of the fuel flow that is received by the fuel cell stack  20  to the oxidizer  38  via the conduit  55  (during the inactive isolation state of the stack  20 , for example), the controller  60  may control the control valves  44  to shut off reactant flow to the stack  20  to isolate the stack  20  pursuant to the technique  120 . The control valves  44  may also provide emergency shutoff of the oxidant and fuel flows to the fuel cell stack  20 . The control valves  44  are coupled between inlet fuel  37  and oxidant  39  lines and the fuel and oxidant manifold inlets, respectively, to the fuel cell stack  20 . The inlet fuel line  37  receives the fuel flow from the fuel processor  22 , and the inlet oxidant line  39  receives the oxidant flow from the air blower  24 . The fuel processor  22  receives a hydrocarbon (natural gas or propane, as examples) from the valve  19  and converts this hydrocarbon into the reformate (a hydrogen fuel flow, for example) that is provided to the fuel cell stack  20 .  
      The fuel cell system  10  may include water separators, such as water separators  34  and  36 , to recover water from the outlet and/or inlet fuel and oxidant ports of the fuel cell stack  20 . The water that is collected by the water separators  34  and  36  may be routed to a water tank (not shown) of a coolant subsystem  54  of the fuel cell system  10 . The coolant subsystem  54  circulates a coolant (de-ionized water, for example) through the fuel cell stack  20  to regulate the operating temperature of the stack  20 .  
      For purposes of isolating the external load  50  from the fuel cell stack  20 , the system  10  may include the switch  29  (a relay circuit, for example) that is coupled between the main output terminal  31  of the stack  20  and an input terminal of the current sensing element  49 . The controller  60  may control operation of switch  29  via an electrical communication line  51 .  
      In some embodiments of the invention, the controller  60  may include at least one microcontroller that includes a read only memory (ROM) that serves as at least part of the memory  63  that stores instructions for the program(s)  65 . Other types of storage mediums may be used to store instructions for the program(s)  65 . Various analog and digital external pins of the microcontroller(s)/microprocessor(s) of the controller  60  may be used to establish communication over the electrical communication lines  46 ,  51 ,  52  and  53 ; and the serial bus  48 . In other embodiments of the invention, a memory that is fabricated on one or more separate die(s) or semiconductor packages may be used as the memory  63  and store instructions for the program(s)  65 . Other variations are possible.  
      While the invention has been disclosed with respect to a limited number of embodiments, those skilled in the art, having the benefit of this disclosure, will appreciate numerous modifications and variations therefrom. It is intended that the appended claims cover all such modifications and variations as fall within the true spirit and scope of the invention.