Abstract:
A technique that is usable with a fuel cell stack includes coupling the fuel cell stack to a load and determining a power that is consumed by the load. The technique includes detecting a change in the power that is consumed by the load and controlling a fuel flow to the fuel cell stack to control a power output of the fuel cell stack to accommodate the change in the power consumed by the load. The technique also includes delaying the beginning of the controlling in response to the detection of the change in the power that is consumed by the load.

Description:
BACKGROUND 
     The invention generally relates to a technique and apparatus to control the transient response of 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 
     O 2 +4H + +4e − →2H 2 O at the cathode of the cell. 
     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 power that the stack needs to supply, and based on this determination, the controller estimates the fuel flow to satisfy the appropriate stoichiometric ratios to produce this power. In this manner, the controller regulates the fuel processor to produce this flow, and in response to the controller determining that a change in the output power is needed, the controller estimates a new rate of fuel flow and controls the fuel processor accordingly. 
     The fuel cell system may provide power to a 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 demanded by the load. Thus, the power that is consumed by the load may not be constant, but rather the power 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 load to vary in a stepwise fashion over time. 
     It is possible that the fuel processor may not be able to adequately adjust its fuel flow output in a timely fashion to respond to a transient in the power that is consumed by the load. In this manner, the rate at which the power that is consumed by the load changes during a transient may be significantly faster than the rate at which the fuel processor can change its fuel output. For example, the time constant of the fuel processor may be in the order of minutes, and the time constant at which the power that is consumed by the load changes during a transient may be in the order of seconds. Due to this discrepancy, it is possible that the output of the fuel processor may significantly lag transients in the power that is consumed by the load, thereby resulting in inefficient operation of the fuel cell system. 
     For example, if the fuel cell system powers a house, one or more appliances may be briefly turned on to momentarily increase the power that is consumed by the appliance(s) to produce a transient. However, by the time the fuel processor responds to counteract this increase, the one or more appliances that were turned on may have been turned off. During the time during which the fuel processor falls behind, it is possible that power from a power grid may provide the power (to the load) that the fuel cell system fails to provide. However, this arrangement may contribute to increased costs associated with powering the load. 
     Thus, there is a continuing need for an arrangement and/or technique to address one or more of the problems that are stated above. 
     SUMMARY 
     In an embodiment of the invention, a technique that is usable with a fuel cell stack includes coupling the fuel cell stack to a load and determining a power that is consumed by the load. The technique includes delaying in response to a detection of a change in the power consumed by the load, and in response to the expiration of the delaying, controlling a fuel flow to the stack to control a power output of the fuel cell stack to accommodate the change in the power that is consumed by the load. 
     Advantages and other features of the invention will become apparent from the following description, from the drawing and from the claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic diagram of fuel cell system according to an embodiment of the invention. 
     FIG. 2 is a flow diagram depicting a technique to control a fuel flow to the fuel cell stack in response to up and down transients according to an embodiment of the invention. 
     FIG. 3 depicts an exemplary waveform of a power consumed by a load of the fuel cell system over time. 
     FIG. 4 depicts an output of the fuel processor in response to the power depicted in FIG. 3 according to an embodiment of the invention. 
     FIG. 5 is a flow diagram depicting a technique to control a fuel flow to the fuel cell stack in response to an up transient according to an embodiment of the invention. 
     FIG. 6 is a flow diagram of a technique to control a fuel flow to the fuel cell stack in response to a down transient according to an embodiment of the invention. 
     FIG. 7 is a flow diagram depicting a technique to detect a sustained increase in power that is demanded by the load according to an embodiment of the invention. 
     FIG. 8 is a flow diagram depicting a technique to detect a sustained decrease in the power that is demanded by the load according to an embodiment of the invention. 
     FIG. 9 is a flow diagram depicting a technique to detect a sustained increase in power that is demanded by the load according to another embodiment of the invention. 
     FIG. 10 is a flow diagram depicting a technique to detect a sustained decrease in the power that is consumed by the load according to another embodiment 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  that is capable of producing power for a load  50  (a residential load, for example) 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  to control the fuel flow that the processor  22  provides to the fuel cell stack  20 . This rate of fuel flow to the fuel cell stack  20 , in turn, controls the level of power that is produced by the stack  20 . As described below, the fuel cell system  10  bases (at least in part) its regulation of the fuel processor  22  on the power that is consumed (or “demanded”) by the load  50 . 
     The power that is consumed by the load  50  varies with time, as the load  50  represents a collection of individual loads (appliances and/or electrical devices that are associated with a house, for example) that may each be turned on and off. As a result, the power that is consumed by the load  50  may change to produce a transient. In the context of this application, a “transient in the power consumed by the load  50 ” refers to a significant change in the power (in the power that is consumed by the load  50 ) that deviates from the current steady state level of the power at the time the transient occurs. The transient may have a time constant that is on the same order or less than the time constant of the fuel processor  22 . 
     Therefore, the fuel processor  22  may not be able to quickly adjust to transients in the power that is consumed by the load  50 . However, as described below, the fuel cell system  10  takes measures to prevent the fuel processor  22  from prematurely responding to the transients until the system  10  verifies that the change in power is sustained and thus, is not temporary in nature. 
     In the context of the application, the phrase “up transient” refers to a positive transient in the power that is consumed by the load  50 , and the phrase “down transient” refers to a negative transient in the power that is consumed by the load  50 . An up or down transient may or may not result in a sustained change in the power that is consumed by the load  50 . As described below, the fuel cell system&#39;s response to up transients may differ from the system&#39;s response to down transients, in some embodiments of the invention. 
     The effect of up and down transients on the fuel cell system  10  may differ, depending on the power connection mode of the system  10 . In this manner, in a first power connection mode, the fuel cell system  10  is connected to furnish power to the load  50  in parallel with a power grid  56 . Therefore, if the fuel cell system  10  is not capable of supplying all of the power that is consumed by the load  50 , the power grid  56  may supplement the system&#39;s output power. This arrangement may be cost ineffective. Therefore, it may be desirable for the fuel processor  22  to increase its output when the load  50  needs more power. However, the increase in power that is consumed by the load  50  may be short in nature, and as a result, it is possible that by the time the fuel processor  22  increases its fuel output, the power that is consumed by the load  50  has returned to the level that existed before the up transient. Thus, the fuel processor  22  may be producing too much fuel that does not match the power that is being consumed by the load  50 . 
     In some embodiments of the invention, when the power that is consumed by the load  50  is not changing, the fuel processor  22  provides a flow rate that establishes a predetermined percentage of the load&#39;s power, and the remaining percentage is provided by the power grid  56 . In this manner, for these embodiments, both the fuel cell system  10  and the power grid  56  provide power to the load  50 . For example, in some embodiments of the invention, during steady state operation, the fuel cell system  10  may provide ninety-five percent of the power that is consumed by the load  50 , and the power grid  56  may provide the remaining five percent of the power. It is noted that when up or down transients occur, the fuel cell system  10  may provide power that deviates from the predetermined percentage until the fuel cell system  10  changes its power output in accordance with the techniques described herein. 
     In a second power connection mode, the fuel cell system  10  may be disconnected from the power grid  56  and include a battery  41  as a source of instant supplemental power for purposes of providing time to allow the fuel processor  22  to increase its output. Therefore, if the fuel cell stack  20  cannot provide adequate power for the load  50  in response to an up transient, the battery  41  may provide the additional power. However, the increase in power that is consumed by the load  50  may be short in nature, and as a result, it is possible that the by the time the fuel processor  22  increases its fuel output, the power that is demanded by the load  50  has returned to the level that existed before the up transient. It is noted that if the increase in power that is consumed by the load  50  is sustained, the fuel cell system  10  eventually responds to boost its power output to prevent depletion of the finite amount of energy that is stored in the battery  41 . 
     Referring also to FIG. 2, to prevent the fuel processor  22  from prematurely responding to up and down transients, in some embodiments of the invention, the system  10  uses a technique  100  to regulate the fuel production of the fuel processor  22  so that the fuel processor  22  only responds to sustained increases and decreases in the power that is consumed by the load  50 . In the technique  100 , the fuel cell system  10  determines (diamond  102 ) whether an up transient has occurred. If so, the fuel cell system  10  responds (block  104 ) to the up transient using a first control technique (described below), as indicated in block  104 . However, if the output power has not increased, the fuel cell system  10  then determines (diamond  106 ) whether a down transient has occurred. If so, then the fuel cell system  10  responds to the down transient using a different, second control technique (described below). Thus, the fuel cell system  10  may use two different control techniques to control the fuel processor  22 : a first control technique for up transients and a second different control technique for down transients. 
     The two different control techniques accommodate the scenario in which the rate at which the fuel processor  22  increases its output may be significantly slower than the rate at which the fuel processor  22  decreases its output. The two different control techniques may also accommodate the scenario in which the up transients occur at a significantly greater frequency than the down transients. 
     Referring to FIG. 1, in some embodiments of the invention, the fuel cell system  10  includes a controller  60  to detect the up and down transients and regulate the fuel processor  22  accordingly. More specifically, in some embodiments of the invention, the controller  60  detects these up and down transients by monitoring the cell voltages, the terminal stack voltage (called “V TERM ”) and the output current of the fuel cell stack  20 . From these measurements, the controller  60  may determine when an up or down transient occurs in the power that is consumed by the load  50 . 
     To obtain the above-described measurements from the fuel cell stack  20 , the fuel cell system  10  may include a cell voltage monitoring circuit  40  to measure the cell voltages of the fuel cell stack  20  and the V TERM  stack voltage; and a current sensor  49  to measure a DC output current from the stack  20 . 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 from the stack  20 , the controller  60  may execute a program  65  (stored in a memory  63  of the controller  60 ) to determine whether an up or down transient has been detected and control the fuel processor  22  accordingly via electrical communication lines  46 . Specific implementations of the technique  100  (according to different embodiments of the invention) are described below. 
     More specifically, referring to FIGS. 3 and 5, in some embodiments of the invention, the program  65 , when executed by the controller  60 , may cause the controller  60  to perform a technique  150  (depicted in FIG. 5) to control the fuel processor  22  in response to up and down transients. In particular, the controller  60  introduces (block  152  of FIG. 5) a first delay in response to an up transient. For example, the power that is demanded by the load  50  may initially reside near output power level called P 1  (see FIG.  3 ), and during the time interval from T 0  to T 1 , the fuel processor  22  may operate at a steady state fuel output level called L 1  (see FIG. 4) to provide the appropriate fuel to sustain the power that is consumed by the load  50  at the P 1  level. 
     As depicted in FIG. 3, the power that is consumed by the load  50  may actually vary slightly about the P 1  level from time T 0  to time T 1 . However, the controller  60  does not respond to slight deviations from the P 1  level. Instead, the controller  60  establishes a hysteresis zone  121  about the P 1  level by establishing upper  121   a  and lower  121   b  thresholds to set the respective upper and lower limits of the zone  121 . As long as the power that is consumed by the load  50  is within the zone  121 , the controller  60  determines no up or down transient has occurred. Otherwise, a variation of the power outside of the zone  121  indicates an up transient (for an increase above the upper threshold  121   a ) or a down transient (for a decrease below the lower threshold  121   b ). 
     As an example, as depicted in FIG. 3, at time T 1 , the power that is consumed by the load  50  increases to a new output level P 2 , a level that is above the upper threshold  121   a  and thus, is recognized by the controller  60  as being an up transient. This increase may be attributable to one or more appliances and/or devices (that are associated with a house, for example) being turned on at about the same time, for example. As noted from FIG. 3, the increase may approximate a step function. 
     The controller  60  does not immediately respond to this increase but rather introduces a delay, or delay interval  125 , from time T 1  until time T 2 , pursuant to block  152  (see FIG.  5 ). As described in more detail below, this delay may have a fixed or variable duration, depending on the particular embodiment of the invention. 
     At the expiration of the delay interval (such as the delay interval  125 ), the controller  60  determines (diamond  154  of FIG. 5) whether there has been a sustained increase in the power that is consumed by the load  50  during the delay interval. For the example that is depicted in FIG. 3, the power that is consumed by the load  50  during the delay interval  125  does not deviate from a hysteresis zone  123  that the controller  60  establishes about the P 2  level. If the output power would have decreased below the upper threshold  121   a,  for example, during the delay interval  125  then the controller  60  would deem this as not being a sustained increase in the power that is consumed by the load  50  and thus, would reset the delay interval without changing the output of the fuel processor  22 . However, as shown, the power that is demanded by the load  50  remains with the zone  123  during the interval  125 , and as a result, the controller  60  increases the fuel output of the fuel processor  22  to respond to the increase in the load  50 , in accordance with block  156  of FIG.  5 . 
     Referring to FIG. 4, thus, from time T 0  to T 1 , the fuel output of the fuel processor  22  is at a constant level L 1 , as the power that is demanded by the load  50  also remains at a nearly constant level. At time T 1 , the fuel output of the fuel processor  22  does not change (although the power that is consumed by the load  50  has changed). At the expiration of the delay interval  125  at time T 2 , the controller  60  controls the fuel processor  22  to ramp its fuel production upwardly until the output of the fuel processor reaches a level L 2 , a level that sustains the P 2  level of power that is being consumed by the load  50 . 
     In some embodiments of the invention, the controller  60  controls the maximum rate at which the fuel processor  22  increases its fuel production to minimize the level of carbon monoxide that may be otherwise produced by causing the fuel processor  22  to change its operating point too rapidly. In this manner, the controller  60  may establish a predefined maximum rate of increase (as indicated by the upward slope  129  in FIG. 4) that permits the fuel processor  22  to ramp upwardly without producing excessive carbon monoxide. The controller  60  may impose a similar limit on the rate of decrease in the fuel processor&#39;s output, as depicted by the constant decreasing slope  130  in FIG.  4 . 
     In some embodiments of the invention, the controller  60  executes the program  65  to perform a technique  160  (depicted in FIG. 6) to perform the second control technique for responding to down transients. Referring to FIGS. 3 and 6, in this manner, the controller  60  may introduce a second delay, or delay interval, (pursuant to the second control technique) when the controller  60  detects a down transient, as depicted in block  162  of FIG.  6 . The controller  60  determines that a down transient has occurred when the power that is consumed by the load  50  decreases below the lower threshold of the associated hysteresis zone, as described above. If the controller  60  determines (diamond  164 ) that this decrease is sustained (i.e., the power that is consumed by the load  50  does not increase above the lower threshold during the delay interval), then the controller  60  decreases (block  166 ) the output of the fuel processor  22  to respond to the sustained decrease in power. 
     As an example, FIG. 3 depicts a down transient that occurs at time T 3 . In response to this down transient, the controller  60  begins measuring a delay interval  126  that lasts from time T 3  until time T 4 . Because the power that is demanded by the load  50  does not increase above the lower threshold  123   b  of the zone  123  during the interval  126 , the controller  60  determines a sustained decrease in the power has occurred and decreases the output of the fuel processor  22  (as indicated by the ramp  130 ) during time T 4  to time T 6 . At time T 6 , the fuel processor  22  provides an output level L 3  to cause the fuel cell stack  20  to provide the appropriate level of power to the load  50 . 
     FIG. 3 also depicts a momentary spike  120  in the power that is consumed by the load  50 . The spike begins at time T 5  and lasts until time T 7 . In response to the increase, the controller  60  introduces another delay interval  128  that begins at time T 5  and extends until time T 7 . However, the delay interval  128  is shorter than the delay interval  125 , as the controller  60  recognizes (at time T 7 ) that the increase in power has not been sustained and therefore, resets the delay and does not increase the fuel output of the fuel processor  22  to accommodate this increase. 
     Thus, pursuant to the technique  160 , if the controller  60  determines (diamond  164 ) that a sustained decrease in the power that is consumed by the load  50  has existed for the duration of the second delay interval, the controller  60  decreases the fuel output of the fuel processor  22  to respond to the decrease in load, as depicted in block  166 . 
     The first delay interval (associated with the first control technique) and the second delay interval (associated with the second control technique) may each have a fixed duration; may each have a variable duration; or one of the delay intervals may have a fixed duration and the other delay interval may have a variable duration, depending on the particular embodiment of the invention. As an example, FIG. 7 depicts a technique  170  that is used in connection with the first control technique and which uses a variable duration for the first delay interval. The technique  170  may be performed by the controller  60  when executing the program  65 . 
     In the technique  170 , the controller  60  measures (block  172 ) the power that is demanded by the load  50  at regular time intervals, the frequency of which is governed by the first control technique. From these sampled measurements, the controller  60  constructs a rolling average of the power that is consumed by the load  50 . For example, the controller  60  may measure the power that is consumed by the load  50  at five minute intervals. Other time intervals may be used. After measuring the power at each time interval, the controller  60  determines (block  174 ) a new rolling average for the power that is consumed by the load  50 . If the controller  60  subsequently determines (diamond  176 ) that the rolling average of the power is above an upper threshold, then the controller  60  sets (block  178 ) a flag indicating the continued increase and controls the fuel processor  22  accordingly. As an example, the upper threshold may represent a predetermined percentage increase from a level of the power averaged over the last several time intervals, for example. Other techniques may be used to set the threshold. Alternatively, the rolling average itself may be used to control the output of the fuel processor  22  without comparing this average to a threshold before taking action with the fuel processor  22 . Other variations are possible. 
     Therefore, due to this technique, increases in the power (that is consumed by the load  50 ) that are relatively short in duration do not effect the rolling average. However, sustained increases in the power increase the rolling average and thus, provoke a change in the output of the fuel processor  22 . 
     Referring to FIG. 8, in a similar manner, the controller  60  may perform a rolling average technique  182  to address decreases in the power that is consumed by the load  50 . The controller  60  may perform the technique  182  when executing the program  65 . 
     In the technique  182 , the controller measures (block  184 ) the power that is consumed by the load  50  at the next regular time interval. The timing of the time intervals (i.e., the frequency at which measurements of the power that is consumed by the load  50  are taken) is governed by the second control technique. After each measurement, the controller  60  uses the measurement to determine (block  186 ) the new rolling average. 
     In some embodiments of the invention, the controller  60  takes the measurements that are used for determining the rolling average that is associated with the second control technique at a higher frequency than the measurements that are used for determining the rolling average that is associated with the first control technique. This difference allows the controller  60  to respond more rapidly to decreases in the power that is consumed by the load  50  than to increases in the power that is consumed by the load  50 . 
     Continuing the description of the technique  182 , if the controller  60  determines (block  188 ) that the average power is below a lower threshold, then the controller  60  sets (block  190 ) a flag indicating the continued decrease and proceeds as described above to control the fuel processor  22  to respond to the sustained decrease in the power that is consumed by the load  50 . Alternatively, the rolling average itself may be used to control the output of the fuel processor  22  without comparing this average to a threshold before taking action with the fuel processor. Other variations are possible. 
     Thus, the controller  60  may use a first rolling average in connection with the first control technique to respond to up transients and a second rolling average in connection with the second control technique to respond to down transients. 
     Instead of using rolling averages to establish the first and second delay intervals, in some embodiments of the invention, the controller  60  may measure a delay interval that has a constant, or fixed, duration. In this manner, the controller  60  may introduce a fixed delay interval that is shorter in duration for responding to sustained decreases in the power that is demanded by the load  50  and introduce a fixed delay interval that is longer in duration for responding to sustained increases in the power that is demanded by the load  50 . 
     More specifically, referring to FIG. 9, the controller  60  may perform a technique  194  (when executing the program  65 ) to control the fuel processor  22  in response to an up transient using a delay interval that has a fixed duration. In the technique  194 , the controller  60  measures (block  196 ) a predefined interval (i.e., the first delay interval that is associated with the first control technique) when the controller  60  determines that an up transient has occurred. If the controller  60  subsequently determines (diamond  198 ) that the increase in the power that is consumed by the load has been sustained during this time interval, then the controller  60  sets (block  199 ) a flag that indicates the continued increase and thereafter, controls the fuel processor  22  accordingly to increase its output to produce the appropriate level of power for the load  50 . 
     Referring to FIG. 10, similar to the above-described technique  194  to control the fuel processor  22  in response to up transients, the controller  60  may use a technique  210  (when executing the program  65 ) that uses a fixed duration delay interval (i.e., the second delay used by the second control technique) in response to the controller  60  detecting a down transient. The duration of this delay interval may be less than the duration of the delay interval that is used in the technique  194 . 
     In the technique  210 , the controller  60  begins measuring (block  212 ) a predefined delay interval (that is associated with the second control technique) in response to a down transient. If the controller  60  determines (diamond  214 ) that a decrease in the power has been sustained, then the controller  60  sets (block  216 ) a flag indicating the continued decrease and decreases the output of the fuel processor  22  accordingly. 
     Referring back to FIG. 1, among the other features of the fuel cell system  20 , the system  20  may include a voltage regulator  30  that regulates the V TERM  stack voltage and converts this voltage into an AC voltage via an inverter  33 . The output terminals  32  of the inverter  33  are coupled to the load  50 . The fuel cell system  10  also includes control valves  44  that 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) and converts this hydrocarbon into the fuel flow (a hydrogen 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 stack  22 . 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 . The fuel cell system  10  may also include an oxidizer  38  to burn any fuel from the stack  22  that is not consumed in the fuel cell reactions. 
     For purposes of isolating the load from the fuel cell stack  20  during a shut down of the fuel cell system  10 , the system  10  may include a 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 the switch  29  via an electrical communication line  50 . 
     In some embodiments of the invention, the controller  60  may include a microcontroller and/or a microprocessor to perform one or more of the techniques that are described herein when executing the program  65 . For example, the controller  60  may include a microcontroller that includes a read only memory (ROM) that serves as the memory  63  and a storage medium to store instructions for the program  65 . Other types of storage mediums may be used to store instructions of the program  65 . Various analog and digital external pins of the microcontroller may be used to establish communication over the electrical communication lines  46 ,  51  and  52  and the serial bus  48 . In other embodiments of the invention, a memory that is fabricated on a separate die from the microcontroller may be used as the memory  63  and store instructions for the program  65 . Other variations are possible. 
     In the connection mode in which the fuel cell system  10  is connected in parallel to the power grid  56 , the controller  60  may activate the switches  58  and  57  (part of a relay circuit, for example) to couple the fuel cell system  10  to the power grid  56 . Thus, due to this connection, when the fuel cell system  10  does not provide all of the power that is consumed by the load  50 , the power grid  56  supplies the additional power to the load  50 . In some embodiments of the invention, the fuel cell system  10  may provide power to the power grid  56  when the fuel cell system  10  provides more power than is consumed by the load  50 . 
     In the connection mode in which the fuel cell system  10  is not connected in parallel with the power grid  56 , the controller  60  may open the switches  57  and  58  to disconnect the power grid  56  from the fuel cell system  10 . In the case that the fuel cell stack  20  does not supply adequate power to the load  50 , the batteries  41  may supplement the power that is provided by the fuel cell stack  20 . In this mode, the controller  60  closes a switch  45  to couple the battery  41  to the remainder of the fuel cell system  10 . When the switch  45  is closed, the output terminal of the battery  41  is coupled to the anode of a diode  43  that has its cathode coupled to the output terminal  31  of the fuel cell stack  20 . Another diode  11  has its anode coupled to the output terminal  31  and its cathode coupled to the cathode of the diode  42 . Thus, when the V TERM  terminal voltage of the fuel cell stack  20  drops below a predefined threshold, the diode  43  conducts, thereby allowing the battery  41  to provide additional power to supplement the power that is provided by the fuel cell stack  20 . 
     Other embodiments are within the scope of the appended claims. For example, in some embodiments of the invention, the second control technique may include not introducing any delays when responding to down transients. Thus, in this manner, for these embodiments the controller may immediately respond to a down transient. The controller  60  may, however, place a limit on the rate at which the fuel flow may decrease, as described above. 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.