Abstract:
A technique that is useable with a fuel cell system includes adjusting operating parameters of a fuel cell system to obtain an optimal reactant stoichiometric ratio and thereby maximize the operating efficiency and/or performance of the system. An initial starting point for the reactant stoichiometric ratio is determined based on the output power provided by a fuel cell stack. Thereafter, the optimal reactant stoichiometric ratio is obtained by adjusting the reactant stoichiometric ratio based upon the observed system operating parameters and their response to the adjustment. In this manner, an optimal reactant stoichiometric ratio is reached and maintained while the fuel cell system is in operation, thus, maximizing the system&#39;s efficiency and performance.

Description:
BACKGROUND 
       [0001]    The invention generally relates to a technique and apparatus to regulate a reactant stoichiometric ratio of a fuel cell system. 
         [0002]    A fuel cell is an electrochemical device that converts chemical energy directly into electrical energy. For example, one type of fuel cell includes a proton exchange membrane (PEM) that permits only protons to pass between an anode and a cathode of the fuel cell. Typically PEM fuel cells employ sulfonic-acid-based ionomers, such as Nafion, and operate in the 60° Celsius (C) to 70° C. temperature range. Another type employs a phosphoric-acid-based polybenziamidazole, PBI, membrane that operates in the 150° C. to 200° C. temperature range. 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: 
         [0000]      H 2 →2H + +2e −  at the anode of the cell, and  Equation 1 
         [0000]      O 2 +4H + +4e − →2H 2 O at the cathode of the cell.  Equation 2 
         [0003]    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. 
         [0004]    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. 
         [0005]    The fuel cell stack is one out of many components of a typical fuel cell system. For example, the fuel cell system may also include a cooling subsystem to regulate the temperature of the stack, a cell voltage monitoring subsystem, a control subsystem, a power conditioning subsystem to condition the power that is provided by the fuel cell stack for the system load, etc. The particular design of each of these subsystems is a function of the application that the fuel cell system serves. 
         [0006]    The fuel cell system also may include a fuel processor that converts a hydrocarbon (natural gas, propane methanol, as examples) into the fuel for the fuel cell stack. To provide output power from the fuel cell stack, the reactant flows (i.e., the fuel and oxidant flows) to the stack must satisfy the appropriate stoichiometric ratios governed by the equations listed above. With respect to the fuel flow provided to the stack, the hydrogen stoichiometric ratio is defined as the ratio between the amount of hydrogen provided to the stack and the amount of hydrogen consumed by the stack. To maximize the efficiency of the stack, the hydrogen stoichiometric ratio should be minimized. Theoretically, the minimum hydrogen stoichiometric ratio is 1.1, which indicates that ten percent of the fuel provided to the stack is not consumed. In practice, however, the minimum achievable hydrogen stoichiometric ratio generally is greater than 1.1 and varies based on the output power provided by the stack to the load. To deal with this variation, a controller of the fuel cell system may monitor the output power of the stack and, based on the monitored output power, estimate the fuel flow to satisfy the hydrogen stoichiometric ratio. The controller regulates the fuel processor to produce this flow, and, in response to the controller detecting a change in the output power, the controller estimates a new rate of fuel flow and controls the fuel processor accordingly. 
         [0007]    Due to non-ideal characteristics of the stack, it may be difficult to precisely predict the rate of fuel flow needed for a given output power. Moreover, as the fuel cell system ages, the fuel flow needed for a given output power may change. To take into account these uncertainties, the controller may build in a sufficient margin of error by causing the fuel processor to provide more fuel than is necessary to ensure that the cells of the stack receive enough fuel and, thus, are not starved. However, such a control technique may be quite inefficient, as the fuel cell stack typically does not consume all of the incoming fuel, leaving unconsumed fuel that may be burned off by an oxidizer of the fuel cell system. As the fuel cell system ages, this control technique may become even more inefficient as it does not take into account the degradation of the fuel cell system. 
         [0008]    Thus, there is a continuing need for an arrangement and/or technique to address one or more of the problems discussed above. 
       SUMMARY 
       [0009]    In an embodiment of the invention, a technique useable with a fuel cell system that provides power to a load includes providing a reactant flow to a fuel cell stack, monitoring a plurality of operating parameters that are indicative of a reactant stoichiometric ratio, and determining a target range for at least one of the operating parameters based on an output power level provided by the fuel cell stack. The technique further includes determining a step size for adjusting the reactant stoichiometric ratio based on a difference between at least one of the monitored parameters and its target range. The technique also includes adjusting the reactant stoichiometric ratio in increments in accordance with the step size until a desired performance level of the system is reached. The desired performance level may be indicated when at least one of the operating parameters is approximately within its target range. 
         [0010]    In another embodiment of the invention, a fuel cell system includes a fuel cell stack to provide power to a load, a fuel processor to provide a fuel flow to the fuel cell stack, and a circuit. The circuit is configured to monitor a plurality of operating parameters of the fuel cell system that are indicative of a hydrogen stoichiometric ratio, determine a target range for at least one of the operating parameters based on the output power level provided by a fuel cell stack, and determine a step size for adjusting the hydrogen stoichiometric ratio based on a difference between at least one of the monitored parameters and its target range. The circuit is further configured to adjust the hydrogen stoichiometric ratio in increments in accordance with the step size until at one of the monitored operating parameters is approximately within its target range. 
         [0011]    In yet another embodiment of the invention, an article comprising a computer readable storage medium that is accessible by a processor-based system stores instructions. When executed by the processor-based system, the stored instructions cause the processor-based system to monitor a plurality of operating parameters of a fuel cell system that are indicative of a reactant stoichiometric ratio, determine a target range for at least one of the operating parameters based on an output power level provided by a fuel cell stack, and determine a step size for adjusting the reactant stoichiometric ratio based on a difference between at least one of the monitored operating parameters and its target range. The instructions further cause the processor-based system to adjust the reactant stoichiometric ratio in increments in accordance with the step size until at least one of the operating parameters is approximately within its target range. 
         [0012]    Advantages and other features of the invention will become apparent from the following drawing, description and claims. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
         [0013]      FIG. 1  is a schematic diagram of a fuel cell system according to an embodiment of the invention. 
           [0014]      FIG. 2  is a flow diagram depicting a technique to adjust the reactant stoichiometric ratio of the fuel cell system of  FIG. 1  according to an embodiment of the invention. 
           [0015]      FIG. 3  is a flow diagram depicting a further technique to adjust the hydrogen stoichiometric ratio of the fuel cell system of  FIG. 1  according to an embodiment of the invention. 
           [0016]      FIG. 4  is a flow diagram depicting yet a further technique to adjust the hydrogen stoichiometric ratio of the fuel cell system of  FIG. 1  according to an embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0017]    Referring to  FIG. 1 , in accordance with an embodiment of the invention, a fuel cell system  10  includes a fuel cell stack  20  (a PEM fuel cell stack, for example) that, in response to fuel and oxidant flows produces power for an electrical load  100 . Powering conditioning circuitry  50  of the fuel cell stack converts a DC stack voltage of the fuel cell stack  20  into the appropriate voltage (DC or AC, depending on the type of load) for the load  100 . For example, the load  100  may be a residential load and, may receive an AC voltage from the fuel cell system  10 . However, in other embodiments of the invention, the fuel cell system  10  may provide a DC output voltage for the case where the load  100  is a DC load. Other variations are possible and are within the scope of the appended claims. 
         [0018]    In accordance with embodiments of the invention, a fuel processor  30  (a reformer, for example) of the fuel cell system  10  receives a hydrocarbon and produces a corresponding fuel flow (called “reformate”) to the fuel cell stack  20 . The fuel flow from the fuel processor  30  may pass, for example, through a flow control  52  (one or more valves and/or a pressure regulator, as examples) to anode inlet  22  of the fuel cell stack  20 . An air blower  34  may produce an air flow (i.e., the oxidant flow) that passes through the oxidant flow control  54  to a cathode inlet  24  of the fuel cell stack  20 . The incoming oxidant flow to the fuel stack  20  passes through the oxidant flow channels of the fuel cell stack  20  to appear as cathode exhaust at a cathode outlet  28  of the stack  20 , and the incoming fuel flow to the stack  20  passes through fuel flow channels of the fuel cell stack  20  to appear as anode exhaust at an anode outlet  26  of the stack  20 . 
         [0019]    In the embodiment illustrated in  FIG. 1 , fuel cell system  10  further includes a controller  40  that is generally configured to control the power produced by fuel stack  20  by controlling the fuel and oxidant flows provided by fuel processor  30  and air blower  34 , respectively. Controller  40  bases its regulation of the fuel and oxidant flows on various measured operating parameters of the fuel cell system  10 . The monitored operating parameters are indicators of various different operating conditions of the fuel cell system  10  and, thus, generally also may be indicators of how efficiently the fuel cell system  10  is operating. These operating parameters include, for instance, cell voltages detected by a cell voltage monitoring circuit  32  that monitors the cell voltages of each of the fuel cells in fuel cell stack  20 , temperatures of various subsystems associated with the fuel processor  30 , etc. 
         [0020]    In one embodiment, controller  40  may obtain indications representative of the various system operating parameters via, for example, communication bus  42  and communication bus  44 . Controller  40  may provide control signals to various subsystems of system  10  in response to the indications of the monitored operating parameters via, for example, communication bus  46 . For instance, the control signals may be provided to adjust the hydrogen stoichiometric ratio (referred to as the “H 2  Stoic”), regulate the efficiency of system  10 , recover from undesirable operating conditions, etc. 
         [0021]    The H 2  Stoic is the ratio between the amount of fuel provided to the stack  20  and the amount of fuel consumed by the stack  20  and, thus, also is an indicator of the operating efficiency of system  10 . An optimal H 2  Stoic is reached when substantially all of the fuel provided to the stack  20  is consumed by the stack  20 . Theoretically, the optimal H 2  Stoic is approximately 1.1, which indicates that ten percent of the fuel provided to the stack is not consumed. In practice, the optimal H 2  Stoic is greater than 1.1 and varies based on the amount of power being provided to load  100 . Typically, the optimal H 2  Stoic at a high power output level is smaller than at a low power output level. In addition to varying with power output level, the optimal H 2  Stoic also tends to increase as the fuel cell system  10  degrades. Accordingly, to achieve an optimal H 2  Stoic for all operating conditions and as system  10  ages, the H 2  Stoic may be adjusted while the system  10  is in operation based on indications of various system operating parameters. 
         [0022]    In one embodiment, system operating parameters that may be used to guide the adjustment of the H 2  Stoic are parameters that are indicative of the performance of stack  20  and the performance of fuel processor  30 . Once an adjustment is made to the H 2  Stoic, further adjustments may be implemented by observing the responses of various operating parameters to the initial adjustment. Operating parameters associated with the performance of stack  20  typically are indicated by the cell voltages measured by cell voltage monitoring system  32 . For instance, various pieces of information derived from the cell voltages may indicate whether the stack  20  is starved of fuel, which indicates that the H 2  Stoic is not at an optimal level. 
         [0023]    With respect to parameters associated with the fuel processor  30 , the fuel processor includes various subcomponents having operating parameters that are indicative of the H 2  Stoic. As an example, the fuel processor  30  may include a steam mixing box  60  to mix the incoming fuel, air and steam streams before the mixture is heated and reacted in an autothermal reformer  62  of the fuel processor  30 . In addition to the steam mixing box  60  and the autothermal reformer  62 , the fuel processor  30  may include, for instance, a preferential oxidation reactor (PrOx)  64 . If the temperature of any of the steam mixing box  60 , the autothermal reformer  62 , or the PrOx  64  is too low, this may be an indication that the fuel processor  30  may not be able to produce enough hydrogen to attain an optimal H 2  Stoic or that high levels of carbon monoxide (i.e., carbon monoxide poisoning) may result in the stack  20 . Thus, upon receipt of indications of parameters indicative of stack  20  performance or reformer  30  performance, controller  40  may implement a routine  200  to adjust certain system parameters and thereby adjust the H 2  Stoic and/or the O 2  Stoic to an optimal level that maximizes the efficiency and/or optimizes the performance of the fuel cell system  10 . 
         [0024]    Such a routine  200  is illustrated in the flow diagram of  FIG. 2 . The routine  200  may be embodied in program instructions  66  stored in a memory  68  in controller  40 . When the program instructions  66  are executed by a processor  30 , the controller  40  operates as described herein to obtain indications of fuel cell system operating parameters and to adjust the operation of system  10  based on those parameters to attain an optimal reactant stoichiometric ratio, which corresponds to the system  10  operating at a performance level that best utilizes the reactant flows while also not resulting in damage to the stack  20 . For instance, this performance level may be deemed reached when the stack  20  consumes substantially all of the reactant provided to the stack while at the same time is not starved (i.e., not enough fuel is provided to the stack). 
         [0025]    The routine  200  illustrated in  FIG. 2  assumes that the system  10  is started up from a powered down state. In accordance with routine  200 , controller  40  provides a predetermined reactant flow to stack  20 , such as a predetermined fuel flow and/or a predetermined oxidant flow. Typically, the predetermined reactant flow is a conservative estimation of the required reactant flow for an assumed level of output power provided by the stack  20 . Generally, this initial level of reactant flow results in the provision of more reactant to the stack  20  than needed. Once the system  10  is powered up by providing a reactant flow to stack  20 , the amount of output power provided by the fuel cell stack  20  is detected (block  204 ). The output power may be detected, for instance, by detecting the amount of current drawn from the stack  20  and providing indications of the detected current to the controller  40 . Based on the detected output power, controller  40  determines a target range for one or more system operating parameters that have been selected for aiding in the adjustment of the reactant stoichiometric ratio (block  206 ). 
         [0026]    Having determined the target range for the operating parameters, those parameters are observed (block  208 ). If any one or more of the operating parameters are outside of the target range (diamond  210 ) (e.g. above or below the target range), then controller  40  determines an appropriate step size for adjusting the reactant stoichiometric ratio to thereby bring the operating parameters within the target range (block  212 ). In one possible embodiment, the step size may be a fixed step size that has been predetermined. In other embodiments, an adaptive or variable step size may be used which may result in better overall system efficiency. For instance, the larger the difference is between the monitored operating parameters and the target range, the larger the step size of the adjustment can be. By adapting the step size based on a comparison between the target range and the monitored value, the reactant stoichiometric ratio may be more quickly brought to its optimal value, thus maximizing the efficiency of the operation of fuel cell system  10 . 
         [0027]    Once the step size of the reactant stoichiometric ratio adjustment has been determined, controller  40  then provides the appropriate control signals to adjust the reactant stoichiometric ratio. For instance, if the monitored operating parameters indicate that a non-optimal amount (either too much or too little) of fuel is being provided to stack  20 , then controller  40  may provide a control signal to fuel processor  30  or flow control  52  to increase or decrease the fuel flow as needed. Alternatively, if the monitored operational parameters indicate that a temperature of a subsystem of the fuel processor  30  is out of range, such that either fuel starvation or carbon monoxide poisoning may result, controller  40  may provide an appropriate control signal to fuel processor  30  to increase or decrease the temperatures of the subsystems and/or to increase or decrease the flow of fuel provided to stack  20  as needed. Yet further, if the monitored operating parameters indicate that a non-optimal amount (either too much or too little) of oxidant is being provided to stack  20 , then controller  40  may provide a control signal to air blower  34  or flow control  54  to increase or decrease the oxidant flow as needed. 
         [0028]    After making the adjustment, the controller  40  observes a response of the system to the adjustment (block  215 ) and determines whether the monitored operating parameters have been brought within their target range (thus indicating that the desired performance level of system  10  has been achieved) (diamond  216 ). If not, then controller  40  continues to increment the adjustment until the target range is reached. Once the operating parameters are within the target range, controller  40  continues to monitor the operating parameters to determine whether further adjustments are needed while the system  10  is operating. 
         [0029]    In some embodiments of the invention, circuitry other than the controller  40  may be used to perform one or more parts of the routine  200 . For instance, in some embodiments, the cell voltage monitoring circuit  32  may determine whether a parameter is out of range and indicate to the controller  40  whether to increase or decrease the reactant stoichiometric ratio based on this determination. In other embodiments, the fuel processor  30  may determine whether an operating parameter is out of range and indicate to the controller  40  whether to increase or decrease the reactant stoichiometric ratio based on this determination. For purposes of simplifying the description below, it is assumed that the controller  40  determines whether the reactant stoichiometric ratio can be improved, although other variations are possible. 
         [0030]    As mentioned above, there are numerous ways for the controller  40  to determine whether the reactant stoichiometric ratio is at an optimal level. For example,  FIG. 3  illustrates a routine  300  that the controller  40  may perform to make decision about the H 2  Stoic based on the performance of the stack  20 . In accordance with routine  300 , the fuel cell system is powered up by providing a fuel flow to stack  20  (block  302 ). The output power level provided to load  100  is then detected (block  304 ). A target range for the cell voltages of stack  20  is then determined based on the detected power level (block  306 ). Controller  40  obtains indications of the cell voltages of stack  20  from, for instance, cell voltage monitoring circuit  32  to determine whether any of these cells are being deprived of sufficient fuel (i.e., an indication that the H 2  Stoic is too low) (block  308 ). For example, if the controller  40  determines that one or more cell voltages are below the minimum threshold of the range (less than 0.2 volt, for example) such that damage to the membranes of those cells may result (diamond  310 ), then the controller  40  determines the appropriate step size for adjusting the fuel flow provided to stack  20  (block  312 ). As mentioned previously, the size of the step may either be fixed at a predetermined size or may be a variable step, the size of which is determined based on the amount of difference between the measured cell voltage and the target range. If, however, at diamond  310 , the controller  40  determines that none of the cell voltages are outside of the target range, then controller  40  returns to block  308  to continue to receive indications of the cell voltages. 
         [0031]    After determining the step size, controller  40  provides control signals to adjust the fuel provided to stack  20  in accordance with the determined step size (block  314 ). Controller  40  then observes the response of the cell voltages to the adjusted fuel flow (block  315 ) and continues to adjust the fuel flow until the cell voltages are within the target range (diamond  316  and block  314 ). Once the cell voltages are within the target range, controller  40  returns to monitoring the cell voltages at block  308 . 
         [0032]    In addition to or as an alternative to observing the cell voltages, controller  40  may look at a cell ratio, which is derived from the measured cell voltages, to determine whether the H 2  Stoic is at an optimal level. The cell ratio is the ratio between the lowest cell voltage in the stack  20  and the average of all the cell voltages. As with a cell voltage being outside of a target range, the cell ratio may be indicative of a non-optimal fuel flow provided to the stack  20  and, thus, a non-optimal H 2  Stoic. A standard deviation of the cell voltages also may be examined to determine whether the H 2  Stoic should be adjusted. Generally, the standard deviation may be used as an indicator of carbon monoxide poisoning, which would affect the manner in which the H 2  Stoic may be adjusted. 
         [0033]      FIG. 4  depicts an alternative routine  400  that the controller  40  may use to determine if the H 2  Stoic may be improved. In the routine  400 , a fuel flow is provided to the stack  20  (block  402 ), an output power is detected (block  404 ), and a target range for one or more operating parameters associated with the fuel processor  30  and the stack  20  are determined based on the output power (block  406 ). The controller  40  then monitors one or more operating parameters of the fuel processor  30 , such as the steam mixing box  60  temperature, the autothermal reformer  62  temperature, and/or the PrOx  64  temperature, and one or more stack operating parameters, such as the cell voltages (block  408 ). If any of these parameters are outside of their target range (e.g., a range of 600 to 700° C. for the autothermal reformer  62 ), this may be an indication that the stack  20  is not at its optimal operating condition. If any parameter is outside of a determined target range, the controller  40  then determines an appropriate step size for adjusting the H 2  Stoic (diamond  410  and block  412 ). For instance, the further the temperature is from the target range, the larger the step size may be. Alternatively, the step size may be a fixed step size. The controller  40  provides a control signal to the fuel processor  30  to adjust the temperature and/or fuel flow based on the determined step size (block  414 ) and then observes a response of the stack  20  to the adjusted parameter (block  415 ), such as the response of one or more cell voltages as indicated by cell voltage monitoring circuit  32 . The controller  40  may continue to adjust the temperature of and/or the fuel flow provided by the fuel processor  30  until all of the cell voltages are within the target range (diamond  416 ). At that point, the controller  40  may return to monitoring one or more operating parameters (block  408 ) and then continue to adjust the H 2  Stoic based on observed changes in those parameters. 
         [0034]    It should be understood that routines  300  and  400  may be implemented separately or in conjunction with each other, various of the steps may be performed in different orders, and fewer or additional steps than those shown in the figures may be performed. In addition, other control loops may be used in combination with either of routine  300  or  400 . For example, the controller  40  may adjust the fuel flow in response to a monitored output power of the fuel cell stack  20 . However, the controller  60  continues to implement the control provided by the general routine  200  to obtain an optimal reactant stoichiometric ratio and thus maximize the efficiency and/or performance of the fuel cell system  10 . 
         [0035]    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.