Abstract:
A technique that is usable with a fuel cell includes generating first and second oxidant flows from a source oxidant flow. The first oxidant flow is communicated to a cathode chamber of the fuel cell, and the second oxidant flow is communicated to a reactor to oxidize an anode exhaust flow, which is provided by the fuel cell. The generation of the first oxidant flow is regulated based on a state of the reactor.

Description:
BACKGROUND 
       [0001]    The invention generally relates to controlling oxidant flows in a fuel cell system. 
         [0002]    A fuel cell is an electrochemical device that converts chemical energy directly into electrical energy. There are many different types of fuel cells, such as a solid oxide fuel cell (SOFC), a molten carbonate fuel cell, a phosphoric acid fuel cell, a methanol fuel cell and a proton exchange membrane (PEM) fuel cell. 
         [0003]    As a more specific example, a PEM fuel cell includes a PEM membrane, which permits only protons to pass between an anode and a cathode of the fuel cell. A typical PEM fuel cell may employ polysulfonic-acid-based ionomers and operate in the 50° Celsius (C) to 75° temperature range. Another type of PEM fuel cell may employ a phosphoric-acid-based polybenziamidazole (PBI) membrane that operates in the 150° to 200° temperature range. 
         [0004]    At the anode of the PEM fuel cell, diatomic hydrogen (a fuel) ionizes to produce 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 protons to form water. The anodic and cathodic reactions are described by the following equations: 
         [0000]      H 2 →2H + +2 e   −  at the anode of the cell, and  Equation 1 
         [0000]      O 2 +4H + +4 e   − →2H 2 O at the cathode of the cell.  Equation 2 
         [0005]    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. 
         [0006]    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. Catalyzed 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. 
       SUMMARY 
       [0007]    In an embodiment of the invention, a technique that is usable with a fuel cell includes generating first and second oxidant flows from a source oxidant flow. The first oxidant flow is communicated to a cathode chamber of the fuel cell, and the second oxidant flow is communicated to a reactor to oxidize an anode exhaust flow, which is provided by the fuel cell. The generation of the first oxidant flow is regulated based on a state of the reactor. 
         [0008]    In another embodiment of the invention, a fuel cell system includes an oxidant source, a diverter, a fuel cell, a reactor and a controller. The oxidant source furnishes a source oxidant flow, and the diverter generates first and second oxidant flows from the source oxidant flow. The fuel cell includes a cathode chamber to receive a first oxidant flow and an anode chamber to provide an anode exhaust flow. The reactor receives the anode exhaust flow and the second oxidant flow and oxidizes the anode exhaust flow. The controller is coupled to the diverter to regulate the generation of the first oxidant flow in response to a state of the reactor. 
         [0009]    In yet another embodiment of the invention, an article includes a computer accessible storage medium to store instructions that when executed cause a processor-system to control a diverter to generate first and second oxidant flows from a source oxidant flow. The first oxidant flow is communicated to a cathode chamber of a fuel cell, and the second oxidant flow is communicated to a reactor to oxidize an anode exhaust flow that is provided by the fuel cell. The instructions when executed also cause the processor-based system to regulate the generation of the first flow based on a state of the reactor. 
         [0010]    Advantages and other features of the invention will become apparent from the following drawing, description and claims. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
         [0011]      FIG. 1  is a schematic diagram of a fuel cell system according to an embodiment of the invention. 
           [0012]      FIG. 2  is a schematic diagram of a control architecture directed to controlling a cathode blower of the fuel cell system of  FIG. 1  according to an embodiment of the invention. 
           [0013]      FIG. 3  is a flow diagram depicting a technique to optimize an oxidant flow to a fuel cell stack of the fuel cell system according to an embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0014]    Referring to  FIG. 1 , an embodiment 10 of a fuel cell system in accordance with the invention includes a single air blower  40 , which furnishes an oxidant flow at its outlet. For purposes of reducing the materials cost of the fuel cell system  10 , the oxidant flow that is provided by the air blower  40  is split, or divided, by a three-way valve  44  for purposes of producing oxidant flows to a cathode chamber of a fuel cell stack  20  and to a reactor, or anode tailgas oxidizer (ATO)  70 . As described herein, a controller  100  of the fuel cell system  10  controls the three-way valve  44  based on a state of the ATO  70  for purposes of minimizing the oxidant flow to the fuel cell stack  20  to optimize the overall system efficiency. 
         [0015]    By controlling the oxidant flow to the fuel cell stack  20 , the molar flow rate of oxidant to the fuel cells of the fuel cell stack  20  are controlled. In this regard, for a stoichiometric mixture according to Equations 1 and 2 (in the Background section), all fuel that flows through the anode chamber of the fuel cell stack  20  is consumed. However, a certain flow molar flow rate of fuel needs to be present in an anode exhaust line  28  of the fuel cell stack  20  for purposes of ensuring proper operation of the ATO  70 , as further described below. By monitoring at least one state of the ATO  70 , the controller  100  operates the three-way valve  44  to ensure that the minimum oxidant flow is provided to the fuel cell stack  20  to maintain the ATO  70  within predefined operating boundaries. 
         [0016]    By ensuring that the minimum oxidant is provided to the fuel cell stack  20  to maintain proper operation of the ATO  70 , the lifetime of the fuel cell stack  20  is maximized. In this regard, a relatively high oxidant flow may serve to dry out the membranes of the fuel cell stack  20 , thereby reducing the lifetime of the stack  20 . Additionally, by minimizing the oxidant flow to the fuel cell stack  20 , the speed of the blower  40  is also minimized, thereby improving the overall efficiency of the fuel cell system  10 . 
         [0017]    The fuel cell stack  20  includes a cathode inlet  22 , which receives the incoming oxidant flow to the stack  20 . The incoming flow passes through the cathode chamber of the fuel cell stack  20 , which represents the flow passageways through the cathodes of the fuel cells of the stack  20 . The oxidant flow into the cathode inlet  22  produces a corresponding cathode exhaust, which exits the fuel cell stack  20  at a cathode outlet  24 . As depicted in  FIG. 1 , the cathode exhaust may be routed through a valve  27  to a cathode humidifier  46 . The cathode humidifier  46  uses the cathode exhaust stream to humidify the incoming oxidant stream to the fuel cell stack. In this regard, the cathode humidifier  46  transfers humidity from the outgoing cathode exhaust to the incoming oxidant stream. As depicted in  FIG. 1 , the cathode humidifier  46  receives its incoming oxidant stream from an outlet of the three-way valve  44 . After being humidified, the oxidant stream passes through a reactant conditioner  50  to the cathode inlet of the fuel cell stack  20 . The cathode exhaust exits the cathode humidifier  46  at an exhaust outlet  47 , which is connected to a junction  43 . 
         [0018]    From the junction  43 , the cathode exhaust may be combined with a flow from another outlet of the three-way valve  44  to form an oxidant flow, which is communicated by an oxidant communication line  90  to the ATO  70 . The fuel cell system  10  also includes a bypass line  80 , which is connected to the junction  43  for purposes of communicating a flow from the junction  43  to an exhaust flow from the ATO  70 . As depicted in  FIG. 1 , the bypass line  80  may include a flow restrictor orifice  82 , which may be a fixed or variable orifice, depending on the particular embodiment of the invention. The function of the bypass line  80  is to limit the maximum oxidant flow through the communication line  90  to the ATO  70 . In this regard, a sufficiently high oxidant flow to the ATO  70  may lower the operating temperature of the ATO  70  outside of an acceptable operating range, thereby reducing system efficiency and possibly producing an unacceptably high level of emissions. 
         [0019]    The fuel for the fuel cell stack  20  is provided by a fuel source  60 , which may be a reformer, hydrogen tank, etc., depending on the particular embodiment of the invention. Thus, the fuel cell source  60  may provide a reformate flow, a pure hydrogen flow, etc., depending on the particular source of fuel for the fuel cell stack  20 . The fuel flow that is provided by the fuel source  60  passes through a three-way valve  64  through the reactant conditioner  50 . From the reactant conditioner  50 , the fuel flow is received into an anode inlet  26  of the fuel cell stack  20 . The anode flow is communicated through the anode chamber of the fuel cell stack  20  for purposes of sustaining the electrochemical reactions inside the fuel cell stack  20 . The fuel flow through the fuel cell stack  20  produces a corresponding anode exhaust, which exits the fuel cell stack  20  at an anode exhaust outlet  28 . As depicted in  FIG. 1 , the anode exhaust from the fuel cell stack  20  may be combined with the oxidant flow from the communication line  90  to form a feed stock flow which is provided to the ATO  70 , such that the ATO  70  oxidizes the anode exhaust. 
         [0020]    As a result of the oxidation inside the ATO  70 , a relatively emission free exhaust flow is produced, which exits the ATO  70  at an outlet  72 . As depicted in  FIG. 1 , in accordance with some embodiments of the invention, an oxygen sensor  90  is located to measure an oxygen content of the ATO&#39;s exhaust flow. More specifically, as described further below, the controller  100  uses the signal that is provided from the oxygen sensor  90  for purposes of controlling the three-way valve  44  and thereby controlling the oxidant flow to the fuel cell stack  20 . 
         [0021]    The controller  100  may take on numerous forms, depending on the particular embodiment of the invention. In general, the controller  100  includes a processor  106 , which may be formed from one or more microprocessors, microcontrollers, computers, or a combination of these components. In general, the processor  106  executes program instructions  104 , which are stored in a memory  102 . The memory  102  may be built into the controller  100  or may be external to the controller  100 , depending on the particular embodiment of the invention. The program instructions  104 , when executed by the processor  106 , cause the controller  100  to perform one or more of the routines to control oxidant flow, which are set forth herein. The memory  102  may also include a table  103 , which sets forth the predicted control parameters, based on the system configuration  10 . For example, in accordance with some embodiments of the invention, the table  103  sets forth the settings for the three-way valve  44  based on the anticipated electrical power that is provided by the fuel cell stack  20  to a load (not depicted in  FIG. 1 ) of the fuel cell system  10 . 
         [0022]    The controller  100  includes various input communication lines  120  for purposes of possibly receiving communications from other controllers, readings from sensors, current and voltage measurements, etc. Thus, through the communication lines  120 , the controller  100  observes various states, operating conditions and measurements of the fuel cell system  10 . Based on these measured parameters and communications, the controller  100  may control various components of the fuel cell system  10 , such as the air blower  40 , the three-way valves  44  and  64 , the orifice  82  (when variable), the valve  27 , electrical power conditioning circuitry (not depicted in  FIG. 1 ), the fuel source  60 , etc. Thus, although the control of the three-way valve  44  is specifically discussed in detail herein, it is noted that the controller  100  may control various other aspects of the fuel cell system  10 , in accordance with the many possible embodiments of the invention. 
         [0023]    Referring to  FIG. 2 , in accordance with some embodiments of the invention, the controller  100  (via the execution of the program instructions  104 ) may implement a control and software architecture  120 . Pursuant to the control and software architecture  120 , the controller  100  controls the oxidant flows to the ATO  70  and fuel cell stack  20  using two control loops: a relatively slower loop  130 , which controls the three-way valve  44  (i.e., controls the division of oxidant flows from the air blower  40  between the ATO  70  and fuel cell stack  20 ); and a relatively faster loop  150 , which controls the speed of the air blower  40 . In the slow loop  130 , the controller  100  performs a routine  140  for purposes of optimizing the oxidant flow to the fuel cell stack  20 . Besides using the result of the routine  140  to control the three-way valve  44 , the controller  100  may also use the result in a feedforward controller routine  144  for purposes of controlling the air blower  40 . 
         [0024]    Regarding the fast loop  150 , the controller  100  controls the speed of the air blower  40  based on several input parameters, one of which may be the feedforward result from the routine  140 . The parameters on which the control of the speed of the air blower  40  is based, may be combined together, as indicated by an adder  160 , which provides the speed control signal for the air blower  40 . In addition to the results provided by the feedforward control routine  144 , the controller  100  may also consider results provided by a routine  164 , which considers feedback from the ATO  70 . In this regard, the routine  164  generates a control input based on a difference between the ATO temperature  70  (measured by a sensor  71  in  FIG. 1 , for example) and a predetermined set point, or threshold, temperature. Thus, the actions of the routine  164  regulate the speed of the air blower  40  based on feedback of the ATO temperature for purposes of regulating the temperature to a predetermined level. 
         [0025]    The regulation of the air blower speed is also based on the result of a routine  168 , which is a feedforward control routine that generates an input to the adder  160  based on an estimated hydrogen flow to the ATO  70 . In this regard, an estimate is made as to the molar flow of hydrogen that exits the anode exhaust from the fuel cell stack  20  and is provided to the ATO  70 . 
         [0026]    The fast loop  150  may also include an oxidant switching control routine  170 , which receives the estimate hydrogen flow to the ATO  70  and a signal from the oxygen sensor  90  (see  FIG. 1 ). 
         [0027]      FIG. 3  depicts a technique  200  that may be performed by controller  100  due to the execution of the oxidant flow optimization routine  140 . Pursuant to the technique  200 , the controller  100  uses the table  103  (see  FIG. 1 ) to select an initial oxidant flow to the fuel cell stack  20 . The table  103  may specify the initial oxidant flow, may specify the controller settings for the three-way valve  44  to achieve this flow, etc., depending on the particular embodiment of the invention. Regardless of the particular content of the table  103  or other associated tables used by the controller  100 , the controller  100  derives an initial setting for the three-way valve  44 , i.e., determines the percentage split between the oxidant flows that are provided to the ATO  70  and the oxidant flow to the cathode chamber of the fuel cell stack  20 . For example, initially, the controller  100 , via the access of the cable  103  may determine that the valve  44  should be set for a 70/30 split in which 70 percent of the air flow is furnished to the ATO  70 , and the remaining 30 percent is furnished to the fuel cell stack  20 . 
         [0028]    Next, pursuant to the technique  200 , the controller  100  checks (block  206 ) the health of the ATO  70 . In this regard, the controller  100  determines whether the ATO  70  is operating within the predefined boundaries. For example, the ATO  70  may be currently receiving too much air, and thus, the controller  100  may need to wait to adjust the valve  44  until the fast loop  150  (see  FIG. 2 ) reduces the speed of the air blower  40 . The controller  100  may use one or more sensors  71  (see  FIG. 1 ) of the ATO  70  as well as possibly external sensors for purposes of ascertaining the health of the ATO  70 . 
         [0029]    After the controller  100  determines (diamond  208 ) that the ATO  70  is healthy, the controller  100  then lowers (block  210 ) the oxidant flow to the fuel cell stack  20  and updates the table  103  accordingly. 
         [0030]    The controller  100  continually lowers the oxidant flow to the fuel cell stack  20  until a fuel rich condition is detected at the ATO  70 . In this regard, when the oxidant flow through the fuel cell stack  20  becomes sufficiently low, a corresponding hydrogen flow is produced at the anode exhaust due to not enough oxygen being present in the cathode chamber of the fuel cell stack  20 . Due to this condition, the ATO  70  may enter a fuel rich condition in which the oxygen content in the exhaust of the ATO  70  is below a certain threshold (such as 1000 parts per million (ppm), for example). The fuel rich condition may be detected by using the oxygen sensor  90 , which indicates the oxygen content of the ATO exhaust flow. 
         [0031]    When the controller  100  determines (diamond  214 ) that a fuel rich condition has occurred, the controller  100  increases the oxidant flow to the fuel cell stack  20  and updates the table, pursuant to block  220 . If a fuel rich condition has not occurred, then control proceeds to block  206  to check the health of the ATO  70  before once again lowering the oxidant flow to the fuel cell stack  20 . 
         [0032]    Thus, to summarize, assuming the ATO  70  is healthy, the controller  100  lowers the oxidant flow to the fuel cell stack  20  until a fuel rich condition at the ATO  70  occurs. As a result of this control technique, the oxidant flow to the fuel cell stack  20  is minimized, thereby improving the system efficiency and extending the stack life. 
         [0033]    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.