Abstract:
A technique that is usable with a fuel cell system includes establishing a path to route an anode exhaust from the fuel cell system back to an anode inlet port of the stack. The technique includes diverting part of a first flow otherwise flowing through the path to produce a diverted flow and combining the diverted flow with a flow that is associated with a cathode of the stack.

Description:
BACKGROUND  
       [0001]     The invention generally relates to purging anode channels of a fuel cell stack.  
         [0002]     A fuel cell is an electrochemical device that converts chemical energy that is 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 relationships: 
 
H 2 →2H + +2 e   −   Eq. 1 
 
 at the anode of the cell, and 
 
O 2 +4H + +4 e   − →2H 2 O  Eq.2 
 
 at the cathode of the cell. 
 
         [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 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 may be used in a fuel cell system that routes the anode exhaust from the stack back into the anode input channels of the stack. This type of arrangement is called a “dead-ended” configuration. A potential problem with this arrangement is that inert gases, such as nitrogen, may accumulate in the fuel cell stack, as no exit path from the stack exists for these gases. Therefore, an inert gas, such as nitrogen, may accumulate to a concentration that degrades the performance of the fuel cell stack.  
         [0006]     Thus, there is a continuing need for an arrangement and/or technique to address one or more of the problems that are set forth above as well as possibly address one or more problems that are not set forth above.  
       SUMMARY  
       [0007]     In an embodiment of the invention, a technique that is usable with a fuel cell system includes establishing a path to route an anode exhaust from the fuel cell system back to an anode inlet port of the stack. The technique includes diverting part of a first flow otherwise flowing through the path to produce a diverted flow; and the method includes combining the diverted flow with a flow that is associated with a cathode of the stack.  
         [0008]     Advantages and other features of the invention will become apparent from the following description, drawing and claims. 
     
    
     BRIEF DESCRIPTION OF THE DRAWING  
       [0009]      FIGS. 1, 2 ,  4 ,  6  and  7  are schematic diagrams of fuel cell systems according to different embodiments of the invention.  
         [0010]      FIGS. 3 and 5  are flow charts depicting techniques to control a fuel cell system according to different embodiments of the invention.  
     
    
     DETAILED DESCRIPTION  
       [0011]     Referring to  FIG. 1 , an embodiment of a fuel cell system  10  in accordance with the invention includes, among other components, a fuel cell stack  20 , a fuel processor  22  (a reformer, for example) and an air blower  24 . The fuel cell stack  20  produces power for a load  50  in response to fuel and oxidant (i.e., reactant) flows that are provided by the fuel processor  22  and the air blower  24 , respectively. More specifically, the fuel cell system  10  includes a controller to, among its other functions (described herein) controls the power that is produced by the fuel cell stack  20  by controlling the fuel processor  22  to regulate the fuel flow that the processor  22  provides to the stack  20 . The fuel flow exits an outlet port  37  of the fuel processor  22  and is pressure-regulated by a pressure regulator  36  before entering the fuel cell stack  20  through an anode inlet port  77 . The oxidant flow leaves the air blower  24  and enters a cathode inlet port  39  of the fuel cell stack  20 .  
         [0012]     In some embodiments of the invention, the fuel cell system  10  establishes an anode circulation path between the anode exhaust  57  and anode inlet  77  ports of the fuel cell stack  20 . More particularly, to create this path, the fuel cell system  10  includes an exhaust gas re-circulation (EGR) blower  58  to pressurize the anode exhaust flow exiting the fuel cell stack  20  (having a pressure of about zero pounds per square inch, gauge (psig), in some embodiments of the invention) to convert this exhaust flow into an anode inlet flow (having a pressure of about one psig, in some embodiments of the invention). As described below, the flow from the outlet port of the EGR blower  58  is routed back into the anode inlet port of the fuel cell stack  20 . Thus, an anode circulation path is formed from the following components: the anode inlet port  77 , anode channels of the fuel cell stack  20 , the anode exhaust port  57 , an inlet port  79  to the EGR blower  58 , the flow path through the blower  58  and the blower&#39;s outlet port  59  that is connected (via a T coupler  75 ) back to the anode inlet port  77 . The T coupler  75  joins an outlet port  73  of the pressure regulator  36 , the outlet port  59 , and the anode inlet port  77  together so that the anode inlet port  77  receives the anode exhaust from the fuel cell stack  20  and fuel from the fuel processor  22 .  
         [0013]     A potential problem with the above-described circulation path is that concentrations of contaminants (inert gases, such as nitrogen) may reach levels that may degrade performance of the system. However, to maintain the concentrations of contaminants at acceptable levels, the fuel cell system  10  includes a valve  80  to establish a small bleed flow from the path that exists between the anode exhaust port  57  and anode inlet port  77 . This bleed flow, in turn, purges contaminants from the anode channels of the fuel cell stack  20 . Because the bleed flow is significantly smaller than the flow exiting the anode exhaust port  57 , only a small amount of fuel (hydrogen) exits through the bleed flow path. As an example, in some embodiments of the invention, the rate of the bleed flow may be about {fraction (1/400)}th that of the rate of the flow exiting the anode exhaust port  57 . Other relative flow rates are possible in other embodiments of the invention. Even though the bleed flow is relatively small, the bleed flow is sufficient to maintain the concentrations of contaminants in the anode circulation path at acceptable levels.  
         [0014]     In some embodiments of the invention, the valve  80  may be a valve that has a fixed cross-sectional flow path. In other words, the cross-sectional flow path of the valve is permanent and therefore, does not vary over time. More specifically, in some embodiments of the invention, the input port to the valve  80  is connected to a T coupler  78  that, in turn, couples the input port of the valve  80 , the anode exhaust port  57  of the fuel cell stack  20  and the inlet port  79  of the EGR blower  58  together. The flow path through the valve  80  provides a bleed flow path for nitrogen gas to escape the anode circulation path between the anode outlet port  57  and the anode inlet port  77  of the fuel stack  20 . Some hydrogen gas may also pass through the bleed flow path. However, the bleed flow is sufficiently very small to release a safe concentration of hydrogen, as shown by way of specific example above. Furthermore, in some embodiments of the invention, the bleed flow is routed to the cathode exhaust plenum to dilute the overall system emissions below hydrogen concentrations that would require special safety or siting considerations.  
         [0015]     More specifically, in some embodiments of the invention, the outlet port of the valve  80  is connected by a conduit  82  and a T coupler  84  to the cathode exhaust flow path. In this manner, the T coupler  84  couples the conduit  82 , the cathode outlet port  56  and a cathode exhaust path  86  (i.e., the path containing the exhaust from the cathode channels of the fuel cell stack  20 ) together, as depicted in  FIG. 1 . Thus, in some embodiments of the invention, the bleed flow path is combined with the cathode exhaust to dilute the concentrations of gases in the bleed flow.  
         [0016]     Many other variations are possible from the arrangement that is depicted in  FIG. 1 . For example, in some embodiments of the invention, the fuel cell system  10  may be replaced by a fuel cell system  100  that is depicted in  FIG. 2 . Referring to  FIG. 2 , the fuel cell system  100  has a design that is similar to the fuel cell system  10 , except that in the fuel cell system  100 , the EGR blower  58  of the fuel cell system  10  is replaced by a variable speed EGR blower  101 . The variable speed EGR blower  101 , in turn, in some embodiments of the invention, is coupled to communication lines  102  that extend to the controller  60 . These communication lines  102 , in turn, permit the controller  60  to regulate the flow rate through the anode circulation path based on one or more system parameters.  
         [0017]     More specifically, in some embodiments of the invention, the controller  60  may control the speed of the EGR blower  101  (and thus, control the flow through the anode re-circulation path) in accordance with a technique  150  that is depicted in  FIG. 3 . Referring to both  FIGS. 2 and 3 , pursuant to the technique  150 , the controller  60  may determine (block  152 ) one or more system parameters for purposes of controlling the speed of the blower  101 . As examples, these system parameters may include a power output of the system  100  (i.e., the power furnished to the load  50 ), an output current of the fuel cell system  100  (i.e., the current provided to the load  50 ), a system voltage (a stack voltage, an output voltage at the load  50 , or some voltage in between the load  50  and the stack  20 , as just a few examples) or a rate of the cathode flow (input or output flow). Depending on the particular embodiment of the invention, the controller  60  may (through the use of various system sensors, for example) determine other and/or different system parameters.  
         [0018]     However, regardless of the system parameters that are determined by the controller  60 , pursuant to the technique  150 , the controller controls (block  154 ) the speed of the blower  101  in response to the determined system parameter(s). For example, in some embodiments of the invention, the controller  60  may increase the speed of the blower  101  to increase the flow through the anode re-circulation path to accommodate an increased output power, current or voltage from the fuel cell system  101 . In response to a decreased output power, current or voltage, in some embodiments of the invention, the controller  60  may decrease the speed of the blower  101  to decrease the flow through the anode re-circulation path. Depending on the particular embodiment of the invention, when determining the appropriate speed of the blower  101 , the controller  60  may assign different weights to different system parameters, i.e., some system parameters may have a greater weight on the speed of the blower  101  than others. Thus, unlike the fuel cell system  10 , the fuel cell system  100  varies the speed of the blower  101  (i.e., controls the flow rate through the anode circulation path) in response to one or more determined system parameters.  
         [0019]     Referring to  FIG. 4  as another example of additional embodiment of the invention, the fuel cell system  10  or  100  may be replaced by a fuel cell system  200 . The fuel cell system  200  has a similar design to the fuel cell system  10 , with the following exceptions. In particular, unlike the fuel cell system  10 , the fuel cell system  200  varies the rate of the bleed flow in response to one or more system parameters.  
         [0020]     To accomplish this, the fuel cell system  200 , in some embodiments of the invention, includes an additional valve  205  in the bleed flow path. Unlike the valve  80 , the cross-sectional area of the valve  205  is not permanently fixed, but rather, the cross-sectional area is controlled by the controller  60  for purposes of regulating the flow rate through the bleed flow path. Thus, in some embodiments of the invention, the controller  60  communicates with the valve  205  via one or more communication lines  209  for purposes of regulating the flow rate through the valve  205 . In some embodiments of the invention, the inlet port to the valve  205  is connected to the outlet port of the valve  80 , and the conduit  82  connects the outlet port of the valve  205  to the T coupler  84 .  
         [0021]     As also depicted in  FIG. 4 , in some embodiments of the invention, the fuel cell system  200  includes a differential pressure sensor  203  that is connected to sense the difference between the pressure appearing at the anode inlet port  77  and the pressure appearing at the anode exhaust port  57 . The differential pressure sensor  203  is coupled to communication lines  210  that extend to the controller  60 . Due to this arrangement, the controller  60  may monitor the differential pressure between the anode inlet and outlet ports of the fuel cell stack  20  and control the bleed flow path accordingly. For example, in some embodiments of the invention, the controller  60  may increase the bleed flow path in response to an increased differential pressure and decrease the bleed flow in response to a decreased differential pressure.  
         [0022]     The differential pressure, however, is one out of many possible system parameters that the controller  60  may evaluate for purposes of regulating the rate of the bleed flow. More specifically, in some embodiments of the invention, the controller  60  may perform a technique  250  that is depicted in  FIG. 5 . Referring to both  FIGS. 4 and 5 , pursuant to the technique  250 , the controller  60  may determine (block  255 ) one or more system parameters. The system parameters may include an output current, an output power, a system voltage, a differential pressure between the anode inlet and outlet ports, etc. In response to the determined system parameter(s), the controller  60  communicates (via the communication lines  209 ) with the valve  205  to control the flow through the flow rate through valve  205  (and thus, control the bleed flow rate), as depicted in block  254 . For example, the case of the controller  60  controlling the valve  205  in response to the differential pressure between the anode inlet and outlet ports is discussed above. As another example, in some embodiments of the invention, the controller  60  may increase the rate of flow through the bleed flow path in response to an increased system output power, current or voltage. The controller  60  may decrease rate of flow through the bleed flow path in response to a decreased output system current, power or voltage, in some embodiments of the invention. Other variations are possible.  
         [0023]      FIG. 4  depicts the valves  80  and  205  as being serially connected, i.e., the fixed flow path of a valve  80  is connected to the variable-sized flow path of the valve  205 . Thus, as depicted in  FIG. 4 , in some embodiments of the invention, the bleed flow path may always provide some level of bleed flow from the anode circulation path. In some embodiments of the invention, however, the fuel cell system may not include the valve  80 . Instead, in these embodiments of the invention, the fuel cell system may include only the valve  205 . Other variations are possible.  
         [0024]     As yet another example of a possible embodiment of the invention,  FIG. 6  depicts a fuel cell system  400  that has similar features to the fuel cell system  10 , except that a bleed flow path is communicated to an inlet of the cathode flow through the fuel cell stack  20  instead of being communicated to the cathode exhaust flow. More specifically, in some embodiments of the invention, the anode outlet port  57  of the fuel cell stack  20  is connected directly to the inlet port of the blower  50 . The bleed flow path, in turn, is connected to the outlet port  59  of the blower  58 . More specifically, a T coupler  89  connects the outlet port  59 , a conduit  90 , and the T coupler  75  together. The conduit  90 , in turn, is coupled in line with a flow restriction valve  91  (replacing the valve  80  of the system  10 ), and the outlet port of the valve  91  is coupled to a T coupler  92 . The T coupler  92 , in turn, couples the cathode inlet port of the fuel cell stack  20 , the conduit  39  and the outlet port of the valve  91  together. Thus, the fuel cell system  400  has a similar design to the fuel cell system  10 , except that the bleed flow path is routed to the cathode inlet flow to the stack  20 , instead of to the cathode outlet flow from the stack  20 .  
         [0025]      FIG. 7  depicts a fuel cell system  420  in accordance with another embodiment of the invention. The fuel cell system  420  has a similar design to the fuel cell system  400  except that the fuel cell system  420  does not include a connection (such as the conduit  90  in the fuel cell system  400 ) from the anode circulation path to the cathode inlet stream. Instead, in the fuel cell system  420 , the inlet of the flow restriction valve  91  is connected to the anode inlet stream. More specifically, in the fuel cell system  420 , the inlet of the valve  91  is connected to an inlet of a T coupler  75 . Another inlet of the coupler  75  is connected to the outlet port  73  of the pressure regulator  36 ; and the outlet of the coupler  75  is connected to the anode inlet port  77 . Thus, to summarize, in the fuel cell system  400  ( FIG. 6 ), the bleed flow path to the cathode inlet port taps into the anode circulation path; and in the fuel cell system  420  ( FIG. 7 ), the bleed flow path originates closer to the anode inlet port.  
         [0026]     Many other variations are possible from the arrangements described above. In this manner, elements from the various fuel cell systems  10 ,  100 ,  200 ,  400  and  420  may be combined in other embodiments of the invention. For example, in some embodiments of the invention, the fixed speed EGR blower  58  of the fuel cell system  200  ( FIG. 4 ) may be replaced by a variable speed blower that the controller  60  controls in accordance with one or more system parameters. Many other variations are possible and are within the scope of the appended claims.  
         [0027]     Referring back to  FIG. 1 , the fuel cell system  10  contains additional features (also contained by the other fuel cell systems  100 ,  200 ,  400  and  420 ), such as power conditioning circuitry  35  that receives a stack voltage (called “V TERM ”) from the fuel cell stack  20  via a stack terminal voltage line  27 . The power conditioning circuitry  35  produces a regulated AC voltage (called “V AC ”) in response to the V TERM  voltage. The V AC  voltage appears across output terminals  32  of the power conditioning circuitry  35  for purposes of providing power to the load  50 . The power conditioning circuitry  35  may include, for example, a voltage regulator to produce a regulated voltage from the V TERM  voltage, and the power conditioning circuitry  35  may include an inverter to convert this regulated voltage into the V AC  voltage.  
         [0028]     Among its other functions, the controller may control the general operations of the fuel cell system  10 , in some embodiments of the invention. The controller  60  may monitor various system parameters and base its control of the system  10  on these monitored parameters. For example, the controller  60  may receive an indication of the stack current via communication lines  53  that extend between the power conditioning circuitry  35  and the controller  60 . The controller  60  may also control the stack current using the communication lines  53 . The controller  60  may, for example, monitor the cell voltages of the fuel cell stack  20  via a cell voltage monitoring circuit  40 . The cell voltage monitoring circuit  40  is coupled to the fuel cell stack  20  through cell voltage monitoring lines  47 ; and the circuit  40  may continually scan the cell voltages of the stack  20  and provide indications of the scanned voltage to the controller  60  via a serial bus  48 .  
         [0029]     The controller  60  executes program instructions  65  that are stored in a memory  63  (of the controller  60 , for example). These program instructions cause the controller  60  to perform one or more routines that are related to controlling the general monitoring and operation of the fuel cell system  10 . 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 (techniques  150  and/or  250 , as examples) 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 electrical communication lines that extend to various components of the fuel cell system  10 , such as electrical communication lines  27  and  53  and the serial bus  48 . Electrical interferences (not shown) may be coupled between these lines and the controller  60 . 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.  
         [0030]     The fuel cell systems depicted in the figures are only examples of a fuel cell system in accordance with some embodiments of the invention, as the fuel cell systems in other embodiments of the invention may include more components, less components, different components and different arrangements than that shown in the figures and described in the specification.  
         [0031]     For example, in some embodiments of the invention, the fuel cell system may include a coolant subsystem to circulate a coolant through the fuel cell stack  20 . The fuel cell system may also include water recovery devices to recover product water from the fuel cell stack  20  and return this water to the coolant subsystem. In this manner, in some embodiments of the invention, the water recovery devices may be connected in the low point of the anode circulation path and/or bleed flow path for purposes of returning water to the coolant subsystem and removing this water from the anode and/or bleed flow paths.  
         [0032]     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.