Abstract:
A technique that is usable with a fuel cell stack includes providing an ejector to combine a first fuel flow from a fuel source with an anode exhaust flow from a fuel cell stack to produce a second fuel flow to an anode inlet plenum of the fuel cell stack. The technique includes regulating communication of the first flow to the ejector based on a pressure of gas in the anode inlet plenum.

Description:
GOVERNMENT LICENSE RIGHTS  
       [0001]     The U.S. Government has a paid-up license in this invention and the right in limited circumstances to require the patent owner to license others on reasonable terms as provided for by the terms of Contract No. DE-SC02-03CH11137 awarded by the Department of Energy. 
     
    
     BACKGROUND  
       [0002]     The invention generally relates to supplying and recirculating fuel in a fuel cell system.  
         [0003]     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° temperature range. Another type employs a phosphoric-acid-based polybenziamidazole, PBI, membrane that operates in the 150° to 200° 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: 
 
H 2 →2H + +2e −  at the anode of the cell, and  Equation 1 
 
O 2 +4H + +4e − →2H 2 O at the cathode of the cell.  Equation 2 
 
         [0004]     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.  
         [0005]     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.  
         [0006]     The fuel cell stack is one out of many components of a typical fuel cell system, such as a cooling subsystem, a cell voltage monitoring subsystem, a control subsystem, a power conditioning subsystem, etc. The particular design of each of these subsystems is a function of the application that the fuel cell system serves.  
         [0007]     For purposes of ensuring that the cells of the fuel cell stack are not “starved” of fuel, the incoming fuel flow to the stack exceeds the stoichiometric ratio that is set forth in Equations 1 and 2 above. Therefore, the anode exhaust flow from the fuel cell stack contains residual fuel. For purposes of maximizing the efficiency of the fuel cell system and establishing a sufficient flow through the fuel cell stack to remove water from its flow channels, the fuel cell system may recirculate the exhaust flow back to the anode inlet of the stack. Conventionally, an exhaust gas recirculation (EGR) blower may be used for this recirculation. However, the EGR blower may significantly contribute to the overall cost of the fuel cell system.  
         [0008]     Thus, there exists a continuing need for better ways to recirculate anode exhaust flow in a fuel cell system.  
       SUMMARY  
       [0009]     In an embodiment of the invention, a technique that is usable with a fuel cell stack includes providing an ejector to combine a first fuel flow from a fuel source with an anode exhaust flow from a fuel cell stack to produce a second fuel flow to an anode inlet plenum of the fuel cell stack. The technique includes regulating communication of the first flow to the ejector based on a pressure of gas in the anode inlet plenum.  
         [0010]     In another embodiment of the invention, a fuel cell system includes a fuel source, a fuel cell stack, an ejector and a control subsystem. The fuel cell source provides a first fuel flow, and the fuel cell stack includes an anode inlet plenum to receive a second fuel flow and an anode outlet to provide an anode exhaust flow. The ejector combines the first fuel flow with the anode exhaust flow to produce the second fuel flow. The control subsystem regulates communication of the first flow to the ejector based on a pressure of gas in the anode inlet plenum.  
         [0011]     Advantages and other features of the invention will become apparent from the following drawing, description and claims. 
     
    
     BRIEF DESCRIPTION OF THE DRAWING  
       [0012]      FIG. 1  is a schematic diagram of a fuel cell system according to an embodiment of the invention.  
         [0013]      FIG. 2  is a waveform of an anode gas plenum pressure illustrating control of a valve of the fuel cell system of  FIG. 1  according to an embodiment of the invention.  
         [0014]      FIG. 3  is a flow diagram depicting a technique to regulate communication of a fuel flow to a gas ejector of the fuel cell system of  FIG. 1  according to an embodiment of the invention.  
         [0015]      FIG. 4  is a more detailed flow diagram depicting a technique to regulate a fuel flow to the ejector according to an embodiment of the invention. 
     
    
     DETAILED DESCRIPTION  
       [0016]     Referring to  FIG. 1 , in accordance with an embodiment of the invention, a fuel cell system  10  includes a fuel cell stack  20 , which produces power for an external load  80  in response to incoming fuel and oxidant flows. The fuel cell stack  20  includes an anode inlet  22  that receives an incoming fuel flow and an oxidant inlet  24  that receives an incoming oxidant flow. The incoming fuel and oxidant flows are communicated inside the fuel cell stack  20  to respective anode and cathode inlet plenums of the fuel cell stack. The plenums are formed at least in part from openings in flow plates of the fuel cell stack. From these plenums, the fuel and oxidant flow through anode and cathode flow channels of the fuel cell stack  20  to promote electrochemical reactions pursuant to Equations 1 and 2 (see Background). The fuel flow produces an anode exhaust at an anode exhaust outlet  26  of the fuel cell stack  20 ; and the oxidant flow produces a cathode exhaust flow at a cathode outlet  28  of the stack  20 .  
         [0017]     As a result of the electrochemical reactions inside the fuel cell stack  20 , the stack  20  produces power, which is conditioned into the appropriate form by power conditioning circuitry  32  for the load  80 . Thus, depending on the particular embodiment of the invention, the power conditioning circuitry  32  may regulate a DC stack voltage from the fuel cell stack  20  into another regulated DC level for the load  80  (for the case where the load  80  is a DC load); and in other embodiments of the invention, the power conditioning circuitry  32  may convert the DC stack voltage into an AC voltage for the load  80  (for the case where the load  80  is an AC load). Thus, many variations are possible and are within the scope of the appended claims.  
         [0018]     In accordance with embodiments of the invention, the fuel cell system  10  includes a gas ejector  40  for purposes of recirculating the anode exhaust flow back to the anode inlet  22  and mixing the anode exhaust flow with an incoming fuel flow from a high pressure fuel source  50 . The gas ejector  40 , unlike an exhaust gas recirculation (EGR) blower, is a passive device, which has a main flow path that communicates the incoming fuel flow from the high pressure fuel source  50 . The main flow path extends from an inlet  39  of the gas ejector  40  to an outlet  41  of the ejector  40 . In response to the communication of the fuel flow through its main flow path, the gas ejector  40  creates a pressure drop at an ejector input  43  that is connected to the anode exhaust outlet  26 . This pressure drop establishes an anode exhaust recirculation flow from the anode exhaust outlet  26 , to the ejector inlet  43  and into the main flow path of the ejector  40 .  
         [0019]     As a more specific example, in accordance with some embodiments of the invention, the gas ejector  40  may be a venturi; and for these embodiments of the invention, the main flow path of the gas ejector  40  may be considered the throat of the venturi, and the ejector inlet  43  is the inlet port of the venture into which a flow may be injected into the throat.  
         [0020]     For purposes of optimizing use of the gas ejector  40 , in accordance with embodiments of the invention that are described herein, the incoming fuel flow from the high pressure source  50  is regulated based on a pressure of the gas in the anode inlet plenum of the fuel cell stack  20 . More particularly, in accordance with some embodiments of the invention, the fuel cell system  10  includes a controller  70  (one or more microprocessors or microcontrollers, as examples) that monitors the pressure of the gas in the anode inlet plenum (via a pressure sensor  42  and associated output signal that appears on its output terminal  44 ) and controls a valve  46  accordingly to regulate the flow from the high pressure fuel source  50  to the main flow path inlet  39  of the gas ejector  40 .  
         [0021]     More specifically, as further described below, the controller  70  controls the valve  46  to maintain the anode inlet plenum pressure within a regulated range. Thus, when the plenum pressure surpasses the regulated range, the controller  70  closes the valve  46 ; and conversely, when the plenum pressure decreases below the regulated range, the controller  70  opens the valve  46  to reestablish communication between the high pressure source  50  and the gas ejector  40  to raise the plenum pressure.  
         [0022]     As depicted in  FIG. 1 , in accordance with some embodiments of the invention, the valve  46  includes at least one control terminal  48  that receives a control signal that is generated (indirectly or directly) by the controller  70  for purposes of controlling the state (open or closed) of the valve  46 . The valve  46  has an input terminal that is coupled to an output terminal  51  of the high pressure fuel source  50 , and the valve  46  includes an output terminal that is coupled to the main flow path inlet  39  of the gas ejector  40 . As a more specific example, in accordance with some embodiments of the invention, the valve  46  may a solenoid valve, although other valves may be used in other embodiments of the invention.  
         [0023]     The controller  70  may include, for purposes of example, a processor  72  that executes program instructions  76  (that are stored in a memory  74 ) for purposes of controlling the valve  46  in accordance with the techniques that are disclosed herein. The controller  70  may include input terminals  77  for purposes of receiving status signals (such as the pressure signal from the pressure sensor  42 ) from the fuel cell system, signals indicative of commands, etc. Furthermore, the controller  70  may include output terminals  79  for purposes of controlling various valves (such as the valve  46 ), motors, etc. of the fuel cell system  10  and for purposes of communicating status messages and commands to other entities.  
         [0024]     Among the other features of the fuel cell system  10 , in accordance with some embodiments of the invention, the fuel cell system  10  may include an oxidant source, such as an air blower  60 , which furnishes the incoming oxidant flow to the oxidant inlet  24  of the fuel cell stack  20 . In accordance with some embodiments of the invention, the fuel cell system  10  may include a cathode recirculation path. Furthermore, in accordance with some embodiments of the invention, the fuel cell system  10  may include a bleed path in the anode exhaust recirculation path; and therefore, a bleed path that is established by an orifice may be coupled to the anode exhaust outlet  26 , in some embodiments of the invention. Thus, many variations are possible and are within the scope of the appended claims. Additionally, as depicted in  FIG. 1 , the fuel cell system  10  may include a coolant subsystem  64  that circulates a coolant through the fuel cell stack  20  for purposes of regulating the temperature of the stack  20 .  
         [0025]     In accordance with some embodiments of the invention, the high pressure source  50  may be a hydrogen tank, and in other embodiments of the invention, the high pressure fuel source may be a reformer. Thus, depending on the particular embodiment of the invention, either a hydrogen fuel flow or a reformate fuel flow may be received at the main flow path inlet  39  of the gas ejector  40 , depending on the state of the valve  46 . In some embodiments of the invention, the high pressure source  50  may have a pressure of 80 pounds per square inch gauge (psig), although other pressures are possible in other embodiments of the invention.  
         [0026]     As depicted in  FIG. 1 , in some embodiments of the invention, the pressure sensor  42  may be coupled in the flow path of a conduit that is located between the outlet  41  of the gas ejector  40  and the anode inlet  22  of the fuel cell stack  20 . In other embodiments of the invention, the pressure sensor  42  may be located inside the anode inlet plenum of the fuel cell stack  20 ; and in yet other embodiments of the invention, the pressure sensor  42  may be located at the outlet  41  of the gas ejector  40 . Thus, many variations are possible and are within the scope of the appended claims.  
         [0027]      FIG. 2  depicts the anode gas plenum pressure in accordance with an embodiment of the invention and illustrates the control of the valve  46 . More specifically, as depicted in  FIG. 2 , the valve  46  may be controlled pursuant to successive switching cycles  100 . Each switching cycle  100  includes an open time  108  in which the valve  46  is open to communicate fuel from the high pressure source fuel  50  and a closed time  110  in which the valve  46  is closed to block the communication of fuel from the high pressure fuel source  50 . During the open time  108 , the anode inlet plenum gas pressure increases until the pressure reaches an upper pressure threshold (called “P H ”), at which time the open time  108  ends and the closed time  110  begins. Thus, when the anode gas plenum pressure reaches the P H  upper pressure threshold, the controller  70  closes the valve  46  to isolate the high pressure fuel source  50  from the anode inlet plenum.  
         [0028]     As depicted in  FIG. 2 , when the valve  46  is closed, the anode gas plenum pressure decreases, a decrease that continues until the pressure reaches a lower pressure threshold (called “P L ” in  FIG. 2 ) at which time the controller  70  once again opens the valve  46  to reestablish communication between the high pressure fuel source  50  and the anode inlet plenum.  
         [0029]     Thus, as depicted in  FIG. 2 , the anode gas plenum pressure cycles between the P H  upper pressure threshold and the P L  lower pressure threshold so that by the above-described operation of the valve  46 , the controller  70  keeps the anode inlet plenum gas pressure within a regulated range that is defined by the P H  and P L  boundaries.  
         [0030]     The incoming fuel flow to the anode inlet  22  of the fuel cell stack  20  is relatively constant during the time when the plenum is charging with pressure (i.e., during the valve open time  108 ), thereby producing relatively constant anode exhaust recirculation flow when the valve  46  is open. Although the load  80  may affect the pressure, the difference between the minimum load pressure and the maximum load pressure is relatively small.  
         [0031]     The duration of the open time  108  is dependent upon such system design parameters as the plenum volume, the P H  upper pressure threshold, the P L  lower pressure threshold and the orifice size of the gas ejector  40 . To a lesser extent, the duration of the open time  108  is dependent upon the load  80 .  
         [0032]     The duration of the closed time  110  is a function of such system design parameters as the plenum volume and fuel cell volume, the load  80 , the P H  upper pressure threshold and the P L  lower pressure threshold. Thus, the P H  upper pressure threshold and the P L  lower pressure threshold are selected based on system design goals and constraint.  
         [0033]     Referring to  FIG. 3 , to summarize, in accordance with some embodiments of the invention, a technique  150  may be used to control incoming flow to the gas ejector  40 . Pursuant to the technique  150 , gas is provided (block  152 ) to the ejector  40  so that the ejector  40  injects anode exhaust gas into the anode intake plenum in response to an incoming fuel flow to the ejector  40  from the high pressure fuel source  50 . The technique  150  includes regulating (block  156 ) incoming fuel flow to the gas ejector based on the gas pressure in the anode inlet plenum.  
         [0034]     The advantages of the technique  150  may include one or more of the following. A single fixed ejector may be operated acceptably with a fuel cell over a wide load range. Anode gas recirculation may be accomplished without the use of a costly gas blower. A high recirculation flow rate may be achieved, thereby effectively moving liquid water from flow plate flow channels. The energy that is present in a high pressure hydrogen source stream (i.e., energy normally lost in a pressure regulator) is used, thereby increasing system efficiency.  
         [0035]      FIG. 4  depicts a more detailed technique  200  to control the valve  46  in accordance with some embodiments of the invention. Pursuant to the technique  200 , the gas pressure in the anode inlet plenum is measured, pursuant to block  204 . A determination is then made (diamond  208 ) whether the pressure in the plenum is greater than or approximately equal to the P H  upper pressure threshold. If so, then the valve  48  is closed, pursuant to block  210 . Otherwise, a determination is made (diamond  214 ) whether the pressure is less than or equal to the lower pressure limit threshold. If so, then the valve  48  is open, pursuant to block  220 . Otherwise, the valve  48  is maintained in its current state, as depicted in block  224 .  
         [0036]     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.