Abstract:
An automated reactant flow control subsystem is provided for a fuel cell system. The subsystem is achieved with a minimum of parts for decreased cost and increased reliability. The subsystem includes a fail-safe solenoid-actuated three-way valve in the fuel line that achieves very low pressure drop and very low parasitic load requirements. The subsystem also includes a fuel bypass system such as a flare, and a controller to automatically interlock the fuel and oxidant streams of the fuel cell.

Description:
BACKGROUND 
     The invention relates to a flow control subsystem for a fuel cell system. 
     A fuel cell is an electrochemical device that converts chemical energy produced by a reaction directly into electrical energy. For example, one type of fuel cell includes a proton exchange membrane (PEM), often called a polymer electrolyte membrane, that permits only protons to pass between an anode and a cathode of the fuel cell. At the anode, diatomic hydrogen (a fuel) is reacted to produce hydrogen protons that pass through the PEM. The electrons produced by this reaction travel through circuitry that is external to the fuel cell to form an electrical current. At the cathode, oxygen is reduced and reacts with the hydrogen protons to form water. The anodic and cathodic reactions are described by the following equations: 
     
       
         H 2 →2H + +2 e   −  at the anode of the cell, and 
       
     
     
       
         O 2 +4H + +4 e   − →2H 2 O at the cathode of the cell. 
       
     
     A typical fuel cell has a terminal voltage near one volt DC. For purposes of producing much larger voltages, several fuel cells may be assembled together to form an arrangement called a fuel cell stack, an arrangement in which the fuel cells are electrically coupled together in series to form a larger DC voltage (a voltage near 100 volts DC, for example) and to provide more power. 
     The fuel cell stack may include flow plates (graphite composite or metal plates, as examples) that are stacked one on top of the other, and each plate may be associated with more than one fuel cell of the stack. The plates may include various surface flow channels and orifices to, as examples, route the reactants and products through the fuel cell stack. Several PEMs (each one being associated with a particular fuel cell) may be dispersed throughout the stack between the anodes and cathodes of the different fuel cells. 
     The fuel cell stack may be part of a fuel cell stack system that supplies electrical power to an electrical load. For example, for a residential fuel cell system, the electrical load may be established by the various power consuming devices of a house. To furnish AC power to the house, the fuel cell system typically converts the DC voltage that is provided by the fuel cell stack into AC voltages. The fuel cell system may include a fuel processor to convert a hydrocarbon (natural gas or propane, as examples) into a reformate flow that furnishes the hydrogen to the fuel cell stack. The fuel cell system may also include an air blower that produces an air flow that furnishes the oxygen to the fuel cell stack. 
     For various reasons, it is desirable in a fuel cell system to be able to bypass reactant flows from the fuel cell stack. For example, upon start-up of a fuel processor, the initial reformate stream may contain high levels of carbon dioxide that would damage the electrode catalysts of the fuel cell stack. On start-up, it may thus be desirable to burn any off-specification fuel in a flare. Bypassing fuel to a flare on start-up may also provide system warm-up capabilities for outdoor systems in cold climates. Other operating modes, such as routine and emergency shut down scenarios, are also provided through fuel bypass capabilities. It will be appreciated that the performance, reliability and efficiency of a fuel cell system is increased by improving the performance reliability and efficiency of the reactant flow control subsystem of the fuel cell system. Likewise, as fuel cell technology is transitioned into consumer products, it is also desired to have such a flow control subsystem that is inexpensive and easy to manufacture. 
     There is a continuing need for an arrangement in a fuel cell system that efficiently and dependably addresses one or more of the issues stated above. 
     SUMMARY 
     An automated reactant flow control subsystem is provided for a fuel cell system. The subsystem is achieved with a minimum of parts for decreased cost and increased reliability. The subsystem includes a fail-safe solenoid-actuated three-way valve in the fuel line that achieves very low pressure drop and very low parasitic load requirements. The subsystem also includes a fuel bypass system such as a flare, and a controller to automatically interlock the fuel and oxidant streams of the fuel cell. 
     In general, in one embodiment, the reactant flow control system includes a three-way valve adapted to selectively switch a fuel stream between a bypass path and a fuel cell stack path. The valve is connected to a supply line, a bypass line, and a stack line, and has a bypass line seating orifice and a stack line seating orifice. A plunger in the valve directly abuts the stack seating orifice when the valve is in a bypass position, and directly abuts the bypass seating orifice when the valve is in a operating position. The bypass position is used to divert the fuel stream away from the fuel cell, for example, to flare off-specification fuel on start up or shutdown of the fuel cell system. The operating position is used to supply the fuel stream to the fuel cell during normal operation. 
     An important feature of the design is that the valve is configured to achieve very low pressure drop when in the operating position. For example, the stack line seating orifice is sized to have a cross-sectional area that is larger than the cross-sectional area of the stack line (for example 120% or larger) such that when the valve is in the operating position, the pressure drop across the valve is less than 5 inches water column (IWC) at a fuel stream flow of 20 cubic feet per minute (CFM). In some embodiments, the stack line seating orifice has about the same cross-sectional area as the stack line, and the pressure drop may be as low as 0.5 IWC at 20 CFM of fuel flow through the valve. Another feature of the design is that the plunger within the valve directly abuts the stack line seating orifice, and the orifice leads directly to the stack line. In this manner, when the valve is in the operating position, the fuel flow through the valve has a more direct path and lower pressure drop than in conventional 3-way valve designs, such as those typical in hydraulic systems where the flow path through such valves is often circuitous and restricted. The valve housing and plunger shape, which generally define the flow path through the valve, are also configured to provide a smooth and direct flow path through the valve to promote laminar flow through the valve. 
     The low pressure drop aspect of the above-described design features makes such a system advantageous for a fuel cell system that is operated at close to atmospheric pressure (for example, less than one atmosphere), since less energy is required to push reactants through the system. 
     Another important feature of the design is that it is configured to achieve failsafe operation with minimum power requirements. The failsafe operation refers to the fact that the plunger in the valve is biased to the bypass position. Thus, as an example, if the overall system were to lose power, the flow control system could bypass the fuel stream to a flare system. The bypass position of the valve can thus be referred to as the non-energized position, and the operating position can thus be referred to as the energized position. 
     In some embodiments, the valve is solenoid actuated by an electromagnetic coil surrounding the stem of a spring-loaded plunger. When a sufficient power is supplied to the coil (for example 10 Watts), the resulting electromagnetic force compresses the plunger spring, placing the valve in the operating position while the power is supplied. In other embodiments, the power requirements of such operation are minimized by utilizing a second coil around the plunger stem. The second coil is used to hold the plunger in the operating position since this requires less power (for example 5 Watts) than the power needed to actuate the plunger. The lower power requirements of the second coil thus replace the higher power requirements of the first coil during normal operation of the system. The power required to maintain operation of the fuel cell system may be referred to as the parasitic load. The two coil approach provides increased system efficiency by reducing the parasitic load on the system. 
     Finally, in some embodiments, the flow control subsystem is associated only with the fuel lines of the system. Whereas a need may exist to be able to bypass off-specification fuel (for example, fuel that is high in carbon monoxide, which would damage the fuel cell electrode catalysts), a similar need may not exist to bypass the oxidant gas. For example, in a start-up or shut-down operating mode, fuel might be bypassed away from the stack to a flare system, and the oxidant stream of the system might continue flowing through the stack on its way to the flare. In other embodiments, the flow control subsystem may be associated with both the fuel lines and the oxidant lines to bypass all reactant flows from the stack when desired. For example, in a PEM system where membrane dry-out is a concern, it may be desirable to bypass sub-saturated oxidant flow from the stack on start-up. 
     Advantages and other features of the invention will become apparent from the following description, from the drawing, and from the claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     FIG. 1 is a schematic diagram of a fuel cell system according to an embodiment of the invention. 
     FIG. 2 is a schematic diagram of a valve of the system of FIG. 1 in a closed mode according to an embodiment of the invention. 
     FIG. 3 is a schematic diagram of a valve of the system of FIG. 1 in an open mode according to an embodiment of the invention. 
    
    
     DETAILED DESCRIPTION 
     Referring to FIG. 1, an embodiment  10  of a fuel cell system in accordance with the invention includes a fuel cell stack  12  that consumes reactants (oxygen and hydrogen) to produce power for a load. As an example, the fuel cell system  10  may be a residential fuel cell system that supplies power to a house. For purposes of furnishing oxygen to the fuel cell stack  12 , the fuel cell system  10  includes an air blower  17  that produces an air flow that is provided to an air flow input line  16 . As an example, the pressure of the air in the air flow input line  16  may be approximately 27.7 inches of water column, 1 psig, at 70° C. maximum. 
     For purposes of furnishing hydrogen to the fuel cell stack  12 , the fuel cell system  10  may include a fuel processor  19  that furnishes a reformate fuel stream (that contains hydrogen) to a reformate flow input line  18 . As an example, the pressure of the reformate flow in the reformate flow input line  18  may vary from about 23 inches to 46 inches of water column at 70° C. maximum, with the variation being attributable to the varying demand required by the fuel cell stack  12  to respond to varying power demands from its load. 
     It is possible that during the course of its operation, the fuel cell system  10  may not be able to sustain power production due to a breakdown or a problem with the fuel cell stack  12  or the overall fuel cell system  10 . For example, the air flow may be substantially interrupted due to, as examples, failure of the air blower  17  or severe clogging of an air filter  21  that filters particulates from the air flow. Continuing the example, eventually, the air flow may decrease to a point at which reactions in the fuel cell stack are not sustainable, and thus, a terminal voltage (present at output terminals  13 ) of the fuel cell stack  12  may significantly drop. Thus, the drop in the terminal voltage or power production may indicate that shut down of the fuel cell system  10  is needed in order for repairs and maintenance to be performed. Thus, for purposes of preventing damage to the system  10  upon this or any other occurrence that disrupts the system&#39;s power generation, the system  10  includes a three-way solenoid valve  50  that responds to a control signal to shut off the reformats flow to the fuel cell stack  12  as described below. 
     More particularly, in some embodiments of the invention, the valve  50  is coupled between the reformate flow input line  18  and a manifold intake line  39  that is in communication with a reformate inlet manifold opening of the fuel cell stack  12  to deliver the reformate flow to the fuel cell stack  12  when the valve  50  is open. 
     The valve  50  is either fully open in its energized operating mode or fully closed in its non-energized bypass mode. In this manner, when the fuel processor  19  first powers up, the valve  50  is in its non-energized bypass mode and diverts the reformate flow from the reformate flow input line  18  to a reformate flow bypass line  32 . The diversion of the reformate flow from the stack  12  continues until a controller  40  of the system  10  determines (via a fuel sensor (not shown), for example) that the quality of the reformate is sufficient, or that the valve should otherwise be energized. In this manner, when the controller  40  determines that the fuel processor  19  is producing quality reformate, the controller  40  causes (via control lines, or wires  43 ) a voltage regulator  44  to provide a sufficient power (10 Watts at voltage of about 48 volts DC, for example) via control lines, or wires  42 , to the valve  50  to open the valve  50 , as described below. It is noted that the energy that is used to energize the valve  50  to cause the valve  50  to open may come from the fuel cell stack  12 . Therefore, as described below, if the system  10  is unable to maintain power production for whatever reason, the energy that is supplied to keep the valve  50  open disappears, an event that automatically places the valve  50  in its bypass mode of operation and shuts off the reformate flow. A solenoid driver  48  may be coupled between the voltage regulator  44  and the valve  50 , in some embodiments of the invention, for purposes of enhancing the current drive capability of the voltage regulator  44 . 
     The valve  50 , in its open mode, closes communication between the reformate flow input  18  and bypass  32  lines and establishes communication between the reformate flow input  18  and manifold intake  39  lines. After the valve  50  opens, the controller  40 , in some embodiments of the invention, causes the voltage regulator  44  to operate in a pulse width modulation (PWM) mode for purposes of communicating a PWM signal to the valve  50  (via the wires  42 ) to lower the average voltage to the valve  50  hold the valve  50  open. As described below, when the valve  50  is in its energized operating mode, the pressure that is exerted by the fluid flowing through the valve  50  aids in maintaining the open state of the valve  50 , thereby reducing the required average voltage to maintain the valve  50  in its open mode of operation. 
     When the average DC voltage that is established by the PWM signal decreases below a threshold level, the valve  50  closes communication between the reformate flow input  18  and manifold intake  39  lines and opens communication between the reformate flow input  18  and reformate flow bypass  32  lines. Because the voltage regulator  44  and driver  46  are both powered by the fuel cell stack  12 , a change in the terminal voltage of the fuel cell stack  12  influences the amplitudes of the voltages that are provided to control the valve  50 . Therefore, in the event that the generation of power by the fuel cell stack  12  is substantially disrupted, the voltage that is furnished to keep the valve  50  open decreases. As a result of this voltage decrease, the valve  50  enters the closed mode, a mode in which the valve  50  closes communication between the input reformate flow line  18  and the manifold intake line  39  and opens communication between the input reformate flow line  18  and the bypass reformate line  32 . As a result, the flow of the reformate to the stack  12  is shut off to effectively shut the power production by the stack  12 , and thus, potential damage to the stack  12  is prevented and bypass control of the fuel is achieved. 
     In some embodiments of the invention, the system  10  may include a three-way solenoid valve  52  that controls communication between the air input line  16 , an air bypass line  35  and an air line  21  that extends to an air inlet manifold opening of the stack  12 . The valve  52  may have a similar design to the valve  50 . In this manner, the valve  52  may establish communication between the air flow input line  16  and the air intake line  21  when the fuel cell stack  12  is furnishing a sufficient voltage and reroute the air from the air input line  16  to an air bypass line  35  when the voltage decreases below a predetermined threshold. 
     The controller  40  may perform functions other than regulating operation of the valve  50 . For example, in some embodiments of the invention, the controller  40  may monitor a current (via a current sensor (not shown)) that is produced by the stack  12  and the cell voltages (via a cell voltage scanning circuit (not shown)) of the stack  12  to determine a power output of the stack  12 . For this power output, the controller  40  may then control (via control lines, or wires  47 ) the rate at which the fuel processor  19  produces the reformate. 
     In some embodiments of the invention, the valve  50  may have a design that is depicted in FIGS. 2 and 3. However, other designs are possible. As shown in FIGS. 2 and 3, the valve  50  may include a plunger assembly  76  that is operated in a manner (e.g., based on the voltage or current that appears across the wires  42 ) to control communication between a fuel inlet port  60  that is in communication with the input reformate flow line  18  (also referred to as the supply line) and a fuel outlet port  62  that is in communication with the manifold intake line  39  (also referred to as the stack line). The plunger assembly  76  also controls communication between the fuel inlet port  60  and a fuel bypass port  64  that is in communication with the bypass line  32 . 
     More particularly, when the valve  50  does not receive the appropriate voltage level to sustain or establish its open mode of operation, the plunger assembly  76  is in a closed position (shown in FIG.  2 ), a position in which the plunger assembly  76  blocks communication between the reformate flow inlet port  60  and the reformate flow outlet port  62  and allows communication between the reformate flow inlet port  60  and the reformate flow bypass port  64 . When the valve  50  does receive an appropriate voltage level, the plunger assembly  76  assumes an open position (shown in FIG.  3 ), a position in which the plunger assembly  76  blocks communication between the reformate flow inlet port  60  and the reformate flow bypass port  64  and allows communication between the reformate flow inlet port  60  and the reformate flow outlet port  62 . 
     In some embodiments of the invention, the plunger assembly  76  operates inside a housing  70  (formed from two housing sections  70   a  and  70   b ) in which is formed the ports  60 ,  62  and  64 . In some embodiments of the invention, the housing  70  forms a generally circularly cylindrical plunger chamber  75  (also referred to as the internal plenum of the valve  50 ) that houses a generally circularly cylindrical plunger head  74  of the plunger assembly  76 . The plunger chamber  75  is in communication with the fuel inlet port  60  that circumscribes an axis  93  and opens to the top side of the valve  50 , as depicted in FIGS. 2 and 3. The plunger head  74  is concentric to and generally moves in a direction along an axis  91  (inside the plunger chamber  75 ) that is circumscribed by the fuel outlet port  62  and is orthogonal to the axis  93 . 
     The plunger head  74  has a closed end with a peripheral beveled surface  79  that mates with a corresponding beveled surface  77  (stack line seating orifice) of the housing  70  to close off the outlet port  62  when the plunger assembly  76  is in the closed position (and the valve  50  is in the closed mode), as depicted in FIG.  2 . Thus, the beveled surface  77  of the housing  70  forms a valve seat. As shown, when the plunger assembly  76  is in the closed position, the reformate may flow from the reformats flow inlet port  60  through an opening  103  of the chamber  75 . The opening  103  may also be referred to as the supply orifice of the valve  50 . The opening  103  leads into a spring chamber  107  (of the housing  70 ) that is always in communication with the bypass port  64 . In some embodiments of the invention, the opening  103  circumscribes the axis  91 , and the port  64  circumscribes an axis  95  that is parallel to the axis  93  and opens on the bottom side of the valve  50 , as depicted in FIGS. 2 and 3. The opening  103  is generally sized to as not to introduce significant pressure drop to fluids flowing through the valve  50 . 
     When the plunger assembly  76  is in the open position (and the valve  50  is in the open mode), the reformate may flow from the fuel inlet port  60  to the outlet port  62 , as depicted in FIG.  3 . When the plunger assembly  76  is in the open position, a closed end of the plunger head  74  seals off the opening  103  to closed off the fuel bypass port  64 . In this manner, this closed end of the plunger head  74  includes a peripheral beveled surface  83  that mates with a corresponding surface (bypass line seating orifice)  81  of the housing  70  that forms a valve seat about the opening  103 . 
     The size relationship between the fuel inlet port  60  and the stack line seating orifice  77  may be configured to minimize pressure drop through the valve  50 . For example, in a prototype of an embodiment of the invention, the fuel inlet port  60  has about the same cross-sectional area as the fuel line  18  to the valve  50 , and the stack line seating orifice  77  has about the same cross-sectional area as the line  39  exiting the valve  50  to the stack. The stack line seating orifice  77  is sized to have a cross-sectional area approximately 190% larger than the cross-sectional area of the stack line  39  such that when the valve  50  is in the energized position, the pressure drop across the valve  50  is less than 5 IWC at a fuel stream flow through the valve  50  of 20 CFM. In some embodiments, the stack line seating orifice  77  has about the same cross-sectional area as the stack line  39 . In other possible embodiments, the pressure drop through the valve  50  may be also be lower, such as being less than 0.5 IWC at 20 CFM of fuel flow through the valve  50 . 
     Another feature of the design is that the plunger head  74  directly abuts the stack line seating orifice  77 , and the orifice  77  leads directly to the stack line  39 . In this manner, when the valve  50  is in the operating position, the fuel flow through the valve has a more direct path and lower pressure drop than in conventional 3-way valve designs, such as those typical in hydraulic systems where the flow path through such valves is often circuitous and restricted. The valve housing and plunger shape, which generally define the flow path through the valve, are also configured to provide a smooth and direct flow path through the valve to promote laminar flow through the valve. 
     For purposes of moving the plunger assembly  76  between the open and closed positions, in some embodiments of the invention, the valve  50  includes a compression spring  80  (a stainless steel compression spring, for example) and an electromagnetic coil  84  (a 48 volt DC coil, for example) that interact with the plunger assembly  76  to form a solenoid-type control. In this manner, the plunger assembly  76  includes a stem that is formed from two stem portions  78  and  82  (described below) and is coaxial with the axis  91 . The stem is connected to the end of the plunger head  74  near the opening  103  and extends through the opening  103  inside the electromagnetic coil  84  that is also coaxial with the axis  91 . 
     More particularly, the stem portion  82  (at the unattached end of the stem) resides inside a generally cylindrical coil chamber  85  (of the housing  70 ) that is coaxial with the axis  91  and connects to the spring chamber at an opening  101 . The coil chamber  85  is circumscribed by the electromagnetic coil  84 . The stem portion  82  exhibits ferromagnetic properties so that longitudinal movement of the stem along the axis is influenced by the current that flows through the coil  84 . The stem portion  82  may have a larger radius about the axis  91  than the other stem portion  78  that is connected between the stem portion  82  and the plunger head  74  and resides in the coil chamber  107 . 
     The stem portion  82  is circumscribed by the spring  80  that has one end connected to the plunger head  74  and the opposite end connected to the housing  70  near the opening  99  where the spring chamber  107  meets the coil chamber  85 . The spring  80  exerts a force (on the plunger assembly  76 ) for purposes of seating the plunger head  74  in the opening  109  to place the valve  50  in the closed mode, as depicted in FIG.  2 . Conversely, the force that is exerted by the electromagnetic coil  84  exerts a force (on the plunger assembly  76 ) for purposes of seating the plunger head  74  in the opening  103  to place the valve  50  in the open mode, as depicted in FIG.  3 . 
     Thus, due to the above-described arrangement, when a sufficient voltage level is applied to the electromagnetic coil  84  (via the wires  42 ), a corresponding current is created in the coil  84  to cause the force that is developed by the coil  84  to overcome the force that is exerted by the coiled spring  80 . As a result, the plunger head  74  is seated in the opening  103  to place the valve in the open mode (shown in FIG.  3 ). Once open, most of the sealing force to maintain the plunger assembly  76  in the open position is obtained from the pressure that is exerted by the flow of the reformate that passes through the valve  50 . When no voltage or not enough voltage is applied to the wires  42 , the force that is exerted by the coiled spring  80  dominates to overcome the combined force that is exerted by the fluid pressure and the coil  84  (if any force is exerted by the coil  84 ) and seat the plunger head  74  in the opening  109  to place the valve in the closed mode (see FIG.  2 ). In a prototype of an embodiment of the invention, the first coil to actuate the plunger was a 48 VDC coil operated at about 8 Watts, and the second coil to hold the plunger in the activated position was a 48 VDC coil operated at about 4 Watts. The invention is not limited by the particular coils that are used, including any special design of the coils with respect to the voltage, current or power required. 
     Among the other features of the valve  50 , in some embodiments of the invention, the housing  70  may be formed out of metal, such as 316 non-magnetic stainless steel, for example. In other embodiments of the invention, the housing  70  may be formed from teflon or PPA-GF45 plastic, as examples. The stem portion  82  of the plunger assembly  76  may be made out of 400 series magnetic stainless steel. The beveled surfaces  77 ,  79 ,  81  and  83  that form the valve seats and the corresponding mating portions of the plunger head  74  may be coated with silicon rubber, for example. In some embodiments of the invention, the entire plunger head  74  may include an outer silicon rubber jacket, and this jacket may have a thickness of about 0.03 inches, for example. Other arrangements are possible. 
     While the invention has been disclosed with respect to a limited number of embodiments, those skilled in the art, having the benefit of this disclosure, will appreciate numerous modifications and variations therefrom. It is intended that the appended claims cover all such modifications and variations as fall within the true spirit and scope of the invention.