Abstract:
A component for reducing the likelihood of ice-related blockage in a fuel cell and methods for starting a fuel cell system. In one embodiment, the component is a separate insert configured with a sharp leading edge such that water droplets present in a reactant fluid that pass through an orifice in the component are conveyed away from an unstable formation at the edge to a more stable formation in an adjacent part of the component. In one form, the component is sized to fit within a valve inlet that in turn is placed in a humid reactant flowpath. In this way, when the fuel cell is operated in cold conditions—such as those associated with temperatures at or below the freezing point of water—the water droplets do not freeze in the area around the orifice such that ice-related blockage of the flowpath does not occur.

Description:
[0001]    This application claims priority to U.S. Provisional Application 61/643,559, filed May 7, 2012. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    This invention relates generally to fuel cells, and more particularly to a humid stream orifice design that does not become blocked under freezing conditions, as well as methods of fuel cell system start-up under frozen conditions such that blockage due to ice formation is inhibited. 
         [0003]    Fuel cells convert a fuel into usable electricity via electrochemical reaction. A significant benefit to such an energy-producing means is that it is achieved without reliance upon combustion as an intermediate step. As such, fuel cells have several environmental advantages over internal combustion engines (ICEs) and related power-generating sources. In a typical fuel cell—such as a proton exchange membrane or polymer electrolyte membrane (in either event, PEM) fuel cell—a pair of catalyzed electrodes are separated by an ion-transmissive medium (such as Nafion™). The electrochemical reaction occurs when a gaseous reducing agent (such as hydrogen, H 2 ) is introduced to and ionized at the anode and then made to pass through the ion-transmissive medium such that it combines with a gaseous oxidizing agent (such as oxygen, O 2 ) that has been introduced through the other electrode (the cathode); this combination of reactants form water as a benign byproduct. The electrons that were liberated in the ionization of the hydrogen proceed in the form of direct current (DC) to the cathode via external circuit that typically includes a load where useful work may be performed. The power generation produced by this flow of DC electricity can be increased by combining numerous such cells to form a fuel cell stack or related assembly that makes up a fuel cell system. 
         [0004]    Various fuel cell system operating conditions can lead to a high water content in one or both of the reactant streams. For example, water generated during operation of the fuel cell system may build up in one or both of the anode stream and cathode stream. In certain operating conditions, it is desirable to remove excess moisture to ensure that ice blockage of key flowpaths is avoided in conditions where such water may be exposed to freezing temperatures or related environmental conditions. Removing water from the fuel cell&#39;s anode loop is especially difficult as it doesn&#39;t have the high volume and velocity gas flow motive force that the cathode loop does as a way to purge any excess water. As such, starting a vehicular fuel cell system that has moisture present in one or both of the reactant fluid streams is hampered under cold ambient conditions if the low temperatures lead to ice or related blockage of the passageways that normally convey reactant to or from the fuel cells or stack. If a flowpath leading to the anode is blocked with ice, the flow of H 2  to the stack is prevented, which in turn leads to a failed cold catalytic heating (CCH) event, and the consequent failure of the vehicle to start. 
         [0005]    One way to promote heating within a fuel cell after a period of rest is known as cold catalytic stack heating (CCSH). This approach allows the flow of hydrogen from the anode to the cathode as a way to promote heating during fuel cell cold starts. For the cold start to be successful, flow of hydrogen must occur within two seconds of start. If the orifice in the valve is blocked with ice, CCSH will not occur and the cold start is aborted. Supplemental energy devices (including those capable of imparting heat or vibration to at-risk components) may be also employed to reduce the likelihood of ice-related blockage to fuel cell components. Nevertheless, such measures significantly increase the cost and complexity of the overall fuel cell system. 
       SUMMARY OF THE INVENTION 
       [0006]    The present invention includes a passive orifice design that remains clear, even under freeze-inducing ambient temperatures. The orifice permits the flow of humid gas under such sub-freezing conditions without the need for supplemental energy devices, thereby simplifying the overall design and reducing the cost, weight, size, reliability and system efficiency. 
         [0007]    According to a first aspect of the invention, a cup-shaped device or component is formed into one or both of the anode flowpath and the cathode flowpath. In a particular configuration, the cup is shaped as a cone with an approximately 2 millimeter diameter orifice in the base. In one form, the wall thickness of such a cup is about 0.08 millimeters thick, and may be made of 1100-series aluminum. In a preferred form, the cup-shaped component is configured as an insert so that it can be rapidly inserted into an existing valve flowpath. In another preferred form, the orifice is volcano-shaped relative to the remainder of the component such that the edge of the orifice defines the initial point of passage of the reactant through the orifice. 
         [0008]    A variation on this aspect includes a fuel cell system made up of—in addition to the aforementioned ice-resistant valve—one or more fuel cells. In situations where there are a plurality of such fuel cells, it will be appreciated that such cells may be arranged as a stack or related fuel cell assembly. Each cell is made up of an anode to accept a hydrogen-bearing reactant, a cathode to accept an oxygen-bearing reactant and a medium (such as the aforementioned Nafion™ (or the like) to form a PEM. Such a configuration promotes the delivery of at least a catalytically-ionized portion of the hydrogen-bearing reactant from the anode to the cathode. Additional components, such as an anode flowpath and a cathode flowpath help to deliver the reactants to the respective sides of the PEM. The valve is fluidly cooperative with one or both of the flowpaths to establish combination of a portion of the hydrogen-bearing reactant and the oxygen-bearing reactant. As with the previous aspect, the valve defines a fluid reactant passageway with a sharp-edged orifice that permits moisture contained within the reactant (whether hydrogen-bearing or oxygen-bearing to deposit on a surface portion of the fluid reactant passageway that is adjacent the orifice. In this way, the deposited moisture (which may be in droplet form) is conveyed away to be collected elsewhere such that upon exposure of the valve to environmental conditions where the moisture may be prone to freezing, such freezing will take place away from the orifice or other parts of the fluid reactant passageway. 
         [0009]    According to another aspect of the invention, a method for starting a fuel cell system is provided. In one embodiment, the cup-shaped device is configured as an insert into the anode flowpath. This method could be applied to a cathode as well. By providing a clear path in a passive way, humid gas under freezing conditions may be delivered without the need for supplemental devices. 
         [0010]    According to yet another aspect of the invention, a method for preventing blockage of a fuel cell system reactant flowpath is provided. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]    The following detailed description of the present invention can be best understood when read in conjunction with the following drawings, where like structure is indicated with like reference numerals and in which: 
           [0012]      FIG. 1  is a cutaway view of a reactant flowpath according to the prior art where the orifice is prone to ice buildup; 
           [0013]      FIG. 2  shows the placement of a solenoid valve relative to a fuel cell stack for facilitating CCH during fuel cell cold starts and related ice blockage-prevention according to an aspect of the present invention; 
           [0014]      FIG. 3  is a cutaway view highlighting the placement of an insert into a reactant flowpath according to an aspect of the present invention; 
           [0015]      FIG. 4  is a cutaway view showing the insert of  FIG. 3  in more detail; and 
           [0016]      FIG. 5  is a cutaway view of an automobile employing a fuel cell system with the ice blockage-prevention features according to an aspect of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0017]    Referring first to  FIG. 1 , a conventional valve  1  according to the prior art for use in a fuel cell system is disclosed. Valve  1  includes a valve body  1 A that defines a fluid reactant passageway  1 B therethrough such that the valve  1  may selectively allow passage of the reactant from the valve inlet  1 C to the valve outlet  1 D and then on to various components within the fuel cell system. An orifice  1 E is integrally formed into the valve body  1 A at either at the inlet (when the valve  1  is situated in a horizontal mount orientation) or as part of a poppet seat of the valve body  1 A (as shown in  FIG. 1 ). In one common configuration, the orifice is of a flat plate variety. Other devices that employ valves in conjunction with fluids exposed to freezing conditions (for example, a household refrigerator ice maker) will typically have the valve placed in a warm ambient environment, and will further typically use an open waterfall (rather than pipe) design to prevent ice blockage. Such a configuration is not available to fuel cell applications in general, and to automotive fuel cell applications in particular where the device with the valve and orifice may be expected to encounter freezing conditions (sometimes for protracted lengths of time), and may have to be in orientations that don&#39;t permit a ready disposal of the water that collects. In configurations where the valve  1  is configured for use in environments where freezing temperatures may be experienced, the relatively large thermal mass of the valve  1  (which is typically made of a dense metal such as iron or the like) makes it susceptible to ice formation and related blockage. 
         [0018]    Referring next to  FIG. 5 , the major components of a vehicle  10  and a fuel cell system  15  used to provide motive power to the vehicle  10  are shown. The system  15  includes one or more fuel cell stacks  20  that receive fuel from a fuel storage system  30  (made up of one or more fuel tanks) that are configured to contain a hydrogen-bearing reactant. Although not shown, an optional fuel processing system may also be used; such a system may include a conversion system (such as a methanation reactor or other such equipment known to those skilled in the art) to change a hydrogen-bearing precursor into a form suitable for catalytic reaction in the fuel cell stacks  20 . It will also be appreciated by those skilled in the art that other fuel delivery and fuel processing systems are available. Likewise, the features of an air delivery system for the oxygen-bearing reactant may be disposed between an oxygen source (such as the ambient atmosphere) and the fuel cell stack  20 . Such a system may include fluid delivery equipment in the form of conduit, valves, compressors, controllers or the like (none of which are shown). As will be appreciated by those skilled in the art, stack  20  is a repeating arrangement of numerous individual fuel cells such that the power output is sufficient to operate the drivetrain  50  through the energy conversion device  40  or other load. 
         [0019]    Other features of vehicle  10  may include an energy conversion device  40  (for example, in the form of an electric motor that acts as a load for the current being generated by fuel cell system  15 ) coupled to a drivetrain  50  (such as a driveshaft or the like) and one or more motive devices  60 , shown notionally as a wheel. Other ancillary equipment may include one or more batteries  70 , as well as electronics  80  in the form of controllers or related system management hardware, software or combinations thereof. While the present system  10  is shown for mobile (such as vehicular) applications, it will be appreciated by those skilled in the art that the use of the fuel cell stack  20  and its ancillary equipment is equally applicable to stationary applications, such as stand-alone power generation equipment or the like. 
         [0020]    Referring next to  FIG. 2 , the general configuration of a fuel cell stack  20  with a valve  28  used to permit selective combination of the anode and cathode reactants (such as for a CCSH event) is shown. Flow channels  22 ,  24  form the part of an anode flowpath and cathode flowpath that act as conduit for delivering reactants to the respective anodes and cathodes of the multiple fuel cells  26  in stack  20 . In the present context, fluid-based passageways, streams, channels, conduit, loops, flowpaths and related terms may be used interchangeably to describe the conveyance of a fluid from one location to another; their meaning should be apparent from the context. In a preferred embodiment, the first reactant being routed through flow channel  22  is a hydrogen-bearing fluid (such as that contained within and delivered from fuel storage system  30 ), while the second reactant being routed through flow channel  24  is air or related oxygen-rich fluid. Each fuel cell  26  within stack  20  includes an anode, cathode and an electrolyte layer (none of which are shown) disposed between anode and cathode. A load (for example, in the form of a motor or related energy conversion device  40 ) is electrically coupled to stack  20  such that a current generated thereby may be used to perform useful work. 
         [0021]    In one form, valve  28  may be formed in one of the flow channels  22 ,  24  of the respective reactants. In another form (as shown), a separate flow channel  27  may be coupled to both the flow channels  22 ,  24  to allow the selective combination, while valve  28  is used in either version to control when such combination is made. In one preferred (although not necessary) embodiment, valve  28  is a solenoid valve that can be powered through an appropriate electrical signal. The electrochemical combination of reactants made possible by valve  28  helps to reduce or eliminate the chance of flowpath ice formation; such an approach is particularly beneficial during fuel cell system  15  startup via CCSH or the like, as this catalytic reaction of the hydrogen and oxygen contained within the reactants produces heat that may be used to raise the temperature of adjacent flowpaths and components. In one form, valve  28  is allowed to remain open long enough (possibly for only a few seconds) to promote the desired combination and subsequent system warm-up via the catalytic reaction. 
         [0022]    Referring next to  FIGS. 3 and 4 , additional details of the construction of valve  28  are shown. As discussed herein, in one form, valve  28  may be configured as a solenoid valve, where an electric current passing through wrapped coil (not shown) can force a magnetically-compliant (for example, iron-based) actuator or related plunger (not shown) to move a flap or related closure mechanism (not shown) in valve  28  in order to regulate the flow of reactant therethrough. Valve body  28 A forms the primary structure, through which a bore forming a fluid reactant passageway  28 B is defined. An inlet  28 C and an outlet  28 D are at respective ends of fluid reactant passageway  28 B. 
         [0023]    A cup-shaped insert  29  is sized to fit within an enlarged region within the inlet  28 C. In one form, the insert  29  defines a slight inward taper along the reactant flow direction F. Furthermore, the insert  29  defines a generally smooth path with gradual (rather than abrupt) surface contour changes. Such shaping helps to promote a continuous flow of a fluid (as well as moisture contained within or separated from) to a desired location for collection or additional downstream movement. In one form, the insert  29  is made of an inexpensive material (for example, an aluminum or aluminum alloy) that can be stamped or otherwise formed in a cost-efficient way to define a passive path for the reactant to flow through. Other manufacturing approaches may be used as well, so long as the surface finish remains very smooth to create a hydrophillic surface that avoids droplet formation and buildup, as well as and keeping the wall thickness very thin (for example, to the thickness mentioned above) to promote rapid warm-up. Thus, while machining the piece and then subjecting it to electropolishing would work, such a method would be cost prohibitive. In a preferred form, the roughness is below a suitable profile or area value as determined by suitable AMSE (for example, ASME Y14.36M), ANSI, ISO (for example, ISO 1302) or related standard. In a more preferred form, such values are an R z  of about 10 and an R t  of about 12. Moreover, the upper rim of the insert  29  is sized to allow a secure snap-fit connection between the insert  29  and a lip on the compatibly-sized and shaped region within the inlet  28 C. An orifice  29 A forms a flow-regulating opening, preferably with a precisely known size to provide a calibrated or measured amount of the fluid flow from a pressure drop that occurs as the fluid passes through it. While the orifice  29 A performs valuable flow or control functions, the very size, shape, orientation and precision needed to establish its flow-regulating function also make it particularly susceptible to the types of ice blockage associated with the remainder of the valve  28 . To alleviate the tendency of having water droplets form on—and remain in the vicinity of—the orifice  29 A, the present inventors shaped it to define an upwardly-projecting edge (or lip)  29 B around its periphery such that it defines a generally three-dimensional structure. In one preferred form, the orifice  29 A and edge  29 B define a volcano-like profile, where the raised edge  29 B is the first part of the orifice  29 A that the incoming flow or reactant encounters along flow direction F. The edge  29 B is preferably very thin, which promotes instability of any water that contacts it Likewise, the volcano-like shape of the insert  29  enhances capillary driven flow of water from the orifice  29 A, as a droplet at the peak of the volcano (i.e., at the edge  29 B) is highly unstable. As such, by the present construction of insert  29 —with its use of a thin metallic configuration and sharp edge  29 B—introduces instability in the water drops by maximizing gas/liquid surface area in a manner generally analogous to putting a drop of water on a needle tip. This condition can be remedied by moving the water droplets to other more energy-compatible surfaces that include a gutter  29 C and corner  29 D that are shaped to provide a smooth transition away from the unstable edge  29 B that make up the insert  29 . In this way, the use of the contoured features discussed above in insert  29  helps avoid the droplets of water that condense out of the reactant in the immediate vicinity about orifice  29 A from remaining there and turning into ice in freezing conditions. Instead, the present insert  29  minimizes surface energy of the condensed droplets by having them collect in locations that reduce the gas/liquid surface area in what is known as the Concus-Finn condition. The surface energy described thereby is also important for describing capillary motion toward such low-energy geometries. This configuration (with its use of smooth, gradual surface changes) promotes neutral surface energy; such promotion is enhanced through the avoidance of machining marks, pitting, waviness or other related undulations along the insert  29  surface that would otherwise fill with water and undesirably change the surface energy (in essence making it become more hydrophilic) for larger drops of water. Further rationale for avoiding or minimizing surface marks is because such marks could also promote capillary movement toward the orifice  29 A. By promoting a neutral surface energy, the present inventors realized that they could discourage the formation of a hydrophilic surface (and the concomitant spillage of water over the orifice  29 A). Similarly, the promotion of a neutral surface would discourage the formation of the opposite (i.e., a hydrophobic surface) that would otherwise tend to reduce capillary motion of the water toward, and retention in, the corner  29 D. 
         [0024]    A gap G is formed between the tapered portion of the insert  29  and the inner wall of the region within the inlet  28 C. In a preferred form (as shown with particularity in  FIGS. 3 and 4 ), the valve  28  is horizontally mounted. In this way, water droplets that form on one side or surface of the insert  29  drain around the sharp edge  29 B and into gutter  29 C and corner  29 C, while the water droplets that form on the other side or surface of the insert  29  drain along the gradual outer contour formed by gutter  29 C to collect in gap G. From here, any accumulated liquid would either get absorbed into the gas flowing through the valve  28 , or accumulate until it ran over the lip formed at the inlet junction between the insert  29  and valve  28  and then down the face of the component that valve  28  is installed in. 
         [0025]    It is noted that recitations herein of a component of an embodiment being “configured” in a particular way or to embody a particular property, or function in a particular manner, are structural recitations as opposed to recitations of intended use. More specifically, the references herein to the manner in which a component is “configured” denotes an existing physical condition of the component and, as such, is to be taken as a definite recitation of the structural factors of the component. Likewise, it is noted that terms like “generally,” “commonly,” and “typically,” when utilized herein, are not utilized to limit the scope of the claimed embodiments or to imply that certain features are critical, essential, or even important to the structure or function of the claimed embodiments. Rather, these terms are merely intended to identify particular aspects of an embodiment or to emphasize alternative or additional features that may or may not be utilized in a particular embodiment. 
         [0026]    For the purposes of describing and defining embodiments herein it is noted that the terms “substantially,” “significantly,” and “approximately” are utilized herein to represent the inherent degree of uncertainty that may be attributed to any quantitative comparison, value, measurement, or other representation. The terms “substantially,” “significantly,” and “approximately” are also utilized herein to represent the degree by which a quantitative representation may vary from a stated reference without resulting in a change in the basic function of the subject matter at issue. 
         [0027]    Having described embodiments of the present invention in detail, and by reference to specific embodiments thereof, it will be apparent that modifications and variations are possible without departing from the scope of the embodiments defined in the appended claims. More specifically, although some aspects of embodiments of the present invention are identified herein as preferred or particularly advantageous, it is contemplated that the embodiments of the present invention are not necessarily limited to these preferred aspects.