Abstract:
A valve for reducing the likelihood of ice-related blockage in a fuel cell and methods for starting a fuel cell system. The valve includes a valve plate and coupling plate that are cooperative with one another within a valve body such that flexural forces imparted to the valve plate from a pressurized fluid are transferred to localized contact surfaces between the valve plate and coupling plate. By concentrating these forces to such a localized area, improvements in the ability of the fluid to initiate and propagate a crack in built-up ice around the valve&#39;s seating region is improved. In this way, fuel cell starting in cold conditions—such as those associated with temperatures at or below the freezing point of water—is also improved.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates generally to valves used in fuel cell systems, and more particularly to valves used in moisture-prone environments in such fuel cell systems such that the valves do not become blocked due to ice buildup 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. 
     A significant benefit to fuel cells as an energy-producing means is that it is achieved without reliance upon combustion as an intermediate step. As such, they 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™). An 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) electricity to the cathode via external circuit that typically includes a motor or related 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. 
     Various fuel cell system operating conditions can lead to high water content in one or both of the reactant streams. In one form, such conditions may arise out of the use of devices such as a water vapor transfer (WVT) unit that helps ensure adequate humidity levels within various parts of the fuel cell stack. In certain operating conditions (including those associated with WVT usage), it is desirable to remove excess moisture to ensure that ice blockage of key flowpaths is avoided in conditions where such moisture may be exposed to freezing temperatures. Avoiding ice blockage is especially important during vehicle starting, where access to electricity for use in ancillary vehicular systems (such as heating, cooling, lighting and other systems) is generally not available until the fuel cell stack is operational. 
     Valves—with their relative movement between adjacent surfaces as a way to provide selective flow—are particularly susceptible to ice blockage, especially between such surfaces that come into intermittent contact with one another during valve opening and closing. One example are check valves, which are frequently used in fuel cell systems to limit reactant backflow into the stack during periods of non-operation of the stack in order to minimize undesirable reactions between catalytic substrates within the stack and an oxygen-bearing or hydrogen-bearing fluid. The type of complete valve closure that is needed to avoid the aforementioned reactions is often difficult to achieve, especially in situations where ice bonds are formed on valve sealing interfaces after a cold soak in a humid environment. Conventional valves (which in one form may be formed as a diaphragm that is responsive to a pressurized reactant bearing against it) exist in a deformed state at temperature for the duration of stack operation; this in turn can lead to warpage or related sustained permanent deformation that exacerbates the sealing or leakage problems. Moreover, stresses imparted to the diaphragm from the reactant is nearly uniform around the diaphragm perimeter; such relatively larger surface contact requires a significantly high reactant force, which in turn delays the onset of ice breakup and the subsequent opening of the valve. In some circumstance, this force may not be sufficient to overcome the built-up ice, leading to the aforementioned failed start. 
     SUMMARY OF THE INVENTION 
     The present invention includes a passive valve design that retains its ability to open and close, even under freeze-inducing ambient temperatures where an ice bond may form. Such a valve may be placed in either or both of the cathode reactant flowpath or the anode reactant flowpath. In fuel cell system configurations where a WVT is employed, such valve (or valves) may be fluidly placed between the WVT and a fuel cell stack to provide the necessary isolation of the stack to help protect the cathode side of the stack from air intrusion during stack off time. Alternatively, an inlet form of the valve may be placed upstream of the dry side of the WVT with an outlet form of the valve downstream of the wet side of the WVT. Because the WVT is capable of producing significant levels of moisture or related humidity within the cathode flowpath, such a valve is especially useful. Likewise, such valves may be used with systems that have no back pressure valve, as well as system where such a backpressure valve is prone to sealing inadequacies. 
     According to a first aspect of the invention, an ice-resistant valve for use in a fuel cell system is disclosed. The valve includes a fluid inlet, a fluid outlet, a body defining a fluid reactant passageway through it that is cooperative with the fluid inlet and the fluid outlet, and an actuation member disposed within the body. The actuation member includes a flexible diaphragm, a coupling plate and a valve plate, where the valve plate defines a seating region cooperative with the body such that during a closed state of the valve, the valve plate substantially prevents the fuel cell reactant from flowing between the fluid inlet and the fluid outlet through the fluid reactant passageway. Likewise, the diaphragm operates such that during an open state of the valve, the valve plate becomes unseated to permit the fuel cell reactant to flow. The valve plate is situated adjacent the coupling plate, and includes one or more tab regions defining a localized connection surface that is smaller than a surface defined by the seating region. In this way, the smaller contact surface means that the initiation of an ice-breaking movement of the valve plate will be easier to effect than under traditionally-seated valves where a substantial entirety of the connecting adjacent surfaces may have ice formed on them. Specifically, the same amount of movement force imparted from the reactant fluid to the valve plate is concentrated in a much smaller interface region under the present invention than it is with a traditionally-seated valve. More particularly, the increased notch sensitivity of the present invention is such that flexural forces induced in the valve plate in response to movement that are in turn in response to the fluid force can be more efficiently used at the localized connection surface of the present invention as a way to initiate a breakup of any ice formed in the seating region. 
     According to another aspect of the invention, a fuel cell system includes one or more fuel cells each of which includes an anode to accept a hydrogen-bearing reactant, a cathode to accept an oxygen-bearing reactant and a medium cooperative with the anode and the cathode such that upon catalytic transformation of at least one of the reactants, the catalytically-transformed reactant travels from one of the anode and the cathode to another of the cathode and the anode through the medium. The system also includes an anode flowpath in fluid communication with the anode and a cathode flowpath in fluid communication with the cathode, each of the anode flowpath and the cathode flowpath cooperative with a fluid inlet configured to receive a fuel cell reactant and a fluid outlet disposed fluidly downstream of the fluid inlet; and at least one valve disposed in at least one of the anode flowpath and the cathode flowpath and defining a fluid reactant passageway therethrough, the at least one valve comprising a fluid inlet, a fluid outlet, a body defining a fluid reactant passageway therethrough that is cooperative with the fluid inlet and the fluid outlet, and an actuation member comprising: a flexible diaphragm selectively cooperative with the body and defining a seating region therebetween; and a valve plate situated adjacent the coupling plate and defining at least one tab region that defines a localized connection surface between the valve plate and at least one of the body and the diaphragm that is smaller than a surface defined by the seating region such that commensurate with or prior to movement of the diaphragm between a closed valve state and an open valve state in response to a load imparted by the fuel cell reactant, a flexural force induced in the valve plate by the load initiates a breakup of any ice formed in the seating region at the localized connection surface. 
     According to another aspect of the invention, a method of inhibiting freeze-related blockage of a reactant flowpath in an automotive fuel cell system, the method comprising: configuring a valve to be fluidly cooperative with the reactant flowpath, the valve defining a body with an actuation member disposed therein, the actuation member comprising: a flexible diaphragm selectively cooperative with the body; and a valve plate situated adjacent the coupling plate and defining at least one tab region that defines a localized connection surface between the valve plate and the coupling plate that is smaller than a surface defined by the seating region; and introducing at least one of a hydrogen-bearing reactant and an oxygen-bearing reactant to the valve plate such that a flexural force induced in the valve plate preferentially initiates a breakup of any ice formed in the seating region at the localized connection surface. In one particular form, the method is for starting the fuel cell system in temperatures where residual water present in the system may be prone to freezing, especially at movement-critical components such as a check valve. By providing a clear path in the check valve, humid gas under freezing conditions may be delivered without the need for supplemental devices (such as a backpressure valve). 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       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: 
         FIG. 1  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; 
         FIG. 2  shows a simplified schematic view of the fuel cell system useable in the automobile of  FIG. 1 , including placement of a check valve in accordance with one embodiment of the present invention; 
         FIGS. 3A through 3C  show a cutaway view of the check valve of  FIG. 2  during various stages of valve operation; 
         FIG. 4  is a cutaway view of the placement of the valve of  FIGS. 3A through 3D  into a housing that is in turn placed in a reactant flowpath; 
         FIG. 5  is a view of the check valve with tab connectors in accordance with one embodiment of the present invention; and 
         FIG. 6  shows an alternate embodiment of a valve with a stepper motor actuation. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring first to  FIG. 1 , the major components of a vehicle  10  and a fuel cell system  100  are shown. Other features of vehicle  10  may include an energy conversion device  20  (for example, in the form of an electric motor that acts as a load for the current being generated by fuel cell system  100 ) coupled to a drivetrain  30  (such as a driveshaft or the like) and one or more motive devices  40 , shown notionally as a wheel. Other ancillary equipment may include one or more batteries  50 , as well as electronics  60  in the form of controllers or related system management hardware, software or combinations thereof. While the present system  100  is shown for mobile (such as vehicular) applications, it will be appreciated by those skilled in the art that it is equally applicable to stationary applications, such as stand-alone power generation equipment or the like. 
     The system  100  is coupled to a fuel storage system  70  (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 discussed below. 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. 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, the stack is a repeating arrangement of numerous individual fuel cells such that the power output is sufficient to operate the drivetrain  30  through the energy conversion device  20  or other load. 
     Referring next to  FIG. 2 , a schematic shows that the fuel cell system  100  is made up of—in addition to the aforementioned ice-resistant valve  160 —one or more fuel cells  110  that collectively form a stack  120 . Each cell  110  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  130  and a cathode flowpath  140 —each with respective inlets  132 ,  142  and outlets  134 ,  144 —help to deliver the reactants to the respective sides of the PEM, while a WVT  150  may be used to provide humidity control within stack  120 . One or more pumps or related pressure-increasing devices  145  may be used to facilitate delivery of the reactant-bearing fluids to the respective anode and cathode flowpaths  130 ,  140 . It should be noted that fuel cell system  100  may either be configured (a) with an expander, in which case no backpressure valve is required, or (b) without an expander. In either case, such a backpressure valve (not shown) may be used to modulate the pressure within the stack. Elevating the cathode pressure increases stack  120  power but consumes electrical power to drive a pump  145  harder. Depending on device efficiencies, operating temperatures and humidification requirements, optimum pressure may be high enough that the energy dissipated in a backpressure valve (which functions in a manner similar to a throttle) could justify the expense of replacing the valve with a more expensive expander (which functions in a manner similar to the exhaust side of a turbocharger) to recover the energy. In some cases both may be needed for proper control. If range and efficiency are most important, high pressure systems with expanders are generally preferred. On the other hand, if cost is paramount, lower pressure systems with backpressure valves are preferred. In extreme cases (i.e., very inexpensive systems with little or no concern for efficiency of operation), neither will be used such that the system will just run at ambient pressure. 
     High operating temperatures require higher pressure to keep the water vapor in the reactants from becoming a significant diluent. Increasing pressure for a given temperature reduces the amount of water required for humidification, thereby reducing the size and cost of the WVT  150 . In one form, the expander may be placed downstream of the WVT  150  and an isolation valve. In such case, the backpressure valve (not shown) could go immediately before or after the expander. Backpressure control can also be accomplished within the expander using a variable nozzle turbine (VNT) expander (not shown). While such a configuration may employ either a common shaft connection between the compressor and turbine, a separate shaft configuration can also be used, depending on cost, complexity and efficiency concerns, where compressor and generator controls on the expander may exacerbate these concerns, making such control prohibitive. 
     Referring next to  FIGS. 4 and 5  in conjunction with  FIG. 2 , the valve  160  may be placed in either or both of the anode flowpath  130  and a cathode flowpath  140  to be fluidly cooperative with one or both. When configured as an outlet valve in general and a cathode stack outlet valve in particular, valve  160  defines a body  161  with a fluid reactant passageway  162  made up of an inlet  163  and an outlet  164 . An actuation member (shown presently in the form of a diaphragm  166 ) is used to permit selective introduction and removal of a reactant-bearing fluid to and from stack  120 . In the present context, fluid-based passageways, streams, channels, conduit, loops, flowpaths and related terms may be used interchangeably to describe the conveyance of the reactant-bearing fluid from one location to another; their meaning should be apparent from the context. In one form, a dimension of the valve  160  could be 50 mm by 36 mm with a 30 mm ID across the inlet  163 . 
     In one form when used in conjunction with a cathode stack outlet, valve  160  may be configured as a diaphragm-actuated stack isolation valve, where the top cavity  180  is vented to the atmosphere, and where the valve  160  opens as the reactant-bearing fluid supply pressure increases through inlet  163 . Because valve  160  is preferably upstream of an expander in system  100 , the pressure drop is only due to turns defined by a reactant passageway (discussed in more detail below); this in turn ensures that the stack vacuum works to close the valve  160 . The stack outlet configuration shown is able to avoid diaphragm inversion, which is the flipping of the diaphragm through the center position. By setting the flow direction F so the vacuum will not act on the rolling part of the diaphragm  166 , such a condition may be avoided. As mentioned above, the valve  160  shown in  FIG. 4  can also be used as an inlet isolation valve, where the flow directions are merely reversed from the ones depicted in the figure. In another embodiment (not shown), the valve  160  could be used as a diaphragm actuated stack isolation valve with a back pressure control function. In this case, the top cavity  180  is either vented to the atmosphere or connected to a WVT supply pressure with a solenoid (such as a 3 port pulse width modulation (PWM) electric solenoid where the various ports include one connected to the chamber, one to vent and one to the WVT supply pressure). Such a solenoid would apply variable amounts of pressure by alternately pressurizing and venting the chamber fast enough to produce an average pressure. Increasing the fraction of the time (duty cycle) of the pressurized portion of the cycle increases the average result. An orifice such as top cavity  180  would assure that total flow loss is not excessive. Both the minimum pressure drop and failure mode pressure are set by spring force, whereas the stack vacuum works to close the valve  160 . The diaphragm  166  (discussed in more detail below) would stay inverted from the shape shown. 
     In one preferred (although not necessary) embodiment, valve  160  is configured as a check valve to isolate the stack  120  from reactant intrusion during times when the stack  120  is not operational. As mentioned above, flexible diaphragm  166  is selectively cooperative with the body  161  to act as an actuation member. Valve  160  further includes a coupling plate  167  and a valve plate  168 ; between them, they cooperate with the body  161  and diaphragm  166  (or other actuation mechanism) to permit the selective breakup of built-up ice at discrete locations on the surface of the a coupling plate  167  and a valve plate  168 . Such an approach—which facilitates a more localized initiation of a crack or related fissure in the built-up ice—will allow a smaller, more simplistic construction of valve  160 . 
     Valve plate  168  defines a seating region  168 A that is adjacently-placed relative to the relatively rigid coupling plate  167  such that during a closed state of valve  160 , the valve plate  168  substantially prevents the reactant-bearing fluid from flowing along flow direction F between the inlet  163  and the outlet  164  through the reactant passageway  162 . In a preferred embodiment, diaphragm  166  is made from a rubber that can flex in response to pressure differentials across its inlet  163  and outlet  164 , while valve plate  168  is also deformable and preferably made from a sheet of plastic material. Likewise, the diaphragm  166  is such that during an open state of valve  160 , the valve plate  168  becomes unseated to permit the reactant to flow. Coupling plate  167  is situated adjacent the diaphragm  166  and includes one or more tab regions  167 A that defines a localized connection surface  170  between it and apertures  168 B formed in a portion of the valve plate  168 . Coupling plate  167  further defines a seating region  167 B that is conformally shaped to accept a complementary lower surface of diaphragm  166 . Significantly, the size of the contact of the localized connection surface  170  is much smaller than a surface defined by the seating region  168 A. In this way, a load L imparted by the introduction of reactant is such that either prior to or commensurate with movement of the diaphragm  166  between the closed and open states, a flexural force (indicated my moment M, see  FIG. 3C  in particular) induced in the valve plate  168  by the load L initiates a breakup of any ice formed in the seating region at the localized connection surface  170  rather than having to break it up along a substantial entirety of the seating region  168 A (which is shown presently as a peripheral surface of valve plate  168 ). Because ice is notch-sensitive, a greater percentage of load L (as well as the concomitant moment M) can be focused on a smaller location (specifically, the localized connection surface  170  that is formed adjacent the tab regions  167 A and apertures  168 B) to promote greater ease in breaking up any ice formation. The present design addresses significant failure mode requirements of system  100 , including a “fail fully or partially closed” situation for a backpressure valve. A bias mechanism  169  that includes a spring  169 A, spring seat  169 B and retainer  169 C may be used to keep valve  160  in a predetermined state in the absence of load L imparted by fluid flow F. 
     In an ideal operation, the system would seal when the valve plate  168  touches the body  161 , but in reality, extra force is needed to deform the surfaces to close gaps due to surface finish or geometry irregularities. The spring  169  provides this extra force and introduces a bias in the sense that some pressure is required to open it. That pressure limits the range of regulation when used for backpressure control. 
     Referring next to  FIGS. 3A through 3C , valve  160  will be particularly discussed in conjunction with its placement in the cathode flowpath  140  of the system  100  of  FIG. 2  (although it will be appreciated that the same applies mutatis mutandis to the anode flowpath  130  as well). Valve  160  protects the cathode-side of the stack  120  from air intrusion during times when stack  120  is not operational. Correct operation of the valve  160  is especially important in situations where there is either no backpressure valve or one that does not seal well. Thus, for example, in situations where either a backpressure valve was designed with clearance which allows oxygen to diffuse into the stack  120  or those using an expander system, both tend to permit a significant amount of diffusion, so it is up to the isolation valve to be the diffusion barrier. Significantly, the localized connection surface  170  made up of the tab regions  167 A of valve plate  168  can be—upon application of load L—flexed against a rigid member that is part of the body  161  placed behind the flexing valve plate  168  such that a moment M is induced in the tab regions  167 A. This has the effect of focusing the pneumatic force generated by the opening pressure of the reactant-bearing fluid that flows along flow direction F on the tab regions  167 A and apertures  168 B rather than around the larger peripheral dimensions of the seating region  168 A as a whole. In one form, a four-fold increase in the maximum stress can be applied to an ice bond over that of a conventional seating arrangement. As mentioned above, once a crack is formed locally at the tab region  167 A, the notch sensitivity and brittle behavior of the ice should cause the balance of the bond along seating region  168 A to fail and allow the valve  160  to open as desired. 
     Referring first to  FIG. 3A , when the valve  160  is in its closed (dormant) state (i.e., prior to the application of a force distributed against its lower surface), valve plate  168  is in an as-formed deflected state. Creep deformation is not a concern as this deflection is not present during operation at creep-inducing temperatures. Radial symmetry is chosen to assure deflection does not produce gaps; as such, in one preferred form, at least the valve plate  168  is of a generally cylindrical shape for the portion that engages with body  161  along the seating region  168 A. As stated above, in one preferred form, valve plate  168  is made from a plastic-based material, including PTFE (Teflon), PEI (Ultem), PTFE coated PEI, or polypropylene. A rubber version may also be used, subject to modifications in the design, such as employing a wider retainer  169 C (especially in its lower engaging portion) around the stem of the diaphragm  166  that is used to engage spring  169 A. In the event a rubber version were used, it would employ material properties to manage the desired tensile strain in a majority of the perimeter with desired tensile stiffness in the tab regions  167 A. In yet another version, a composite-based fabric-reinforced rubber sheet with fibers oriented along the tab regions  167 A could also be used. The slight deformation shown in  FIG. 3A  to the valve plate  168  is used to address height tolerances between the apertures  168 B and the tab regions  167 A of the coupling plate  167 , as well as flatness tolerances between them. Referring next to  FIG. 3B , when the valve  160  starts to open to an active state in response to a rising pressure differential imparted by load L across the diaphragm  166  and valve plate  168 , at least the valve plate  168  deflects beyond neutral to a relatively planar (i.e., flat) shape. Referring next to  FIG. 3C , as the valve plate  168  continues to deflect upward, additional pneumatic forces start to appear at the tab regions  167 A due to bending stiffness and tensile stiffness of the valve plate  168  in order to produce moment M. In one form, a 100 kPa fluid pressure differential over a 30 mm diameter valve plate  168  generates 16 lbs of total force. The deflection of the valve plate  168  closes any and begins to apply a peel force at both tab regions  167 A. One form of such peel forces is in the form of tensile force T that extends along the outward edges of tab regions  167 A and the corresponding localized connection surface  170  that is formed around these regions and the apertures  168 B. 
     In another version (not shown) a double-diaphragm valve configuration may be used. Adding another atmosphere referenced diaphragm to the valve would make it easier to open when there is a vacuum inside the stack  120 . Such a configuration would be especially beneficial to the inlet side of the stack  120 , where the necessity of such inclusion depends on the dead head pressure capability of the compressor  145  or the presence of a compressor recirculation valve or stack bypass valve (neither of which are shown) to allow the compressor  145  to avoid being dead-headed when the valve is closed. Thus, the double diaphragm valve would be used if the dead-headed supply pressure was not adequate to open the single diaphragm version discussed above while at a partial vacuum. Such could also be used on the anode side of stack  120 , especially for breaking ice formed on an anode drain/purge valve. 
     Referring next to  FIG. 6 , a variation on a valve  260  that is actuated by a stepper motor  200  is disclosed. Motor  200  includes a housing  202 , bearing  204 , drive key  206 , coils  208 , rotor  210 , shaft  212  and threads  214 . A connector  216  secures the rotatable shaft  212  to the coupling bar  218  and valve plate  220 . In the present figure, valve plate  220  corresponds generally to valve plate  168 , while valve seat  222  corresponds to the seal area of body  161  of  FIGS. 3A  though  3 C,  4  and  5 . Unlike valve  160  discussed above, the simplified valve  260  does not require the pressure balancing diaphragm  166  of  FIG. 4  to act as an actuating member; instead, the motor  200  can act as the actuating member. Such a configuration may be particularly useful in situations where stack cooling may produce significant vacuum levels (for example, up to about 40 kPa) inside the stack  120 . While such levels may not inhibit a traditional outlet check valve  260  operation (as it would be self energizing in that direction), a traditional check valve  260  disposed on the inlet side would require a 40 kPa spring to keep it from being pulled open; such a spring would then introduce 40 kPa pressure drop during stack operation. The valve  260  could take advantage of an atmosphere referenced diaphragm assist as a way to meliorate this potential problem. 
     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. 
     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. 
     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.