Abstract:
A passive restriction passageway (for example, a passive capillary valve or a restricting orifice) positioned to drain accumulated liquid from a fuel cell reactant flow channel is used in conjunction with a control element for periodically adjusting the pressure across the passageway. The control element intermittently adjusts pressure across the passageway to enable liquid flow through the passageway. The restriction passageways and the adjustment of pressure periodically move liquid water through the passageways to drain liquid buildup from the reactant supply channels. Together, these features enable sustained performance from the fuel cell during operation and also prevent damage to the fuel cell when the fuel cell is exposed to freezing temperatures (especially after shutdown of the fuel cell).

Description:
FIELD OF THE INVENTION 
     The present invention relates to fuel cell power systems and methods for consistent provision of gaseous reactant feeds to the power system; in this regard, the invention controls liquid water blockage of gaseous flow in channels supplying reactant to the fuel cell stack. 
     BACKGROUND OF THE INVENTION 
     Fuel cell power systems convert a fuel and an oxidant to electricity. One fuel cell power system type of keen interest employs use of a proton exchange membrane (hereinafter “PEM”) to catalytically facilitate reaction of fuels (such as hydrogen) and oxidants (such as air/oxygen) into electricity. The PEM is a solid polymer electrolyte that facilitates transfer of protons from the anode to the cathode in each individual fuel cell of the stack of fuel cells normally deployed in a fuel cell power system. 
     In a typical fuel cell assembly (stack) within a fuel cell power system, individual fuel cells have flow fields with inlets to fluid manifolds; these collectively provide channels for the various reactant and cooling fluids reacted in the stack to flow into each cell. Gas diffusion assemblies then provide a final fluid conduit to further disperse reactant fluids from the flow field space to the reactive anode and cathode; these diffusion sections are frequently advantageously embedded as a part of the design of collector electrodes pressing against the reactive anode and cathode. 
     PEM fuel cell stacks are typically designed with serpentine flow fields. Serpentine flow fields are desirable as they effectively distribute reactants over the active area of an operating fuel cell, thereby improving performance and stability. On the other hand, effective operation of a PEM requires operation of the flow field channels and gas diffusion assemblies in non-flooded states. However, in this regard, an operational problem arises as certain portions of serpentine flow fields accumulate liquid water during fuel cell operation. This liquid water is undesirable, as it alters flow distribution of the reactant gases, and it can also remain in the stack even after a considerable purge. In freezing conditions, water plugs remaining in a channel after stack shutdown are a basis for severe mechanical damage to the fuel cell as the remaining liquid water is transformed into ice. U-bend (180-degree turn) portions of the serpentine flow channels are particularly prone to such water build up. 
     What is needed, is a way of preventing significant water build up during operation of a fuel cell power system along with a way of removing any water build up when it does occur. The present invention is directed to fulfilling this and other related needs in a fuel cell. 
     SUMMARY OF THE INVENTION 
     The present invention is for a fuel cell and/or method of operating a fuel cell (where the fuel cell has at least one membrane electrode assembly in reactive interface to at least one oxidant reactant flow channel carrying an oxidant reactant and to at least one fuel reactant flow channel carrying a fuel reactant) where the fuel cell has a passive restriction passageway in fluid connection between a liquid accumulation portion of one reactant flow channel and a liquid reception portion of one reactant flow channel and a control element intermittently adjusting the pressures between the accumulation and reception portions so that liquid flows from the liquid accumulation portion to the liquid reception portion as a result of the pressure adjustment. The liquid accumulation portion contains fluid at a first pressure, and the liquid reception portion contains fluid at a second pressure. The first pressure is operationally maintained to be greater than the second pressure; but the control element intermittently adjusts the first pressure in relation to the second pressure to provide a low pressure difference between the first pressure and the second pressure at a first setting of the control element, and a high pressure difference between the first pressure and the second pressure at a second setting of the control element. The passive restriction passageway provides a fluid passageway having a length and a cross sectional area sufficient to preclude flow of the liquid between the liquid accumulation portion and the liquid reception portion at the lower pressure difference, but sufficient to enable flow at the higher of the pressure differences. 
     In one aspect of the invention, the passive restriction passageway is a capillary passive restriction passageway. In another aspect of the invention, the passive restriction passageway is a restriction orifice. 
     In further aspects of the invention, the liquid accumulation portion and the liquid reception portion are both within the same channel; one of the channels is a serpentine channel; and/or one of the channels has a channel curvature subtended by angularity of at least 90 degrees. 
     In still further aspects of the invention, the control element is a control module controlling any of a variable speed compressor, a valve controlling pressure in the liquid accumulation portion, and/or a valve for controlling pressure in the liquid reception portion. The control module may also measure inputs from sensor(s) measuring either of the pressures and/or the speed of the compressor. 
     In yet further aspects of the invention, the channels and restriction passageway have surfaces of differing wetting (hydrophilic or hydrophobic) character. 
     In considering benefits from the invention, the restriction passageways and the adjustment of pressure periodically move liquid water through the passageways to drain liquid buildup from the reactant supply channels. Together, these features enable sustained performance from the fuel cell during operation and also prevent damage to the fuel cell when the fuel cell is exposed to freezing temperatures (especially after shutdown of the fuel cell). 
     Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the preferred embodiment of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will become more fully understood from the detailed description and the accompanying drawings, wherein: 
         FIG. 1  shows a fuel cell power system overview; 
         FIG. 2  shows detail in a portion of a PEM fuel cell stack within the fuel cell stack assembly of the fuel cell power system of  FIG. 1 ; 
         FIG. 3  shows blocked reactant flow channels as determined from a fuel cell stack test; 
         FIG. 4A  and  FIG. 4B  present two types of passive restriction passageways; 
         FIG. 4C  presents a passive restriction passageway connecting a U-bend portion and a straight portion of the same channel or of two different channels; 
         FIG. 5  shows a first flow channel plate design with incorporated passive restriction passageways; 
         FIG. 6  shows a second flow channel plate design with incorporated passive restriction passageways; and 
         FIG. 7A  and  FIG. 7B  show cross-sectional detail in the passive restriction passageways of the channel plate designs of  FIG. 5  and  FIG. 6 . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The following description of the preferred embodiment(s) is merely exemplary in nature and is in no way intended to limit the invention, its application, or uses. 
     The invention is further understood with reference to a generic fuel cell power system. Therefore, before further describing the invention, a general overview of the power system within which the improved fuel cells of the invention operate is provided. 
     As shown in  FIG. 1 , fuel cell power system  100  includes a hydrogen source  112  in the form of stored hydrogen or a fuel processor for providing a hydrogen stream  120 . 
     Concurrent with the feeding of hydrogen stream  120  through control valve  162  into the anode chamber of fuel cell stack  122 , oxygen in the form of air in stream  124  is fed into the cathode chamber of fuel cell stack  122 . Stream  124  is first compressed in (variable speed) compressor  136  and fed through pipe  138  to control valve  142 . Pressure sensor  144  measures the condition of pressure in stream  124  as applied to cathode sides of the PEM in fuel cell stack  122 . A pressure sensor  146  measures the condition of pressure in stream  120  as applied to anode sides of the PEM in fuel cell stack  122 . 
     The hydrogen stream  120  and the oxygen from oxidant stream  124  react in fuel cell stack  122  to produce electricity. Electrical power sensor  180  measures the condition of electrical power generated by fuel cell stack  122  through a measure of voltage, amperage, or wattage, or a combination of these. 
     Anode exhaust (or effluent)  126  from the anode side of fuel cell stack  122  flows through backpressure control valve  176  to combustor  130 . Cathode exhaust (or effluent)  128  from the cathode side of fuel cell stack  122  flows through backpressure control valve  178  to combustor  130 . Pressure sensor  172  measures the condition of pressure in stream  128 . Pressure sensor  174  measures the condition of pressure in stream  126 . 
     Control valve  162 , compressor  136 , control valve  142 , pressure sensor  146 , electrical power sensor  180 , backpressure control valve  178 , backpressure control valve  176 , pressure sensor  172 , pressure sensor  174 , and pressure sensor  144  are in signal connection to control module  164 . Control module  164  regulates conditions in streams  124 , stream  128 , stream  126 , and/or stream  120  by operating control valve  162 , compressor  136 , backpressure control valve  178 , backpressure control valve  176 , and control valve  142  in response to signals from any of pressure sensor  146 , electrical power sensor  180 , pressure sensor  172 , pressure sensor  174 , and pressure sensor  144 . In one embodiment, compressor  136  is a variable speed compressor operating essentially as a control valve regulating oxidant feed stream  124  to fuel cell stack  122 . Controller program  166  (also denoted as “software” and/or “executable logic” and/or an “executable program” as a data scheme holding data and/or formulae information and/or program execution instructions) is provided in control module  164  for controlling operation of power system  100 . In one embodiment, computer  164  and controller program  166  are provided as an ASIC (application-specific integrated circuit). 
     Fuel cell power system  100  may be stationary or may be an auxiliary power system in a vehicle. In a preferred embodiment, however, fuel cell power system  100  powers a vehicle such as a passenger car, truck, or van. 
     Turning now to  FIG. 2 , a partial PEM fuel cell stack  200  of fuel cell stack  122  is schematically depicted as having a pair of membrane electrode assemblies (MEAs)  208  and  210  separated from each other by a non-porous, electrically-conductive bipolar plate  212 . Each of MEAs  208 ,  210  have a cathode face  208   c ,  210   c  and an anode face  208   a ,  210   a . MEAs  208 ,  210  and bipolar plate  212  are stacked together between non-porous, electrically-conductive, liquid-cooled end plates  214  and  216  (the bipolar plate  212  may be liquid cooled as well). Plates  212 ,  214 ,  216  each include respective flow fields  218 ,  220 ,  222  established from a plurality of flow channels formed in the faces of the plates for distributing fuel and oxidant gases (i.e., H 2  &amp; O 2 ) to the reactive faces of MEAs  208 ,  210 . Nonconductive gaskets or seals  226 ,  228 ,  230 ,  232  provide sealing and electrical insulation between the several plates of fuel cell stack  200 . 
     Porous, gas permeable, electrically conductive sheets  234 ,  236 ,  238 ,  240  press up against the electrode faces of MEAs  208 ,  210  and serve as primary current collectors for the respective electrodes. Primary current collectors  234 ,  236 ,  238 ,  240  also provide mechanical supports for MEAs  208 ,  210 , especially at locations where the MEAs are otherwise unsupported in the flow field. Suitable primary current collectors include carbon/graphite paper/cloth, fine mesh noble metal screens, open cell noble metal foams, and the like which conduct current from the electrodes while the mesh and/or open cell portions function as gas diffusers in allowing reactant gases to pass therethrough. 
     Bipolar plate  214  presses up against primary current collector  234  on cathode face  208   c  of MEA  208 , bipolar plate  216  presses up against primary current collector  240  on anode face  210   a  of MEA  210 , and bipolar plate  212  presses up against primary current collector  236  on anode face  208   a  of MEA  208  and against primary current collector  238  on cathode face  210   c  of MEA  210 . 
     An oxidant gas  262  such as air/oxygen is supplied to the cathode side of the partial fuel cell stack  200  from air compressor  136  and line  124  via appropriate supply plumbing  248 . In a preferred embodiment, oxygen  262  is in air supplied to the cathode side from the ambient. A fuel such as hydrogen, or hydrogen containing reformate,  264  is supplied to the anode side of fuel cell  200  from stream  120  via appropriate supply plumbing  244 . 
     Exhaust plumbing (not shown) for both the H 2  and O 2 /air sides of MEAs  208 ,  210  is also provided for removing anode effluent from the anode flow field and the cathode effluent from the cathode flow field. Coolant plumbing  250 ,  252  is provided for supplying and exhausting liquid coolant to bipolar plates  212 ,  214 ,  216 , as needed. 
     It is to be noted that fuel cell stack  200  shows two fuel cells with plate  212  shared between the two fuel cells and plates  214  and  216  shared between one of the shown fuel cells and, in each case, another fuel cell not depicted in  FIG. 2 . In this regard, a “fuel cell” within a fuel cell stack is not physically fully separable insofar as any particular fuel cell in the stack shares at least one side of a bipolar plate with another cell. 
       FIG. 3  shows blocked reactant flow channels  300  as determined from a fuel cell stack test after operation of a fuel cell and subsequent freezing of the fuel cell before disassembly. In this regard,  FIG. 3  is derived from a photo-image of a flow channel field plate used in the test. In this regard, blocked reactant flow channels  300  are an example of the plurality of flow channels formed in the faces of any of plates  212 ,  214 ,  216  for distributing fuel and oxidant gases (i.e., H 2  &amp; O 2 ) to the reactive faces of MEAs  208 ,  210 . Channels  302 ,  304 , and  308  each have a bend, for example a U-bend (180-degree turn) portion. Although a 180 degree U-bend is shown, it should be understood that other bend angles or shapes that accumulate water may also benefit from the present invention. More specifically, each of channels  302 ,  304 , and  306  have a portion with a channel curvature subtended by angularity of at least 90 degrees or with other shapes that accumulate water. An ice plug  312  defines a liquid accumulation portion of channel  302 . An ice plug  310  defines a liquid accumulation portion of channel  304 . An ice plug  308  defines a liquid accumulation portion of channel  306 . In this regard, each of ice plugs  312 ,  310 , and  308  derive from accumulations of liquid water in the respective liquid accumulation portions of channels  302 ,  304 , and  306  during operation of the fuel cell stack. 
       FIG. 4A  presents a capillary passive restriction passageway design  400 , and  FIG. 4B  presents a restriction orifice passive restriction passageway design  410 . Capillary passive restriction passageway design  400  provides a capillary passive restriction passageway  406  having a length L in fluid connection between a liquid accumulation portion in the U-bend of channel  402  and a liquid reception portion of an adjacent channel or a different portion of the same channel  404  (illustrated in a U-bend configuration, but which can have any shape including a straight or curved configuration). See  FIG. 4C . The liquid accumulation portion of channel  402  contains fluid at a pressure greater than the pressure of the liquid reception portion of channel  404 . Similarly, restriction orifice passive restriction passageway design  410  provides a restriction orifice passive restriction passageway  412  in fluid connection between a liquid accumulation portion in the U-bend of channel  408  and a liquid reception portion of channel  409  (also in a U-bend or other configuration). The liquid accumulation portion of channel  408  contains fluid at a pressure greater than the pressure of the liquid reception portion of channel  409 . 
     As passive restriction valves, capillary passive restriction passageway  406  and restriction orifice passive restriction passageway  412  each have a respective length L and a cross sectional area designed to preclude flow of liquid water between the respective liquid accumulation portions of channels  402  and  408  and the respective liquid reception portions of channels  404  and  409  when the pressure across the respective passageways is below a low pressure threshold. But the respective length and a cross sectional area of either of capillary passive restriction passageway  406  and restriction orifice passive restriction passageway  412  is also designed to enable flow of liquid water between the respective liquid accumulation portions of channels  402  and  408  and the respective liquid reception portions of channels  404  and  409  when the pressure across the respective passageways is above a high pressure threshold. 
     Capillary passive restriction passageway  406  and restriction orifice passive restriction passageway  412  therefore enable flow fields such as flow fields  218 ,  220 ,  222  to have a small drainage feature either in the U-bends of plate channels or in other areas in the stack flow field where water is known to accumulate. The small drainage feature is, in other words, a by-pass channel cutting across the land to a companion flow channel. In one embodiment, the companion flow channel is a subsequent portion of the channel having the liquid accumulation portion. In an alternative embodiment, the companion flow channel is an independent channel from the channel having the liquid accumulation portion.  FIG. 4C  illustrates a detail view of a capillary passive restriction passageway  406  disposed between a channel  414  and a straight section  416  with arrows illustrating the water drainage direction. 
     As previously noted, when the pressure across the restrictive passageway is below a low pressure threshold, the by-pass “valve” is “shut” to liquid passage so that the portion of the channel in fluid connection to the by-pass fills with water as reactant flow proceeds through the main reactant channel ( 402  or  408 ). Then, as water accumulates in the channel (in the U-bend), the pressure drop across capillary passive restriction passageway  406  (or restriction orifice passive restriction passageway  412 ) is increased (through use of control module  164 ) to cause flow of the liquid through the passageway. In the cathode case, the shift in pressure is enabled by, without limitation, increasing the speed of variable compressor  136  (as shown on the cathode of  FIG. 1 ), lowering the back pressure of line  128  by use of valve  178 , and/or increasing pressure of line  124  by use of valve  142 . In the anode case, the shift in pressure is enabled by, without limitation (as shown on the anode of  FIG. 1 ), lowering the pressure of line  126  by use of valve  176 , and/or increasing pressure of line  120  by use of valve  162 . As should be appreciated, control module  164  effects the pressure shift, in one embodiment, in conjunction with measurements from sensors  180 ,  144 ,  146 ,  172 , and  174  as appropriate. 
     This pressure differential shift across the by-pass restriction passageway pushes the water out of the U-bend where it is accumulating and into a liquid receiving portion of a channel, so that the liquid water is more advantageously positioned for subsequent removal from the fuel cell stack. The magnitude of the pressure shift (extended pulse) need only be several kPa, (for example, without limitation, less than 10 kPa), which is a function of geometry, surface properties of the capillary valve surface, viscosity and surface tension of the liquid. 
     In one embodiment, control module  164  executes the pressure adjustment at some pre-determined timed interval. In this regard, for example and without limitation in one embodiment, a pressure adjustment (pressure shift) to the higher level of pressure difference (enabling liquid flow) occurs every 90 seconds for a high pressure difference duration of about 15 seconds. In an alternative embodiment, control module  164  executes the pressure adjustment when power output sensed via power measurement  180  drops below a threshold value. In yet other embodiments, control module  164  executes the pressure adjustment when the pressure or a pressure difference as sensed by any of or a combination of pressure sensors  146 ,  174 ,  172 , and  144  shifts beyond a threshold value. 
     In one embodiment, capillary passive restriction passageway  406  (or restriction orifice passive restriction passageway  412 ) have fluid contact surfaces of a hydrophilicity different from the hydrophilicity of the fluid contact surface of the liquid accumulation portion of the channel ( 402  or  408 , respectively) to which they are in fluid connection. In one embodiment, for example and without limitation, a polymeric fluorocarbon provides the surface of the capillary passive restriction passageway. This consideration provides a further degree of freedom in by-pass drain design for the flow fields so that a certain pressure difference threshold is effective in enabling liquid passage from accumulation sections of the flow channels. In general the requirement for capillary passive restriction passageway  406  or restriction orifice passive restriction passageway  412  dimensioning and/or surfacing is that: (a) under normal, static operating pressure, the size and shape of the orifice must be small enough that the surface tension of the water present is too great to allow drainage of the water through the orifice; and that (b) an acceptable pressure rise is defined and programmed (in program  166  of control module  164 ) to periodically (and/or upon water buildup) create a pressure gradient which will overcome this surface tension and “push” the water through capillary passive restriction passageway  406  or restriction orifice passive restriction passageway  412  to drain the channel. 
     Efficient operation of a fuel cell is best enabled when the pressure drop across the MEA ( 208 ,  210 ) is maintained to a close tolerance. Accordingly, in one embodiment, control module  164  executes the pressure adjustment to, for example, the anode side of fuel cell  122  when the pressure is adjusted for purge on the cathode of fuel cell  122  so that the pressure difference across the MEA remains optimally balanced (for example, without limitation, to a difference of less than 3 kPa) during the purging of liquid. In this regard, control module  164  uses measurements as sensed by pressure sensors  146 ,  174 ,  172 , and/or  144 , or a combination thereof, to control pressures through use of valves  142 ,  162 ,  176 , and/or  178 , or a combination thereof. 
       FIG. 5  shows one embodiment of a flow channel plate design  500  with incorporated passive restriction passageways. Capillary passive restriction passageway  406  instances are shown at capillary passive restriction passageway instances  502 ,  504 ,  506 , and  508  with further capillary passive restriction passageway instances disposed diagonally in  FIG. 5  between instances  502  and  504  and also between  506  and  508 . In  FIG. 5 , instances  502 - 504  connects U-bends of adjacent channels and instances  506 - 508  connect U-bends and L-bends of the same channel. 
       FIG. 6  shows an alternative embodiment of a flow channel plate design  600  with incorporated passive restriction passageways. Capillary passive restriction passageway  406  instances are shown at capillary passive restriction passageway instances  602 ,  604 ,  606 , and  608  with further capillary passive restriction passageway instances disposed diagonally in  FIG. 6  between instances  602  and  604  and also between  606  and  608 . In flow channel plate design  600 , as differentiated from flow channel plate design  500 , the passive restriction passageways are disposed between U-bends in one flow channel and a straight portion of a second channel where the straight portion is proximate to two channel curvatures subtended by fluid flow arc angles of 90 degrees in a non-reversing serpentine curvature segment of a channel. 
     Either of designs  500  or  600  are preferably manifested a bi-polar plate in rounded, square, rectangular, or triangular channels. Plates derived from designs  500  and/or  600  are preferably manufactured by either stamping, etching, or molding procedures. 
       FIG. 7A  and  FIG. 7B  show cross-sectional detail in one embodiment of the passive restriction passageways of the channel plate designs of either  FIG. 5  and/or  FIG. 6 .  FIG. 7A  shows cross-sectional detail along the elongation axis of capillary passive restriction passageway  706  between liquid accumulation portion  702  of one channel and liquid reception portion  704  of a second channel.  FIG. 7B  shows cross-sectional detail across the elongation axis of capillary passive restriction passageway  706 . 
     In one embodiment, fluid contact surface  760  of capillary passive restriction passageway  706  has a surface property different from the surface property of fluid contact surface  762  of liquid accumulation portion  702 . This can be enabled by any of different polishing approaches, different applied coatings and/or alloy finishing, and/or different material portions within plate  701 . In one embodiment, capillary passive restriction passageway  706  has a depth  708  of 0.20 mm, width  712  of 1.4 mm, and elongation of about 1.7 mm. 
     The passive restriction passageway further enables efficient water removal at shutdown of the fuel cell stack. This has value in avoiding corrosion within the fuel cell and has value in preparation of the fuel cell for storage at freezing temperatures. In this regard, control module  164  effects a series of pressure pulsations at shutdown along with purges of substantial duration to insure that all liquid water has been thoroughly removed from the fuel cell stack. 
     The described preferred embodiments provide efficient water removal at shutdown to prepare the fuel cell stack for freezing conditions. In this regard, a relatively non-disruptive periodic pressure shift is periodically used to implement a “clearing” effect to sustain fuel cell performance in normal operation. However, a long duration purge and/or a series of pressure shifts and pulses are used when the fuel cell is operationally shut down to insure that liquid water has been thoroughly removed. 
     The description of the invention is merely exemplary in nature and, thus, variations that do not depart from the gist of the invention are intended to be within the scope of the invention. Such variations are not to be regarded as a departure from the spirit and scope of the invention.