Abstract:
System for thermally conditioning, humidifying and filtering reactant feed gases supplied to a stack of fuel cells using an evaporative element, a water spray mechanism and a heat exchanger. The evaporation element also functions as a filter. The evaporative element may take the form of a removable packing (filter media). Use of a controller to manage the rate of nebulized water addition to the feed gas stream is also described. Benefits in filter efficiency and extended filter service life along with volume, weight, and cost reduction in a fuel cell system are realized.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates to gas conditioning of a fuel cell power system, and, in particular, to an apparatus and method for conditioning the temperature, humidity and/or purity of reactant gases supplied to a stack of fuel cells.  
         BACKGROUND OF THE INVENTION  
         [0002]    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.  
           [0003]    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 distribution 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.  
           [0004]    Effective operation of a PEM requires a balanced supply of water in the polymer of a PEM to maintain its proton conductivity while maintaining the flow field channels and gas diffusion assemblies in non-flooded operational states. In this regard, the hydrogen is supplied to the anode face of the MEA and reacts with the catalyst thereon to form hydrogen cations and free electrons. The oxidant, typically oxygen or oxygen-containing air, is supplied to the cathode face of the MEA and reacts with hydrogen cations that have crossed the proton exchange membrane to form water. Thus, the fuel cell generates both electricity and water through the electrochemical reaction, and the water is removed with the cathode effluent, dehydrating the PEM of the fuel cell unless the water is otherwise replaced. It is also to be noted that the inlet air flow rate to the cathode will generally evaporate water from the proton exchange membrane at an even higher rate than the rate of water generation (and commensurate dehydration of the PEM) via reaction at the cathode.  
           [0005]    When hydrated, the polymeric PEM possesses “acidic” properties that provide a medium for conducting protons from the anode to the cathode of the fuel cell. However, if the PEM is not sufficiently hydrated, the “acidic” character diminishes, with commensurate reduction of the desired electrochemical reaction of the cell. Hydration of a fuel cell PEM also assists in temperature control within the fuel cell, insofar as the heat capacity of water provides a heat sink.  
           [0006]    There is also a need to maintain the flow field channels and gas diffusion assemblies in a non-plugged state respective to any particulates which might be in the gaseous oxidant and fuel fluids which feed the cell; this concern is especially relevant to the oxidant in fuel cell power systems deployed on vehicles when the oxidant is air, since the condition of air varies from location to location, and the vehicle clearly has a purpose of providing transportation from location to location. As is generally appreciated, filters are traditionally used in vehicles to provide clean air to both fuel cells and, for that matter, to most internal combustion engines traditionally used to power vehicles.  
           [0007]    There is also a need to provide thermal conditioning of feed gases to the fuel cell stack. In this regard, it is desirable to maintain the temperature of the feed gases within an operating range. However, the ambient conditions of the environment as well as the operating conditions of the fuel cell system may cause the feed gases to be outside of the desired temperature range.  
           [0008]    In addition to issues in water balance, filtration and temperature conditioning of feed gases, another issue in fuel cell design for use in vehicles is directed to the efficient use of space. In this regard, space in a vehicle is precious and design approaches which represent an efficient use of space in the vehicle clearly benefit the utility of the vehicle; this leads toward integration of the humidifying system or gas conditioning system into each of the fuel cells, as provided.  
           [0009]    Accordingly, there is a need for a fuel cell power system which includes full humidification of the feed gases (especially the oxidant), high capture filtration of particulates in the feed gases, and thermal conditioning of feed gases commensurate with full humidification, in such a way that a minimum of space is needed for the humidification, cooling and filtration operations.  
         SUMMARY OF THE INVENTION  
         [0010]    One aspect of the present invention provides a fuel cell power system having a stacked series of fuel cells and including an integral evaporative element for evaporating water into the feed gas stream from a two-phase feed gas stream of feed gas and nebulized water. The evaporative element provides a medium for mass transfer of the nebulized water to fully humidify the feed gas stream. In addition and especially when wetted with water, the evaporative element provides filtration of solid particulates from the feed gas stream. Furthermore, the evaporative element provides a temperature conditioning function for controlling the inlet temperature of the feed gas stream. In one preferred embodiment, the invention also provides a removable evaporative element in the form of a packing or filter media and for use of a controller to manage the rate of nebulized water added to the feed gas stream.  
           [0011]    Another aspect of the present invention provides a fuel cell power system having a stacked series of fuel cells and including an integral heat exchange element for thermally conditioning the feed gas stream entering the individual fuel cells. The heat exchange element provides a heat transfer means for extracting heat from or adding heat to the feed gas stream. Such thermal conditions may be based upon the present operating state and the desired operating conditions of the fuel cell power system.  
           [0012]    Yet another aspect of the present invention provides a fuel cell power system having a stacked series of fuel cells and including an integral water injection mechanism for introducing water into the feed gas stream prior to entry into the individual fuel cells. The water injection mechanism can be used to increase the relative humidity of the feed gas stream and is preferably used in conjuction with an evaporative element. The water injection mechanism can also be used to provide thermal conditioning of the feed gas stream depending on the temperature difference between the water and the feed gas stream.  
           [0013]    While described herein with respect to a cathode feed stream, the invention also provides for use of an evaporative element, a heat exchange element and/or a water injection mechanism for thermally conditioning, humidifying and filtering the fuel gas feed to the fuel cell.  
           [0014]    There are several benefits which are derived from the present invention. Because of the relatively extensive surface area of the evaporative element, the invention provides for high filter efficiency and extended filter service life; and per the integration of a heat exchanger, filter, and water injector into one unit supported by the fuel cell stack plates, the invention provides a basis for volume, weight, and cost reduction in a fuel cell system.  
           [0015]    The invention is further appreciated from a consideration of the Figures and the Detailed Description Of The Preferred Embodiments. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0016]    [0016]FIG. 1 shows a fuel cell power system overview;  
         [0017]    [0017]FIG. 2 shows schematic representation of a portion of a PEM fuel cell stack within the fuel cell stack assembly of the fuel cell power system of FIG. 1;  
         [0018]    [0018]FIG. 3 is a side cross-sectional view showing detail in an integrated feed air humidifier, filter and cooler for a PEM fuel cell stack according to the present invention;  
         [0019]    [0019]FIG. 4 is a top cross-sectional view of the fuel cell stack taken along line A-A in FIG. 3; and  
         [0020]    [0020]FIG. 5 is an end cross-sectional view of the fuel cell stack taken along line B-B in FIG. 3. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0021]    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 fuel cell power system within which the improved fuel cells of the invention operate is provided. In the system, a hydrocarbon fuel is processed in a fuel processor, for example, by reformation and partial oxidation processes, to produce a reformate gas which has a relatively high hydrogen content on a volume or molar basis. Therefore, reference is made to hydrogen-containing as having relatively high hydrogen content. The invention is hereafter described in the context of a fuel cell fueled by an H 2 -containing reformate regardless of the method by which such reformate is made. It is to be understood that the principles embodied herein are applicable to fuel cells fueled by H 2  obtained from any source, including reformable hydrocarbon and hydrogen-containing fuels such as methanol, ethanol, gasoline, alkaline, or other aliphatic or aromatic hydrocarbons.  
         [0022]    As shown in FIG. 1, a fuel cell power system  100  includes a fuel processor  112  for catalytically reacting a reformable hydrocarbon fuel stream  114 , and water in the form of steam from a water stream  116 . In some fuel processors, air is also used in a combination partial oxidation/steam reforming reaction. In this case, fuel processor  112  also receives an air stream  118 . The fuel processor  112  may contain one or more reactors wherein the reformable hydrocarbon fuel in stream  114  undergoes dissociation in the presence of steam in stream  116  and air in stream  118  (optionally oxygen storage tank  118 ) to produce the hydrogen-containing reformate exhausted from fuel processor  112  in reformate stream  120 . Fuel processor  112  typically also includes one or more secondary reactors, such as water-gas shift (WGS) and/or preferential oxidizer (PrOx) reactors that are used to reduce the level of carbon monoxide in reformate feed gas stream  120  to acceptable levels, for example, below 20 ppm. H 2 -containing reformate  120  is fed through the anode chamber of fuel cell stack system  122 . At the same time, oxygen in the form of air in an oxidant feed gas stream  124  is fed into the cathode chamber of fuel cell stack system  122 . The hydrogen from reformate stream  120  and the oxygen from oxidant stream  124  react in fuel cell stack system  122  to produce electricity.  
         [0023]    Anode exhaust (or effluent)  126  from the anode side of fuel cell stack system  122  contains some unreacted hydrogen. Cathode exhaust (or effluent)  128  from the cathode side of fuel cell stack system  122  may contain some unreacted oxygen. These unreacted gases represent additional energy recovered in combustor  130 , in the form of thermal energy, for various heat requirements within power system  100 .  
         [0024]    Specifically, a hydrocarbon fuel  132  and/or anode effluent  126  are combusted, catalytically or thermally, in combustor  130  with oxygen provided to combustor  130  either from air in stream  134  or from cathode effluent stream  128 , depending on power system  100  operating conditions. Combustor  130  discharges exhaust stream  154  to the environment, and the heat generated thereby is directed to fuel processor  112  as needed.  
         [0025]    Turning now to FIG. 2, a two-cell PEM fuel cell stack  200  of fuel cell stack system  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 . 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 .  
         [0026]    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. 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 .  
         [0027]    An oxidant gas such as air/oxygen is supplied to the cathode side of fuel cell stack  200  from air source/storage tank  118  and line  124  via appropriate supply plumbing  242 . In a preferred embodiment, oxygen tank  118  is eliminated, and air is supplied to the cathode side from the ambient via a pump or compressor. A fuel such as hydrogen is supplied to the anode side of fuel cell  200  from storage tank  420  via appropriate supply plumbing  244 . In a preferred embodiment, hydrogen tank  420  is eliminated and the anode feed stream is supplied from a reformer (as described with reference to FIG. 1) via line  120  after catalytically dissociating hydrogen from hydrocarbon fuel  114 .  
         [0028]    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  214 ,  216 , as needed.  
         [0029]    It is to be noted that fuel cell stack  200  shows two fuel cells with plate  212  being shared between the two fuel cells. In practice, the number of individual cells in a fuel cell stack is dictated by the particular application and may include many individual fuel cells.  
         [0030]    Turning now to FIGS.  3 - 5 , a plurality of plates  302  similar to plates  212  are shown in FIG. 3 as generally defining the edges of flow channels for inputting feed gas stream to fuel cells in the fuel stack  300 . Collectively, FIGS.  3 - 5  show detail in an integrated feed gas humidifier, filter, and cooler for a PEM fuel cell stack embodiment according to the present invention.  
         [0031]    [0031]FIG. 3 also shows a heat exchanger  361  in the form of a tube and fin radiator element receiving input coolant flow  366  into pipe  363  and having attached individual heat exchange fins  371  which defines a fluid circuit providing cooling to the oxidant feed gas entering the fuel cell stack from manifold  362 . FIG. 4 also shows that input coolant flow  366  occurs as a parallel flow with a serpentine leg discharging as coolant discharge flow  367 . Coolant header  380  is shown in FIG. 5 as a source of coolant for supplying individual instances of flow  366  inter-cell coolant flow from coolant lines  250 ,  252  represented in plates  214 ,  216  of FIG. 2. Coolant flow  366  may be in place of or in addition to the inter-cell coolant flow. While the terms “cooling” and “coolant” has been used herein, a skilled practitioner will appreciate that the heat exchanger may also affect a temperature increase in the feed gas stream depending on the temperature difference between the coolant flow and the feed gas stream.  
         [0032]    The evaporative element  364  is shown in FIGS.  3 - 5 . Spray nozzles  374   a,    374   b,    374   c,    374   d  provide a flow of water droplets (collectively as nebulized water) within the oxidant gas flow to establish a two-phase flow of nebulized water and air (oxygen) in manifold  362  to evaporative element  364 . As presently preferred, evaporative element takes the form of a demisting packing which is sized and designed to provide a medium for evaporation of the water in nebulized form into humidity for the oxidant gas stream.  
         [0033]    A differential pressure transducer  381  is schematically shown to monitoring the pressure drop across the evaporative element for one fuel cell oxidant inlet. In practice a number of such transducer  381  may be used to define a representative profile of the pressure drop across the full expanse of evaporative element  364 . A measurement signal from differential pressure transducer  381  is provided to control circuit  384 . Flow transducer  382  is also optionally provided to control circuit  384  for indicating the water injected into the fuel cell stack  300 . Control circuit  384  adjusts the flow of water stream  372  via positioning of control valve  383  in response to measurements from transducer  381 ,  382 . In one embodiment, valve  383  operates in a similar manner to a fuel injection valve (as normally used in an internal combustion engine) with operational frequency and resultant intermittent spraying from spray nozzles  374   a,    374   b,    374   c,    374   d  defined in real-time or near real-time by conditions as measured by transducer  381 ,  382 . In another embodiment, control circuit  384  has input measurements from the temperature (not shown) of the oxidant gas as an input in the control decision logic executed by computer  384 . Drainage lines (not shown) may be employed at the edges of filter  364  help in flooding control.  
         [0034]    As presently preferred, evaporative element  364  is a filter capable of filtering particles of, about, 10 microns and, most preferably, of less than about 2 microns diameter, and the water droplets of the nebulized water have a diameter between about 30-50 microns. Possible filter materials include conventional polyester fiber/mesh used for air filtration or other suitable moisture-resistant filter papers. Evaporative element  364  is a removable filter held in place by a frame with support rack  369  as best seen in FIG. 5 for receiving, holding, and releasing (upon withdrawal) the framed filter. This embodiment provides for a filter and evaporative element which is periodically replaceable in the fuel cell power system. Such replacement may be based on periodic scheduled maintenance or alternately when the pressure drop across the filter becomes unacceptably high.  
         [0035]    Water is preferably added to the oxidant stream by nebulizing the water which is entrained in the air to the fuel cell. To this end, nozzles  374   a ,  374   b ,  374   c ,  374   d  are located in the water supply line  372  to nebulize the water, but remain a sufficient distance from evaporative element  364  at the oxidant inlet ends of the fuel cells to hydrate the area adjacent all fuel cells in the fuel cell stack.  
         [0036]    In another aspect, water is nebulized in the manifold  362  in a plurality of water mass flow increments with respect to either position and/or time, so that pressure drop across evaporative element  364  from the nebulized water is sufficiently controlled to preserve the operation of the fuel cell power system. In this regard, spray nozzles  374  have a base throughput for nebulizing a flow of water into a spray; this base throughput corresponds to one of the nebulized water mass flow increments in the plurality of nebulized water mass flow increments. Control circuit  384  adjusts the nebulized water mass flow increments to maintain pressure drop control over evaporative element  364 , either through analog control of the flow to nozzle  374  or through pulsed width modulation control (in a manner similar to fuel injection flow in an internal combustion engine and especially if the flow were to diminish below that needed by a spray nozzle to nebulize). In an alternative embodiment, nebulized water is provided through a system that does not depend upon throughput or flow rate (such as a sonic mister or thermal vaporizer) or is controlled by the water pump head pressure.  
         [0037]    As used herein, “water” means water that, in compositional nature, is useful for operation of a fuel cell power system. While certain particulates are acceptable in the water, they will further accelerate plugging of evaporative element  364  in addition to the plugging caused by particulates in the oxidant gas. In a fuel cell, such plugging could be caused by trace mineral precipitates from the essentially entrained nebulized or particulate water. Preferably, sufficient pre-filtering of the air and water is suggested in extending the life of the evaporative element  364  between replacements.  
         [0038]    A number of nozzle designs could be employed for providing the fine water spray, with an atomizer type nozzle being preferred in the fuel cell power system for providing a spray characterized by a volume mean diameter of between about 30 microns and about 50 microns and a flow rate of approximately 0.5 gallon per second at a pressure drop of approx. 10 bar and a temperature in the range of 5 to 60 degrees Celsius. However, a skilled practitioner will appreciate that the design and operating parameters of the nozzle are dependent on the system conditions such as power, temperature and pressure, and thus may vary for a given application.  
         [0039]    In one alternative embodiment, each individual spray nozzle  374  is separately valved and controlled to provide a maximum number of controllable water mass flow increments, with each increment being the essentially predictable and constant flow where each spray nozzle will deliver its functionally suitable spray pattern at the pressure drop available. In another embodiment, a group of spray nozzles  374  are controlled at the water supply line  372 . In yet another embodiment a group of spray nozzles may be mixed with other spray nozzles, which are individually controlled. It should be apparent from the above that a number of different arrangements of nozzles, supply lines and valving can achieve the provision of mass flow of nebulized water in a plurality of nebulized water mass flow increments.  
         [0040]    In some cases also, spray nozzle  374  may be of different sizes to enable either pre-defined spray concentration profiles or to facilitate passage and flushing of particulates from within water pipe  372  to avoid clogging of any spray rack water nozzle  374 . In this regard, the internal clearances on larger throughput nozzles would accommodate the passage of particulates more readily than the internal clearances on smaller throughput nozzles.  
         [0041]    The present invention has been described above in conjunction with conditioning the cathode feed gas stream. In another embodiment, the present invention can be employed to condition the anode feed gas stream. In the context of designing for the flow and character of the fuel gas, the design of the fuel gas cooler, humidifier and filter is similar to that shown in FIGS.  3 - 5 .  
         [0042]    As should be apparent from a consideration of the foregoing, integration of filtration, mass transfer and heat transfer operations in the preferred embodiments are achieved both from utilization of filtration, mass transfer and heat transfer components to take advantage of structural support offered by the stack components and also from a unification of these elements and the functions preferred thereby into a common space within the fuel cell stack. This integration of evaporative cooler, filter, humidifier, heat exchanger and existing stack structure provides a basis for volume, weight, and cost reduction in a fuel cell power system.  
         [0043]    The invention is described herein in a discussion of preferred embodiments, and a skilled practitioner will readily appreciate that various aspects of the preferred embodiment may be omitted or substituted from the embodiments described herein without departing from the spirit and scope of the invention. Accordingly, the invention should only be limited by the claims set forth below.