Abstract:
A fuel cell assembly ( 2 ) includes a vessel ( 4 ) containing a gas-permeable, porous housing ( 16 ). A fuel cell stack ( 14 ), including cells ( 42 ) and interconnect plates ( 44 ), is contained within the porous housing. Each interconnect plate has oxidant and fuel sides ( 64, 60 ) adjacent to the cathode ( 58 ) and anode ( 62 ) of adjacent cells. Fuel ( 68 ) is supplied to the fuel side at positions ( 78 ) midway between the center ( 82 ) and the periphery ( 80 ) of the fuel side. Reaction products ( 90 ) are withdrawn from the center of the fuel side. Flue gas ( 100 ) is withdrawn from the center ( 98 ) of the oxidant side. Air is preheated as it passes through the porous housing to the fuel cell stack. The preheated air combusts residual fuel ( 110 ) flowing radially outwardly from the periphery of the stack to further heat the air to the stack operating temperature to eliminate any external preheating of the air. Corrugations ( 66, 67 ) on the interconnect plates act as flow deflectors and form the electrical contact surfaces for adjacent cells. The fuel cell stack is preferably oriented horizontally and is allowed to thermally expand and contract in a substantially free manner, to minimize damage to the cells, until the fuel cell stack is close to an operating temperature.

Description:
This application is a Division of Ser. No. 08/786,954 filed Jan. 23, 1997, now U.S. Pat. No. 5,851,689. 
    
    
     BACKGROUND OF THE INVENTION 
     A fuel cell is an electric cell that converts the chemical energy of a fuel, typically hydrogen, directly into electric energy in a continuous process. Although fuel cells can be used with a variety of fuels and oxidants, they almost exclusively combine hydrogen and oxygen to form water vapor. Fuel cells include an anode in contact with the fuel, a cathode in contact with the oxygen and an electrolyte sandwiched between the anode and cathode. Each cell creates less than one volt so that a series or stack of fuel cells are used to convert fuel into usable energy. Interconnect plates are used between each cell to keep the fuel and oxygen separated and to electrically connect the anode of one cell to the cathode of an adjacent cell. 
     One source of hydrogen is natural gas. A common way to obtain hydrogen from natural gas is by using a reformer which combines natural gas and steam at a high temperature, such as 760° C., to obtain the hydrogen. Some fuel cells operate using a separate, external reformer to create the hydrogen; other fuel cells combine the function of a reformer into the fuel cell itself by operating the fuel cell at a high enough temperature, as well as other appropriate design considerations. 
     One type of fuel cell uses radial flow configurations for solid oxide fuel cells. In one design, disclosed in M. Petrik et al., “Stack Development Status of the Interscience Radial Flow (IRF) SOFC”, An EPRI/GRI Fuel Cell Workshop on Fuel Cell Technology Research and Development, Atlanta, Ga., Mar. 22-23, 1994, the fuel and air are fed to each cell through a pair of holes at the center region of the cell. The fuel and air then flow radially outwardly to the edge of the cell. This flow configuration requires seals to segregate the fuel and air at the feed points and also runs the risk of temperature excursions at the center of the cell where both rich fuel and rich oxygen exist. In another configuration, disclosed in M. Prica et al., “Contoured PEN Plates for Improved Thermomechanical Performance in SOFCs”, Proceedings of the Second European Fuel Cell Forum, Vol. 1, pp. 393-402, Oslo, Norway, May 6-10, 1996, the fuel and air are fed to the center of each cell through a pair of needles. These gases then flow radially to the cell edge. This flow configuration eliminates the gas seal requirement but still has problems with regard to temperature excursion. In another configuration, disclosed in European Patent 0,635,896 A1, the fuel is fed to the center of the cell by a feed needle while air is fed to the entire cathode area by distribution nozzles. The spent fuel and spent air are collected at the cell edge. This configuration eliminates the need for a gas seal and does not have temperature excursion problems. It does, however, require a complex gas nozzle distribution system. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to a fuel cell assembly, typically a solid oxide fuel cell assembly, which requires no gas seal, no large axial clamping force, no separate air preheater, no external reformer or pre-reformer and no external steam supply for reforming. The assembly has no gas leakage; cell cracking or other cell damage is minimized and the simplicity of the entire system is greatly enhanced. The present invention provides a relatively low cost and high reliability fuel cell assembly. 
     The fuel cell assembly includes a vessel defining an inside. A gas-permeable, porous housing is preferably located within the inside of the vessel. A fuel cell stack is housed within the interior of the porous housing. Air, or other oxygen-containing gas, is supplied to the fuel cell stack by delivering air into the region between the vessel and porous housing, the air preferably passing through the wall of the porous housing to reach the fuel cell stack. 
     The fuel cell stack includes a plurality of alternating cells and interconnect plate assemblies. Each cell has an anode surface and a cathode surface. Each interconnect plate assembly has an oxidant side adjacent to the cathode surface of one cell and a fuel side adjacent to the anode surface of another cell. Fuel is supplied to the fuel side preferably at a plurality of fuel exits positioned midway between a central region of the fuel side and the periphery of the fuel side. 
     A reaction products collection conduit has a reaction products entrance at the central region of the fuel side for withdrawing reaction products away from the fuel side. A flue gas collection conduit has a flue gas entrance at a central region of the oxidant side for the flow of flue gas away from the oxidant side. 
     The porous housing is heated by the heat generated by the fuel cell stack. This allows the air passing through the porous housing to be preheated to, for example, 500° C. to 800° C. as the air enters a residual reaction products combustion region defined between the interior of the porous housing and the exterior or periphery of the stack. Residual reaction products which pass radially outwardly from the peripheral edge of the interconnect plate assembly combust with the heated air passing through the porous housing. This acts to heat the air flowing to the oxidant side of the interconnect plate to a desired temperature, typically about 700° C. to 1000° C., so to eliminate any need to preheat the air entering the assembly. The desired temperature will depend upon the desired or required operating temperature for the stack. Also, since the air passes through the porous housing, the porous housing remains relatively cool on the outside surface for both safety and efficiency. 
     The gas flow along both the fuel side and oxidant side of the interconnect plate is preferably directed by flow deflectors. These flow deflectors are preferably created by corrugating the interconnect plate. The corrugations not only act as flow deflectors but also form the electric contact surfaces with the cathode and anode surfaces of adjacent cells. The oxidant side preferably has radially-oriented flow deflectors while the fuel side preferably has both radially-oriented and rotary-oriented flow deflectors. 
     The fuel stack is preferably oriented horizontally, that is with the wafer-like interconnect plates and cells oriented vertically. The fuel cell stack is preferably allowed to thermally expand and contract in a substantially free manner until the fuel cell stack is within 50° C., or less, of an operating temperature. This freedom of movement during most of the temperature changes helps to minimize cracking or other damage to the cells and interconnect plates. The outer surface portion of the corrugations are preferably plated with a soft metal, such as silver on the fuel side and gold on the oxidant side, to provide good electrical contact with adjacent cells and to help prevent cracking or other damage to the cells. 
     The present invention differs from conventional fuel cell systems with regard to fuel and oxygen flow primarily because of the use of split fuel flow on the fuel side of the interconnect plate and the radially inward flow of the oxidant gas (typically air) on the oxidant side of the interconnect plate. Because no rich fuel and rich oxidant coexist at any point, the present invention eliminates the temperature excursion problems associated with conventional fuel cell assemblies without the need for complex gas distribution nozzles. The gas distribution method provides other advantages as well. A portion of the spent fuel (fuel reaction products) can be collected by a collection conduit at the central region of the fuel side; the reaction water product in this spent fuel stream is used as a source of reforming steam so that no external steam generation and boiler feed water treatment are required. The split fuel flow also distributes the fuel quickly to the entire fuel side of the interconnect plate and thus to the anode surface of the cell. This helps prevent the cell from local overcooling by the highly endothermic reforming reaction which occurs. As a result, the stack can readily incorporate the reforming internally without the need for an external reformer. 
     With the present invention waste heat from the stack can be transferred from the fuel cell stack to the porous housing; this heat in the porous housing is transferred to the air passing in through the porous housing to provide a very effective air preheating. Due to the radial flow of the air through the porous housing, stack cooling along the length of the stack is uniform. Also, the provision of inwardly directed air tends to contain the heat adjacent to the stack so that relatively low temperature vessels or enclosures can be used with essentially no heat losses. 
     The residual reaction products (spent fuel) of many conventional fuel cell systems are disposed of by burning the spent oxidant at the edge of the cell. With the present invention a portion of the residual reaction products which exits about the periphery of the interconnect plate is burned at the cell edge upon contact with the heated air passing through the porous housing. This provides a final preheating of the air to the cell operating temperature. This direct heating by combustion eliminates the need for an expensive high temperature heat exchanger. 
     In many fuel cell stack designs the cells need to be clamped together to provide a tight gas seal and to minimize the electrical contact resistance. This clamping inhibits or prevents free expansion and contraction during stack heating and cooling and can result in thermally induced stresses. Therefore, the mechanical force exerted by the clamping device can cause the cells to crack or otherwise fail. The use of a sealless stack design eliminates the need for clamping to provide gas seals. The use of gold and silver provides soft, conforming electric contact surfaces at the operating temperature (typically 700-1000° C.). The use of gold also prevents oxidation at the oxidant side of the interconnect plate. While gold could be used on the fuel side of the interconnect plate, silver is less expensive and the use of gold is not considered necessary because of the lack of oxidation problems at the fuel side. 
     The present invention reduces some of the expense associated with fuel cell stacks. Conventional fuel cell stacks often use ceramic interconnect plates to match the coefficient of thermal expansion of the ceramic cells. However, to meet mechanical strength requirements, ceramic interconnect plates must be made relatively thick. This thickness requirement, in conjunction with the high material cost for ceramics, can make the cost of the interconnect plates prohibitively high. To eliminate this problem, metal interconnect plates can be used; these interconnect plates need to be doped with special materials, such as yttri or alumina, to adjust the coefficient of thermal expansion. These special alloys are also expensive and tend to be brittle. The free expansion stack aspect of the invention eliminates this problem with conventional fuel cell assemblies by allowing the use of common stainless steel, such as  316 , for the interconnect plates. 
     Other features and advantages of the invention will appear from the following description in which the preferred embodiment has been set forth in detail in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a simplified, partially schematic, cross-sectional isometric view of a fuel cell assembly made according to the invention; 
     FIG. 2 illustrates the porous housing of FIG. 1 with compartment plates at either end; 
     FIG. 2A a somewhat enlarged, broken away portion of the corner of the porous housing of FIG. 2 illustrating a perforated feed tube for the initial heat-up of the stack; 
     FIG. 2B is a simplified end view of the right-hand compartment plate of FIG. 2 showing the positioning of various pipes and tubes between the corrugations at the periphery of the compartment plate; 
     FIG. 3 is a somewhat simplified view of four interconnect plates and five cells; 
     FIG. 4 is an enlarged, exploded isometric view showing the fuel side of an interconnect plate and the cathode surface of a cell within a section of the porous housing; 
     FIG. 5 is a view similar to that of FIG. 4 illustrating the oxidant side of the interconnect plate of FIG. 4; 
     FIG. 5A is an enlarged view of a portion of the peripheral edge of the interconnect plate of FIG. 5; and 
     FIG. 5B is a cross-sectional view taken along line  5 B— 5 B of FIG. 5A illustrating the flow deflecting corrugations. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Fuel cell assembly  2  is shown in FIG. 1 to include a vessel  4 , typically made of carbon steel. Vessel  4  has a pair of air inlets  6 ,  8  through which air from a blower  10  enters into the inside  12  of vessel  4 . 
     Fuel cell assembly  2  includes a fuel cell stack assembly  14 , shown best in FIG. 2, comprising a gas-permeable, porous housing  16 . Housing  16  is typically made of a metal sponge material, such as stainless steel, which permits air, or other oxidant gas, to pass through its outer peripheral surface  18  and into the interior  20  of housing  16 . The ends  22 ,  24  are sealed by insulating plates  26 ,  28 . Plates  26 ,  28 , typically made of ceramic materials, are both electrical and thermal insulators. Fuel stack assembly  14  also includes compartment plates  30 ,  32  secured to the outside surfaces of insulating plates  26 ,  28 . Compartment plates  30 ,  32  have scalloped outer edges  34  which engage the inside surface  35  of vessel  4 . Plate  30  is fixed to inside surface  35  while plate  32  slidably engages inside surface. Scalloped edges  34  permit air flow entering inside  12  of vessel  4  from air inlet  6 ,  8  to flow evenly about outer surface  18  of porous housing  16 . This is suggested by the air flow arrows in FIG.  1 . Scalloped edges  34  also provide openings for the passage of various tubes and conduits as will be discussed in more detail below. Compartment plate  32  has a compression spring  36  extending axially from the compartment plate and captured between compartment plate  32  and vessel  4  in the region surrounding air inlet  8 . The purpose of spring  36  will be discussed below. Assembly  14  is supported within vessel  4  by three horizontal support bars  38 , the support bars being secured at either end to inside surface  35  of vessel  4 . 
     Returning now to FIG. 1, fuel cell stack assembly  14  is seen to include a fuel cell stack  40  housed within interior  20  of porous housing  16 . Stack  40  is made up of alternating cells  42  and interconnect plates  44  see FIGS,  3  and  4 . Cells  42  are preferably ceramic cells and interconnect plates  44  are preferably stainless steel. Each cell  42  and interconnect plate  44  has its axial dimension substantially enlarged relative to its diameter for ease of illustration. A typical diameter of cell  42  and interconnect plate  44  would be 2 to 12 inches (5 to 30 cm). The axial dimension  46  of interconnect plate  44 , see FIG. 5B, is about 0.25 to 1 inch (0.63 to 2.5 cm) while the axial dimension of cell  42  is about 10 to 1000 microns. Other dimensions can also be used. 
     Each cell  42  includes an anode layer  48  and a cathode layer  50  between which an electrolyte layer  52  is sandwiched. The outside diameter of anode layer  48  and electrolyte layer  52  are about equal while cathode layer  50  has a smaller outside diameter to leave a peripheral, annular space  54  about cathode layer  50 . A metal ring  56 , typically made of stainless steel, is sized to fit within annular space  54 . Ring  56  is used to help keep fuel from contacting cathode surface  58  of cathode layer  50  as discussed below. Anode layer  48  is typically about 200 microns thick, electrolyte layer  52  is about 20 microns thick and cathode layer  50  is about 5-10 microns thick. Such a thin cell  42  is possible because of the way in which thermal expansion and contraction of stack  40  of cells  42  and interconnect plates  44  are accommodated, as will be discussed below. 
     Referring now primarily to FIGS. 4,  5 ,  5 A and  5 B, interconnect plate  44  is seen to include a fuel side  60 , shown in FIG. 4, facing anode surface  62  of an adjacent cell  42  (not shown in FIG. 4) and oxidant side  64 , shown in FIG. 5, facing cathode surface  58  of an adjacent cell  42 , shown in FIG.  4 . Interconnect plate  44  is preferably about 500-1000 microns thick and has corrugations  66 ,  67  formed about its entire surface. Radially-oriented corrugations  66 , as shown best in FIG. 5B, alternate between convex and concave shapes on each surface  58 ,  62  to provide radially-oriented flow deflectors  66  for fluid flow along fuel side  60  and along oxidant side  64 . However, rotary-oriented flow deflectors  67  are formed only on fuel side  60 . 
     To enhance good electrical contact between interconnect plate  44  and the adjacent cells  42 , the outermost portions of corrugations are plated with good electrical conductors such as gold or silver. Gold and silver are also preferred because they both provide relatively soft contact layers for contact with cathode surface  58  and anode surface  62 . The corrugations on fuel side  60  are preferably plated with silver while the corrugations on oxidant side  64  are preferably plated with gold to keep the contact surfaces from oxidizing on the oxidant side. 
     Fuel, typically in the form of natural gas  68 , flows through a pair of fuel feed tubes  70 ,  72  which then flows through a pair of gas feed needles  74 ,  76 , one set of needles  74 ,  76  for each interconnect plate  44 . Each gas feed needle  74 ,  76  has a fuel exit  78  positioned along fuel side  60  midway between an outer periphery  80  of interconnect plate  44  and a central region  82  of fuel side  60 . Corrugations  66 ,  67  cause fuel  68  to flow in both radial and rotary directions as indicated by the various arrows in FIG.  4 . 
     A spent fuel or reaction products collection conduit  84  extends along the length of fuel cell stack  40  overlying the entire fuel cell stack while feed tubes  70 ,  72  extend along the stack on either side of the stack. A collection needle  86  extends downwardly from collection conduit  84  for each interconnect plate  44 . Each collection needle has a reaction products entrance  88  at its distal or lower end positioned at central region  82  of fuel side  60 . By the time fuel  68  passes from fuel exit  78  to central region  82  it is mostly spent. The reaction products  90  are drawn away from central region  82  through entrance  88  of collection needle  86 . Reaction products  90  contain a mixture of carbon dioxide and water, as well as an amount of unused fuel in the form of carbon monoxide and hydrogen. The use of this will be discussed below. 
     FIG. 5 illustrates a collection needle  92  extending upwardly from a flue gas collection conduit  94  situated directed below the center of interconnect plate  44 . Interconnect plate  44  and adjacent cells  42  are vertically supported by a bulge or enlarged region  95  of collection needle  92 . Collection needle  92  has a flue gas entrance  96  adjacent to the central region  98  of oxidant side  64 . Flue gas  100  is withdrawn from stack  40  through flue gas collection conduit  94 . The flow of the oxidant gas is thus from an annular region  102  defined between the interior wall  104  of porous housing  16  and stack  40 . The oxidant gas, typically air, passes through porous housing  16  as indicated by arrows  106 . After passing through porous housing  16  this air is heated to, for example, 500° to 800° C. due to the heat generated by fuel cell stack  40  that has been transferred to porous housing  16 . The outside surface  108  of porous housing  16  remains relatively cool due to the passage of air through the wall, the air heating up as it passes through. The reaction products  110  flowing radially outwardly from fuel side  60  of interconnect plate  44  contain some combustible gases. Upon entering region  102  these combustible gases react with heated air in region  102  and combust, thus further raising the temperature of the oxidant gas (air) to about 700° C. to 1000° C. The relatively small amount of combustible products within reaction products  110  do not significantly adversely affect the oxygen content of the gas within region  102 , which gas is then pulled into the region between cathode surface  58  of cell  42  and oxidant side  64  of interconnect plate  44 . The gas flows along oxidant side  64  guided by radially-oriented flow deflectors  67 . 
     Metal ring  56  helps prevent unburned fuel from contacting cathode surface  58 , which would cause cathode reduction. Reaction products  90  (spent fuel) are fed to an ejector  112 , see FIG. 1, located in the head compartment  114  of the inside  12  of vessel  4 . A hydrocarbon fuel feed, typically natural gas  68 , is used as the motive gas for ejector  112 . The functioning of ejector  112  will be discussed in more detail below. Fuel cell stack  40  includes a pair of end plates  116 ,  118  which function as the terminals of the fuel cell stack. Lines  120 ,  122  are connected to end plates  116 ,  118  for access to the electrical current created by fuel cell stack  40 . 
     Mechanical compression spring  36  is sized so that it exerts a compression force on slidable plate  32  only when stack  40  is within about 50° C. of its operating temperature, that is at or near the end of the preheat cycle. The extreme axial thinness of cells  42  relative to the much greater axial thickness of interconnect plates  44  cause stack  40  to expand and contract axially as if it were made entirely of the interconnect plates. Therefore, applying a compression force on plate  32  applies a compression force on stack  40  because porous housing  16 , tubes and conduits  70 ,  72 ,  84 ,  94 , and interconnect plate  44  are all made of materials with the same coefficient of thermal expansion, preferably stainless steel. Spring  36  is constantly cooled by air entering inlet  8  to ensure spring  36  retains its elasticity. 
     During startup, stack  40  is preheated using hot gases generated from burning natural gas  68  with air at startup burner  124 . See FIG.  1 . Startup burner  124  is supplied with natural gas through a valve  125  along a line  126  and with air from blower  10  through a valve  127 , and along a line  128 . Hot exhaust gas, used for preheating, passes from burner  124  through a line  130  which connects to a circular feed tube  132  located adjacent to insulation plate  26 . See FIGS. 2A and 2B. Circular feed tube  132  has numerous perforations  134  through which the heated gas flows into annular region  102  of interior  20 . To keep the hot gases within porous housing  16 , blower  10  is operated to direct a sufficient amount of air into inside  12  of vessel  4  so that the air pressure outside of porous housing  16  is slightly greater than the air pressure within the porous housing. 
     During this preheating it is desired to keep anode surface  62  from oxidizing. To do so, nitrogen from a nitrogen storage bottle  136  is directed through a valve  137  along a line  138 , into line  126 , out of line  126  and through valves  139 ,  141  along lines  140 ,  142 . Lines  140 ,  142  have heat exchange coils  144 ,  146  formed along their lengths, coils  144 ,  146  being situated along conduit  94 . Thus nitrogen, which is heated within coils  144 ,  146 , passes from ejector  112  through an outlet line  148 , outlet line  148  flowing into fuel feed lines  70 ,  72 . The nitrogen then passes through exits  78  of needles  74 ,  76  situated between fuel side  60  and anode surface  62 . This keeps anode surface  62  blanketed with nitrogen to keep the anode surface from oxidizing. Gas (a mixture of nitrogen from bottle  136  and heated exhaust gas from burner  124 ) is withdrawn from interior  20  of porous housing  16  through flue gas entrance  96  of collection needle  92  and then through collection conduit  94 . 
     When fuel cell stack  40  reaches operating temperature it is ready to accept natural gas  68  or other feed fuel; however, no fuel cell reaction product water is available to recycle through to provide the reforming steam at this time. The required startup reforming steam is generated by a once-through flashing of boiler feed water supplied from a water storage drum  150 . Water passes through a valve  151  and along a line  152 ; line  152  has a heat transfer coil  154  along its length housed within conduit  94 . Passing the water through the coil  154  causes the water to be flashed into steam by the passage of the startup flue gas (generated by startup burner  124 ). 
     Once the fuel cell stack  40  reaches the operating temperature and sufficient startup reforming steam has been generated, valves  127 ,  125 ,  137  and  151  (which are open only during startup operations) are closed and blower  10  blows air into inside  12  of vessel  4  through air inlets  6 ,  8 . Natural gas  68  is pumped through line  126 , through line  142  for passage through ejector  112  and through lines  140 ,  152  to bypass ejector and to flow into outlet line  148 . Valves  139 ,  141  are used to control the proportion of natural gas flowing through ejector  112  along line  142  and bypassing ejector  112  along line  152 . 
     Natural gas  68  passes through line  148  and into lines  70 ,  72  for delivery to fuel side  60  of each interconnect plate  44 . The natural gas then is deflected so to pass in both rotary and radial directions, both radially inwardly and outwardly. Simultaneously, air is being drawn through porous housing  16  and is being created as it is pulled through the porous housing. Final preheating of this air  106  occurs by the combustion of reaction products  110  within annular region  102 . The now completely preheated air is drawn into the region between oxidant side  64  and cathode surface  58  of each fuel cell. This radially inward movement is caused by the passage of flue gas  100  from flue gas entrance  96  of collection needle  92  located adjacent the central region  98  of side  64  of each interconnect plate  44 . Flue gas  100  is quite hot, typically about 700° C. to 1000° C. and effectively preheats natural gas  68  as the natural gas passes through heat exchanger tubes  144  and  146 . To reduce the temperature of the flue gas passing the heat exchangers  144  and  146 , a portion of the flue gas can bypass the heat exchangers along line  162 ; also, blower  10  can introduce ambient air into collection conduit  94  through a valve  159  and along a line  160  during startup operations. 
     Reaction products from fuel side  60  are collected at central region  82  of the fuel side through entrance  88  of collection needle  86 . Collection needle  86  feeds reaction products  90  into reaction products collection conduit  84  which intersects. Reaction products  90  are recycled through ejector  112  with natural gas  68  being used as the motive gas passing through line  142 . The discharge through line  148  is a mixture of natural gas  68  and reaction products  90 . 
     Porous housing  16  has openings  156 ,  158 , shown in FIG. 2, formed for the passage of fuel feed tube  72  and reaction products collection conduit  84 . Other holes are also formed for fuel feed tube  70 , flue gas collection conduit  94  and line  130 . 
     All of the tubes and conduits entering into porous housing  16  are thermally insulated. Similarly, equipment within compartment  114 , including ejector  112 , startup burner  124  and the various tubes and lines are all thermally insulated. Thermal losses from these components are recovered by the act of preheating the air flowing into head compartment  114  from blower  10 . 
     Modifications and variations can be made to the disclosed embodiment without departing from the subject of the invention as defined in the following claims. For example, stack  40  can be compressed axially by a temperature-actuated biasing element which applies a chosen axial force to the stack only when a predetermined temperature is reached. An axial compression force could also be provided pneumatically instead of mechanically.