Abstract:
There is provided a stack of a fuel cell system in which one or more electricity generators including separators disposed at both sides of a membrane-electrode assembly are stacked, the stack comprising heat releasing means for releasing heat generated from the electricity generators. The heat releasing means have different heat release rates depending on the positions of the associated electricity generators in the stack. In particular, the heat releasing means associated with the electricity generators located near the center of the stack have a higher heat release rate in order to maintain a more even temperature gradient across the stack.

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
       [0001]     This application claims priority to and the benefit of Korean Patent Application No. 10-2005-0007919, filed on Jan. 28, 2005 in the Korean Intellectual Property Office, the entire content of which is incorporated herein by reference.  
       FIELD OF THE INVENTION  
       [0002]     The present invention relates to a fuel cell system, and more particularly to a stack of a fuel cell system with improved cooling efficiency, and a fuel cell system having the same.  
       BACKGROUND OF THE INVENTION  
       [0003]     In general, a fuel cell is an electricity generating system for directly converting chemical energy into electric energy through an electrochemical reaction between oxygen contained in air and hydrogen contained in hydrocarbon-grouped materials such as methanol, ethanol, and natural gas. Specifically, fuel cells have a feature such that electricity is generated through an electrochemical reaction between a fuel gas and an oxidizing gas without combustion and with heat as a byproduct thereof.  
         [0004]     Such fuel cells may be classified into phosphate fuel cells working at temperatures of about 150° C. to 200° C., molten carbonate fuel cells working at high temperatures of about 600° C. to 700° C., solid oxide fuel cells working at high temperatures of 1000° C. or more, and polymer electrolyte membrane fuel cells and alkali fuel cells working at room temperature or at temperatures of 100° C. or less, depending upon the kinds of electrolyte used. All such fuel cells basically work under the same principle, but may differ from one another in the kinds of fuel, the operating temperatures, the catalysts, and the electrolytes.  
         [0005]     Recently developed polymer electrolyte membrane fuel cells (hereinafter referred to as a PEMFCs) have excellent output characteristic, low operating temperatures, and fast starting and response characteristics compared to other fuel cells. Accordingly, PEMFCs have a wide range of applications such as for mobile power sources for vehicles, for distributed power sources for homes or buildings, and for small-sized power sources for electronic apparatuses.  
         [0006]     A PEMFC system basically requires a fuel cell main body (hereinafter referred to as a stack for the purpose of convenience), a reformer for reforming fuel to generate hydrogen gas, a fuel tank, and a fuel pump for supplying fuel to the reformer. In a PEMFC, the fuel stored in the fuel tank is supplied to the reformer by means of pumping power of the fuel pump. Then, the reformer reforms the fuel and generates the hydrogen gas to the stack. In the stack, hydrogen gas and oxygen electrochemically react with each other, thereby generating electrical energy.  
         [0007]     Another type of fuel cell is a direct methanol fuel cell (hereinafter referred to as a DMFC) in which liquid-state methanol fuel is supplied directly to the stack. Unlike a PEMFC, a DMFC does not require a reformer.  
         [0008]     In the fuel cell systems described above, the stack that generates electricity has a stacked structure of several or several tens of unit cells, each of which has a membrane-electrode assembly (hereinafter referred to as an MEA) and separators (or bipolar plates). The MEA includes an anode electrode and a cathode electrode attached to either side of an electrolyte membrane. The separator simultaneously functions as a passage through which oxygen and hydrogen gas required for the reaction of the fuel cell are supplied, and as a conductor connecting the anode electrode and the cathode electrode of each MEA in series.  
         [0009]     Through the bipolar plate, fuel gas containing hydrogen is supplied to the anode electrode and oxygen gas containing oxygen is supplied to the cathode electrode. An oxidation reaction of the fuel gas takes place in the anode electrode and a reduction reaction of the oxygen gas takes place in the cathode electrode. Due to movement of electrons, electricity, heat, and water can be produced simultaneously.  
         [0010]     In such a fuel cell system, when the operating temperature deviates from an appropriate range, the performance of the electrolyte membrane deteriorates and safety thereof cannot be ensured. Furthermore, in a serious case, the fuel cell may be damaged. Therefore, a cooling means using air or water is often provided within the fuel cell system in order to continuously remove heat generated inside the stack when operating the fuel cell system.  
         [0011]     However, the adaptation of the same cooling scheme to all parts of the stack in a conventional cooling scheme makes it difficult to effectively cool all unit cells of a stack which may have substantially different temperature distributions depending on their positions within the stack.  
         [0012]     Specifically, a unit cell located at a central portion among unit cells of the stack tends to have a temperature that is higher than that of a unit cell located near either end of the stack.  
         [0013]     In the conventional fuel cell system, since heat generated in each of the stacked unit cells is not uniformly released, the performance of the stack deteriorates, thereby decreasing the overall efficiency of the fuel cell system.  
       SUMMARY OF THE INVENTION  
       [0014]     The present invention is directed to a fuel cell system that is capable of quickly releasing heat generated from the entire stack and of uniformly maintaining the temperature of the entire stack within an appropriate range by changing the heat-emission structure depending on location within the stack.  
         [0015]     According to the present invention, a fuel cell stack and a fuel cell system are provided that are capable of uniformly maintaining temperature distribution throughout the entire stack by quickly absorbing heat locally generated in the stack.  
         [0016]     According to one embodiment of the present invention, a fuel cell stack is provided, the stack comprising an electricity generating unit including a plurality of unit cells, and a cooling unit supplying specific unit cells with coolant to correspond to differences in heat-emission depending on the different positions of each of the unit cells.  
         [0017]     According to the invention, the electricity generating unit may generate more heat toward a central portion of the stack, so the cooling unit may provide an increased amount of coolant toward a central portion of the stack.  
         [0018]     According to another embodiment of the present invention, a stack of a fuel cell is provided in which one or more electricity generators including separators disposed at both sides of a membrane-electrode assembly (MEA) are stacked, the stack comprising a heat releasing means for releasing heat generated from each electricity generator, wherein the heat releasing means have different heat release rates depending on the positions of the electricity generators in the stack.  
         [0019]     In general, the heat releasing means are constructed such that the heat release rate of the electricity generator increases toward a central portion of the stack.  
         [0020]     According to such a construction, since the temperature of an electricity generator located at a central portion of the stack tends to be higher than those of the electricity generators located at the outer portions thereof, the heat release rates of the heat releasing means located near the central portion of the stack are higher than those of the heat releasing means located at the outer portions. Therefore, the heat generated at the central portion of the stack can be quickly released and the temperature of the entire stack can be uniformly controlled.  
         [0021]     According to one embodiment, the heat releasing means includes flow passages which are formed in the separators and through which a coolant flows. The heat release rate may be varied by varying the sizes of the flow passages depending on the positions of the electricity generators within the stack.  
         [0022]     According to another embodiment, the heat releasing means may include cooling plates with holes through which coolant may flow. The size of the holes may be varied to vary the heat release rate.  
         [0023]     According to yet another embodiment, the heat releasing means may include flow grooves which are formed in separators at portions corresponding to non-active regions of the MEA and through which coolant flows. The heat release rate may be varied by making the size of each of the flow grooves different depending on the positions of the electricity generators within the stack.  
         [0024]     According to still another embodiment, the heat releasing means may optionally include heat conductive media which are attached to the separators and which have heat conductivity that is higher than those of the separators. The heat release rates may be varied by making the size of each of the heat conductive media different depending on the positions of the electricity generators within the stack.  
         [0025]     According to yet another embodiment, the heat releasing means may include fans for directing cooling air to the electricity generators. The heat release rates may be varied by varying the amount of cooling air provided to the different electricity generators within the stack. This can be done by varying the number of fans or the sizes of the fans at different locations in the stack. Alternatively, the output from individual fans can be varied such as by changing the pitch of the fan blades of certain fans, or by changing the speeds of certain fans.  
         [0026]     According to still another embodiment of the invention, a fuel cell stack is provided in which one or more electricity generators including separators disposed at both sides of an MEA are stacked. For this embodiment, flow passages through which coolant flows are formed in the separators, and the sizes of the flow passages are varied depending on the positions of the electricity generators. In this embodiment, the flow passages are formed at side surfaces of separators not opposing the MEA. Further, each of the flow passages may generally be formed in the shape of a channel or a hole.  
         [0027]     When the flow passages are formed in the shape of a channel, a part of the channel is formed at one surface of the separator and a part of the channel is formed at one surface of another adjacent separator disposed opposite to and adhered to the separator. By such a construction, two channels are joined to become one hole.  
         [0028]     In general, the sizes of the flow passages formed in the electricity generators located at the central portion are relatively larger than those of the flow passages of the electricity generator located at the outer sides of the stack.  
         [0029]     The flow passages may have a cross section in the shape of a tetragon or circle, but they are not limited to any specific shape.  
         [0030]     According to still another embodiment of the present invention, a fuel cell stack is provided in which one or more electricity generators including separators disposed at both sides of an MEA are stacked, cooling plates are disposed between the electricity generators, holes through which coolant may flow are formed in the cooling plates, and the sizes of the holes formed in the cooling plates are varied depending on the positions of the electricity generators.  
         [0031]     It is preferable that the size of the hole formed in the electricity generator located at a central portion is relatively larger than that of the hole formed in the electricity generator located at the outer sides.  
         [0032]     The flow passages may have a cross section in the shape of a tetragon or a circle, but they are not limited to any specific shape.  
         [0033]     According to another aspect of the present invention, there is provided a stack of a fuel cell in which one or more electricity generators including separators disposed at both sides of an MEA are stacked, flow grooves through which coolant flows are formed in separators at portions corresponding to non-active regions of the MEA, and the sizes of the flow grooves are varied depending on the positions of the electricity generators.  
         [0034]     Here, the non-active region means a region through which air or hydrogen gas does not flow and in which the hydrogen gas does not react with the air.  
         [0035]     The flow grooves form one or more flow lines between the MEA and the separator when they are stacked, and the coolant flows along these flow lines.  
         [0036]     Further, the flow grooves are not especially limited to any specific positions in the separators if they are formed in a region other than a region to which hydrogen gas or air is supplied, and preferably are formed in the whole region outside the region to which hydrogen gas or air is supplied.  
         [0037]     According to another aspect of the present invention, a stack of a fuel cell is provided in which one or more electricity generators including separators disposed at the sides of an MEA are stacked, heat conductive media having higher heat-conductivity than that of the separator are attached to the separators, and the size of each of the heat conductive media is made different depending on the positions of the electricity generators.  
         [0038]     Here, the heat conductive media may be made of a heat-conductive material. Suitable examples are metal plates made from a metal such as aluminum, copper, and iron.  
         [0039]     The heat conductive media may be attached to one side surface of each of the separators or inserted into the separators as one layer. Further, one or more heat conductive media may be inserted into the separators at a predetermined distance as a plurality of layers.  
         [0040]     The heat conductive media may optionally have one or more holes which are formed in a central portion of the heat conductive media to be connected to a coolant supply unit and through which a coolant flows.  
         [0041]     Where holes are formed in the heat conductive media, the size of each of the holes may be made different depending on the positions of the electricity generators within the stack. The holes may be formed in the shapes of channels.  
         [0042]     According to another embodiment of the present invention, a stack of a fuel cell is provided in which one or more electricity generators including separators are disposed at both sides of an MEA. Fans are provided for directing cooling air to the electricity generators. The amount of cooling provided to different electricity generators can be varied by varying the amount of cooling air provided to the different electricity generators within the stack. This can be done by varying the number of fans or the sizes of the fans at different locations in the stack. Alternatively, the output from individual fans can be varied such as by changing the pitch of the fan blades of certain fans, or by changing the speeds of certain fans.  
         [0043]     According to this embodiment, the fans may be disposed at a housing constituting the external frame of the stack. Here, the stack may be assembled by use of separators disposed at its outermost sides as end plates or by using additional end plates. The stack may use cooling air, cooling water, or some other coolant as the coolant supplied to the heat releasing means.  
         [0044]     According to another embodiment of the present invention, a fuel cell system comprises: a stack in which one or more electricity generators including separators disposed at both sides of an MEA are stacked; a fuel supply unit for supplying hydrogen-containing fuel to the electricity generators; an air supply unit for supplying air to the electricity generators; and a coolant supply unit for supplying a coolant to the electricity generators. The stack includes heat releasing means for releasing heat generated from each electricity generator as described in the various embodiments above, and the heat release rate is made different depending on the positions of the electricity generators.  
         [0045]     In general, the heat release rate of the heat releasing means located at a central portion is larger than that of the heat releasing means located at the outer side.  
         [0046]     The fuel cell system may further comprise a reformer which reforms fuel supplied from the fuel supply unit to generate hydrogen gas. The fuel cell system may employ a PEMFC scheme or may employ a DMFC scheme.  
         [0047]     The fuel cell system may employ any one of a number of different coolants including air and water. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0048]     The above and other features and advantages of the present invention will become more apparent by describing in detail exemplary embodiments thereof with reference to the attached drawings, in which:  
         [0049]      FIG. 1  is a schematic diagram illustrating the entire structure of a fuel cell system according to an embodiment of the present invention;  
         [0050]      FIG. 2  is an exploded perspective view illustrating a stack of a fuel cell system according to an embodiment of the present invention;  
         [0051]      FIG. 3  is an exploded perspective view illustrating a stack according to another embodiment of the present invention;  
         [0052]      FIG. 4  is an exploded perspective view illustrating a stack according to another embodiment of the present invention;  
         [0053]      FIG. 5  is an exploded perspective view illustrating a stack according to another embodiment of the present invention;  
         [0054]      FIG. 6  is an exploded perspective view illustrating a stack according to another embodiment of the present invention;  
         [0055]      FIG. 7  is a schematic diagram illustrating a heat emission structure for the stack shown in  FIG. 2 ;  
         [0056]      FIG. 8  is a schematic diagram illustrating a heat emission structure for the stack shown in  FIG. 3 ;  
         [0057]      FIG. 9  is a schematic diagram illustrating a heat emission structure for the stack shown in  FIG. 4 ;  
         [0058]      FIG. 10  is a schematic diagram illustrating a heat emission structure for the stack shown in  FIG. 6 ; and  
         [0059]      FIG. 11  is a schematic diagram illustrating a heat emission structure for a stack according to another embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION  
       [0060]     Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings such that the embodiments can be easily put into practice by those skilled in the art. However, since the present invention can be embodied in various forms, the present invention is not limited to the embodiments described below.  
         [0061]      FIG. 1  is a schematic diagram illustrating a structure of a fuel cell system according to an embodiment of the present invention.  
         [0062]     Referring to  FIG. 1 , the fuel cell system  100  according to the present invention comprises a stack  10  in which a number of electricity generators  11  for converting chemical energy into electric energy through a chemical reaction between hydrogen and oxygen are stacked, a fuel supply unit  30  for supplying the hydrogen-containing fuel to the electricity generators  11 , an air supply unit  40  for supplying air to the electricity generators  11 , and a coolant supply unit  70  for supplying a coolant to the stack  10  in order to control the temperature of the electricity generators  11 .  
         [0063]     The fuel supply unit  30  includes a fuel tank  31  in which a hydrogen-containing liquid-fuel is stored, and a fuel pump  33  connected to the fuel tank  31  to discharge the stored fuel to the stack  10  through an optional reformer  20  disposed between the fuel supply unit  30  and the stack  10 . The reformer  20  is connected to the fuel supply unit  30  via a first supply line  91 , and to the stack  10  via a second supply line  92 .  
         [0064]     When the fuel cell system of the present invention employs a DMFC scheme for supplying liquid fuel directly to a stack to generate electricity, the reformer is excluded, unlike in the above-mentioned PEMFC scheme. Hereinafter, the present invention will be described with reference to a fuel cell system employing the PEMFC scheme which employs the reformer  20 , but the present invent is not limited to the PEMFC scheme.  
         [0065]     The reformer  20  generates hydrogen gas from the liquid fuel, which is required for generating electricity at the stack, and reduces the concentration of CO contained in the hydrogen gas. Generally, the reformer  20  includes a reforming section for reforming the liquid fuel to generate hydrogen gas, and a CO concentration reducing section for reducing the concentration of CO. The reforming section converts the fuel into reformed gas rich in hydrogen through a catalytic reaction such as steam reformation, partial oxidation, autothermal reaction. The CO reducing section reduces the concentration of CO in the reformed gas using a catalytic reaction such as a water-gas shift method, a preferential oxidation method, etc., or purification of hydrogen using a separating membrane.  
         [0066]     In this embodiment, the fuel includes hydrocarbon fuels, which can be easily loaded and stored. Examples include methanol, ethanol, natural gas, etc. The fuel may further include a mixture of water and a hydrocarbon fuel such as methanol, ethanol, natural gas, etc. Hereinafter, methanol, ethanol, and natural gas are referred to as “liquid fuels” for the purpose of convenience.  
         [0067]     Pure oxygen gas stored in an additional storage unit or external air containing oxygen may be used as an oxygen source. Hereinafter, for convenience, the invention will be described with reference to an example in which external air is used, however, the invention is not so limited.  
         [0068]     The air supply unit  40  includes an air pump  41  which is connected to the stack  10 , and which draws in external air and supplies it to the stack  10 . The stack  10  is connected to the air supply unit  40  through a third supply line  93 .  
         [0069]     Further, the coolant supply unit  70  includes a pump  71  which draws in a coolant and produces the coolant to the stack  10  through a fourth supply line  94 . Any one of a number of different coolants may be used. Examples include cooling water which can be provided in either a liquid phase or a gaseous phase. In this embodiment, however, it will be described with reference to an example in which air is used as the coolant, which can be easily obtained in nature.  
         [0070]     Next, in the fuel cell system having the described structure, the stack  10  which generates electricity using fuel and air supplied from the fuel supply unit  30  and the oxygen supply unit  40  is cooled using a coolant supplied from the coolant supply unit  70  will be described with reference to FIGS.  2  to  5 .  
         [0071]      FIG. 2  illustrates the stack according to the present embodiment. The stack  10  includes a plurality of electricity generators  11  which are supplied with hydrogen gas reformed through the reformer  20  and external air, and which generate electricity through an oxidation and reduction reaction.  
         [0072]     Each of the electricity generators  11  is a unit cell for generating electricity.  
         [0073]     The electricity generators  11  include an MEA  12  which oxidizes/reduces hydrogen gas and air, and separators  13  which supply hydrogen gas and air to the MEA  12 .  
         [0074]     Each of the electricity generators  11  is constructed such that the separators  13  are disposed at both sides of the MEA  12 , and the separators  13  are attached to the MEA  12 . The stack  10  is comprised of the plurality of electricity generators  11  successively disposed.  
         [0075]     The MEA  12  is generally constructed such that an electrolyte membrane is disposed between an anode electrode and a cathode electrode which constitute both side surfaces of the MEA  12 . The anode electrode is supplied with reformed gas through the separator  13 . The anode electrode includes a catalytic layer separating reformed gas into electrons and hydrogen ions, and a gas diffusion layer for smooth movement of the electrons and the reformed gas. The cathode electrode is supplied with air through the separator  13 . The cathode electrode includes a catalytic layer for forming water by a reaction of the electrons, the hydrogen ions and hydrogen of the air, and a gas diffusion layer for smooth movement of the electrons and the oxygen. The electrolyte membrane is made of a solid polymer electrolyte having a thickness of 50 to 200 μm and functions to move hydrogen ions generated at the catalytic layer of the anode electrode to the catalytic layer of the cathode electrode.  
         [0076]     The separators  13  function to serially connect the anode electrode to the cathode electrode and to provide passages for supplying hydrogen gas and air required for the oxidation and reduction reactions of the MEA  12  to the anode and cathode electrodes. The separators  13  have flow channels  13   a  formed on the surfaces thereof to supply gas required for the oxidation and reduction reaction of the MEA  12 .  
         [0077]     More specifically, the separators  13  are disposed at both sides of the MEA  12  with the MEA  12  interposed therebetween, and are closely attached to the anode and cathode electrodes of the MEA  12 . The separators  13  have flow channels  13   a  formed on the surface closely attached to the anode and cathode electrodes of the MEA  12 . The flow channels  13   a  supply hydrogen gas to the anode electrode and supply air to the cathode, respectively.  
         [0078]     The stack  10  having the above-mentioned configuration generates electricity and water through a reaction such as in the following equations. 
 
Anode reaction: H 2 →2H + +2 e   − 
 
Cathode reaction: ½O 2 +2H + +2 e   − →H 2 O 
 
Total reaction: H 2 +½O 2 →H 2 O+current+heat 
 
         [0079]     Referring to the equations, hydrogen gas and air are supplied to the anode and cathode electrodes of the MEA  12  through the separator  13 , respectively. When the hydrogen gas flows through the anode electrode, hydrogen is decomposed into electrons and protons (hydrogen ions) at the catalytic layer. When protons move through the electrolyte membrane, electrons, oxygen, and protons are reacted together and water is generated at the cathode electrode with the help of a catalytic agent. Here, electrons generated at the anode electrode cannot move through the electrolyte membrane and move to the cathode electrode through an external circuit. Through such a process, electricity and water are generated, and heat as a byproduct is generated at the stack  10  through a chemical reaction between the hydrogen gas and oxygen.  
         [0080]     During operation of the stack  10 , heat is generated from each electricity generator  11 . The coolant supply unit  70  is operated to remove the heat generated from each electricity generator  11 . According to this embodiment, the coolant supply unit  70  supplies cooling air to the stack  10 .  
         [0081]     In the stack  10  according to the present embodiment, the temperature of the entire stack  10  is suitably maintained by circulating the cooling air supplied from the coolant supply unit  70  through the inside of the stack  10 . To do so, flow passages  14  through which air flows are formed in the separators  13 .  
         [0082]     The sizes of the flow passages  14  are varied depending on the positions of the electricity generators  11  within the stack  10 . In general, the sizes of the flow passages  14  increase from the outside ends of the stack  10  toward the central portion of the stack  10 .  
         [0083]      FIG. 7  illustrates the differences in sizes of the flow passages  14  depending on the positions of the respective separators within the stack  10 . In  FIG. 7 , a plurality of electricity generators  11  including the MEA  12  and the separators  13  are stacked to form the stack  10 . The sizes of the flow passages  14  increase toward a central portion of the stack  10  from both side surfaces.  
         [0084]     Here, the differences in sizes between the flow passages located at the outermost right or left side and the flow passages located at a central portion is not limited to any specific value.  
         [0085]     Here, the size of the flow passage  14  means an individual sectional area of each of the flow passages  14  formed on one separator  13 , or the sum of the sectional areas of the all flow passages  14  formed on one separator  13 . The sectional area can be defined as a sectional area which substantially determines the flow rate.  
         [0086]     In this embodiment of the present invention, the sizes of the flow passages  14  are different because the heat-emission temperatures of the electricity generators  11  located at a central portion of the stack  10  tend to be higher than those of the electricity generators  11  located at the sides when operating the fuel cell system. When cooling air is supplied to the stack  10  from the coolant supply unit  70  through the flow passages  14 , a greater amount of cooling air can be supplied to the electricity generator located at a central portion of the stack  10  compared to one located nearer the outer sides of the stack  10 , thereby increasing cooling effect.  
         [0087]     Here, the flow passages  14  include a plurality of channels  14   a  and  14   b  formed at side surfaces opposite to side surfaces on which the flow channels  13   a  are formed. In the present embodiment, the flow passages  14  are constructed such that channels  14   a  formed on one separator  13  of one electricity generator  11  and channels  14   b  formed on one separator  13  of another electricity generator  11  opposite to each other are joined together.  
         [0088]     The temperature of the stack  10  can be lowered by releasing heat generated from each electricity generator  11  by the action of cooling air supplied from the coolant supply unit  70  through the flow passages  14  formed by the channels  14   a  and  14   b.    
         [0089]     As mentioned above, since the sizes of the flow passages  14  are made different depending on the positions of the electricity generators within the stack  10 , a greater amount of air is supplied to a central portion of the stack  10  compared to the outer sides, whereby more heat can be removed from the central portion of the stack. Therefore, it is possible that a uniform temperature distribution can be obtained through the entire region of the stack  10 .  
         [0090]      FIG. 3  illustrates a stack according to another embodiment of the present invention which employs cooling plates.  
         [0091]     As shown in  FIG. 3 , electricity generators  53  including separators  52  which are disposed at both sides of MEAs  51  and which are attached to the MEAs  51  are successively stacked within a stack  50 . Cooling plates  54 , which have holes  54   a  through which air flows for cooling down the stack  50  are disposed between the electricity generators  53 . The sizes of the holes  54   a  formed in the cooling plates  54  increase toward a central portion from the outer side depending on the stacked positions of the electricity generators  53  within the stack  50 .  
         [0092]     When the fuel cell system  100  shown in  FIG. 1  employs the stack  50 , cooling air supplied from the coolant supply unit  70  flows through the holes  54   a  formed in the cooling plates  54 , whereby the temperature of the entire stack  50  can be maintained uniformly.  
         [0093]     The sizes of the holes  54   a  formed in the cooling plate  54  increase toward a central portion of the stack  50  from the outer side, so the stack  50  can be effectively cooled down corresponding to heat-emission conditions that vary depending on the locations of the electricity generators  53  within the stack  50 .  
         [0094]      FIG. 8  illustrates the differences in sizes of the holes  54   a  formed in the cooling plates  54  depending on the positions in the stack  50 . The stack  50  is constructed such that the cooling plates  54  are interposed between the electricity generators  53  including the MEA  51  and separators  52 . The sizes of the holes  54   a  formed in the cooling plates  54  increase toward a central portion of the stack  50  from both side surfaces.  
         [0095]     Here, the differences in sizes between the holes located at the outermost sides and the holes located at the central portion are not limited to any specific value.  
         [0096]     Further, the size of the hole  54   a  means the individual sectional areas of the holes  54   a  formed in one cooling plate  54 , or the sum of the sectional areas of the all holes  54   a  formed in one cooling plate  54 . The sectional area can be defined as a sectional area which substantially determines the flow rate.  
         [0097]     The cooling plates  54  may be formed with the same area or plate thickness through the entire stack  50 , irrespective of the size of the holes  54   a  that vary depending on the position in the stack  50 , or may be formed with different areas or thicknesses corresponding to the size of the holes  54   a  that vary depending on their position in the stack  50 .  
         [0098]     The separator  52  may be made of graphite, and it is preferable that the cooling plate  54  be made of a material with higher heat conductivity than that of the separator  52 . Suitable materials include aluminum, copper, and iron.  
         [0099]     According to this embodiment, it is noted that more heat tends to be generated from the electricity generators  53  toward a central portion of the stack  50 . Therefore, the holes  54   a  having different sizes as mentioned above are formed in the cooling plates  54  disposed between the electricity generators  53  to achieve a greater amount of cooling at the central portions of the stack  50  through the holes  54   a . Therefore, an electricity generator  53  located at the central portion of the stack can quickly release a greater amount of heat compared to an electricity generator  53  located at the outer side. Accordingly, it is possible to obtain a more uniform temperature distribution through the entire stack  50 .  
         [0100]      FIG. 4  illustrates a stack according to another embodiment of the present invention. As shown in  FIG. 4 , a stack  60  is constructed such that one or more electricity generators  63 , each including separators  62  disposed at both sides of an MEA  61 , are stacked.  
         [0101]     Flow grooves  64  through which a coolant such as cooling air flows are formed in the separators  62  at portions corresponding to non-active regions  61   a  of the MEA  61 , and the size of each of the flow grooves  64  is made different depending on its location within the stack  60 .  
         [0102]     The flow grooves  64  form passages between the separators  62  and the MEAs  61  adhered to the separators  62  through which the coolant such as cooling air is circulated to remove heat generated from the electricity generators  63 .  
         [0103]     The sizes of the flow grooves  64  increase toward a central portion of the stack  60  from the outer sides.  
         [0104]     The non-active region  61   a  is a region other than an active region  61   b , the active region  61   b  being the region through which air or hydrogen gas flows. That is, the non-active region  61   a  is the region where hydrogen gas and air do not react.  
         [0105]     In the stack  60  shown in  FIG. 4 , the active region  61   b  is formed at a central portion of the MEA  61 , and the non-active region  61   a  is formed on the periphery of the active region  61   b  to surround it. The flow grooves  64  are formed at positions corresponding to the non-active regions  61   a , that is, at the upper and lower sides of the separator in the figure.  
         [0106]     The position in which the flow groove  64  is formed is not especially limited as long as it is formed at a non-active region other than an active region to which hydrogen or air is supplied, and it is preferable that the flow groove  64  is formed through all regions other than the active region.  
         [0107]     The flow groove  64  may be formed in the shape of channel, and is connected to coolant supply opening  62   a  and coolant discharge opening  62   b.    
         [0108]     A coolant such as cooling air is supplied through the supply opening  62   a  and flows through the flow groove  64  of the separator  62 , and is circulated out through the discharge opening  62   b.    
         [0109]     As mentioned above, since a plurality of electricity generators  63  are stacked to construct the stack  60 , the supply and discharge openings  62   a  and  62   b  formed at each of the separators  62  are formed at the same positions, and supply and discharge openings  61   c  and  61   d  are formed in the MEA  61  disposed between the separators  62  at positions corresponding to the supply and discharge openings  62   a  and  62   b , thereby forming one supply opening and one discharge opening.  
         [0110]     Reference numeral  62   c  indicates flow channels formed at the active region of the separator  62  to supply hydrogen and oxygen to the MEA  61 .  
         [0111]     As shown in  FIG. 9 , a plurality of electricity generators  63  including the MEA  61  and the separators  62  are stacked to constitute the stack  60 , and the sizes of the flow grooves  64  formed on the separators  62  increase toward a central portion of the stack  60  from both sides thereof.  
         [0112]     Here, the difference in size between the flow grooves  64  located at the outermost side of the stack  60  and the flow grooves  64  located at a central portion of the stack  60  is not limited to any specific value.  
         [0113]     Further, the size of the flow groove  64  may be considered as a sectional area or a volume of passage formed by the flow groove  64  and the MEA  61  which is disposed at the outside of the flow groove  64  and is attached to the separator  62 .  
         [0114]     According to this embodiment, it is noted that the amount of heat generated from each electricity generator  63  increases toward a central portion of the stack  60 . Therefore, the flow grooves  65  having different sizes as mentioned above are formed on the separators  62 , whereby a greater amount of cooling air is supplied to the electricity generators  63  located at a central portion of the stack  60  through the flow grooves  64 . Therefore, the electricity generator  63  located at the central portion of the stack can quickly release a greater amount of heat compared to the electricity generator  63  located at the outer side. Therefore, it is possible to obtain a uniform temperature distribution through the entire stack  60 .  
         [0115]      FIG. 5  illustrates a stack according to another embodiment of the present invention, in which separators employ a heat conductive medium.  
         [0116]     As shown in  FIG. 5 , a plurality of electricity generators  83  including separators  82  disposed at both sides of an MEA  81  are stacked to constitute a stack  80 .  
         [0117]     A metal plate  84  having higher heat-conductivity than that of the separator  82  is attached to each separator  82 , and the sizes of the metal plates  84  are different depending on the positions of the electricity generators  83  within the stack  80 . That is, the thicknesses of the metal plates  84  become larger toward a central portion of the stack  80  from the outer sides.  
         [0118]     According to this configuration, heat generated from the electricity generators  83  is quickly absorbed and released by the metal plates  84  that have higher heat-conductivity than those of the separators  82 , so the stack  80  can release heat more quickly compared to another stack including only the separators  13 . Further, since the thickness of the metal plates  84  positioned at the central position of the stack  80  are greater than those of the metal plates  84  positioned at the outer side, the stack  80  can effectively release more heat from the thicker plates, whereby a uniform temperature distribution can be maintained throughout the entire region.  
         [0119]     In the present embodiment, the metal plate  84  is formed in the shape of a thin plate and is disposed at an outer side surface of the separator  82 , that is, at a surface opposite to a surface in contact with the MEA  81 . The thicknesses of the metal plates  84  are not limited to any specific values.  
         [0120]     The separator  82  may be made of graphite. It is preferable that the metal plates  84  are made of a material which has higher heat-conductivity than that of the separator  82 . Exemplary materials include aluminum, copper, and iron.  
         [0121]     Further, the metal plate  84  may optionally have a plurality of holes through which a coolant such as cooling air may be supplied from the coolant supply unit in order to enhance the heat-emission effect. Such an embodiment is illustrated in further detail in  FIG. 6 .  
         [0122]     As shown in  FIG. 6 , the stack  180  is constructed such that the metal plates  184 , which are made of a heat conductive medium, are disposed between the electricity generators  183 . Holes  184   a  are formed on each metal plate  184 . The size of each of the holes  184   a  increases toward a central portion of the stack  180  from the outer sides, depending on the position of the electricity generators  183  within the stack  180 . Here, the thickness of each of the metal plates  184  may be the same, or may be different as shown in  FIG. 5 .  
         [0123]      FIG. 10  illustrates the differences in the sizes of the holes  184   a  depending on their position within the stack  180  shown in  FIG. 6 . As shown in  FIG. 10 , a plurality of electricity generators  183  including MEAs  181  and separators  182  are stacked to constitute the stack  180 , and the sizes of the holes  184   a  formed on the metal plates  184  attached to the separators  182  increases toward a central portion of the stack  180  from both sides thereof.  
         [0124]     Here, the difference in size between the holes  184   a  formed in the metal plate  184  located at the outermost side of the stack  180  and the holes  184   a  formed in the metal plate  184  located at a central portion are not limited to any specific values.  
         [0125]     Further, the size of the holes  184   a  may be considered to be an individual sectional area of each of the holes  184   a  formed in any one of the metal plates  184  or the sum of sectional areas of all the holes  184   a  formed in any one of the metal plates  184 .  
         [0126]     According to this embodiment, it is noted that the amount of heat generated from the electricity generator  183  increases toward a central portion of the stack  180 . Therefore, holes  184   a  having different sizes as mentioned above are formed on the metal plates  184 , so a greater amount of cooling air is supplied to the electricity generators  183  located at a central portion of the stack  180  through the holes  184   a . Therefore, the electricity generator  183  located at the central portion of the stack  180  can release a greater amount of heat compared to the electricity generators  183  located at the outer sides.  
         [0127]     In this embodiment, channels  184   b  formed in a metal plate  184  corresponding to any one of electricity generators  183 , and channels  184   b  formed in a metal plate  184  corresponding to an adjacent electricity generator  183  are joined together while both the metal plates  184  are closely adhered to each other to construct the stack  180  in which the holes  184   a  are formed.  
         [0128]     The coolant such as cooling air removes the heat generated from the electricity generators  183  by releasing the heat outside after it passes through the holes  184   a.    
         [0129]     As mentioned above, the size of each of the holes  184   a  in the stack  180  is made different depending on its position within the stack  180 , and a greater amount of air is supplied to the metal plate  184  located at a central portion of the stack  180  than those located at the outer sides. Therefore, more heat generated at the central portion of the stack  180  can be removed, providing a uniform temperature distribution throughout all the regions of the stack  80 .  
         [0130]      FIG. 11  illustrates a heat-emission structure of a stack according to still another embodiment of the present invention.  
         [0131]     As shown in  FIG. 11 , one or more electricity generators  113  including separators  112  disposed at both sides of an MEA  111  are stacked in a stack  110 , and the stack  110  includes a plurality of fans  115  which are disposed at a housing  114  surrounding the stacked electricity generators  113  and which direct a coolant (for example, cooling air) to the electricity generators  113 . The amount of cooling air provided to the different electricity generators within the stack can be varied such as by varying the number of fans or the sizes of the fans at different locations in the stack. Alternatively, the output from individual fans can be varied such as by changing the pitch of the fan blades of certain fans, or by changing the speeds of certain fans.  
         [0132]     According to the embodiment shown, the number of fans may be varied depending on the positions of the electricity generators  113  within the stack  110 . The number of fans  115  increases toward a central portion of the stack  110  from the outer side thereof to correspond to the increased amount of heat generated by the electricity generators  113  at this location.  
         [0133]     According to this configuration, cooling air supplied to the stack  110  from the coolant supply unit  70  (shown in  FIG. 1 ) is directed in greater quantities toward the electricity generator located at a central portion of the stack than toward the electricity generator located at the outer side, whereby the electricity generator  113  showing a high temperature distribution at the central portion of the stack  110  can be effectively cooled down. Therefore, it is possible to maintain a uniform temperature distribution throughout all the regions of the stack  110 .  
         [0134]     The difference between the number of fans  115  located at the outermost position of the stack  110  and the number of fans  115  located at a central position of the stack  110  is not limited to any specific value.  
         [0135]     According to the present embodiment, the number of fans  115  corresponding to the electricity generators  113  disposed at corresponding positions is made different depending on the position of the electricity generators  113  within the stack  110 , such that air supplied to the stack  110  from the coolant supply unit  70  is directed in large quantities toward the central portion of the stack  110 . Therefore, the heat of the electricity generator  113  located at the central portion of the stack  110  can be lowered more than the heat of the electricity generator  113  located at the outer side, whereby a uniform temperature distribution can be obtained throughout all the regions of the stack  110 .  
         [0136]     According to the present invention described above, the temperature of the stack at a central portion thereof can be significantly lowered, such that it is possible to have a uniform temperature distribution through the entire stack and to maintain the temperature of the stack at an appropriate level.  
         [0137]     Further, the cooling effect of the stack can be enhanced by changing the flow rate of the coolant depending on the amount of heat generated corresponding to location within the stack.  
         [0138]     Although exemplary embodiments of the present invention have been described, the present invention is not limited to the exemplary embodiments, but may be modified in various forms without departing from the scope of the appended claims, the detailed description, and the accompanying drawings of the present invention. Therefore, it is natural that such modifications belong to the scope of the present invention.