Patent Publication Number: US-2012028156-A1

Title: Stack having uniform temperature distribution and method of operating the same

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims the benefit of Korean Patent Application No. 10-2010-0074386, filed on Jul. 30, 2010 in the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference. 
     BACKGROUND 
     1. Field 
     Aspects of the present disclosure relate to a heat source, and more particularly to a stack having a uniform temperature distribution, as well as a method of operating the same. 
     2. Description of the Related Art 
     When a fuel cell stack is in a normal operation, the temperature of the interior of the fuel cell stack increases due to the heat generated from the electrochemical reaction in the fuel cells. The increase in temperature of the fuel cell stack is limited to a predetermined level by disposing cooling plates at predetermined intervals. 
     However, a thermal gradient having a parabolic shape is formed between cooling plates. Since heat is transferred to the outside through end plates, a thermal gradient is also formed in the interior of the fuel cell stack. The temperature of fuel cells near the end plates, that is, the operating temperature of the end cells, is lower than that of cells in a central region of the fuel cell stack. Due to the temperature difference between the end plates and the central region of the fuel cell stack, non-uniformity of performance of the fuel cells included in the fuel cell stack may increase and lifetime of the fuel cells included in the fuel cell stack may decrease. 
     In particular, the temperature near an inlet of a stack in which a cold working fluid flows and the temperature near an outlet of a stack area are lower than that of the rest of a stack. 
     The temperature of a bipolar plate in a fuel cell stack may be preheated to 100° C. or above to avoid the possibility of phosphoric acid leak due to the condensation of steam generated from an electrochemical reaction. For this, the fuel cell stack may be preheated before applying a load to the fuel cell stack. 
     However, in the case of conventional fuel cell stack, the configuration of the fuel cell stack is complicated and the total volume of the fuel cell stack is increased since the preheating to 100° C. or above takes a long time and an additional heat source for preheating is needed. 
     SUMMARY 
     Aspects of the present invention provide a stack having a uniform temperature distribution without an additional heat source. 
     Aspects of the present invention provide a method of operating the stack. 
     According to an aspect of the present invention, there is provided a method of operating a stack having a plurality of cells and a plurality of cooling plates, the method including: supplying a working fluid to a first group of the cooling plates; and re-supplying the working fluid passed through the first group of the cooling plates to a second group of the cooling plates, wherein the first and second groups are divided according to an operating temperature in the stack. 
     The stack may be a fuel cell stack and the cells are fuel cells. 
     The first group of the cooling plates may be located on both ends of the fuel cell stack and the second group of the cooling plates may be located in a central region of the fuel cell stack. 
     The first group of the cooling plates may be located in a central region of the fuel cell stack and the second group of the cooling plates may be located on both ends of the fuel cell stack. 
     Each cooling plate of the fuel cell stack may belong to one of the first group and the second group. 
     The working fluid supplied to the first group may be preheated by circulating in the fuel cell stack. 
     The working fluid supplied to the first group may be preheated at the outside of the fuel cell stack using a heating device. 
     The method may further include controlling a flow rate of the working fluid supplied to the second group of the cooling plates. 
     The working fluid passed through the first group of the cooling plates may be divided into two streams in directions different from each other. 
     The stack may be a battery pack and the cells are battery cells. 
     According to another aspect of the present invention, there is provided a stack including: a plurality of cells; first and second groups of cooling plates located between the cells, each cooling plate including a working fluid inlet and a working fluid outlet; a working fluid supply manifold in fluid communication with one or more of the working fluid inlets of the first group of cooling plates to supply a working fluid to the first group of cooling plates; and a working fluid resupply manifold in fluid communication with one or more of the working fluid outlets of the first group of cooling plates and one or more of the working fluid inlets of the second group of cooling plates to resupply the working fluid passed through the first group of cooling plates to the second group of cooling plates. 
     The stack may be a fuel cell stack and the cells may be fuel cells. 
     The stack may further include a working fluid outlet manifold in fluid communication with one or more of the working fluid outlets of the second group of cooling plates to convey the working fluid passed through the second group of cooling plates to outside of the fuel cell stack. 
     The first group of cooling plates may be located on both ends of the fuel cell stack and the second group of cooling plates may be located in a central region of the fuel cell stack. 
     The number of fuel cells per cooling plate in a central region of the fuel cell stack may be greater than that in both end regions of the fuel cell stack. 
     The number of fuel cells per cooling plate on one end of the fuel cell stack may be greater than that on the other end of the fuel cell stack. 
     The stack may further include a flow controller on the working fluid resupply manifold. 
     Each cooling plate in the fuel cell stack may belong to one of the first group and the second group. 
     The working fluid supply manifold may be in fluid communication with the working fluid inlet of each of the first group of cooling plates. 
     The working fluid resupply manifold may be in fluid communication with the working fluid outlet of each of the first group of cooling plates. 
     The working fluid resupply manifold may be in fluid communication with the working fluid inlet of each of the second group of cooling plates. 
     The stack may be a battery pack and the cells are battery cells. 
     Additional aspects and/or advantages of the invention will be set forth in part in the description which follows and, in part, will be obvious from the description, or may be learned by practice of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and/or other aspects and advantages of the invention will become apparent and more readily appreciated from the following description of the embodiments, taken in conjunction with the accompanying drawings, of which: 
         FIG. 1  is a perspective view of a fuel cell stack according to an embodiment; 
         FIGS. 2 and 3  are front views of cooling plates of a first cooling plate group included at both ends of the fuel cell stack of  FIG. 1 ; 
         FIG. 4  is a front view of a cooling plate of a second cooling plate group that is included in a central region of the fuel cell stack between cooling plates of the first cooling plate groups of the fuel cell stack of  FIG. 1 ; 
         FIG. 5  is a perspective view of a fuel cell stack according to another embodiment; 
         FIG. 6  is a cross-sectional view taken along a direction vertical to the y-axis direction of the fuel cell stack of  FIG. 5 ; 
         FIG. 7  is a perspective view of a fuel cell stack according to another embodiment; 
         FIG. 8  is a cross-sectional view taken along a direction vertical to the y-axis direction of the fuel cell stack of  FIG. 7 ; 
         FIG. 9  is a cross-sectional view showing another embodiment in which cooling plates and bipolar plates are symmetrically disposed about the center of a fuel cell stack; 
         FIG. 10  is a magnified cross-sectional view of a predetermined region of  FIG. 9 ; 
         FIG. 11  is a cross-sectional view showing another embodiment in which constituent elements are asymmetrically disposed about the center of a fuel cell stack; 
         FIG. 12  is a cross-sectional view showing another embodiment in which the number of bipolar plates between cooling plates is different in an inner region of the fuel cell stack; 
         FIGS. 13 through 29  are cross-sectional views showing methods of operating (circulating working fluid) a fuel cell stack according to other embodiments; 
         FIG. 30  is a graph showing simulated results for a method of operating a fuel cell stack according to a conventional method and a method of operating a fuel cell stack according to embodiments; 
         FIGS. 31 and 32  are graphs respectively showing simulated results of temperature changes according to time with respect to a conventional method of operating a fuel cell stack and a method of operating a fuel cell stack according to embodiments; and 
         FIG. 33  is a graph showing simulated results of temperature distribution in a fuel cell stack according to temperature variation of a working fluid in a method of operating a fuel cell stack according to an embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     Reference will now be made in detail to embodiments, examples of which are illustrated in the accompanying drawings, wherein like reference numerals refer to the like elements throughout. 
     First, a fuel cell stack according to an embodiment will now be described.  FIG. 1  is a perspective view of a fuel cell stack according to an embodiment. Referring to  FIG. 1 , the fuel cell stack  40  includes first cooling plate group  42  and  44  and a second cooling plate group  48 . The first cooling plate group  42  and  44  may include cooling plates included in a region where an already measured operating temperature is relatively lower than the rest of the regions in the fuel cell stack  40 . For example, as depicted in  FIG. 1 , the first cooling plate group  42  and  44  may include cooling plates located on both ends of the fuel cell stack  40 . However, this is only an example, and the first cooling plate group may include cooling plates in other predetermined regions according to an already measured operating temperature. 
     Here, the “operating temperature” may denote the temperature of a fuel cell stack when the fuel cell stack is in normal operation For example, during operation of a fuel cell stack, the temperature of an end or both ends of the fuel cell stack may be lower than that of a central region of the fuel cell stack due to the incoming and outgoing of a working fluid (deionized water, oil, silicone oil, mineral oil, ethylene glycol or propylene glycol) through the end or both ends of the fuel cell stack. Accordingly, temperature deviation between fuel cells according to positions of the full cells may occur in the fuel cell stack. In this way, the operating temperature may denote the temperature measured inside the fuel cell stack during an operation of the fuel cell stack. 
     A plurality of cooling plates of a fuel cell stack may be divided at least into two cooling plate groups according to an already-measured operating temperature inside the fuel cell stack. An example may be the first cooling plate group  42  and  44  and second cooling plate group  48 . 
     Although not shown in  FIG. 1 , as shown in  FIGS. 9 ,  11 , and  12 , end plates  70  and  72  (refer to  FIG. 9 ) are included on outer sides of the first cooling plate group  42  and  44 . The first cooling plate group  42  and  44  may include cooling plates located near the end plates  70  and  72 . For example, the first cooling plate group  42  and  44  may include two cooling plates, one on each end of the fuel cell stack  40 , but not limited thereto, and may include at least one cooling plate located on both ends thereof where the operating temperature is relatively low. 
     Also, the second cooling plate group  48  may include a plurality of cooling plates included between the both ends of the fuel cell stack  40 , that is, in a central region of the fuel cell stack  40 . Cooling plates of the first cooling plate group  42  and  44 , and second cooling group  48  may absorb heat from neighboring fuel cells. Working fluid paths are formed on surfaces of the cooling plates of the first cooling plate group  42  and  44  and second cooling group  48 . A working fluid that passes through the first cooling plate group  42  and  44  and second cooling group  48  flows through the working fluid paths. The working fluid absorbs heat from fuel cells adjacent to the corresponding cooling plates while moving through the working fluid paths. The working fluid paths may be included on a side or both sides of the cooling plates. 
     As described above, the cooling plates of the first and second cooling plate groups  42 ,  44 , and  48  basically absorb heat from adjacent fuel cells. However, since preheated working fluid may be supplied to the cooling plates included in a location where the operating temperature is relatively low, the cooling plates may also perform a function of supplying heat to the fuel cells adjacent thereto. An example of this case is that a preheated working fluid is supplied to cooling plates located on an end or both ends of a fuel cell stack when the fuel cell stack starts up. 
     Next, for convenience of explanation,  FIG. 1  depicts that the second cooling plate group  48  includes four cooling plates. However, the second cooling plate group  48  may include a larger number of cooling plates. Also,  FIG. 1  depicts that the cooling plates of the first and second cooling plate groups  42 ,  44 , and  48  are arranged in a row. A plurality of bipolar plates may be included between the cooling plates. However, for simplicity of drawing, the bipolar plates are omitted in  FIG. 1 . 
     The fuel cell stack  40  may include a plurality of cooling manifolds, for example, first through fourth cooling manifolds  45 ,  46 ,  50 , and  52 . The first through fourth cooling manifolds  45 ,  46 ,  50 , and  52  may include a working fluid supply manifold that is used for a path supplying a working fluid to the fuel cell stack  40 , first to a selected portion of the cooling plates (for example, the second cooling plate group  48 ) and next to the other portion of the cooling plates (for example, the first cooling plate group  42  and  44 ). The working fluid supply manifold may be the second cooling manifold  46 . 
     In  FIG. 1 , if the flow of the working fluid is reversed, the first cooling manifold  45  may be the working fluid supply manifold and the second cooling manifold  46  may be a working fluid outlet manifold. Here, “via the cooling plates” denotes that, as described above, the working fluid flows along the working fluid paths formed on the cooling plates. 
     Referring to  FIG. 1 , each of the first through fourth cooling manifolds  45 ,  46 ,  50 , and  52  may be connected to a portion or all of the cooling plates of the first and second cooling plate groups  42 ,  44 , and  48 . A portion of the first through fourth cooling manifolds  45 ,  46 ,  50 , and  52  may be working fluid supply manifolds which are used as paths for supplying a working fluid to working fluid inlet units of the cooling plates, another portion of the first through fourth cooling manifolds  45 ,  46 ,  50 , and  52  may be working fluid outlet manifolds which are used as paths for conveying the working fluid discharged from working fluid outlet units of the cooling plates to the outside, and still another portion of the first through fourth cooling manifolds  45 ,  46 ,  50 , and  52  may be working fluid resupply manifolds which are used as paths for moving or re-supplying the working fluid discharged from the working fluid outlet units of the cooling plates to other cooling plates of the fuel cell stack  40 . 
     A working fluid supplied to the second cooling plate group  48  in the central region of the fuel cell stack  40  via the second cooling manifold  46 , which is a working fluid supply manifold, flows to the third cooling manifold  50  through working fluid outlet units of the cooling plates of the second cooling plate group  48 . The working fluid discharged from the working fluid outlet units of the cooling plates of the second cooling plate group  48  may flow to the first cooling plate group  42  and  44  via the third and fourth cooling manifolds  50  and  52 . Thus, the third and fourth cooling manifolds  50  and  52  may be working fluid resupply manifolds for moving or re-supplying the working fluid discharged from the second cooling plate group  48  to the first cooling plate group  42  and  44 . The third cooling manifold  50  may be connected to the fourth cooling manifold  52  by extending to the outside of the fuel cell stack  40 . 
     The working fluid may be oil, silicone oil, mineral oil, ethylene glycol, propylene glycol or deionized water. The first cooling manifold  45  may be a path for discharging a working fluid discharged from the first cooling plate group  42  and  44  to the outside of the fuel cell stack  40 . Accordingly, the first cooling manifold  45  may be a working fluid outlet manifold. The first cooling manifold  45  may be connected to at least a lower left-end of the first cooling plate group  42  and  44 . At the same time, the first cooling manifold  45  may be connected to a lower left-end of the second cooling plate group  48 . For these connections, the lower left-ends of the first and second cooling plate groups  42 ,  44 , and  48  may protrude to be connected to the first cooling manifold  45 . The first cooling manifold  45  may be a path for conveying the working fluid that flows into the first cooling plate group  42  and  44  to the outside of the fuel cell stack  40 . Thus, although the first cooling manifold  45  may be connected to the second cooling plate group  48 , working fluid paths (not shown) formed on the cooling plates of the second cooling plate group  48  do not contact the first cooling manifold  45 . Accordingly, the working fluid that flows into the second cooling plate group  48  is not discharged through the first cooling manifold  45 . The working fluid circulated in the cooling plates in the fuel cell stack  40  is conveyed to the outside of the fuel cell stack  40  through the first cooling manifold  45 . The second cooling manifold  46  is a path for supplying a working fluid into the fuel cell stack  40  from the outside. The working fluid conveyed to the outside of the fuel cell stack  40  through the first cooling manifold  45  may be re-supplied to the fuel cell stack  40  through the second cooling manifold  46  after passing through an external circulation path. The second cooling manifold  46  is connected to the lower right-ends of the first and second cooling plate groups  42  and  48 . For these connections, the lower right-ends of the first and second cooling plate groups  42  and  48  may protrude to be connected to the second cooling manifold  46 . A working fluid flowing into the fuel cell stack  40  through the second cooling manifold  46  is supplied to the second cooling plate group  48 . However, the second cooling manifold  46  does not contact the working fluid paths of the first cooling plate group  44 . Thus, the working fluid coming in through the second cooling manifold  46  is not supplied to the first cooling plate group  44 . The third cooling manifold  50  is connected to an upper left-end of the second cooling plate group  48 , and is connected to the upper left-ends of the first cooling plate group  44 . For this connection, the upper left-ends of the cooling plates of the second cooling plate group  48  and the first cooling plate group  44  may protrude to be connected to the third cooling manifold  50 . The third cooling manifold  50  may be a path for discharging the working fluid that passed through the second cooling plate group  48 . A working fluid that flows into the third cooling manifold  50  flows into the fourth cooling manifold  52  as indicated by an arrow  51 . For this, a connection manifold (not shown) for connecting the third cooling manifold  50  to the fourth cooling manifold  52  may be included between the third cooling manifold  50  and the fourth cooling manifold  52 . The connection manifold may be installed outside the fuel cell stack  40  or inside the end plates (for example,  72  in  FIG. 9 ) of the fuel cell stack  40 . The fourth cooling manifold  52  is connected to upper right-ends of the first and second cooling plate groups  42 ,  44 , and  48 . For these connections, the upper right-ends of the first and second cooling plate groups  42 ,  44 , and  48  may protrude to be connected to the fourth cooling manifold  52 . A working fluid that flows into the fourth cooling manifold  52  through the third cooling manifold  50  and the connection manifold is not supplied to the second cooling plate group  48 , but is supplied to the first cooling plate group  42  and  44 . The working fluid supplied through the fourth cooling manifold  52  flows to the outside of the fuel cell stack  40  through the first cooling manifold  45  after passing through the first cooling plate group  42  and  44 . In  FIG. 1 , arrows indicate the moving direction of the working fluid. 
     Meanwhile, a working fluid may flow in a reverse direction. For example, the working fluid may be supplied through the first cooling manifold  45 . At this point, the first cooling manifold  45  may be a working fluid supply manifold. A working fluid supplied to the first cooling manifold  45  may flow to the outside of the fuel cell stack  40  via the first cooling plate group  42  and  44 , the fourth cooling manifold  52 , the connection manifold, the third cooling manifold  50 , the second cooling plate group  48 , and the second cooling manifold  46 . At this point, the second cooling manifold  46  is a working fluid outlet manifold. 
     Alternatively, if the connection manifold connects the third cooling manifold  50  to the first cooling manifold  45  instead of connecting the third cooling manifold  50  to the fourth cooling manifold  52 , the working fluid supplied to the third cooling manifold  50  may flow to the outside of the fuel cell stack  40  via the first cooling plate group  42  and  44  and the fourth cooling manifold  52  after moving to the first cooling manifold  45  via the connection manifold. At this point, the fourth cooling manifold  52  may be a working fluid outlet manifold. 
     Also, alternatively, the connection manifold may connect the third cooling manifold  50  to the first cooling manifold  45 , and the working fluid may inflow through the fourth cooling manifold  52  from the outside of the fuel cell stack  40 . In this case, the working fluid may flow sequentially through the fourth cooling manifold  52 , the first cooling plate groups  42  and  44 , the first cooling manifold  45 , the connection manifold, the third cooling manifold  50 , the second cooling plate group  48 , and the second cooling manifold  46 . At this point, the second cooling manifold  46  may be a working fluid outlet manifold. 
       FIG. 2  is a front view of a cooling plate of the first cooling plate group  42  connected to the first, second, and fourth cooling manifolds  45 ,  46 , and  52  of the fuel cell stack  40  of  FIG. 1 . Referring to  FIG. 2 , the cooling plate  42  includes a main plate  42 D having a structure through which a working fluid can flow, for example, a wick structure, and protrusion units  42 A,  42 B, and  42 C respectively formed on a lower right-end, an upper right-end, and a lower left-end of the main plate  42 D. In  FIG. 2 , arrows indicate directions of working fluid flow. 
       FIG. 3  is a front view of the cooling plate of the first cooling plate group  44  connected to the first, third and fourth cooling manifold  45 ,  50 , and  52  of the fuel cell stack  40  in  FIG. 1 . Referring to  FIG. 3 , the cooling plate  44  includes a main plate  44 D having, for example, a wick structure, and protrusion units  44 A,  44 B, and  44 C respectively formed on an upper left-end, an upper right-end, and a lower left-end of the main plate  44 D. In  FIG. 3 , arrows indicate directions of working fluid flow. 
       FIG. 4  is a front view of a cooling plate of the second cooling plate group  48  of  FIG. 1 . Referring to  FIG. 4 , the cooling plate  48  includes a main plate  48 D and protrusion units  48 A,  48 B,  48 C, and  48 E respectively formed on an upper right-end, a lower right-end, and an upper left-end, and a low left-end of the main plate  48 D. In  FIG. 4 , arrows indicate directions of working fluid flow. 
       FIG. 5  is a perspective view of a fuel cell stack  60  according to another embodiment. Referring to  FIG. 5 , the fuel cell stack  60  includes a plurality of cooling plates  62  and fifth through eighth cooling manifolds  64 ,  66 ,  67 , and  69 . The fifth and sixth cooling manifolds  64  and  66  are included within the fuel cell stack  60 . The fifth and sixth cooling manifolds  64  and  66  are respectively provided on and under the cooling plates  62 , and are separated from the cooling plates  62 . The seventh cooling manifold  67  is connected to upper left-ends of the cooling plates  62 . For this connection, protrusion units  62 A (refer to  FIG. 6 ) may be included on upper left-ends of the cooling plates  62 . The eighth cooling manifold  69  is connected to lower right-ends of the cooling plates  62 . For this connection, protrusion units  62 B (refer to  FIG. 6 ) may be included on lower right-ends of the cooling plates  62 . 
     A working fluid may be supplied to the fuel cell stack  60  from the outside through the fifth cooling manifold  64 . The working fluid may be supplied to the fuel cell stack  60  through the sixth cooling manifold  66  instead of through the fifth cooling manifold  64 . The fuel cell stack  60  may include both of the fifth and sixth cooling manifolds  64  and  66 , but one of the fifth and sixth cooling manifolds  64  and  66  may be omitted. In other words, one of the fifth and sixth cooling manifolds  64  and  66  is optional. If the fuel cell stack  60  includes only the fifth cooling manifold  64 , the working fluid may flow sequentially through the fifth cooling manifold  64 , the seventh cooling manifold  67 , the cooling plates  62 , and the eighth cooling manifold  69  as indicated by solid-line arrows. 
     When the fuel cell stack  60  includes both the fifth and sixth cooling manifolds  64  and  66 , the working fluid supplied to the fifth cooling manifold  64  flows sequentially through the sixth cooling manifold  66 , the eighth cooling manifold  69 , the cooling plates  62 , and the seventh cooling manifold  67  as indicated by dashed-line arrows. 
     In the above-described flow process, the working fluid absorbs heat generated from fuel cells included between the cooling plates  62  of the fuel cell stack  60  while passing through the fifth cooling manifold  64  or the fifth and sixth cooling manifolds  64  and  66 . Accordingly, the working fluid that passes through the fifth cooling manifold  64  or the fifth and sixth cooling manifolds  64  and  66  may be preheated. The preheated working fluid flows into the cooling plates  62  through the seventh cooling manifold  67  or the eighth cooling manifold  69 . Thus, the start-up time of the fuel cell stack  60  can be reduced by reducing the preheating time of the fuel cell stack  60 , and also the temperature deviation and the voltage deviation inside the fuel cell stack  60  can be reduced. 
     For the working fluid flow described above, a connection manifold may be included between the fifth and seventh cooling manifolds  64  and  67 . Also, a connection manifold may be included between the fifth and sixth cooling manifolds  64  and  66 . Also, a connection manifold may be included between the sixth and eighth cooling manifolds  66  and  69 . The connection manifolds may be included within end plates provided in the fuel cell stack  60  or outside the fuel cell stack  60 . 
       FIG. 6  is a cross-sectional view taken along a direction vertical to the y-axis direction of the fuel cell stack  60  of  FIG. 5 . Referring to  FIG. 6 , it is seen that the fifth and sixth cooling manifolds  64  and  66  respectively are included on and under the cooling plates  62 , and are separated from the cooling plates  62 . 
       FIG. 7  is a fuel cell stack  40   a  according to another embodiment. The fuel cell stack  40   a  in  FIG. 7  corresponds to the combined fuel cell stack  40  of  FIG. 1  and the fuel cell stack  60  of  FIG. 5 . Like reference numerals are used to indicate elements that are substantially identical to the elements of  FIGS. 1 and 5 , and thus the descriptions thereof will not be repeated. 
     Referring to  FIG. 7 , the fuel cell stack  40   a  further includes at least one of fifth and sixth cooling manifolds  64  and  66  besides the first through fourth cooling manifolds  45 ,  46 ,  50 , and  52 . If only the fifth cooling manifold  64  is included, a working fluid supplied to the fifth cooling manifold  64  flows sequentially through the fourth cooling manifold  52 , the first cooling plate group  42  and  44 , the first cooling manifold  45 , the second cooling manifold  46 , the second cooling plate group  48 , and the third cooling manifold  50 . Alternatively, the working fluid supplied to the fifth cooling manifold  64  may flow sequentially through the third cooling manifold  50 , the second cooling plate group  48 , the second cooling manifold  46 , the first cooling manifold  45 , the first cooling plate group  42  and  44 , and the fourth cooling manifold  52 . For the working fluid flow described above, the fuel cell stack  40   a  may include a connection manifold (not shown) that connects the fifth cooling manifold  64  to the fourth cooling manifold  52  or the third cooling manifold  50 . Also, the fuel cell stack  40   a  may include a connection manifold (not shown) that connects the first cooling manifold  45  to the second cooling manifold  46 . 
     When the fuel cell stack  40   a  includes both the fifth and sixth cooling manifolds  64  and  66 , a working fluid supplied to the fifth cooling manifold  64  may flow sequentially through the sixth cooling manifold  66 , the second cooling manifold  46 , the second cooling plate group  48 , the third cooling manifold  50 , the fourth cooling manifold  52 , the first cooling plate group  42  and  44 , and the first cooling manifold  45 . In this case, an inlet and an outlet of the working fluid may be on the same surface or on different surfaces of the fuel cell stack  40   a . If the working fluid flows into the sixth cooling manifold  66 , the working fluid may flow sequentially through the fifth cooling manifold  64 , the fourth cooling manifold  52 , the first cooling plate group  42  and  44 , the first cooling manifold  45 , the second cooling manifold  46 , the second cooling plate group  48 , and the third cooling manifold  50 . In this case, an inlet and an outlet of the working fluid may be on the same surface or on different surfaces of the fuel cell stack  40   a . Also, if the working fluid flows into the sixth cooling manifold  66 , the working fluid may flow sequentially through the fifth cooling manifold  64 , the third cooling manifold  50 , the second cooling plate group  48 , the second cooling manifold  46 , the first cooling manifold  45 , the first cooling plate group  42  and  44 , and the fourth cooling manifold  52 . Also, in this case, an inlet and an outlet of the working fluid may be on the same surface or on different surfaces of the fuel cell stack  40   a.    
       FIG. 8  is a cross-sectional view taken along a direction vertical to the y-axis direction of the fuel cell stack  40   a  of  FIG. 7 . Referring to  FIG. 8 , the fifth and sixth cooling manifolds  64  and  66  respectively are located on and under the first and second cooling plate groups  42 ,  44 , and  48  and are separated from the first and second cooling plate groups  42 ,  44 , and  48 . A working fluid may absorb heat generated from fuel cells (not shown) included between the first and second cooling plate groups  42 ,  44 , and  48  while passing through at least one of the fifth and sixth cooling manifolds  64  and  66 . Accordingly, the working fluid that flows through the fifth and sixth cooling manifolds  64  and  66  may be preheated. In this way, since the preheated working fluid flows in advance through one of the first and second cooling plate groups  42 ,  44 , and  48 , the preheated working fluid may prevent the temperature of the fuel cells from rapidly changing due to the inflow of a cold working fluid. Also, the slow increase in temperature of the fuel cells located on both ends of the fuel cell stack  40   a , that is, near the end plates, can be prevented. Also, since the heat generated from the fuel cell stack  40  is used, an additional heat source for heating the working fluid is unnecessary. Accordingly, the volume of the fuel cell stack  40   a  can be reduced. 
       FIG. 9  is a cross-sectional view showing another embodiment in which cooling plates and bipolar plates are symmetrically disposed about the center of a fuel cell stack.  FIG. 9  shows an example configuration of the fuel cell stack, and for convenience, first through eighth cooling manifolds  45 ,  46 ,  50 ,  52 ,  64 ,  66 ,  67 , and  69  described above are omitted in  FIG. 9 . 
     Referring to  FIG. 9 , the fuel cell stack includes first and second end plates  70  and  72 , first and second current collection plates  74  and  76 , a plurality of cooling plates  80  and  82 , and a plurality of bipolar plates  84  which are between the first and second end plates  70  and  72 . The first and second end plates  70  and  72  may include connection manifolds  70 A and  72 A described above. One of the first and second current collection plates  74  and  76  may be an anode plate and the other one may be a cathode plate. The first current collection plate  74  is connected to the first end plate  70 , and the second current collection plate  76  is connected to the second end plate  72 . The cooling plates  80  and  82  are arranged between the first and second current collection plates  74  and  76 . The cooling plates  80  correspond to the first cooling plate group, and the cooling plates  82  correspond to the second cooling plate group. The cooling plates  80  and  82  are separated from each other. The bipolar plates  84  are included between the cooling plates  80  and  82 . Also, the bipolar plates  84  are included between the first and second current collection plates  74  and  76  and the cooling plates  80 . The number of bipolar plates  84  included between the first and second current collection plates  74  and  76  and the cooling plates  80  is less than the number of bipolar plates  84  included between the cooling plates  80  and  82 . The number of bipolar plates  84  included between the cooling plates  80  and  82  may be equal to the number of bipolar plates  84  included between the cooling plates  82 . Also, the number of bipolar plates  84  included between the first and second current collection plates  74  and  76  and the cooling plates  80  may be equal. Under this consideration, it is seen that the first and second current collection plates  74  and  76 , the cooling plates  80  and  82 , and the bipolar plates  84  are symmetrically arranged about the center between the first and second end plates  70  and  72 . 
       FIG. 10  is a magnified cross-sectional view of a predetermined region  86  that includes a contact surface between two adjacent bipolar plates  84  of  FIG. 9 . Referring to  FIG. 10 , a fuel cell, that is, a membrane electrode assembly (MEA)  89  is included between two bipolar plates  84  which are adjacent to each other. The MEA  89  includes an anode  90 , a cathode  92 , and a membrane  94  included between the anode  90  and the cathode  92 . The bipolar plates  84  may be connected to the anode  90  and the cathode  92 . Although not shown in  FIG. 10 , a fuel supply path may be formed on a surface of the bipolar plates  84  facing the anode  90  and an oxygen supply path for supplying oxygen may be formed on a surface of the bipolar plates  84  facing the cathode  92 . 
       FIG. 11  is a cross-sectional view showing another embodiment in which constituent elements are asymmetrically disposed about the center of a fuel cell stack.  FIG. 11  shows an example configuration of a fuel cell stack and, for convenience, the first through eighth cooling manifolds  45 ,  46 ,  50 ,  52 ,  64 ,  66 ,  67 , and  69  described above are omitted in  FIG. 11 . 
     The basic configuration of the fuel cell stack of  FIG. 11  may be the same as that of the fuel cell stack of  FIG. 9 . However, in the fuel cell stack of  FIG. 11 , the number and arrangement of elements included between the first and second current collection plates  74  and  76  may differ from that of the fuel cell stack of  FIG. 9 . 
     Referring to  FIG. 11 , the number of cooling plates  80   a ,  80   b , and  80   c  corresponding to the first cooling plate group described above may be asymmetrical about the center of the fuel cell stack. In  FIG. 11 , an arrow indicates the direction of working fluid in flow. 
     More specifically, while two cooling plates  80   a  and  80   b  are near the first end plate  70 , that is, in a location close to the first current collection plate  74 , and one cooling plate  80   c  may be near the second end plate  72 , that is, in a location close to the second current collection plate  76 . The configuration and location relationship of the cooling plates  82  included between the first and second end plates  70  and  72  and the number of bipolar plates  84  between the cooling plates  82  may be the same as that of the fuel cell stack of  FIG. 9 . The number of bipolar plates  84  between the two cooling plates  80   a  and  80   b  included near the first end plate  70  and the number of bipolar plates  84  between the first current collection plate  74  and the cooling plate  80   b , for example, two, may be smaller than the number of bipolar plates  84  provided between the cooling plates  82  included in the center region between the first and second end plates  70  and  72 , that is, the center region of the fuel cell stack. Also, the number of bipolar plates  84  included between the second current collection  76  and the cooling plate  80   c  included near the second end plate  72  is less than the number of bipolar plates  84  included between the cooling plates  82  located in a central region of the fuel cell stack, but may be greater than the number of bipolar plates  84  included near the first end plate  70 . In other words, in both ends of the fuel cell stack, the number of fuel cells per cooling plate in the end-side (the right-end of the fuel cell stack) that does not have a working fluid inlet is larger than that in the end-side (the left-end of the fuel cell stack) that has a working fluid inlet. 
       FIG. 12  is a cross-sectional view showing another embodiment in which the number of bipolar plates between cooling plates is different from above inside the fuel cell stack.  FIG. 12  shows another example of arrangement of constituent elements in the fuel cell stack. 
     Referring to  FIG. 12 , the arrangement of cooling plates  80  and  82  between the first and second end plates  70  and  72  may be the same as the fuel cell stack of  FIG. 9 . However, the number of bipolar plates  84  between the cooling plates  80  and  82  may be different according to locations of the bipolar plates  84 . For example, the number of bipolar plates  84  between the cooling plates  80  and  82  is largest in the central region between the first and second end plates  70  and  72 , that is, in the central region of the fuel cell stack, and is gradually less towards the first and second end plates  70  and  72 , that is, both ends of the fuel cell stack. In other words, the number of fuel cells per cooling plates  80  and  82  is larger in the central region of the fuel cell stack than in both ends of the fuel cell stack. 
     In this arrangement, since the number of fuel cells near the first and second end plates  70  and  72  is small, the time for increasing the temperature of fuel cells near the first and second end plates  70  and  72  can be reduced. Accordingly, the start-up time of the fuel cell stack can be reduced. 
       FIGS. 13 through 29  are cross-sectional views showing methods of operating (circulating working fluid) a fuel cell stack, according to other embodiments of the present invention. In  FIGS. 13 through 29 , each of a plurality of horizontal lines corresponds to one of the first through fourth cooling manifolds  45 ,  46 ,  50 , and  52 . Vertical lines on both ends represent the first cooling plate group  42  and  44 , and vertical lines between the both ends represent the second cooling plate group  48 . Arrows on the horizontal lines and the vertical lines indicate directions of working fluid flow. 
     Referring to  FIG. 13A , a working fluid supplied to the fuel cell stack  40  via a second cooling manifold L 1 , which is a working fluid inlet manifold, may flow first through a portion of cooling plates and afterwards through the remaining portion of the cooling plates of the fuel cell stack  40 . 
     More specifically, a working fluid supplied to the fuel cell stack  40  may flow first through a second cooling plate group L 2  included in a central region R 1  of the fuel cell stack  40 . The second cooling manifold L 1  may be connected to working fluid inlets of cooling plates that constitute the second cooling plate group L 2 . When the second cooling manifold L 1  is used as a path for discharging the working fluid supplied to the fuel cell stack  40 , the working fluid inlet of the cooling plate may be a working fluid outlet of the cooling manifold. The working fluid that is moved along the second cooling plate group L 2  may flow along a third cooling manifold L 3 , and may flow to a first cooling manifold L 5  along a connection manifold L 4  provided on the outside of the fuel cell stack  40 . The working fluid moved to the first cooling manifold L 5  may flow to the outside of the fuel cell stack  40  along a fourth cooling manifold L 7  after moving upwards via first cooling plate group L 61  and L 62  located on both ends of the fuel cell stack  40 . The fourth cooling manifold L 7  may be a working fluid outlet manifold. The working fluid flow described above may be a working fluid flow in normal operation with a load, that is, operation under a load. 
     The flow of a working fluid may be reversed in a start-up operation of the fuel cell stack  40 . For example, a working fluid may be supplied through the fourth cooling manifold L 7  and may pass first through the first cooling plate group L 61  and L  62  located on both ends of the fuel cell stack  40 , and afterwards may pass through the second cooling plate group L 2  and be discharged to the outside of the fuel cell stack  40  through the second cooling manifold L 1 . In a start-up operation, the fourth cooling manifold L 7  may be a working fluid inlet manifold and the second cooling manifold L 1  may be a working fluid outlet manifold. A working fluid supplied through the fourth cooling manifold L 7  may be working fluid preheated by a preheating device at the outside of the fuel cell stack  40 . 
     In a start-up operation of the fuel cell stack  40 , if there is a large temperature deviation between cooling plates of the first cooling plate group L 61  and L 62  included on both ends of the fuel cell stack  40 , for example, if the temperature of a region where the cooling plates of the first cooling plate group L 61  are located is lower than that of a region where the cooling plates of the first cooling plate group L 62  are located, the flow of a working fluid preheated for start-up may be controlled to flow first through the cooling plates L 61  and then through the cooling plates of the first cooling plate group L 62 . 
     When a start-up operation and a load operation are considered, the third cooling manifold L 3 , the connection manifold L 4 , and the first cooling manifold L 5  may form a working fluid-moving manifold that moves (or supplies) a working fluid discharged from one selected portion (for example, the second cooling plate group L 2 ) to the remaining portion (for example, the first cooling plate group L 61  and L 62 ) of the cooling plates. 
     The working fluid supplied to the fuel cell stack  40  may be preheated while circulating in the fuel cell stack  40 . For example, as depicted in  FIGS. 5 and 7 , in the case of fuel cell stack  60  or  40   a  in which the fifth cooling manifold  64  and/or sixth cooling manifold  66  are included, a working fluid that passes through the fifth cooling manifold  64  and/or the sixth cooling manifold  66  in a start-up operation may be preheated by heat generated from fuel cells included in the fuel cell stack  60  or  40   a . This case may be applied to other embodiments described below. 
     Alternatively, when the temperature deviation between fuel cells is increased due to the increase in the number of unit fuel cells in the fuel cell stack  40 , that is, the increase in the number of bipolar plates, the flow of the working fluid may be controlled to pass first through the cooling plates adjacent to fuel cells having a large temperature deviation among the cooling plates of the second cooling plate group L 2  included in the central region of the fuel cell stack  40 . 
     For example, referring to  FIG. 13B , a working fluid supplied through the second cooling manifold L 1  may flow first through a central region R 1  of the fuel cell stack  40 . However, among the cooling plates in the central region R 1  of the fuel cell stack  40 , the working fluid may be supplied first to the four cooling plates located in the center of the central region R 1  of the fuel cell stack  40 . The working fluid that is supplied first to the four cooling plates in the central region R 1  of the fuel cell stack  40  may be supplied to the rest of the cooling plates located in the central region R 1  of the fuel cell stack  40  after passing through a cooling manifold LL 1 , a connection manifold LL 2 , and a cooling manifold LL 3 . The working fluid supplied to the rest of the cooling plates located in the central region R 1  of the fuel cell stack  40  flows to the first cooling plate group L 61  and L 62  through cooling manifolds L 51  and L 52 . The working fluid that moved to the first cooling plate group L 61  and L 62  may be discharged to the outside of the fuel cell stack  40  through the fourth cooling manifold L 7 . 
     The method of moving a working fluid, in which the working fluid is sequentially supplied to both ends of the fuel cell stack  40  and to the central region R 1  of the fuel cell stack  40  or vice versa, may be applied to methods of operating a working fluid according to another embodiment of the present invention. 
     When a start-up operation and a load operation are considered, in  FIG. 13B , the cooling manifolds LL 1  and LL 3  and the connection manifold LL 2  may form a manifold that moves or resupplies the working fluid discharged from one selected portion of the cooling plates and to the rest portion of the cooling plates located in the central region R 1  of the fuel cell stack  40 . Also, the cooling manifolds L 51  and L 52  may form a manifold that moves or resupplies the working fluid discharged from selected cooling plates in the central region R 1  of the fuel cell stack  40  and to the cooling plates of the first cooling plate group L 61  and L 62  located on both ends of the fuel cell stack  40 . Also, the fourth cooling manifold L 7  may be a working fluid outlet manifold or a working fluid inlet manifold. 
     The description of cooling manifolds described above may be applied to the following embodiments.  FIG. 14  shows a case in which a working fluid flows sequentially through cooling plate group. Referring to  FIG. 14 , a working fluid may inflow to a cooling manifold, for example, a second cooling manifold L 1  below the first and second cooling plate groups L 61 , L 62 , and L 2 , may flow along the second cooling plate group L 2 , may flow to the right inside the fuel cell stack  40  along a cooling manifold above the second cooling plate group L 2 , for example, the third cooling manifold L 3 , and may flow downwards along the cooling plates of the first cooling plate group L 62  located on the right-hand end of the inside of the fuel cell stack  40 . Next, the working fluid may flow to the left side of the inside of the fuel cell stack  40  along the first cooling manifold L 5 , and then, may flow upwards along the cooling plate L 61  located on the left-end of the inside of the fuel cell stack  40 , and afterwards, may be discharged to the left side of the fuel cell stack  40  as shown in  FIG. 14 . 
     In the working fluid flow depicted in  FIGS. 13A ,  13 B, and  14 , the temperature of the working fluid increases while passing through the second cooling plate group L 2  of the central region R 1  of the fuel cell stack  40 . Since the working fluid having an increased temperature flows along the first cooling plate group L 61  and L 62  located on both ends of the inside of the fuel cell stack  40 , during start-up of the fuel cell stack  40 , the start-up time of the fuel cell stack  40  can be reduced by shortening the time for increasing the temperature of fuel cells located on both ends of the inside of the fuel cell stack  40 , that is, near the first and second end plates  70  and  72  (refer to  FIG. 9 ). Also, a stable temperature range of, for example, about 140 to about 160° C. can be maintained during normal operation with a load. 
     Referring to  FIG. 15 , a working fluid inflows from the left side of the fuel cell stack  40  to a cooling manifold, for example, the second cooling manifold L 1  below the first and second cooling plate groups L 61 , L 62 , and L 2 , flows along the second cooling plate group L 2 , flows to the left and right sides of the fuel cell stack  40  along a manifold, for example, the third cooling manifold L 3  above the second cooling plate group L 2 , and flows downwards along the first cooling plate group L 61  and L 62  located on left and right-ends in the fuel cell stack  40 , and afterwards, is discharged to the right side of the fuel cell stack  40  through the first cooling manifold L 5  as shown in  FIG. 15 . Accordingly, the working fluid inlet and the working fluid outlet are located on opposite sides of the fuel cell stack  40 . 
     Referring to  FIG. 16 , the working fluid inflows to the second cooling manifold L 1 , flows along the second cooling plate group L 2 , flows to the left side of the inside of the fuel cell stack  40  along the third cooling manifold L 3  on the second cooling plate group L 2 , and flows downwards along the cooling plate L 61  located on a left-end of the inside of the fuel cell stack  40 . Next, the working fluid flows to the right side of the inside of the fuel cell stack  40  along the first cooling manifold L 5  and flows upwards along the cooling plate L 62  located on a right-hand end of the inside of the fuel cell stack, and afterwards, is discharged to the right side of the fuel cell stack  40  as shown in  FIG. 16 . The working fluid inlet and the working fluid outlet are located on opposite sides of the fuel cell stack  40  and have different vertical locations from each other. 
     Referring to  FIG. 17 , the working fluid inflows from the left side of the fuel cell stack  40  to the second cooling manifold L 1 , flows along the second cooling plate group L 2 , flows to left and right sides of the inside of the fuel cell stack  40  along a manifold, for example the third cooling manifold L 3  located above the second cooling plate group L 2 , and flows downwards along the first cooling plate group L 61  and L 62  located on both left and right-ends of the inside of the fuel cell stack  40 . Afterwards, as shown in  FIG. 17 , the working fluid is discharged to the left side of the fuel cell stack  40  through the first cooling manifold L 5 . Accordingly, the working fluid inlet and the working fluid outlet are located on the same surface of the fuel cell stack  40 . 
     Referring to  FIG. 18 , the composition of a fuel cell stack and the flow of a working fluid may be basically the same as that described with reference to  FIG. 14 . However, in  FIG. 18 , a flow controller  96  is included on the working fluid flow path between the first cooling plate group L 61  and L 62  and the second cooling plate group L 2 . The flow controller  96  is placed between the right-end of the third cooling manifold L 3  and the upper end of the cooling plate L 62  located on the right-hand end of the inside of the fuel cell stack  40 . Accordingly, the amount of working fluid that flows into the first cooling plate group L 61  and L 62  located on both ends of the inside of the fuel cell stack  40  from the third cooling manifold L 3  may be controlled by the flow controller  96 . In the case of the flow of  FIG. 18 , the flow rate of the total amount of working fluid may be controlled by the flow controller  96  since the working fluid flows sequentially through the first cooling plate group L 62  and L 61 . The flow controller  96  may be, for example, an electronic proportional valve, a servo valve, a 3-way valve, or an ON/OFF valve. 
     Referring to  FIG. 19 , the working fluid inflows to the second cooling manifold L 1  and flows to the second cooling plate group L 2 . Next, the working fluid flows through the third cooling manifold L 3  and a flow controller  96  provided on the left side of the fuel cell stack  40 . A portion of the working fluid that passed through the flow controller  96  is discharged to the outside of the fuel cell stack  40  through the cooling plate L 61  located on the left-end of the inside of the fuel cell stack  40  and the first cooling manifold L 5 , and the other portion of the working fluid is discharged to the outside of the fuel cell stack  40  through the fourth cooling manifold L 7  and the cooling plate L 62  located on the right-end of inside of the fuel cell stack  40 . Accordingly, the working fluid inlet and the working fluid outlet are located on opposite sides of the fuel cell stack  40 . 
     Referring to  FIG. 20 , the working fluid inflows to the second cooling manifold L 1 , flows to the second cooling plate group L 2 , and then, passes through the third cooling manifold L 3  and a flow controller  96  provided on the left side of the fuel cell stack  40 . The working fluid that passed through the flow controller  96  flows through the cooling plate of the first cooling plate group L 61  located on the left-hand end of the inside of the fuel cell stack  40  and the first cooling manifold L 5 , and afterwards, is discharged to the right side of the fuel cell stack  40  through the cooling plate L 62  located on the right-hand end of the inside of the fuel cell stack  40 . In this case, the working fluid inlet and the working fluid outlet are located on opposite sides and have different vertical positions from each other. 
       FIG. 21  has a basic configuration that is similar to that of  FIG. 19 . However, the location of the flow controller  96  and the discharge direction of the working fluid are different from the configuration of  FIG. 19 . 
     Referring to  FIG. 21 , the working fluid that flows in the right direction through the third cooling manifold L 3  passes through a flow controller  96  provided on the right side of the fuel cell stack  40 . A portion of the working fluid that passed through the flow controller  96  is discharged to the left side of the inside of the fuel cell stack  40  through the cooling plate L 62  located on the right-end of the fuel cell stack  40  and the first cooling manifold L 5 . The other portion of the working fluid is discharged to the left side of the fuel cell stack  40  through the fourth cooling manifold L 7  and the cooling plate L 61  located on the left side of the inside of the fuel cell stack  40 . Accordingly, the working fluid inlet and the working fluid outlet are located on the same surface of the fuel cell stack  40 . 
       FIGS. 22 through 25  show cases of dividing working fluid streams using a flow controller. Referring to  FIG. 22 , the working fluid inflows to the second cooling manifold L 1  and flows through the second cooling plate group L 2  and the third cooling manifold L 3 . The working fluid that passed through the third cooling manifold L 3  is divided into two streams in directions different from each other. One of the streams flows in a direction towards the outside of the fuel cell stack  40  through the cooling plate L 62  located on the right-hand end of the inside of the fuel cell stack  40  and the other stream flows in a direction towards the left side of the fuel cell stack  40  where the working fluid is discharged to the outside of the fuel cell stack  40  through the flow controller  98 , a connection manifold L 8  which is a working fluid flow path formed on the outside of the fuel cell stack  40 , and the cooling plate L 61  located on a left-end of the inside of the fuel cell stack  40 . The connection manifold L 8  connects the flow controller  98  to the cooling plate L 61  located on the inside of the fuel cell stack  40 . The flow controller  98  may be the same as that of  FIGS. 18 through 21 . When the temperature of the cooling plate L 62  located on the right-hand end is lower than that of the cooling plate L 61  located on the left-hand end of the inside of the fuel cell stack  40 , by controlling the flow controller  98 , the amount of working fluid supplied to the cooling plate L 62  may be greater than that supplied to the connection manifold L 8 . In this way, temperature distribution in the fuel cell stack  40  can be maintained uniform, and, in particular, the start-up time of the fuel cell stack  40  can be reduced by increasing the temperature of fuel cells located near the first cooling plate group L 61  and L 62 , and in a normal operation, all fuel cells in the fuel cell stack  40  can be maintained in a predetermined stable temperature range. 
     Referring to  FIG. 23 , the working fluid inflows to the second cooling manifold L 1  and flows through the second cooling plate group L 2  and the third cooling manifold L 3 . The working fluid that passed through the third cooling manifold L 3  is divided into two streams in directions different from each other. One of the streams flows in a direction to be discharged towards the right side of the fuel cell stack  40  through the cooling plate L 62  located on the right-hand end of the inside of the fuel cell stack  40 , the other stream flows in a direction towards the right side of the fuel cell stack  40  where the working fluid is discharged to the outside of the fuel cell stack  40  through the flow controller  98 , the fourth cooling manifold L 7 , and the cooling plate L 61  located on a left-end of inside of the fuel cell stack  40 , and the first cooling manifold L 5 . In this embodiment, the two streams are discharged in one direction by reuniting in the course of a discharging process. 
     Referring to  FIG. 24 , the working fluid inflows to the second cooling manifold L 1  and flows towards the left side in the fuel cell stack  40  through the second cooling plate group L 2  and the third cooling manifold L 3 . The working fluid that passed through the third cooling manifold L 3  is divided into two streams in directions different from each other. One of the streams flows in a direction towards the left side of the fuel cell stack  40  where the working fluid is discharged to the outside of the fuel cell stack  40  through the cooling plate L 61  located on the left-hand end of the inside of the fuel cell stack  40 , and the other stream flows in a direction towards the right side of the fuel cell stack  40  where the working fluid is discharged to the outside of the fuel cell stack  40  through the flow controller  98  provided on the left side of the fuel cell stack  40 , the connection manifold L 9  formed on the outside of the fuel cell stack  40 , and the cooling plate L 62  located on the right-hand end of the inside of the fuel cell stack  40 . The connection manifold L 9  connects the flow controller  98  to the cooling plate L 62  located on the right-hand end of the inside of the fuel cell stack  40 . 
     Referring to  FIG. 25 , a working fluid inflows to the second cooling manifold L 1  and flows towards the left side in the fuel cell stack  40  through the second cooling plate group L 2  and the third cooling manifold L 3 . The working fluid that passed through the third cooling manifold L 3  is divided into two streams in directions different from each other. One of the streams flows in a direction towards the left side of the fuel cell stack  40  where the working fluid is discharged to the outside through the cooling plate L 61  located on the left-hand end of the inside of the fuel cell stack  40 , and the other stream flows in a direction towards the left side of the fuel cell stack  40  where the working fluid is discharged to the outside of the fuel cell stack  40  through the flow controller  98 , the fourth cooling manifold L 7 , the cooling plate L 62  located on the right-hand end of the inside of the fuel cell stack  40 , and the first cooling manifold L 5 . In this embodiment, the two streams are discharged in one direction by reuniting in the course of the discharge process. 
       FIGS. 26 and 27  respectively show the controlling of a working fluid divided into two streams in directions different from each other by using two flow controllers. Referring to  FIG. 26 , the working fluid inflows to the second cooling manifold L 1  and flows towards the right side of the fuel cell stack  40  through the second cooling plate group L 2  and the third cooling manifold L 3 . At this point, the third cooling manifold L 3  may extend to the right-outside of the fuel cell stack  40 . The working fluid that passed through the third cooling manifold L 3  is divided into two streams in directions different from each other. One of the streams flows in a direction towards the right side of the fuel cell stack  40  where the working fluid is discharged to the outside via the first flow controller  100  and the cooling plate L 62  located on the right-hand end of the inside of the fuel cell stack  40 , and the other stream flows in a direction towards the left side of the fuel cell stack  40  where the working fluid is discharged to the outside via the second flow controller  102 , the connection manifold L 8  formed on the outside of the fuel cell stack  40 , and the cooling plate L 61  located on the left-end of inside of the fuel cell stack  40 . The connection manifold L 8  connects the second flow controller  102  to the cooling plate L 61 . The first and second flow controllers  100  and  102  may be the same types as the flow controller  96  of  FIGS. 18 through 21 . Since the fuel cell stack  40  includes the first and second flow controllers  100  and  102 , the working fluid streams divided into two directions can be independently controlled. 
     Referring to  FIG. 27 , the working fluid inflows to the second cooling manifold L 1  and flows towards the left side of the fuel cell stack  40  through the second cooling plate group L 2  and the third cooling manifold L 3 . In this embodiment, the third cooling manifold L 3  may extend to the left-outside of the fuel cell stack  40 . The working fluid that passed through the third cooling manifold L 3  is divided into two streams in directions different from each other. One of the streams flows in a direction towards the left side of the fuel cell stack  40  where the working fluid is discharged to the outside via the first flow controller  100  and the cooling plate L 61  located on a left-end of inside of the fuel cell stack  40 , and the other stream flows in a direction towards the right side of the fuel cell stack  40  where the working fluid is discharged to the outside via the second flow controller  102 , the connection manifold L 9  located on the outside of the fuel cell stack  40 , and the cooling plate L 62  located on the right-hand end of the inside of the fuel cell stack  40 . The connection manifold L 9  connects the second flow controller  102  to the cooling plate L 62  located inside of the fuel cell stack  40 . 
       FIGS. 28 and 29  show the flow of a working fluid in a fuel cell stack and a heat exchanger on the outside thereof. Referring to  FIG. 28 , the working fluid inflows to the second cooling manifold L 1  through the heat exchanger  110 . The working fluid is discharged to the outside through the heat exchanger  110  after passing through the second cooling plate group L 2 , the third cooling manifold L 3 , the fourth cooling manifold L 7 , and the first cooling plate group L 61  and L 62  and the first cooling manifold L 5 . The third cooling manifold L 3  and the fourth cooling manifold L 7  may protrude to the outside of the fuel cell stack  40 . Portions of the third cooling manifold L 3  and the fourth cooling manifold L 7  protruding to the outside of the fuel cell stack  40  may be connected to each other at the outside of the fuel cell stack  40 . The working fluid that flows to the first cooling plate group L 61  and L 62  via the fourth cooling manifold L 7  is in a heated state from heat generated in the cell stack  40 . Accordingly, the working fluid being discharged to the outside through the heat exchanger  110  has a temperature higher than that of the working fluid that flows into the second cooling manifold L 1  through the heat exchanger  110 . Therefore, heat exchange occurs between the working fluid that is discharged to the outside through the heat exchanger  110  and the working fluid that flows into the heat exchanger  110 . Accordingly, the temperature of the working fluid that flows into the second cooling manifold L 1  is increased, thereby minimizing the temperature instability in a normal operation. 
     Referring to  FIG. 29 , the working fluid flows into the second cooling manifold L 1  through the heat exchanger  110 , and flows again through the heat exchanger  110  via the second cooling plate group L 2  and the third cooling manifold L 3 . In this embodiment, heat exchange can be achieved between the working fluid that flows into the heat exchanger  110  through the third cooling manifold L 3  and the working fluid that has a relatively low temperature and passes through the heat exchanger  110  in order to be supplied to the second cooling manifold L 1 . The working fluid that passes through the third cooling manifold L 3  and the heat exchanger  110  is discharged to the right-outside of the fuel cell stack  40  via the first cooling manifold L 5 , the first cooling plate group L 61  and L 62 , and the fourth cooling manifold L 7 . The third cooling manifold L 3  may protrude to the left-outside of the fuel cell stack  40 . Also, the first cooling manifold L 5  may protrude to the left-outside of the fuel cell stack  40  where the heat exchanger  110  is installed. The first cooling manifold L 5  may be connected to an extended portion L 31  of the third cooling manifold L 3  via the heat exchanger  110 . 
     Simulation results with respect to a fuel cell stack and a method of operating the fuel cell stack will now be described. In the simulations, a surface heater and two sheets of carbon paper were used instead of an MEA of the fuel cell stack. The fuel cell stack includes a total of 48 cells, and a cooling plate is included every six cells. Also, the simulation was designed such that the temperature of working fluid was increased by using a 900 W external heater and heat generated during normal operation with a load was replaced by heat generated from the surface heater. In order to prove the superiority of the fuel cell stack and the method of operating the fuel cell stack according to an embodiment of the present invention, the working fluid was allowed to flow in a method according to the current embodiment, for example, the method described with reference to  FIG. 13 . 
       FIG. 30  is a graph showing the temperature distribution in a fuel cell stack when a working fluid was allowed to flow according to a conventional method and a method according to an embodiment. In  FIG. 30 , the curve indicated by open circles, o, represents the simulation result of the conventional method, and a graph indicated by closed squares, ▪ represents the simulation result of the method according to an embodiment of the present invention. The X-axis represents positions of bipolar plates, for example, “3” on the X-axis indicates the third bipolar plate when bipolar plates and cooling plates were arranged from one end of the fuel cell stack to the opposite end of the fuel cell stack. “10c” and “16c”, etc. on the X-axis indicates positions of cooling plates located between bipolar plates. For example, “10c” denotes a cooling plate positioned 10 th  sequence in a complete arrangement that includes bipolar plates and cooling plates. The Y-axis represents temperatures as a function of the positions of the bipolar plates and cooling plates. The same descriptions of the X-axis and Y-axis apply to the graphs of  FIGS. 31 through 33 . 
     When the graphs in  FIG. 30  are compared, the temperature difference between the maximum temperature and the minimum temperature was approximately 14° C. in the method of operating a fuel cell stack according to the current embodiment, but that in the conventional method was approximately 29° C. The standard deviation of temperature in the method of operating a fuel cell stack according to the current embodiment was approximately 3.1° C. while that in the conventional method was approximately 6.7° C. Also, in the method of operating a fuel cell stack according to the current embodiment, the temperature of both ends of inside of the fuel cell stack, that is, near end plates, was higher than that in the conventional method. 
     Table 1 summarizes the simulation results. 
     
       
         
           
               
               
               
             
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 Conventional 
                 Current 
               
               
                   
                 method 
                 embodiment 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
            
               
                   
                 Maximum temperature (° C.) 
                 155 
                 152 
               
               
                   
                 Minimum temperature (° C.) 
                 126 
                 138 
               
               
                   
                 Maximum − minimum (° C.) 
                 29 
                 14 
               
               
                   
                 Standard deviation (° C.) 
                 6.7 
                 3.1 
               
               
                   
                   
               
            
           
         
       
     
     As seen in Table 1, the method of operating a fuel cell stack according to the current embodiment had a smaller temperature difference between maximum and minimum temperatures in a fuel cell stack and had a smaller standard deviation when compared to the conventional method. These results indicate that the temperature distribution in a fuel cell stack and in a method of operating the fuel cell stack according to the current embodiment is much more uniform and stable than in a conventional method. 
       FIG. 31  is a graph showing temperature change in a fuel cell stack as a function of elapsed time in a conventional method, and  FIG. 32  is a graph showing temperature change in a fuel cell stack as a function of elapsed time in a method of operating a fuel cell stack according to the current embodiment. Referring to  FIG. 31 , the temperature of cells near end plates was approximately 78° C. at an elapsed time of 30 minutes in the conventional method. 
     However, referring to  FIG. 32 , in the method of operating a fuel cell stack according to the current embodiment, the temperature of cells near end plates was approximately 90° C. That is, the temperature of cells near end plates was improved by approximately 12° C. in the method according to the current embodiment when compared to the conventional method. 
     Also, when  FIG. 31  and  FIG. 32  are compared, it can be seen that the time for all of the bipolar plates in a fuel cell stack to reach 100° C. was shorter in the method according to the current embodiment than in the conventional method. Also, in connection with the temperature of cells located between cooling plates included on both ends within the fuel cell stack, in the case of the conventional method, the temperature distribution had a parabolic shape at each elapsed time. However, in the method according to the current embodiment, it is seen that all cells had a uniform temperature distribution. 
       FIG. 33  is a graph showing the temperature distribution in a fuel cell stack according to the temperature variation of a working fluid that is supplied through a heat exchanger  110  as in the case of  FIGS. 28 and 29 . 
     In  FIG. 33 , the first through fourth curves G 1 , G 2 , G 3 , and G 4  respectively represent the results of temperature distributions when the working fluid that inflowed to the fuel cell stack had temperatures of 73, 88, 104, and 122° C., respectively. 
     When the first through fourth curves G 1 , G 2 , G 3 , and G 4  are compared, it can be seen that the average temperature increased as a function of the increase in the temperature of the working fluid that inflowed. Also, the difference between the maximum and minimum temperatures in the method according to the current embodiment was less than that in the conventional method. 
     A fuel cell stack and a method of operating the same which reduce a temperature deviation according to a position in a stack by using heat generated in the stack when the stack operates has been described. The stack for reducing the temperature deviation in the stack and the method of operating the stack are not limited to a fuel cell stack and may be applied to a heat source that generates heat. For example, the afore-described stack and method may be applied to a heat source having any of various stacks and a method of operating the heat source. In addition, the stack and the method of operating the stack may also be applied to a battery pack that is an example of a heat source. 
     Although a few embodiments of the present invention have been shown and described, it would be appreciated by those skilled in the art that changes may be made in this embodiment without departing from the principles and spirit of the invention, the scope of which is defined in the claims and their equivalents.