Abstract:
A secondary battery has: a box body having a heat insulating structure, the box body having an opening on the upper surface thereof and an assembled battery housed therein; a lid body having a heat insulating structure, the lid body sealing the opening of the box body; and a duct which is installed at least between the box body and the lid body and inside which a fluid circulates.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a Continuation of International Application No. PCT/JP2014/077542 filed on Oct. 16, 2014, which is based upon and claims the benefit of priority from Japanese Patent Application No. 2013-216123 filed on Oct. 17, 2013, the contents all of which are incorporated herein by reference. 
     
    
     TECHNICAL FIELD 
       [0002]    The present invention relates to a secondary battery having a cooling apparatus. 
       BACKGROUND ART 
       [0003]    In general, frequency adjustment in an electric power system and adjustment of power demands and power supplies in the electric power system are carried out using a plurality of power generators, storage batteries, etc., equipped in the electric power system. Further, in most cases, adjustment in the difference between the generated electric power from natural energy based power generators and its planned output electric power, and reduction in the changes of electric power generated by the natural energy based power generators are also performed using the power generators, storage batteries, etc. In comparison with general electric power generators, the storage batteries can change the electric power output at high speed, and can be used effectively in frequency adjustment of the electric power system, adjustment of the difference between the generated electric power from natural energy based power generators and its planned output electric power, and adjustment of power demands and power supplies in the electric power system. 
         [0004]    In this regard, as a storage battery operated at high temperature connected to the electric power system, a sodium-sulfur battery (hereinafter referred to as the NaS battery) is used, for example. This NaS battery is a secondary battery containing metal sodium and sulfur as active materials in an isolated manner using a solid electrolyte tube. When the NaS battery is heated at high temperature of about 300° C., a certain amount of energy is generated by an electrochemical reaction of both of the melted active materials of these metal sodium and sulfur. Normally, the NaS battery is formed by assembling a plurality of battery cells upright, and used in a form of a module battery including a plurality of battery cells connected together. That is, the module battery has structure where circuits (strings) each formed by connecting a plurality of battery cells in series are connected in parallel to form a block, and at least two blocks are connected in series, and placed in a heat insulating container. 
         [0005]    In use of the NaS battery, a plurality of heat insulating containers are stacked (in stages) to form one module string, and a plurality of the module strings are arranged in parallel to form one package. A control device for controlling each module battery of the package is provided (for example, see Japanese Laid-Open Patent Publication No. 2004-055373). 
         [0006]    According to the disclosure of Japanese Laid-Open Patent Publication No. 2004-055373, as a method of adjusting the temperature in a package, an air intake opening connected to the outside is provided in a lower portion or a side portion of the package, an exhaust opening connected to the outside is provided in an upper portion of the package, and the opening ratio of the exhaust opening is changed. 
       SUMMARY OF INVENTION 
       [0007]    In the case where the load on the electric power system is high, e.g., in summer season or in winter season, since the electric discharge output is high or the electric discharge time period is long, the amount of heat generated in the NaS battery may exceed the heat loss of the vacuum heat insulating container containing the NaS battery, and heat may be accumulated in the vacuum heat insulating container undesirably. If heat is accumulated due to the imbalance between the heat generation and the heat loss, the internal temperature of the heat insulating container of the NaS battery is increased excessively. If the temperature exceeds 360° C. while the NaS battery has battery capacity, malfunction may occur disadvantageously. In this regard, for example, it may be considered to adopt vacuum heat insulating structure for the box body containing the battery assembly, and adopt atmospheric heat insulating structure for the lid body closing the opening of the box body, to avoid heat accumulation in the heat insulating container. 
         [0008]    On the other hand, in the case where the load on the electric power system is low, e.g., in spring season or in autumn season, and for example, at the time of leveling power generation such as wind force power generation, since the electric discharge time period is short, the amount of heat generated in the NaS battery may drop below the heat loss of the heat insulating container. In this case, in order to keep operation of the NaS battery in the desired state, energy consumption of the heater used for maintaining the temperature of the battery module may become excessive. If energy consumption of the heater becomes excessive, the charging/discharging efficiency representing the ratio of the amount of discharging electric energy to the amount of consumed electrical energy of the NaS battery (amount of charging electrical energy+ amount of heater electrical energy) is degraded undesirably. In this regard, it may be considered to adopt vacuum heat insulating structure for both of the box body and the lid body of the heat insulating container, and utilize the accumulated heat in the heat insulating container to suppress degradation in the charging/discharging efficiency. 
         [0009]    As described above, for the optimum operation of the secondary battery, it may be considered to change the heat insulating structure of the heat insulating container depending on the level of the load. However, adoption of such a scheme for every heat insulating container is difficult, and unrealistic. 
         [0010]    The present invention has been made to take the problems of this type into account, and an object of the present invention is to provide a secondary battery which makes it possible to perform optimum operation without changing physical heat insulating structure of the heat insulating container depending on the level of the load. 
         [0000]    [1] A secondary apparatus according to the present invention includes a box body, a lid body, and a duct. The box body has heat insulating structure. An opening is formed in an upper surface of the box body. The box body contains a battery assembly formed by a plurality of battery cells. The lid body has insulating structure closing the opening of the box body. The duct is provided at least between the box body and the lid body, and configured to allow fluid to flow through the duct. That is, in the present invention, the duct is provided, and the fluid is taken from the outside into the duct, the fluid flows through the duct, and the fluid is discharged to the outside from the duct. The outside herein means the outside of the secondary battery. 
         [0011]    Firstly, in the case where the load on the electric power system is low, e.g., in spring season and in autumn season, and for example, at the time of leveling power generation such as wind force power generation, since the electric discharge time period is short, in conventional approaches, occasionally, the amount of heat generated in the NaS battery dropped below the heat loss of the heat insulating container. In this case, the charging/discharging efficiency may be lowered undesirably. However, in this secondary battery, since both of the box body and the lid body have insulation structure, heat accumulated in the box body can be utilized. Even if heater electrical energy is used, energy consumption is small, and degradation in the charging/discharging efficiency can be suppressed. 
         [0012]    On the other hand, in the case where the load on the electric power system is high, e.g., in summer season or in winter season, since the electric discharge output is high or the electric discharge time period is long, the amount of heat generated in the NaS battery may exceed the heat loss of the secondary battery undesirably. In particular, in the secondary battery, since both of the box body and the lid body have insulation structure with high heat insulation performance, the internal temperature of the box body may be increased excessively by the heat accumulated in the box body. However, this secondary battery has the duct provided at least between the box body and the lid body for allowing fluid to flow through the duct. Therefore, the cooled fluid from the outside of the secondary battery flows through the duct in the secondary battery, and absorbs the heat. Further, the hot fluid is released to the outside of the secondary battery, and the heat in the box body is released to the outside of the box body. That is, the heat in the box body is released. Therefore, even in the case where the container has high heat insulating property like a container including a box body and a lid body both having vacuum heat insulating structure, the inside of the box body is cooled efficiently. Consequently, even if the electric discharge output is high, or electric discharge time period is long, the temperature in the box body can be maintained in a predetermined temperature range, and it becomes possible to operate the battery assembly in the box body in the optimum operating environment. 
         [0000]    [2] In the present invention, the duct may be made of metal, and the secondary battery may include a plate member at least having electrically insulating property provided between the battery assembly and the duct. In this case, even if the duct is made of metal, it is possible to prevent contact between the duct and the battery assembly, and avoid short circuiting among the battery cells.
 
[3] In the present invention, the duct may be provided under a lower surface of the lid body, and sand may be provided between the duct and the battery assembly. In the structure, thermal conductivity of the sand is low. Since influence of the temperature of the fluid in the duct is absorbed when the heat is transmitted to the battery assembly by conduction of the heat in the height direction, the temperature in the height direction of the battery cells in the battery assembly becomes uniform to a greater extent.
 
[4] In the present invention, sand may be filled in a gap between the box body and the battery assembly, and between the box body and the lid body, and the duct may include a plurality of fins extending toward at least the battery assembly. The types of the fins include, for example, plain plate fins, corrugated fins (wavy fins), interrupted fins, etc.
 
         [0013]    In this case, even if the sand is filled between the box body and the lid body, it is possible to enhance conduction of the heat from the battery assembly to the duct, and efficiently lower the temperature in the box body. Further, by attaching the fins, the structural rigidity of the duct becomes high, and it is possible to prevent thermal deformation due to the box body having high temperature and the fluid having low temperature in the channel. Therefore, in the duct, it is possible to maintain the preferred height range over the entire channel of a portion (heat transportation section) for transporting the heat at least generated in the box body together with the fluid. 
         [0000]    [5] In the present invention, the duct includes a metal fluid inlet section into which the fluid is supplied, a metal heat transportation section provided downstream of the fluid inlet section, between the lid body and the box body, and configured to transport heat generated at least in the box body together with the fluid, and a metal heat releasing section provided downstream of the heat transportation section, and configured to release the heat to outside together with the fluid. 
         [0014]    In the structure, the fluid supplied into the heat transportation section through the fluid inlet section flows between the box body and the lid body. Normally, the heat generated in the battery assembly is transmitted upward (toward the lid body). Therefore, after the heat is transmitted upward, the heat is transported toward the downstream side together with the fluid flowing through the duct, and the heat is released to the outside of the box body through the heat releasing section. 
         [0000]    [6] In this case, the lid body may include a ceiling wall and an eaves, and the lid body may be provided to close the opening formed in the upper surface of the box body. The fluid inlet section may be provided along a first side wall of the box body, and oriented between the eaves of the lid body and the first side wall of the box body. The heat transportation section may be provided between the ceiling wall of the lid body and the box body. Further, the heat releasing section may be provided from a position between a second side wall of the box body opposite to the first side wall and the eaves of the lid body along the second side wall of the box body. 
         [0015]    The duct can be provided easily between the box body and the lid body both having vacuum heat insulating structure. Moreover, the heat transportation section can be provided at the portion where the heat generated from the battery assembly is transmitted. 
         [0000]    [7] Further, at least the first side wall and the second side wall of the box body may be made of metal, the fluid inlet section may be spaced from the first side wall of the box body, and the heat releasing section may be provided in contact with the second side wall of the box body. 
         [0016]    Assuming that the fluid inlet section contacts the first side wall, when the heat in the box body is transmitted to the fluid inlet section through the first side wall, and transmitted to the duct, the temperature of the fluid supplied in the duct may be increased, and the fluid may not perform its function as a coolant undesirably. Further, by providing the fluid inlet section spaced from the first side wall of the box body, it is possible to avoid such a disadvantage, and it is possible to supply the fluid which serves as a coolant to the heat transportation section. Further, since the heat releasing section is provided in contact with the second side wall of the box body, the heat transported through the heat transportation section is released from the heat releasing section together with the fluid, and the heat is transmitted to the second side wall of the metal box body, and diffused to the outside. Therefore, heat can be released efficiently. 
         [0000]    [8] In this case, buffer material may be interposed between the fluid inlet section and the first side wall of the box body. It is possible to provide the fluid inlet section spaced from the first side wall of the box body.
 
[9] Preferably, the buffer material may have a heat insulating function. In this case, the buffer material may be heat insulating material. It is possible to prevent the heat in the box body from being transmitted to the fluid inlet section through the first side wall.
 
[10] The fluid inlet section may include a fluid supply section to which fluid from an external fan is supplied and a fluid guide section configured to guide the fluid supplied to the fluid supply section to the heat transportation section. The fluid in the fluid guide section may be guided in a direction along a surface having a normal line direction in which the fluid is supplied to the fluid supply section, and toward the ceiling wall of the lid body. In the structure, the fluid outputted from the fan can flow toward the heat transportation section smoothly.
 
[11] In this case, the fluid supply section may include an air chamber, and a channel at an outlet of the air chamber may be narrowed. In the structure, the pressure of the supplied fluid is increased to a constant level in the air chamber, and the fluid can flow at a constant flow rate without any drift (bias) of the flow.
 
[12] In the present invention, preferably, the temperature of the fluid at the heat releasing section is higher than the temperature of the fluid at the fluid inlet section by +60° C. or less. More preferably, the temperature of the fluid at the heat releasing section is higher than the temperature of the fluid at the fluid inlet section by +40° C. or less. By adopting a small temperature slope between the fluid inlet section and the heat releasing section, it becomes possible to suppress the temperature difference among the battery cells in the box body, and control deterioration of the battery cells to become uniform. Further, since the temperature of the fluid released to the outside and the room temperature are substantially the same, it becomes possible to prevent burns at the time of maintenance operation.
 
[13] In this case, the width direction of the duct may be the parallel direction of the battery cells, and the length direction of the duct may be the serial direction of the battery cells.
 
[14] In the present invention, the height of the channel of each of the fluid inlet section, the heat transportation section, and the heat releasing section may be in a range of 10 to 30 mm. In the case where the heights are less than 10 mm, the pressure loss becomes large. Therefore, in the embodiment where a fan, etc. is used as a fluid driving machine for forced cooling, a large driving capability is required for the fan. In this case, the size of the fan, and electrical energy required for driving the fan are increased. Consequently, energy density of the battery and the system efficiency are lowered, and the cost is increased undesirably. In the case where the heights exceed 30 mm, convection flows are generated in the heat transportation section. Consequently, heat is radiated from the box body easily, and the heat insulating performance may be impaired undesirably.
 
[15] Further, in the present invention, the lid body and the duct may be formed integrally. In the structure, in contrast to the case where the lid body and the duct are provided as separate components, since two metal members overlapped between the lid body and the duct can be integrated into one metal member, the surface area (heat radiation area) of the metal member having good thermal conductivity is reduced. Further, since the number of components or parts is reduced, assembling operation of the secondary battery is simplified. The number of assembling steps is reduced, and reduction of the time required for assembling operation is achieved.
 
[16] Further, in the present invention, the lid body may include a ceiling wall and an eaves, the lid body may be configured to close the opening formed in the upper surface of the box body, the fluid inlet section may be provided along a first side wall of the box body, and oriented between the eaves of the lid body and the first side wall of the box body, the heat transportation section may be provided between the ceiling wall of the lid body and the box body, spirally from a central portion to a peripheral portion of the box body, and the heat releasing section may be provided from a position between the first side wall of the box body and the eaves of the lid body along the first side wall of the box body.
 
[17] Alternatively, in the present invention, the lid body may include a ceiling wall and an eaves, and the lid body may be provided to close the opening formed in the upper surface of the box body, the fluid inlet section may include a through hole formed at a central portion of the ceiling wall in the lid body, the heat transportation section may be provided between the ceiling wall of the lid body and the box body, and is configured to transport heat generated at least in the box body together with the fluid supplied through the through hole, and the heat releasing section may at least include a first heat releasing section provided from a position between a first side wall of the box body and the eaves of the lid body along the first side wall of the box body, and a second heat releasing section provided from a position between the second side wall of the box body opposite to the first side wall and the eaves of the lid body along the second side wall of the box body.
 
[18] The lid body may include a ceiling wall and an eaves, and the lid body may be configured to close the opening formed in the upper surface of the box body, the fluid inlet section may include a first through hole formed in one side wall of the eaves of the lid body, the heat transportation section may be provided between the ceiling wall of the lid body and the box body and transport the heat generated at least in the box body toward the heat releasing section together with the fluid supplied through the first through hole, and the heat releasing section may include a second through hole formed in another side wall opposite to the one side wall of the eaves in the lid body.
 
[19] In the present invention, the secondary battery may include a heater provided in the box body, and heat insulating material provided between the duct and the box body, the heat insulating material being narrower than an opening area of the box body. The box body may include a first side wall and a second side wall opposite to each other, and the heat insulating material may be provided adjacent to the first side wall of the box body where the fluid is supplied into the box body by the duct.
 
         [0017]    If the cooled fluid is supplied to the duct, since the temperature of the fluid is low, a large amount of heat is taken away depending on the temperature difference. Consequently, in the box body, an area around the first side wall as the fluid inlet side is cooled, and the temperature distribution in the box body may be degraded undesirably. In this case, the heater is energized. Since the fluid is supplied, and the heater is energized, the system efficiency may be degraded undesirably. 
         [0018]    In this regard, in the case where the inlet side of the fluid is adjacent to the first side wall, by providing heat insulating material between the duct and the box body, it becomes possible to decrease the temperature of the area around the first side wall. Consequently, even in the middle of supplying the fluid, it becomes no longer necessary to energize the heater. Accordingly, since it is possible to suppress degradation of the system efficiency, this structure is more preferable. 
         [0019]    In the secondary battery according to the present invention, it is possible to perform optimum operation without changing heat insulating structure of the heat insulating container depending on the level of the load. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0020]      FIG. 1A  is a vertical cross sectional view showing structure of a secondary battery according to an embodiment of the present invention with partial omission: 
           [0021]      FIG. 1B  is a plan view showing a box body of the secondary battery as viewed from above; 
           [0022]      FIG. 2A  is an exploded vertical cross sectional view showing structure of a secondary battery according to an embodiment of the present invention; 
           [0023]      FIG. 2B  is a front view showing an example of structure of an air inlet section; 
           [0024]      FIG. 3  is a circuit diagram showing a battery assembly accommodated in the box body with partial omission; 
           [0025]      FIG. 4  is a perspective view showing an example of a duct; 
           [0026]      FIG. 5  is a block diagram showing an example of a cooling controller and a heater controller; 
           [0027]      FIG. 6  is a vertical cross sectional view showing a lid body, the box body, and the duct as viewed in a direction in which the air is supplied; 
           [0028]      FIG. 7A  is a cross sectional view showing a secondary battery according to a first modified embodiment with partial omission; 
           [0029]      FIG. 7B  is a plan view showing a box body and a duct according to the first modified embodiment as viewed from above; 
           [0030]      FIG. 8  is a vertical cross sectional view showing a secondary battery according to a second modified embodiment with partial omission; 
           [0031]      FIG. 9A  is a vertical cross sectional view showing a secondary battery according to a third modified embodiment with partial omission; 
           [0032]      FIG. 9B  is a plan view showing a box body of the secondary battery as viewed from above; 
           [0033]      FIG. 10  is a vertical cross sectional view showing a state where partially exposed heat insulating material is provided in the third modified embodiment with partial omission; 
           [0034]      FIG. 11A  is a vertical cross sectional view showing a state where heat insulating material is provided along each of a third side wall and a fourth side wall in the third modified embodiment as viewed in a direction in which air is supplied; 
           [0035]      FIG. 11B  is a plan view showing a box body of a secondary battery according to the third modified embodiment as viewed from above; 
           [0036]      FIG. 12A  is a plan view showing a box body where heat insulating material is provided along each of a first side wall and a second side wall in the third modified embodiment as viewed from above; 
           [0037]      FIG. 12B  is a plan view showing a box body where heat insulating material is provided along each of first to fourth side walls in the third modified embodiment as viewed from above; 
           [0038]      FIG. 13A  is a vertical cross sectional view showing a secondary battery according to a fourth modified embodiment with partial omission; and 
           [0039]      FIG. 13B  is a vertical cross sectional view showing a secondary battery according to a fifth modified embodiment with partial omission. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0040]    Hereinafter, embodiments of a secondary battery according to the present invention applied to a NaS battery will be described with reference to  FIGS. 1A to 13B . 
         [0041]    As shown in  FIGS. 1A to 2B , a secondary battery  10  according to an embodiment of the present invention includes a base frame  12  made of, e.g., steel material, a box body  14  placed, and fixed on the base frame  12 , a battery assembly  18  made up of a large number of battery cells  16  accommodated in the box body  14 , and a lid body  20  closing an opening of the box body  14 . For example, each of the battery cells  16  has a cylindrical shape, and the battery cells  16  are accommodated in the box body  14  such that the axial direction of the battery cells  16  is oriented in the vertical direction. 
         [0042]    Further, heaters  22  used for raising the temperature in the box body  14  are provided along a bottom surface and inner wall surfaces of the box body  14 , respectively. Further, in order to attend to damages and abnormal heating of the battery cells  16  or leakage of active material, etc., silica sand  24  is filled in a gap between the box body  14  and the battery assembly  18 . 
         [0043]    For example, the box body  14  has a substantially rectangular parallelepiped shape, and includes four side walls and a bottom wall. An opening is formed in an upper surface of the box body  14 . For example, the box body  14  is made of plate material of stainless steel. The box body  14  itself has a box shape including a hollow area  26 . The hollow area  26  is a hermetical space which is sealed in an air-tight manner. The hollow area  26  is connectable to the external space by a vacuum valve (not shown). A porous vacuum heat insulating board  28  formed by solidifying glass fiber into a plate shape using adhesive is loaded in the hollow area  26  to achieve vacuum heat insulating structure of the box body  14 . 
         [0044]    The lid body  20  includes a ceiling wall  30  and an eaves (projecting edge portions), and the lid body  20  is provided to close the opening in the upper surface of the box body  14 . In the same manner as in the case of the box body  14 , the lid body  20  is made of plate material of stainless steel. The lid body  20  itself has a box shape including a hollow area  34 . The hollow area  34  is a hermetical space which is sealed in an air-tight manner. The hollow area  34  is connectable to the external space by a vacuum valve (not shown). A porous vacuum heat insulating board  36  formed by solidifying glass fiber into a plate shape using adhesive is loaded in the hollow area  34  to achieve vacuum heat insulating structure of the lid body  20 . 
         [0045]    As shown in  FIG. 3 , the battery assembly  18  is formed by connecting two or more blocks  42  in series from a positive electrode external terminal  38  to a negative electrode external terminal  40 . Each of the blocks  42  is formed by connecting two or more circuits (strings  44 ) in parallel, and each of the strings  44  is formed by connecting two or more battery cells  16  in series. The positive electrode external terminal  38  extends to the outside through a first side wall  46   a  of the box body  14 , and the negative electrode external terminal  40  extends to the outside through a second side wall  46   b  of the box body  14  (side wall opposite to the first side wall  46   a ). That is, the serial direction of the battery cells  16  is a direction from the first side wall  46   a  toward the second side wall  46   b . The parallel direction of the battery cells  16  is a direction from a third side wall  46   c  to a fourth side wall  46   d  (side wall opposite to the third side wall  46   c ). 
         [0046]    The heaters  22  noted above at least include a bottom surface heater  22   x  provided at the bottom surface of the box body  14 , a first side surface heater  22   a  provided at the inner wall surface of the first side wall  46   a , and a second side surface heater  22   b  provided at the inner wall surface of the second side wall  46   b . It is a matter of course that side surface heaters (third side surface heater  22   c  and fourth side surface heater  22   d ) may be provided at the inner wall surface of the third side wall  46   c  and the inner wall surface of the fourth side wall  46   d , respectively. 
         [0047]    In order to measure the temperature in the box body  14 , a bottom surface temperature sensor  102   a  (denoted by a white circle in  FIGS. 1A and 1B ) is provided at the center of the bottom surface of the box body  14 , at a position remote from the bottom surface heater  22   x  by a distance in a range of, e.g., 3 mm to 15 mm. Further, a side surface temperature sensor  102   b  (denoted by a black circle in  FIGS. 1A and 1B ) is provided at the center of the inner wall surface of the first side wall  46   a  in the lateral direction and vertical direction, at a position remote from the first side surface heater  22   a  by a distance in a range of, e.g., 3 mm to 15 mm. It is a matter of course that the side surface temperature sensor  102   b  may be provided at the center of the inner wall surface of the second side wall  46   b  in the lateral direction and vertical direction, at a position remote from the second side surface heater  22   b  by a distance in a range of, e.g., 3 mm to 15 mm. 
         [0048]    Further, as shown in  FIGS. 1A to 2B , the secondary battery  10  includes a metal duct  50 , a plate member  52 , a fan  54 , and a cooling controller  56 , and a heater controller (see  FIG. 5 ). The metal duct  50  is provided between at least the box body  14  and the lid body  20 , and air  48  flows inside the duct  50 . The plate member  52  is provided between the battery assembly  18  and the duct  50 , and at least has electrically insulating property. The fan  54  is provided outside the box body  14 , and sends the air  48  to the duct  50 . The cooling controller  56  implements drive control of the fan  54 . The heater controller  57  implements energizing control of the heater  22 . That is, in the embodiment of the present invention, the duct  50  is provided under the lower surface of the lid body  20 , and the silica sand  24  is provided between the duct  50  and the battery assembly  18 . In the embodiment of the present invention, as fluid flowing through the duct  50 , though the air  48  is used, other gases such as a nitrogen gas or a helium gas may be used. Instead of providing only one fan  54  for the secondary battery  10 , a plurality of fans  54  may be provided and connected to the drive circuit, with a view to provide redundancy at the time of occurrence of a failure. 
         [0049]    The duct  50  includes a metal air inlet section  58  (fluid inlet section), a metal heat transportation section  60 , and a metal heat releasing section  62 . The air  48  is supplied into the air inlet section  58 . The metal heat transportation section  60  is provided downstream of the air inlet section  58 , between the lid body  20  and the box body  14 , and transports the heat generated at least in the box body  14  together with the air  48 . The heat releasing section  62  is provided downstream of the heat transportation section  60 , and releases the heat to the outside together with the air  48 . 
         [0050]    The air inlet section  58  is provided along the first side wall  46   a  of the box body  14 , and the air inlet section  58  is oriented between the eaves  32  of the lid body  20  and the first side wall  46   a  of the box body  14 . In particular, buffer material  64  (heat insulating material) is interposed between the air inlet section  58  and the first side wall  46   a  of the box body  14 , and the air inlet section  58  is spaced from the first side wall  46   a  of the box body  14 . Preferably, the buffer material  64  has a heat insulating function. In the embodiment, heat insulating material is used as the buffer material  64 . 
         [0051]    This air inlet section  58  includes an air supply section  66  (fluid supply section) and an air guide section  68  (fluid guide section). The air  48  from the external fan  54  is supplied into the air supply section  66 . The air guide section  68  is connected to the air supply section  66 , and guides the air  48  supplied to the air supply section  66  to the heat transportation section  60 . The air  48  in the air guide section  68  is guided in a direction along a surface  69  of the air guide section  68  having a normal line direction in which the air  48  is supplied to the air supply section  66 , and toward the ceiling wall  30  of the lid body  20 . 
         [0052]    The air supply section  66  has an air chamber  70 . As shown in  FIG. 4 , the air chamber  70  has a channel  71 , and the channel  71  has a rectangular cross section  72  having a normal line direction which is the main flow direction of the air  48 . The surface area of the cross section  72  (long side  73  of the rectangular cross section  72 ) is increased gradually toward the air guide section  68 . The air guide section  68  has a channel  74 , and the channel  74  has a rectangular cross section  75  having a normal line direction which is the main flow direction of the air  48 . The surface area of the cross section  75  is constant toward the heat transportation section  60 . The channel herein means a space (space surrounded by inner wall surfaces) where the air  48  flows. Hereinafter, the term “channel” is used in this meaning. The height ha (length of the short side  76 ) of the channel  71  of the air chamber  70  and the height hb (length of the short side  77 ) of the channel  74  of the air guide section  68  are in a range of 10 to 30 mm. In particular, the height ha of the channel  71  of the air chamber  70  is larger than the height hb of the channel  74  of the air guide section  68  (ha&gt;hb). 
         [0053]    As shown in  FIG. 1A , the heat transportation section  60  is provided between the ceiling wall  30  of the lid body  20  and the box body  14 . The shape of a lower surface  60   a  of the heat transportation section  60  has a rectangular shape which is the same as the shape of the opening of the box body  14 . The size of the lower surface  60   a  is substantially the same as the size of the opening of the box body  14 . Further, a plurality of fins  78  are provided at the lower surface  60   a  of the heat transportation section  60  (surface facing the battery assembly  18  (or the plate member  52 )). The fins  78  extend toward the battery assembly  18  (or the plate member  52 ). 
         [0054]    The types of the fins  78  include, for example, plain plate fins, corrugated fins (wavy fins), interrupted fins, etc. In the case of providing the plurality of fins  78  at the lower surface  60   a  of the heat transportation section  60 , for example, a plurality of plain plate fins or corrugated fins extending in the width direction of the heat transportation section  60  (parallel direction of the battery cells  16 ) may be arranged in the length direction of the heat transportation section  60  (serial direction of the battery cells  16 ), and conversely, a plurality of plain plate fins or corrugated fins extending in the length direction of the heat transportation section  60  (serial direction of the battery cells  16 ) may be arranged in the width direction of the heat transportation section  60  (parallel direction of the battery cells  16 ). Further, in the case of providing a plurality of interrupted fins, the interrupted fins may be arranged in the length direction of the heat transportation section  60  (serial direction of the battery cells  16 ) while aligning the direction of the plate surface of each interrupted fin with the width direction of the heat transportation section  60  (parallel direction of the battery cells  16 ), and vice versa. It is a matter of course that the interrupted fins may be arranged in a state where the direction of the plate surface of each interrupted fin may be oriented randomly. In the case of arranging the fins  78  in the width direction of the heat transportation section  60  (parallel direction of the battery cells  16 ), one fin  78  is provided for each range of s×1/1 to ¼ where “s” denotes the number of the battery cells  16  in the serial direction. 
         [0055]    As shown in  FIG. 4 , the heat transportation section  60  has a channel  80 , and the channel  80  has a rectangular cross section  82  having a normal line direction which is the main flow direction of the air  48 . The surface area of the cross section  82  is constant toward the heat releasing section  62 . The height hc (length of the short side  83 ) of the channel  80  of the heat transportation section  60  is in a range of 10 to 30 mm as in the case of the air guide section  68  described above. 
         [0056]    Further, in the channel  80  of the heat transportation section  60 , as shown in  FIG. 2B , a plurality of support sections  84  for maintaining the shape of the channel  80  are provided. As the support sections  84 , for example, plain plate members, corrugated members (wavy members), interrupted members, etc. can be used. In the case of providing a plurality of support sections  84  in the channel  80  of the heat transportation section  60 , for example, a plurality of plain plate members or corrugated members extending in the length direction of the heat transportation section  60  (serial direction of the battery cells  16 ) may be arranged in the width direction of the heat transportation section  60  (parallel direction of the battery cells  16 ). Further, in the case of providing a plurality of interrupted members, the interrupted members may be arranged in the width direction of the heat transportation section  60  (parallel direction of the battery cells  16 ) while aligning the direction of the plate surface of each interrupted member with the length direction of the heat transportation section  60  (serial direction of the battery cells  16 ). It is a matter of course that the interrupted members may be arranged in a state where the direction of the plate surface of each interrupted member may be oriented randomly. Further, as the support sections  84 , for example, L-shaped metal members or U-shaped metal members may be used suitably. In the case of providing the support sections  84  in the width direction of the heat transportation section  60  (parallel direction of the battery cells  16 ), the support sections  84  of the heat transportation section  60  are arranged at interval or 200 to 1000 mm. 
         [0057]    As shown in  FIGS. 1A and 2A , the heat releasing section  62  is provided from a position between the second side wall  46   b  of the box body  14  and the eaves  32  of the lid body  20  along the second side wall  46   b  of the box body  14 . In particular, the heat releasing section  62  is provided in contact with the second side wall  46   b  of the box body  14 . As shown in  FIG. 4 , the heat releasing section  62  has a channel  86 , and the channel  86  has a rectangular cross section  88  having a normal line direction which is the main flow direction of the air  48 . The surface area of the cross section  88  is constant toward the downstream side. The height hd (length of the short side  90 ) of the channel  86  of the heat releasing section  62  is in a range of 10 to 30 mm. The total length of the heat releasing section  62  is the same as the height of the eaves  32  of the lid body  20 . Further, in use, the heat releasing section  62  having the 0 mm length (i.e., the terminal portion of the heat releasing section  62  is cut, and an opening at the cutting plane forms the heat releasing section  62 ) is preferable as well. 
         [0058]    As shown in  FIG. 5 , the cooling controller  56  includes a fan drive determination section  94  and a fan drive circuit  96 . The heater controller  57  includes a bottom surface heater drive determination section  104 , a bottom surface heater drive circuit  106 , a side surface heater drive determination section  108 , and a side surface heater drive circuit  110 . 
         [0059]    When the temperature Di detected by the bottom surface temperature sensor  102   a  becomes a predetermined upper limit threshold temperature Dth 1  (e.g., 330 to 340° C.) or more, the fan drive determination section  94  outputs a start-up signal Sd to the fan drive circuit  96 . When the temperature Di becomes a predetermined lower limit threshold temperature Dth 2  or less, the fan drive determination section  94  outputs a stop signal Ss to the fan drive circuit  96 . The fan drive circuit  96  drives the fan  54  based on the input of the start-up signal Sd, and supplies the air  48  to the duct  50 . Further, the fan drive circuit  96  stops the fan  54  based on the input of the stop signal Ss, and stops supply of the air  48  to the duct  50 . 
         [0060]    When the temperature Di detected by the bottom surface temperature sensor  102   a  becomes a predetermined lower limit threshold temperature Dth 3  (e.g., 300° C.) or less, the bottom surface heater drive determination section  104  outputs a start-up signal Sbd to the bottom surface heater drive circuit  106 . When the temperature Di exceeds the predetermined lower limit threshold temperature Dth 3 , the bottom surface heater drive determination section  104  outputs a stop signal Sbs to the bottom surface heater drive circuit  106 . The bottom surface heater drive circuit  106  energizes the bottom surface heater  22   x  based on the input of the start-up signal Sbd, and stops energization of the bottom surface heater  22   x  based on the input of the stop signal Sbs. 
         [0061]    When the temperature Dk detected by the side surface temperature sensor  102   b  becomes the predetermined lower limit threshold temperature Dth 3  (e.g., 300° C.) or less, the side surface heater drive determination section  108  outputs a start-up signal Std to the side surface heater drive circuit  110 . When the temperature Dk exceeds the predetermined lower limit threshold temperature Dth 3 , the side surface heater drive determination section  108  outputs a stop signal Sts to the side surface heater drive circuit  110 . The side surface heater drive circuit  110  energizes the first side surface heater  22   a  to the fourth side surface heater  22   d  based on the input of the start-up signal Std, and stops energization of the first side surface heater  22   a  to the fourth side surface heater  22   d  based on the input of the stop signal Sts. 
         [0062]    It is a matter of course that the following control method can be adopted suitably as well. 
         [0063]    Specifically, in this control method, the secondary battery  10  is equipped with the duct  50 , and the air  48  for the duct  50  is regulated according to the required amount of heat radiation. As this control method, for example, the following first to fourth control methods can be adopted. 
         [0064]    In the first control method, control is implemented by regulating the wind flow rate based on the change of the rotation number of the fan, or regulating the average wind flow rate based on the ON/OFF duty ratio of the drive time of the fan in order to control the amount of heat radiation based on temperature. For example, using the value of the internal temperature of the secondary battery  10 , when the temperature is low, the wind flow rate is decreased, and when the temperature is high, the wind flow rate is increased. 
         [0065]    In the second control method, control is implemented by estimating the required amount of heat radiation and regulating the wind flow rate based on the change of the rotation number of the fan, or regulating the average wind flow rate based on the ON/OFF duty ratio of the drive time of the fan. For example, by estimating electric discharge waveform of the battery calculated from load fluctuations or power generation fluctuations estimated beforehand, when the electric discharge output is high or the electric discharge time period is long, the wind flow rate is increased. 
         [0066]    In the following third and fourth control methods, control is implemented by estimating degradation in the resistance of the secondary battery  10 . 
         [0067]    Specifically, in the third control method, the resistance value of the secondary battery  10  is calculated from the voltage value/current value during operation, and the amount of internal heat generation is calculated based on an estimated electric discharge waveform to estimate the required amount of heat radiation for regulating the wind flow rate based on the change of the rotation number of the fan, or regulating the average wind flow rate based on the ON/OFF duty ratio of the drive time of the fan. 
         [0068]    In the fourth control method, the resistance value of the secondary battery  10  is calculated from the temperature during operation and the operation cycle, and the amount of internal heat generation is calculated based on an estimated electric discharge waveform to estimate the required amount of heat radiation for regulating the wind flow rate based on the change of the rotation number of the fan, or regulating the average wind flow rate based on the ON/OFF duty ratio of the drive time of the fan. 
         [0069]    Next, operation of the secondary battery  10  will be described. Firstly, in the case where the load on the electric power system is low, e.g., in spring season or in autumn season, and for example, at the time of leveling power generation such as wind force power generation, since the electric discharge time period is short, in conventional approaches, occasionally, the amount of heat generated in the NaS battery dropped below the heat loss of the heat insulating container. In this case, as described above, the charging/discharging efficiency may be lowered undesirably. However, in this secondary battery  10 , since both of the box body  14  and the lid body  20  have vacuum insulation structure, heat accumulated in the box body  14  can be utilized. Even if heater electrical energy is used, energy consumption is small, and degradation in the charging/discharging efficiency can be suppressed. 
         [0070]    On the other hand, in the case where the load on the electric power system is high, e.g., in summer season or in winter season, since the electric discharge output is high or the electric discharge time period is long, the amount of heat generated in the NaS battery may exceed the heat loss of the secondary battery  10  undesirably. In particular, in the secondary battery  10 , since both of the box body  14  and the lid body  20  have vacuum insulation structure, the internal temperature of the box body  14  may be increased excessively by the heat accumulated in the box body  14 . However, in this secondary battery  10 , when the temperature in the box body  14  reaches the upper limit threshold temperature Dth 1  or more, the cooling controller  56  drives the fan  54 . Thus, the fan  54  is driven to supply the cooled air  48  to the duct  50 . Since the air  48  is supplied into the duct  50 , the heat in the box body  14  moves to the air  48  in the heat transportation section  60 , and the air  48  becomes hot. The hot air  48  is released to the outside of the box body  14  through the heat releasing section  62 . That is, the heat in the box body  14  is released. Accordingly, the inside of the box body  14  is cooled forcibly. Even if both of the box body  14  and the lid body  20  have good heat insulating property, the inside of the box body  14  is cooled efficiently. Consequently, Even if the electric discharge output is high, or electric discharge time period is long, the temperature in the box body  14  can be maintained in a temperature range of about 300 to 360° C., and it becomes possible to operate the battery assembly  18  in the box body  14  in the optimum operating environment. 
         [0071]    The temperature of the air  48  in the heat releasing section  62  is higher than the temperature of the air  48  in the air inlet section  58  preferably by +60° C. or less, and more preferably by +40° C. or less. By adopting a small temperature slope between the air inlet section  58  and the heat releasing section  62 , it becomes possible to suppress the temperature difference in the horizontal direction (serial direction) of the battery cells  16  to 15° C. or less. It becomes possible to reduce the temperature difference among the battery cells  16  resulting from cooling, and enable the battery assembly  18  to exert its performance effectively, and control deterioration of the battery cells  16  to become uniform. Further, since the temperature of the air released to the outside and the room temperature are substantially the same, it becomes possible to prevent burns at the time of maintenance operation. 
         [0072]    In particular, since the plate member  52  at least having electrically insulating property is provided between the battery assembly  18  and the heat transportation section  60  of the duct  50 , it becomes possible to prevent contact between the metal duct  50  and the battery assembly  18 , and avoid short circuiting among the battery cells  16 . 
         [0073]    In the embodiment of the present invention, the duct  50  is provided under the lower surface of the lid body  20 , and the silica sand  24  is provided between the duct  50  and the battery cells  16 . In this case, thermal conductivity of the silica sand  24  is low. Since influence of the temperature of the air  48  in the duct  50  is absorbed when the heat is transmitted to the battery assembly  18  by conduction of the heat in the height direction, the temperature in the height direction of the battery cells  16  in the battery assembly  18  becomes uniform to a greater extent. 
         [0074]    Further, since a heat insulating layer like the silica sand  24  mentioned above is present between the duct  50  and the upper portion of the battery cells  16 , when the air  48  flows into the duct  50  for cooling, the temperature difference among the battery cells  16  in the vertical direction is suppressed to ±15° C. In this case, the ratio of the thermal resistance (R 1 /R 2 ) is larger than 8 (R 1 /R 2 &gt;8) where R 1  denotes the thermal resistance of the heat insulating layer such as the silica sand  24 , and R 2  denotes the thermal resistance of the battery cells  16 . 
         [0075]    Since the plurality of fins  78  extending toward the battery assembly  18  (plate member  52 ) are provided at the lower surface  60   a  of the heat transportation section  60  of the duct  50 , even if the silica sand  24  is filled between the box body  14  and the lid body  20 , it is possible to enhance conduction of the heat from the battery assembly  18  to the duct  50 , and efficiently lower the temperature in the box body  14 . 
         [0076]    Further, it may be possible that the upper surface of the silica sand  24  becomes uneven due to vibrations of transportation or earthquake, and the surface contact between the duct  50  and the silica sand  24  become non-uniform. However, since the plurality of fins  78  are provided, it is possible to achieve uniform conduction of the heat in the box body  14  in the horizontal direction, and regulate the temperature difference among the battery cells  16  in the battery assembly  18  within ±15° C. Consequently, it becomes possible to reduce the temperature difference among the internal battery cells  16  resulting from cooling, and allows each of the battery cells  16  of the battery assembly  18  to exert its performance. Accordingly, degradation of the battery cells  16  is suppressed. 
         [0077]    The duct  50  includes the metal air inlet section  58 , the metal heat transportation section  60 , and the metal heat releasing section  62 . The air is supplied into the air inlet section  58 . The metal heat transportation section  60  is provided downstream of the air inlet section  58 , between the lid body  20  and the box body  14 , and transports the heat generated at least in the box body  14  together with the air  48 . The heat releasing section  62  is provided downstream of the heat transportation section  60 , and releases the air to the outside together with the air  48 . In the structure, the air  48  supplied into the heat transportation section  60  through the air inlet section  58  flows between the box body  14  and the lid body  20 . Normally, the heat generated in the battery assembly  18  is transmitted upward (toward the lid body  20 ). Therefore, after the heat is transmitted upward, the heat is transported toward the downstream side together with the air  48  flowing through the duct  50 , and the heat is released to the outside of the box body  14  through the heat releasing section  62 . 
         [0078]    Further, since the air inlet section  58  is provided along the first side wall  46   a  of the box body  14  toward a position between the eaves  32  of the lid body  20  and the first side wall  46   a  of the box body  14 , the heat transportation section  60  is provided between the ceiling wall  30  of the lid body  20  and the box body  14 , and the heat releasing section  62  is provided from the position between the second side wall  46   b  of the box body  14  and the eaves  32  of the lid body  20  along the second side wall  46   b  of the box body  14 , the duct  50  can be provided easily between the box body  14  and the lid body  20  both having vacuum heat insulating structure. Moreover, the heat transportation section  60  can be provided at the portion where the heat generated from the battery assembly  18  is transmitted. 
         [0079]    In this case, the air inlet section  58  is spaced from the first side wall  46   a  of the box body  14 . Assuming that the air inlet section  58  contacts the first side wall  46   a , when the heat in the box body  14  is transmitted to the air inlet section  58  through the first side wall  46   a , and transmitted to the duct  50 , the temperature of the air  48  may be increased, and the air  48  may not perform its function as coolant undesirably. However, by providing the air inlet section  58  remotely from the first side wall  46   a  of the box body  14 , it is possible to avoid such a disadvantage, and it is possible to supply the air  48  which serves as a coolant to the heat transportation section  60 . In particular, by providing the buffer material  64  (heat insulating material) between the air inlet section  58  and the first side wall  46   a  of the box body  14 , it is possible to provide the air inlet section  58  remotely from the first side wall  46   a  of the box body  14  easily. Further, since the heat releasing section  62  is provided in contact with the second side wall  46   b  of the box body  14 , the heat transported through the heat transportation section  60  is released from the heat releasing section  62  together with the air, and the heat is transmitted to the second side wall  46   b  of the metal box body  14 , and diffused to the outside. Therefore, heat can be released efficiently. 
         [0080]    Further, the air inlet section  58  includes the air supply section  66  (air chamber  70 ) and the air guide section  68 . The air from the external fan  54  is supplied into the air supply section  66 . The air guide section  68  is connected to the air supply section  66 , and guides the air  48  supplied to the air supply section  66  to the heat transportation section  60 . The air  48  in the air guide section  68  is guided in the direction along the surface  69  having a normal line direction in which the air  48  is supplied to the air supply section  66 , and toward the ceiling wall  30  of the lid body  20 . In the structure, the air  48  outputted from the fan  54  can flow toward the heat transportation section  60  smoothly. 
         [0081]    In particular, by adopting the narrow channel of the outlet of the air chamber  70  (which also serves as the inlet of the air guide section  68 ), the air  48  outputted from the fan  54  is stored in the air chamber  70  to increase the pressure at a certain level, and the air  48  can be released from the outlet. 
         [0082]    Further, the heat transportation section  60  has the channel  80 , and the channel  80  has a rectangular cross section  82  having a normal line direction which is the main flow direction of the air  48 . The surface area of the cross section  82  is constant toward the heat releasing section  62 . The pressure of the supplied air  48  is increased to a constant level in the air chamber  70 , and the air  48  can flow toward the heat releasing section  62  at a constant flow rate without any drift (bias) of the flow. Likewise, the heat releasing section  62  has the channel  86 , and the channel  86  has the rectangular cross section  88  having a normal line direction which is the main flow direction of the air  48 . The surface area of the cross section  88  is constant toward the downstream side. Therefore, the air supplied at the constant pressure level can flow toward the outside of the duct  50  at a constant flow rate without any drift (bias) of the flow. Further, the air guide section  68  and the heat releasing section  62  are oriented downward in comparison with the heat transportation section  60 . In the structure, when the fan  54  is not driven, heat movement due to spontaneous (natural) convection of the air is suppressed, and it becomes possible to maintain the high heat insulation property of the secondary battery  10 . 
         [0083]    Further, in this secondary battery  10 , as shown in  FIG. 3 , the plurality of battery cells  16  of the battery assembly  18  are connected in series in the main flow direction of the air  48 . In this regard, for the purpose of explanation, one of the blocks  42  will be taken into consideration. Among a plurality of strings  44  of the block  42 , battery cells  16  adjacent to the first side wall  46   a  are cooled sufficiently, and battery cells  16  remote from the first side wall  46   a  are cooled to a lesser degree. Such a difference in the cooling degree is the same in each of the plurality of strings  44  forming one block  42 . Therefore, all of the strings  44  are used for charging/discharging, and substantially no variation occurs in the characteristics and degradation degrees, etc. Therefore, in the case of providing the duct  50 , preferably, the width direction of the duct  50  is aligned with the parallel direction of the battery cells  16 , and the length direction of the duct  50  (main flow direction of the air  48 ) is aligned with the serial direction of the battery cells  16 . 
         [0084]    Further, in the secondary battery  10 , the heights hb, hc, and hd of the channels of the air guide section  68 , the heat transportation section  60 , and the heat releasing section  62  are in the range of 10 to 30 mm. In the case where the heights are less than 10 mm, the pressure loss becomes large, and a large driving capability is required for the fan  54 . In this case, the size of the fan  54 , and electrical energy required for driving the fan  54  are increased. Consequently, energy density of the battery and the system efficiency are lowered, and the cost is increased undesirably. In the case where the heights exceed 30 mm, convection flows are generated in the heat transportation section  60 . Consequently, heat is radiated from the box body  14  easily, and the heat insulating performance is degraded. Therefore, it is preferable that the heights hb, hc, and hd of the channels of the air guide section  68 , the heat transportation section  60 , and the heat releasing section  62  are in the range of 10 to 30 mm. 
         [0085]    In the heat transportation section  60  provided in the horizontal direction, the central portion of the channel  80  tends to be bent easily, and the height hc of the channel  80  may not be maintained in the above range of 10 to 30 mm due to the weight of the lid body  20  having vacuum heat insulating structure. Therefore, it is preferable to provide a plurality of support sections  84  in the channel  80  of the heat transportation section  60 . In this manner, the height hc of entire channel  80  of the heat transportation section  60  can be maintained in the range of 10 to 30 mm. Further, it becomes possible to support the lid body  20  by the heat transportation section  60 . In this case, the support sections  84  in the heat transportation section  60  are arranged in the serial direction of the battery cells  16 , and the fins  78  at the lower surface  60   a  of the heat transportation section  60  are arranged in the parallel direction of the battery cells  16 . In this manner, the heat transportation section  60  can be supported by the support sections  84  and the fins  78  in a crisscross pattern. Thus, it is possible to improve the effect of preventing thermal deformation of the duct  50 . 
         [0086]    Further, preferably, one fin  78  is provided for each range of s×1/1 to ¼ where “s” denotes the number of the battery cells  16  in the serial direction. In the case where the number of fins  78  is small, the cooling degree may vary among the battery cells  16  in the battery assembly  18 . As a result, the temperature difference among the battery cells  16  becomes large, and each of the battery cells  16  cannot exert its performance sufficiently, and some of the battery cells  16  may be deteriorated. If the number of the fins  78  is large, large quantity of material is required for producing the fins  78 , and the number of assembling steps is increased disadvantageously. 
         [0087]    Further, preferably, the support sections  84  of the heat transportation section  60  may be arranged at interval or 200 to 1000 mm. Preferably, the height hc of the heat transportation section  60  is in a range of 10 to 30 mm. The box body  14  has internal temperature in a range of 300 to 360° C., and the box body  14  is used for long time at this high temperature. Therefore, in consideration of thermal deformation or thermal expansion, it is preferable to increase the number of the support sections  84  as many as possible. However, if the number of the support sections  84  is excessively large, large quantity of material is required and the number of assembling steps is increased undesirably. Preferably, in consideration of thermal deformation, the height hc of the heat transportation section  60  is 20 or more times as large as the change in the tolerable height. 
         [0088]    Further, as shown in  FIG. 6 , at least in the heat transportation section  60  of the duct  50 , preferably, the lateral width Wa of the channel  80  as a passage of the air (length in the parallel direction of the battery cells  16 ) is shorter than the total width in the parallel direction of the battery cells  16 , by the distance corresponding to a range of one to two rows of the battery cells  16 . In this case, in the heat transportation section  60 , partition plates  112  are provided on both of left and right sides as viewed in the direction in which the air is supplied (portions adjacent to the third side wall  46   c  and the fourth side wall  46   d ). The air flows along the partition plates  112 . In this manner, the lateral width Wa of the channel  80  as a passage of the air  48  becomes shorter than the total width in the parallel direction of the battery cells  16 , by the distance corresponding to a range of one to two rows of the battery cells  16 . In this regard, in the case where the lateral width Wa of the channel  80  in the heat transportation section  60  is the same as the total width in the parallel direction of the battery cells  16 , by heat radiation from the side surfaces (the third side wall  46   c  and the fourth side wall  46   d ) of the box body  14  and heat radiation resulting from the air  48  flowing through the duct  50 , the battery cells  16  adjacent to the side surfaces of the box body  14  are cooled to a greater extent in comparison with the battery cells  16  arranged at the central portion, the temperature of the battery cells  16  of the battery assembly  18  in the parallel direction may become non-uniform undesirably. Therefore, as described above, the lateral width Wa of the channel  80  of the duct  50  at least in the heat transportation section  60  is shorter than the total width in the parallel direction of the battery cells  16 , by the distance corresponding to the range of one to two rows of the battery cells  16 . More specifically, the lateral width Wa of the channel  80  is shortened to a degree where both sides of the duct  50  in the parallel direction of the battery cells  16  do not reach the side surfaces (the third side wall  46   c  and the fourth side wall  46   d ) of the box body  14 . Thus, the battery cells  16  adjacent to the side surfaces of the box body  14  are not cooled greatly, and it becomes possible to achieve the uniform temperature in the parallel direction of the battery cells  16  in the battery assembly  18 . 
         [0089]    Further, it is preferable to adopt structure where lateral width Wb of the duct  50  from the air inlet section  58  to a position adjacent to the inlet of the heat transportation section  60  (length in the parallel direction of the battery cells  16 : see  FIG. 4 ) is shorter than the total width in the parallel direction of the battery cells  16  by the distance corresponding to a range of one to two rows of the battery cells  16 . Further, it is preferable to adopt structure where the lateral width We (see  FIG. 4 ) from a position adjacent to the outlet of the heat transportation section  60  to the heat releasing section  62  is shorter than the total width in the parallel direction of the battery cells  16  by the distance corresponding to a range of zero to one row of the battery cells  16 . In this manner, it becomes possible to achieve the uniform temperature in the serial direction of the battery cells  16  in the battery assembly  18 . 
         [0090]    Next, several modified embodiments of the secondary battery  10  according to the embodiment of the present invention will be described with reference to  FIGS. 7A to 13B . 
         [0091]    Firstly, as shown in  FIGS. 7A and 7B , a secondary battery  10   a  according to a first modified embodiment has substantially the same structure as the above described secondary battery  10 . However, the secondary battery  10   a  according to the first modified embodiment is different from the secondary battery  10  in the following point. 
         [0092]    Specifically, the heat transportation section  60  is provided between the ceiling wall  30  of the lid body  20  and the box body  14 , and as shown in  FIG. 7B , the heat transportation section  60  is provided spirally from the central portion to the peripheral portion of the box body  14 . For example, the channel may have a circular shape or a rectangular shape in cross section. The heat transportation section  60  is provided from a position between the first side wall  46   a  of the box body  14  and the eaves  32  of the lid body  20  along the first side wall  46   a  of the box body  14 . 
         [0093]    By adopting the channel of the heat transportation section  60  in a spiral pattern having a large channel length, for example, in the case of forcibly cooling the plurality of battery cells  16  using one fan  54 , even if there is some difference in the channel length among the plurality of battery cells  16 , since there is no large difference in the air resistance, it is possible to supply the air equally to each of the battery cells  16 . Further, by directly supplying the low temperature air to the center of the module having the high temperature, cooling can be performed intensively, and it become possible to achieve the uniform temperature of the box body  14 . 
         [0094]    Next, as shown in  FIG. 8 , a secondary battery  10   b  according to a second modified embodiment has substantially the same structure as the above described secondary battery  10 . However, the secondary battery  10   b  according to the second modified embodiment is different from the secondary battery  10  in the following point. 
         [0095]    Specifically, the lid body  20  and the duct  50  are formed integrally.  FIG. 8  shows an example where the duct  50  is incorporated into the hollow area  34  of the lid body  20  to achieve the integral structure. Contact portions between the lid body  20  and the duct  50  are sealed in an air-tight manner, and vacuum heat insulating structure of the lid body  20  is maintained. The contact portions between the lid body  20  and the duct  50  includes, for example, portions between the lid body  20  and the air guide section  68  of the duct  50 , between the lid body  20  and the fins  78  of the duct  50 , and between the lid body  20  and the heat releasing section  62  of the duct  50 . Other possible methods for achieving the integral structure include welding methods, connecting methods using stud bolts, etc. 
         [0096]    By adopting the integral structure of the lid body  20  and the duct  50 , in contrast to the case where the lid body  20  and the duct  50  are provided as separate components, since two metal members overlapped between the lid body  20  and the duct  50  can be integrated into one metal member, the surface area of the metal portion having good thermal conductivity (heat radiation area) is reduced, and improvement in the thermal efficiency is achieved. Further, since the number of components or parts is reduced, assembling operation of the secondary battery  10   b  is simplified. The number of assembling steps is reduced, and reduction of the time required for assembling operation is achieved. 
         [0097]    Next, as shown in  FIGS. 9A and 9B , a secondary battery  10   c  according to a third modified embodiment has substantially the same structure as the above described secondary battery  10 . However, the secondary battery  10   c  according to the third modified embodiment is different from the secondary battery  10  in that heat insulating material  114  is provided between the box body  14  and the lid body  20 . 
         [0098]    In the box body  14 , the heat insulating material  114  is provided at a position adjacent to the first side wall  46   a  as the fluid inlet side. In an example shown in  FIG. 9A , the heat insulating material  114  is provided between the silica sand  24  and the duct  50 , and in particular, the heat insulating material  114  is embedded in the silica sand  24 . It is a matter of course that, as shown in  FIG. 10 , the heat insulating material  114  may be partially embedded in the silica sand  24 . 
         [0099]    Next, the size of the heat insulating material  114  will be described with reference to  FIG. 9B . As a premise of the explanation, it should be noted that La denotes the distance from the outer surface of the third side wall  46   c  of the box body  14  to the outer surface of the fourth side wall  46   d , and Lb denotes the distance from the inner side surface of the third side wall  46   c  to the inner side surface of the fourth side wall  46   d . Lc denotes the distance in the heat insulating material  114  extending from the third side wall  46   c  to the fourth side wall  46   d  (parallel direction y of the battery cells  16 ), and Ld denotes the distance in the heat insulating material  114  extending from the first side wall  46   a  to the second side wall  46   b  (serial direction x of the battery cells  16 ). Further, Le denotes the distance of the box body  14  from the inner side wall of the first side wall  46   a  to the inner side surface of the second side wall  46   b.    
         [0100]    In this regard, the following size relationship is satisfied. 
         [0000]        Lc≧La , or 
         [0000]        Lb&lt;Lc&lt;La , or 
         [0000]    
       
      
       Lc≦Lb  
      
     
         [0000]        Ld&lt;Le/ 2 
         [0101]    Gaps  116  are formed where no heat insulating material  114  is provided. Therefore, for example, in the case where the heat insulating material  114  is placed on the silica sand  24 , or in the case where the heat insulating material is partially embedded in the silica sand  24 , the contents in the box body  14  such as the silica sand  24  are exposed through the gaps  116 . As describe above, it is possible to enhance conduction of the heat from the battery assembly  18  to the duct  50  can be achieved through the fins  78  of the duct  50 . 
         [0102]    It should be noted that the heat insulating material  114  may be placed, or may not be placed on an upper end surface of the box body  14 . For example, ceramic fiber, etc. can be used for the heat insulating material  114 . 
         [0103]    If the cooled air  48  is supplied to the duct  50  by operation of the fan  54 , since the temperature of the air  48  is low, a large amount of heat is taken away depending on the temperature difference. In the box body  14 , an area around the first side wall  46   a  as the fluid inlet side is cooled. Consequently, normally, the temperature Dk detected by the side surface temperature sensor  102   b  becomes the predetermined lower limit threshold temperature Dth 3  or less (e.g., 300° C.), and the first side surface heater  22   a  to the fourth side surface heater  22   d  are energized. In this case, since the fan  54  is driven by the fan drive circuit  96 , and the first side surface heater  22   a  to the fourth side surface heater  22   d  are energized by the side surface heater drive circuit  110 , the system efficiency may be degraded undesirably. 
         [0104]    Thus, the heat insulating material  114  is provided between the box body  14  and the lid body  20  to suppress transmission of the heat of the outer surface of the first side wall  46   a  to the inside of the box body  14 . Consequently, it becomes possible to avoid a situation where the first side surface heater  22   a  to the fourth side surface heater  22   d  are energized even during driving of the fan  54  by the fan drive circuit  96 . Accordingly, since it is possible to suppress degradation of the system efficiency, this structure is more preferable. 
         [0105]    Further, as shown in  FIG. 6 , in the case where the lateral width Wa of the channel  80  where the air  48  flows in at least the heat transportation section  60  of the duct  50  is shorter than the total width in the parallel direction of the battery cells  16  by the distance corresponding to a range of one to two rows of the battery cells  16 , using the partition plates  112 , since the air  48  does not flow through spaces  117  outside the partition plates  112 , almost no heat is taken away by the air  48  normally. However, in the case where the duct  50  is made of material having good thermal conductivity such as metal, the decrease in the temperature by the air  48  may be transmitted to the inside of the box body  14  through the metal portions outside the partition plates  112 . In this case, heat balance is degraded, and the system efficiency is degraded. 
         [0106]    Therefore, as shown in  FIGS. 11A and 11B , in addition to the above described heat insulating material  114  (see  FIG. 11B ), preferably, heat insulating material  114   a  is provided along the third side wall  46   c , and heat insulating material  114   b  is provided along the fourth side wall  46   d . In the structure, since the decrease in the temperature of the air  48  is not transmitted to the inside of the box body  14  through the metal portions outside the partition plates  112 , it is possible to suppress degradation of the heat balance. 
         [0107]    Further, assuming that the flow rate of the air  48  flowing through the duct  50  is constant, if the distance Le from the inner side surface of the first side wall  46   a  to the inner side wall surface of the second side wall  46   b  (see  FIGS. 9B and 12A ) of the box body  14  is short, the amount of heat transmitted to the air  48  is reduced. In the state where the temperature of the air  48  is low, the air  48  reaches the heat releasing section  62  adjacent to the second side wall  46   b . Therefore, in the box body  14 , since the area around the second side wall  46   b  on the fluid output side is cooled, the heat balance is degraded, and the energy efficiency is degraded. Thus, as shown in  FIG. 12A , in addition to the above described heat insulating material  114 , it is preferable to provide heat insulating material  114   c  along the second side wall  46   b . In the structure, even if the above described distance Le is short, the decrease in the temperature by the air  48  is not transmitted easily to the inside of the box body  14 . Therefore, it is possible to suppress degradation of the heat balance. 
         [0108]    It is a matter of course that, as shown in  FIG. 12B , in addition to the above described heat insulating material  114 , the heat insulating materials  114   a ,  114   b , and  114   c  may be provided along the third side wall  46   c , the fourth side wall  46   d , and the second side wall  46   b . Though the number of heat insulating materials  114  is increased, this structure is effective in suppressing degradation of the heat balance. 
         [0109]    Next, as shown in  FIG. 13A , the secondary battery  10   d  according to a fourth modified embodiment has substantially the same structure as the above described secondary battery  10 . However, the secondary battery  10   d  according to the fourth modified embodiment is different from the secondary battery  10  in the following point. 
         [0110]    That is, the air inlet section  58  includes a through hole  120  formed in a central portion of the ceiling wall  30  of the lid body  20 . The heat transportation section  60  is provided between the ceiling wall  30  of the lid body  20  and the box body  14 , and transports heat generated at least in the box body  14  toward the heat releasing section  62  together with the air  48  supplied through the through hole  120 . The heat releasing section  62  includes a first heat releasing section  62 A provided from a position between the first side wall  46   a  of the box body  14  and the eaves  32  of the lid body  20  along the first side wall  46   a  of the box body  14 , and the second heat releasing section  62 B provided at a position between the second side wall  46   b  of the box body  14  and the eaves  32  of the lid body  20  along the second side wall  46   b  of the box body  14 . 
         [0111]    As shown in  FIG. 13B , a secondary battery  10   e  according to a fifth modified embodiment has substantially the same structure as the above described secondary battery  10 . However, the secondary battery  10   e  according to the fifth modified embodiment is different from the secondary battery  10  in the following point. 
         [0112]    Specifically, the air inlet section  58  includes a first through hole  122 A formed in one side wall of the eaves  32  of the lid body  20 . The heat releasing section  62  includes a second through hole  122 B formed in the other side wall opposite to one side wall of the eaves  32  of the lid body  20 . The heat transportation section  60  is provided between the ceiling wall  30  of the lid body  20  and the box body  14 , and transports heat generated at least in the box body  14  toward the second through hole  122 B of the heat releasing section  62  together with the air  48  supplied through the first through hole  122 A. 
         [0113]    The above described fourth and fifth modified embodiments are preferable in the case where the load on the electric power system is high, e.g., in summer season or in winter season, i.e., in the case where the electric discharge output is high or the electric discharge time period is long. However, since the lid body  20  has the through holes, the heat insulating performance is low. Therefore, it is preferable to close the through holes in the case where the load on the electric power system is low, e.g., in spring season or autumn season, and in the case where the electrical discharge time period is short, for example, at the time of leveling power generation such as wind force power generation 
         [0114]    It is a matter of course that the secondary battery according to the present invention is not limited to the embodiments described above, and various structures can be adopted without deviating the gist of the present invention. For example, in the above-described embodiments, both the box body and the lid body adopt vacuum insulation structure. Alternatively, both the box body and the lid body may adopt atmospheric heat insulating structure. Instead, it is a matter of course that the lid body may adopt atmospheric heat insulating structure and the box body may adopt vacuum insulation structure, or that the lid body may adopt vacuum insulation structure and the box body may adopt atmospheric heat insulating structure.