Patent Publication Number: US-2017352894-A1

Title: Electrolyte-circulating battery

Description:
TECHNICAL FIELD 
     The present invention relates to an electrolyte-circulating battery such as a redox flow battery. In particular, the present invention relates to an electrolyte-circulating battery in which a sealing structure of an electrolyte can be easily formed and which has a good assembling workability. 
     BACKGROUND ART 
     An example of large-capacity storage batteries that store the electric power derived from natural energy, for example, obtained by photovoltaic power generation or wind power generation, is an electrolyte-circulating battery such as a redox flow battery (RF battery). An RF battery is typically connected between a power generation unit (for example, a photovoltaic power generator, a wind power generator, or a typical power plant) and a load (for example, a power consumer) with an AC/DC converter therebetween, stores the electric power generated in the power generation unit by charging, and supplies the stored electric power to the load by discharging. 
     For example, as illustrated in the operation principle diagram of an RF battery of  FIG. 8 , an RF battery  1  includes a battery cell  100  which is separated into a positive electrode cell  102  and a negative electrode cell  103  by a membrane  101  that allows hydrogen ions to permeate. The positive electrode cell  102  contains a positive electrode  104  therein and is connected, via a supply duct  108  and a discharge duct  110 , to a positive electrode electrolyte tank  106  that stores a positive electrode electrolyte. Similarly, the negative electrode cell  103  contains a negative electrode  105  therein and is connected, via a supply duct  109  and a discharge duct  111 , to a negative electrode electrolyte tank  107  that stores a negative electrode electrolyte. The positive and negative electrode electrolytes are supplied from the supply ducts  108  and  109  to the electrode cells  102  and  103  by pumps  112  and  113  disposed in halfway positions of the supply ducts  108  and  109 , respectively, and discharged from the electrode cells  102  and  103  to the electrode electrolyte tanks  106  and  107  through the discharge ducts  110  and  111 , thereby being circulated through the electrode cells  102  and  103 , respectively. The RF battery  1  circulates the electrolytes in this manner to perform charging and discharging by using the difference in redox potential between ions contained in the positive electrode electrolyte and ions contained in the negative electrode electrolyte. In  FIG. 8 , vanadium ions are illustrated as ions contained in the electrolytes of the electrodes, the solid-line arrows indicate charging, and the dotted-line arrows indicate discharging. 
     The battery cell  100  is typically formed inside a structure referred to as a cell stack  200  illustrated in the lower drawing of  FIG. 9 . As illustrated in the upper drawing of  FIG. 9 , the cell stack  200  includes a stacked body formed by stacking a cell frame  120 , a positive electrode  104 , a membrane  101 , and a negative electrode  105  in that order. The cell frame  120  includes a bipolar plate  121  and a frame  122  that surrounds a peripheral edge of the bipolar plate  121 . In the case of this structure, one battery cell  100  is formed between bipolar plates  121  of adjacent cell frames  120 , and a positive electrode  104  (positive electrode cell  102 ) and a negative electrode  105  (negative electrode cell  103 ) of adjacent battery cells  100  are disposed on the front side and the back side of each bipolar plate  121 . 
     The flow of the electrolyte of each of the electrodes in the battery cell  100  is performed through liquid-supplying manifolds  131  and  132  and liquid-discharging manifolds  133  and  134  which are formed on the frame  122 . The positive electrode electrolyte is supplied from the liquid-supplying manifolds  131  to the positive electrode  104  through guide grooves formed on one surface side (front side of the sheet) of the frame  122 . As indicated by the arrows in the upper drawing of  FIG. 9 , the positive electrode electrolyte passes from the lower side to the upper side of the positive electrode  104  and is discharged to the liquid-discharging manifolds  133  through guide grooves formed on the frame  122 . Similarly, the negative electrode electrolyte is supplied from the liquid-supplying manifolds  132  to the negative electrode  105  through guide grooves formed on the other surface side (back side of the sheet) of the frame  122 . The negative electrode electrolyte passes from the lower side to the upper side of the negative electrode  105  and is discharged to the liquid-discharging manifolds  134  through guide grooves formed on the frame  122 . A ring-shaped sealing member  140  such as an O-ring is disposed between the frames  122  to suppress leakage of an electrolyte from the battery cell  100 . 
     For example, Patent Literature 1 describes that leakage of an electrolyte in a cell is prevented by providing an inner peripheral seal groove and an outer peripheral seal groove each having a rectangular cross-sectional shape on both surfaces of a frame body (frame) at positions facing each other, and arranging an inner peripheral seal and an outer peripheral seal in the inner peripheral seal groove and the outer peripheral seal groove, respectively. O-rings are used as the inner peripheral seal and the outer peripheral seal to come in contact with a membrane under pressure and to seal the electrolyte. 
     CITATION LIST 
     Patent Literature 
     PTL 1: Japanese Unexamined Patent Application Publication No. 2002-367659 
     SUMMARY OF INVENTION 
     Technical Problem 
     A sealing member such as an O-ring formed of an elastic material is soft and has a long string-like shape, and thus the shape of the O-ring is not easily determined. Accordingly, even when the O-ring is fitted in a seal groove, the O-ring may easily become detached from the seal groove. An example means for suppressing this detachment of the O-ring is to fix the O-ring to the seal groove with an adhesive. However, it is necessary to apply the adhesive onto the seal groove in advance, and production efficiency decreases with an increase in the number of production steps. 
     The present invention has been made in view of the circumstances described above. An object of the present invention is to provide an electrolyte-circulating battery in which a sealing structure of an electrolyte can be easily formed and which has good production efficiency. 
     Solution to Problem 
     An electrolyte-circulating battery according to an embodiment of the present invention includes a cell frame including a bipolar plate in contact with an electrode that forms a battery cell, and a frame that surrounds a peripheral edge of the bipolar plate; and a sealing member that is disposed on the frame and that prevents an electrolyte supplied to the battery cell from leaking out of the frame. The frame has a seal groove in which the sealing member is fitted. The seal groove includes a narrow section that causes the sealing member to elastically deform to prevent the sealing member from becoming detached from the seal groove. The narrow section has a width that is uniform in a depth direction of the seal groove. 
     Advantageous Effects of Invention 
     According to the electrolyte-circulating battery, a sealing structure of an electrolyte can be easily formed, and good production efficiency is realized. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a schematic plan view illustrating a cell frame included in a redox flow battery according to Embodiment 1. 
         FIG. 2  is a partially enlarged view illustrating a seal groove of the cell frame illustrated in  FIG. 1  in an enlarged manner. 
         FIG. 3  is an (a)-(a) sectional view of the seal groove illustrated in  FIG. 2 . 
         FIG. 4  is a partially enlarged view illustrating, in an enlarged manner, a narrow section in a seal groove of a cell frame included in a redox flow battery according to Embodiment 2. 
         FIG. 5  is a partially enlarged view illustrating, in an enlarged manner, a narrow section in a seal groove of a cell frame included in a redox flow battery according to Embodiment 3. 
         FIG. 6  is a partially enlarged view illustrating, in an enlarged manner, a narrow section in a seal groove of a cell frame included in a redox flow battery according to Embodiment 4. 
         FIG. 7  is a partially enlarged view illustrating, in an enlarged manner, a narrow section in a seal groove of a cell frame included in a redox flow battery according to Embodiment 5. 
         FIG. 8  is an operation principle diagram of a redox flow battery. 
         FIG. 9  is a schematic structural view of a cell stack included in a redox flow battery. 
     
    
    
       
     
       
         
           
               
             
               
                   
               
               
                 Reference Signs List 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
            
               
                   
                 1 RF battery 
               
               
                   
                 2 cell frame 
               
            
           
           
               
               
            
               
                   
                 3 bipolar plate 
               
               
                   
                 4 frame 
               
            
           
           
               
               
               
            
               
                   
                 41 liquid supply-side piece 
                 42 liquid discharge-side piece 
               
            
           
           
               
               
            
               
                   
                 43 connection piece 
               
            
           
           
               
               
               
            
               
                   
                 44 liquid-supplying manifold 
                 45 liquid-discharging manifold 
               
            
           
           
               
               
            
               
                   
                 44a, 45a manifold seal groove 
               
            
           
           
               
               
               
            
               
                   
                 46 liquid supply guide groove 
                 47 liquid discharge guide groove 
               
            
           
           
               
               
            
               
                   
                 48 seal groove 
               
            
           
           
               
               
               
            
               
                   
                 48i inner peripheral seal groove 
                 48o outer peripheral seal 
               
            
           
           
               
            
               
                 groove 
               
            
           
           
               
               
               
            
               
                   
                 48L long straight-line section 
                 48S short straight-line 
               
            
           
           
               
            
               
                 section 
               
            
           
           
               
               
            
               
                   
                 48C curved section 
               
            
           
           
               
               
            
               
                   
                 49 narrow section 49a, 49b, 49c, 49d, 49e protruding piece 
               
            
           
           
               
               
            
               
                   
                 5 sealing member 
               
               
                   
                 100 battery cell 
               
            
           
           
               
               
               
               
            
               
                   
                 101 membrane 
                 102 positive electrode cell 
                 103 negative electrode 
               
            
           
           
               
            
               
                 cell 
               
            
           
           
               
               
               
            
               
                   
                 104 positive electrode 
                 105 negative electrode 
               
            
           
           
               
               
               
            
               
                   
                 106 positive electrode electrolyte tank 
                 107 negative electrode electrolyte tank 
               
            
           
           
               
               
               
            
               
                   
                 108, 109 supply duct 
                 110, 111 discharge duct 
               
            
           
           
               
               
            
               
                   
                 112, 113 pump 
               
               
                   
                 120 cell frame 
               
            
           
           
               
               
               
            
               
                   
                 121 bipolar plate 
                 122 frame 
               
            
           
           
               
               
               
            
               
                   
                 131, 132 liquid-supplying manifold 
                 133, 134 liquid-discharging manifold 
               
            
           
           
               
               
            
               
                   
                 140 sealing member 
               
               
                   
                 200 cell stack 
               
               
                   
                   
               
            
           
         
       
     
     DESCRIPTION OF EMBODIMENTS 
     Description of Embodiments of the Present Invention 
     First, the content of embodiments of the present invention will be listed and described. 
     (1) An electrolyte-circulating battery according to an embodiment of the present invention includes a cell frame including a bipolar plate in contact with an electrode that forms a battery cell, and a frame that surrounds a peripheral edge of the bipolar plate; and a sealing member that is disposed on the frame and that prevents an electrolyte supplied to the battery cell from leaking out of the frame. The frame has a seal groove in which the sealing member is fitted. The seal groove includes a narrow section that causes the sealing member to elastically deform to prevent the sealing member from becoming detached from the seal groove. The narrow section has a width that is uniform in a depth direction of the seal groove. 
     According to the structure described above, detachment of the sealing member from the seal groove can be suppressed by simply fitting the sealing member in the narrow section of the seal groove. This is because since the seal groove includes the narrow section that causes the sealing member to elastically deform, the sealing member is locally compressed, and a frictional force can be applied to the compressed portion in a longitudinal direction and in a depth direction of the seal groove. Therefore, an adhesive is not necessary to fix the sealing member to the seal groove, and a step of applying an adhesive is unnecessary. The sealing structure of an electrolyte can be firmed without increasing the number of production steps. Accordingly, the structure described above realizes good production efficiency of the electrolyte-circulating battery. 
     In addition, since the width of the narrow section is uniform in the depth direction, production efficiency of the frame having the seal groove can be increased. This is because the seal groove including the narrow section can also be formed at the same time when the frame is produced, and thus the number of production steps is not increased. As in the structure described above, an example of the shape of a groove capable of suppressing detachment of the sealing member from the seal groove by simply fitting the sealing member in the seal groove is a trapezoidal shape or substantially trapezoidal shape whose width gradually increases in the depth direction, such as a dovetail groove. However, in the case of a frame having such a dovetail groove, the seal groove cannot be formed at the same time when the frame is produced, and thus it is necessary to separately perform grooving, resulting in an increase in the number of production steps. Although the frame is typically produced by, for example, injection molding, it is difficult to form a groove having a shape of a dovetail groove or the like by injection molding. This is because, in order to produce a frame having a dovetail groove, after a frame that does not have a dovetail groove is produced, it is necessary to form a groove by, for example, cutting so that the groove has a trapezoidal shape or a substantially trapezoidal shape. 
     (2) In an embodiment of the electrolyte-circulating battery, a width Wn (mm) of the narrow section may be 0.60 D or more and 0.97 D or less where D represents a diameter (mm) of the sealing member in a non-compressed state. 
     According to the structure described above, since the width Wn (mm) of the narrow section is 0.60 D or more, it is easy to prevent the sealing member from degrading in early use due to crushing or excessive compression. Since the width Wn (mm) of the narrow section is 0.97 D or less, detachment of the sealing member from the seal groove is easily prevented. 
     (3) In an embodiment of the electrolyte-circulating battery, the narrow sections may be disposed at intervals of 50 mm or more and 500 mm or less in a longitudinal direction of the seal groove. 
     According to the above structure, since a plurality of narrow sections are disposed at an interval of 50 mm or more, the interval of the narrow sections in the longitudinal direction of the seal groove is not excessively small. Therefore, the operation of fitting the sealing member in the seal groove is unlikely to be complicated. Since a plurality of narrow sections are disposed at an interval of 500 mm or less, detachment of the sealing member is easily prevented. 
     (4) In an embodiment of the electrolyte-circulating battery, the seal groove may include a curved section that curves in a longitudinal direction, and the narrow section may be disposed in the curved section. 
     According to the above structure, since the narrow section is disposed in the curved-line section (curved section) of the seal groove from which the sealing member becomes detached more easily than the straight-line section of the seal groove, detachment of the sealing member is effectively prevented. 
     Details of Embodiments of the Present Invention 
     Embodiments of the present invention will now be described with reference to the drawings. It is intended that the present invention is not limited to these examples but is defined by the claims described below, and that the scope of the present invention includes the meaning of equivalents of the claims and all modifications within the scope of the claims. Herein, a description will be made using a redox flow battery (RF battery) as an example of an electrolyte-circulating battery. 
     Embodiment 1 
     An RF battery according to Embodiment 1 includes, as in the existing RF battery described with reference to  FIGS. 8 and 9 , a cell stack  200  that includes a stacked body of cell frames and battery cells  100 , a positive electrode electrolyte tank  106  that stores a positive electrode electrolyte circulated through a positive electrode cell  102  of the battery cell  100 , and a negative electrode electrolyte tank  107  that stores a negative electrode electrolyte circulated through a negative electrode cell  103  of the battery cell  100 . The positive electrode electrolyte and the negative electrode electrolyte are respectively circulated through supply ducts  108  and  109  and discharge ducts  110  and  111  by pumps  112  and  113  disposed between the corresponding supply duct and the discharge duct. A main feature of the RF battery according to Embodiment 1 lies in the fixing structure of a sealing member to a cell frame, the structure being capable of preventing an electrolyte from leaking. Specifically, the RF battery according to Embodiment 1 includes a cell frame having a different structure from the cell frame  120  ( FIG. 9 ) of the existing RF battery, and thus the difference will be mainly described below with reference to  FIGS. 1 to 3  ( FIGS. 8 and 9 , as required). Components the same as those of the existing RF battery are assigned the same reference numerals as those in  FIGS. 8 and 9 , and a description of those components is omitted. 
     [Cell Frame] 
     A cell frame  2  includes a bipolar plate  3  and a frame  4  that surrounds a peripheral edge of the bipolar plate  3 . There are two types of the cell frame  2 , namely, an intermediate cell frame disposed between adjacent battery cells  100  ( FIG. 9 ) of the stacked body, and an end cell frame disposed on both ends of the stacked body. The front surface and the back surface of the bipolar plate  3  of the intermediate cell frame contact a positive electrode  104  of one battery cell  100  and a negative electrode  105  of the other battery cell  100 . One surface of the bipolar plate  3  of the end cell frame contacts one of the positive electrode and the negative electrode of a battery cell  100 . Regarding the structure of the front and back (positive electrode side/negative electrode side) surfaces of the cell frame  2 , the intermediate cell frame and the end cell frame have the same structure. 
     (Bipolar Plate) 
     The bipolar plate  3  partitions adjacent battery cells  100  ( FIGS. 8 and 9 ) in principle. The bipolar plate  3  is a rectangular plate. The bipolar plate  3  may be formed of a material that allows an electric current to pass but does not allow an electrolyte to pass. In addition, the bipolar plate is preferably formed of a material having acid resistance and an appropriate rigidity. Examples of such a material include conductive materials containing carbon. Specific examples thereof include conductive plastics formed from graphite and a polyolefin organic compound or a chlorinated organic compound. The bipolar plate may be formed of a conductive plastic in which part of graphite is replaced with at least one of carbon black and diamond-like carbon. Examples of the polyolefin organic compound include polyethylene, polypropylene, and polybutene. Examples of the chlorinated organic compound include vinyl chloride, chlorinated polyethylene, and chlorinated paraffin. By forming the bipolar plate  3  using the material described above, the electrical resistance of the bipolar plate  3  can be reduced and good acid resistance is achieved. 
     (Frame) 
     The frame  4  forms a region functioning as a battery cell  100  ( FIGS. 8 and 9 ) inside thereof. The frame  4  includes a liquid supply-side piece  41  having liquid-supplying manifolds  44  through which an electrolyte is supplied to the inside of the battery cell  100 , and a liquid discharge-side piece  42  having liquid-discharging manifolds  45  through which an electrolyte is discharged to the outside of the battery cell  100 . An end of the liquid supply-side piece  41  and an end of the liquid discharge-side piece  42  are connected to each other with a pair of connection pieces  43  facing each other and orthogonal to the liquid supply-side piece  41  and the liquid discharge-side piece  42 . The frame  4  has a rectangular frame shape. The liquid supply-side piece  41  and the liquid discharge-side piece  42  form long pieces of the rectangular frame, and the connection pieces  43  form short pieces of the rectangular frame. In plan view of the cell frame  2 , when a direction in which the liquid supply-side piece  41  and the liquid discharge-side piece  42  face each other is defined as a vertical direction, and a direction orthogonal to the vertical direction is defined as a lateral direction, the liquid supply-side piece  41  is located on the lower side in the vertical direction and the liquid discharge-side piece  42  is located on the upper side of the vertical direction. Specifically, the electrolyte flows in the direction from the lower side of the frame  4  in the vertical direction toward the upper side of the frame  4  in the vertical direction. A seal groove  48  is formed in a peripheral edge of the frame  4  so as to be located outside the liquid-supplying manifolds  44  and the liquid-discharging manifolds  45 . The arrow at the center of  FIG. 1  indicates that sealing members  5  are fitted in the seal groove  48 . 
     &lt;Seal Groove&gt; 
     Sealing members  5  described below are fitted in the seal groove  48 . The seal groove  48  is formed on both surfaces of the frame  4 . Alternatively, the seal groove  48  is formed on one surface side (the front-surface side of the sheet of  FIG. 1 ) and is not formed on the other surface side (the back-surface side of the sheet of  FIG. 1 ) of the frame  4 . In the case where the seal groove  48  is formed on both surfaces, when cell frames  2  are stacked, a sealing member  5  in the seal groove  48  is pressed by a sealing member fitted in a seal groove on another surface of an adjacent cell frame and seals the cell frames with each other. In this case, a membrane  101  ( FIGS. 8 and 9 ) is sandwiched between the sealing members. In the case where the seal groove  48  is formed on a surface and is not formed on another surface, when cell frames  2  are stacked, a sealing member  5  in the seal groove  48  is pressed by another surface of a frame of an adjacent cell frame and seals the cell frames with each other. In this case, the seal groove  48  is formed on one surface side (the front-surface side of the sheet of  FIG. 1 ) of the frame  4  and is not formed on the other surface side (the back-surface side of the sheet of  FIG. 1 ) of the frame  4 . The seal groove  48  includes an inner peripheral seal groove  48   i  and an outer peripheral seal groove  48   o . The seal groove  48  may include any one of the inner peripheral seal groove  48   i  and the outer peripheral seal groove  48   o . The structures of the two seal grooves  48   i  and  48   o  are the same. 
     The seal groove  48  is formed so as to have a ring shape along the outer shape (long dimensions of the pieces  41  to  43 ) of the frame  4 . The seal groove  48  includes long straight-line sections  48 L that are formed in the liquid supply-side piece  41  and the liquid discharge-side piece  42  and that are linear in the longitudinal direction, short straight-line sections  48 S that are formed in the connection pieces  43  and that are linear in the longitudinal direction, and curved sections  48 C that are formed at corners of the frame  4 , that connect between a long straight-line section  48 L and a short straight-line section  48 S, and that curve in the longitudinal direction of the seal groove  48 . The seal groove  48  includes narrow sections  49 . 
     Each of the narrow sections  49  causes a sealing member  5  to elastically deform to prevent the sealing member  5  from becoming detached from the seal groove  48 . The term “narrow section  49 ” refers to a section having a narrower groove width than other sections in the longitudinal direction of the seal groove  48 . A width Wn of the narrow section  49  is less than a diameter D (mm) of the sealing member  5  in the non-compressed state. In other words, the width of each wide section other than the narrow sections  49  is equal to or more than the diameter D of the sealing member  5 . Therefore, in the wide section, a space is formed between the sealing member  5  and the seal groove  48 . The sealing member  5  can be easily fitted in the wide sections, and detachment of the sealing member  5  from the seal groove  48  can be suppressed by simply fitting the sealing member  5  in the narrow sections  49 . This is because the narrow sections  49  can apply a suitable frictional force to the sealing member  5  in the longitudinal direction of the sealing member  5  and in the depth direction of the seal groove  48 . 
     Each of the narrow sections  49  has a width Wn that is uniform in the depth direction of the seal groove  48  ( FIG. 3 ). This structure can increase production efficiency of the frame  4  having the seal groove  48 . The reason for this is as follows. The frame  4  can be typically produced by, for example, injection molding. In the case where the width Wn of the narrow section  49  is uniform in the depth direction of the seal groove  48 , the seal groove  48  can also be formed at the same time when the frame  4  is produced. Thus, it is not necessary to separately perform grooving or the like, and the number of production steps does not increase. For example, if the seal groove has a trapezoidal shape or substantially trapezoidal shape whose width gradually increases in the depth direction, as in the case of a dovetail groove, from the viewpoint of removing a molded body from a mold, the seal groove cannot be formed at the same time when the frame is produced. Thus, it is necessary to separately perform grooving, resulting in an increase in the number of production steps. This is because, after a frame that does not have a dovetail groove is produced, it is necessary to perform grooving by, for example, cutting so as to form a seal groove having a trapezoidal shape or a substantially trapezoidal shape. 
     The width Wn (mm) of the narrow section  49  can be appropriately selected in accordance with the diameter D (mm) of the sealing member  5  in the non-compressed state so that the narrow section  49  does not cause the sealing member  5  to excessively elastically deform and does not cause the sealing member  5  to become detached. Herein, the term “width Wn” of the narrow section  49  refers to a width of a portion of the seal groove  48 , the portion having the narrowest width. The width Wn of the narrow section  49  is preferably 0.60 D or more and 0.97 D or less though it depends on the material of the sealing member  5 . When the width Wn of the narrow section  49  is 0.60 D or more, it is easy to prevent the sealing member  5  from degrading in early use due to crushing or excessive compression by the inner wall of the narrow section  49 . When the width Wn of the narrow section  49  is 0.97 D or less, detachment of the sealing member  5  from the seal groove  48  is easily prevented. The width Wn of the narrow section  49  is more preferably 0.88 D or more and 0.97 D or less. The width Wn of the narrow section  49  satisfying 0.60 D or more and 0.97 D or less means that the compressibility of the sealing member  5  in the width direction is 3% or more and 40% or less. The compressibility in the width direction is defined as “{(diameter D of sealing member  5 −width Wn of narrow section  49 )/(diameter D of sealing member  5 )×100}”. 
     A depth d (mm) of the seal groove  48  is preferably 0.6 D or more and 0.9 D or less though it depends on the material of the sealing member  5 . When the depth d of the seal groove  48  is 0.6 D or more, detachment of the sealing member  5  from the seal groove  48  is easily prevented. In addition, since a region of the sealing member  5  exposed from the seal groove  48  is not excessively large, it is easy to prevent the sealing member  5  from being crushed by the adjacent cell frame or a sealing member provided on the adjacent cell frame (hereinafter, may be referred to as “adjacent member”). When the depth d of the seal groove  48  is 0.9 D or less, the region of the sealing member  5  exposed from the seal groove  48  can be appropriately ensured, and the sealing member  5  is easily and sufficiently compressed by the adjacent member. The depth d of the seal groove  48  is more preferably 0.7 D or more and 0.8 D or less. The depth d of the seal groove  48  satisfying 0.6 D or more and 0.9 D or less means that the compressibility of the sealing member  5  in the depth direction is 10% or more and 40% or less. The compressibility in the depth direction is defined as “{(diameter D of sealing member  5 −depth d of seal groove  48 )/(diameter D of sealing member  5 )×100}”. 
     A total (Wn+d) of the width Wn of the narrow section  49  and the depth d of the seal groove  48  is preferably 1.2 D or more and 1.8 D or less. When the total (Wn+d) of the narrow section  49  is 1.2 D or more and 1.8 D or less, the sealing member  5  is sufficiently easily compressed, crushing of the sealing member  5  is easily prevented, and detachment of the sealing member  5  from the seal groove  48  is easily prevented. The total (Wn+d) of the narrow section  49  is more preferably 1.6 D or more and 1.7 D or less. Specifically, the total of the compressibility in the width direction and the compressibility in the depth direction preferably satisfies 20% or more and 80% or less. This total of the compressibility particularly preferably satisfies 40% or less. 
     A volume-occupying ratio of the sealing member  5  in the seal groove  48  is preferably 100% or less. This volume-occupying ratio is defined as (VS/VG)×100 where VG (mm 3 ) represents the volume of the seal groove  48  and VS (mm 3 ) represents the volume of the sealing member  5  in the non-compressed state. When the volume-occupying ratio is 100% or less, it is easy to prevent the sealing member  5  from being crushed, and it is possible to suppress the application of an excessive pressure onto the seal groove  48  when the sealing member  5  is compressed, and thus crushing of the seal groove  48  is also easily prevented. Since the volume-occupying ratio is 100% or less, when the sealing member  5  is compressed, a space is formed between the sealing member  5  and the seal groove  48  in a portion (wide section) other than the narrow sections  49 . The volume-occupying ratio of the sealing member  5  in the seal groove  48  is preferably 50% or more. In this case, leakage of an electrolyte is more easily prevented over the entire perimeter of the frame  4 . The volume-occupying ratio is particularly preferably 50% or more and 90% or less. 
     The narrow section  49  is formed in each of the long straight-line section  48 L, the short straight-line section  48 S, and the curved section  48 C ( FIG. 1 ). The seal groove  48  may include only a single narrow section  49 . Alternatively, as in this example, when the seal groove  48  includes a plurality of narrow sections  49  at appropriate positions over the entire perimeter of the seal groove  48 , detachment of the sealing member  5  is easily prevented. The narrow section  49  may be provided in at least one of the long straight-line section  48 L, the short straight-line section  48 S, and the curved section  48 C. However, the narrow section  49  is preferably provided in all the three sections, as in this example. A plurality of narrow sections  49  are preferably provided at opposite positions and/or diagonal positions in the seal groove  48 . As in this example, the narrow sections  49  are more preferably disposed in the long straight-line sections  48 L and the short straight-line sections  48 S located at opposite positions and in the curved sections  48 C located at diagonal positions. Forming the narrow sections  49  in the curved sections  48 C is effective to prevent the sealing member  5  from becoming detached because the sealing member  5  becomes easily detached from the curved sections  48 C compared with the long straight-line sections  48 L and the short straight-line sections  48 S. In the curved sections  48 C, the narrow sections  49  are formed at the center or substantially the center of the curved sections  48 C in the longitudinal direction (the center or substantially the center of the are). 
     The interval of adjacent narrow sections  49  in the longitudinal direction of the seal groove  48  is determined so that, for example, detachment of the sealing member  5  can be prevented and the operation of fitting the sealing member  5  in the seal groove  48  is unlikely to be complicated. Specifically, the interval is preferably 50 mm or more and 500 mm or less, and more preferably 100 mm or more and 200 mm or less in the longitudinal direction of the seal groove  48 . 
     The interval of adjacent narrow sections  49  may be uniform over the entire perimeter of the seal groove  48  or may be uniform in each of the liquid supply-side piece  41 , the liquid discharge-side piece  42 , and the connection pieces  43 . In the latter case, as long as an interval L 1  between narrow sections  49  in the long straight-line section  48 L is uniform and an interval L 2  between narrow sections  49  in the short straight-line section  48 S is uniform, the interval L 1  need not be the same as the interval L 2  ( FIG. 2 ). An interval L 3  and an interval L 4  are preferably uniform, the interval L 3  being a distance between a narrow section  49  of the curved section  48 C and, among narrow sections  49  of the long straight-line section  48 L, a narrow section  49  adjacent to the narrow section  49  of the curved section  48 C, and the interval L 4  being a distance between the narrow section  49  of the curved section  48 C and, among narrow sections  49  of the short straight-line section  48 S, a narrow section  49  adjacent to the narrow section  49  of the curved section  48 C. However, the interval L 3  and the interval L 4  may become non-uniform as a result of making the interval L 1  and the interval L 2  uniform. The interval L 3  and the interval L 4  are respectively defined as a length of the long straight-line section  48 L and a length of the short straight-line section  48 S in the longitudinal direction. 
     In this embodiment, the interval L 1  between the narrow sections  49  in the long straight-line section  48 L is a regular interval of 150 mm, and the interval L 2  between narrow sections  49  in the short straight-line section  48 S is a regular interval of 120 mm. The interval L 3  is about 115 mm, and the interval L 4  is about 90 mm. 
     The form of the narrow section  49  is not particularly limited as long as the width of the narrow section  49  is narrower than the width of other portions of the seal groove  48 . In this embodiment, as illustrated in the circle shown by the one-dotted chain line in  FIG. 2  in an enlarged manner, the narrow section  49  is formed of protruding pieces  49   a  that face each other in the width direction of the seal groove  48  and that protrude in an are shape in a direction in which the protruding pieces  49   a  face each other. In the partially enlarged view of  FIG. 2 , for the sake of convenience of explanation, the sealing member  5  is omitted. In the circle shown by the one-dotted chain line in the figure, the sealing member  5  is indicated by the two-dotted chain line. 
     The outline shape of each of the protruding pieces  49   a  can be appropriately selected. In this embodiment, the protruding piece  49   a  has an outline corresponding to a shape of an arc in which an apex is curved. Since the apex of the protruding piece  49   a  is curved, a frictional force can be applied to the sealing member  5  without damaging the sealing member  5 . The bend radius of the protruding piece  49   a  can be appropriately selected. For example, the bend radius of the protruding piece  49   a  is preferably 1 mm or more and 50 mm or less, and particularly preferably 5 mm or more and 20 mm or less. 
     Examples of the material of the frame  4  include materials that satisfy acid resistance, electric insulation, and mechanical properties. Examples of such materials include fluororesins such as polytetrafluoroethylene, polypropylene resins, polyethylene resins, and vinyl chloride resins. In this embodiment, the frame  4  is formed of a rigid vinyl chloride resin. 
     [Sealing Member] 
     The sealing members  5  prevent an electrolyte supplied to the battery cell  100  ( FIGS. 8 and 9 ) from leaking out of the frame  4  ( FIG. 1 ). Specifically, each of the sealing members  5  is disposed in the seal groove  48  and, when cell frames  2  are stacked as described above, the sealing member  5  is pressed by a sealing member fitted in a seal groove on another surface of a frame of an adjacent cell frame or by another surface of a frame of an adjacent cell frame and elastically deformed, thereby sealing the cell frames  2  with each other. 
     For example, an O-ring is used as the sealing member  5 . The cross-sectional shape of the sealing member  5  is a circular shape in the non-compressed state. As schematically illustrated in  FIG. 3 , when the sealing member  5  is fitted in the seal groove  48 , the cross-sectional shape of the sealing member  5  is, in the narrow section  49 , an elliptical shape whose major axis extends in the depth direction and is, in portions other than the narrow section  49 , a circular shape similar to that in the non-compressed state. 
     Examples of the material of the sealing member  5  include elastic materials having good acid resistance. Suitable examples of the material of the sealing member  5  include rubbers such as ethylene-propylene-diene rubber (EPDM), fluororubber, and silicone rubber. 
     [Others] 
     The liquid supply-side piece  41  has liquid supply guide grooves  46  each extending between the corresponding liquid-supplying manifold  44  and the bipolar plate  3  to guide an electrolyte from the liquid-supplying manifold  44  to the bipolar plate  3 . Similarly, the liquid discharge-side piece  42  has liquid discharge guide grooves  47  each extending between the corresponding liquid-discharging manifold  45  and the bipolar plate  3  to guide an electrolyte from the bipolar plate  3  to the liquid-discharging manifold  45 . The liquid supply guide grooves  46  and the liquid discharge guide grooves  47  are covered with a plastic protective plate (not shown) when the stacked body is formed. With this structure, the electrolyte is allowed to flow between the liquid-supplying manifold  44  and the bipolar plate  3  and between the liquid-discharging manifold  45  and the bipolar plate  3  without leaking from the liquid supply guide groove  46  and the liquid discharge guide groove  47 . 
     Manifold seal grooves  44   a  and  45   a  ( FIG. 1 ) in which sealing members (not shown) are fitted are preferably formed in the outer peripheries of the liquid-supplying manifolds  44  and the liquid-discharging manifolds  45  that respectively supply and discharge the electrolyte to the other surface side (the back-surface side of the sheet in  FIG. 1 ). The manifold seal grooves  44   a  and  45   a  preferably have narrow sections (not shown) similar to the narrow sections  49  described above. In such a case, leakage of the electrolyte from the liquid-supplying manifolds  44  and the liquid-discharging manifolds  45  can be suppressed. 
     [Operation and Effect] 
     According to the RF battery of Embodiment 1, since the seal groove  48  includes the narrow section  49  that causes the sealing member  5  to elastically deform, detachment of the sealing member  5  from the seal groove  48  can be suppressed by simply fitting the sealing member  5  in the narrow section  49  of the seal groove  48 . This is because a frictional force can be applied, in the longitudinal direction and in the depth direction of the seal groove  48 , to a local position of the sealing member  5  deformed by the narrow section  49 . Therefore, an adhesive is not necessary to fix the sealing member  5  to the seal groove  48 , and a step of applying an adhesive is unnecessary. The sealing structure of an electrolyte can be formed without increasing the number of production steps. In addition, since the width of the narrow section  49  is uniform in the depth direction, production efficiency of the frame  4  having the seal groove  48  can be increased. This is because the seal groove  48  including the narrow section  49  can be formed at the same time when the frame  4  is produced, and thus the number of production steps is not increased. Accordingly, the structure described above realizes good production efficiency of the RF battery. 
     Embodiment 2 
     In Embodiment 2, as illustrated in  FIG. 4 , a narrow section  49  can be formed by protruding pieces  49   b  each having an outline corresponding to a shape of an angle bracket (&gt;). In the angle bracket (&gt;) shape, two sides adjacent to an apex form a corner. The lengths of the two sides are equal to each other. An angle θ formed by the two sides can be appropriately selected and may be, for example, an acute angle. In particular, the angle θ is preferably an obtuse angle. When the angle θ formed by the two sides is an obtuse angle, the sealing member  5  is unlikely to be damaged, and detachment of the sealing member  5  from the seal groove  48  can be prevented by applying a frictional force to the sealing member  5 . The angle θ formed by the two sides is preferably, for example, 60° or more and 160° or less, and particularly preferably 100° or more and 140° or less. 
     Embodiment 3 
     In Embodiment 3, as illustrated in  FIG. 5 , a narrow section  49  can be formed by protruding pieces  49   c  each having an outline corresponding to a shape of a bracket (]). In the bracket (]) shape, the width of a seal groove  48  does not change in the longitudinal direction of the seal groove  48 . When the protruding piece  49   c  has an outline corresponding to the shape of the bracket (]), the contact area between the narrow section  49  and the sealing member  5  can be increased, and detachment the sealing member  5  from the seal groove  48  is easily prevented. A length a of the bracket (]) shape in the longitudinal direction of the seal groove  48  is preferably 5 mm or more and 50 mm or less. When the length a of the bracket (]) shape is 5 mm or more, the contact area between the protruding piece  49   c  and the sealing member  5  can be increased. When the length a of the bracket (]) shape is 50 mm or less, the length of the narrow section  49  is not excessively long, and the operation of fitting the sealing member  5  in the narrow section  49  is unlikely to be complicated. The length a of the bracket (]) shape is particularly preferably 10 mm or more an 20 mm or less. 
     Embodiment 4 
     In Embodiment 4, as illustrated in  FIG. 6 , narrow sections  49  can be formed by providing, on only one surface of a seal groove  48  in the width direction, protruding pieces  49   d  that protrude from the one surface toward the other surface, and a flat surface on the other surface of the seal groove  48 . Examples of the outline shape of each of the protruding pieces  49   d  include an arc shape ( FIG. 2 ), an angle bracket (&gt;) shape ( FIG. 4 ), and a bracket (]) shape ( FIG. 5 ) that are respectively similar to those of the protruding pieces  49   a  to  49   c  in Embodiments 1 to 3. In this embodiment, the protruding piece  49   d  has an outline corresponding to a shape of an arc similar to that of the protruding piece  49   a  in Embodiment 1. An interval L 5  between adjacent protruding pieces  49   d  in the longitudinal direction of the seal groove  48  preferably satisfies a range similar to the interval L 1  between the protruding pieces  49   a  in Embodiment 1 and is preferably uniform. 
     Embodiment 5 
     In Embodiment 5, as illustrated in  FIG. 7 , narrow sections  49  can be formed by protruding pieces  49   e  alternately formed in a shifted manner in the longitudinal direction of a seal groove  48  and a flat surface that faces the protruding pieces  49   e . Examples of the outline shape of each of the protruding pieces  49   e  include an arc shape ( FIG. 2 ), an angle bracket (&gt;) shape ( FIG. 4 ), and a bracket (]) shape ( FIG. 5 ). In this embodiment, the protruding piece  49   e  has an outline corresponding to a shape of an arc similar to that of the protruding piece  49   a  in Embodiment 1. An interval L 6  between protruding pieces  49   e  that are alternately disposed preferably satisfies a range similar to the interval L 1  between the protruding pieces  49   a  in Embodiment 1 and is preferably uniform. Specifically, an interval L 7  between protruding pieces  49   e  on one surface of the seal groove  48  in the width direction and an interval L 8  between protruding pieces  49   e  on the other surface of the seal groove  48  are each preferably about double the interval L 1  between the protruding pieces  49   a  in Embodiment 1, that is, preferably 100 mm or more and 1,000 mm or less, and preferably 200 mm or more and 400 mm or less. In addition, the interval L 7  and the interval L 8  are each preferably uniform. 
     MODIFICATIONS 
     All the narrow sections  49  provided in the seal groove  48  may have the same shape, as described in Embodiment 1. Alternatively, the seal groove  48  may have narrow sections  49  having various shapes described in Embodiments 1 to 5 in combination. 
     INDUSTRIAL APPLICABILITY 
     The electrolyte-circulating battery according to an embodiment of the present invention can be suitably used in applications for the purpose of stabilizing variations in power output, storing generated power during oversupply, load leveling, and the like for new-energy power generation such as photovoltaic power generation or wind power generation. The electrolyte-circulating battery according to an embodiment of the present invention can also be additionally provided in a general power plant and suitably used as a large-capacity storage battery as a countermeasure against an instantaneous voltage drop/power failure and for the purpose of load leveling.