Abstract:
The present invention relates to a housing assembly for at least two fuel cells, comprising: 
     a hollow profile-like body ( 12 ) extending in an axial direction (z) and being adapted to laterally encompass at least two fuel cells stacked on one another in axial direction (z), and 
     a fastening mechanism to interconnect the body ( 12 ) with at least one end plate ( 18, 28 ) adapted to support the at least two fuel cells.

Description:
[0001]    This application claims priority from European Patent Application No. 11173446.3 filed Jul. 11, 2011, the entire disclosure of which is incorporated herein by reference. 
       FIELD OF THE INVENTION 
       [0002]    The present invention relates to fuel cell design and in particular to a housing adapted to receive a plurality of fuel cells stacked on one another. 
       BACKGROUND AND PRIOR ART 
       [0003]    By means of electrochemical fuel cells, reactants, like a stream of hydrogen and a stream of oxygen can be converted into electric power and water. For example with polymer electrolyte fuel cell designs, an electrolyte membrane is typically interposed between an anode and a cathode to form a membrane electrode assembly (MEA). Since a single fuel cell only provides a rather limited maximum voltage, a pre-determined number of fuel cells are typically stacked on one another or next to each other to form a fuel cell stack, wherein adjacently disposed fuel cells are electrically contacted in series. 
         [0004]    With proton exchange membrane fuel cells (PEMFC) but also with other fuel cell designs when making use of gaseous reactants it is necessary to precisely control the gas flow within the stack. In order to provide a gas tight sealing, a fuel cell stack sufficiently equipped with sealing elements is typically compressed along the stacking direction, e.g. by way of two end plates arranged at opposite ends of the fuel cell stack that are directly interconnected with each other by way of tension bolts. 
         [0005]    However, such a fuel cell design is rather difficult to assemble since the various fuel cells, MEAs, sealing elements and separating structures may be twisted in a plane perpendicular to the stacking direction. As well, the entire fuel cell stack may be rather prone to torsion forces in use. 
         [0006]    From document EP 1 936 729 A1 a casing for a fuel cell stack is known. There, the stack body is held in a casing including end plates. The casing also includes a plurality of separate side plates provided on sides of the stack body. End plates and side plates are coupled by coupling pins, wherein each of the side plates has a plurality of separate coupling portions adapted to receive the coupling pins. In effect, tension forces between the end plates have to be transferred via a plurality of coupling pins and corresponding coupling portions that may form a weakening spot being subject to considerable point loading in use. Moreover, the mutual interconnection of end plates and side plates is quite laborious. Also, an interconnection of side plates further requires separate assembly of angle profiles in a respective corner region. 
         [0007]    It is therefore an object of the present invention to provide a housing assembly of improved stability, especially against torsion effects. Moreover, the housing assembly should be easy and intuitive to assemble. It should be long lasting, robust and light weight and also cost-efficient in production and assembly. 
       SUMMARY OF THE INVENTION 
       [0008]    The present invention provides a housing assembly for at least two fuel cells, preferably for a stack of fuel cells comprising several tens or even more than hundred individual fuel cells stacked on one another. The housing assembly comprises a hollow profile-like body extending in an axial direction (z) and being adapted to laterally encompass at least two fuel cells being stacked on one another in axial direction. The housing assembly further comprises a fastening mechanism to interconnect the body with at least one end plate which is adapted to support the at least two fuel cells. 
         [0009]    By providing a hollow profile-like body, the number of components required to form a closed casing for a fuel cell stack substantially reduces to the hollow profile body and to at least one end plate adapted to close said body after a number of fuel cells has been arranged inside said body to form a respective stack of fuel cells. The hollow profile-like body may comprise a somewhat tubular or cylindrical shape and may have a closed bottom face adapted to support the fuel cells stacked therein. By way of the fastening mechanism, the top end of the profile body can be closed in order to provide a substantially closed casing structure for the fuel cell stack. 
         [0010]    By having a single hollow profile-like body, the housing assembly only comprises a single side wall element to form a lateral enclosure for the fuel cell stack. Hence, various side wall sections as for instance described in EP 1 936 729 A1 do no longer have to be interconnected. Also, the assembly process for interconnecting the at least one end plate with the hollow profile-like body becomes much easier and is therefore less time consuming. 
         [0011]    According to a preferred embodiment, the body is adapted to transfer an axially directed tension or compression force between two end plates arranged at opposite axial end sections of the body. Consequently, the profile body not only provides a protective structure for the fuel cells to be stacked therein but also serves to transfer tension or compression forces between oppositely disposed end plates in order to establish and to maintain a gas tight sealing of gas flow channels extending through the stack. Hence, the hollow profile-like body may also replace and substitute the functionality of tension bolts ordinarily extending between oppositely disposed end plates of a fuel stack. 
         [0012]    By having a substantially laterally closed hollow body, the entire fuel cell stack can be effectively stabilized against lateral and axial loads. 
         [0013]    In a further preferred aspect, a first end plate is entirely inserted in the hollow body and comprises a laterally outwardly extending bearing structure which is adapted to mate with a correspondingly shaped laterally inwardly extending counter-bearing structure of a first axial end of the body. By an entire insertion of the end plate into the body the end plate is at least circumferentially encompassed by the body. However, the end plate may at least partially protrude from the axial end of the body. By providing mutually corresponding bearing and counter-bearing structures on the end plate as well as at the inside facing side wall of the body, an axial stop or bearing for the end plate relative to the body can be provided. 
         [0014]    By having such a unidirectional abutment and stop function between first end plate and body, e.g. a downwardly directed axial force acting on the end plate can be entirely transferred to the body via the mutually corresponding bearing structures. Additional or separate fastening means are therefore not required to fix the first end plate relative to the surrounding body. 
         [0015]    According to a further preferred aspect, the fastening mechanism comprises a closed frame or frame element to surround a second axial end of the body. The frame element comprises a laterally inwardly extending bearing structure which is adapted to mate with a correspondingly shaped laterally outwardly extending counter-bearing structure of the second end of the body. Typically, the counter-bearing structure of the body comprises a flange-like contour. The inner diameter formed by the bearing structure of the frame element substantially corresponds with or is at least slightly larger than the outer diameter or cross section of the body. Since the inner diameter or cross section of the bearing structure of the frame element is smaller than the outwardly extending counter-bearing structure of the second end of the body, by way of a mutual axially directed abutment of bearing and counter-bearing structures, axially directed compression or tension forces can be transferred from the frame element to the body and vice versa. 
         [0016]    Bearing- and correspondingly counter-bearing structures may comprise a plurality of laterally inwardly and outwardly extending stop elements. For providing a rather homogenous force distribution across the interface of frame element, end plate(s) and body, it is of further benefit, when bearing and counter-bearing structures comprise circumferentially extending rims or respective recesses. This way, axially directed tension or compression forces can be transferred across said interface regions in a material-protecting way. Point loads rising above maximum admissible levels can be effectively avoided. 
         [0017]    In a further preferred aspect, the frame element comprises at least two laterally outwardly protruding portions that substantially overlap with corresponding fastening sections of a second end plate as seen in an axial projection. Since the frame element is unidirectionally fixed to the body by the above-mentioned mutually corresponding bearing- and counter-bearing structures, the frame element serves as a tension element to cooperate with the end plate in order to transfer a respective axially directed tension from the end plate to the body. Mutually corresponding laterally outwardly protruding portions of the frame element and the second end plate are intended to be rigidly interconnected with each other in axial direction. It is advantageous, when fastening sections of the second end plate are aligned with respective protruding portions of the frame element in axial direction. 
         [0018]    Generally, a large variety of interconnecting mechanisms, e.g. clamping devices can be generally used here. In a further preferred aspect, the protruding portion of the frame element comprises a screw hole to receive an axially extending screw means extending through a respective hole of the corresponding fastening section of the second end plate. The screw means can comprise conventional tension bolts to be inserted through the holes of the end plate&#39;s fastening sections and to be screwed into corresponding screw holes of the frame element. This way, a considerable pressure can be applied to the stack of fuel cells sandwiched between oppositely disposed end plates. A respective counter force can be provided via the hollow profile-like body operably engaged with the oppositely arranged end plates in order to transfer an axially directed tension force between said end plates. 
         [0019]    According to a further preferred aspect, the body engages with the first end plate and/or with the frame element all along the lateral circumference of the body. Correspondingly, respective bearing and/or counter-bearing structures of body, first end plate and/or frame element also extent all along the lateral circumference thereof. 
         [0020]    In a further aspect, the body also comprises at least one elongated slit or slot intersecting a side wall portion in axial direction (z). By way of such a through opening of the hollow profile-like body, access to various membrane electrode assemblies or to selected bipolar plates can be attained, which is beneficial to monitor the voltage across the fuel cell stack. 
         [0021]    Furthermore and according to another preferred aspect, the body is made of an electrically insulated material. This way, additional insulated sheets or insulating structures become superfluous and the electrically conducting components of the fuel cell stack can be directly arranged inside the body. Hence, the body itself is ready to receive fuel cells in a stacked configuration. 
         [0022]    According to another preferred embodiment, the body of the housing assembly comprises a fiber enhanced or fiber reinforced plastic material and/or a fibrous composite material. Depending on the mechanical load, and temperature stress, a variety of plastic or composite materials can be used for the hollow profile-like body. Generally, even fiber reinforced plastic components can be manufactured in various geometries, e.g. by way of extruding or injection molding manufacturing processes. This way, a single pieced hollow profile-like body of arbitrary geometry and shape can be manufactured in a rather cost-efficient way. 
         [0023]    For the arrangement of the various fuel cells inside the profile body it is of further benefit, when the body at least at its inward facing side wall sections comprises a symmetry breaking feature mating with the lateral or outer shape of the respective fuel cells to be arranged therein. This way, a mounting or installation direction is pre-defined by the structure or geometry of the profile body. A wrong or inadmissible arrangement of fuel cells in the body can be effectively prevented. 
         [0024]    In a further preferred embodiment, the hollow body comprises a circular symmetric, an oval shaped or an at least in sections convex shaped outer cross-sectional profile. A rounded, oval or circular cross-sectional profile of the body is particular beneficial in terms of lateral load distribution. Accordingly, it is beneficial that the outer shape or outer cross section of the hollow body is free of edges, that are generally prone to mechanical impact. 
         [0025]    In a further preferred aspect, a stack of fuel cells is disposed in the hollow profile-like body of the housing assembly. The fuel cell stack is typically clamped in axial direction (z) by way of the two end plates, wherein a counter force between said oppositely disposed end plate is transferred from the first or bottom end plate to the body, from the body to the frame element and from the frame element to the second or top end plate. 
         [0026]    Moreover and according to an additional but independent aspect, the invention also relate to a fuel cell stack encompassed in a housing assembly as described above. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0027]    In the following, preferred embodiments of the invention will be described in detail by making reference to the drawings in which: 
           [0028]      FIG. 1  shows a perspective illustration of the housing assembly, 
           [0029]      FIG. 2  shows a side view of the housing assembly according to  FIG. 1 , 
           [0030]      FIG. 3  illustrates a cross section along the intersection line A-A of  FIG. 2 , 
           [0031]      FIG. 4  shows an enlarged view of the upper interface between the body and the upper end plate according to  FIG. 3 , 
           [0032]      FIG. 5  provides an enlarged illustration of the lower interface section according to  FIG. 3 , 
           [0033]      FIG. 6  shows another side view of the housing assembly, and 
           [0034]      FIG. 7  is illustrative of a lateral cross section of the housing assembly along B-B of  FIG. 6 . 
       
    
    
     DETAILED DESCRIPTION 
       [0035]      FIG. 1  illustrates a housing assembly  10  to accommodate a stack of fuel cells. The housing assembly  10  comprises a rather elongated, almost tubular shaped hollow profile-like body  12  extending in an axial direction (z). The hollow body  12  is closed by way of two end plates  18 ,  28  arranged at a top end and at bottom end of the body  12  as can be seen in the cross section of  FIG. 3 . The end plates  18 ,  28 , typically comprising structural metal plates are adapted to support the fuel cell stack to be sandwiched there between. Additionally, the end plates  18 ,  28  provide electrical contacting to the stack of fuel cells, which is not explicitly illustrated in the present set of Figures. 
         [0036]    As shown in detail in  FIG. 5 , the lower and first end plate  18  is entirely inserted into the profile body  12 . A laterally extending margin of the first end plate  28  thereby gets in direct abutment or contact with the circumferential side wall of the hollow body  12 . In order to provide axially directed force transmission between the end plate  18  and the body  12 , the end plate  28  comprises a laterally or radially outwardly extending bearing structure  28   a  which is adapted to mate and to cooperate with a correspondingly shaped inwardly extending counter-bearing structure  12   b  provided at the lower end of the body  12 . 
         [0037]    The radially protruding components counter-bearing structure  12   b  and bearing structure  28   a  are preferably integrally formed with the body  12  and/or with the end plate  28 , respectively. By way of the mutual axial abutment of the first end plate  28  and the lower end section  12   b  of the body  12 , downward directed forces acting on the end plate  28  can be effectively and entirely transferred to the body  12 . 
         [0038]    The upper end of the body  12  is depicted in  FIG. 4  in an enlarged view. The opposite and upper end of the body  12  comprises a laterally outwardly extending counter-bearing structure  12   a  that mates with a correspondingly shaped laterally or radially inwardly extending bearing structure  14   a  of a closed ring or frame element  14 , which, as illustrated in  FIG. 1  entirely encompasses the circumference (w) of the body  12 . With this unidirectional axial abutment, an upwardly directed force acting on the frame element  14  can be effectively transferred to the body  12 . 
         [0039]    In an assembly process, initially, the first end plate  28  has to be inserted into the body  12  from the top of the body  12  until the first end plate  28  engages with the radially inwardly extending counter-bearing structure  12   b  of the body  12  as shown in  FIG. 5 . Thereafter, several fuel cells can be stacked in the body  12  on top of the first end plate  28 . The body  12  can be inserted into the surrounding frame element  14 , which is to be moved until it engages with the radially outwardly extending bearing structure  12   a  of the body  12  as depicted in  FIG. 4 . In a final step of assembly, the second top end plate  18  is to be mounted on the stack of fuel cells. This end plate  18  comprises laterally outwardly extending fastening sections  20  that substantially overlap with correspondingly shaped and laterally outwardly extending protruding portions  24  of the frame element  14 . 
         [0040]    The portions  24  of the frame  14  and the outwardly extending sections  20  of the end plate mutually overlap in an axial projection in order to enable insertion of a screw through a hole  22  of the fastening section  20  to be further screwed in a screw hole  26  of the protruding portion  20  of the frame element  14 . This way, an axial distance between frame element  14  and upper end plate  18  can be varied and a corresponding axially directed pressure can be applied onto the stack of fuel cells disposed between the two end plates  18 ,  28 . With their axially inward facing surfaces  34 ,  32 , a required pressure can be directly exerted to the stack of fuel cells. 
         [0041]    The sideview according to  FIG. 2  further illustrates a longitudinal slit  16  providing access to the various fuel cells or bipolar plates in order to monitor electric properties, in particular the voltage across the fuel cell stack during operation. 
         [0042]    As illustrated in the cross section of  FIG. 7  along the intersecting line B-B of  FIG. 6 , the cross-sectional profile of the hollow profile-like body  12  is almost rectangular but comprises rounded corners  38 . Also, side wall sections extending between adjacent corner portions  38  are of substantially convex shape. Apart from the illustrated cross section, it is conceivable to have a circular symmetric, an oval or even a polygonal shaped cross section of the body  12  with rounded corner sections  38 . Accordingly, the surrounding ring  14  or frame element  14  then also comprises a corresponding geometry and shape. 
         [0043]    As further depicted in  FIG. 6 , a lug  30  extending in axial direction (z) and protruding from the body  12  serves as a conducting element by way of which electric current available at a current collector plate can be passed to an endplate. The current collector plate and the endplate, being not explicitly illustrated in  FIG. 6 , are electrically insulated e.g. by way of a plastic overmoulding structure. 
         [0044]    In the cross section according to  FIG. 7  also a symmetry breaking feature  36  is indicated comprising a radially inwardly protruding rib that mates with a correspondingly shaped recess of a fuel cell being not explicitly illustrated here. 
         [0045]    In the cross section according to  FIG. 7 , the end plate  18  as well as the frame element  14  are shown from the bottom. In particular, in the portion of the frame element  14  located radially inwardly compared to the  12  hollow profile-like body  12 , various gas transporting channels  40  are indicated that extend in axial direction (z). 
       LIST OF REFERENCE NUMERALS 
       [0000]    
       
           10  housing assembly 
           12  hollow profile-like body 
           14  frame element 
           16  slit 
           18  end plate 
           20  fastening section 
           22  through hole 
           24  protruding portion 
           26  screw hole 
           28  end plate 
           30  lug 
           32  support surface 
           34  contact surface 
           36  symmetry breaking item 
           38  rounded corner 
           40  transport channel