JP4452585B2 - Fuel cell stack - Google Patents

Description

  The present invention includes a unit cell in which an electrolyte / electrode structure in which a pair of electrodes are provided on both sides of an electrolyte is sandwiched between separators, and a stacked body in which a plurality of the unit cells are stacked is contained in a box-shaped casing. It relates to a battery stack.

  For example, a polymer electrolyte fuel cell employs an electrolyte membrane (electrolyte) made of a polymer ion exchange membrane. A fuel cell is configured by sandwiching an electrolyte membrane / electrode structure in which an anode side electrode and a cathode side electrode are provided on both sides of the electrolyte membrane with a separator.

  In this fuel cell, a fuel gas, for example, a gas mainly containing hydrogen (hereinafter also referred to as hydrogen-containing gas) is supplied to the anode side electrode, while an oxidant gas, for example, A gas or air mainly containing oxygen (hereinafter also referred to as oxygen-containing gas) is supplied. In the fuel gas supplied to the anode side electrode, hydrogen is ionized on the electrode catalyst and moves to the cathode side electrode side through the electrolyte membrane. Electrons generated during that time are taken out to an external circuit and used as direct current electric energy.

  Normally, this fuel cell is used as a fuel cell stack in which a predetermined number (for example, several tens to several hundreds) is stacked in order to obtain a desired power generation. In this fuel cell stack, the stacked fuel cells need to be reliably pressurized and held in order to prevent an increase in the internal resistance of the fuel cell and a decrease in the sealing performance of the reaction gas.

  Therefore, for example, a fuel cell stack of Patent Document 1 is known. As shown in FIG. 15, the fuel cell stack includes a stacked body 2 in which a plurality of unit cells 1 are stacked, and auxiliary plates 4 a with end plates 3 and 3 interposed at both ends in the stacking direction of the stacked body 2. 4b are arranged.

  A pair of fastening bands 5 and 5 are disposed along both side portions of the laminate 2. Cylindrical connecting portions 6 are provided at the ends of the fastening bands 5 and 5 and the auxiliary plates 4a and 4b so that the respective holes are aligned in a straight line. The fastening bands 5 and 5 and the auxiliary plates 4a and 4b are integrally connected by inserting the metal pin 7 into each connecting portion 6.

  A plurality of bolts 8 are screwed onto the auxiliary plate 4a, while a plurality of disc springs 9 are disposed on the auxiliary plate 4b. Therefore, when the bolt 8 is screwed in, the end plate 3 is pressed downward, and the disc spring 9 disposed on the auxiliary plate 4b is compressed, which is necessary for the laminated body 2 via the pair of end plates 3. The fastening pressure is applied.

  However, in Patent Document 1 described above, a plurality of bolts 8 that are dedicated parts are provided in order to adjust the tightening load of the fuel cell stack. Accordingly, there is a problem that the weight of the fuel cell stack increases due to the addition of the dedicated parts, and the fuel cell stack increases in size in the stacking direction of the unit cells 1.

  Therefore, for example, a fuel cell stack disclosed in Patent Document 2 is known. In this fuel cell stack, an end plate is disposed outside a stacked body in which a predetermined number of single cells are stacked with a current collecting electrode (terminal plate) interposed therebetween, and the end plate is attached to a casing by a hinge mechanism. It is connected. The casing includes a plurality of panels (side plates) disposed between the end plates in the vertical and horizontal directions.

  Thereby, in patent document 2, while the volt | bolt 8 of patent document 1 becomes unnecessary, a thin end plate can be used, and size reduction and weight reduction of the whole fuel cell stack are achieved easily.

JP 2001-135344 A (FIG. 5) JP 2002-298901 A (FIG. 1)

  By the way, in said patent document 2, a comparatively big clearance gap may generate | occur | produce between each side plate which comprises a casing, and the side part of a laminated body, for example. For this reason, when an impact is applied to the fuel cell stack, there is a problem that the stack is displaced with respect to the stacking direction or vibration is caused in the stack. On the other hand, a gap is considerably small between each side plate and the side part of the laminated body, and the side plate and the laminated body may be in contact with each other more than necessary.

  The present invention solves this type of problem, and with a simple configuration, the gap between the side plate constituting the casing and the laminate is well adjusted, and the contact state between the side plate and the laminate is arbitrarily set. An object of the present invention is to provide a fuel cell stack that can be set.

  The present invention includes a unit cell in which an electrolyte / electrode structure in which a pair of electrodes are provided on both sides of an electrolyte is sandwiched between separators, and a stacked body in which a plurality of the unit cells are stacked is contained in a box-shaped casing. It is a battery stack.

The casing includes end plates disposed at both end portions in the stacking direction of the laminate, a plurality of side plates disposed at the side portions of the laminate, and connection pins that connect the end plates and the side plates. At least one of the side plates, by the center of the connecting pin in relation to the neutral plane of the side plate is offset inwardly, are bending deformation in the direction toward the stack.

  Here, the neutral surface of the side plate is a surface that is assumed to have theoretically zero stress and strain when bending stress acts on the side plate, assuming that the deformation of the cross section of the side plate is very small. A surface that does not expand or contract). Note that the center of the connecting pin is disposed on the neutral surface of the side plate means that the neutral axis of the side plate (a straight line where the neutral surface intersects the cross section) and the axis of the connecting pin are on the same plane. In other words.

  Moreover, it is preferable that at least one of the side plates is a ribbed panel in which the ribs are integrally or individually configured. This is because the rigidity of the side plate itself can be improved satisfactorily.

  In the present invention, when a tightening load is applied to the laminate, when the load acts on the connecting pins along the stacking direction, the center of the connecting pin is offset with respect to the neutral surface of the side plate. Is bent and deformed in the stacking direction.

At that time, the center of the connecting pin is offset in the direction (inner side) approaching the laminate with respect to the neutral surface of the side plate, and the side plate is bent and deformed toward the laminate. For this reason, the side plate can be in contact with the side portion of the laminated body to hold the laminated body. For example, it effectively reduces the occurrence of displacement or vibration in the laminated body due to an external impact. Is possible.

  FIG. 1 is a partially exploded schematic perspective view of a fuel cell stack 10 according to the first embodiment of the present invention, and FIG. 2 is a partial sectional side view of the fuel cell stack 10.

  As shown in FIG. 1, the fuel cell stack 10 includes a stacked body 14 in which a plurality of unit cells 12 are stacked in the horizontal direction (arrow A direction). A terminal plate 16a, an insulating plate 18 and an end plate 20a are sequentially disposed at one end in the stacking direction (arrow A direction) of the stacked body 14 toward the outside. At the other end in the stacking direction of the stacked body 14, a terminal plate 16b, an insulating spacer member 22 and an end plate 20b are sequentially disposed outward. The fuel cell stack 10 is integrally held by a casing 24 including end plates 20a and 20b each having a rectangular shape as end plates.

  As shown in FIGS. 2 and 3, each unit cell 12 includes an electrolyte membrane / electrode structure (electrolyte / electrode structure) 30, and a thin plate-shaped first and second sandwiching the electrolyte membrane / electrode structure 30. Second metal separators 32 and 34 are provided. Instead of the first and second metal separators 32 and 34, for example, a carbon separator may be used.

  An oxidant gas supply for supplying an oxidant gas, for example, an oxygen-containing gas, communicates with each other in the arrow A direction at one end edge of the unit cell 12 in the long side direction (the arrow B direction in FIG. 3). A communication hole 36a, a cooling medium supply communication hole 38a for supplying a cooling medium, and a fuel gas discharge communication hole 40b for discharging a fuel gas, for example, a hydrogen-containing gas, are provided.

  The other end edge in the long side direction of the unit cell 12 communicates with each other in the direction of arrow A, and a fuel gas supply communication hole 40a for supplying fuel gas, and a cooling medium discharge communication hole for discharging the cooling medium. 38b and an oxidizing gas discharge communication hole 36b for discharging the oxidizing gas are provided.

  The electrolyte membrane / electrode structure 30 includes, for example, a solid polymer electrolyte membrane 42 in which a perfluorosulfonic acid thin film is impregnated with water, and an anode side electrode 44 and a cathode side electrode 46 that sandwich the solid polymer electrolyte membrane 42. With.

  The anode side electrode 44 and the cathode side electrode 46 are uniformly coated on the surface of the gas diffusion layer with a gas diffusion layer (not shown) made of carbon paper or the like and porous carbon particles carrying a platinum alloy on the surface. And an electrode catalyst layer (not shown) formed. The electrode catalyst layers are formed on both surfaces of the solid polymer electrolyte membrane 42.

  A fuel gas flow path 48 that connects the fuel gas supply communication hole 40 a and the fuel gas discharge communication hole 40 b is formed on the surface 32 a of the first metal separator 32 facing the electrolyte membrane / electrode structure 30. The fuel gas channel 48 is constituted by, for example, a plurality of grooves extending in the arrow B direction. On the surface 32b of the first metal separator 32, a cooling medium flow path 50 that connects the cooling medium supply communication hole 38a and the cooling medium discharge communication hole 38b is formed. The cooling medium flow path 50 is configured by a plurality of grooves extending in the arrow B direction.

  The surface 34a of the second metal separator 34 facing the electrolyte membrane / electrode structure 30 is provided with, for example, an oxidant gas flow path 52 composed of a plurality of grooves extending in the direction of arrow B, and this oxidant gas. The flow path 52 communicates with the oxidant gas supply communication hole 36a and the oxidant gas discharge communication hole 36b. A cooling medium flow path 50 is integrally formed on the surface 34 b of the second metal separator 34 so as to overlap the surface 32 b of the first metal separator 32.

  A first seal member 54 is integrally formed on the surfaces 32 a and 32 b of the first metal separator 32 around the outer peripheral end of the first metal separator 32. The first seal member 54 surrounds the fuel gas supply communication hole 40a, the fuel gas discharge communication hole 40b, and the fuel gas flow path 48 on the surface 32a so as to communicate with each other, and on the surface 32b, the cooling medium supply communication hole 38a, The medium discharge communication hole 38b and the cooling medium flow path 50 are surrounded and communicated with each other.

  A second seal member 56 is integrally formed on the surfaces 34 a and 34 b of the second metal separator 34 around the outer peripheral end of the second metal separator 34. The second seal member 56 surrounds and communicates the oxidant gas supply communication hole 36a, the oxidant gas discharge communication hole 36b, and the oxidant gas flow path 52 on the surface 34a, while the cooling medium supply communication hole on the surface 34b. 38a, the cooling medium discharge communication hole 38b, and the cooling medium flow path 50 are surrounded and communicated.

  As shown in FIG. 2, a seal 57 is interposed between the first and second seal members 54 and 56 in order to prevent the outer periphery of the solid polymer electrolyte membrane 42 from directly contacting the casing 24. Is done.

  As shown in FIGS. 1 and 2, plate-like terminal portions 58a and 58b protruding in the surface direction are formed at the ends of the terminal plates 16a and 16b. For example, a load such as a traveling motor is connected to the terminal portions 58a and 58b.

  As shown in FIG. 1, the casing 24 includes end plates 20 a and 20 b that are end plates, a plurality of side plates 60 a to 60 d disposed on the side of the laminated body 14, and end portions of the side plates 60 a to 60 d that are close to each other. Angle members (for example, L angles) 62a to 62d that connect the portions, and connecting pins 64a and 64b having different lengths that connect the end plates 20a and 20b and the side plates 60a to 60d, respectively. The side plates 60a to 60d are composed of thin metal plates.

  Two first connection portions 66a and 66b are formed to project from the upper and lower sides of the end plates 20a and 20b, respectively, and one first connection portion 66c and 66d is formed to project from each side of each side. The Holes 67a to 67d are formed through the first connecting portions 66a to 66d. Mount bosses 68a and 68b are formed at the lower ends of the respective sides of the end plates 20a and 20b. The boss portions 68a and 68b are fixed to a mounting portion (not shown) via a bolt or the like, so that the fuel cell stack 10 is mounted on, for example, a vehicle.

  Two second connecting portions 70a and 70b are formed at both ends in the longitudinal direction (arrow A direction) of the side plates 60a and 60c arranged on both sides in the arrow B direction of the laminate 14. Three second connecting portions 72a and 72b are formed at both ends in the longitudinal direction of the side plates 60b and 60d disposed on the upper and lower sides of the laminated body 14, respectively. Holes 71a and 71b are formed in the second connecting portions 70a and 70b, and holes 73a and 73b are formed in the second connecting portions 72a and 72b.

  Between the second connection portions 70a and 70b of the side plates 60a and 60c, first connection portions 66c and 66d on both sides of the end plates 20a and 20b are arranged, and a short connection pin 64a is integrally formed therewith. The side plates 60a and 60c are attached to the end plates 20a and 20b.

  Similarly, the second connecting portions 72a and 72b of the side plates 60b and 60d are alternately arranged with the first connecting portions 66a and 66b on the upper and lower sides of the end plates 20a and 20b, and a long connecting pin 64b is provided on these. The side plates 60b and 60d are attached integrally to the end plates 20a and 20b.

  The side plates 60a to 60d are formed with a plurality of screw holes 74 at both edges in the short direction, respectively, while the sides 76 of the angle members 62a to 62d are formed with holes 76 corresponding to the screw holes 74. Is done. When the screws 77 inserted into the holes 76 are screwed into the screw holes 74, the side plates 60a to 60d are fixed to each other via the angle members 62a to 62d. Thereby, the casing 24 is configured (see FIG. 4).

  In addition, while forming screw holes in the angle members 62a to 62d and forming holes in the side plates 60a to 60d and arranging the angle members 62a to 62d inward of the side plates 60a to 60d, these are integrated. Alternatively, it may be screwed.

  As shown in FIGS. 2 and 5, at least in the side plate 60b, the thickness center PC of the side plate 60b coincides with the neutral surface NS of the side plate 60b, and the center O of the connecting pin 64b corresponds to the side plate 60b. The neutral plane NS is offset by a distance h toward the laminated body 14 (inward side). A load in the direction of arrow A acts on the pair of connecting pins 64b, and the side plate 60b is provided with a deformation portion 78 that bends and deforms toward the stacked body 14 along the stacking direction (arrow A direction) (see FIG. 5).

  As shown in FIG. 2, the side plate 60d is configured in the same manner as the side plate 60b described above, and a detailed description thereof is omitted. Further, the side plates 60a and 60b are configured similarly to the side plate 60b as necessary.

  As shown in FIGS. 1 and 2, the spacer member 22 has a rectangular shape set to a predetermined size so as to be positioned on the inner periphery of the casing 24. The spacer member 22 is adjusted in thickness in order to absorb a variation in the length of the stacked body 14 in the stacking direction and to apply a desired tightening load to the stacked body 14. Note that the spacer member 22 may not be used if the variation in the length of the stacked body 14 in the stacking direction can be absorbed by the elasticity of the first and second metal separators 32 and 34 themselves.

  The operation of the fuel cell stack 10 configured as described above will be described below.

  In the fuel cell stack 10, first, as shown in FIG. 4, an oxidant gas such as an oxygen-containing gas is supplied to the oxidant gas supply communication hole 36a of the end plate 20a, and hydrogen is supplied to the fuel gas supply communication hole 40a. Fuel gas such as contained gas is supplied. Further, a coolant such as pure water or ethylene glycol is supplied to the coolant supply passage 38a. For this reason, in the stacked body 14, the oxidant gas, the fuel gas, and the cooling medium are supplied in the arrow A direction to the plurality of unit cells 12 stacked in the arrow A direction.

  As shown in FIG. 3, the oxidant gas is introduced into the oxidant gas flow path 52 of the second metal separator 34 through the oxidant gas supply communication hole 36 a, and along the cathode side electrode 46 of the electrolyte membrane / electrode structure 30. Move. On the other hand, the fuel gas is introduced into the fuel gas channel 48 of the first metal separator 32 through the fuel gas supply communication hole 40 a and moves along the anode side electrode 44 of the electrolyte membrane / electrode structure 30.

  Therefore, in each electrolyte membrane / electrode structure 30, the oxidant gas supplied to the cathode side electrode 46 and the fuel gas supplied to the anode side electrode 44 are consumed by an electrochemical reaction in the electrode catalyst layer, Power generation is performed.

  Next, the oxidant gas consumed by being supplied to the cathode side electrode 46 flows along the oxidant gas discharge communication hole 36b, and then is discharged to the outside from the end plate 20a. Similarly, the fuel gas consumed by being supplied to the anode side electrode 44 is discharged to the fuel gas discharge communication hole 40b, flows, and is discharged from the end plate 20a to the outside.

  The cooling medium flows in the direction of arrow B after being introduced into the cooling medium flow path 50 between the first and second metal separators 32 and 34 from the cooling medium supply communication hole 38a. The cooling medium cools the electrolyte membrane / electrode structure 30, and then moves through the cooling medium discharge communication hole 38b and is discharged from the end plate 20a.

  In this case, in the first embodiment, as shown in FIGS. 2 and 5, for example, in the side plate 60b, the center O of the connecting pin 64b is closer to the laminate 14 side than the neutral surface NS of the side plate 60b. Offset by distance h. Therefore, when a tightening load in the stacking direction is applied to the fuel cell stack 10, a load acts on the side plate 60b in the direction of the arrow A via the pair of connecting pins 64b, and the side plate 60b is connected to the stacked body 14. It is bent and deformed to the side (see FIG. 5).

  Thereby, the side plate 60b can contact the laminated body 14a and hold the laminated body 14 via the deformation portion 78 that is bent and deformed to the laminated body 14 side. Therefore, it is possible to effectively reduce the occurrence of the deviation in the stacking direction, the vibration in the direction of the arrow B and the direction of the arrow C, and the like in the stack 14 with a simple configuration. In addition to ensuring sealing performance, the effect of stable power generation can be obtained.

Figure 6 is a cross-sectional side view of the fuel cell stack 90 that relate to the present invention. The same components as those of the fuel cell stack 10 according to the first embodiment are denoted by the same reference numerals, and detailed description thereof is omitted .

  The casing 92 that constitutes the fuel cell stack 90 includes side plates 94a and 94b that are arranged vertically. As shown in FIGS. 6 and 7, in the side plate 94a, a side (outside side) where the center O of the connecting pin 64b is separated from the laminated body 14 with respect to the neutral surface NS (plate thickness center PC) of the side plate 94a. ) By a distance h1. The side plate 94b is configured in the same manner as the side plate 94a, and a detailed description thereof is omitted. Although not shown, side plates corresponding to the side plates 60a and 60c are disposed on both sides of the laminate 14, and are configured in the same manner as the side plate 94a as necessary.

In this case , as shown in FIG. 7, when a tightening load is applied to the laminate 14, the side plate 94a is pulled in the direction of arrow A via the pair of connecting pins 64b. At that time, the center O of the connecting pin 64b is offset by a distance h1 toward the side away from the stacked body 14 with respect to the neutral surface NS of the side plate 94a. Accordingly, the side plate 94a is bent and deformed on the side away from the laminated body 14 to be provided with a deformed portion 96.

For this reason, the side plate 94 a is separated from the side portion of the laminated body 14, and a desired gap is formed between the side plate 94 a and the side portion of the laminated body 14. Thus, the side plate 94a and the laminated body 14, the effect is obtained that it is possible to prevent the contact than necessary.

FIG. 8 is a partially exploded schematic perspective view of a fuel cell stack 100 according to the second embodiment of the present invention, and FIG. 9 is a sectional side view of the fuel cell stack.

  The casing 102 that constitutes the fuel cell stack 100 includes side plates 104 a to 104 d that are disposed on the side portions of the stacked body 14. At least the side plate 104b extends in the stacking direction of the stacked body 14 and is integrally formed with two rib portions 106 to form a ribbed panel. The rib part 106 protrudes to the side away from the laminated body 14, and the rib part 106 improves the rigidity of the side plate 104b itself.

  As shown in FIG. 9, the center O of the connecting pin 64b coincides with the thickness center PC of the side plate 104b, and the center O of the connecting pin 64b is the laminate 14 with respect to the neutral plane NS of the side plate 104b. It is offset to the side by a distance h2. The side plate 104d is configured similarly to the side plate 104b described above, and a detailed description thereof is omitted. The side plates 104a and 104b are provided with a single rib portion 106 as necessary.

In the second embodiment, since the two rib portions 106 are integrally formed on the side plate 104b to form a panel with ribs, the rigidity is effectively improved as compared with the planar panel. Moreover, the center O of the connecting pin 64b is offset from the neutral surface NS of the side plate 104b by the distance h2 toward the laminate 14 side. For this reason, when the load of the lamination direction acts between the connection pins 64b, the side plate 104b is bent and deformed to the laminated body 14 side (see FIG. 10). Therefore, in the second embodiment, the same effect as in the first embodiment can be obtained.

Figure 11 is a cross-sectional side view of the fuel cell stack 110 that are related to the present invention.

  The casing 112 constituting the fuel cell stack 110 includes side plates 114 a and 114 b disposed above and below the stacked body 14. At least the side plate 114a extends in the direction of arrow A, and a plurality of, for example, two rib portions 116 are integrally formed to form a ribbed panel.

  The rib part 116 protrudes to the laminated body 14 side. The center O of the connecting pin 64b coincides with the thickness center PC of the side plate 114a, and the center O of the connecting pin 64b is a distance h3 on the side away from the stacked body 14 with respect to the neutral surface NS of the side plate 114a. Only offset. The side plate 114b is configured in the same manner as the side plate 114a.

In this case , as shown in FIG. 12, when a load in the direction of arrow A acts on the pair of connecting pins 64b, the side plate 114a is bent and deformed to the side away from the laminated body 14 .

In the second embodiment, but the rib portion 106 is integrally formed with the side plate 1 0 4 b, and alternatives to the side plate 104 b, can be used side plates 12 0 shown in FIG. 13.

As shown in FIG. 13, in the third embodiment, the side plate 120 includes a substantially planar panel 122, and two rib members 124 are joined to an outer surface 122 a of the panel 122. That is, the side plate 120 is configured substantially in the same manner as the side plate 104b used in the second embodiment by joining the rib member 124 to the panel 122.

As shown in FIG. 14, the side plate 130 includes a generally planar panel 132, the inner surface 132a of the panel 132, two rib members 134 are joined. Thus, the side plate 130 is similarly configured substantially the side plate 114a.

1 is a partially exploded schematic perspective view of a fuel cell stack according to a first embodiment of the present invention. 2 is a cross-sectional side view of the fuel cell stack. FIG. It is a disassembled perspective explanatory drawing of the unit cell which comprises the said fuel cell stack. It is a perspective view of the fuel cell stack. It is explanatory drawing of the side plate which comprises a casing. It is a cross-sectional side view of a fuel cell stack that are related to the present invention. It is explanatory drawing of the side plate which comprises a casing. It is a partial schematic perspective view of a fuel cell stack according to a second embodiment of the present invention. 2 is a cross-sectional side view of the fuel cell stack. FIG. It is explanatory drawing of the side plate which comprises a casing. It is a cross-sectional side view of a fuel cell stack that are related to the present invention. It is explanatory drawing of the side plate which comprises a casing. It is a disassembled perspective explanatory drawing of the side plate which comprises the fuel cell stack which concerns on the 3rd Embodiment of this invention. It is an exploded perspective view showing a side plate of a fuel cell stack that are related to the present invention. 2 is an explanatory diagram of a fuel cell stack of Patent Document 1.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10, 90, 100, 110 ... Fuel cell stack 12 ... Unit cell 14 ... Laminated body 16a, 16b ... Terminal plate 18 ... Insulating plate 20a, 20b ... End plate 22 ... Spacer member 24, 92, 102, 112 ... Casing 30 ... Electrolyte membrane / electrode structure 32, 34 ... metal separator 42 ... solid polymer electrolyte membrane 44 ... anode side electrode 46 ... cathode side electrode 48 ... fuel gas passage 50 ... cooling medium passage 52 ... oxidant gas passage 54, 56 ... Seal members 60a-60d, 94a, 94b, 104a-104d, 114a, 114b, 120, 130 ... Side plates 62a-62d ... Angle members 64a, 64b ... Connecting pins 106, 116 ... Ribbed parts 122, 132 ... Panel 124, 134: Rib member

Claims (2)

A fuel cell stack comprising a unit cell in which an electrolyte / electrode structure having a pair of electrodes provided on both sides of an electrolyte is sandwiched between separators, and a stacked body in which a plurality of the unit cells are stacked is housed in a box-shaped casing. And
The casing includes end plates disposed at both ends in the stacking direction of the stacked body,
A plurality of side plates disposed on the side of the laminate;
A connecting pin for connecting the end plate and the side plate;
With
At least one of the side plates has the center of the connecting pin offset inward with respect to the neutral surface of the side plate, and the side plate is pulled along the stacking direction of the stack, thereby fuel cell stack, characterized that you are bent and deformed in the direction to close.   2. The fuel cell stack according to claim 1, wherein at least one of the side plates is a ribbed panel in which the ribs are integrally or individually formed.

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