WO2009078045A1

WO2009078045A1 - Gasketing system for fuel cells having a v-shaped profile

Info

Publication number: WO2009078045A1
Application number: PCT/IT2007/000881
Authority: WO
Inventors: Sergio Fasce; Thomas Coltella; Luca Rimassa
Original assignee: Ansaldo Fuel Cells S.P.A.
Priority date: 2007-12-18
Filing date: 2007-12-18
Publication date: 2009-06-25
Also published as: EP2243186A1

Abstract

The invention relates to a gasketing system for fuel cell stack, preferably a molten carbonate fuel cell stack, with external manifolding comprising a dielectric frame that is adapted to be coupled with a manifold housing by means of complementary- section shape coupling, wherein the frame comprises at least one oblique wall and the housing comprises a recess having a corresponding oblique wall. The frame may comprise a plurality of modular segments interconnected each other through a slide-fit connection system.

Description

DESCRIPTION

"Gasketing system for fuel cells having a V-shaped profile"

[0001] BACKGROUND OF THE ART [0002] A molten carbonate (MCFC) stack consists of a set of individual MCFC cells being serially electrically coupled to each other. The cells are separately fed in a continuous and independent manner by a fuel fluid at the anode and by an oxidizing fluid at the cathode. The fuel generally consists of a hydrogen-rich mixture, while the oxidant mainly consists of the oxygen contained in the air.

[0003] A distinctive feature for the MCFC stacks relates to the system for feeding the fuel and oxidizing gases to the cells: this can take place either by internal or external manifolding.

[0004] In the case of internal manifolding, the gas channels are directly obtained inside the cell stack. On the contrary, in the case of an external manifolding, the manifolding and collection of the gases are achieved by means of four manifolds which are held in contact with side walls of the cell package.

[0005] The gasketing against gas leakage in those area of the manifolds resting on the cells is achieved by introducing suitable gaskets between the contact surfaces .

[0006] The requirements which the manifold gaskets must meet are indicated herein below:

- sealing against gas leakage: is achieved both using very-low porosity materials and employing compressed fibrous materials so as to limit the gas escape paths between the fibres;

- thermal resistance: the materials used must resist the high operation temperatures of the MCFCs; - chemical resistance in harsh environments: the materials must be capable of resisting both the corrosion by the electrolyte, and the reducing and oxidizing gases that are supplied to the anode and cathode side, respectively; - mechanical strength: except for those portions where gasket compression is desired, the material used must mechanically resist the load applied by the manifold;

- electrical resistance: considering that each cell is at an electric potential which is different from the other cells in the stack, the gaskets must be electrically insulating in order not to cause short circuits between the cells through the manifolds.

[0007] In MCFC stacks, the gas gasketing systems or gaskets are obtained by suitably coupling a frame of dielectric material, felts and ceramic material cloths. The felts and cloths compensate for the unevenness of the surfaces being in contact with each other and become gas impermeable when compressed. On the other hand, the task of the frame is to ensure the electrical insulation between cells and manifolds.

[0008] The frame typically consists of a ceramic material (for example, alumina) being provided with, on the one hand, extremely low electric permeability, and on the other, with excellent high-temperature resistance. It must be sufficiently thick to ensure that a minimum safety distance is maintained between manifold and cell package in order to avoid possible electric arcs. [0009] Instead, the felts and cloths are sandwiched between the dielectric frame and the metallic parts, whether of the manifold or stack. They are typically made of ceramic materials according to the quality of the surfaces which are in contact therewith. [0010] Felts are more deformable, thus more suitable to be employed on the uneven surface of the cell stack. On the contrary, cloths are used for the sealing on planar, even surfaces .

[0011] The average operating temperature of the stacks is 650° C, while the assembly takes place at room temperature (25° C), with a thermal differential of 625° C (650° C - 25° C = 625° C) .

[0012] The coefficient of linear thermal expansion of alumina (the material of the dielectric frame) and steel (the material of the manifold) are approximately one half the other.

[0013] Taking into account the high thermal differential and the different expansion coefficients of the frame and manifold, the ceramic frame must have such a structure as to compensate for the higher expansion of the steel which the manifold is made of.

[0014] The solution generally adopted in the prior art for the dielectric frame provides that the frame consists of a plurality of alumina rectangular bars that are connected by slidable keys (US 4,414,294) at their ends. [0015] The keys inserted in the alumina bars with a small clearance coupling on one hand allow the relative sliding which is required to compensate for the different thermal expansion, and ensure, on the other hand, a good sealing for the gases. [0016] In order to standardize the relative sliding between the various bars, each of them is connected to the manifold at the central point thereof, by means of small keyed pins on the manifold which are inserted within suitable holes provided in the alumina bars. The angular areas of the alumina frame have gasketing systems comprising two keys that are arranged adjacent to the angle.

[0017] Another example of dielectric frame and gasketing system is described in the patent application WO2006071349, in which the alumina frame angular areas have gasketing systems comprising a key arranged proximate to the angle.

[0018] Another problem which is encountered during the operation of the stack is the occurrence of a decrease of the whole height of the cell package.

[0019] Since the manifolds height remains constant, the stack lowering results in a relative sliding between manifold and cells which takes significant values at the end plates of the same stack. [0020] The friction force which is generated during the sliding of the end plates relative to the manifolds and the dielectric frames being integral thereto generates a torque which tends to rotate the dielectric bars that are arranged along the manifold horizontal lengths, about their transversal axis.

[0021] This rotation or "turnover" phenomenon causes, upon time, break-ups in those bars that arranged along the frame horizontal lengths and angular areas. [0022] In fact, in prior art configurations, where the frame angular areas have gasketing systems comprising one or two keys that are arranged proximate to the angle, the frame angles are constrained in their position, thus the rotation is prevented.

[0023] Consequently, all the torque which is derived from this phenomenon will be released in these areas, causing possible break-ups of the gasket dielectric element and consequent gas leakage.

[0024] Therefore, the problem at the heart of the invention is to provide a dielectric frame configuration and a gasketing system for fuel cell stack which are provided with an external manifolding, which allows either reducing or eliminating the phenomenon of break-ups of those bars arranged along the frame horizontal lengths and the angular areas of the same frame. [0025] Such problem is solved by a frame and a gasketing system for a fuel cell stack being provided with external manifolding as set forth in the annexed claims. [0026] Further features and advantages of the dielectric frame and the gasketing system according to the invention will be appear more clearly from the description set forth below of preferred exemplary embodiments thereof, which are given by way of non-limiting example, with reference to the annexed Figures, in which: [0027] Figure 1 is a top view of an assembled dielectric frame; [0028] Figure 2 is a perspective view of a side portion of the frame of Figure 1 partially assembled;

[0029] Figure 2A is a perspective view of a side portion of the frame of Figure 1, assembled and not expanded; [0030] Figure 2B is a perspective view of a side portion of the frame of Figure 1, assembled and expanded;

[0031] Figure 3 is a perspective view of an angular portion of Figure 1, according to an embodiment of the invention;

[0032] Figure 4 is a perspective view of the angular portion of Figure 3 turned over, then rotated at 90° on the plane;

[0033] Figure 5 is an exploded, perspective view of an angular portion of Figure 1, according to an embodiment of the invention; [0034] Figure 6 is an exploded, perspective view of the gasketing system;

[0035] Figure 7 is an perspective view assembled of the gasketing system;

[0036] Figure 8 is a sectional view of a detail of the structure of Figure 8 along the VII plane of Figure 7;

[0037] Figure 9 is a scheme of Figure 8 in which the forces involved are outlined.

[0038] With reference to Figure 1, the numeral 1 generally designates a dielectric material frame that is assembled according to the invention. [0039] The rectangular- or squared-shape frame 1 comprises a plurality of modular segments which are interconnected to each other by means of a slide-fit connection system to yield the frame. [0040] In particular, the modular segments comprise a plurality of segments 2 which are connected to each other and to angular segments 3 through a slide-fit connection system to yield the frame. [0041] The angular segments 3 form, two by two, an angular portion 4, and are joined by means of a slide-fit connection system 5.

[0042] With reference to Figures 2 and 8, each segment 2 has an upper wall 6 that is joined, on the one hand, to an inner side wall 7, which is in turn joined to a lower wall 8 opposite the upper part 6 and, on the other hand, to an outer side part 9. The outer side wall and the lower wall are joined by an oblique joining wall. [0043] As shown in Figure 8, the segment 2 has a transversal section S comprising a first portion Sl of substantially rectangular shape adjacent to a second portion S2 of substantially trapezoidal shape. In particular, the portion S2 is in the shape of a right trapezium. [0044] In other words, the segment 2 has a substantially half V-shaped sectional profile with flat base. [0045] The joining wall 10, in the gasketing system of the invention, provides a mechanical coupling, for example abutting, with a manifold housing 25, as it will be explained herein below. [0046] Each segment 2 ends, at each end, with opposite end surfaces 12.

[0047] The inner side wall 7 has at least two projections 7A, of a rectangular or squared shape, having a length 1 and a height h. [0048] The inner side wall 7 further comprises at least two recesses 7B, of a substantially rectangular or squared shape, having a length 1' and a height h' . [0049] The segment 2 is symmetric relative to its middle plane . [0050] For example, with reference to the Figure 2, each projection 7A is located substantially proximate to each end of the inner side wall 7, while the recesses 7B are located substantially proximate to the central portion of said wall, suitably spaced apart from each other by the two projections 7A.

[0051] On the contrary, the configuration described above can also be provided with each recess 7B being located substantially proximate to each end of the inner side wall 7, with the projections 7A located substantially proximate to the central portion of said wall, suitably spaced apart from each other by the two recesses 7B. [0052] The recesses 7B are adapted to house in a slide-fit manner, at least with reference to the heights (the heights h and h' are adapted to create a fitting relative to each other) , respective projections 7A of the inner side wall of a segment 2 set aside, while the projections 7A are adapted to be received in respective recesses 7B of the inner side wall of a segment set aside. Thus a fitting assembly of the segments 2 is accomplished, with reference to the heights, and slidable along the lengths 1 and 1' .

[0053] With reference to the Figure 2A, the segments 2 are advantageously assembled in an offset manner, that is each segment 2 is slide-fit connected to two segments 2. [0054] During the assembly step, a projection 7A and the recess 7B immediately adjacent thereto, of a first segment 2, are fitted, at least relative to the heights, within a respective recess 7B and a projection 7A immediately adjacent thereto, of a second segment 2, whereas the second projection 7A and the recess immediately adjacent thereto of the first segment 2 are fitted, relative to the heights, within respective recess and projection immediately adjacent thereto of a third segment 2. [0055] Figure 2A shows the segments 2 as being assembled as described above, at room temperature.

[0056] Figure 2B, on the other hand, represents the segments 2 being assembled as described above, at the operational temperature of the stack (about 650 C) . [0057] When passing from the frame assembling temperature of 25 C (room temperature) to the operational temperature of the cell stack of about 650 C, the frame 1 in ceramic material, preferably alumina, and the manifold in metallic material, preferably steel, undergoes a temperature differential of 625 C.

[0058] Since the expansion coefficient of the manifold metallic material is twice that of the frame ceramic material, the higher expansion of the steel, and conseguently the greater size reached by the manifold as compared with the frame, is compensated by the capability of the inventive frame to expand, increasing the size thereof due to the slide-fit structure of the segments 2. [0059] In fact, since the recesses 7B have a length 1' greater than the length 1 of the projections 7A, when the frame is assembled and the projections are housed in the respective recesses and the temperature is brought to about 650^° C, the projections 7A slide within the recesses 7B, thus resulting in an increase in the frame size. In this configuration, as shown in Figure 2B, the end surfaces 12 of the segments 2 are spaced apart by a distance equal to the sliding space of the projections in the recesses.

[0060] As shown in Figure 8, when two segments 2 are coupled with the slide-fit system of the invention, the respective inner side walls 7 abut against each other.

[0061] With reference to Figure 8, a first segment 2 has a transversal section S comprising a first portion Sl of a substantially rectangular shape adjacent to a second portion S2 of a substantially trapezoidal shape. In particular, the portion S2 has the shape of a right trapezium.

[0062] A second segment 2 has a transversal section S' comprising a first portion S3, similar to Sl, of a substantially rectangular shape (also taking into account the portion corresponding to the projection-recess coupling) , adjacent to a second portion S4, symmetric to S2, of a substantially trapezoidal shape.

[0063] S2 and S4 define a section portion which is defined by the two joining walls 10 and the two lower walls 8 of the two coupled segments 2, having the shape of a isosceles trapezium.

[0064] In other words, the two coupled segments 2 have a flat based V-shaped sectional profile. [0065] After the frame and manifold have been assembled to yield the gasketing system 31 of the invention, the section portion as defined by S2 and S4 represents at least one engagement portion of the frame assembled to the housing 25 of the manifold 24, as will be set forth in greater detail herein below. [0066] With reference to Figures 3, 4, and 5, an angular portion is generally designated with the numeral 4. [0067] The angular portion comprises at least two angular segments 3A and 3B, which have an upper wall 13 which on the one hand is joined to an inner side wall 14 that is, in turn, joined to a lower wall 15 opposite the upper part 13, and on the other hand to an outer side wall 16, respectively. The outer side wall and the lower wall are joined by an oblique joining wall 17. [0068] By "inner side wall 14" is meant the wall of the segment 3A or 3B that is adapted to be coupled to the inner side wall 7 of a segment 2. By "outer side wall 16 of the segment 3A or 3B" is meant the wall opposite the inner side wall 14. [0069] Each angular segment 3A and 3B ends, at each end, with opposite end surfaces 18.

[0070] The angular segments 3A and 3B have, respectively, first portions 3A' and 3B' , each having a transversal section (P with reference to the end surface 18 of Fig. 3 or 4) substantially corresponding to that of a segment 2, and second end portions 3A' ' and 3B' ' having a transversal section (Q with reference to the end surface 18 in Fig. 5) substantially corresponding to the section as defined by two mutually coupled segments 2 (sections Sl to S4 in Fig. 8) . [0071] The transversal section Q of the second end portions 3A' ' and 3B' ' has a size being substantially twice the size of the section P of the first portions 3A' and 3B' of the angular segments 3A and 3B. [0072] The first portions 3A' and 3B' and the second end portions 3A' ' and 3B' ' of the angular segments 3A and 3B are uninterruptedly connected to yield an individual piece. Therefore, the second end portions 3A'' and 3B'' have, respectively, a connection surface 23 with the first portion 3A' or 3B' of the angular segment. [0073] The second end portion 3A' ' of the segment 3A results to be provided with a groove 19 which is obtained substantially in the centre of the end surface 18 of the second end portion 3A' ' . The groove extends parallel to the side walls 14, 16. [0074] The second end portion 3B' ' of the segment 3B is provided with a groove 20 obtained in the inner side wall 14 (Fig. 5) or the outer side wall 16 (Figs. 3 and 4) substantially at the end of the second portion 3B' ' . The groove 20 extends transversally to the inner side wall 14 of the segment 3B. [0075] The grooves 19 and 20 define a housing seat 21 for a key 5. The groove has preferably an oblong shape, though other shapes may also be provided, for example a rectangular shape. [0076] The key is dimensioned so as to allow for the coupling in abutment of the two angular segments 3A and 3B, thus providing an angular portion 4 of the frame 1. [0077] After the frame has been assembled, the key 5 is fitted within the housing seat 21 as defined by the grooves 19 and 20.

[0078] Preferably, the key 5 is a parallelepiped and could have, in the outermost parts thereof, the shape of the grooves 19 and 20. [0079] At the stack operating temperature, the key-housing seat angular coupling system allows the angular portion 4 to expand, i.e. to increase its size, due to the angular segments 3A and 3B sliding in opposite directions. [0080] Thereby, the frame matches the increased manifold dimensions due to the higher expansion coefficient of the manifold metallic material compared to the ceramic material of the frame, as explained above. [0081] The inner side wall 14 of the angular segments has at least one projection 7A of a preferably rectangular or squared shape, having a length 1 and a height h, and at least one recess 7B of a preferably rectangular or squared shape, having a length 1' and a height h' . [0082] The projection length 1 is lesser than the recess length 1' so as to accomplish, as previously described, a fitting coupling with reference to the heights between a projection and a recess, which also allows the frame expansion at the stack operating temperature, due to the sliding of the projection in the recess along the length 1 and 1' . [0083] The at least one recess 7B and the at least one projection 7A being provided on the inner side surface 14 of each segment 3A and 3B, are fittingly coupled with reference to the heights h and h' with a projection and a recess that are provided on the inner side surface 7 of a segment 2, respectively. [0084] Thereby, the inner side wall 7 of the segment 2 abuts against the inner side wall 14 of the angular segment 3A and 3B.

[0085] With reference to Figures 6, 7, and 8, the numeral 31 designates the gasketing system of the invention comprising a frame 1 and a manifold 24.

[0086] The manifold 24 comprises a wall 24A having an inlet hole 24B for the reducing and oxidizing gases to feed the cell stack. The wall 24A has preferably a squared or rectangular shape. [0087] From the four sides of the wall 24A, four walls 24C perpendicularly extend, which have two sides 24C opposite each_ other, and two sides 24C ' opposite each other.

[0088] Each side 24C of a wall 24C matches with a respective side 24C of the adjacent wall, whereas a side 24C ' matches with a side of the wall 24A. [0089] The walls 24C are uninterruptedly connected, via the sides 24C and 24C ' , respectively, to the other walls and wall 24A, thus forming four upper angles 24D opposite to four lower angles 24D' .

[0090] A side 24C" corresponds to a side of the wall 24A, while the opposite side 24C ' is integral, throughout its length, to a housing 25 having a shape complementary to at least one portion of the transversal sections S2 and S4 of two segments 2 that are connected by the slide-fit system as described before.

[0091] At the four upper angles 24D, the housing 25 has a shape complementary to the transversal section Q of the end portions 3A' ' and 3B' ' of the angular segments 3A and 3B.

[0092] In particular, the housing 25 comprises a lower surface 25A integral to the side 24C" of the wall 24C and having a width L greater than the thickness I/ of the wall 24C. [0093] From the lower surface 25A, two side walls 25B perpendicularly extend, which are opposite each other and end in an upper surface 25C opposite the lower surface. [0094] The upper surface 25C has a cavity 26 having a bottom wall 26A and two side walls 26B which extend from said bottom wall to the upper surface 25C of the housing 25 so as to define, for the cavity 26, a preferred trapezoidal profile.

[0095] Advantageously, the trapezoidal profile of the cavity 26 results to be substantially complementary to the sections S2 a S4 of two coupled segments 2. At the angles 24D, the cavity 26 has a preferably trapezoidal profile substantially complementary to the transversal section Q of the portion 3A' ' or 3B' ' of the angular segment 3A or 3B. [0096] Accordingly, the housing 25 fittingly houses the frame 1 achieved by assembling the segments 2 and the angular segments 3A and 3B so that the joining walls 10 of the segments 2 and the joining walls 17 of the angular segments 3A and 3B are abutted against, optionally with the interposition of ceramic cloth or felt gaskets, the bottom wall 26A and the side walls 26B of the cavity 26. [0097] In a preferred embodiment, between the frame 1 and the cavity 26 of the manifold housing 25, a felt 27 and a cloth 28 in ceramic material are interposed to the purpose of ensuring a better sealing and imperviousness to gas leakage, and of standardizing the pressure on the whole contact surface.

[0098] The felt and cloth have a shape substantially equivalent to that of the frame 1. The cloth 28 comprises two parts which are separated and located, respectively, on the side walls 26B. However, a cloth configuration can be provided which comprises an individual part which is thus also arranged, beside on the walls 26B, on the bottom wall 26A under the felt. [0099] In particular, the felt prevents possible gas infiltrations in the junction points between two adjacent segments from flowing through the channel located between the segments and the housing and escaping from the subsequent junction. [00100] A cloth and/or felt is also interposed between the upper walls 6 and 13, respectively, of the segments 2 and angular segments 3A or 3B, which are mounted so as to form the frame 1, and the cell stack (not shown) . [00101] Accordingly, the gasketing system 31 for fuel cells with external manifolding of the invention comprises a manifold for fuel cell stack having a housing of a shape complementary to the frame transversal section, and the same frame which couples to the manifold by abutted mechanical coupling. [00102] In a preferred aspect, the gasketing system also comprises a felt and a cloth that are interposed between the frame and the manifold housing. [00103] The gasketing system of the invention is preferably for molten carbonate fuel cell (MCFC) stacks. [00104] The frame and gasketing system of the invention allow overcoming some drawbacks encountered when using prior art frames and gasketing systems or gaskets. [00105] In particular, the frame and gasketing system of the invention determines a reduction in the phenomenon of the frame horizontal and angular lengths turnover, thus decreasing the occurrence of break-ups of the frame. [00106] Therefore, the coupling system between the frame 1 and manifold 24 of the present invention is an improvement, as compared with the previous solutions adopted in the art, in terms of: higher sealing to gas leakage;

• higher mechanical strength;

• lower risk of breaking of the components, both at the angles and along the frame sides where the segments tend to turn over, then breaking, due to the forces associated to the stack-manifold relative sliding. [00107] This is due to the fact that the sealing or frame-manifold coupling system of the present invention allows releasing the pressures which generate during the cells operation, between the manifold and the cell stack in an efficient manner, by limiting or completely eliminating the break-ups due to frame torsions. [00108] In fact, with reference to Fig. 9, during the cell stack operation, the manifold is pushed towards the cells with a force Fm which is transmitted to the dielectric frame in the orthogonal direction to the contact surfaces between the frame 1 and the housing 25 of the manifold 24. [00109] Accordingly, the force Fm turns into the force Ft,d with reference to the oblique surface, and in the force Ff, d with reference to the bottom surface 26A of the cavity 26.

[00110] The forces Ff, d, and Ft,d, which are respectively associated with the base and oblique surface, press the ceramic cloths and felts comprised between the manifold housing and the dielectric frame, thus generating a sealing against gas leakage in this area. [00111] The force Ft,d has two components: a component Fa and a component Fn.

[00112] The forces Fa and Ff, d act upon the felt and the cloth which are interposed between the frame and the cells of the fuel stack (the felt and the cell stack are not shown in the Figures) . Such forces press the ceramic cloths and felts, thus ensuring a sealing against 1 eakage in these areas.

[00113] The second component Fn is the force normal to the contact surfaces between two faced segments, which ensures just the contact between the pieces. This force, in combination with the very accurate finish of the contacted surfaces, ensures the sealing against gas leakage between a segment and the other.

[00114] Just due to the coupling type between the frame and the manifold housing, a better distribution of the forces involved during the stack operation, and consequently better results in terms of sealing against gases and break-ups of the dielectric components are achieved. [00115] An improved stability of the dielectric frame further allows using frames having higher thicknesses, thus reducing the occurrence of electric arcs. [00116] Another advantage of the frame of the invention is due to the angular connection, which is provided by a single key. [00117] In other terms, the angles of the inventive frame are not formed with rigid pieces having an angular shape connected to the segments by means of key-housing groove modular connection systems, as in the prior art. [00118] Instead, the angles of the inventive frame are formed by the junction, by means of a key-groove connection system, between two angular segments. [00119] As compared with known solution, this angular junction has a higher degree of freedom, and allows releasing the mechanical stresses due to the turning over torque generated by the pressures between the stack and manifold during the cell operation.

[00120] This innovative angular connection contributes, in association with the frame-manifold coupling system, to eliminate, or anyhow greatly reduce, break-ups in the angular areas.

[00121] Beside the aforementioned improvements, the slide-fit coupling system between the segments of the invention offers further advantages:

• possibility of using modular segments, which are geometrically equal each other, thus interchangeable

(except for the angular members) independently from the row, horizontal or vertical arrangement, and type of frame. This definitely simplifies the frame assembling steps and makes the use of the components versatile and flexible, also in the case these had already been used;

• reduction in the risk of break-ups, the probability of being able to reuse several times the dielectric material segments increases, thus sparing the supply costs .

Claims

CLAIMS l.A gasketing system (31) for fuel cell stack with external manifolding comprising a dielectric frame (1) which is suitable to couple with a housing (25) of a manifold (24) by complementary section shape coupling, characterized in that said frame comprises at least one oblique wall (10, 17) and said housing comprises a recess (26) having a correspondent oblique wall (26B) .

2. The gasketing system according to claim 1, wherein said frame (1) comprises a plurality of modular segments interconnected each other through a slide-fit connection system.

3. The system according to claim 2, wherein said modular segments comprise a plurality of segments (2) which are connected to each other and to angular segments (3) through a slide-fit connection system.

4. The system according to claim 3, wherein said angular segments (3) form, two by two, angular portions (4) .

5. The system according to claim 3 or 4, wherein said angular segments (3) are joined through a slidable connection system with joining key (5) .

6. The system according to any claim 2 to 5, wherein each segment (2) has an upper wall (6) that on one hand is joined to an inner side wall (7) which is, in turn, joined to a lower wall (8) opposite the upper wall (6), and on the other hand to an outer side wall (9), and wherein said outer side wall and said lower wall are joined by said oblique joining wall (10) .

7. The system according to any claim 2 to β, wherein said segment (2) has a transversal section (S) comprising a first portion (Sl) of substantially rectangular shape adjacent to a second portion (S2) of substantially trapezoidal shape.

8. The system according to claim 7, wherein said portion (S2) has the shape of a right trapezium.

9. The system according to any claim 2 to 8, wherein said segment (2) has a sectional profile that is substantially half V-shaped with flat base.

10. The system according to any claim 1 to 9, wherein said segment (2) is symmetric relative to its median plane .

11. The system according to any claim 6 to 10, wherein said inner side wall (7) has at least two projections (7A) of a rectangular or squared shape having a length (1) and a height (h) .

12. The system according to any claim 6 to 11, wherein said inner side wall (7) comprises at least two recesses (7B) of a substantially rectangular or squared shape, having a length (1') and a height (h' ) .

13. The system according to the claims 11 and 12, wherein said at least two projections (7A) are substantially located proximate to each end of the inner side wall (7), and said at least two recesses (7B) are located substantially proximate to the central portion of said wall.

14. The system according to any claim 11 to 13, wherein, after the frame has been assembled, said recesses (7B) are adapted to fittingly house, at least with reference to the heights, respective projections (7A) of the inner side wall of a segment (2) that is placed aside, while the projections (7A) are adapted to be received within respective recesses (7B) of the inner side wall of a segment that is placed aside.

15. The system according to claim 14, wherein said segments (2) are fittingly assembled, with reference to the heights (h, h' ) , and slidingly along the lengths (1) and (I' ) .

16. The system according to claim 15, wherein said segments (2) are assembled in an offset manner, i.e. each segment (2) is slide-fit connected to two segments (2 ) .

17. The system according to claim 16, wherein said offset assembling of the segments (2) comprises the fitting, at least relative to the heights, of a projection (7A) and the recess (7B) immediately adjacent thereto, of a first segment (2) , to a respective recess (7B) and to a projection (7A) immediately adjacent thereto, of a second segment (2), while the second projection (7A) and the recess immediately adjacent thereto of the first segment (2) are fitted, at least relative to the heights, to the respective recess and the projection immediately adjacent thereto of a third segment (2) .

18. The system according to any claim 14 to 17, wherein the respective inner side walls (7) of two segments

(2) are coupled in abutment.

19. The system according to any claim 14 to 18, wherein, after the frame has been assembled, two segments (2) that are slide-fit coupled have, respectively, a first transversal section (S) comprising a first portion (Sl) of a substantially rectangular shape adjacent to a second portion (S2) of substantially trapezoidal shape, and a second transversal section (S') comprising a first portion (S3) , similar to (Sl) , of substantially rectangular shape, adjacent to a second portion S4, symmetric to S2, of substantially trapezoidal shape.

20. The system according to claim 19, wherein said sections (S2) and (S4) define a sectional portion being defined by said two joining walls (10) and said two lower walls (8) of said two coupled segments (2), having the shape of an isosceles trapezium.

21. The system according to the claims 19 or 20, wherein said two coupled segments (2) have a V-shaped sectional profile with flat base.

22. The system according to any claim 1 to 21, wherein said frame (1) comprises a plurality of angular portions (4), each comprising at least two angular segments (3A) and (3B) having, on the one hand, respectively, an upper wall (13) being joined to an inner side wall (14) that is, in turn, joined to a lower wall (15) opposite the upper part (13), and on the other hand to an outer side part (16), wherein said outer side wall and said lower wall are joined by an oblique joining wall (17) .

23. The system according to claim 22, wherein each angular segment (3A) and (3B) ends, at each end, with opposite end surfaces (18).

24. The system according to claim 22 or 23, wherein said angular segments (3A) and (3B) have, respectively, first portions (3A') and (3B') having each transversal section (P) substantially corresponding to that of a segment (2) and second end portions (3A'') and (3B'') having a transversal section (Q) substantially corresponding to the sections Sl to S4 as defined by two segments (2) being coupled to each other.

25. The system according to claim 24, wherein said transversal section (Q) of said second end portions (3A'') and (3B") has a size that is substantially twice the section (P) of said first portions (3A' ) and (3B') of said angular segments (3A) and (3B) .

26. The system according to claim 24 or 25, wherein said second end portions (3A'') and (3B'') have, respectively, a connection surface (23) with said first portion (3A') or (3B') of the angular segment.

27. The system according to any claim 24 to 26, wherein said second end portion (3A'') of the segment (3A) is provided with a groove (19) that is obtained substantially in the centre of the end surface (18) of the second end portion (3A'') .

28. The system according to claim 27, wherein said groove extends parallel to the side walls (14, 16) of the angular segment .

29. The system according to any claim 24 to 28, wherein said second end portion (3B'') of the segment (3B) is provided with a groove (20) that is obtained in the inner side wall (14) or outer side wall (16) substantially at the end of the second portion (3B'') -

30. The system according to claim 29, wherein said groove (20) extends transversally to the inner side wall (14) of the angular segment (3B) .

31. The system according to claim 29 or 30, wherein said grooves (19, 20) define a housing seat (21) for a joining key (5) .

32. The system according to any claim 22 to 31, wherein said inner side wall (14) of the angular segments has at least one projection (7A) and at least one recess (7B) .

33. The system according to any claim 1 to 32, wherein said manifold (24) comprises a housing (25) of a shape complementary to at least one portion of the transversal sections S2 and S4 of two coupled segments (2) of the frame.

34. The system according to claim 33, wherein said housing (25) has a shape complementary to the transversal section (Q) of the end portions (3A' ' ) and (3B'') of said angular segments (3A) and (3B) .

35. The system according to claim 33 or 34, wherein said housing (25) comprises a recess (26) having a trapezoidal profile.

36. The system according to claim 35, wherein said recess (26) comprises a bottom wall (26A) and two side walls (26B) which extend from said bottom wall to an upper surface (25C) of the housing (25) .

37. The system according to claim 36, wherein, after the system has been assembled, said bottom wall (26A) and said side walls (26B) of said recess (26) of the housing (25) are coupled in abutment with the joining walls (10) of the segments (2) and joining walls (17) of the angular segments (3A) and (3B) .

38. The system according to any claim 1 to 37, wherein between said frame (1) and said housing (25) of the manifold a felt (27) and a cloth (28) in ceramic material are interposed.

39. The system according to any claim 1 to 38 for molten carbonate fuel cell (MCFC) stacks with external manifolding.

40. A frame (1) for fuel cell stack with external manifolding comprising a plurality of modular segments.

41. The frame according to claim 40, wherein said plurality of modular segments comprises a plurality of segments (2) that are connected to each other and angular segments (3) through a slide-fit connection system, wherein said angular segments (3) form, two by two, angular portions (4) .

42. The frame according to claim 40, wherein said segment (2) is symmetric relative to the median plane thereof.