Compression assembly, solid oxide fuel cell stack, a process for compression of the solid oxide fuel cell stack and its use

Abstract
A compression assembly for distributing an external compression force to a solid oxide fuel cell stack, the compression assembly comprising a force distributing plate, and a force distributing layer so that when said compression assembly is mounted together with the solid oxide fuel cell stack, the external compression force is exerted on the force distributing plate and the force distributing layer is provided next to a surface of at least one end plate, opposite to the surface facing the solid oxide fuel cells. The force distributing layer has a rigid frame extending next to a region of a sealing area of the solid oxide fuel cell stack. One or more resilient elements are placed inside the space enclosed by the rigid frame and positioned next to an electrochemically active area of the solid oxide fuel cell stack, so that when the compression assembly mounted with the solid oxide fuel cell is in use, the force distributing layer provides an unequally pressure distribution across the region of the sealing area and the electrochemically active area.
Description

BRIEF DESCRIPTION OF THE DRAWINGS


FIG. 1 shows the different components of a SOFC stack of the invention.



FIG. 2 shows a vertical section of a SOFC stack in another embodiment of the invention.



FIG. 3 shows a vertical section through a SOFC stack in an embodiment of the invention, where the resilient element is air.



FIG. 4 shows an embodiment of the invention, where the resilient elements are springs.



FIG. 5 shows another embodiment of the invention, where the resilient elements are springs.



FIG. 6 shows the distribution of forces in the SOFC stack.





DETAILED DESCRIPTION OF THE INVENTION

In SOFC many sealing materials require a higher compression pressure to create a gas tight seal than the required compression pressure to create electrical contact on the electrochemically active area. Subjecting the electrochemically active area to compression forces that are too high lead to its deformation. When utilizing the compression assembly of the invention the compression forces are separated into several regions so that the electrochemically active area can be compressed with a pressure that is, for instance, smaller than the pressure present in the sealing regions.


This is an advantage because separation of the compression forces makes it possible to choose a suitable compression pressure for the electrochemically active area independent of the compression pressure required in the region of the sealing material.


According to the inventive process, the overall compression force on the fuel cell stack is provided by exerting an external force on force transmitting plates situated at each end of the fuel cell stack. The external force is transmitted through the force transmitting plate and at one or both ends of the fuel cell stack distributed to a force distributing layer comprising a frame extending in the region of the sealing area of the SOFC and one or more resilient elements placed inside the space enclosed by the frame and positioned above the electrochemically active area of the SOFC. Between the force distributing layer and the first solid oxide fuel cell an end plate is placed.


The outer dimensions of the frame, i.e. length and width, are of the same magnitude as those of a single solid oxide fuel cell. In one embodiment the inner dimensions, i.e. inner length and width of the frame, are chosen to provide a surface area covered by the frame corresponding to the sealing area of the solid oxide fuel cell.


The frame is made from a material of greater rigidity than the one or more resilient elements. This is an advantage since it allows the exertion of a greater compression pressure via the frame in the sealing area region compared to the pressure exerted on the electrochemically active area via the one or more resilient elements.


The one or more resilient elements are more flexible than the frame. The force exerted on the force transmitting plate is thereby divided into separate areas with different pressures on the frame and the resilient elements. The flexible material for the one or more resilient elements can be any element that is more flexible than the frame. Examples are materials based on mica or ceramic fibres. Fibrous metallic materials are also suitable. Compressed air or springs of, for instance, metal can also be used.


The one or more resilient elements must cover a surface approximately corresponding to the inner dimensions of the frame. An arbitrary thickness can be chosen since the resilient elements are flexible in nature.


Separating the compression forces in this manner is an advantage as it allows the maintenance of the compression force in situations when temperature gradients cause thermal expansion of the SOFC stack. This allows the SOFC stack to be operated with larger temperature gradients, for instance higher current density, and the stack can be made with a larger number of cells. Higher current density and increased number of cells reduces the overall cost of the SOFC system and increases the power output per stack. SOFC stacks comprising the compression assembly of the invention are therefore particularly suitable for the generation of power.


In an embodiment of the invention the force distributing layer is only situated on the first end plate adjacent to the first solid oxide fuel cell in the stack.


In another embodiment of the invention the force distributing layer is situated on both end plates of the SOFC stack. In an embodiment of the invention the force distributing layer comprises a frame and one or more resilient elements in the form of metal springs. The metal springs are supported by one or more positioning elements provided with apertures or holes in the region of the electrochemically active area in which the metal springs can be introduced. The metal springs provide compression force separated from both the force transmitted through the one or more spring positioning elements and from the force transmitted through the frame. The spring positioning elements can for instance be one or more plates provided with apertures or holes in the region of the electrochemically active area for positioning the metal springs.


Suitable compression pressures that can be exerted in the region of the electrochemically active area are in the range of 0.05 to 3 bars.


Suitable compression pressures that can be exerted in the region of the cell sealing area are in the range of 0.05 to 40 bars.


These pressures depend on interconnect geometries, sealing materials and fuel cell operating gas pressures.


In a further embodiment of the invention the force distributing layer comprises a frame and a plurality of resilient elements of flexible material.


In another embodiment of the invention the force distributing layer comprises a frame and a plurality of springs. The spring positioning element is in this embodiment not required, when a sufficient number of springs are present. A suitable number of springs are 4 to 100.


In a further embodiment of the invention the force distributing layer comprises a frame and a resilient element of compressed air or a flexible material.


In the following figures different embodiments illustrating the invention are described.



FIG. 1 shows the different components of a SOFC stack according to an embodiment of the invention. External compression force is exerted on force transmitting plate 1. The force is thus transmitted to the force distributing layer comprising frame 2 and one or more resilient elements 3 placed inside the space 4 enclosed by frame 2 and positioned above the electrochemically active area 18 of the solid oxide fuel cell 5. The force distributing layer is followed by planar end plate 6, which in turn is followed by spacer-interconnect assembly 7 and finally by solid oxide fuel cell 5. The frame 2 is positioned adjacent to the sealing area 17 such that when the external force is exerted on the frame 2 part of it is transferred to the sealing area and another part to the electrochemically active area. The number of solid oxide fuel cells depends on the power to be produced by the solid oxide fuel cell stack. The number of solid oxide fuel cells may e.g. be between one and 75 such as between 5 and 75. Consequently, the height of the solid oxide fuel stack depends on the number of solid oxide fuel cells. For example may the height of a solid fuel cell stack comprising 75 fuel cells be about 9 cm excluding each of the rigid frames 2 each having a height about 1 cm. The electrochemically area may e.g. be between 2000 cm2 and 15000 cm2 e.g. 9000 cm2 and the sealing area between 800 cm2 to 6000 cm2.



FIG. 2 shows a vertical section through of a SOFC stack according to another embodiment of the invention. The force transmitting plate 1 is subjected to an external compression force which is transmitted to the force distributing layer comprising frame 2 and resilient element 3 and thereby to spacer-interconnect assembly 7 and solid oxide fuel cells 5 placed in electrical series. Each spacer-interconnect assembly 7 has gas channels for transfer of either hydrogen (or another fuel such as methane), oxygen or air to the anode or cathode, respectively.


In this embodiment of the invention two force distributing layers are present. A force distributing layer is placed adjacent to each of the two planar end plates 6. The resilient element 3 can consist of a plurality of elements of flexible material.



FIG. 3 is a vertical section through a SOFC stack showing a force distributing layer comprising a frame and a resilient element of compressed air. In this embodiment the force transmitting plate and the frame have been integrated to form an integrated frame 8. Inlets 9 for compressed air to the space 4 enclosed by integrated frame 8 are shown. Air pressures of for example 100-1000 mbar gauge can be used.



FIG. 4 shows another embodiment of the invention, where the force distributing layer comprises a frame and one or more resilient elements in the form of metal springs 12. A spring positioning plate 10 provided with apertures or holes 11 in the region of the electrochemically active area in which the metal springs 12 can be placed. The metal springs 12 provide compression force separated from both the force transmitted through spring positioning plate 10 and from the force transmitted through frame 2.



FIG. 5 shows another embodiment of the invention, where the force distributing layer comprises a frame 2 and the resilient elements are a plurality of metal springs 12. In this embodiment a spring positioning plate is not required due to the presence of many metal springs supporting each other, for instance in a number between 4 and 100.



FIG. 6 shows the distribution of forces within the SOFC stack when an external force 13 is exerted on the force transmitting plate 1 thereby providing a compression load to a fuel cell stack. Different compression pressures for different parts of a solid oxide fuel cell stack are simultaneously exerted on the stack. The compression pressures, indicated by arrows, for the electrochemically active area 15 and the sealing area 14 of the solid oxide fuel cell are not equal. The pressure exerted on the resilient element 3 is of a lower magnitude than the pressure exerted on the frame 2, while maintaining good electrical contact within the solid oxide fuel cell stack during operation and at the same time ensuring a gas tight stack.

Claims
  • 1. Compression assembly for distributing an external compression force to a solid oxide fuel cell stack, said compression assembly comprising a force distributing plate, and a force distributing layer so that when said compression assembly is mounted together with the solid oxide fuel cell stack, the external compression force is exerted on said force distributing plate and said force distributing layer is provided next to a surface of at least one end plate, opposite to the surface facing the solid oxide fuel cells, said force distributing layer having a rigid frame extending next to a region of a sealing area of the solid oxide fuel cell stack, one or more resilient elements placed inside the space enclosed by said rigid frame and positioned next to an electrochemically active area of the solid oxide fuel cell stack, so that when said compression assembly mounted with the solid oxide fuel cell is in use, said force distributing layer provides an unequally pressure distribution across the region of the sealing area and the electrochemically active area.
  • 2. The compression assembly according to claim 1, wherein the pressure distribution by said one or more resilient elements is around 875 PA when the solid oxide fuel cell is in use.
  • 3. The compression assembly according to claim 1, wherein said one or more resilient elements allow for a compression between 0.1 mm and 0.2 mm more in a region in the middle of the force distributing layer than near the sides of the force distributing layer.
  • 4. Compression assembly according to claim 1, wherein said one or more resilient elements are arranged in one or more positioning elements.
  • 5. Compression assembly according to claim 1, wherein said one or more resilient elements are selected from the group of compressed air, a fibrous ceramic material and a fibrous metallic material.
  • 6. Compression assembly according to claim 1, wherein said one or more resilient elements comprises a material based on mica.
  • 7. Compression assembly according to claim 6, wherein at least one of said one or more resilient elements is a sheet made of mica.
  • 8. Compression assembly according to claim 7, wherein the thickness of the mica sheet has a thickness between 0.8-1.2 mm.
  • 9. Compression assembly according to claim 1, wherein said one or more resilient elements comprises at least one metal spring.
  • 10. Compression assembly according to claim 1, wherein said one or more resilient elements is at least one metal spring, wherein said at least one metal spring is arranged in one or more positioning elements.
  • 11. A solid oxide fuel cell stack comprising a compression assembly according to claim 1.
  • 12. A solid oxide fuel cell stack comprising an end plate, one or more solid oxide fuel cells, a compression assembly having a force distributing plate on which an external compression force is exerted, and a distributing layer provided next to a surface of said end plate opposite to its surface facing said one or more solid oxide fuel cells, said force distributing layer having a rigid frame extending next to the region of a sealing area of said solid oxide fuel cell stack, one or more resilient elements placed inside a space enclosed by said rigid frame and positioned next to an electrochemically active area of said solid oxide fuel cell stack, so that when said solid oxide fuel cell is in use said force distributing layer provides an unequally pressure distribution across the region of the sealing area and the electrochemically active area.
  • 13. Solid oxide fuel cell according to claim 12, wherein said force transmitting plate is provided with a clamp pressure, such that said rigid frame via said force transmitting plate is provided with a clamp pressure between 70%-90% of said clamp pressure of said clamp pressure of said force transmitting plate.
  • 14. Method for compressing a solid oxide fuel cell stack at both ends of the stack, the process comprising the steps of stacking a plurality of solid oxide fuel cells in electrical series thereby providing a region of a electrochemically active area and a sealing area, placing each end of the solid oxide fuel cell stack adjacent to an end plate surface, such that the surface of at least one of the end plates is opposite to the surface facing the solid oxide fuel cells,providing a force distributing layer of one or more resilient elements and a rigid frame above the region of the electrochemically active area and the sealing area of the solid oxide fuel cell stack and applying an external force to the force distributing layer, whereby a resulting compression pressure is distributed unequally across the region of the sealing area and the electrochemically active area, and the compression pressure exerted in the region of the sealing area is greater than the compression pressure exerted on the electrochemically active area of the solid oxide fuel cell stack.
Priority Claims (1)
Number Date Country Kind
PA 2006 00978 Jul 2006 DK national
Parent Case Info

This is a continuation-in-part of U.S. application Ser. No. 11/486,060, filed Jul. 14, 2006.

Continuation in Parts (1)
Number Date Country
Parent 11486060 Jul 2006 US
Child 11822647 US