Patent Application
20080014492
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 utilising 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.
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.
