The present invention relates to a fuel cell system in accordance with the preamble of claim 1.
Fuel cell systems are known in which a stack of fuel cells is constructed with layers of a plurality of fuel cells which are each separated from one another by bipolar plates. The bipolar plates here have a plurality of functions:
For supplying media to and draining media from the bipolar plates to the actual fuel cells (these are for example MEAs (Membrane Electron Assembly), each having a gas-diffusion layer formed for example from a carbon mat and orientated towards the bipolar plates), the bipolar plates have openings for cooling or for supplying and draining media.
In particular with respect to the gas-diffusion layer, difficulties regularly arise here. Up to now it has been usual to design the sealing between the bipolar plates or respectively between the bipolar plates and the fuel cell in that for example an elastomer seal is inserted into a groove of the bipolar plate. By exertion of compressive stress (for example by means of tightening straps) on the fuel cell stack, compression of the seal occurs by which means a sealing effect for the openings is intended to be achieved.
Now a problem with the inserted gas-diffusion layer is that it is generally designed as a graphite fibre mat or graphite paper. Standard industrial graphite fibre mats have a nominal thickness of e.g. 340 μm; however the production tolerance is around ±40 μm. The graphite fibres which form the mat are themselves brittle and not resilient. Moreover it is also not recommended here to balance production tolerances of the graphite fibre mat by compressing the mat since the gas permeability of the gas diffusion layer is severely impaired by this and thus the operation of the fuel cell is restricted. On the other hand it is necessary, however, to exert a minimum pressure through the bipolar plate on the entire gas-diffusion layer so that there is adequate electrical conductivity through the gas-diffusion layer.
It can therefore be summarised that with the previous elastomer seals either an imperfect sealing effect or a non-optimal operation of the fuel cell had to be accepted. Added to this, especially in the case of fuel cells operated with molecular hydrogen, losses of H2 occur due to the gas diffusing through the elastomer seal.
The object underlying the present invention, therefore, is to achieve reliable sealing of the openings in a fuel cell stack with the lowest possible costs.
This object is accomplished by a fuel cell system according to claim 1.
Because resilient bead arrangements are provided at least in regions to seal the openings, reliable sealing is achieved over a long resilient path (a wide range of elastic compressibility) of the bead arrangement. “Openings” are understood in the present application as a region of practically any type which is to be sealed. This can be for example a port for a reaction gas or a coolant. It can however also be e.g. the electrochemically active region in which e.g. the gas-diffusion layer is arranged or screw holes are provided. The resilient bead arrangement constantly permits the balancing of production tolerances of e.g. gas-diffusion layers within a wide tolerance range and yet the provision nevertheless of an optimal sealing effect.
Advantageous embodiments of the invention are described in the dependent claims.
A very advantageous embodiment of the invention provides for the bead arrangement of thin coating for the micro-sealing having a thickness of between 1 μm and 400 μm. The coating is advantageously formed from an elastomer such as silicon, viton or EPDM; these are applied for example in a screen printing process or by CIPG (cure-in-place gasketing; i.e. elastomer introduced in a fluid state at the location of the seal and cured there). What is achieved by these measures is that, for example, the hydrogen diffusion through the seal is reduced to a very low amount.
A further advantageous embodiment of the invention provides for the bead arrangement to contain a complete bead or a half bead. Here it is also possible to provide both forms within one bead arrangement, since according to the course of the bead arrangement in the plane, other elasticities can prove to be sensible, e.g. in narrow radii a different bead geometry could be more sensible compared to straight courses of the bead arrangement.
A further advantageous embodiment provides for the bead arrangement to be formed from steel. Steel offers the advantage that its machining is possible with conventional tools in a very cost-effective way, moreover e.g. methods for coating steel with thin elastomer layers are well tried. The good elastic properties of steel make it possible to form well the wide range of elastic compressibility offered by the invention. Here it is particularly favourable for the bead arrangement to be attached to the bipolar plate. This gives on the one hand the possibility that the bipolar plate is designed overall as a formed steel part (which is perhaps provided in regions with a coating to resist corrosion). However it is also possible that the bipolar plate is designed as a composite element of two steel plates with a plastics material plate lying between them. In each case the good production possibilities of steel can be exploited; it is possible to provide the bead arrangement within a process step which is taking place anyhow (e.g. the imprinting of a flow field). Thus very low costs arise; there are also no additional error sources due to extra components, such as additionally inserted elastomer seals.
However, it is also possible according to the invention to provide the bead arrangement from other metals, such as steel, nickel, titanium or aluminium for example. The choice of preferred metal here depends also on the desired electrical properties or on the desired degree of corrosion resistance.
Thus it is possible to adapt the compression characteristic of the bead e.g. to a gas-diffusion layer. This does not however have to apply only to gas-diffusion layers; the bead characteristics can in general be well adapted to components which have less elasticity. The beaded seal can be designed flexible and thus can be well used by all fuel cell manufacturers without high re-equipping costs.
A further advantageous embodiment provides for the bead arrangement to have a stopper which limits the compression of the gas-diffusion layer to a minimum thickness. This is an incompressible part of the bead arrangement or respectively a part, the elasticity of which is much lower than that of the actual bead. By this means the degree of deformation in the bead region is limited so that the bead cannot be pressed completely flat.
A further advantageous embodiment provides for the bead arrangement to be disposed on a separate component from the bipolar plate. This is particularly propitious when the bipolar plates consist of a material such as graphite which is unsuitable for bead arrangements. The separate component is then placed on the bipolar plate or integrated by gluing, clicking in, welding, soldering in, or by injection mold-around, so that altogether a sealing connection is provided between the separate component and the bipolar plate.
Finally a further advantageous embodiment provides for the bead arrangement to be realised from an elastomer bead. Such a bead can be applied in a screen printing process. This serves both micro- and macro-sealing. The bead also assumes the function of adapting the compression to a gas-diffusion layer.
Further advantageous embodiments of the present invention are given in the remaining dependent claims.
The present invention is now explained with the aid of several figures. These show:
a to 1c the type of setup of a fuel cell stack,
a + 2b embodiments of bead arrangements according to the invention,
c a plan view of a bipolar plate according to the invention,
a to 3d several bead arrangements with stoppers.
a shows the setup of a fuel cell arrangement 12 such as is shown in
In
Ports for the drain of media 5a and for the discharge of media 5b lie within the sealing region and are sealed by the bead 11 from additional ports, for instance the ports for cooling 4 (which have their own bead for sealing). The sealing effect here takes place on all the beads by exertion of compression force on the fuel cell stack 1 in the direction 6 of the layering (see
In order to limit the compression of the gas-diffusion layer, the bead construction is designed with a stopper. This stopper, which can be designed as a crimped portion, a a wave stopper or also as a trapezoid stopper, will be described in more detail below in the description of
The bipolar plate 3 is here designed as a metal part. In respect of the ease of manufacture and the advantages of steel in connection with bead arrangements, reference is made to what has already been said.
If the bipolar plate is formed e.g. from a metal which is not suitable for the production of appropriate bead geometries having the necessary elasticity, the bead region can be formed from some other suitable material (e.g. steel). By joining processes such as welding, soldering, gluing, riveting, clicking in, connection of the separate bead component to the bipolar plate then takes place. If the bipolar plates are made of a material other than metal, for example from graphite, graphite composite or plastics material, the bead region can be designed as a frame formed from a suitable material. By joining methods such as melting, injection mold-around, welding, soldering, gluing, riveting, clicking in, the base material of the bipolar plate, which contains the flow field, is connected to a bead sealing frame, which contains the beads, in a gas- or fluid-tight manner.
a and 2b show two embodiments of a bead arrangement according to the invention. In
In addition, the use of a full bead or respectively a half bead offers the possibility of adapting the elasticity within wide limits.
a shows the bead arrangement 7 in the uncompressed state. When mechanical compressive stress is exerted on the fuel cell stack, compression takes place in direction 6, such that the bead arrangement 7 or respectively the bead 11 forms a gas-tight lateral seal for the closed space 10 in respect of the gas-diffusion layer.
b shows a further bead arrangement, the bead arrangement 7′. The only difference between this arrangement and that of
c shows a plan view of an additional embodiment 3′ of a bipolar plate according to the invention. Here the bead arrangements can be recognised in the plan view by a broad line. The bead arrangements here serve to seal a plurality of ports.
a to 3d show various bead arrangements which each have a stopper. This stopper serves to limit the deformation of a bead in such a way that it cannot be compressed below a certain specific height.
Thus
Number | Date | Country | Kind |
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101 58 772.4 | Nov 2001 | DE | national |
Filing Document | Filing Date | Country | Kind |
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PCT/EP02/13251 | 11/25/2002 | WO |