IMPROVED ENCLOSED FUEL CELL STACK ROW

Abstract

Solid polymer electrolyte fuel cell stacks require a significant nominal compressive loading for proper operation and sealing. This loading is typically provided using relatively thick end plates and tight straps. In certain fuel cell applications, one or more solid polymer electrolyte fuel cell stacks are secured in larger enclosures (e.g. for isolation and crashworthiness in automotive applications). The enclosures however can themselves be sturdy enough to provide the necessary loading on the fuel cell stacks within. The present invention takes advantage of that to allow for use of thinner end plates and/or weaker straps which would otherwise be insufficient for use.

Claims

1. An enclosed fuel cell stack row comprising at least one solid polymer electrolyte fuel cell stack and an enclosure, wherein each fuel cell stack comprises: a series stack of a plurality of solid polymer electrolyte fuel cells; a pair of end plates at either end of the series stack; and at least one stack strap encircling the series stack and the end plates; wherein each pair of end plates applies compressive force to each respective series stack at a nominal loading for fuel cell stack operation and wherein the enclosure is capable of supporting the nominal loading for each fuel cell stack in the fuel cell stack row, characterized in that each aligned fuel cell stack is compressed to a fixed loading less than the nominal loading by the combination of end plates and the at least one stack strap in each fuel cell stack which are insufficient per se to provide the nominal loading for each fuel cell stack.

2. The enclosed fuel cell stack row of claim 1 wherein the deflection of the end plates per se under the nominal loading on the series stack would exceed 1.5 mm and the enclosure supports each pair of end plates so as to provide the nominal loading on each series stack.

3. The enclosed fuel cell stack row of claim 1 wherein the elongation of the at least one strap per se under the nominal loading on the series stack would exceed 0.5 mm and the enclosure supports each pair of end plates so as to maintain the nominal loading on each series stack.

4. The enclosed fuel cell stack row of claim 2 wherein an end plate in the pair of end plates is made of thermoplastic material and has a thickness less than about 4 cm.

5. The enclosed fuel cell stack row of claim 2 wherein an end plate in the pair of end plates is made of steel and has a thickness less than about 0.8 cm.

6. The enclosed fuel cell stack row of claim 2 wherein an end plate in the pair of end plates is made of aluminum and has a thickness less than about 2.5 CM.

7. The enclosed fuel cell stack row of claim 3 wherein the at least one stack strap is made of plastic material and has a cross-sectional area less than about 20 cm.sup.2.

8. The enclosed fuel cell stack row of claim 3 wherein the at least one stack strap is made of steel and has a cross-sectional area less than about 1 cm.sup.2.

9. The enclosed fuel cell stack row of claim 3 wherein the at least one stack strap is made of aluminum and has a cross-sectional area less than about 1 cm .

10. The enclosed fuel cell stack row of claim 1 wherein the nominal loading for fuel cell stack operation is greater than or about 27 kN.

11. A method of assembling the enclosed fuel cell stack row of claim 1 comprising: aligning the plurality of fuel cells and the pair of end plates for each fuel cell stack in a jig for alignment and compression; compressing the aligned series stack and pair of end plates in the jig to a fixed loading less than the nominal loading; encircling the compressed and aligned series stack and pair of end plates with the at least one stack strap; securing the stack strap, thereby forming each fuel cell stack; inserting each fuel cell stack in the enclosure; and compressing each fuel cell stack to the nominal loading.

12. The method of claim 11 wherein the fixed loading is less than or about 15 kN.

13. The method of claim 11 wherein the at least one stack strap is of fixed length and the encircling step comprises sliding the stack strap over the compressed and aligned series stack and pair of end plates.

14. The method of claim 11 wherein the at least one stack strap is made of plastic material and the securing step comprises heat sealing the ends of the stack strap together.

15. The method of claim 11 wherein the enclosure comprises jacking screws between one of the end plates and each fuel cell stack and the step of compressing each fuel cell stack comprises tightening the jacking screws.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0027] FIG. 1 shows a cutaway view of an enclosed fuel cell stack row of the invention.

DETAILED DESCRIPTION

[0028] Herein, the following definitions have been used. In a quantitative context, the term “about” should be construed as being in the range up to plus 10% and down to minus 10%.

[0029] The term “insufficient per se” is used herein with reference to certain mechanical properties of specific fuel cell components and is intended to refer to the relevant mechanical properties of that specific fuel cell component—on its own—without additional support, additional supporting components, or other mechanical assistance provided to that specific fuel cell component. In a like manner, terms such as deflection of the end plates “per se” or elongation of a strap “per se” are intended to refer to that mechanical property of that specific component—on its own—without additional support, additional supporting components, or other mechanical assistance provided.

[0030] The term “nominal loading” refers to the minimal compression force which is required to ensure that the fuel cell stack works properly with respect to gas tightness and electrical contact.

[0031] A cutaway view of an assembled enclosed fuel cell stack row of the invention is shown in FIG. 1. Here, the enclosed fuel cell stack row has been designed to be suitable for automotive application. In the cutaway view of enclosed fuel cell stack row 1 of FIG. 1, only a single fuel cell stack 2 is shown to reduce clutter. However, a typical such stack row would comprise several fuel cell stacks similar to that shown. Fuel cell stack 2 comprises a series stack of a plurality of solid polymer electrolyte fuel cells (not called out in FIG. 1), a pair of end plates 3a, 3b at either end of stack 2, and two stack straps 4 that encircle stack 2 and end plates 3a, 3b for securing the numerous components in place. Fuel cell stack 2 and other stacks in the stack row are all enclosed in enclosure 5. For automotive purposes, enclosing the fuel cell stacks in enclosure 5 serves to provide isolation and crashworthiness. Further, it provides waterproofing, dustproofing, and means for mounting into a vehicle.

[0032] In prior art embodiments, the end plates and stack straps employed in the assembly of a fuel cell stack not only secure the stack adequately for handling purposes but also have to provide sufficient loading or compressive force to the stack to effect and maintain adequate seals and sufficient loading to the components in the stack for other functional purposes (e.g. to obtain low contact resistances between components). Thus, the mechanical properties of the endplates must be such that they do not deflect an unacceptable amount under the desired nominal loading for an operating fuel cell stack. Further, the mechanical properties of the stack straps must be such that they do not stretch or elongate an unacceptable amount under the desired nominal loading for an operating fuel cell stack.

[0033] The present invention however takes advantage of situations where enclosure 5 is already or can be made sturdy enough to significantly support much of the nominal loading required on the fuel cell stacks within. In such situations, the mechanical requirements for either or both of the end plate hardware and the compression stack strap hardware can be relaxed. Thus for instance, thinner and/or weaker end plates and/or compression straps may be employed which allow for weight and size reduction of these components in the stack. Further, alternative weaker material choices might be considered (e.g. plastics). Such advantages may be obtained without sacrificing alignment or final net loading on the stacks. Thus inventive fuel cell stack row 1 in FIG. 1 differs from a prior art fuel cell stack row in that either end plates 3a, 3b are thinner and/or stack straps 4 are smaller in cross-sectional area than their counterparts in the prior art. Further however, a suitable hardware arrangement or configuration must be incorporated in the design of fuel cell stack row 1 such that enclosure 5 supports the nominal loading on each fuel cell stack 2 inside. In FIG. 1, a series of jacking screws 6 provide this support between enclosure 5 and fuel cell stack 2.

[0034] Enclosed fuel cell stack row 1 can be assembled in very much the same way as fuel cell stack rows of the prior art are assembled, except that the nominal loading on fuel cell stack 2 and other stacks within is provided after assembly of the individual fuel cell stacks as opposed to during assembly of the individual fuel cell stacks. For instance, enclosed fuel cell stack row 1 may be made by first assembling individual fuel cell stacks using thinner end plates 3a, 3b and/or lower cross-sectional area stack straps 4. As in the prior art, this is done by aligning the fuel cells for each fuel cell stack and the pair of end plates for each fuel cell stack in a jig for alignment and compression. Each aligned series stack and pair of end plates is compressed in the jig, but to a fixed loading less than the nominal loading. An appropriate number of stack straps are then installed so as to encircle the compressed and aligned series stack and pair of end plates, and then the stack straps are secured in place, thereby forming each fuel cell stack. During assembly, the fixed loading employed is sufficient to readily maintain alignment of the stack components during subsequent handling until assembly of the stack is completed (and particularly until nominal loading is applied and straps are secured around the stack). However, the fixed loading is preferably substantially less than the nominal loading required in order to allow for the greatest possible reduction in weight and/or size of the end plate and strap hardware.

[0035] Note that an appropriate stack strap or straps for use in the invention can be of fixed length. In such a case, the encircling step can comprise sliding the stack strap over the compressed and aligned series stack and pair of end plates. Alternatively, the stack straps can be made of plastic and initially can have excess length. In such a case, the securing step can comprise heat sealing the ends of the stack strap together.

[0036] After assembling the individual fuel cell stacks, each stack is inserted in the enclosure and compressed to the nominal loading. As shown in FIG. 1, the required nominal loading may be obtained by appropriate use of jacking screws 6. In such an embodiment, the step of compressing each fuel cell stack can comprise appropriately tightening jacking screws 6.

[0037] After the fuel cell stacks 2 are properly installed in enclosure 5 and are compressed to the nominal loading, straps 4 may no longer be needed for normal operation of fuel cell stack row 1. Advantageously however, straps 4 may simply be left loosely in place so that stacks 2 remain restrained and so that alignment is maintained in the event that enclosure 5 is removed for some reason thereafter (e.g. for replacement of a stack, for analysis, or for maintenance purposes). Alternatively, the straps could be cut and removed if desired.

[0038] The preceding figure shows an exemplary embodiment of the invention but those skilled in the art will appreciate that numerous alternative embodiments may be contemplated (e.g. embodiments comprising more than two stack straps 4 per fuel cell stack 2, or comprising different means than jacking screws 6 for providing support for the nominal loading on fuel cell stacks 2).

[0039] The following Example has been included to illustrate certain aspects of the invention but should not be construed as limiting in any way.

EXAMPLES

[0040] In an exemplary embodiment for automotive applications, the nominal loading for normal functioning and operation of a typical solid polymer electrolyte fuel cell stack is about 27 kN. A commonly accepted deflection limit for the end plates under such loading is about 1.5 mm. And a commonly accepted elongation limit for the securing stack straps is about 0.5 mm.

[0041] Given the preceding, it is possible to calculate the minimum thickness requirements for various materials that might be considered for use in end plates so that the preceding acceptable deflection limit can be achieved. For end plates made of thermoplastic, steel, and aluminum respectively, the minimum thicknesses required are calculated to be about 4, 0.8, and 2.5 cm respectively.

[0042] End plates made of these materials with thicknesses less than these values would not be expected to be sufficient per se to support the nominal loading.

[0043] In a like manner, it is also possible to calculate the minimum cross-sectional area requirements for various materials that might be considered for use in stack straps so that the preceding acceptable elongation limit can be achieved. For stack straps made of plastic, steel, and aluminum respectively, the minimum cross-sectional areas required are calculated to be about 20, 1, and 1 cm.sup.2 respectively. Stack straps made of these materials with cross-sectional areas less than these values would not be expected to be sufficient per se to support the nominal loading.

[0044] In this exemplary embodiment, it has been found that a 15 kN loading is sufficient to maintain alignment of the numerous components for subsequent handling during assembly of the fuel cell stacks. In fact, even lower fixed loadings may prove to be sufficient. But at least a 15 kN fixed loading—which is substantially less than the 27 kN nominal loading—is sufficient loading until assembly of the individual stacks is completed. Thus, thinner end plates and/or stack straps with smaller cross-sectional areas that are commensurate with this lower fixed loading requirement may be employed in this exemplary embodiment of the invention.

[0045] All of the above U.S. patents, U.S. patent application publications, U.S. patent applications, foreign patents, foreign patent applications and non-patent publications referred to in this specification, are incorporated herein by reference in their entirety.

[0046] While particular elements, embodiments and applications of the present invention have been shown and described, it will be understood, of course, that the invention is not limited thereto since modifications may be made by those skilled in the art without departing from the spirit and scope of the present disclosure, particularly in light of the foregoing teachings. Such modifications are to be considered within the purview and scope of the claims appended hereto.