A fuel-cell is a device which transforms chemical energy in the form of fuel (e. g., natural gas, bio-gas, methanol, diesel fuel, etc.) directly into electrical energy by way of an electrochemical reaction. Like a battery, a fuel-cell contains two electrodes, an anode and a cathode. Unlike a battery the fuel-cell will produce electrical power as long as fuel and oxidant are delivered to the anode and cathode, respectively. The major advantage of fuel-cells over more traditional power generation technologies (e. g., IC engine generators, gas or steam turbines, etc.) is that the fuel-cell converts chemical to electrical energy without combusting the fuel. The efficiency of the fuel-cell is, therefore, not thermodynamically limited, as are heat engines, by the Carnot cycle. This allows fuel-cell based systems to operate at a far higher efficiency than traditional power plants thereby reducing fuel usage and byproduct emissions. Additionally, due to the controlled nature and relatively low temperature of the chemical reactions in a fuel-cell, the system produces nearly zero pollutant emissions of hydrocarbons, carbon monoxide, nitrogen oxides and sulfur oxides.

Fuel-cells are typically arranged in stacked relationship. A fuel-cell stack includes many individual cells and may be categorized as an internally manifolded stack or an externally manifolded stack. In an internally manifolded stack, gas passages for delivering fuel and oxidant are built into the fuel-cell plates themselves.

In an externally manifolded stack, the fuel-cell plates are left open on their ends and gas is delivered by way of manifolds or pans sealed to the respective faces of the fuel- cell stack. The manifolds thus provide sealed passages for delivering fuel and oxidant

gasses to the fuel-cells, thereby preventing those gasses from leak e environment or to the other manifolds. The manifolds must perform tms tunction under the conditions required for operation of the fuel-cell and for the duration of its life.

An important aspect of the performance of a fuel-cell stack manifold is the gas seal established between the manifold edge and the stack face. As the stack face is typically electrically conductive and has an electrical potential gradient along its length and the manifold is typically constructed from metal, a dielectric insulator is needed to isolate the manifolds from the fuel-cell stack. As the dielectric insulator is typically constructed from ceramic which tends to be brittle, care must be taken in how the manifolds are compressed against the stack face so as not to damage the dielectrics.

Another requirement of fuel-cell stack manifolds relates to the fact that typically a fuel-cell stack will shrink over its life as the cell components creep and densify at high temperature. For a tall fuel-cell stack the total height may decrease by 2-3 inches. This means that continuous, metal manifolds cannot be fixed to both the top and bottom of the stack but must, instead, be allowed to slide relative to the stack face. Therefore, the retention system employed to compress the manifolds against the fuel-cell stack must maintain adequate normal force to prevent gas leakage while allowing the manifolds to slide along the stack face. In addition, the forces cannot be so great that the dielectric insulators are caused to break.

Due to its inherently non-uniform temperature distribution, a tall fuel-cell stack also tends to bow. Horizontal deflection of the top of the stack at high temperature can be as much as 1-2 inches relative to the base of the stack. This places a further burden on the manifolds and retention system which are required to flex with the bowing stack in order to maintain tight gas seals.

Fuel-cells operate at temperatures above ambient (Polymer Electrolyte Fuel- cells,"PEFC" :"80°C ; Phosphoric Acid Fuel-cells,"PAFC": ~200° ; Molten Carbonate Fuel-cells,"MCFC" :-650°C ; Solid Oxide Fuel-cells,"SOFC": ~1000°C) Therefore, the selection of materials and the mechanical design must allow the components to last for the life of the fuel-cell stack (typically years). Component stress and corrosion must be considered relative to the environment in which these components

must perform. In the case of MCFC and SOFC the temperatures a 1 and the lifetime long enough that long term creep of metallic components must be considered in their design.

The current state of the art fuel-cell manifold retention system used by the assignee of the subject application for carbonate fuel-cell stacks includes rigid mechanical members and springs which attempt to apply exclusively normal load to the rails of the manifolds. The components are constructed from high-temperature, corrosion-resistant materials such as nickel-based alloys and stainless steels. Leaf springs are used to transfer the tension developed in the rigid members to the manifold rails in multiple locations. This is done to provide as uniform a pressure under the manifold rails as possible. As the leaf springs see a relatively high stress level, they are intricately designed with tight dimensional tolerances so as to minimize this stress and extend their life.

The current design uses a large quantity of different parts to satisfy the requirements for a uniformly distributed normal load application to the manifold as well as to allow both stack shrinkage and stack bowing. The selected materials, intricacy of the geometry and large number of parts used in this design make it expensive, heavy and difficult to install. Also, the current system is designed to function completely independently from the manifolds and, as such, results in certain redundancies of material which add to the cost, weight and complexity of the fuel-cell stack.

Other fuel-cell stack manifold retention systems are described in various issued patents. A number of these patents are listed below: US 4,345,009 US 4,670,361 JP 6-129075 US 4,212,929 US 4,849,308 US 4,407,904 US 4,337,571 US 4,794,055 US 4,794,055 US 4,873,155 US 4,260,067 US 5,811,202 US 4, 467, 018 US 4, 444, 851 US 4,738,905 US 4,490,442 It is, therefore, an object of the present invention to provide a fuel-cell stack manifold retention system which does not suffer from the above disadvantages.

It is a further object of the present invention to provide a fu manifold retention system which is less costly, less complex and easier to assemble.

It also an object of the present invention to provide a fuel-cell stack manifold retention system which better allows for stack bowing and stack shrinkage.

It is yet another object of the present invention to provide a fuel-cell stack retention system which is lighter in weight and results in more effective gas sealing.

Summary of the Invention In accordance with the principles of the present invention, the above and other objectives are realized in a fuel-cell stack manifold retention system which includes a number of strap members for retaining the stack manifolds against the corresponding stack faces. Each strap member extends over an extent of a manifold and an attachment assembly attaches the strap members to the fuel-cell stack. The strap members are further adapted to expand, contract and slide. In this way the strap members are able to hold the manifolds against the faces of the fuel-cell stack so as to accommodate changes in the fuel-cell stack geometry caused by temperature changes and material creep.

In the embodiment of the invention to be disclosed hereinbelow, each strap member is connected to one manifold, extends around the adjacent manifold and the attachment assembly connects the strap members to the opposite side of the stack.

Also, in the disclosed embodiment, the strap members are spring loaded to provide the expansion and contraction. Additionally, the attachment assembly includes truss members for supporting the length of the strap members extending over the manifolds and yoke assemblies and hook members for holding the ends of the strap members.

Certain of these components are fixedly secured, to the manifolds, as by welding, or they can be designed as part of the manifolds themselves.

Brief Description of the Drawings The above and other features and aspects of the present invention will become more apparent upon reading the following detailed description in conjunction with the accompanying drawings, in which: FIGS. 1-4 show various views of a fuel-cell stack including a fuel-cell stack manifold retention system in accordance with the principles of the present invention; FIG. 5 shows pictorially the path followed by certain of the strap members of the fuel-cell stack manifold retention system of FIGS. 1-4; FIGS. 6-8 show the configuration of various ends of the strap members of the fuel-cell stack manifold retention system of FIGS. 1-4; FIG. 9 illustrates the configuration of a part of the yoke assembly of the fuel- cell stack manifold retention system of FIGS. 1-4; FIG. 10 illustrates the dielectric interface of the yoke assembly used in the fuel-cell stack manifold retention system of FIGS. 1-4; FIG. 11 shows predicted characteristics for the strap member tension and the manifold gasket pressure for the fuel-cell stack manifold retention system of FIGS. 1- 4; and FIG. 12 illustrates a modification of the fuel-cell stack retention system of FIGS. 1-4.

Detailed Description FIGS. 1-4 show a fuel-cell stack 1 including a fuel-cell stack retention system 100 in accordance with the principles of the present invention. The fuel-cell stack 1 comprises a plurality of stacked fuel-cells to which process gases are fed and exhaust gases extracted via gas manifolds. End plates 11 and 12 are also situated at the top and bottom of the stack.

For the illustrative fuel-cell stack of FIGS. 1-4, three manifolds 21,22, and 23 are utilized. These manifolds serve as the fuel inlet and outlet manifolds and the oxidant-outlet manifold, respectively, and lie adjacent the stack faces 2,3 and 4. A fourth manifold serving as the oxidant inlet is associated with the fourth stack face 5 and is provided by a vessel into which the stack is inserted. This vessel is not shown in the figures.

It should be noted that the principles of the invention apply well to stacks having all their manifolds adjacent respective stack faces. Moreover, the principles of the invention are intended to apply to stacks having any number of faces and manifolds.

In the case shown, each of the manifolds 21,22 and 23 abuts its respective stack face 2,3 and 4 via top and bottom horizontal rails and left and right vertical rails. The rails of each manifold are required to provide a gas sealing interface with the respective stack face under the various operating conditions of the stack 1. As illustrated, the manifold 21 has vertical rails 21A, 21B and horizontal rails 21C, 21D; the manifold 22 has vertical rails 22A, 22B and horizontal rails 22C, 22D; and the manifold 23 has vertical rails 23A, 23B and horizontal rails 23C, 23D.

While not visible, between each manifold rail and its respective stack face is a dielectric insulator. These insulators electrically isolate the manifolds from the stack and their integrity must be preserved in realizing the gas seal between the manifolds and the stack faces.

In accordance with the principles of the present invention, the stack manifold retention system 100 includes strap members 101 and a strap attachment assembly 200. The strap members 101 hold the manifolds 21,22 and 23 in sealing engagement with the stack faces and the attachment assembly 200 attaches the strap members 101 to the stack 1.

Each strap member 101 extends over an extent of a corresponding manifold and is adapted to expand, contract and slide to accommodate changes in the stack geometry and the stack components resulting from the operating conditions of the stack. In this way, the strap members 101, can be made to exert a desired pressure against the horizontal and vertical rails of the manifolds 21,22 and 23, thereby sealing the rails against the stack faces, while still maintaining the integrity of the dielectric insulators.

In the illustrative case shown, the fuel-cell stack 1 is rectangular, i. e., the stack faces 4 and 5 are of equal width b greater than the equal width a of the stack faces 2 and 3, and the strap members 101 have been routed only around the shorter faces 2 and 3 of the stack. This minimizes the length of the strap members and, therefore, the

creep that occurs in the strap members over time. Moreover, the st 101 not only hold the surrounded manifolds 21 and 22 in place, but also the manifold 23.

More particularly, six strap members 101 are distributed along the length of each of the manifolds 21 and 22. Each strap member 101, furthermore, has an extent which extends over the width of the respective manifold and each strap member 101 has end sections 101A and 101B.

Each strap member can be designed as a multi-wire rope (cable) or as laminated, flat strips or as any other member providing the required flexibility to encompass and slide relative to the manifolds. To this end, each strap member 101 can be sheathed in a flexible ceramic tube (braided) in order to both reduce friction between the strap member and its supporting surface and to prevent corrosion debris from falling from the strap member into the surrounding environment. Alternately, the strap members can be coated with an appropriate high temperature, corrosion resistant coating.

The sliding function of the strap members is advantageous in permitting the extent of the straps extending around the manifolds to be increased and decreased as thermal expansion and contraction of the fuel-cell stack results in a change in the perimeter of the stack. In particular, the variation in length of the path occupied by a strap member is accounted for through relative sliding of the strap with respect to the manifolds. In this way, a desired load can be applied to all manifolds regardless of the temperature of the stack.

To permit each strap member 101 to expand and contract its respective length or extent, each strap member is spring loaded. More particularly, each strap member 101 is provided with a spring assembly 102 at its end portion 101A. The spring assembly 102 holds its respective strap member 101 taught and is designed to apply the proper load at temperature to achieve the desired manifold pressure against the dielectric insulator at the stack face. Moreover, as can be appreciated, each spring assembly makes up for differences in strap member path length due to thermal expansion of the system and also takes up slack in the strap member caused by thermal contraction or other effects such as material creep at high temperatures.

In the disclosed embodiment, as seen more clearly in FIG. 6, the spring assembly 102 includes a coil spring 102A in compression. The spring 102A is in

surrounding relationship with its respective strap member at the en ç and has one end attached to a stop member 102B. The stop member 102B also surrounds the strap member end section 101A and is fixedly attached to its terminal end. A second stop member 102C is located at the other end of the spring and also surrounds the end section of the strap member.

As shown, the stop members 102B and 102C are washers, with the stop member 102C being a self-aligning washer. The spherical shape of each washer allows it to pivot to make up for small differences in strap angle due to manufacturing and assembly variations. Cylindrical and other curved shapes can also be used.

Likewise, other types of springs may be used for the spring 102A. Thus, for example, extension, belleville, torsion and leaf spring configurations could also be employed.

Also, it is important that each strap member 101 run parallel to the axis of its associated spring. This insures that no lateral loads are applied that may cause the spring to tip and also insures that high bending stresses are not imparted to the strap end.

The path for routing the each strap member 101 around its respective manifold depends on the pressure distribution desired at the manifold rails. In this embodiment, a uniform and equal pressure is desired on all areas of the manifold rails.

This being the case, for the strap members which apply pressure to the vertical rails of the manifolds, i. e., for the strap members which are situated between the uppermost and lowermost strap members, the strap members are routed so that sections of the strap members adjacent the vertical rails of adjacent manifolds should form a 45° angle with the associated faces of the stack. This results in the strap tension being equally distributed to all vertical manifold rails equally. In particular, the force applied to each vertical rail will equal n12/2 times the tension in the strap member.

The strap members routed to apply pressure to the horizontal rails of the manifolds, i. e., the uppermost and lowermost strap members surrounding a manifold, on the other hand, should be routed at an angle determined by the aspect ratio of the widths of the corresponding adjacent stack faces. This is depicted pictorially in FIG.

5.

For the stack 1 of FIG. 1, where adjacent stack faces are of hs a and b, i. e., for a stack with a rectangular footprint, the angle as measured from the shorter face is equal to the arctangent of the ratio of the shorter face width a to the longer face width b. This angle, also depicted in FIG. 5, allows the tension in the strap member to be split between the shorter and longer horizontal manifold rails such that equal sealing pressure is achieved. As can also be appreciated, from this figure, where the stack faces are of equal length, i. e., for a stack with a square footprint, the routing angle 8 = 45°.

In order for the strap members 101 to be able to retain the manifolds 21,22 and 23 in place in the manner described above, the attachment assembly 200 includes truss members 201, yoke assemblies 203 and hook members 204. The truss members each comprise an upper curved section 201A and leg members 201B. Each yoke assembly includes a belt member 203A having at each of its ends a spring support ear or member 203B.

The truss members 201 extend over the width of the respective manifolds 21 and 22. The hook members 204 are connected to the vertical rails 23A and 23B of the manifold 23. Preferably, the truss members and hook members are integral with their respective manifolds. This can be achieved by welding the truss members and hook members to the manifolds. Alternatively, the truss members and hook members could be designed integrally as parts of their respective manifolds.

As shown more clearly in FIGS. 7 and 8, the manifold horizontal rails 23C and 23D have slots 202A and the hook members 204 include slots 204A for capturing the ends 101B, i. e., the dead ends, of the associated strap members 101. These strap members 101 then pass over the respective curved sections 201A of the truss members 201. The curvature of each truss section 201A is such as to realize the above-mentioned desired routing of the strap members 101 to achieve substantially equal, constant pressure at the manifold rails.

In particular, each section 201A has the curvature of a circular sector. In this way, each strap member can be made to leave the section 201A at a tangent point which provides the desired angle for the strap member 101 as it passes adjacent the stack faces, as discussed above.

As also discussed above, the dead end 101B of each strap n captured in the slot of either the hook member 204 or one of the horizontal rails 23C or 23D.

These dead ends of the strap members are designed to pivot to make up for small differences in strap angle due to manufacturing and assembly variations. This is realized by configuring the dead ends to be cylindrical in shape, although other shapes such as, for example, spherical shapes, could also be used.

The spring assemblies 102 of the strap members 101 are brought to the face 5 of the fuel-cell stack and supported there. In particular, in order to support the spring assemblies 102 in their proper positions and orientations, while maintaining electrical isolation from the stack face, the above-mentioned yoke assemblies 203 are employed.

More particularly, the belt member 203A of each yoke assembly 203 spans the width of the face 5 of the stack. Additionally, each belt member 203A and its corresponding spring support ears 203B, which hold the spring assemblies 102 (see, FIG. 6), are designed such that the spring assemblies are properly oriented under all load conditions and no lateral loading or rotation is applied to the dielectric assemblies 205 associated with the yoke assembly 203.

The dielectric assemblies 205 are more fully discussed below and electrically isolate the spring support ear from and allow sliding of the spring support ear relative to the stack of face 5. To accomplish this, each spring support ear 203B is made sufficiently stiff so as not to deflect significantly under load. In addition, the height h from the knee of the spring support ear to the belt member 203A is chosen to provide zero moment about the knee.

In particular, as shown in FIG. 9, the height, h, is found by taking the perpendicular length, 1. from the strap member 101 to the knee of the support ear 203B and dividing by the cosine of the angle which the strap member forms with the adjacent stack face (shown as the angle 6 in FIG. 9). This geometry provides zero moment at the contact point of the spring support ear with the dielectric assembly, thereby allowing the spring to maintain proper orientation and placing the belt member 203A in pure tension. This, in turn, allows the belt member 203A to be made relatively thin and light.

As above-mentioned, each spring support ear 203B is elect d from and allowed to slip relative to the fuel-cell stack face 5 by use of a dielectric assembly 205. As shown in FIG. 10, each dielectric assembly 205 includes first and second ceramic slabs 205A and 205B. These slabs are separated by a layer of ceramic cloth 205C which is situated between the slabs.

In the present illustrative case, one slab 205A is attached to the ear support at the knee, while the other is fixed to the stack face, and the cloth situated therebetween. As the stack shrinks, relative motion between the stack and ear occurs at the ceramic cloth plane. This plane provides a low friction surface with no chance of corroding and fusing.

As can be appreciated, the uppermost and bottommost strap members 101, and associated truss members 201 and yoke assemblies 203, apply the desired uniform loading pressure to the horizontal rails of the manifolds 21 and 22. In order for the horizontal rails of the manifold 23 to receive a similar holding pressure, they must be designed with sufficient stiffness so as not to deflect significantly under load at operating pressure.

As an alternative to configuring the horizontal rails 23C and 23D to hold the dead ends of the strap members 101, the horizontal rails can be made flat and tension bars having the slots 202A for capturing the dead ends of the strap members 101 can be situated against the rails. These tension bars can be configured to provide uniform loading of the rails by either one of two techniques. One technique contemplates machining each bar to give it an appropriate profile so that the bar applies uniform pressure as the member deflects. A second simpler technique is to place shims of a specific thickness under each bar so as to yield uniform pressure under each shim after the bar deflects.

As can be appreciated, the highest stress components in the fuel-cell stack retention system 100 are the flexible strap members 101 and springs 102A. For these components to have reasonable weight and cost they must be designed with relatively high stress. Due to the high temperature and long life of the fuel-cells of the stack 1, long term creep of the strap members 101 and relaxation of the springs 102A must be accounted for and designed into the retention system 100. A mathematical model incorporating the high temperature creep of the strap members 101, relaxation of the

springs 102A and thermal expansion of all components has been d optimize the strap members and spring design and provide adequate manifold gasket (dielectric insulator) pressure for the duration of stack life. Based on this model, a representative curve showing predicted strap tension and manifold gasket pressure for the duration of stack life has been generated and is shown in FIG. 11.

The creep of materials at high temperature is a non-linear phenomenon increasing exponentially with both temperature and stress level. This fact is used advantageously in the retention system 100. Due to the arrangement of the springs and strap members, the creep of both components becomes a self-limiting process (see FIG. 11). As each strap member creeps, the deflection of the spring decreases and subsequently so does the stress in the strap member and the spring. As the spring relaxes, so does the load applied to the strap member, thereby reducing the rate of creep in both components. By properly selecting the geometry of the strap member and the spring and specifying the starting spring deflection, an adequate dielectric insulator pressure is maintained through the life of the fuel-cell stack. At the beginning of stack life, a higher pressure is required to insure that the manifolds do not slip downward while the stack is shipped. Once installed, the pressure need only be enough to prevent gas leaks.

FIG. 12 illustrates an alternative embodiment of the fuel-cell stack retention system of FIGS. 1-4 in which a manifold 24, acting as an oxidant inlet manifold, is situated adjacent the face 5 of the fuel-cell stack 1. The manifold 24 abuts the face 5 via vertical rails 24A, 24B and horizontal rails 24C, 24D.

In this embodiment, the spring support ears or members 203B are formed integrally with the manifold 24, either by welding or in the manifold forming. The spring support ears 203B are thus able to support the spring assemblies 102, without the need for the belt members 203A and the dielectric assemblies 205.

In all cases it is understood that the above-described arrangements are merely illustrative of the many possible specific embodiments which represent applications of the present invention. Numerous and varied other arrangements can be readily devised in accordance with the principles of the present invention without departing from the spirit and scope of the invention.