CELL RETENTION DESIGN AND PROCESS

A system and method for reducing the relative movement between adjacent fuel cells within a fuel cell stack includes an improved strategy for distributing an acceleration load over a fuel cell stack while maintaining stack performance after exposure to high acceleration loads. The system comprises a fuel cell stack comprising a plurality of fuel cells enclosed by a housing. A curable material occupies at least a portion of a lateral space located between the edges of each fuel cell in the stack and an interior wall of the housing. Upon occurrence of high acceleration loads within the housing, the curable material transmits the acceleration load from the housing to more evenly distribute the load to the edges of the fuel cells. A plurality of dams may be secured between the housing and the fuel cell stack forming channels for receiving the curable material.

DETAILED DESCRIPTION

The following detailed description and appended drawings describe and illustrate various embodiments of the present disclosure. The description and drawings serve to enable one skilled in the art to make and use the invention, and are not intended to limit the scope of the invention in any manner.

Referring first toFIG. 1, vehicle2is shown, according to embodiments shown and described herein. Vehicle2(for example, a car, bus, truck, or motorcycle) includes a fuel-cell based propulsion system100made up of an electric motor150that receives its electric power from a fuel cell stack200that includes numerous individual fuel cells6. The propulsion system100may include one or more fuel storage gas vessels210,220, as well as power converters or related electronics300, electrical storage devices (e.g., batteries310, ultra-capacitors or the like) and controllers that provide control over its operation, and any number of valves, compressors, tubing, temperature regulators, and other ancillary equipment.

Any number of different types of fuel cells6may be used to make up the stack200of the propulsion system100; these cells6may be of the metal hydride, alkaline, electrogalvanic, or other variants. In one preferred (although not necessary) form, the fuel cells6are PEM fuel cells as discussed above. Stack200includes multiple such fuel cells6combined in series and/or parallel in order to produce a higher voltage and/or current yield. The produced electrical power from propulsion system100may then be supplied directly to electric motor150or stored within a battery310, capacitor or related electrical storage device (not shown) for later use by vehicle2. It will be understood that the fuel cell system shown and described herein may be used for purposes other than motor vehicles.

Referring toFIGS. 2a, b, and3, a system and method for reducing the relative movement between adjacent fuel cells6within a fuel cell stack200in vehicles2powered by a fuel-cell based propulsion system100is shown. Severe load changes due to a disruptive event, which includes high acceleration or deceleration of the vehicle2, an impact involving the vehicle2, or similar impact to the fuel cell stack200itself, such as a vertical fall, can damage the fuel cell stack200or disassemble the stack200by causing the individual fuel cells6to move relative to one another. The likelihood of damage is heightened when a fuel-cell based propulsion system100has been prepared for a freeze start due to reduced or minimal compressive load between adjacent fuel cells6that in turn produce concomitant reduction in surface friction. To protect against such movement or damage, a system and method for improved distribution of acceleration load is shown and described. The system comprises a fuel cell stack200comprising a plurality of fuel cells6in an adjacently facing relationship enclosed by a housing8. A lateral space10, also known as an air gap, is located between the edges of each fuel cell6in the stack200and an interior wall12of the housing8. A curable material14occupies at least a portion of the lateral space10and provides a bridge between the edges of the fuel cells6and the interior wall12of the housing8giving structural support to the fuel cell stack200. The curable material14may either be expandable or may possesses a high lateral stiffness such that the curable material14resists flexing laterally when a load is applied in that direction. During repairs, the spacing between the fuel cells6of the fuel cell stack200increases as the diffusion media decompresses; an expandable curable material14allows the fuel cells6to expand in both its through-the-thickness dimension, as well as through its edgewise dimension, and then be recompressed without damage to the fuel cells6or other components of the fuel-cell based propulsion system100. Alternatively, when a high stiffness curable material14is used, the edges of the fuel cells6must flex to accommodate the increase in fuel cell stack200length. The curable material14may be a foam, liquid, gel, or epoxy. Upon an occurrence of the disruptive event within the housing8, the curable material14transmits an acceleration load from the housing8to more evenly distribute the load to the edges of the fuel cells6. The curable material14facilitates even distribution of the load despite differing tolerances of the many components of the fuel cell system and protects the fuel-cell based propulsion system100. The curable material14provides support during a disruptive event, even when the car is parked and the stack is not running, and can either be attached to or restrained by the housing8. It will be understood that the system and method as shown and described protects against movement or damage to the fuel cell stack200in any direction of compression of the fuel cell stack200in relation to the orientation of the vehicle.

The lateral space10accommodates the hydration and thermal expansion needs of the stack200and further makes the stack more accessible for repairs. The lateral space10increases when a fuel-cell based propulsion system has been prepared for freeze starts because the elimination of water in the stack200to avoid freezing of the fuel cell stack results in the contraction of the GDM between individual fuel cells6.

According to another aspect of the present disclosure, a plurality of dams16, which may be composed of a thermoset foam such as polyurethane, a thermoplastic foam such as polystyrene, or flexible cross section as used in bulb seals, are secured between the housing8and the fuel cell stack200, wherein the plurality of dams16form channels for receiving the curable material14. The dams16may occupy the entire area defined by the size of the interior wall12of the housing8, may occupy a plurality of small areas, or any area in-between. The plurality of dams16may be secured to the housing8or to an intermediate material between the housing8and the fuel cell stack200. The intermediate material may be an insulating panel18affixed to the interior wall12of the housing8. The plurality of dams16may be situated perpendicular to the orientation of the fuel cell stack200or situated parallel to the stack200orientation and may vary in number and size. The insulating panel18serves to disrupt buoyancy convention flow between the fuel cell stack200and the housing8by minimizing the lateral space10. The insulating panel18also provides electrical insulation. The dams16may also be composed of other materials such as, but not limited to, non-structural elastomeric materials, low durometer foams able to fill between cells (˜1 mm gap) to form an adequate seal to contain the material being applied, or any other structural material that may be applied in a state that allows for sealing between cells.

According to another aspect of the present disclosure, a method of reducing the relative movement between adjacent fuel cells6within a fuel cell stack200during a disruptive event is disclosed. The method includes configuring a fuel cell-based propulsion system100as described above and injecting the curable material14into at least a portion of the lateral space10to provide a bridge between the edges of the fuel cells6and the interior walls12of the housing8. The housing8comprises a plurality of injection ports20for receiving the curable material14. Once the system100is fully assembled, the curable material14is injected into the lateral space10between the interior walls12of the housing8and the fuel cell stack200via the plurality of injection ports20. In embodiments including dams16, the injection ports20are in fluid communication with the dams16. The method further comprises controlling the amount, extent, and curing of the curable material14wherein the curable material14is viscous enough to allow complete fill without leakage and curing of the material14can occur in process without off-line dwell time. Curing can be accomplished at room temperature with two-part mixing systems such as epoxies, or with single component systems at elevated temperature. Alternatively, moisture curing systems such as room temperature vulcanizing (RTV) silicone can be used.

According to another aspect of the present disclosure, the curable material14possesses certain other properties, including thermal insulating properties, electrical insulating properties, elastomeric properties, flowability properties, mechanical properties (i.e., stiffness, compliance, modulus of elasticity, strength, etc.) or the like. The thermal insulating properties must be sufficient to limit heat loss from the stack. The electrical insulating properties must be sufficient to avoid excessive shunt currents between the neighboring plates. The elastic properties must be adequate to allow expansion or plate flexure during decompression. The flowability must be sufficient to allow the material to penetrate the gaps between the cells to generate intimate contact, yet not escape from the dammed area. The mechanical properties must be sufficient to carry the maximum acceleration loads. In one exemplary form, the curable material14possesses an electrical resistivity of 1.3×1014ohms-cm; a viscosity of 30,000 cps; a compressibility of 90 Shore A; a shear strength of between 1250-4500 psi; a coefficient of thermal expansion of between 85-147×10−6in/° C.; and a thermal conductivity of 0.104 btu-ft/ft2-hr-° F.

In one embodiment of the present disclosure, as shown inFIG. 4, using an injector24, the curable material14is injected into the lateral space10as a liquid that then solidifies into a structural material. The curable material14should possess a low enough viscosity to allow the material14to flow between the fuel cells6but be viscous enough to not spill beyond the lateral space10. A release agent or a low surface energy material may be used on the insulating panel18if bonding to the insulating panel18is undesirable.

According to another aspect of the present disclosure, the method further comprises a release liner22, shown inFIGS. 2aandb, situated between the curable material14and an intermediate component (either the housing8or the insulating panel18) so that the system may be disengaged for stack200decompression and recompression associated with repairs.

An alternative embodiment of the present invention is shown inFIG. 5, which includes U-shaped dams16that are loaded directly into the lateral space10once the fuel-cell based propulsion system100is assembled as described above. The open portion of the U-shaped dam16is oriented upward to allow very flowable materials to be poured into place. An injection molded filler block can then be placed into the uncured material to provide an improved thermal barrier and transmit the mechanical loads to the housing.

Another alternative embodiment of the present invention is shown inFIG. 6, which includes dams16for containing curable material14installed within the lateral space10across the corners created by adjoining interior walls12of the housing8as described above. In this embodiment, the dam16is filled with material before being brought into contact with the fuel cell stack200.

Another alternative embodiment of the present disclosure includes a pre-formed drop-in insulating panel18which may be installed in the lateral space10once the fuel-cell based propulsion system100is assembled as described above.

While certain representative embodiments and details have been shown for purposes of illustrating the invention, it will be apparent to those skilled in the art that various changes may be made without departing from the scope of the invention, which is defined in the appended claims.