Patent Application
20140272662
The present disclosure relates generally to an improved strategy for distributing an acceleration load over a fuel cell stack, and more particularly to a way to improve fuel cell systems to secure the position of the fuel cells within a fuel cell stack and maintain stack performance after exposure to high acceleration loads.
Fuel cell systems produce electrical energy through the oxidation and reduction of a fuel and an oxidant. Hydrogen, for example, is a very appealing fuel source because it is clean and it can be used to produce electricity efficiently in a fuel cell. The automotive industry has expended significant resources in the development of hydrogen fuel cells as a source of power for vehicles. Vehicles powered by hydrogen fuel cells would be more efficient and would generate fewer emissions than today's vehicles employing internal combustion engines.
In a typical fuel cell system, hydrogen or a hydrogen-rich gas is supplied as a reactant through a flowpath to the anode side of a fuel cell while oxygen (such as in the form of atmospheric oxygen) is supplied as a reactant through a separate flowpath to the cathode side of the fuel cell. Catalysts, typically in the form of a noble metal such as platinum, are placed at the anode and cathode to facilitate the electrochemical conversion of the reactants into electrons and positively charged ions (for the hydrogen) and negatively charged ions (for the oxygen). In one well-known fuel cell form, the anode and cathode may be made from a layer of electrically-conductive gaseous diffusion media (GDM) with the catalysts deposited thereon to form a catalyst coated diffusion media (CCDM). An electrolyte layer (also called an ionomer layer) separates the anode from the cathode to allow the selective passage of ions from the anode to the cathode while simultaneously prohibiting the passage of the generated electrons; instead, the electrons are forced to flow through an external electrically-conductive circuit (such as a load) to perform useful work before recombining with the charged ions at the cathode. The combination of the positively and negatively charged ions at the cathode results in the production of non-polluting water as a by-product of the reaction. In another well-known fuel cell form, the anode and cathode may be formed directly on the electrolyte layer to form a layered structure known as a membrane electrode assembly (MEA).
The proton exchange membrane (PEM) fuel cell has shown particular promise for vehicular and related mobile applications. The electrolyte layer of a PEM fuel cell is a solid proton-transmissive membrane, such as a perfluorosulfonic acid membrane (PFSA) (a commercial example of which is Nafion™). Regardless of whether the above MEA-based approach or CCDM-based approach is employed, the presence of an anode separated from a cathode by an electrolyte layer forms a single PEM fuel cell; many such single cells can be combined to form a fuel cell stack, increasing the power output thereof. Multiple stacks can be coupled together to further increase power output.
Fuel cell stacks placed within vehicles must be able to withstand severe load changes from acceleration and deceleration of the vehicle. In particular, in order to continue to perform after exposure to high acceleration loads during events such as a vehicle crash, the position of the fuel cells that make up the stack must be retained. In the event of high acceleration, deceleration, or impact of the vehicle, a high shearing force may cause sliding between cells of the stack. If the cells near the ends of the stack slide in a direction perpendicular to the cell stacking direction, the cell stack may disassemble. Additionally, a stack that has been prepared for a freeze start often possesses a reduced compressive load for retention as a result of reduced membrane swell and thermal contraction, leaving the cells vulnerable to being displaced by lateral accelerations.
Typically, the fuel cell stack is enclosed in a housing that must be able to accommodate fuel cell expansion and contraction based on changing hydration levels and temperature. Current methods of generating retention forces include friction at the end cells of the entire stack generated by stack compression and friction at the end cells of modules, transmitted to the housing through contact with the module frame. The primary disadvantage of retention with the end cell friction is that it is limited by the compressive load multiplied by the coefficient of friction for the interfaces. Because the surface of the GDM is typically treated with polytetrafluoroethylene (PTFE), the coefficient of friction may be so low as to generate insufficient friction to retain the entire stack during severe accelerations. Using an adhesive to retain only the end cell merely moves the weakness in the retention system to the next cell interface. Using adhesives to bond all the interfaces would prohibit stack disassembly for repair or diagnostic analysis.
This inadequate friction force is addressed in prior art by dividing the stack into a series of short stack modules sharing a common compression system. The frames at the ends of each module can then be externally supported by the housing. The retention force at the end cell of each module is still limited as discussed above. Moreover, while the reduced mass of the module requires proportionally less retention force, the part count and complexity of the system are increased thereby adding cost and mass of the module frames to the system.
In accordance with the instant disclosure, an improved strategy for distributing an acceleration load over a fuel cell stack, and a method to secure the position of the fuel cells within a fuel cell stack and maintain stack performance after exposure to high acceleration loads is shown and described.
According to one aspect of the present disclosure, a method for reducing the relative movement between adjacent fuel cells within a fuel cell stack during a disruptive event is described. The method includes configuring a fuel cell system to comprise a fuel cell stack comprising a plurality of fuel cells in an adjacently facing relationship enclosed within a housing such that a lateral space exists between the edges of each fuel cell in the stack and an interior wall of the housing. The method further comprises injecting a curable material into at least a portion of the lateral space to provide a bridge between the edges of the fuel cells and the interior wall. Upon occurrence of a disruptive event within the housing, the curable material works to more evenly distribute the acceleration load by transmitting the load from the housing to the edges of fuel cells.
According to another aspect of the present disclosure, a fuel cell system is described. The fuel cell system includes a fuel cell stack comprising a plurality of fuel cells in an adjacently facing relationship enclosed within a housing such that a lateral space exists between the edges of each fuel cell in the stack and an interior wall of the housing. The system further comprises a curable material contained in at least a portion of the lateral space to provide a bridge between the edges of the fuel cells and the interior wall. Upon occurrence of a disruptive event within the housing, the curable material transmits an acceleration load from the housing to more evenly distribute the acceleration load to the edges of fuel cells.
The following detailed description of the preferred embodiments of the present invention can be best understood when read in conjunction with the following drawings, where like structure is indicated with like reference numerals and in which:
a is a perspective view of a fuel cell housing according to one embodiment of the present invention with the fuel cell stack removed;
b is an exploded, detailed perspective view of the fuel cell housing of
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 to
Any number of different types of fuel cells 6 may be used to make up the stack 200 of the propulsion system 100; these cells 6 may be of the metal hydride, alkaline, electrogalvanic, or other variants. In one preferred (although not necessary) form, the fuel cells 6 are PEM fuel cells as discussed above. Stack 200 includes multiple such fuel cells 6 combined in series and/or parallel in order to produce a higher voltage and/or current yield. The produced electrical power from propulsion system 100 may then be supplied directly to electric motor 150 or stored within a battery 310, capacitor or related electrical storage device (not shown) for later use by vehicle 2. It will be understood that the fuel cell system shown and described herein may be used for purposes other than motor vehicles.
Referring to
The lateral space 10 accommodates the hydration and thermal expansion needs of the stack 200 and further makes the stack more accessible for repairs. The lateral space 10 increases when a fuel-cell based propulsion system has been prepared for freeze starts because the elimination of water in the stack 200 to avoid freezing of the fuel cell stack results in the contraction of the GDM between individual fuel cells 6.
According to another aspect of the present disclosure, a plurality of dams 16, 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 housing 8 and the fuel cell stack 200, wherein the plurality of dams 16 form channels for receiving the curable material 14. The dams 16 may occupy the entire area defined by the size of the interior wall 12 of the housing 8, may occupy a plurality of small areas, or any area in-between. The plurality of dams 16 may be secured to the housing 8 or to an intermediate material between the housing 8 and the fuel cell stack 200. The intermediate material may be an insulating panel 18 affixed to the interior wall 12 of the housing 8. The plurality of dams 16 may be situated perpendicular to the orientation of the fuel cell stack 200 or situated parallel to the stack 200 orientation and may vary in number and size. The insulating panel 18 serves to disrupt buoyancy convention flow between the fuel cell stack 200 and the housing 8 by minimizing the lateral space 10. The insulating panel 18 also provides electrical insulation. The dams 16 may 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 cells 6 within a fuel cell stack 200 during a disruptive event is disclosed. The method includes configuring a fuel cell-based propulsion system 100 as described above and injecting the curable material 14 into at least a portion of the lateral space 10 to provide a bridge between the edges of the fuel cells 6 and the interior walls 12 of the housing 8. The housing 8 comprises a plurality of injection ports 20 for receiving the curable material 14. Once the system 100 is fully assembled, the curable material 14 is injected into the lateral space 10 between the interior walls 12 of the housing 8 and the fuel cell stack 200 via the plurality of injection ports 20. In embodiments including dams 16, the injection ports 20 are in fluid communication with the dams 16. The method further comprises controlling the amount, extent, and curing of the curable material 14 wherein the curable material 14 is viscous enough to allow complete fill without leakage and curing of the material 14 can 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 material 14 possesses 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 material 14 possesses an electrical resistivity of 1.3×1014 ohms-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 85-147×10−6 in/° C.; and a thermal conductivity of 0.104 btu-ft/ft2-hr-° F.
In one embodiment of the present disclosure, as shown in
According to another aspect of the present disclosure, the method further comprises a release liner 22, shown in
An alternative embodiment of the present invention is shown in
Another alternative embodiment of the present invention is shown in
Another alternative embodiment of the present disclosure includes a pre-formed drop-in insulating panel 18 which may be installed in the lateral space 10 once the fuel-cell based propulsion system 100 is 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.
