Chemistry: electrical current producing apparatus – product – and – Having magnetic field feature
Reexamination Certificate
1998-08-10
2001-08-14
Kalafut, Stephen (Department: 1745)
Chemistry: electrical current producing apparatus, product, and
Having magnetic field feature
C429S006000, C429S006000
Reexamination Certificate
active
06274258
ABSTRACT:
BACKGROUND OF THE INVENTION
A fuel cell is an electric cell that converts the chemical energy of a fuel, typically hydrogen, directly into electric energy in a continuous process. Although fuel cells can be used with a variety of fuels and oxidants, they almost exclusively combine hydrogen and oxygen to form water vapor. Fuel cells include an anode in contact with the fuel, a cathode in contact with the oxygen and an electrolyte sandwiched between the anode and cathode. Each cell creates less than one volt so that a series or stack of fuel cells are used to convert fuel into usable energy. Interconnect plates are used between each cell to keep the fuel and oxygen separated and to electrically connect the anode of one cell to the cathode of an adjacent cell.
One source of hydrogen is natural gas. A common way to obtain hydrogen from natural gas is by using a reformer which combines natural gas and steam at a high temperature, such as 760° C., to obtain the hydrogen. Some fuel cells operate using a separate, external reformer to create the hydrogen; other fuel cells combine the function of a reformer into the fuel cell itself by operating the fuel cell at a high enough temperature, as well as other appropriate design considerations.
One type of fuel cell uses radial flow configurations for solid oxide fuel cells. In one design, disclosed in M. Petrik et al., “Stack Development Status of the Interscience Radial Flow (IRF) SOFC”, An EPRI/GRI Fuel Cell Workshop on Fuel Cell Technology Research and Development, Atlanta, Ga., Mar. 22-23, 1994, the fuel and air are fed to each cell through a pair of holes at the center region of the cell. The fuel and air then flow radially outwardly to the edge of the cell. This flow configuration requires seals to segregate the fuel and air at the feed points and also runs the risk of temperature excursions at the center of the cell where both rich fuel and rich oxygen exist. In another configuration, disclosed in M. Prica et al., “Contoured PEN Plates for Improved Thermomechanical Performance in SOFCs”, Proceedings of the Second European Fuel Cell Forum, Vol. 1, pp. 393-402, Oslo, Norway, May 6-10, 1996, the fuel and air are fed to the center of each cell through a pair of needles. These gases then flow radially to the cell edge. This flow configuration eliminates the gas seal requirement but still has problems with regard to temperature excursion. In another configuration, disclosed in European Patent 0,635,896 A1, the fuel is fed to the center of the cell by a feed needle while air is fed to the entire cathode area by distribution nozzles. The spent fuel and spent air are collected at the cell edge. This configuration eliminates the need for a gas seal and does not have temperature excursion problems. It does, however, require a complex gas nozzle distribution system.
SUMMARY OF THE INVENTION
The present invention is directed to a fuel cell assembly, typically a solid oxide fuel cell assembly, which requires no gas seal, no large axial clamping force, no separate air preheater, no external reformer or pre-reformer and no external steam supply for reforming. The assembly has no gas leakage; cell cracking or other cell damage is minimized and the simplicity of the entire system is greatly enhanced. The present invention provides a relatively low cost and high reliability fuel cell assembly.
The fuel cell assembly includes a vessel defining an inside. A gas-permeable, porous housing is preferably located within the inside of the vessel. A fuel cell stack is housed within the interior of the porous housing. Air, or other oxygen-containing gas, is supplied to the fuel cell stack by delivering air into the region between the vessel and porous housing, the air preferably passing through the wall of the porous housing to reach the fuel cell stack.
The fuel cell stack includes a plurality of alternating cells and interconnect plate assemblies. Each cell has an anode surface and a cathode surface. Each interconnect plate assembly has an oxidant side adjacent to the cathode surface of one cell and a fuel side adjacent to the anode surface of another cell. Fuel is supplied to the fuel side preferably at a plurality of fuel exits positioned midway between a central region of the fuel side and the periphery of the fuel side.
A reaction products collection conduit has a reaction products entrance at the central region of the fuel side for withdrawing reaction products away from the fuel side. A flue gas collection conduit has a flue gas entrance at a central region of the oxidant side for the flow of flue gas away from the oxidant side.
The porous housing is heated by the heat generated by the fuel cell stack. This allows the air passing through the porous housing to be preheated to, for example, 500° C. to 800° C. as the air enters a residual reaction products combustion region defined between the interior of the porous housing and the exterior or periphery of the stack. Residual reaction products which pass radially outwardly from the peripheral edge of the interconnect plate assembly combust with the heated air passing through the porous housing. This acts to heat the air flowing to the oxidant side of the interconnect plate to a desired temperature, typically about 700° C. to 1000° C., so to eliminate any need to preheat the air entering the assembly. The desired temperature will depend upon the desired or required operating temperature for the stack. Also, since the air passes through the porous housing, the porous housing remains relatively cool on the outside surface for both safety and efficiency.
The gas flow along both the fuel side and oxidant side of the interconnect plate is preferably directed by flow deflectors. These flow deflectors are preferably created by corrugating the interconnect plate. The corrugations not only act as flow deflectors but also form the electric contact surfaces with the cathode and anode surfaces of adjacent cells. The oxidant side preferably has radially-oriented flow deflectors while the fuel side preferably has both radially-oriented and rotary-oriented flow deflectors.
The fuel stack is preferably oriented horizontally, that is with the wafer-like interconnect plates and cells oriented vertically. The fuel cell stack is preferably allowed to thermally expand and contract in a substantially free manner until the fuel cell stack is within 50° C., or less, of an operating temperature. This freedom of movement during most of the temperature changes helps to minimize cracking or other damage to the cells and interconnect plates. The outer surface portion of the corrugations are preferably plated with a soft metal, such as silver on the fuel side and gold on the oxidant side, to provide good electrical contact with adjacent cells and to help prevent cracking or other damage to the cells.
The present invention differs from conventional fuel cell systems with regard to fuel and oxygen flow primarily because of the use of split fuel flow on the fuel side of the interconnect plate and the radially inward flow of the oxidant gas (typically air) on the oxidant side of the interconnect plate. Because no rich fuel and rich oxidant coexist at any point, the present invention eliminates the temperature excursion problems associated with conventional fuel cell assemblies without the need for complex gas distribution nozzles. The gas distribution method provides other advantages as well. A portion of the spent fuel (fuel reaction products) can be collected by a collection conduit at the central region of the fuel side; the reaction water product in this spent fuel stream is used as a source of reforming steam so that no external steam generation and boiler feed water treatment are required. The split fuel flow also distributes the fuel quickly to the entire fuel side of the interconnect plate and thus to the anode surface of the cell. This helps prevent the cell from local overcooling by the highly endothermic reforming reaction which occurs. As a result, the stack can readily incorporate the reforming internally without the need for
Kalafut Stephen
Nexant, Inc.
Townsend and Townsend / and Crew LLP
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