The present invention relates to fuel cells; more particularly, to devices for controlling air flow through fuel cells; and most particularly, to a baffle for distributing cathode air flowing through a fuel cell.
Fuel cells for combining hydrogen and oxygen to produce electricity are well known. A known class of fuel cells includes a solid-oxide electrolyte layer through which oxygen anions migrate; such fuel cells are referred to in the art as “solid-oxide” fuel cells (SOFCs).
In some applications, for example, as an auxiliary power unit (APU) for an automotive vehicle, an SOFC is preferably fueled by “reformate” gas, which is the effluent from a catalytic hydrocarbon oxidizing reformer. Reformate typically includes amounts of carbon monoxide (CO) as fuel in addition to molecular hydrogen. The reforming operation and the fuel cell operation may be considered as first and second oxidative steps of the liquid hydrocarbon, resulting ultimately in water and carbon dioxide. Both reactions are exothermic, and both are preferably carried out at relatively high temperatures, for example, in the range of 700° C. to 1000° C.
A complete fuel cell stack assembly includes a plurality of components known in the art as interconnects, which electrically connect the individual fuel cells. In a typical SOFC stack assembly, a space is provided for flow of air between each interconnect and the cathode of the adjacent fuel cell. The flowing air serves two purposes: first, of course, is to provide oxygen for the fuel cell reaction with hydrogen; and second is to provide cooling of the stack to prevent overheating from the exothermic reactions of the cells.
The direction of flow of air along the cathode through the cathode air flow space may be considered a first direction of flow. In an SOFC, the direction of flow of reformate through an analogous anode reformate flow space may be in the same direction as the cathode air flow (co-flow), transverse of the cathode air flow (cross-flow), or opposite to the cathode air flow (counter-flow). The entering and exiting temperatures of the cathode air and the reformate may be very different and are affected by the relative flow volumes of the two gases, and will differ significantly depending upon the direction and volume of reformate flow relative to the direction and volume of cathode air flow. As a result, a wide range of temperatures may pertain over the interconnect and cathode surfaces.
It is highly desirable that the distribution of temperatures in directions both along and transverse of the cathode surface be as uniform as possible because the fugacity of oxygen anion through the cathode, and hence the rate of reaction of the cell at the anode side, is a direct function of gas temperature. Preventing cold spots in the cathode will optimize the electric output of the cell. Further, the stack assembly can be subjected to serious and potentially destructive thermal stresses where temperatures along the interconnects and cathodes are highly unbalanced.
What is needed in the art is a means for distributing air flowing through the cathode air flow spaces of a solid oxide fuel cell assembly such that temperature variations of the cathode surfaces are acceptably small, and preferably zero.
It is a principal object of the present invention to minimize temperature variations of the cathode surfaces in a fuel cell stack.
It is a further object of the present invention to increase the electric output of a fuel cell stack.
It is a further object of the present invention to reduce thermal stresses within a fuel cell stack.
Briefly described, a fuel cell stack in accordance with the invention is provided with perforated baffles disposed within the cathode air flow spaces of the stack for the distribution of air across the interconnect and cathode surfaces in a predetermined pattern. A baffle comprises at least one element inclined to the air flow direction and having a pattern of perforations for the passage of air therethrough. A baffle may include one or more additional elements to form, for example, a V shape within the cathode air flow space. The perforations may be in the form of slots, holes, or any other shape as desired. The pattern of perforations may be varied as need both longitudinally and transversely of the baffle element to modulate air flow both longitudinally and transversely to provide uniform surface temperatures of the cathodes.
The present invention will now be described, by way of example, with reference to the accompanying drawings, in which:
Referring to
A baffle in accordance with the invention comprises at least one element 28 inclined to the direction of flow 30 of cathode air 20 and having a pattern of perforations for the passage of air therethrough. A baffle may include one or more additional elements 32 to form, for example, a V shape within the cathode air flow space as shown in
Referring to
Referring to
A baffle 10 in accordance with the invention may be formed of any suitable material, and is preferably formed of ceramic or by stamping from stainless steel sheeting. Further, because radiative heat transfer between the interconnect and the cathode can be a significant factor at operating temperatures of the fuel cell, baffle 10 may be coated or treated 44 with materials as are known in the art of heat transfer control to adjust the absorptivity and reflectivity of baffle 10 and/or interconnect 16. Such coating or treatment 44 (
While the invention has been described by reference to various specific embodiments, it should be understood that numerous changes may be made within the spirit and scope of the inventive concepts described. Accordingly, it is intended that the invention not be limited to the described embodiments, but will have full scope defined by the language of the following claims.