1. Field of the Invention
This invention relates to an apparatus and process for equalizing the clamping force applied to a fuel cell stack, in particular, fully internally manifolded molten alkali metal carbonate fuel cell stacks.
Generally, a fuel cell stack is comprised of a stacked plurality of individual fuel cell units separated by inert or bipolar ferrous metal separator plates. Individual cells are sandwiched together and secured into a single stacked unit to achieve desired fuel cell energy output. Each individual fuel cell unit generally includes an anode and cathode electrode, a common electrolyte tile, and a fuel and oxidant gas source. Both fuel and oxidant gas are introduced through manifolds to their respective reactant chambers between the separator plate and the electrodes.
In the manufacture of the individual components of the fuel cell units, variations do occur in the thicknesses of the components, which variations would be expected to occur during the normal manufacturing process. In fuel cell stacks having a relatively small cross sectional area, one square foot or less, compensation for such nonuniformity is generally accomplished by selection and orientation of the components such that variations in thickness can generally be matched to produce a uniform fuel cell stack height. However, in fuel cell stacks with greater horizontal cross section and vertical height, as is necessary to obtain commercially practical fuel cell stacks, this matching of so called "highs and lows" in various components becomes a very difficult, if not impossible task, even when such variations are within manufacturing tolerance.
This invention provides an electrically conducting vertical force equalizer which may be used in any number required within the fuel cell stack itself to compensate for the small vertical dimensional differences across the horizontal plane of the fuel cell stack to reduce internal electrical resistance caused by contact between cell stack components.
2. Description of the Prior Art
Commercially viable molten carbonate fuel cell stacks may contain up to about six hundred individual fuel cell units, each having a planar area in the order of eight square feet. Such fuel cell stacks can be approximately ten feet tall presenting serious problems in the application of a clamping force necessary to force the individual fuel cell units and their respective components together. Due to the thermal gradients between cell assembly and cell operating conditions, differential thermal expansions, and the necessary strength of materials used for the various fuel cell components, close tolerances are required. However, even when the components are manufactured within the required closed tolerances, variations in component thickness inevitably occur. One apparent solution to the problems associated with variations in thickness of the fuel cell unit components is taught by U.S. Pat. No. 4,689,280 in which end plate resilience to a fuel cell stack is provided by a combination flat and corrugated plate structure which make up a "dummy" cell adjacent the end plates. The structure relies solely upon the mechanical resilience provided by the cooperation of the flat and corrugated plate and only suggests that such structures be internally adjacent to the end plates. U.S. Pat. No. 4,973,531 teaches a hollow body filled with air and/or oil external to rigid cell holders to apply even pressure to the external side of the rigid cell holders as the cell is heated to operating temperature.
Other attempts to solve the problems associated with varying thicknesses of cell components are focused only on the edges of the fuel cell components. Resilient edge sealing flanges are taught by U.S. Pat. No. 4,609,595 and 4,514,475 which teach flat and corrugated type spring sheets within hollow sealing flanges rendering compressible sealing flanges to accommodate tolerances in thicknesses of cell components and U.S. Pat. No. 4,604,331 which teaches bellow type edge sealing flanges which may have internal mechanical means to increase their stiffness, to maintain sealing integrity by the resilience of the sealing flanges.
Other known prior art which may affect the compression of a fuel cell stack include U.S. Pat. No. 4,874,678 which teaches solid oxide electrolyte cell stacks with metal fiber felt shock absorbent between the cells; U.S. Pat. No. 4,687,717 which teaches a lithium/iron sulfide battery with hollow separator plates having solid particle or fiber internal support material; U.S. Pat. No. 2,925,456 which teaches hollow electric accumulator separators having internal reinforcement to maintain their shape; U.S. Pat. No. 3,703,417 which teaches hot sealed flexible envelope separators for batteries; U.S. Pat. No. 3,404,041 which teaches an impregnated fibrous battery separator; U.S. Pat. No. 3,647,554 which teaches a flexible film applied to a rigid separator for batteries; U.S. Pat. No. 4,225,654 which teaches fuel cell separators of elastic synthetic material, such as synthetic rubber; U.S. Pat. No. 4,064,321 which teaches fuel cell separators of thin sheets of polyvinyl chloride having internal compartments filled with gas and orifices for ionic conduction between electrodes; U.S. Pat. No. 4,543,303 which teaches a water/gas separator to separate product water and exhaust oxidant in each cell by using a microporous valve metal structure; and U.S. Pat. No. 4,400,447 which teaches a consumable anode cell having a combined current collector and separator.