Patent Application
20090004531
The present invention relates to fuel cell stacks; more particularly, to a fuel cell stack having multiple parallel fuel cells; and most particularly to a solid oxide fuel cell stack comprising a plurality of fuel cell cassettes arranged in series electric flow wherein each cassette includes at least two fuel cells arranged in parallel electric flow.
In practical fuel cell systems, the output of a single fuel cell is typically less than one volt, so connecting multiple cells in series is required to achieve useful operating voltages. Typically, a plurality of fuel cell stages, each stage comprising a single fuel cell unit, are mechanically stacked up in a “stack” and are electrically connected in series electric flow from the anode of one cell to the cathode of an adjacent cell via intermediate stack elements known in the art as interconnects and separator plates.
A solid oxide fuel cell (SOFC) comprises a cathode layer, an electrolyte layer formed of a solid oxide bonded to the cathode layer, and an anode layer bonded to the electrolyte layer on a side opposite from the cathode layer. In use of the cell, air is passed over the surface of the cathode layer, and oxygen from the air migrates through the electrolyte layer and reacts in the anode with hydrogen being passed over the anode surface, forming water and thereby creating an electrical potential between the anode and the cathode of about 1 volt. Typically, each individual fuel cell is mounted, for handling, protection, and assembly into a stack, within a metal frame referred to in the art as a “picture frame”, to form a “cell-picture frame assembly”.
To facilitate formation of a prior art stack of fuel stages wherein the voltage formed is a function of the number of fuel cells in the stack, connected in series, a known intermediate process for forming an individual fuel cell stage joins together a cell-picture frame assembly with an anode interconnect and a metal separator plate to form an intermediate structure known in the art as a fuel cell cassette (“cassette”). The thin sheet metal separator plate is stamped and formed to provide, when joined to the mating cell frame and anode spacers, a flow space for the anode gas. Typically, the separator plate is formed of ferritic stainless steel for low cost.
In forming the stack, the cell-picture frame assembly of each cassette is sealed to the perimeter of the metal separator plate of the adjacent cassette to form a cathode air flow space and to seal the feed and exhaust passages for air and hydrogen against cross-leaking or leaking to the outside of the stack.
The power output P of a fuel cell stack is the product of the voltage V and current I,
P=IV (Eq. 1)
The voltage is a function of the number of fuel cells connected in series in the stack, while the current is a function of the active area of the individual fuel cells. Thus, in designing a fuel cell system, to increase the power output requires an increase in either the number of fuel cells, or the individual fuel cell area, or both.
There are tradeoffs in the number of cells and the surface area of the cells to achieve a desired power level.
Adding more cells in series to increase stack voltage is relatively straightforward, but the reliability of each cell-to-cell connection becomes more critical since the overall reliability of a stack of N cells is a function of the reliability of each connection raised to the Nth power. Also, the resistive losses at the cell-to-cell junctures increase with each connection, and the proportion of system volume required for manifolding of the inlet and return gases increases.
On the other hand, increasing the cell active area to increase the stack amperage by increasing the areal extent of each cell presents many challenges. The cell is a planar ceramic structure, so as the size increases the thickness must also increase to preserve the same level of mechanical strength (that is, resistance to breakage) which significantly increases the cost and size (volume) of the cell per unit area of electric generating capacity. In addition, the manufacturing defect rate is determined by the number of defects per cell, not per unit area, so as the area of a cell increases the number of defects per cell will increase, which adversely affects the overall manufacturing rejection rate in both cell manufacturing and stack manufacturing. Also, as the surface area increases at a constant length-to-width ratio (currently preferred aspect ratio of a prior art fuel cell is about 3:2), the thermal differences across the cell will increase, or the pressure drop will increase, or the gas channel height (and thus overall stack height) will increase, or some intermediate combination of all of these effects must occur. Alternatively, the width or length may be increased while maintaining the same length or width, but this departure from a prior art near-square cell shape makes firing of the ceramic cell very difficult while maintaining acceptable flatness and uniform shrinkage.
What is needed in the art is a means to increase the power output of a fuel cell stack without increasing either the number of cell-to-cell connections or the size of individual fuel cells.
It is a principal object of the present invention to increase the power output of a fuel cell stack.
Briefly described, a fuel cell stack in accordance with the invention comprises a plurality of serially-assembled fuel cell stages preferably formed as individual cassette units. Each stage comprises a plurality of fuel cell units arranged electrically in parallel, such that each stage has the voltage drop of a single fuel cell unit. The assembled stack thus comprises a plurality of internal fuel cell stacks arranged in parallel. The voltage of the plurality of internal stacks is the same as for a prior art single-cell stack of the same number of stages, but the current and hence the power output is multiplied over that of a single-cell stack by the number of internal fuel cell stacks.
Preferably, each cassette includes a plurality of windows for receiving a plurality of individual fuel cell units; a plurality of anode and cathode interconnects; and a single separator plate.
The present invention will now be described, by way of example, with reference to the accompanying drawings, in which:
Corresponding reference characters indicate corresponding parts throughout the several views. The exemplifications set out herein illustrate currently preferred embodiments of the invention, and such exemplifications are not to be construed as limiting the scope of the invention in any manner.
Referring to
To facilitate formation of a prior art stack 26 of individual fuel cells connected in series wherein the voltage formed is a function of the number of individual fuel cell modules in the stack, an intermediate process joins together each cell-picture frame assembly 24 with a separator plate 28 and a first solid (anode) interconnect 30 to form an intermediate structure known as a fuel cell cassette 32. The thin sheet metal separator plate 28 is stamped and formed to provide, when joined to the mating cell frame 22 and anode spacers 29, a flow space for the anode gas 20. Preferably, the separator plate 28 is formed of ferritic stainless steel for low cost. Anode interconnect 30 is placed between the separator plate 28 and the anode surface 31 of the cell within the cassette 32. The solid anode interconnect 30 is typically a woven wire mesh of uniform thickness and is solid in the direction perpendicular to the cell surface in a multitude of points.
A second solid (cathode) interconnect 35, installed during final assembly against cathode surface 34, provides a cathode air flow space. Interconnect 35 also is typically a woven wire mesh of uniform thickness and solid in the direction perpendicular to the cell surface in a multitude of points.
During the final prior art stack assembly process, a glass perimeter seal 42 is disposed between adjacent of the cassettes 32, and the stack under pressure is brought to operating temperature and allowed to settle to its final form. The separator plate and cell frame may deform slightly, providing a compliant assembly, until the cells and interconnects are resting on one another, under load, which prevents further motion.
The present invention provides the capability to increase the active fuel cell area in a cassette without increasing the size of an individual fuel cell element. Alternatively, each fuel cell element can be sized for an optimum combination of cost, manufacturability, and mechanical robustness, largely independent of the cassette active area requirement. Multiple cells are then arranged into a single cassette to achieve the desired active area per cassette.
The cell picture frame has a plurality of openings also referred to herein as “windows”, to accept a plurality of fuel cells in a single frame which is then assembled to a single separator plate, with interconnects and anode spacers, to form a multiple-cell cassette having the desired active area per cassette. The invention thus provides an optimum combination of cost per unit power, volume per unit power, manufacturability, and mechanical robustness.
Cell cost is driven largely by surface area and thickness: for a given thickness and manufacturing discard rate, two cells would cost approximately the same as one cell of the same area. However, an increase in active area requires the thickness of a single cell to be increased for the required mechanical strength, and there would be a higher discard rate as well. Multiple cells in a single frame have slightly less active area than comparable single cells, due to the need for divider bars (in effect, window “mullions”), and more components to assemble than stacks having large single cells, but the additional cost is more than offset by savings in thickness and discard rate.
On the other hand, cassette and stack cost are driven largely by the number of components. The cost of stamping one larger cassette is only slightly more than the cost of stamping one smaller cassette and is much less than the cost of stamping two smaller cassettes. The total number of components, and therefore the assembly cost, is much less for a large stack with multiple cells in a cassette than for multiple stacks of single-cell cassettes having the same power capability. A single stack with more single-cell cassettes is also less reliable and manufacturable due to the large number of electrical and mechanical connections as discussed above.
Regarding mechanical robustness, the picture frames and separator plates preferably are fabricated of ferritic stainless steel which has very little strength at the elevated operating temperature of an SOFC stack. Therefore, the multiple cells are relatively independent of each other mechanically although they reside in a single cassette. In this way, stresses induced by the operating environment (such as thermal cycling, vibration, and the like) are absorbed independently by smaller, more robust cells.
Referring to
In first embodiment 132, the fuel cell elements, having a length-to-width aspect ratio of about 3:2, are arranged with their short sides adjacent in the adjacent windows 123a,123b.
Referring now to
It will be seen that a fuel cell stack formed in accordance with either first embodiment 132 or second embodiment 232 has two internal parallel electric generating stacks and thus has twice the surface area of a prior art stack having the same number of cassettes, and thus has twice the current and hence power generation capability at the same stack voltage.
Higher pluralities of fuel cell elements in each cassette are possible within the scope of the present invention, to generate even more power at the same stack voltage. Referring to
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.
The present invention was supported in part by a US Government Contract, No. DE-FC26-02NT41246. The United States Government may have rights in the present invention.
