Patent Application
20110129754
The present invention relates to solid oxide fuel cells; more particularly, to solid oxide fuel cell stacks having features for improving assembly, electrical certification of individual fuel cells, and long-term mechanical integrity; and most particularly, to an improved method and structure for forming a solid oxide fuel cell stack assembly having features for maintaining alignment of cassettes during sintering, for measuring temperatures at various locations within the stack, for measuring voltage performance of each cassette, and for maintaining compressive load on the stack after sintering and subsequent use of the fuel cell stack assembly.
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 to combine with hydrogen atoms to produce electricity and water; 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, respectively, 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 prior art fuel cell stack assembly includes a plurality of individual fuel cell units known in the art as cassettes or repeating units. Typically, each cassette includes an interconnect which electrically connects the individual fuel cell to the next cassette in the stack to form one half of the fuel cell electric circuit.
In addition, a typical prior art SOFC stack may be based upon a manifold that provides gas-tight attachment to the SOFC stack and distributes the fuel gas and combustion air streams to and from the stack's internal manifolds.
In the prior art, as disclosed for example in U.S. Pat. No. 7,001,685, issued Feb. 21, 2006, a fuel cell stack is formed as a standalone unit in a load frame having self-contained spring means for maintaining a compressive load on the stack at all times as required by the sintered glass gas seals between the cassettes. The load frame subassembly is then mounted onto a manifold. A shortcoming of this system is that a bottom plate is required for the load frame, adding to the complexity and cost. Further, the finished height of the stack, and hence the compressive load within, is governed by spacer supports of fixed length within which a leaf spring arrangement is operative. Such supports also add to complexity and cost, and the compressive load will vary by thermal expansion according to the temperature of the stack.
In the prior art, it is also known to provide at least one dummy cassette in a fuel cell stack, as disclosed in Published US Patent Application US 2009/0004532 (“the '532 application”), published Jan. 1, 2009. It is disclosed to include an inoperative dummy cassette in place of a standard cassette at one or both ends of the stack. It has been observed in SOFC stacks formed of planar cassettes that the endmost cells in the stack perform substantially differently from those in the remainder of the stack. Specifically, the end cells typically exhibit 20-40% lower voltage output than do the rest of the cells. Such lower performing cells may limit the operation of the overall stack. For example, it is undesirable to operate a cell below about 0.5 volts for risk of damaging the cell. If the top and bottom cassettes of a stack are operating at 0.5 volts at a current level at which the rest of the cassettes are operating at 0.8 volts, the stack average voltage is well above the desirable average of 0.7 volts. No more current load may be imposed on the stack, which would be desirable to bring the stack average voltage to 0.7, without causing the top and bottom cassettes to operate at less than 0.5 volts. Failure of the top or bottom cassette due to its operating voltage being less than 0.5 volts can lead to overall stack failure. In the '532 application, the dummy cassette is simply an inert spacer, and no broader use is contemplated in the cited reference.
It is known in the art that maintaining alignment of fuel cell cassettes in a stack is an important requirement. It is further known that when sintering a stack of cassettes having glass seals the cassettes are prone to slipping out of alignment.
Because a single defective fuel cell in a stack can lead to electrical failure of the entire stack, it is desirable to be able to test each fuel cell at will, both during manufacture of the stack and at any time during the working life of the stack.
What is needed in the art is an SOFC assembly wherein the long-term compressive state is reliably and inexpensively maintained; wherein the thermal condition at various points within the fuel cell stack may be monitored at will; wherein the fuel cells are reliably and inexpensively aligned during manufacture; and wherein the electrical performance of each fuel cell in the stack may be tested at will.
It is a principal object of the present invention to improve the manufacturability and working lifetime of a solid oxide fuel cell system.
Briefly described, a solid oxide fuel cell system in accordance with the present invention comprises a plurality of fuel cell cassettes connected in electrical series and bonded together by a plurality of glass seals to form a gas-tight fuel cell stack. At least one dummy cassette containing at least one sensor, such as a thermocouple, is disposed within the fuel cell stack. Each fuel cell and dummy cassette has at least one location tab for receiving an alignment rod, which may be temporary, to maintain stack alignment during sintering in manufacture, and each fuel cell cassette has electrical terminals extending from a side of said stack for performance testing. The distribution manifold is attached to the cassette at a first end of the stack, and a spring subassembly is disposed against the stack at an opposite end thereof from the distribution manifold and is attached to the manifold by tie rods to maintain a compressive load on the stack after sintering and subsequent use to prevent unloading and rupture of the glass seals.
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
In an operating SOFC stack, measuring the gas stream temperatures can be inaccurate and misleading, since there is averaging of gas stream temperature and heat loss from a cell to an adjacent thermocouple. Inserting a thermocouple into an electrochemically active cell unit may affect stack performance, may electrically short adjacent cell repeating units, and may lead to gas leaks. However, knowing the actual cell temperature and temperature distribution at one or more levels of a stack improves understanding of stack operation and can improve stack control.
Stack 12 incorporates one or more dummy cassettes 26 (combination of 26a,
In one aspect of the invention, thermocoupled dummy cassette 26 (TDC) consists of two or three principal components.
A first or carrier plate 30 is thicker than the sheathed thermocouple 28 and has channels 32 formed into it that locate the thermocouples. One end of each channel is located at the desired measurement point, the other end opens to an edge 34 of plate 30. The channels follow a smooth path from the edge to the measuring point so that the thermocouples can be easily inserted after the TDC plates are joined into an assembly, or replaced if necessary. Sensor leads 29 extend from the ends of channels 32. First plate 30 is substantially solid sheet metal, with the exception of the narrow thermocouple channels, for good electrical and thermal conductivity. A second or upper plate 36 provides an electrical contacting surface and sealing surface to one adjacent active repeating unit above the TDC. In cases where channels 32 are formed through carrier plate 30, a third or lower plate 38 may be used to provide an electrical contacting surface and sealing surface to the other adjacent active repeating unit below the TDC. All three plates 30,36,38 have through-holes 40 that correspond to the gas supply and return chimneys in the active repeating units of stack 12. The two or three plates are metallurgically bonded together (brazed or welded) so that the holes for the gas supply and return chimneys are hermetically sealed; therefore, the gas streams pass through the TDC without leaking into or out of the TDC. The plates are also joined in such a way as to provide a highly electrically conductive path through the TDC. Since the TDC is relatively thin in the vertical or axial (Z) direction, it is a very good thermal conductor in the Z direction, but relatively poor in the X-Y plane; this enables the TDC to accurately reflect the temperature and temperature distribution in the X-Y plane of the adjacent active repeating units.
Stack 12 is assembled on a distribution manifold 16 preferably fabricated by casting from stainless steel (to match the CTE of the stack components) and finish-machined to final dimensions. A dummy cassette 26, is assembled adjacent an electrochemically active cell repeating unit 14 with a glass seal interposed therebetween. The glass seal is the same type as is used between functional electrochemically active cell repeating units 14 in the SOFC stack; the seal contains the fuel gas and air streams between repeating units and provides an adhesive, electrically insulating, mechanical bond between repeating units. The glass seal provides the same gas-tight bond joint between manifold 16 and the first component of the stack which effectively bonds the completed stack to the manifold. Manifold 16 then serves as the build platform for stack 12, a supporting carrier for stack 12 after assembly, and as a mounting interface between stack 12 and the SOFC system hardware. Manifold 16 also provides for a simple gas-tight attachment to the SOFC system plumbing and distributes the fuel gas and air streams to the stack internal manifolds.
During the stack assembly process, the glass seals are added in a green state: unsintered glass particles in an organic binder carrier material. The glass seals, along with the stack assembly, are then subjected to a high temperature sintering process to achieve their final dimension and gas-tight bonding properties. During this process, the seals shrink substantially in thickness and become somewhat liquidous as the organic carrier is destroyed. Therefore the stack assembly shrinks substantially in height, and all the repeating units must be restrained to prevent them from “floating” laterally out of their intended positions. To achieve good locational control of the stack assembly components (active repeating units, dummy cassettes, current collectors, etc.), at least one, and preferably two, assembly alignment tab 22 is added to the exterior perimeter of the components; for example, a first tab with a hole and a second tab with a slot at opposite locations to assure correct orientation of each cassette. Ceramic rods (not shown) are inserted through these features and into a locating hole in a reference element such as the base manifold to provide guiding and locating during the sintering process. The rods may be removed after sintering if desired.
After sintering, stack 12 is tested at elevated temperature to verify proper function of the stack. In order to do so, the voltage of each repeating unit must be measured at open circuit and with electrical load. To accomplish this, each repeating unit 14 is provided with at least one, and preferably two, voltage terminal 24 formed from the metal structure of that unit. For testing, a mating terminal (not shown) is preassembled with a mechanical joint or metallurgical bond to the voltage sensing test equipment, preferably computerized, using high volume wiring harness assembly techniques. The mating terminal is also mechanically joined or metallurgically bonded to voltage terminal 24. A low resistance (particularly at the high operating temperature of the stack) joint between the sensing wire and repeating unit is required for accurate voltage measurement (to 0.01 volt). After stack assembly and test, some or all of the voltage leads may be left in place for stack performance monitoring in the SOFC system during usage thereof.
After sintering, the glass seals provide sturdy bonded joints between the components of the completed stack assembly. However, when stack 12 is cooled to room temperature from its operating temperature of 700° C. to 800° C., residual temperature gradient-induced stresses within the components may cause tensile stresses within the glass joints that exceed the tensile strength of the joint. Since the glass seal joints are much stronger in compression than in tension, it is desirable to maintain a compressive load on the SOFC stack (and thus on the seal joints) at all times through the remainder of its life. In the prior art, this was accomplished with an end plate held in place with bolts and torqued to provide a clamping load. However, due to the large difference in coefficient of thermal expansion (CTE) between the SOFC stack operating temperature and room temperature, even a relatively small difference in CTE between the bolts and the stack could result in either an excessively high clamp load or no clamp load at all. To overcome this problem, the present stack assembly is provided with a low profile spring subassembly 18 that provides a continuous compressive load even at SOFC operating temperatures. The present arrangement is simplified considerably over a prior art arrangement disclosed in U.S. Pat. No. 7,001,685 and described above. The present arrangement is bolted directly to the distribution manifold rather than to a base plate and comprises first and second leaf springs 42,44 fabricated from metal alloys with high temperature creep resistance that are assembled one on top of the other. Two springs are used to achieve the desired spring rate while keeping the spring stresses below the creep limit. Depending on load and spring rate requirements, one spring may be sufficient or more than two may be required. In any case, the uppermost spring 44 is larger than the footprint of stack 12, and the end is formed, or a stiffener added, to prevent bending perpendicular to the desired bending direction. Tie rods 20 which pass through upper spring 44 and are is anchored to distribution manifold 16 and are tensioned to pull the ends of spring 44 to a desired deflection to thereby load the spring assembly. Tie rods 20 may be screws, threaded rod, or headed fixed length rods fabricated from a high temperature metal alloy. Lower spring 42 applies load to a stiff load plate 46 fabricated from a high temperature metal alloy or low cost ceramic (such as alumina or ZTA) to distribute the spring load uniformly over the stack footprint area.
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.
