Patent Application
20090090637
1. Field of the Invention
The present invention relates to electrolyzer cell stacks and more particularly to methods for increasing the reliability and fault tolerance of electrolyzer cell stacks.
2. Background
In the future, renewable energy sources will ideally supply a large portion of the energy required to sustain our society. Because renewable energy may come in various forms, systems and methods are needed to efficiently convert this renewable energy into a form that is convenient and useable with different applications. For example, it may be necessary to convert electricity generated from wind and solar power to fuels such as hydrogen or synthesis gas (hydrogen and carbon monoxide) to make it easier to store and transport.
One method of converting electricity to hydrogen or synthesis gas is to use an electrolyzer cell stack, such as a solid oxide electrolyzer cell (SOEC) stack. Such a stack may include numerous individual cells electrically stacked in a series configuration. Although each cell individually may be quite reliable with a low probability of failure (e.g., 1/10,000,000 chance of failure), stacking multiple cells together significantly compounds the probability of failure. This is because the stack will perform only as well as the least reliable cell. Thus, a failed cell (e.g., a cell acting as an open circuit) will cause the stack as a whole to fail. Similarly, a highly resistive cell will reduce the performance of every other cell in the stack. Due to the increasing probability of failure, some feel it is impractical to continue to increase the number of cells in electrolyzer cell stacks beyond a certain level.
One possible solution to this problem may be to redesign each layer of the cell stack to include multiple independent cells arranged into an array. Interconnects may be placed between each layer to electrically connect the cells within each layer into a parallel configuration. Thus, multiple paths may be provided for electricity to flow through the stack. This allows the current to take an alternative path through the stack to avoid an open or highly resistive cell in one or more of the layers.
Nevertheless, this solution may be inefficient in the way it utilizes the area of each layer in the stack and may increase the complexity of each layer. This solution may also make it difficult to seal the area between the cells of each layer. Specifically, this solution may require that a seal be placed around each cell in the layer as well as around the entire array of cells in the layer. Thus, this solution may be difficult and costly to implement.
In view of the foregoing, what is needed is a method for increasing the reliability and fault tolerance of electrolyzer cell stacks, such as SOEC stacks, that is both simple and inexpensive to implement. Ideally, such a method could be used with conventional electrolyzer cell stacks having a single cell between each interconnect.
Consistent with the foregoing, and in accordance with the invention as embodied and broadly described herein, one embodiment of a method for increasing the reliability of an electrolyzer cell stack includes providing multiple electrolyzer cell stacks, such as multiple solid oxide electrolyzer cell stacks. Each electrolyzer cell stack includes multiple cells electrically connected in series. An external power source may be used to provide an electrical current through the electrolyzer cell stacks to cause the electrolyzer cell stacks to produce a fuel. In the event that one or more cells of the electrolyzer cell stacks fail, the method includes electrically routing all or part of the current previously traveling through the failed cell through one or more cells of another electrolyzer cell stack. In selected embodiments, a failure may include a condition which makes a cell act as an open circuit or a condition which increases the resistance of a cell.
In another aspect of the invention, a method for increasing the reliability of an electrolyzer cell stack may include providing multiple electrolyzer cell stacks. Each electrolyzer cell stack includes multiple cells separated by electrically conductive interconnects. The method includes generating, using an external power source, an electrical current through each of the electrolyzer cell stacks to produce a fuel. The method further includes electrically connecting an interconnect of a first electrolyzer cell stack to an interconnect of a second electrolyzer cell stack located at a substantially equivalent electrical potential. This allows current to flow from the first electrolyzer cell stack to the second electrolyzer cell stack in the event a cell fails or increases in resistance.
In yet another aspect of the invention, a method for increasing the reliability of an electrolyzer cell stack includes providing multiple electrolyzer cell stacks, where each stack includes multiple cells separated by electrically conductive interconnects. The method may further include generating, using an external power source, an electrical current through each of the electrolyzer cell stacks in order to produce a fuel. The method may further include electrically connecting selected interconnects of one electrolyzer cell stack to selected interconnects of another electrolyzer cell stack. The interconnects that are connected together are located at substantially equivalent electrical potentials of the respective electrolyzer cell stacks. In selected embodiments, only interconnects at specific intervals are connected. Thus, the method may include electrically connecting every nth interconnect of an electrolyzer cell stack to every nth interconnect of another electrolyzer cell stack.
In order to describe the manner in which the above-recited features and advantages of the present invention are obtained, a more particular description of apparatus and methods in accordance with the invention will be rendered by reference to specific embodiments thereof, which are illustrated in the appended drawings. Understanding that these drawings depict only typical embodiments of the present invention and are not, therefore, to be considered as limiting the scope of the invention, apparatus and methods in accordance with the present invention will be described and explained with additional specificity and detail through the use of the accompanying drawings in which:
It will be readily understood that the components of the present invention, as generally described and illustrated in the Figures herein, could be arranged and designed in a wide variety of different configurations. Thus, the following more detailed description of the embodiments of apparatus and methods in accordance with the present invention, as represented in the Figures, is not intended to limit the scope of the invention, as claimed, but is merely representative of certain examples of presently contemplated embodiments in accordance with the invention. The presently described embodiments will be best understood by reference to the drawings, wherein like parts are designated by like numerals throughout.
Reference throughout this specification to features, advantages, or similar language does not imply that all of the features and advantages that may be realized with the present invention should be or are in any single embodiment of the invention. Rather, language referring to the features and advantages is understood to mean that a specific feature, advantage, or characteristic described in connection with an embodiment is included in at least one embodiment of the present invention. Thus, discussion of the features and advantages, and similar language, throughout this specification may, but do not necessarily, refer to the same embodiment.
Furthermore, the described features, advantages, and characteristics of the invention may be combined in any suitable manner in one or more embodiments. One skilled in the relevant art will recognize that the invention can be practiced without one or more of the specific features or advantages of a particular embodiment. In other instances, additional features and advantages may be recognized in certain embodiments that may not be present in all embodiments of the invention.
Reference throughout this specification to “one embodiment,” “an embodiment,” or similar language means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, appearances of the phrases “in one embodiment,” “in an embodiment,” and similar language throughout this specification may, but do not necessarily, all refer to the same embodiment.
In the following description, numerous specific details are presented to provide a thorough understanding of embodiments of the invention. One skilled in the relevant art will recognize, however, that the invention can be practiced without one or more of the specific details, or with other methods, components, materials, and so forth. In other instances, well-known structures, materials, or operations such as vacuum sources are not shown or described in detail to avoid obscuring aspects of the invention.
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To convey gases to the electrodes 104a, 104b corrugated and perforated layers 108, 110, that are also electrically conductive, may be placed adjacent to each of the electrodes 104a, 104b. These layers 108, 110 may be used to create open space to facilitate gas flow to the electrodes 104a, 104b and may be positioned perpendicular to one another to facilitate gas flow in two perpendicular directions. The layers 108, 110 may also be formed in various configurations and may be designed for parallel or possibly counter-flow of the gases. For example, steam, carbon dioxide, or a combination thereof may flow to the lower electrode 104b through the space created by the corrugated layer 110. These gases may be converted to a fuel such as hydrogen, carbon monoxide, or a combination thereof, which may flow away from the electrode 104b through the same space. Similarly, oxygen may be generated at the other electrode 104a where it may flow through open space created by the layer 108. Depending on the electrode and electrolyte assembly, various gases may be generated during the reaction on the electrodes. Thus, a proton conducting electrolyte would leave oxygen on the side where the steam entered while an oxygen conducting electrolyte would leave hydrogen on the side where the steam enters.
Electrically conductive interconnect plates 112a, 112b may be placed adjacent to each corrugated layer 108, 110 to physically separate each cell 100, provide an electrically conductive path between each cell 100, and create a barrier to prevent passage of gases between adjacent cells 100. Edge rails 114a, 114b may be used to seal the sides of the cell 100 by abutting against the interconnect plates 112a, 112b and the ceramic electrolyte layer 102. The upper and lower sets of rails 114a, 114b may be aligned perpendicular to one another to accommodate gas flow in two directions (cross-flow). It is also possible to align the interconnect plates to allow parallel flow of the gases or by proper manifold arrangements to create a counter-flow of the gases.
In selected embodiments, an interconnect 112a may include a tab 116 or other projection 116 to conduct electrical current to and from an electrolyzer cell stack. As will be explained in more detail hereafter, this tab 116 may be used to connect the interconnect plate 112a to a similarly positioned interconnect plate 112a of another electrolyzer cell stack. As will be further explained, this provides an alternate path for electrical current to flow in the event a cell 100 in one of the electrolyzer cell stacks fails.
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Tabs 116a-e extending from the cell stack 130a may be electrically connected to similarly positioned tabs 116a-e extending from the other cell stack 130b. This may be accomplished using conductors 140a-e such as wires, bus bars, or the like. In selected embodiments, the conductors 140a-e and tabs 116a-e may be integrated to provide an uninterrupted conductive path between the stacks 130a, 130b.
The conductors 140a-e may connect interconnects 112a that are at roughly equivalent electrical potentials in each of the stacks 130a, 130a and may be used to wire the cells 100 of each stack 130a, 130b in parallel. When the stacks 130a, 130b are functioning correctly, very little if any current will flow through the conductors 140a-e since the electrical potential at both ends of the conductors 140a-e will be substantially equal. The conductors 140a-e may also even out any electrical potential imbalances that may exist at different levels within the cell stacks 130a, 130b.
In the event a condition occurs which causes an electrical potential imbalance in the stacks, the conductors 140a-e will transfer current from the higher potential interconnect to the lower potential interconnect, thereby transferring electrical current between the stacks 130a, 130b. For example, if a cell 100 fails such that it acts as an open circuit or becomes highly resistive, current will flow from one stack 130a to the other in order to bypass the defective cell 100. After the defective cell 100 has been bypassed, current will flow back to the stack 130a with the defective cell 100. Thus, the wiring of the stacks 130a, 130b greatly reduces the probability that a defective cell 100 or cells 100 in either stack 130a, 130b will take down the entire stack 130a, 130b.
In selected embodiments, the tabs 116a-e and conductors 140a-e may be provided at every nth interconnect 112a to provide a coarse-grained parallelism. For example, every fourth, fifth, or sixth interconnect 112a of the stacks 130a, 130b may be electrically connected. This granularity may be adjusted by increasing or decreasing the number of interconnects 112a between each tab 116a-e and conductor 140a-e. In other embodiments, every interconnect of the stacks 130a, 130b may be connected together to provide a fine-grained parallelism.
Although the illustrated embodiment shows a pair of electrolyzer stacks 130a, 130b in an integrated architecture, it should be recognized that the system and method disclosed herein may be used to link more than two stacks 130a, 130b. For example, groups of two, three, four, or more stacks 130a, 130b may be linked together using the tabs 116a-e and conductors 140a-e disclosed herein. In certain embodiments, interconnects 112a from multiple stacks may be linked together by connecting to a common bus.
The present invention may be embodied in other specific forms without departing from its essence or essential characteristics. The described embodiments are to be considered in all respects only as illustrative, and not restrictive. The scope of the invention is, therefore, indicated by the appended claims, rather than by the foregoing description. All changes within the meaning and range of equivalency of the claims are to be embraced within their scope.
