Abstract:
A method for increasing the reliability of an electrolyzer cell stack includes providing multiple electrolyzer cell stacks. Each electrolyzer cell 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 to produce a fuel. The method may further include 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 one electrolyzer cell stack to another in the event a cell fails or creates a point of high resistance.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to electrolyzer cell stacks and more particularly to methods for increasing the reliability and fault tolerance of electrolyzer cell stacks. 
         [0003]    2. Background 
         [0004]    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. 
         [0005]    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. 
         [0006]    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. 
         [0007]    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. 
         [0008]    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. 
       SUMMARY OF THE INVENTION 
       [0009]    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. 
         [0010]    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. 
         [0011]    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. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]    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: 
           [0013]      FIG. 1  is an exploded perspective view of one embodiment of an electrolyzer cell having an electrically conductive tab integrated into an interconnect thereof; 
           [0014]      FIG. 2  is a perspective view of an assembled electrolyzer cell containing the components illustrated in  FIG. 1 ; 
           [0015]      FIG. 3  is a perspective view of one embodiment of an electrolyzer cell stack in accordance with the invention; and 
           [0016]      FIG. 4  is a perspective view of multiple electrolyzer cell stacks integrated into a reliable, fault-tolerant architecture in accordance with the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0017]    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. 
         [0018]    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. 
         [0019]    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. 
         [0020]    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. 
         [0021]    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. 
         [0022]    Referring to  FIG. 1 , one embodiment of an electrolyzer cell  100  in accordance with the invention is illustrated. Particular reference is made herein to solid oxide electrolyzer cells (SOECs) with the understanding that the methods disclosed herein may also be applicable to other types of electrolyzer cells. As shown, an electrolyzer cell  100  may, in certain embodiments, include an electrolyte layer  102 , such as an ionically conductive ceramic layer  102 . The electrolyte may also be an ionically conductive liquid or a membrane of appropriate material. Electrodes  104   a ,  104   b  may be deposited, such as by screen printing or otherwise installed on each side of the electrolyte layer  102 . The electrodes  104   a ,  104   b  may be porous to facilitate the flow of gas therethrough. 
         [0023]    To convey gases to the electrodes  104   a ,  104   b  corrugated and perforated layers  108 ,  110 , that are also electrically conductive, may be placed adjacent to each of the electrodes  104   a ,  104   b . These layers  108 ,  110  may be used to create open space to facilitate gas flow to the electrodes  104   a ,  104   b  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  104   b  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  104   b  through the same space. Similarly, oxygen may be generated at the other electrode  104   a  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. 
         [0024]    Electrically conductive interconnect plates  112   a ,  112   b  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  114   a ,  114   b  may be used to seal the sides of the cell  100  by abutting against the interconnect plates  112   a ,  112   b  and the ceramic electrolyte layer  102 . The upper and lower sets of rails  114   a ,  114   b  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. 
         [0025]    In selected embodiments, an interconnect  112   a  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  112   a  to a similarly positioned interconnect plate  112   a  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. 
         [0026]    Referring to  FIG. 2 , when assembled, the cell  100  may include channels  120   a ,  120   b  extending from front-to-back and side-to-side to carry gases in and out of the cell  100 . Similarly, the tab  116  may extend from the assembled cell  100  to facilitate connection to a wire or other conductor. 
         [0027]    Referring to  FIG. 3 , as mentioned, multiple cells  100  may be stacked to create an electrolyzer cell stack  130 . As shown, selected interconnects  112   a  may be provided with tabs  116   a - e  to conduct current to and from the interconnects  112   a . Tabs  116   a ,  116   e  located on either end of the stack  130  may connect the stack  130  to an external power source. This power source may be used to apply a potential to the stack  130  and generate an electrical current through the stack  130 . The intermediate tabs  116   b - d , on the other hand, are used primarily to transfer current to and from a similarly positioned tab of another electrolyzer cell stack, as will be explained in more detail in association with  FIG. 4 . 
         [0028]    Referring to  FIG. 4 , as shown, two or more stacks  130   a ,  130   b  may be integrated to create a reliable, fault-tolerant, electrolyzer cell stack architecture in accordance with the invention. In selected embodiments, the stacks  130   a ,  130   b  may share part of a manifold system. For example, channels  132   a ,  132   b  and manifolds  134   a ,  134   b  located at or near the ends of the stacks  130   a ,  130   b  may be used to deliver steam, carbon dioxide, or the like to the stacks  130   a ,  130   b . A central shared channel  132   c  and manifold  134   c  may output a fuel such as hydrogen, carbon monoxide, or a combination thereof collected from the stacks  130   a ,  130   b.    
         [0029]    Tabs  116   a - e  extending from the cell stack  130   a  may be electrically connected to similarly positioned tabs  116   a - e  extending from the other cell stack  130   b . This may be accomplished using conductors  140   a - e  such as wires, bus bars, or the like. In selected embodiments, the conductors  140   a - e  and tabs  116   a - e  may be integrated to provide an uninterrupted conductive path between the stacks  130   a ,  130   b.    
         [0030]    The conductors  140   a - e  may connect interconnects  112   a  that are at roughly equivalent electrical potentials in each of the stacks  130   a ,  130   a  and may be used to wire the cells  100  of each stack  130   a ,  130   b  in parallel. When the stacks  130   a ,  130   b  are functioning correctly, very little if any current will flow through the conductors  140   a - e  since the electrical potential at both ends of the conductors  140   a - e  will be substantially equal. The conductors  140   a - e  may also even out any electrical potential imbalances that may exist at different levels within the cell stacks  130   a ,  130   b.    
         [0031]    In the event a condition occurs which causes an electrical potential imbalance in the stacks, the conductors  140   a - e  will transfer current from the higher potential interconnect to the lower potential interconnect, thereby transferring electrical current between the stacks  130   a ,  130   b . 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  130   a  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  130   a  with the defective cell  100 . Thus, the wiring of the stacks  130   a ,  130   b  greatly reduces the probability that a defective cell  100  or cells  100  in either stack  130   a ,  130   b  will take down the entire stack  130   a ,  130   b.    
         [0032]    In selected embodiments, the tabs  116   a - e  and conductors  140   a - e  may be provided at every nth interconnect  112   a  to provide a coarse-grained parallelism. For example, every fourth, fifth, or sixth interconnect  112   a  of the stacks  130   a ,  130   b  may be electrically connected. This granularity may be adjusted by increasing or decreasing the number of interconnects  112   a  between each tab  116   a - e  and conductor  140   a - e . In other embodiments, every interconnect of the stacks  130   a ,  130   b  may be connected together to provide a fine-grained parallelism. 
         [0033]    Although the illustrated embodiment shows a pair of electrolyzer stacks  130   a ,  130   b  in an integrated architecture, it should be recognized that the system and method disclosed herein may be used to link more than two stacks  130   a ,  130   b . For example, groups of two, three, four, or more stacks  130   a ,  130   b  may be linked together using the tabs  116   a - e  and conductors  140   a - e  disclosed herein. In certain embodiments, interconnects  112   a  from multiple stacks may be linked together by connecting to a common bus. 
         [0034]    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.