Abstract:
An energy store is provided, having at least one stack, each stack having at least one storage cell, which in turn has an air electrode and a storage electrode. The storage electrode is adjacent to channels, which contain a storage medium and water vapor. The channels have a larger cross-sectional area than the cross-section of the storage medium, which condition causes unhindered spreading of a reaction gas in the area between the storage electrode and the storage medium.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is the U.S. National Stage of International Application No. PCT/EP2013/068710 filed 10 Sep. 2013, and claims the benefit thereof. The International Application claims the benefit of German Application No. DE 102012217290.4 filed 25 Sep. 2012. All of the applications are incorporated by reference herein in their entirety. 
     
    
     FIELD OF INVENTION 
       [0002]    The invention relates to an electrical energy store. 
       BACKGROUND OF INVENTION 
       [0003]    For the storage of excess electrical current which occurs, for example, when current is being generated by means of renewable energy sources or by power stations operated in the optimum efficiency range and for which there is temporarily no demand in the network, various technical alternatives are adopted. One of these is the rechargeable oxide battery (ROB). ROBs are conventionally operated at temperatures of between 600° C. and 800° C. In this case, oxygen, which is supplied at an air electrode of the electrical cell, is converted into oxygen ions, transported through a solid electrolyte and brought to the opposite storage electrode. A redox reaction takes place there, which, depending on the charging or discharging process, absorbs or generates electrical current. On account of the high temperatures required for this process, the choice of material for the cell materials used and the design of the cell parts and also the arrangement of the storage medium are highly complex. In particular, the individual components deteriorate after a plurality of redox cycles which are operated at said operating temperatures. 
       SUMMARY OF INVENTION 
       [0004]    An object of the invention, therefore, is to provide an electrical energy store which, as compared with the prior art, has higher reliability, along with constant energy absorption capacities. 
         [0005]    A solution for achieving this object lies in an electrical energy store according to the claims. 
         [0006]    The energy store according to aspects of the invention as claimed has at least one storage cell which, if appropriate, may be an integral part of a stack, the storage cell comprising an air electrode and a storage electrode, and the storage electrode having adjacent to it ducts in which, on the one hand, a storage medium is positioned and in which, moreover, steam, in particular a steam/water mixture, is contained. The energy store is distinguished in that the ducts have a larger cross-sectional area than the cross section of the storage medium contained therein. The effect of this is that an additional space, which is designated hereafter as a gas duct, is provided along the duct between the storage electrode and the storage medium. 
         [0007]    This gas duct has a plurality of advantageous effects. Since a shuttle gas, which, in particular, contains hydrogen and water and which is dealt with in more detail hereafter, can freely flow and diffuse in this gas duct, the result of this is that all regions, even regions in the middle of the duct, are always supplied homogenously with the shuttle gas which, for example, is necessary for initial reduction. A homogenous supply of the storage medium over the entire duct length ensures optimum utilization of the entire storage capacity of the energy storage cell and therefore of the electrical energy store. This leads, in turn, to a constant energy absorption capacity of the energy store and, last but not least, to a longer service life. Furthermore, by means of these gas ducts, a lack of shuttle gas, which may be due, for example, to a leak of the overall energy store, can be compensated. Furthermore, when the energy store is being put into operation or during maintenance work upon it, an appropriate reaction gas, such as, for example, pure hydrogen, can be introduced from outside, in order to achieve an overall reduction of the storage medium which is advantageous when the store is deactivated. 
         [0008]    It became apparent that it was appropriate if the storage medium is arranged at a distance from the storage electrode so that the gas duct is located between the storage electrode and the storage medium. 
         [0009]    For this purpose, it is expedient, in turn, to provide a fixing device which fixes the storage medium to the duct walls. A fixing device is understood, in principle, to mean any fixing means which enables the storage medium to be arranged in the ducts such that a distance from the storage electrode or the duct walls can be ensured. This may, in particular, also be a clamping device, for example an undercut in the ducts or mechanical fastening. 
         [0010]    It became apparent that it was appropriate to configure the fixing device in the form of a net, in particular of a metallic net. This metallic net, in turn, may be nickel-based. The net in this case runs essentially parallel to the storage electrode in the ducts. In this case, it is advantageously fastened to two duct walls running parallel to one another. 
         [0011]    The fastening of the net to the duct walls may take place, for example, by means of a spot welding method, but it may also be fastened to a frame which, in turn, has two frame flanks which run parallel to the duct walls and between which the net is fastened. 
         [0012]    The cross-sectional area of the ducts may be at least 5% larger than the cross-sectional area of the storage medium. This, in turn, means that the gas ducts have a cross-sectional area which amounts to at least 5% of the cross-sectional area of the storage medium. This duct cross section of at least 5% of the storage medium has proved to be expedient in order to ensure sufficient freedom of movement of the shuttle gas along the gas duct. 
         [0013]    Furthermore, it became apparent that it was expedient if the storage medium has diffusion ducts which have a diameter of more than 0.5 mm and which run essentially perpendicularly to the storage electrode. The diffusion ducts, which are macroscopic with respect to a porosity of the storage medium, ensure, in turn, the gas transport of the shuttle gas even into the volume material of the storage medium. 
         [0014]    Further features and further advantageous embodiments of the invention are explained in more detail by means of the following figures. These embodiments are merely by way of example and do not constitute any restriction of the scope of protection. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]    In the figures: 
           [0016]      FIG. 1  shows a diagrammatic illustration of a cell of a rechargeable oxide battery, 
           [0017]      FIG. 2  shows an exploded illustration of a stack, as seen from above, 
           [0018]      FIG. 3  shows an exploded illustration of the stack from  FIG. 2 , as seen from below, 
           [0019]      FIG. 4  shows an enlarged diagrammatic illustration of a cross section of a duct with a storage medium, 
           [0020]      FIG. 5  shows a diagrammatic cross section through a duct with a fixing device, and 
           [0021]      FIG. 6  shows a cross section, as in  FIG. 5 , with a fixing device in alternative embodiments. 
       
    
    
     DETAILED DESCRIPTION OF INVENTION 
       [0022]    The working of a rechargeable oxide battery (ROB) will first be described diagrammatically by means of  FIG. 1 , in as much as is necessary for the present description of the invention. In a conventional set-up of an ROB, a process gas, in particular air, is injected via a gas supply  20  at a positive electrode  6  which is also designated as an air electrode, oxygen being extracted from the air. The oxygen passes in the form of oxygen ions O 2−  through a solid electrolyte  7 , resting against the positive electrode, to a negative electrode  10  which is also designated as a storage electrode. If, then, a dense layer of the active storage material were present on the storage electrode at the negative electrode  10 , the charging capacity of the battery would quickly be exhausted. 
         [0023]    For this reason, it is expedient to use as energy storage medium at the negative electrode  10  a storage medium  9  composed of porous material which contains a functionally acting oxidizable material as an active storage material, for example in the form of iron and iron oxide. 
         [0024]    Via a redox pair, for example H 2 /H 2 O, which is gaseous in the operating state of the battery, the oxygen ions transported through the solid electrolyte  7  are transported through pore ducts of the porous storage medium  9 , which pore ducts the active storage material comprises. Depending on whether a charging or a discharging operation is taking place, the metal or the metal oxide (iron/iron oxide) is oxidized or reduced, and the oxygen required for this purpose is delivered or transported back to the solid electrolyte  7  by means of the gaseous redox pair H 2 /H 2 O. This mechanism is designated as a shuttle mechanism. 
         [0025]    The advantage of iron as oxidizable material, that is to say as active storage material, is that, during its oxidation process, it has approximately the same open-circuit voltage of about 1 V as the redox pair H 2 /H 2 O. 
         [0026]    One advantage of the ROB is that it can be extended almost unrestrictedly in modular form, using its smallest unit, to be precise the storage cell  4 . A small battery for stationary domestic use can therefore be produced in just the same way as a large-scale plant for storing the energy of a power station. 
         [0027]    A plurality of the storage cells  4  described in  FIG. 1  are combined into what is known as a stack  2 . The set-up of a stack  2  and the arrangement of the storage cells  4  in the stack  2  are made clear by means of the exploded illustrations in  FIG. 2  and  FIG. 3 .  FIG. 2  illustrates the set-up of a stack which is seen from above and in this case is assembled in order from the bottom upward. The stack  2  comprises first a baseplate  24  which, if appropriate, is composed of a plurality of individual plates which, in turn, have functional structurings and depressions, for example, for the air routing. This assembly of individual plates, which is not described in any more detail here, to form the baseplate  24  takes place, for example, by means of a brazing method. 
         [0028]    The baseplate  24  has an air supply  20  and an air discharge  22 , and, as already described, as a result of the assembly of individual plates, ducts for the air supply, which cannot be seen here, are integrated in the baseplate  24 . Furthermore, the baseplate  24  has centering bolts  29 , onto which further components of the stack  2  are then attached and centered. There follows as the next layer an electrode structure  25  which comprises, in particular, the already described positive electrode  6 , the solid electrolyte  7  and the storage electrode  10 . This is a self-supporting ceramic structure, onto which the individual functional regions, such as the electrodes or the solid electrolyte, are attached by a thin film method. 
         [0029]    There follows as a further layer a seal  26  which is composed, for example, of a high temperature-resistant glass frit which seals off the individual plates of the stack  2  at the operating temperatures of the battery. The next following plate is what is known as an interconnector plate  27  which has two functionally acting sides. In this example, an air side  34  of a storage cell  4  and the air supply ducts, not illustrated in any more detail here, are introduced on the lower side of said interconnector plate, as seen with respect to  FIG. 2 . At its top side (storage side  32 ), the interconnector plate  27  has ducts  12 , into which the storage medium  9  is introduced. The top side of the interconnector plate  27  in  FIG. 2  has the same structure as the top side of the baseplate  24 . Here, too, the ducts  12  for introducing the storage medium  9  are provided. This side having the ducts  12  in each case faces the storage electrode  10  of the storage cell  4 . 
         [0030]    A further iteration of the sequence of the electrode structure  25 , the sealing  26  and a then following closing plate  28 , to form the overall set-up of the stack  2 , is illustrated by way of example in  FIG. 2 . In principle, of course, a series of further iterations of these components, in particular a plurality of sequences of interconnector plates, may also follow, so that a stack conventionally has between 10 and 20 layers of storage cells  4 . 
         [0031]    The same stack  2  which is described in  FIG. 2  is illustrated in the opposite viewing direction in  FIG. 3 . In  FIG. 3 , the baseplate  24  is viewed from below, being followed, in turn, by the electrode structure  25  of the sealing  26 , and the interconnector plate  27  can in this case likewise be seen from below, the eye being directed here to the side which faces the air electrode (air side  34 ). This example illustrates on the interconnector plate four separate regions on the air side  34  which form the subdivision into four individual storage cells  4  per stack layer (in this case the subdivision into four storage cells is to be considered merely as an example). The storage cell  4  is therefore composed in this example of one quarter of the area of the respective interconnector plate or the baseplate  24  or the top plate  28 . Furthermore, the respective cell  4  is formed by sequence of the respective air side  34 , seal  26 , electrode structure  25  and again in each case one quarter of the storage side  32  of the baseplate  24  or of the interconnector plate  27 . The air side  34  is in this case supplied with air as the process gas by an air supply device  8  (also called a manifold) which is not illustrated in any more detail here and which in this case comprises a plurality of components in the stack. 
         [0032]    As already stated, an oxidation or reduction reaction takes place between the shuttle gas, which is usually composed of a mixture of steam and hydrogen, and the storage medium, which contains an iron oxide or ion. In order to keep the concentration of shuttle gas as constant as possible at every point of the storage medium, so that a continuous reaction of the overall storage medium can be implemented, it is expedient, according to  FIG. 4 , to arrange the storage medium  9  in the duct  12  in such a way as to give rise parallel to the storage electrode  10  to a gas duct  50  in which the shuttle gas can spread out, as unobstructed as possible, by diffusion and flow. 
         [0033]    In this case, the duct  12  has overall a cross-sectional area which is composed of the lengths under the curly brackets  13  and  13 ′. By contrast, the storage medium  9  has a smaller cross section which is composed of the lengths under the curly brackets  11  and  11 ′, the length  11 ′ in this embodiment being equated with the length  13 ′. Thus, between the storage medium  9  and the storage electrode  10 , the gas flow duct  50  is formed, in which the shuttle gas H 2 O/H 2  can flow and diffuse, unobstructed, along the storage medium, so that a constant partial pressure of the shuttle gas prevails on the surface of the storage medium  9 . 
         [0034]    In order to fix the storage medium  9  in the ducts  12 , a fixing device is provided, which, in principle, may be present in various embodiments. For example, the ducts  12  may have undercuts, not illustrated here, which clamp the storage medium  9  at the desired location in the duct  12 . In principle, movable fixing noses may also be arranged in the ducts  12  and likewise exert a clamping action with respect to the storage medium  9 . However, due to punctiform load upon the per se brittle material of the storage medium  9 , undesirable damage to the storage medium  9  may occur. Moreover, the storage medium may experience longitudinal expansions owing to chemical transformation during the operation of the storage cell. In this case, punctiform fixing of the storage medium  9  is likewise unbeneficial, since it may entail considerable compressive loads or compressive stresses in the storage medium  9 . It therefore became apparent that it was expedient to attach in the duct  12  a net  40  which, in particular, is of nickel-based form, this net having some stretchability and thus preventing punctiform load upon the storage medium  9 , the storage medium  9  nevertheless being arranged at a fixed location in the duct  12 . This net  40  may also be designated as a fixing device  36 . 
         [0035]    The term “net” is to be understood in principle to mean a fabric which may be configured, for example, in the form of a braided fabric, but also in the form of a knit. In principle, the term “net” also embraces individual tensioned wires which run between the duct walls  38 . Irrespective of the configuration of the net, it proved appropriate to have a mesh width which lies between 30 μm and 300 μm, the wire thickness for example lying between 10 μm and 100 μm. With the mesh width described, in conjunction with the wire thickness described, the storage medium  9  is held securely in the ducts, and with these parameters the net has a sufficiently high elasticity, although no material of the storage medium  9  creeps through the meshes. 
         [0036]    As illustrated in  FIG. 5 , the net  40  is fastened by spot welding to walls  38  of the ducts  12  between the electrode structure  25  which has the storage electrode  10  in the direction of the storage side  32  of the interconnector plate  27 . For this purpose, first, the storage medium  9  is introduced into the ducts  12  of the interconnector plate  27 , and subsequently a net is drawn over the storage side  32  and is pressed into the depressions of the ducts. Thereafter, a corresponding number of weld spots are placed in each case at regular intervals along the ducts by means of a spot welding apparatus, so that the net  40  lies directly on the storage medium  9  and presses the latter sufficiently firmly into the ducts  12 , the net being sufficiently elastic to compensate reduction-induced and thermally induced changes in volume of the storage medium. The storage medium  9  is thus fixed to the duct walls  38  in the ducts  12  by the net  40 . 
         [0037]    In an alternative embodiment according to  FIG. 6 , a frame  42  is provided, which projects partially into the ducts  12  and to the frame flanks  46  of which the net  40  is fastened. The frame  42  is likewise laid onto the storage side  32  of the interconnector plate  27  and may be individual frames for each duct  12  or an overall frame which covers the entire interconnector plate  27  and has in each case individual depressions in the ducts  12 . 
         [0038]    Moreover, it may be expedient to introduce macroscopic diffusion ducts  48  in the storage medium  9 , for example by means of bores or spacers, during the manufacturing process (also pore formers). The term “macroscopic” is understood in this case to mean that the diffusion ducts  48  have an at least tenfold diameter with respect to the conventional porosity of the storage medium  9 , so that sufficient shuttle gas can flow, unobstructed, through the diffusion ducts  48  deep into the interior of the storage medium  9 . These diffusion ducts  48 , which conventionally have an extent of more than 0.5 mm, advantageously run in their preferential direction away from the storage electrode  10 . In this case, they may stand perpendicularly to the storage electrode  10 , as illustrated in  FIG. 6 , or they may run at angles, as illustrated in  FIG. 5 .