Abstract:
An electrical energy store having at least one stack with in each case at least one storage cell. Each storage cell has an air electrode, which is connected to an air supply apparatus, and a storage electrode. The storage electrode adjoins channels which contain a storage medium and a steam/hydrogen mixture. A reservoir of steam/hydrogen is provided, the reservoir being directly connected to the channels.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is based on and hereby claims priority to International Application No. PCT/EP2013/055680 filed on Mar. 19, 2013 and German Application No. 10 2012 205 077.9 filed on Mar. 29, 2012, the contents of which are hereby incorporated by reference. 
     
    
     BACKGROUND 
       [0002]    The invention relates to an electrical energy store. 
         [0003]    For the storage of excess electrical current, which occurs, for example, when current is generated by renewable energy sources or by power plants operated in the optimal efficiency range, and for which there is temporarily no demand in the network, various technical alternatives are adopted. One of these is the rechargeable metal air battery (rechargeable oxide battery, ROB). ROBs are usually operated at temperatures of between 600° C. and 800° C. In this case, oxygen, which is supplied at a (positive) air electrode of the electrical cell, is converted into oxygen ions, is transported through a solid electrolyte and is brought to the opposite negative electrode. A reaction with a gaseous redox pair takes place there, which absorbs or emits electrons, depending on the charging or discharging process, the oxygen absorbed or emitted by the gaseous redox pair being transferred by the diffusion of the components of the redox pair to a porous, that is to say gas-permeable, and likewise oxidizable and reducible storage medium. 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 become impaired after several redox cycles which are run at said operating temperatures. 
       SUMMARY 
       [0004]    One potential object, therefore, is to provide an electrical energy store, based on an ROB, which, as compared with the related art, ensures a cost-effective, simply assembled and temperature-resistant set-up of a stack or of a storage cell. 
         [0005]    The inventors propose an electrical energy store. The electrical energy store has at least one stack each with at least one storage cell which comprises, in turn, an air electrode which is connected to an air supply device, a negative electrode and a storage medium arranged in the immediate vicinity of the latter. The negative electrode is therefore designated hereafter as the store-side electrode or, in brief, as the storage electrode. The storage electrode itself does not comprise the storage medium. Adjacent to the storage electrode are ducts which contain the porous storage medium and also a hydrogen/water vapor mixture. The composition of the latter is given, in the nonloaded state, by establishing the chemical equilibrium with the storage material and, during charging and discharging, will deviate from this to a greater or lesser extent, depending on the load. The proposed energy store is distinguished in that a reservoir having a water vapor/hydrogen mixture is provided, which is connected directly to the ducts. Any gas losses due to leakages are in this case compensated in that the reservoir is connected to a vapor line which maintains the pressure in the reservoir. Hydrogen or a mixture of water vapor and of hydrogen could just as well be fed in at this point, since the mixture suitable for the charging state of the store is always set automatically. 
         [0006]    The described set-up of the electrical energy store is a technically simple solution, since various levels of the electrical energy store can be placed one above the other, so that what is known as a stack composed of a plurality of storage cells is obtained. This can be placed in a water vapor/hydrogen reservoir in a simple way, so that a complicated supply of this gas to the individual cells can be dispensed with. 
         [0007]    In a further advantageous refinement, a stack has a wall, the ducts with the storage medium being accessible by being open toward this wall. The water vapor/hydrogen mixture can thereby pass, for example by diffusion, through the orifices in the stack wall into the ducts if the stack is placed directly in the gas reservoir. 
         [0008]    In an advantageous refinement, the electrical energy store has a plurality of stacks which are surrounded, in turn, by a common water vapor/hydrogen reservoir. 
         [0009]    In this case, the water vapor/hydrogen reservoir, occupied by one or more stacks, is thermally insulated outwardly. It thus forms what is known as a hot box. 
         [0010]    Advantageously, an overpressure, preferably in the hectopascal range (1 hPa-100 hPa), may also be present in the water vapor/hydrogen reservoir. This ensures that there is always sufficient water vapor/hydrogen mixture available for reaction and the inflow or inward diffusion of air from outside the reservoir or the hot box due to leakages, which could adversely influence the reaction with the storage material owing to the introduction of inert nitrogen, is obstructed. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]    These and other objects and advantages of the present invention will become more apparent and more readily appreciated from the following description of the preferred embodiments, taken in conjunction with the accompanying drawings of which: 
           [0012]      FIG. 1  shows a diagrammatic illustration of a cell of a rechargeable oxide battery, 
           [0013]      FIG. 2  shows an exploded illustration of a stack, seen from above, 
           [0014]      FIG. 3  shows an exploded illustration of the stack from  FIG. 2 , seen from below, 
           [0015]      FIG. 4  shows a stack with outwardly open ducts for the storage medium, 
           [0016]      FIG. 5  shows a stack in a gas-tight reservoir filled with water vapor/hydrogen mixture and having thermal insulation, and 
           [0017]      FIG. 6  shows an electrical energy store with four stacks in a water vapor/hydrogen reservoir having thermal insulation. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0018]    Reference will now be made in detail to the preferred embodiments of the present invention, examples of which are illustrated in the accompanying drawings, wherein like reference numerals refer to like elements throughout. 
         [0019]    The type of action of a rechargeable oxide battery (ROB) will first be described diagrammatically with reference to  FIG. 1 , in as much as this is necessary for the present description. 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  bearing against the positive electrode, to a negative electrode  10  which, as described, is also designated as a storage electrode. If, then, a dense layer of active storage material were present on the negative electrode  10 , that is to say on the storage electrode, the charging capacity of the battery would quickly be exhausted. 
         [0020]    For this reason, it is expedient to use on the negative electrode  10 , as energy storage medium, a storage medium  9  which is composed of porous material and which contains a functionally acting oxidizable material as an active storage material, preferably in the form of iron and iron oxide. 
         [0021]    Via a redox pair, for example H 2 /H 2 O, which is gaseous in the operating state of the battery, the oxygen transported in the form of O 2−  ions through the solid electrolyte  7  is transported from the negative electrode, leaving behind the electrons, through pore ducts in the porous storage medium  9  into the depth of the active storage material. Depending on whether there is a discharging or a charging operation, the metal or the metal oxide (iron/iron oxide) is oxidized or reduced, and the oxygen required for this purpose is delivered by the gaseous redox pair H 2 /H 2 O or is transported back to the solid electrolyte  7 . This oxygen transport mechanism carried by the gaseous redox pair is designated as a shuttle mechanism. 
         [0022]    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 off-load voltage of about 1 V as the redox pair H 2 /H 2 O in the case of a partial pressure ratio of 1, otherwise increased resistance arises to the transport of oxygen by the diffusing components of this redox pair. 
         [0023]    One advantage of the ROB is that it can be extended to a virtually unlimited extent in modular form by the repetition of its smallest unit, to be precise the storage cell  4 . A small battery for stationary domestic use and a large-scale plant for storing the energy of a power station can thus be produced. 
         [0024]    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 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 the order from the bottom upward. The stack  2  comprises in the first place a base plate  24  which, if appropriate, is assembled from a plurality of individual plates which, in turn, have functional structurings and depressions, for example, for the routing of air. This assemblage of individual plates, which is not described in any more detail here, to form the base plate  24  is carried out, for example, by a brazing method. 
         [0025]    The base plate  24  has an air supply  20  and an air discharge  22 . As already described, ducts, not shown here, for air supply are integrated in the base plate  24  as a result of the assemblage of individual plates. Furthermore, the base plate  24  has centering bolts  29 , by which further components of the stack  2  can then be attached in a centered manner. The next layer which follows is an electrode structure  25  which, in particular, comprises the already described positive electrode  6 , solid electrolytes  7  and storage electrode  10 . This is a self-supporting ceramic structure, to which the individual functional regions, such as the electrodes and the solid electrolyte are attached by a thin-film method. 
         [0026]    A further layer which follows is a seal  26  which is composed, for example, of a glass frit which is resistant to high temperature and which seals off the individual plates of the stack  2  when the battery is at the operating temperature. The next following plate is what is known as an interconnector plate  27  which has two functionally acting sides. On its lower side  34 , as seen with respect to  FIG. 2 , are located the air supply ducts, not illustrated in any more detail here, which are adjacent to the positive electrode  6  of a storage cell  4 . On its topside (store side), the interconnector plate  27  has ducts  12  into which the storage medium  9  is introduced. The topside of the interconnector plate  27  in  FIG. 2  has the same structure as the topside of the base plate  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 . 
         [0027]      FIG. 2  illustrates by way of example a further level of the sequence of an electrode structure  25  and a seal  26  under a closing plate  28 , to form the overall set-up of the stack  2 . In principle, of course, a series of further levels of these structural parts may also follow, so that a stack usually has between  10  and more layers of storage cells  4 . 
         [0028]      FIG. 3  illustrates the same stack  2 , which is described in  FIG. 2 , in the opposite viewing direction. In  FIG. 3 , the view is of the base plate  24  from below, followed, in turn, by the electrode structure  25  and the seal  26 . The interconnector plate  27  can then likewise be seen from below, the view in this case being directed toward the air side  34  which faces the air electrode (air side  34 ). In this example, four separate regions are illustrated on the interconnector plate on the air side  34  and correspond to a subdivision into four individual storage cells  4  per stack level (although this subdivision into four storage cells must be considered as being purely by way of example). In this example, therefore, the storage cell  4  is composed of a quarter of the area of the respective interconnector plate or base plate  24  or cover plate  28 . Furthermore, the respective cell  4  is formed by a sequence of the respective air side  34 , seal  26 , electrode structure  25  and again in each case a quarter of the storage side  32  of the base plate  24  or interconnector plate  27 . The air side  34  is in this case supplied with air as process gas by a stack-internal air distribution device  8  (also called a manifold) which is not illustrated in any more detail here and comprises a plurality of levels of the stack. 
         [0029]      FIG. 4  illustrates a stack  2  according to  FIGS. 2 and 3  in its assembled form. The air supply  20  and the air discharge  22  can be seen from outside, and dashed lines illustrate diagrammatically the air distribution device  8  arranged inside. The air distribution device  8  in this case comprises both the air inlet  20 , the air outlet  22  and ducts, not designated in any more detail, in the base plate  24  and in the lateral region of the assembled stack  2 . 
         [0030]    Arranged separately from these in atmospheric terms are the ducts  12  which are in each case located on the storage side  32  of the base plate  24  or of the interconnector plate  27 . As can be seen in more detail in  FIG. 5 , the storage medium  9  is introduced in the ducts  12 . In the embodiment present here, the ducts  12  are accessible by being open with respect to a stack wall  16 . Thus, as illustrated diagrammatically in  FIG. 5 , the stack  2  can be brought into a closed space in which, in turn, an H 2 /H 2 O atmosphere prevails. This closed space having the H 2 /H 2 O atmosphere is designated as a reservoir  14 . The reservoir  14  in this case preferably has an overpressure of the reaction pair H 2 /H 2 O of about 1-100 hPa. This ensures that, in the event of possible leakage of the reservoir  14 , no nitrogen and oxygen penetrate into the reservoir  14  from outside, but instead, in this case, H 2 /H 2 O can escape from the reservoir harmlessly, while this loss can be compensated by a current flux and a water vapor supply  30  via a valve  31 . It will be possible just as well to supply, instead of pure water vapor, a hydrogen/water vapor mixture or else even pure hydrogen. The overall arrangement according to  FIG. 5  with the stack  2  and with the reservoir  14 , which is surrounded by thermal insulation  18 , which is also designated as what is known as a hot box, is designated as an electrical energy store  1 . 
         [0031]    In this case, even more complex electrical energy stores  1  in which a plurality of stacks  2  are arranged ( FIG. 6 ) are expedient. In this case, a central air supply  20  and a central air discharge  22  may be provided, the individual stacks  2  thereby being supplied with air as process gas by a corresponding line  20 . Virtually the operating temperature of the electrical energy store, which usually lies between 600° C. and 800° C., prevails in the hot box  18 . In principle, further arrangements for heat storage, for example a latent heat store, which is not illustrated here, may also be arranged in the hot box  18 . 
         [0032]    The invention has been described in detail with particular reference to preferred embodiments thereof and examples, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention covered by the claims which may include the phrase “at least one of A, B and C” as an alternative expression that means one or more of A, B and C may be used, contrary to the holding in  Superguide v. DIRECTV,  69 USPQ2d 1865 (Fed. Cir. 2004).