Method for operating a redox flow battery

Abstract

The aim of the invention is to allow a redox flow battery (1) to be operated with the highest possible degree of efficiency. This is achieved in that the positive flow rate (M+) of the positive electrolyte (15a) and / or the negative flow rate (M-) of the negative electrolyte (15b) is varied during the operation of the redox flow battery (1) on the basis of the current electrical stack resistance (Rstack) of the at least one cell stack (10) in order to reduce the difference between the current electrical stack resistance (Rstack) of the at least one cell stack (10) and a specified optimum stack resistance (Ropt).

Description

[0001] Method for operating a redox flow battery

[0002] The present invention relates to a method for operating a redox flow battery with at least one cell stack consisting of a plurality of individual cells, wherein each individual cell comprises a positive half-cell with a positive electrode and a negative half-cell with a negative electrode, wherein a positive electrolyte flows through the positive half-cell during operation of the redox flow battery and a negative electrolyte flows through the negative half-cell during operation of the redox flow battery, wherein the positive electrolyte is circulated through the positive half-cells by a positive electrolyte pump with a positive flow rate and the negative electrolyte is circulated through the negative half-cells by a negative electrolyte pump with a negative flow rate. The invention also relates to a redox flow battery operated according to this method.

[0003] A redox flow battery is an electrochemical energy storage device for electrochemically based energy storage and typically consists of storage tanks for storing positive and negative electrolytes, as well as pumps and lines for circulating the electrolytes through one or more cell stacks containing a number of individual cells. The individual cells of the cell stack are each formed by a positive half-cell and a negative half-cell arranged side by side, with the positive and negative half-cells of an individual cell separated from each other by a semipermeable membrane, typically an ion exchange membrane. The semipermeable membrane is, for example, a cation and / or anion exchange membrane, e.g., based on sulfonated tetrafluoroethylene polymer (PTFE). The positive half-cell contains a positive electrode located in a frame, through which the positive electrolyte flows.The negative half-cell contains a negative electrode located in a frame, through which the negative electrolyte flows. The positive and negative electrolytes are circulated separately through the half-cells. The positive and negative electrodes are usually made of porous graphite felts through which the electrolyte can flow. Electrode plates, such as bipolar plates, are arranged between adjacent individual cells of the cell stack as current collectors. These plates are usually made of a composite material made of carbon and plastic.On the axial outer sides of the axially outer individual cells of the cell stack, current collectors are located on the electrode plates. These current collectors provide an electrical contact to the outside, allowing an electrical voltage to be tapped across the entire cell stack (discharging the redox flow battery) or to be applied to the cell stack (charging the redox flow battery). The cell stack is terminated on each axial outer side by an end plate, which holds the cell stack together.

[0004] The electrolyte of a redox flow battery is a liquid and essentially comprises an electrochemical redox couple consisting of a first redox element and a second redox element, each in the form of a redox-active element or ion, or combinations of redox-active elements and / or ions with different electrical charges (oxidation numbers). There are numerous redox-active elements or ions, or combinations of redox-active elements and / or ions, that can be used as redox couples in a redox flow battery. The redox elements of a redox couple are usually dissolved in the electrolyte. The electrolyte is usually an aqueous acid, such as aqueous sulfuric acid. Redox flow batteries with a wide variety of combinations of redox couples are known. Some non-exhaustive examples of known combinations of redox couples are: 2+ / V 3+ vs. V4+ / V 5+ (in a vanadium redox flow battery), V 2+ / V 3+ vs. Br / CIBr2, Br2 / Br vs. S / S 2 ', Br / Br2vs. Zn 2+ / Zn, Ce 4+ / Ce 3+ vs. V 2+ / V 3+ , Fe 3+ / Fe 2+ vs. Br2 / Br, Fe 3+ / Fe 2+ vs. Cr 3+ / Cr 2+ , Mn 2+ / Mn 3+ vs. Br2 / Br, Fe 3+ / Fe 2+ vs. Ti 2+ / Ti 4+ and others. The redox elements can be contained in the electrolyte in various chemical compounds, for example in the form of sulfates, such as vanadium sulfate, or chlorides, such as vanadium chloride. The vanadium-based redox flow battery is the most widely used, which is why the following discussion will primarily focus on the vanadium redox flow battery, without restricting its generality.

[0005] In a vanadium-based redox flow battery, the positive electrolyte in the charged state consists of a redox couple in the form of a vanadium species with the oxidation number +4 (also known as V lv or V 4+ referred to) and a vanadium species with the oxidation number +5 (also known as V v or V 5+ The negative electrolyte, in the charged state, consists of a redox pair in the form of a vanadium species with the oxidation number +2 (also known as V" or V 2+ referred to) and a vanadium species with the oxidation number +3 (also known as V 111 or V 3+ ) - whereby the negative electrolyte has a more negative electrochemical potential than the positive electrolyte.

[0006] During operation of a redox flow battery, electrical energy is delivered to a consumer or absorbed from an energy source. In the example of a vanadium-based redox flow battery, the following well-known chemical reaction (redox reaction) occurs during charging / discharging in a single cell of the redox flow battery: y5+ _|_ 2+ y4+ _|_ 3 + During the charging process, the reaction is initiated by an external current or voltage source, which applies an electrical voltage to a single cell (or the entire cell stack).

[0007] This structure and function of a redox flow battery is well known, for example from WO 2018 / 087220 A1 or WO 2014 / 131702 A1.

[0008] To operate a redox flow battery, the electrolytes must be circulated through the cell stack. This requires electrolyte pumps, which require energy to operate. The inherent energy demand of the electrolyte pumps reduces the efficiency of the redox flow battery. During operation of the redox flow battery, the circulated flow rate of electrolyte must not be too small, as this could lead to depletion of redox elements in the individual cells, which can reduce the efficiency of the cell stack and the redox flow battery. In the worst case, depletion of redox elements can also cause damage to the individual cells. On the other hand, the flow rate of electrolyte should not be greater than necessary, as this would reduce efficiency due to the increased inherent energy demand for circulating the electrolytes.

[0009] It is therefore an object of the present invention to provide a method by which the efficiency of the redox flow battery can be kept at the highest possible value.

[0010] This object is achieved by changing the positive flow rate and / or the negative flow rate during operation of the redox flow battery as a function of a current electrical stack resistance of the at least one cell stack in order to change the current electrical stack resistance to a predetermined optimal stack resistance.

[0011] It was discovered that the relationship between electrolyte flow rate and stack resistance can be used to control the electrolyte flow rate. Maintaining the stack resistance at an optimal level ensures that neither too little nor too much electrolyte is circulated. This allows the efficiency of the redox flow battery to be maintained at the highest possible level within the intended operating range.

[0012] For a redox flow battery, it is advantageous if the positive electrolyte pump and the negative electrolyte pump supply a plurality of cell stacks with the positive electrolyte and the negative electrolyte, respectively. To ensure optimal electrolyte flow rate adjustment, the current stack resistance of each cell stack is determined, and a representative stack resistance is calculated from the determined current stack resistances of the cell stacks, preferably as a median value or as the mean value of the determined stack resistances of the cell stacks. The representative stack resistance is used as the current stack resistance for changing the positive flow rate and / or the negative flow rate. This allows the stack resistances of all cell stacks in the same electrolyte circuit to be brought as close as possible to the optimal stack resistance.By using the mean or median, outliers at the bottom of the cell stacks are not overrepresented.

[0013] Depending on the battery chemistry (redox elements, electrolyte), it may be advantageous to increase the positive flow rate and / or the negative flow rate if the current stack resistance is greater than the specified optimal stack resistance, and to decrease the positive flow rate and / or the negative flow rate if the current stack resistance is less than the specified optimal stack resistance. This is particularly advantageous for a vanadium redox flow battery. However, it could also be the other way around for other battery chemistries.

[0014] The current stack resistance of a cell stack can be easily determined from a known stack voltage and stack current. The stack voltage and stack current will be determined during operation of the redox flow battery anyway and will be known.

[0015] To adjust the stack resistance, it is advantageous to determine a deviation as a function of the stack resistance and the specified optimal stack resistance. From the determined deviation, a correction factor can then be easily determined, which can be used to change a control parameter for controlling the electrolyte pumps to alter the positive flow rate and / or the negative flow rate. This provides a simple way to change the flow rate depending on the stack resistance.

[0016] If a change in the stack resistance is determined from the stack resistance and a previously determined stack resistance, the determined change in the stack resistance can be taken into account when determining the correction factor. This can be used, for example, to determine whether a change in the flow rate is appropriate in the current operating state of the redox flow battery.

[0017] It is particularly advantageous to consider the aging of the redox flow battery in the specified optimal stack resistance. This allows aging factors to be taken into account when controlling the electrolyte pumps, and the flow rate can also be adjusted depending on the aging condition. While a redox flow battery allows for many years of operation, aging can impair the efficiency of the redox flow battery. By considering aging in the optimal stack resistance, the impact of aging on efficiency can be minimized.

[0018] In a simple implementation, a resistance characteristic curve is used for the redox flow battery. This curve maps the initial stack resistance of the redox flow battery as an optimal stack resistance depending on at least one operating parameter characterizing the redox flow battery's operating state. During operation of the redox flow battery, the optimal stack resistance is then determined from the resistance characteristic curve depending on the current operating state.

[0019] Aging can be taken into account with the resistance characteristic curve by changing the optimal stack resistance determined from the resistance characteristic curve by an aging factor of the redox flow battery and using the changed optimal stack resistance as the specified optimal stack resistance.

[0020] The present invention will be explained in more detail below with reference to Figures 1 to 7, which show exemplary, schematic and non-limiting advantageous embodiments of the invention.

[0021] Fig.1 the basic operating principle of a redox flow battery,

[0022] Fig.2 a redox flow battery with a cell stack,

[0023] Fig.3 shows the structure of a cell stack of a redox flow battery,

[0024] Fig.4 a redox flow battery with several cell strings consisting of several cell stacks,

[0025] Fig.5 a resistance characteristic of the redox flow battery,

[0026] Fig.6 shows a change in the flow rate of electrolyte during charging and discharging of a redox flow battery, and

[0027] Fig.7 shows the change in stack resistance when taking into account the aging of the redox flow battery.

[0028] In order to better understand the invention, the structure and operation of a redox flow battery 1 is briefly explained below with reference to Fig.1 to Fig.4.

[0029] Fig. 1 shows a schematic structure of a redox flow battery 1 using a single cell 2 of a cell stack 10 to explain the well-known functional principle of a redox flow battery 1. For better explanation and illustration, Fig. 1 shows only a single cell 2 of a cell stack 10 of a redox flow battery 1, whereby a cell stack 10 will generally have a plurality of single cells 2 arranged side by side in the cell stack 10.

[0030] A single cell 2 consists of two half-cells 2a, 2b, which form a positive reaction chamber 3a and a negative reaction chamber 3b. The two half-cells 2a, 2b, or the positive reaction chamber 3a and the negative reaction chamber 3b, are separated by a semipermeable, in particular ion-selective, membrane 4. The reaction chambers 3a, 3b are formed, for example, in recesses 6a, 6b of frames 5a, 5b. A cell electrode 7a, 7b is arranged in each of the frames 5a, 5b, in the reaction chambers 3a, 3b, or in the recesses 6a, 6b, which is in contact with the respective electrolyte 15a, 15b located in the half-cell 2a, 2b. Electrolytes 15a, 15b with different electrical charges (positive and negative electrolyte) flow through the recesses 6a, 6b and, if applicable, the cell electrodes 7a, 7b arranged therein of a single cell 2.Each of the electrolytes 15a, 15b contains a redox pair with specific, time-varying concentrations (depending on the state of charge) of redox elements. The semipermeable, particularly ion-selective, membrane 4 can be made, for example, of sulfonate-modified polytetrafluoroethylene (PTFE), with the trade name Nation™, and enables ions to achieve charge equalization between the positive reaction space 3a and the negative reaction space 3b (or between the electrolytes 15a, 15b contained therein). Power connections 11, 12 are also provided on a cell stack 10 of a redox flow battery 1 in order to tap an electrical cell stack voltage Vz applied to the cell stack 10 via a consumer 14 (discharging the redox flow battery 1) or to apply an electrical cell stack voltage Vz to the cell stack 10 (charging the redox flow battery 1).

[0031] An electrical load 14 can take any form. Based on the electrical voltage, current, or power requirements of the electrical load 14, a cell stack 10 can be configured in a redox flow battery 1 to provide the necessary electrical voltage and / or the necessary electrical current. Redox flow batteries 1 are often used as stationary energy storage devices, for example, to serve as emergency power systems for industrial plants, storage systems for renewable energy (photovoltaics, wind power), and the like. Consequently, depending on the application, a person skilled in the art can design or select a cell stack 10, or a parallel and / or serial connection of multiple cell stacks or cell strings consisting of multiple cell stacks, and redox pairs in a redox flow battery 1.

[0032] The electrolytes 15a, 15b are stored in storage tanks 13a, 13b and are circulated from there through the cell stack 10 by electrolyte pumps 9a, 9b, specifically through a half-cell 2a, 2b of a single cell 2 of the cell stack 10. For this purpose, a supply line 16a, 16b and a discharge line 17a, 17b are provided for each electrolyte 15a, 15b, which are connected via electrolyte connections 22a, 22b, 23a, 23b to the corresponding electrolyte channels 18a, 18b, 19a, 19b (see Fig. 3) in the cell stack 10.

[0033] In a cell stack 10 with several adjacent individual cells 2, an electrode plate 8, such as a bipolar plate, is arranged between each two adjacent individual cells 2 (Fig. 3). At the outer ends of the cell stack 10, a power connection 11, 12 can be located on the outer electrode plates 8 or on the outer half-cells 2a, 2b (or cell electrodes 7a, 7b) of the cell stack 10, which can be electrically contacted from the outside.

[0034] The typical structure of a cell stack 10 of a redox flow battery 1 is explained in more detail with reference to Figs.2 and Fig.3.

[0035] A cell stack 10 of a redox flow battery 1 comprises at least one individual cell 2, generally a plurality of individual cells 2, which in turn are each formed from two frames 5a, 5b of half-cells 2a, 2b. A frame 5a, 5b is preferably made of a plastic, such as an elastomer, such as a polyolefinic thermoplastic elastomer (TPE or TPO), such as Santoprene®, or a thermoplastic vulcanate (TPV), in particular using an injection molding process. In the stack direction R (in the direction in which the individual cells 2a, 2b are arranged next to one another), between two frames 5a, 5b of an individual cell 2 in the cell stack 10, a semipermeable membrane 4, typically an ion exchange membrane (either a cation or anion exchange membrane, e.g. Nation®), is arranged.The membrane 4 separates the reaction spaces 3a, 3b, recesses 6a, 6b of the half-cells 2a, 2b of a single cell 2, the cell electrodes 7a, 7b arranged therein, and the electrolyte liquids 15a, 15b contained therein. Between each two individual cells 2 adjacent in the stack direction R, an electrode plate 8, e.g., a bipolar plate, is arranged in the cell stack 10. The electrode plate 8 is, as shown in Fig. 3, inserted into mutually facing recesses 32 in the frames 5a, 5b. The frames 5a, 5b have central recesses 6a, 6b that run through in the stack direction R, each forming a reaction space 3a, 3b and in which cell electrodes 7a, 7b, e.g., mats made of carbon fiber, are arranged, as shown in Fig. 3.

[0036] The differently charged electrolytes 15a, 15b are pumped through the recesses 6a, 6b in the frames 5a, 5b through the individual cells 2, with an electrolyte 15a, 15b with a different electrical charge flowing through the cell electrode 7a, 7b of each half-cell 2a, 2b of an individual cell 2. The electrolytes 15a, 15b are supplied and removed from the outside via electrolyte connections 22a, 22b, 23a, 23b and are then distributed internally via an electrolyte channel system provided in the frames 5a, 5b with electrolyte channels 18a, 18b, 19a, 19b. The electrolyte connections 22a, 22b, 23a, 23b are provided, for example, on an end plate 24 of the cell stack 10, as shown in Fig.3, although other arrangements of the electrolyte connections 22a, 22b, 23a, 23b, for example on an end frame 20, are also possible.

[0037] The cell stack 10 can be closed off in the stack direction R by an end frame 20 at each of its two axial ends. An electrically conductive current collector 21 is arranged in the end frame 20, e.g., in a recess on one end face of the end frame 20, and is connected to an externally routed electrical power connection 11, 12. In the illustrated embodiment, the current collector 21 rests against the last electrode plate 8 of the last individual cell 2 to establish electrical contact. However, the current collector 21 or a power connection 11, 12 could also be designed differently. Likewise, the end frame 20 could be omitted from the cell stack 10.

[0038] In the illustrated embodiment, the cell stack 10 is arranged between two rigid end plates 24 and pressed together by clamping means 25. The clamping means 25 are designed, for example, with through-reaching bolts 26, nuts 27, washers 28, and springs 29, as shown in Fig. 2. However, the cell stack 10 can also be held together in other ways; in particular, the clamping means 25 can be designed differently. The two end plates 24 can also be arranged between two pressure plates 30, which are pressed together by the clamping means 25, as shown in Fig. 2. To prevent the frames 5a, 5b from settling due to the contact pressure of the clamping means 25, a spacer 31 can also be provided between the end plates 24.

[0039] However, the present invention is not limited to a specific embodiment of a half-cell 2a, 2b, a single cell 2, a cell stack 10, or the redox flow battery 1. Nor is the invention limited to a specific electrolyte 15a, 15b or to specific redox elements in the electrolytes 15a, 15b. The above statements regarding a redox flow battery 1 serve merely to facilitate understanding of the invention.

[0040] From the above description and the basic operating principle, it is also obvious that there is a positive electrolyte 15a and a negative electrolyte 15b, as well as a positive half-cell 2a and a negative half-cell 2b of a single cell 2 of a cell stack 10 and a positive electrolyte pump 9a and a negative electrolyte pump 9b.

[0041] A redox flow battery 1 can also comprise a plurality of cell stacks 10, as explained with reference to Fig. 4. In the embodiment according to Fig. 4, two cell strings 40 are provided, wherein each cell string 40 comprises a plurality of cell stacks 10, for example as described above. The cell stacks 10 in a cell string 40 can be electrically connected in series via the power connections 11, 12 of the cell stacks 10 (not shown in Fig. 4 for reasons of clarity). However, a different electrical connection of the cell stacks 10 in a cell string 40 is also possible, such as a parallel connection or a mixture of series and parallel connections. In the embodiment of Fig. 4, the cell strings 40 are electrically connected in parallel (not shown in Fig. 4 for reasons of clarity). However, a different electrical connection of the cell strings 40 is also possible, such as a parallel connection or a mixture of series and parallel connections.Such an electrical interconnection of cell strings 40 can also be referred to as a single battery 41. A redox flow battery 1 can comprise several such single batteries 41, wherein the single batteries 41 can in turn be electrically interconnected by a series or parallel connection, or a combination of a series and parallel connection. This allows a redox flow battery 1 to be realized with the desired electrical output voltage and the desired electrical power.

[0042] Each cell stack 10 is supplied with a positive electrolyte 15a and a negative electrolyte 15b. A storage tank 13a for the positive electrolyte 15a and a storage tank 13b for the negative electrolyte 15b are provided for this purpose. The positive electrolyte 15a is circulated by a positive electrolyte pump 9a, and the negative electrolyte 15b by a negative electrolyte pump 9b.

[0043] In the case of multiple individual batteries 41, a storage tank 13a for the positive electrolyte 15a and a storage tank 13b for the negative electrolyte 15b, as well as a positive electrolyte pump 9a and a negative electrolyte pump 9b, can be provided for each individual battery 41. Thus, the individual batteries 41 would be electrically connected to one another, but would be designed with separate electrolyte circuits. The present invention relates to an electrolyte circuit comprising at least one cell stack 10, typically several cell stacks 10, which can also be connected to form several cell strings 40.

[0044] In the minimal configuration, the redox flow battery 1 comprises at least one cell stack 10 consisting of a plurality of individual cells 2, wherein each individual cell 2 comprises a positive half-cell 2a with a positive cell electrode 7a and a negative half-cell 2b with a negative cell electrode 7b, wherein the positive half-cell 2a is flowed through by the positive electrolyte 15a during operation of the redox flow battery 1 and the negative half-cell 2b is flowed through by the negative electrolyte 15b during operation of the redox flow battery 1. For a specific electrical current in the cell stack 10, a specific flow rate M of electrolytes 15a, 15b is required through the cell stack 10. The flow rate M+ of the positive electrolyte 15a and the flow rate M- of the negative electrolyte 15b are adjusted by the associated positive electrolyte pump 9a and the negative electrolyte pump 9b.Operating the redox flow battery 1 at maximum flow rate would enable maximum current at every operating point and would be conceivable in principle, but would make battery operation inefficient because at many operating points of the redox flow battery 1, neither the maximum current nor the maximum flow rate is required, but rather only a lower electrical current depending on the currently set electrical power. This would result in more electrolyte 15a, 15b being circulated than would be required for the operation of the redox flow battery 1 itself. The inherent energy requirement for operating the electrolyte pumps 9a, 9b would therefore reduce the efficiency of the redox flow battery 1. On the other hand, measurements on redox flow batteries 1 have shown that the performance of cell stacks 10 deteriorates with insufficient flow of electrolytes 15a, 15b.It has also been shown that if the flow rate is too low, irreparable damage to the cell stack 10 can occur due to the destruction of individual cells 2 of the cell stack 10. The reason for this is, for example, that if the flow rate M is insufficient, the electrochemically convertible redox elements in the individual cells 2 are consumed faster than they are supplied, thus leading to depletion of redox elements. This can increase the electrical resistance and the generated waste heat, and only hydrogen is produced at the negative half-cell 2a, while the electrode material in the negative half-cell 2a is oxidized and damaged. Apart from that, it has been shown that the minimum required flow rate M+, M- changes with the aging of the cell stack 10 of the redox flow battery 1.In particular, it was found that the efficiency of the cell stacks 10 of a redox flow battery 1 deteriorates with increasing age and higher flow rates M+, M- are required.

[0045] If the redox flow battery 1 is not to be operated with a maximum or excessive flow of electrolytes 15a, 15b, a control unit 60 (Fig. 4) is required, which controls the flow rate M+, M- of the electrolytes 15a, 15b in such a way that, on the one hand, the efficiency of the redox flow battery 1 is not reduced by the increased energy demand of the electrolyte pumps 9a, 9b, and, on the other hand, the flow rate M+, M- does not become too low to risk damage to the cell stack 10. The control unit 60 should preferably also take aging effects into account.

[0046] Based on measurements and investigations on redox flow batteries 1, it was found that the stack resistance R staCk is suitable for controlling the flow rate M+, M- at electrolytes 15a, 15b because it was recognized that the stack resistance Rstack depends on the flow rate M+, M- at electrolytes 15a, 15b.

[0047] According to the invention, a control unit 60 is therefore provided, which is provided to change the positive flow rate M+ of the positive electrolyte circulated through the cell stacks 10 by the positive electrolyte pump 9a and / or the negative flow rate M- of the negative electrolyte 15b circulated through the cell stacks 10 by the negative electrolyte pump 9b as a function of a current electrical stack resistance Rstack of the at least one cell stack 10 of the redox flow battery 1, in order to determine the difference between a current electrical stack resistance Rstack and a predetermined optimal stack resistance R opt, preferably to reduce the current electrical stack resistance Rstack to the specified optimal stack resistance R op t. The optimal stack resistance R op t ensures that the cell stack 10 is operated with a flow rate M+, M- that is neither too high nor too low. Due to the optimal stack resistance R opt Preferably, aging effects of the cell stack 10 are also taken into account.

[0048] The stack resistance Rstack is the electrical resistance of a cell stack 10 and is essentially composed of the electrical resistances of the cell electrodes 7a, 7b, the semipermeable membranes 4 and the electrolytes 15a, 15b in the cell stack 10.

[0049] The electrode component of the stack resistance Rstack is primarily composed of the contact resistance between the electrode components, such as current collectors, bipolar plates, and cell electrodes, as well as the design, material, and structure of the cell electrodes 7a, 7b. The electrode component of the stack resistance Rstack can change over time. The influence of the cell electrode 7a, 7b is greatest here and can change most over time due to aging. The electrical resistance of a membrane 4 can also increase during operation of the redox flow battery 1 due to aging phenomena, but in a simplified approach, it can also be considered constant.The electrical resistance of the electrolyte 15a, 15b, in turn, depends on the stoichiometrically available species of the redox elements and on the reaction kinetics of the redox reaction, i.e., it depends on the mass transfer, the well-known Nernst equation, the electrical overpotential, and the activation overpotential, which means that this resistance component also depends on the operation of the redox flow battery 1. In principle, however, no change in the stack resistance R is necessary. staC k due to operation, but manufacturing-related differences in the components of the cell stack 10 alone can lead to a certain variance in individual stack resistances R sta ck. It has been shown that the stack resistance R staC k is significantly influenced by the flow rate M+, M- of electrolyte 15a, 15b. A high flow rate M+, M- reduces the stack resistance R stack and increases the electrical current through the cell stack 10. Conversely, a low flow rate M+, M- increases the stack resistance R staC k and reduces the electrical current through the cell stack 10. A high stack resistance Rstack is therefore an indication of a possibly too low flow rate M+, M-. Therefore, a primary goal of the invention is to control the flow rate M+, M- so that the stack resistance Rstack does not become too high.

[0050] The stack resistance R s t aCk of a cell stack 10 but also the performance of a cell string 40 consisting of several cell stacks 10 connected electrically in series. The cell stack 10 with the highest stack resistance Rstack thus determines the possible electrical current in the cell string 40 and thus limits the possible charging and discharging power of the cell string 40 and the redox flow battery 1 . As already explained above, the flow rate M+, M- should not be too high, which would reduce the stack resistance Rstack, because the efficiency of the redox flow battery 1 would decrease due to the inherent energy requirement of the electrolyte pump 9a, 9b.

[0051] The present invention described below provides a method for operating the cell stack 10 with the most optimal flow rate M+, M- possible, i.e. neither with too low a flow rate M+, M- nor with too high a flow rate M+, M-.

[0052] The current stack resistance Rstack of a cell stack 10 can be calculated using Ohm's law from the measured current stack voltage V sta ck and the measured current stack current

[0053] Istack can be determined from For each cell stack 10 in an electrolyte circuit The current stack resistance Rstack of the redox flow battery 1 can thus be determined at any time. During operation of the redox flow battery 1, knowledge of the current stack voltage Vstack and the current stack current Istack can be assumed as basic parameters for the operation of the redox flow battery 1.

[0054] The measured stack voltage Vstack can also be related to a reference voltage, for example, the open-circuit voltage Vocv of a single cell 2, which indicates the difference in the electrical potentials of the two electrolytes 15a, 15b. The above equation then expands to , with the number n of single cells 2 in the cell stack.

[0055] 10. However, a different reference voltage could also be used. The open-circuit voltage Vocv can also be measured on the redox flow battery 1, for example, with a known measuring cell 45 as shown in Fig. 4. The measuring cell 45 is constructed like a single cell 2 of the cell stack 10 and, like the single cell 2, is flowed through by the electrolytes 15a, 15b. A voltage measurement 46 between the two half-cells of the measuring cell 45 then provides the open-circuit voltage Vocv.

[0056] The optimal stack resistance R opt can be determined from a known resistance characteristic curve 50 of the cell stack 10. The stack resistance Rstack is not constant across all operating points of the redox flow battery 1, but is also influenced by the operation of the redox flow battery 1. In particular, the stack resistance R s tack depends on the charge level SoC of the electrolytes 15a, 15b of the cell stack 10 and the electrolyte temperature t E. The resistance characteristic curve 50 is an empirically determined curve or a curve calculated from a mathematical model of the cell stack 10 or a curve determined in a simulation of the redox flow battery 1. The resistance characteristic curve 50 represents the initial state of the redox flow battery 1, i.e., the initial stack resistance Rstack, opt.o, which is determined, for example, after the redox flow battery 1 has been manufactured. The resistance characteristic curve 50 represents the optimal state, i.e., the state where no or at least only negligible depletion phenomena of redox elements occur in the individual cells 2 of the cell stack 10, which is ensured at each point on the resistance characteristic curve 50 by a specific flow of electrolytes 15a, 15b through the individual cells 2. The resistance characteristic curve 50 thus specifies, for each charge level SoC and each electrolyte temperature t E the optimal stack resistance.

[0057] Fig.5 shows an example of such a resistance characteristic curve 50. In it, the stack resistance Rstack, opt, o for different electrolyte temperatures t E plotted against the SoC charge level. However, it would also be possible to use stack resistance Rstack, Op t,o for different charge levels SoC over the electrolyte temperature t E This results in a set of curves with several characteristics 51 for different electrolyte temperatures t E or charge levels SoC. Between the electrolyte temperatures t E or charge levels SoC can of course be interpolated. The resistance characteristic 50 could also be represented as a three-dimensional map with the stack resistance Rstack, Op t,o over the electrolyte temperatures t E and the charging level SoC must be implemented.

[0058] The resistance characteristic curve 50 can be stored in the control unit 60 as a table. Interpolation can be performed between table entries. The resistance characteristic curve 50, or the individual characteristic curves 51, can also be approximated by a mathematical function by adapting the function to the resistance characteristic curve 50 as best as possible. Such curve approximation methods are well known. In this case, only the determined mathematical function, or various mathematical functions for different electrolyte temperatures t E or charge levels SoC, are stored.

[0059] The resistance characteristic curve 50 can also differ for the charging and discharging operation of the redox flow battery 1, which is why there can be different resistance characteristic curves 50 for the charging and discharging operation. In this case, only the resistance characteristic curve 50 corresponding to the current operation must be used.

[0060] However, the relationship between the degree of charge SoC, the electrolyte temperature tE and the optimal stack resistance opt does not necessarily have to be in the form of a resistance characteristic curve 50. In various redox flow batteries 1, the optimal stack resistance R op t also from other or additional operating parameters of the redox flow battery 1 than the charge level SoC and the electrolyte temperature t E , as in a vanadium redox flow battery 1 . The optimal stack resistance R op t can also be in another suitable form depending on the charge level SoC and the electrolyte temperature t E , or other or additional operating parameters of the redox flow battery 1 . For example, the optimal stack resistance R op t may also depend on operating parameters such as the concentration of electrochemically active substances (redox elements) or the pH value of the electrolyte.

[0061] Furthermore, it should be noted that at least one of the electrolyte pumps 9a, 9b is controlled in such a way that a flow rate M+, M- is set so that the current electrical stack resistance Rstack is adjusted to the predetermined stack resistance R op t is adjusted. It is advantageous if both electrolyte pumps 9a, 9b are adjusted depending on the stack resistance R sta ck so that both flow rates M+, M- are optimally adjusted.

[0062] In order to achieve the optimal stack resistance R op t, in an advantageous embodiment, the resistance characteristic 50 is used to calculate the current operating point (SoC, t E ) associated stack resistance R opt read out, if necessary by interpolation. The stack resistance Ro obtained from the resistance characteristic curve 50 Pt is then used to control the flow rates M+, M- of the electrolytes 15a, 15b. At least one of the electrolyte pumps 9a, 9b is controlled in such a way that a flow rate M+, M- is set such that the current electrical stack resistance Rstack, which can be determined as explained above, is adjusted to the optimal stack resistance R obtained from the resistance characteristic curve 50. op t changes.

[0063] A possible strategy for controlling the electrolyte pumps 9a, 9b is described below. A discrete-time implementation is described below, although this is not necessarily the case. The time step t k is therefore not to be understood as limiting. The following functions and parameters therefore also apply without the time t k Validity.

[0064] At specified time intervals t k, for example in the second or minute range, the current stack voltage V staC k(tk) of the cell stack 10 (or the cell stacks 10 in the electrolyte circuit) is determined and from this the current stack resistance R sta ck(tk) (or the current stack resistances R s tack(tk)). A current time t k The optimal stack resistance Ropt(tk) is determined, for example, from the resistance characteristic curve 50 or specified in another way. This allows the deviation AR(t k ) of the current stack resistance R s tack(tk) (or the current stack resistance R s tack(tk)) from the current optimal stack resistance Ropt(tk). In general, for a cell stack 10, the deviation AR(t k ) as a function f from the current stack resistance R s tack(tk) and the current optimal stack resistance Ropt(tk), i.e. AR(t k ) = f(R s tack(tk), R opt(tk)), for example in the form

[0065] From the deviation AR(t k ) a correction factor SPkorr(tk) can be determined, with which a control parameter SPa, SPb for controlling at least one electrolyte pump 9a, 9b is changed in order to change the flow rate M+, M-. The control parameter SPa, SPb can be adjusted in the next time step t k +i. If the time step t k is sufficiently long, then the control parameter SPa, SPb can also be set in the same time step t k Therefore, the target control parameter SP SO II the time step is not specified.

[0066] In general, the control parameter SPa, SPb is a function sp from a current control parameter SPa|bist(tk), here at time t k , and the correction factor SPkorr(tk), i.e. SPa|b = sp(SPa|bj St(tk), SPkorr(tk)). The control parameter SPa, SPb results, for example, as the sum of the respective current control parameter SPa|b ist and the correction factor SPkorr, alSO SPa| b = SPa|bist(tk) + SPkorr(tk)-

[0067] The notation a|b is a shortened notation and refers to the respective size for both the positive electrolyte 1a and the negative electrolyte 15b.

[0068] The correction factor SPkorr(tk) is a function korr of the deviation AR(t k ), so SPcorr(tk) = corr(AR(t k The correction factor SPkorr(tk) in percent can be used, for example, of the current control parameter SPj S t in percent.

[0069] From the current stack resistance R sta ck(tk) and a predetermined stack resistance Rstack.past, for example R s tack,past = Rstack(tk-x), preferably with x=1, a change AR stack(tk) of the stack resistance can be determined, for example from R R ck tk ■ The change in the stack resistance AR sta ck(tk) can also if used to determine the correction factor SPkorr to prevent the stack resistance from changing too much. The correction factor SPkorr(tk) would then be a function korr of the deviation AR(t k ) and the change in the stack resistance AR s tack(tk), so SPcorr(tk) = corr(AR(t k ), AR s tack(tk)). The correction factor SPkorr can then be used, for example, parameters SPa, SPb can be determined using the given function sp.

[0070] The use of the change in stack resistance ARstack in the form R — R

[0071] AR si.acK . = stack - past - tack_ i n the determination of the correction factor SPkorr has the additional stack, past

[0072] Advantage that the flow rate M+, M- is only changed when the stack resistance changes. If there is no change, = 0, then the

[0073] The correction factor SPkorr is zero, and the flow rate M+, M- remains unchanged. This prevents unnecessary energy consumption for the circulation of the electrolytes 15a, 15b.

[0074] When determining the correction factor SPkorr(tk), additional parameters can also be included to influence the correction. For example, a gain factor p, preferably p>1, can be selected as needed. This would result in SPkorr = korr(AR, p) or

[0075] This is explained below using some examples.

[0076] Assume that the current control parameter SPa|b ist = 70%, meaning that the electrolyte pumps 9a, 9b are operated at 70% of their maximum flow. The current optimal stack resistance R op t(tk) was specified as 50mQ (for example from the resistance characteristic curve 50) and as the current stack resistance R staC k a value of 55mQ was determined. The stack resistance at the previous time R sta ck(tk-1) is 56 mQ. The gain factor p is set to 5. In this example, the current stack resistance R staC k is therefore greater than the optimal stack resistance R op t(tk). The current flow is therefore too low and these values ​​a correction factor SPkorr of 0.89%, from which the control parameter SPa|b S oii to 70.89%. The electrolyte pumps 9a, 9b are now controlled with this target control parameter SPsoii, which increases the flow.

[0077] In another example, let the stack resistance at the previous time be R stack(tk-1) is 54 mQ smaller, and all other values ​​are assumed to be the same. This would result in a correction factor SPkorr of -0.93% and a control parameter SPa|b of 69.07%, which would reduce the flow rate.

[0078] Fig. 6 shows the effect of the inventive control of the electrolyte pumps 9a, 9b. The upper diagram shows the discharge operation of the redox flow battery 1 and the lower diagram the charge operation of the redox flow battery 1. In both diagrams, the stack resistance R s t aC k, as well as the flow rates M+, M- of the electrolytes 15a, 15b, each over the charge level SoC. During discharge operation, it can be seen that with decreasing charge level SoC, the risk of depletion increases, which is also reflected in increasing stack resistance R staCk. As a result, the flow rates M+, M- of the electrolytes 15a, 15b increase with decreasing charge level SoC. The same essentially applies to charging operation, where the stack resistance Rstack also increases due to the depletion of the available redox elements for the redox reaction.

[0079] However, it is immediately apparent from Fig. 6 that the electrolyte pumps 9a, 9b are operated with high to maximum flow rates M+, M- only at the edge of the charge level SoC, i.e., at high or low charge levels. Over a wide range of the charge level, the electrolyte pumps 9a, 9b are operated with a low flow rate, which is the maximum flow rate, thereby increasing the efficiency of the redox flow battery 1.

[0080] As a rule, in a redox flow battery 1, several cell stacks 10 are supplied by a common electrolyte circuit, such as a cell string 40 consisting of several cell stacks 10 that are supplied by an electrolyte circuit, as in Fig. 4. However, since such a common electrolyte circuit only has one electrolyte pump 9a, 9b for the positive electrolyte 15a and the negative electrolyte 15b, the electrolyte pumps 9a, 9b cannot simultaneously change the stack resistances Rstack of all cell stacks 10 in the electrolyte circuit independently of one another. In order to be able to apply the invention nevertheless, in such a case, a representative stack resistance is determined as the current stack resistance R from the stack resistances Rstack, N of the N cell stacks 10 in the common electrolyte circuit. stack, which is used to control the flow rates of the electrolyte pumps 9a, 9b as described above. The representative stack resistance R sta ck can, for example, be the median value of all determined stack resistances Rstack, N of the N cell stacks 10, or also an average value of all determined stack resistances Rstack, N of the N cell stacks 10. Any suitable average value can be used as the average value, such as the arithmetic mean or the geometric mean. In this case, too, the invention ensures that the stack resistances R sta ck,N of the N cell stacks 10 not too far from the optimal stack resistance R op t may differ.

[0081] During the operation of the redox flow battery 1, the stack resistance Rstack changes due to aging phenomena, especially after many years of battery operation. Typically, the stack resistance R staCk compared to the initial stack resistance Rstack, o at the start of the redox flow battery 1. The current stack resistance Rstack can thus be expressed with the initial stack resistance Rstack, o and an aging factor A as R stack = R stack 0 -A . This aging factor A is therefore included in every measurement of the cell voltage Vstack, or in other words, this aging factor A is included in the measured cell voltage Vstack. The measured cell voltage Vstack can therefore also be expressed with a known initial cell voltage Vstack, o as

[0082] V t t = V t , n -A . The stack resistance RstiaCTcukix erg CJ is then made up of ■ A or ■A n- V ocr ) The aging factor A can thus be calculated from the determined stack resistance Rstack and the known initial stack resistance R sta ck,o can be determined.

[0083] For example, the aging factor A can be expressed in the form -4 = 1 + * stack - stack ^ be determined den. For Rstack, o Rstack the aging factor A=1 can be set.

[0084] The aging factor A can be determined regularly. However, since aging will only become noticeable very slowly, it is sufficient to determine the aging factor at relatively long intervals, such as monthly, quarterly, or annually. Of course, there is nothing wrong with determining the aging factor A more frequently.

[0085] With the known aging factor A, the optimal stack resistance R op t, for example obtained from the resistance characteristic curve 50, can be adjusted, for example as

[0086] Alternatively, the characteristic curves 51 of the resistance characteristic curve 50 can be approximated by a mathematical function and then the optimal stack resistance R optThe aging factor A can be taken into account in the mathematical function. The characteristic curves 51 can be calculated, for example, using the function -

[0087] R = R stat;k t 0 SoC)-e f ' Bs ) -A. Where g=2100 [°K] is a constant and t E N is a given normalized electrolyte temperature. Rstack, Op t,o is the initial optimal stack resistance at the normalized electrolyte temperature. If the electrolyte temperature during operation of the redox flow battery 1 ranges between 25°C and 45°C, then the normalized electrolyte temperature can be set to 37°C, for example. The aging factor A (>1) shifts the characteristic curves 51 , whereby the optimal stack resistance R opt increases with age, which is especially true for a vanadium redox flow battery 1. Depending on the type of redox flow battery 1, the optimal stack resistance Rop t but also decrease with increasing age A.

[0088] In the given optimal stack resistance R opt The aging of the redox flow battery 1 is therefore already shown, ie the optimal stack resistance R opt over the operating time of the redox flow battery 1.

[0089] Fig.7 shows an example of the influence of the aging factor A on the stack resistance Rstack. It shows a charging process of the redox flow battery 1 . During the charging process, a change in the aging factor A is detected. This determines the specified optimal stack resistance R opt increased, which increases the flow rate M+, M-, which in turn leads to a reduction in the stack resistance R sta ck leads.

[0090] The degree of charge (SoC) can be assumed to be known during operation of a redox flow battery 1, since the degree of charge (SoC) is one of the most important battery parameters and is likely always determined. There are also various known approaches for determining the degree of charge (SoC), which need not be discussed here. The electrolyte temperature t E can be easily measured using a temperature sensor.

Claims

Patent claims 1. A method for operating a redox flow battery (1) with at least one cell stack (10) consisting of a plurality of individual cells (2), wherein each individual cell (2) comprises a positive half-cell (2a) with a positive electrode (7a) and a negative half-cell (2b) with a negative electrode (7b), wherein a positive electrolyte (15a) flows through the positive half-cell (7a) during operation of the redox flow battery (1), and a negative electrolyte (15b) flows through the negative half-cell (2b) during operation of the redox flow battery (1), wherein the positive electrolyte (15a) is circulated through the positive half-cells (2a) by a positive electrolyte pump (9a) with a positive flow rate (M+) and the negative electrolyte (15b) is circulated through the negative half-cells (2b) by a negative electrolyte pump (9b) with a negative flow rate (M-). is circulated, characterized in thatthat the positive flow rate (M+) and / or the negative flow rate (M-) during operation of the redox flow battery (1) depends on a current electrical stack resistance (R, sta ck) of the at least one cell stack (10) is changed in order to compensate for the difference between the current electrical stack resistance (Rstack) of the at least one cell stack (10) and a predetermined optimal stack resistance (R op t), preferably to reduce the current electrical stack resistance (Rstack) to the specified optimal stack resistance (R opt ) to change.

2. Method according to claim 1, characterized in that the positive electrolyte pump (9a) and the negative electrolyte pump (9b) supply a plurality N of cell stacks (10) with the positive electrolyte (15a) and the negative electrolyte (15b), that the current stack resistance Rstack, N of each of the N cell stacks (10) is determined and a representative stack resistance is formed from the determined current stack resistances Rstack, N of the N cell stacks (10), preferably as a median value or as the mean value of the determined stack resistances Rstack, N of the N cell stacks (10), and that the representative stack resistance is used as the current stack resistance (Rstack) for changing the positive flow rate (M+) and / or the negative flow rate (M-).

3. Method according to claim 1 or 2, characterized in that the positive flow rate (M+) and / or the negative flow rate (M-) is increased if the current stack resistance (Rstack) is greater than the predetermined optimal stack resistance (Ropt) and the positive flow rate (M+) and / or the negative flow rate (M-) is reduced if the current stack resistance (Rstack) is smaller than the predetermined optimal stack resistance (R op t).

4. Method according to one of claims 1 to 3, characterized in that the current stack resistance (R s t aC k) a cell stack (10) from a known stack voltage V stack and a known stack current l staC k is determined.

5. Method according to one of claims 1 to 4, characterized in that a deviation AR as a function f from the current stack resistance (R s t aCk) and the specified optimal stack resistance (R op t), so AR = f(R s tack, R op t), preferably in the form that the determined deviation AR is a correction factor R opt SPkorr is determined, and that a control parameter (SPa, SPb) for controlling at least one of the electrolyte pumps (9a, 9b) is changed with the correction factor SPkorr in order to change the positive flow rate (M+) and / or the negative flow rate (M-).

6. Method according to claim 5, characterized in that the control parameter (SPa, SPb) is determined with a function sp from a current control parameter SPa|b is t and the correction factor SPkorr, i.e. SPa|b = sp(SPa|b ist (tk), SPkorr(tk)), preferably as a sum of the current control parameter SPa|b ist and the correction factor SPkorr, i.e. SPa|b = SPa|b ist + SPcorr.

7. Method according to one of claims 5 or 6, characterized in that from the current stack resistance (R staC k) and a predetermined stack resistance R s t a ck, pas t a change AR staC k of the stack resistance is determined, preferably R — R ( t from Ml stack (t k ) = stack - past - stack k k , and that the determined change in stack resistance of the ARstack is taken into account when determining the correction factor SPkorr.

8. Method according to one of claims 1 to 7, characterized in that in the predetermined optimal stack resistance (R op t) ageing of the redox flow battery is taken into account.

9. Method according to one of claims 1 to 7, characterized in that a resistance characteristic (50) is used for the redox flow battery (1), which determines the initial stack resistance (RstaC k,o P t,o) depending on at least one operating parameter of the redox flow battery (1) characterizing an operating state of the redox flow battery (1) as an optimal stack resistance (R opt ) and that during operation of the redox flow battery (1) the optimal stack resistance (R opt ) is determined from the resistance characteristic curve (50) depending on a current operating state of the redox flow battery (1).

10. The method according to claim 9, characterized in that the resistance characteristic (50) represents the initial stack resistance (Rstack, Op t,o) as a function of a degree of charge (SoC) of the redox flow battery (1) and of an electrolyte temperature (ts) of the positive electrolyte (15a) or negative electrolyte (15b).

11. The method according to claim 9 or 10, characterized in that the optimal stack resistance determined from the resistance characteristic (50) is changed by an aging factor (A) of the redox flow battery (1) and the changed optimal stack resistance is used as a predetermined optimal stack resistance (R opt ) is used.

12. The method according to claim 9 or 10, characterized in that the resistance characteristic (50) comprises at least one characteristic (51) which is described by a mathematical function, wherein the mathematical function contains an aging factor (A) of the redox flow battery (1) which changes the at least one characteristic (51) depending on the aging factor (A), and in that the optimal stack resistance (R op t) is calculated as a function of a current aging factor using the mathematical function.

13. Redox flow battery with at least one cell stack (10) consisting of a plurality of individual cells (2), wherein each individual cell (2) comprises a positive half-cell (2a) with a positive electrode (7a) and a negative half-cell (2b) with a negative electrode (7b), wherein a positive electrolyte (15a) flows through the positive half-cell (2a) during operation of the redox flow battery (1) and a negative electrolyte (15b) flows through the negative half-cell (2b) during operation of the redox flow battery (1), wherein a positive electrolyte pump (9a) is provided to circulate the positive electrolyte (15a) at a positive flow rate (M+) through the positive half-cells (2a) and a negative electrolyte pump (9b) is provided to circulate the negative electrolyte (15b) at a negative flow rate (M-) through the negative half-cells (2b) to circulate, characterized inthat a control unit (60) of the redox flow battery (1) is provided, which is designed to control the positive flow rate (M+) and / or the negative flow rate (M-) during operation of the redox flow battery (1) as a function of a current electrical stack resistance (R, sta ck) of the at least one cell stack (10) in order to reduce a difference between the current electrical stack resistance (Rstack) of the at least one cell stack (10) and a predetermined optimal stack resistance (Ropt), preferably in order to reduce the current electrical stack resistance (Rstack) to a predetermined optimal stack resistance (R opt ) to change.

14. Redox flow battery according to claim 13, characterized in that the control unit (60) is designed to control the positive flow rate (M+) and / or the negative flow rate (M-) if the current stack resistance (Rstack) is greater than the specified optimal stack resistance (R opt ) and to reduce the positive flow rate (M+) and / or the negative flow rate (M-) when the current stack resistance (R sta ck) is smaller than the specified optimal stack resistance (R op t).

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