Method for monitoring the state of a redox flow battery system

Abstract

The invention relates to a method for monitoring the state of a redox flow battery system on the basis of vanadium, wherein the battery system comprises at least two battery modules (1), a bidirectional converter (6) and a control device (7), wherein the battery modules are connected in series and are connected to the bidirectional converter, and wherein each battery module comprises a cell arrangement having a plurality of redox flow cells, a measurement device (5) for detecting a potential difference, and a reservoir (3) for storing negative and positive electrolyte and for supplying the cell arrangement with electrolyte, and wherein the method comprises the following steps: S1: identifying at least one battery module with a suspicion of shifted electrolytes; S2: switching off the pumps of the at least one identified battery module at a time t1 while the battery system is in the "discharge" operating state; S3: repeatedly detecting potential difference values at the at least one identified battery module up to a (later) time t2; S4: determining the AOS of the at least one identified battery module from the potential difference values detected in step S3.

Description

Method for monitoring the condition of a redox flow battery system The invention relates to a method for operating a vanadium-based redox flow battery system. The invention particularly relates to redox flow battery systems with a high output voltage. The operating method relates to the condition monitoring of the battery system with regard to its state of health (SoH). To achieve a high output voltage in redox flow battery systems, several cells are usually connected electrically in series. This arrangement is called a stack. However, this cannot be continued indefinitely, as otherwise the shunt current caused by the electrolyte liquid would become unacceptably high. The output voltage can, however, be further increased if several stacks are connected in series, with each stack having a separate tank unit. Such a unit, comprising a stack and its associated separate tank unit, is called a battery module. The series connection of several battery modules is conventionally referred to as a string. The invention therefore relates to a battery system comprising several battery modules, the battery system being designed such that the battery modules are connected in series, i.e. form a string, during charging and discharging of the system. The SoH of a redox flow battery system can be negatively influenced by various effects. An electrolyte imbalance, which results from an unequal ion concentration in the negative and positive electrolytes, can negatively influence the SoH. Such an imbalance is usually described by the so-called average oxidation state (AOS). An AOS that deviates from +3.5 indicates such an imbalance. Such an imbalance can already exist from the start of operation or become greater over the course of operation. The latter can be caused by vanadium oxidation, other chemical side reactions, and crossover at the stack membranes. Such an electrolyte imbalance is also often referred to as electrolyte displacement. A battery module affected by this is hereinafter referred to as a "battery module with displaced electrolyte." Another possibility for an electrolyte imbalance is that the volume of the negative electrolyte can differ from the volume of the positive electrolyte. The total electrolyte volume of a battery module and / or the electrolyte volume located in the cells can exhibit such a deviation. In the latter case, an air bubble in one or more cells, for example, can be the cause. An imbalance caused by different electrolyte volumes can also increase over time. An electrolyte imbalance based on a deviation in the total electrolyte volume of a battery module cannot be detected using the method according to the invention. Methods for monitoring the condition of a redox flow battery system are known from the prior art. For example, DE 10 2020 123 170 A1 discloses such a method in which a battery module is disconnected from the series connection, and at least a partial volume of the electrolyte of the disconnected battery module is discharged, with a potential difference being repeatedly recorded. The AOS is then determined from any steps present in the potential difference curves. The object of the invention is to provide an alternative method for monitoring the condition of a redox flow battery system, which makes it possible to detect an imbalance of the electrolyte which is not due to a deviation of the total electrolyte volume of a battery module, and which can be carried out more easily than the methods known from the prior art. The object is achieved according to the invention by an embodiment according to the independent claim. Further advantageous embodiments of the present invention can be found in the subclaims. The invention is explained below with reference to the figures. The figures show in detail: Fig.1 Battery module Fig.2 Battery system Fig.3 Time course of the terminal voltage during the method according to the invention in a first embodiment; Fig.4 Time course of the terminal voltage during the method according to the invention in a second embodiment; Figure 1 shows a schematic representation of a battery module on the left. The battery module is designated 1. The battery module comprises a cell arrangement, designated 2, and a tank device, designated 3. The cell arrangement 2 is an arrangement of a plurality of redox flow cells, which can be arranged in any desired manner. For example, it could be a single cell stack, a series connection of several stacks, a parallel connection of several stacks, or a combination of series and parallel connections of several stacks. The tank device 3 serves to store electrolyte fluid and to supply the cell arrangement 2 with electrolyte fluid. For this purpose, the tank device 3 comprises at least two tanks for negative and positive electrolyte, a pipe system for connecting the tanks to the cell arrangement 2, and pumps for conveying the electrolyte, one of which is designated 4.Figure 1 shows two separate pumps 4. The electrolyte fluid could just as easily be pumped with a double-head pump, i.e., with two pumps 4 driven by a common motor. The tank device 3 is designed to supply all cells of the cell array 2 with electrolyte fluid. Thus, if the pumps 4 pump electrolyte fluid, the fluid flows through all cells of the cell array 2. The battery module 1 shown in Figure 1 comprises a measuring device for providing a potential difference, which is designated by 5. With the Measuring device 5 measures a potential difference which is formed between a first potential of the negative electrolyte and a second potential of the positive electrolyte. In the measuring device 5 for providing a potential difference, the electrodes for tapping the said potentials are located in corresponding cells of the cell arrangement 2. The potential difference formed depends on the number of cells connected in series between the electrodes for tapping the potentials. Since redox flow battery modules usually include a measuring device which can detect and provide a potential difference between the two outer electrodes of the cell arrangement 2, it is easiest if this measuring device is used. The said potential difference between the two outer electrodes is referred to as the terminal voltage. On the right-hand side of Figure 1, a symbolic representation of the battery module 1 is shown. This symbolic representation will be used below. To carry out a special embodiment of the method according to the invention, each battery module 1 comprises a device for reversing the polarity. The outer terminals, which are indicated in Figure 1 by the small circles, are connected crosswise with respect to the inner terminals, i.e. the electrical connections of the cell arrangement 2, so that for a given electrical polarity of the cell arrangement the polarity of the outer terminals is reversed. This device for reversing the polarity is indicated on the right-hand side of Figure 1 by the small rectangle, which is designated by 8. Figure 2 shows a schematic representation of a battery system. The battery system comprises at least two battery modules, one of which is designated 1, a bidirectional power conversion system (PCS), designated 6, and a control device, designated 7. The battery modules 1 are connected in series and to the converter 6. Figure 2 shows four battery modules, with the dashed lines in the series circuit indicating any number of additional modules. The converter 6 connects the battery system to the grid or to a higher-level electrical system. The control device 7 is designed such that it can detect the operating state of the converter 6 and control the pumps 4 in the battery modules 1. The operating states of the converter 6 include, for example, the states "charging the battery system" and "discharging the battery system." Optionally, the control device 7 can be designed such that it can additionally detect the measured values ​​of the measuring devices for providing a potential difference 5 of the battery modules 1. The method according to the invention is described below. The terminal voltage is used as the potential difference. Furthermore, a sign convention is used, according to which the terminal voltage of the battery modules has a positive sign during normal operation of the battery system. A similar convention applies when a potential difference other than the terminal voltage is used. The inventive method for monitoring the condition of a redox flow battery system comprises the following steps in the specified order: - S1 : Identification of at least one battery module 1 with suspected electrolyte displacement; - S2: Switching off the pumps 4 of the at least one identified battery module 1 at a time ti while the battery system is in the “discharging” operating state; - S3: Repeated acquisition of terminal voltage values ​​on at least one identified battery module 1, until a (later) time t2, wherein the pumps 4 remain switched off; - S4: Determination of the AOS of at least one identified battery module 1 from the terminal voltage values ​​detected in step S3, wherein the length of the time interval At= t2-ti is selected such that at time t2 the terminal voltage of the at least one identified battery module 1 is negative, but overcharging of the cell in the cell arrangement 2 of the at least one identified battery module 1 electrolyte is avoided, and wherein the battery system is discharged in the time interval [ti,t2]. For a detailed description of step S4, please refer to sections

[0027] until

[0030] DE 10 2020 123 170 A1. The aforementioned sections are considered part of this document. The terminal voltage values ​​recorded in step S3 serve as potential difference values. Furthermore, the determination of the SoH or AOS described in DE 10 2020 123 170 A1 is analogously applied to the range in which the terminal voltage is negative. This is explained in more detail below in connection with Figure 3. The first step, i.e. the identification of at least one battery module with displaced electrolytes, can take place during any operating state of the battery system, i.e. both while the battery system is being charged and while the battery system is being discharged. The other steps, on the other hand, can only be carried out when the battery system is being discharged. The identification step is a step which checks whether the subsequent steps of the method according to the invention are to be carried out on a battery module. This means that the method according to the invention in the narrower sense consists of the steps named after the identification step. In some cases, the term “method according to the invention” is used in this narrower sense in the following explanations. This is the case when it is clear from the context that one or more battery modules have already been identified. A battery module is suspected of having a shift in electrolyte if its usable capacity has been reduced. This can be detected, for example, by the battery module in question being charged or discharged faster than the other battery modules in the battery system, even though all battery modules are being charged or discharged using the same current (series connection of the battery modules). The electrochemical processes that occur in the degraded battery module in question during the implementation of the method according to the invention are explained in more detail with reference to Figure 3. Figure 3 shows the temporal progression of the terminal voltage of a battery module with displaced electrolytes during the implementation of the method according to the invention. Before time t1, the battery module in question participates in the discharge process of the battery system like any other battery module in the battery system. The terminal voltage drops over time because the electrolyte pumped through the cell arrangement by the pumps is partially discharged while it remains in the cell arrangement. When the pumps are switched off at time t1, the supply of electrolyte to the cell arrangement ceases, and the electrolyte permanently present in the cell arrangement during this state is therefore discharged much more quickly because the discharge current flowing through the series-connected battery modules does not change or changes only negligibly as a result of the process. The terminal voltage of the degraded battery module drops correspondingly quickly.When the terminal voltage reaches zero, the electrolyte in the cell array is charged in reverse, causing the terminal voltage of the battery module in question to become negative. This is an electrochemical characteristic of the vanadium electrolyte. However, the charging process cannot be continued indefinitely with the opposite sign, as otherwise the electrolyte in the cell array would be overcharged, resulting in damage to the battery module. Therefore, the pumps are switched on again at a selected time t2. Fresh electrolyte now flows into the cell array. This supplied electrolyte has a charge level similar to that which existed shortly before time t1, so the terminal voltage rises again to the (positive) value corresponding to t1.In the curve shown in Figure 3, the pumps are operated at the same flow rate as before time t1. However, this is not a necessary condition. Rather, it simply indicates that the battery module in question returns to normal operation at time t2. Should, for any reason, the battery module's normal operation be interrupted at that time, If the pumping rate at time t2 requires a different pumping rate than that prevailing shortly before time t1, the pumps would be operated at the required pumping rate at time t2. Alternatively, the battery module in question can be removed from the series connection of the battery system, e.g., if the AOS determination has produced a result that necessitates immediate maintenance. In the area between time t1 and the zero crossing, a plateau can be seen in the terminal voltage curve Vki(t), which is marked with A. In the area between the zero crossing and time t2, i.e. in the negative range of the terminal voltage, a further plateau can be seen in the terminal voltage curve Vki(t), which is marked with B. Any plateau A that may be present in the positive range of the terminal voltage can be used to determine the SoH or AOS in the same way as described in DE 10 2020 123 170 A1. Any plateau B that may be present in the negative range of the terminal voltage can be used in an analogous manner to determine the AOS. The method according to the invention enables a redundant determination of the AOS and thus a higher reliability of the determination of the AOS and thus of the SoH because the AOS is determined in both the positive and the negative range of the terminal voltage.For this purpose, the time t2 must be chosen so that a possibly existing plateau B in the negative range of the terminal voltage can be detected. Figure 4 shows the temporal progression of the terminal voltage of a battery module with shifted electrolytes during the implementation of the method according to the invention in a further embodiment. In contrast to the embodiment according to Figure 3, the pumps are not switched on at time t2. Instead, the polarity of the respective battery module is reversed at time t2. This leads to the sign of the current flowing through the respective battery module being reversed. As a result, the battery module passes through the states passed through in the time interval [t1, t2] in the time interval [t2, ts] in reverse order. Therefore, the plateaus A and B also occur in the time interval [t2, ts], whereby This enables quadruple redundant SoH determination. At time ts, the terminal voltage is positive again. At time ts, the polarity of the battery module in question is reversed again, and the pumps are switched on. Alternatively, the battery module in question can be removed from the series connection of the battery system at time ts, e.g., if the AOS determination has produced a result that requires immediate maintenance. To implement the method according to Figure 4, the battery module in question must include a polarity reversal device. The terminal voltage curve shown in Figure 4 results when the polarity reversal device is arranged between the measuring device for providing the terminal voltage and the outer terminals of the battery module. In this arrangement, the detected terminal voltage is not affected by the polarity reversal. The polarity reversal device could also be arranged between the measuring device for providing the terminal voltage and the inner terminals of the battery module. In this case, the time curve of the detected terminal voltage would exhibit corresponding jumps at times t2 and ts. Implementing the method according to the invention results in a variation in the total voltage of the battery system that differs from the variation in the total voltage during normal operation of the battery system. This is particularly the case when the method according to the invention is implemented on more than one battery module simultaneously. To ensure normal operation of the battery system even during implementation of the method according to the invention, the battery system must be designed in such a way that it can compensate for the variation in the total voltage caused by the method according to the invention. The measuring device 5 for providing a potential difference must be designed in such a way that it can detect both positive and negative potential differences and provide the corresponding measured values. If the terminal voltage is used as the potential difference for the method according to the invention, then this condition naturally applies to the measuring device for providing the terminal voltage. To increase the reliability of the SoH or AOS determination according to the invention, the discharge current flowing through the battery system can be reduced in the time interval [ti, t2] or [ti, ts]. This increases the width of plateaus A and B, which facilitates the evaluation of the potential difference or terminal voltage curves and increases measurement accuracy. At very low currents, measurement accuracy suffers again. The inventors have recognized that, for common battery systems, optimum measurement accuracy is achieved when the discharge current is in a range of 10% to 50% of the nominal current of the battery system. List of reference symbols 1 battery module 2 cell arrangement 3 tank device 4 Pump 5 Measuring device for detecting a potential difference 6 inverters 7 Control device 8 Polarity reversal device

Claims

Patent claims 1. A method for monitoring the condition of a vanadium-based redox flow battery system, wherein the battery system comprises at least two battery modules (1), a bidirectional converter (6), and a control device (7), wherein the battery modules (1) are connected in series and to the bidirectional converter (6), and wherein each battery module (1) comprises a cell arrangement (2) with a plurality of redox flow cells, a measuring device (5) for detecting a potential difference, and a tank device (3) for storing negative and positive electrolyte and for supplying the cell arrangement (2) with electrolyte, and wherein the control device (7) is designed such that it can detect operating states of the converter (6) and control the pumps (4) in the battery modules (1), characterized in that the method comprises the following steps: S1 : Identification of at least one battery module (1 ) with suspected electrolyte displacement; S2: Switching off the pumps (4) of the at least one identified battery module (1) at a time ti while the battery system is in the “discharging” operating state; S3: Repeated detection of potential difference values ​​on the at least one identified battery module (1) until a (later) time t2, wherein the pumps of the at least one identified battery module (1) remain switched off; S4: Determination of the AOS of the at least one identified battery module (1) from the potential difference values ​​recorded in step S3, wherein the length of the time interval At=t2-ti is selected such that at time t2 the potential difference of the at least one identified battery module (1) is negative, but overcharging of the electrolyte in the cell arrangement (2) of the at least one identified battery module (1) is avoided, and wherein the battery system is discharged in the time interval [ti, t2], and wherein the determination of the AOS in step S4 is carried out redundantly from positive and negative potential difference values.

2. The method according to claim 1, wherein each battery module (1) comprises a device (8) for reversing the polarity, and wherein the method additionally comprises the following steps: S5: Reversing the polarity of the at least one identified battery module (1) at time t2j S6: Repeated detection of potential difference values ​​at the at least one identified battery module (1) until a (later) time ts, wherein the pumps of the at least one identified battery module (1) remain switched off; S7: Determination of the AOS of the at least one identified battery module (1) from the potential difference values ​​recorded in step S6, wherein the length of the time interval At=ts-t2 is selected such that at time ts the potential difference of the at least one identified battery module (1) is positive, but overcharging of the electrolyte in the cell arrangement (2) of the at least one identified battery module (1) is avoided, and wherein the battery system is discharged in the time interval [t2,ts], and wherein the determination of the AOS in step S7 is carried out redundantly from positive and negative potential difference values.

3. The method according to claim 1 or 2, wherein the current intensity of the discharge current in the time interval [ti, t2] is in a range of 10% to 50% of the nominal current of the battery system.

4. The method according to claim 2 or 3, wherein the current intensity of the discharge current in the time interval [t2, ts] is in a range of 10% to 50% of the nominal current of the battery system.

5. Method according to one of the preceding claims, wherein the potential difference is the terminal voltage.

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