Method for operating a redox flow battery system

Abstract

The invention relates to a method for operating a redox flow battery system comprising at least two battery modules (1), wherein the redox flow battery system comprises a measuring device (4, 5) for providing a measured variable which constitutes a measurement of the charge state of each battery module (1), and the method has the following steps: - cyclically operating the redox flow battery system; - detecting measurement values using the measuring device (4, 5); and - carrying out at least two balancing interventions on a battery module (1) during a half-cycle, wherein the last balancing intervention carried out in the half-cycle is an overcompensation balancing intervention type, and an SoC2 value which is used for the balancing intervention is predicted using a previously trained AI.

Description

[0001] Operating procedure for a redox flow battery system

[0002] The invention relates to a method for operating a redox flow battery system, wherein the redox flow battery system comprises a plurality of battery modules connected in series, and wherein the method relates to the reduction or elimination of imbalances occurring between battery modules connected in series during charging and discharging of the battery system.

[0003] Methods for reducing or eliminating imbalances that occur between series-connected battery modules during charging and discharging of the battery system are known from the prior art. For example, DE 102020 108 053 ​​A1 discloses such a method. Measures to reduce or eliminate these imbalances are commonly referred to as "balancing."

[0004] The object of the present invention is to further optimize the methods known from the prior art.

[0005] This object is achieved according to the invention by a method according to the independent claim. Further advantageous embodiments of the present invention can be found in the subclaims.

[0006] The invention is explained below with reference to the figures. The figures show in detail:

[0007] Fig.1 Battery module

[0008] Fig.2 Battery system

[0009] Fig.3 Charge / discharge cycles without balancing

[0010] Fig.4 Charge / discharge cycles with balancing by “decoupling”

[0011] Fig.5 Charge / discharge cycles with balancing by “misuse”

[0012] Fig. 6 Charge / discharge cycles with multiple balancing steps per half cycle according to the present invention 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, a tank device, designated 3, and a measuring device for recording a controlled variable. 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 the electrolyte and to supply the cell arrangement 2 with electrolyte.For this purpose, the tank device 3, with few exceptions, comprises at least two tanks, a pipe system connecting the tanks to the cell arrangement 2, and pumps for pumping the electrolyte. Figure 1 shows two separate pumps. The electrolyte could equally well be pumped with a double-head pump, i.e., with two pumps driven by a common motor. The tank device 3 is designed such that it can supply all cells of the cell arrangement 2 with electrolyte.

[0013] The battery module 1 shown in Figure 1 comprises two measuring devices for providing a measured value that represents a measure of the state of charge of the associated battery module (SoC). The measuring device, designated by 4, is a measuring device for providing the so-called open circuit voltage (OCV). The OCV value is a measure of the state of charge of the battery module (SoC). The measuring device, designated by 5, is a measuring device for providing the terminal voltage of the cell arrangement 2 and thus also of the battery module 1. When the battery module 1 is charging or discharging, the terminal voltage differs from the open circuit voltage by the voltage that drops across the internal resistance of the cell arrangement 3. With the charging or discharging current and the internal resistance of the cell arrangement 2 known, the terminal voltage represents a measure of the state of charge of the battery module.An alternative to determining the OCV value is so-called Coulomb counting, which also represents a measure of the state of charge of the battery module. This requires a measuring device to provide the current flowing through the modules connected in series. Such a measuring device for Coulomb counting could therefore also be implemented outside the battery modules 1, so that a battery module 1 optionally comprises a measuring device for providing a controlled variable. In any case, a battery system according to the invention comprises at least one measuring device for providing a measured variable that represents a measure of the state of charge for each battery module 1 of the battery system.

[0014] On the right side of Figure 1, a symbolic representation of the battery module 1 is shown. This symbolic representation will be used below.

[0015] 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 diagram indicating any number of additional modules. The converter 6 connects the battery system to the grid or to a higher-level electrical system. The series connection of battery modules shown is also referred to as a battery string.

[0016] In a battery system according to Figure 2 with completely identical battery modules 1, no harmful imbalance could arise. However, real battery modules 1 differ due to manufacturing variations and aging processes. Furthermore, different operating conditions, e.g. temperature differences, can cause the individual modules to behave differently (since the internal resistance of the modules depends, among other things, on the temperature of the module - see below). For these reasons, real battery modules have different efficiency values ​​and different internal resistances. For a given charging or discharging current, higher efficiency leads to the final state of the respective battery module being reached more quickly. Since the same current flows through all battery modules 1 in the series circuit according to Figure 2, the high-efficiency modules reach the final state more quickly than the low-efficiency modules.To avoid damage, the charging or discharging process must be aborted as soon as a module reaches its respective final state. If this effect is not compensated for, the usable storage capacity of such a battery system decreases with each cycle ("capacity fading"). The different internal resistance of the modules has a similar effect. There are upper and lower limits for the terminal voltage, which must not be exceeded or undercut. Even with identical efficiency, a module with a higher internal resistance will reach the respective limit of the terminal voltage more quickly during charging or discharging than a module with a lower internal resistance. If the first module reaches a limit, the respective process must be aborted, which also leads to a reduction in the usable capacity of the battery system. Alternatively, the system's output could also be reduced.In any case, these effects lead to a degradation of the system. Balancing is intended to reduce or completely eliminate these effects in order to maintain the usable capacity of the battery system at a consistently high level or to eliminate the degradation described. On the other hand, successful balancing enables the use of cells with a comparatively high variation in efficiency and / or internal resistance, which naturally translates into reduced production costs.

[0017] The control device 7 is designed to perform the necessary actions for balancing, which are described in detail below. The control device 7 can be a separate unit or part of a higher-level control system that provides additional control functions of the battery system in addition to balancing.

[0018] The method according to the invention is explained below for the case where the battery modules have different efficiency values, i.e., the modules of a battery string reach the minimum or maximum state of charge (SoC) at different rates. In the graphics shown, the minimum state of charge is marked as 0% and the maximum state of charge as 100%. Furthermore, for the sake of simplicity, the charging / discharging cycles shown are presented for the case where charging and discharging occur at the same constant power.

[0019] It should be noted that the 0% and 100% values ​​of the state of charge each refer to the usable range of the individual battery modules, whereby for the sake of simplicity it is assumed that the usable capacity of the battery modules connected in series is the same.

[0020] The method according to the invention relates to the cyclic operation of a battery system, whereby the term “cyclic operation” is defined very broadly. This means operation in which discharge phases alternate with charging phases. The two aforementioned phases each form a half-cycle. In each half-cycle, there can be any number of time periods of any length in which the battery system is in standby mode. A standby state is understood to be the state in which the current through the battery string is zero. A half-cycle therefore begins with a reversal of the current direction through the battery string and ends with the subsequent reversal of the current direction. A half-cycle does not have to extend over the entire state of charge range (i.e. from 0% to 100%), but can begin at any first state of charge value S0C1 and end at any other state of charge value S0C2.It is clear that during a charging half-cycle, S0C1 < S0C2, and during a discharging half-cycle, S0C1 > S0C2. Therefore, the fact that the half-cycles depicted in the figures of the application generally extend over the entire state of charge range is for the sake of clarity and should not be understood as limiting.

[0021] Figure 3 shows two charge / discharge cycles of two battery modules connected in series with different efficiency values. For clarity, the efficiency difference is chosen to be very high. In real battery systems, the efficiency differences are much smaller. In Figure 3, the SoC curve of the battery module with the higher efficiency is shown as a solid line, and the SoC curve of the battery module with the lower efficiency is shown as a dashed line.

[0022] The more efficient battery module reaches the 100% SoC value when the less efficient battery module is not yet fully charged. Since both battery modules are subjected to the same current, the current must still be reversed, thus completing the charging half-cycle. Discharge begins before the less efficient battery module is fully charged. Due to this unequal starting point for discharging and the lower efficiency, the less efficient battery module reaches the 0% SoC value when the more efficient battery module is not yet fully discharged. Since the described effects act cumulatively, the SoC curves of the two battery modules diverge further and further with increasing cycle numbers, and the usable capacity of the battery system continues to decrease.

[0023] The negative effect described can be avoided by implementing balancing measures. The various types of possible balancing interventions are described in more detail below.

[0024] Figure 4 shows two diagrams, each with a charging cycle of two battery modules connected in series with different efficiencies. A balancing intervention is performed in each half-cycle. The balancing interventions are of the "decoupling" type. With this type of balancing intervention, one or more battery modules are decoupled from the series connection for a certain period of time, so that they no longer participate in the charging or discharging of the remaining battery modules during this time. Ideally, the SoC curve for the decoupled modules is horizontal (self-discharge can be neglected).

[0025] During the charging half-cycle, the more efficient battery module is decoupled, and during the discharging half-cycle, the less efficient battery module is decoupled. The difference between the two diagrams lies in the different decouple times.

[0026] In the left diagram, the relevant battery module is only decoupled until the two curves equalize. For example, during charging, the efficient battery module is decoupled until the less efficient battery module reaches the same state of charge as the more efficient battery module. The associated balancing interventions can be classified as "equalization" type.

[0027] In the right-hand diagram, the battery module in question is decoupled for a longer period, causing the two battery module curves to intersect. For example, during the charging half-cycle, the more efficient battery module is decoupled until the less efficient battery module has gained a sufficiently large "head start" in charging, so that the more efficient battery module catches up with the less efficient battery module just at 100% SoC. The associated balancing interventions can be classified as "overcompensation."

[0028] It should be noted that the degree of overcompensation naturally depends on the SoC2 value at which the respective half-cycle ends. As mentioned above, SoC2 does not have to be 0% or 100%, but can also assume any value in between. For example, if the charging half-cycle were to end at SoC2=80%, the more efficient battery module would have to be decoupled for a shorter time than in the example shown in Figure 4.

[0029] If balancing interventions of the "decoupling" type are to be used, it is advantageous if the battery string is designed so that a maximum of one battery module can be decoupled at a time when the charging or discharging power is at full. If lower charging or discharging power is required, several battery modules can be decoupled simultaneously. This reduces the time required for balancing. To carry out balancing interventions of the "equalization" type, it is sufficient if the battery management system controlling the interventions has the SoC values ​​of the battery modules available. For balancing interventions of the "overcompensation" type, the battery management system must perform a type of extrapolation. For this, the battery management system also requires the gradient of the SoC curves of the battery modules in question. These gradients can be determined in a known way from the SoC measured values ​​for different points in time.Well-known smoothing filters can be applied to the slope values ​​thus determined to minimize the influence of measurement errors. So-called moving-average filters or a filter extended with second-order exponential smoothing have proven effective.

[0030] Figure 5 shows a further type of balancing intervention. Here, the battery modules in question are not decoupled from the series connection, but rather a consumer is connected in parallel to the battery module in question. This results in the current flowing through the battery string branching out in such a way that a battery module affected by such a balancing intervention only receives part of the charging or discharging current. As a result, it is charged or discharged less quickly than without this intervention. This means that the SoC curve in question is less steep during the intervention than before or after the intervention. Figure 5 again shows two diagrams, the left-hand diagram showing interventions of the “adjustment” type and the right-hand diagram showing interventions of the “overcompensation” type. Balancing interventions that come about by connecting a consumer in parallel could also be casually referred to as “misuse” because part of the charging or discharging current is still flowing through the battery module.discharge current is diverted to another purpose. It is advantageous if the loads connected in parallel are elements that belong to the battery system and must be supplied with energy anyway. For example, the pumps of a battery module affected by such an intervention can be supplied with energy during the intervention by the diverted charging or discharging current, whereas they would otherwise be fed with energy from another energy source. It should be noted that an "overcompensation" type intervention must be initiated in good time before the end of the relevant half-cycle is reached. This is particularly true for an "misuse" type intervention, since in the case of "misuse" the battery module affected by the intervention also has a non-zero SoC gradient.Such an intervention of the “decoupling” type can in principle be carried out shortly before the SoC2 value is reached, whereby the intervention then becomes more and more similar to an intervention of the “adjustment” type, the shorter the time interval before the SoC2 value is reached.

[0031] The diagrams in Figures 4 and 5 show one balancing intervention in each half-cycle. However, more than one intervention can be performed per half-cycle if necessary. Such interventions are generally triggered by the battery management system when there are deviations between the SoC values ​​of the battery modules that exceed a predefined threshold. For example, the minimum and maximum values ​​of the measured SoC values ​​can be considered. The variance of these values ​​could just as easily be calculated and used to trigger balancing interventions. The smaller the threshold is selected, the closer the SoC curves of the battery modules are kept together due to the increased number of balancing interventions performed.

[0032] It should also be noted that SoC curves, as shown in Figure 5, can also be generated using "decoupling" type interventions. The interventions are triggered by the battery management system at a high frequency and with a correspondingly short duration. This means that the flattened curve shown in Figure 5 during the intervention is approximated by a large number of small steps. Such a high-frequency sequence of decoupling and coupling steps is considered a single balancing intervention and classified with a type corresponding to the respective SoC curve (i.e., "equalization" or "overcompensation").

[0033] It is also conceivable that the balancing interventions are only ever carried out during charging or only during discharging. The inventors have recognized that the balancing methods known from the prior art can be improved by performing at least two balancing interventions during a half-cycle, with the last balancing intervention performed in the half-cycle being of the "overcompensation" type. This can ensure that the SoC curves of the battery modules coincide at the end of the half-cycle. However, the inventors have also recognized that a balancing intervention of the "overcompensation" type can only be carried out in such a way that it leads to the desired result if the battery management system controlling the intervention "in advance" the SoC2 value at which the currently ongoing half-cycle will end.The inventors have recognized that a reliable prediction of the said SoC2 value can be made by a pre-trained AI (artificial intelligence).

[0034] The AI ​​can be trained in various ways. One possibility is that the training takes place on the same battery system on which the method according to the invention is to be carried out at a later point in time. For this purpose, for example, only balancing interventions of the "adjustment" type can be carried out during the training phase. Or if balancing interventions of the "overcompensation" type are carried out, the required SoC2 value is predicted in a different way, i.e., not with the aid of the AI. For example, the SoC2 value of the penultimate half-cycle can simply be used, or an average value from several past half-cycles of the same type (i.e., either discharging or charging).

[0035] Another possibility for training the AI ​​is to use historical data from another battery system that is as similar as possible. Another possibility for training is the use of artificially generated data. In this case, certain known characteristics of the battery system on which the AI ​​is to be used can be used for the artificial generation. The AI ​​can also be further trained while it is being used for the method according to the invention, i.e. the AI ​​can continuously "learn" during the course of the method according to the invention. Therefore, it is not necessary to train the AI ​​perfectly in advance; it is sufficient to carry out the training to such an extent that the prediction quality of the AI ​​is at least as good as the simple prediction options mentioned above (using the second-to-last half-cycle or calculating an average value from several past half-cycles of the same type).

[0036] To predict the required SoC2 value, the AI ​​advantageously uses not only correlation variables from the battery system in question, but also correlation variables determined by the environment, such as weather data and weather forecast data from the area covered by the electrical grid to which the battery system is connected. This is particularly advantageous when the battery system is used for grid regulation.

[0037] The inventors have further recognized that it is advantageous if balancing interventions of the "decoupling" type are only performed when the SoC values ​​of the battery modules are in a range of 20% to 80%, i.e., in an SoC range in which the charging or discharging current is moderate. The switching operations required for the interventions can thus be performed with less wear.

[0038] Figure 6 shows two diagrams showing the SoC curve of two battery modules in which balancing interventions are carried out according to the method according to the invention. In the left-hand diagram, interventions of the "decoupling" type are used. In the right-hand diagram, the interventions shown can be of the "misuse" type or can be caused by high-frequency "decoupling" (see above). In both diagrams, the 20%-80% SoC range has been marked by two additional horizontal dashed lines. All balancing interventions take place in the SoC range between these lines, i.e., the intervention always ends in this range (important for the interventions in the right-hand diagram, where the SoC value of the affected battery module changes during the intervention). It should be noted that in the right-hand diagram, this criterion is only advantageous if the curve shown is caused by high-frequency decoupling.In the left diagram, three interventions occur per half-cycle, while in the right diagram, only two interventions occur per half-cycle. The last intervention per half-cycle is of the "Overcompensation" type, while the other interventions are of the "Adjustment" type. The S0C2 value concluding the half-cycles is 0% or 100%, respectively. As mentioned above, any other SoC2 value can also complete the half-cycles.

[0039] It should be noted that during a half-cycle, the types “decoupling” and “misappropriation” can also occur mixed.

[0040] Finally, it should be noted that the battery management system controlling the interventions is a computer program running in the control device 7. This means that the control device 7 controls the balancing interventions and, for this purpose, records the SoC measured values ​​of the individual battery modules 1. However, other measured values ​​(see above) can be used as control variables, e.g., the measured values ​​of the terminal voltages of the battery modules 1, since these are scaled with the SoC values. The K1 used to predict the SoC2 value is part of the computer program.

[0041] List of reference symbols

[0042] 1 battery module 2 cell arrangement

[0043] 3 Tank facility

[0044] 4 Measuring device for determining the OCV

[0045] 5 Measuring device for determining the terminal voltage

[0046] 6 Bidirectional converter (PCS) 7 Control device

Claims

Patent claims 1. A method for operating a redox flow battery system comprising at least two battery modules (1), a bidirectional inverter (6), and a control device (7), wherein the battery modules (1) are connected in series and to the inverter (6), and wherein each battery module (1) comprises a cell assembly (2) with a plurality of redox flow cells and a tank device (3) for storing electrolyte and supplying electrolyte to the cell assembly (2), and wherein the redox flow battery system comprises a measuring device (4, 5) for providing a measured quantity which is a measure of the state of charge (SoC) of each battery module (1), and wherein the method comprises the following steps: - Cyclic operation of the redox flow battery system; - Recording measured values ​​with the measuring device (4, 5); - Performing at least two balancing interventions on a battery module (1) during a half-cycle; characterized in that the last balancing intervention performed in the half-cycle is of the type "overcompensation", and a SoC2 value used for this balancing intervention is predicted using a pre-trained AI.

2. Method according to claim 1, wherein at least one balancing intervention is of the type "uncoupling" and the balancing interventions are only carried out when the battery module (1) in question is in a state of charge between 20% and 80%.

3. Method according to claim 1 or 2, wherein at least one balancing intervention performed is of the "misappropriation" type.

4. Redox flow battery system designed to carry out the method according to any one of claims 1 to 3. Computer program for executing the steps of the method according to one of claims 1 to 3. Data carrier on which a computer program according to claim 5 is stored.

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