Method for monitoring status of redox flow battery system

Abstract

The invention relates to a method for monitoring the state of a vanadium-based redox flow battery system, in which the battery system comprises at least two battery modules (1), a bi-directional converter (6) and a control device (7), in which the battery modules are connected in series and to the bi-directional converter, and in which the control device (7) is connected to the bi-directional converter. Each battery module comprises a cell device having a plurality of redox flow cells, a measuring device (5) for detecting a potential difference, and a reservoir (3) for storing a negative electrolyte and a positive electrolyte and for supplying the cell device with the electrolyte, the method comprises the following steps: S1, identifying at least one battery module suspected to be subjected to electrolyte transfer; s2, turning off the pump of the at least one identified battery module at a time t1 while the battery system is in a "discharge" operating state; s3, repeatedly detecting the potential difference value of at least one identified battery module until (later) time t2; and S4, determining the AOS of at least one identified battery module according to the potential difference value detected in the step S3.

Description

[0001] This invention relates to a method for operating a vanadium-based redox flow battery system. More specifically, it relates to a redox flow battery system with a high output voltage. The operating method involves monitoring the state of health (SoH) of the battery system.

[0002] To achieve high output voltage in a redox flow battery system, several cells are typically connected in series. This arrangement is called a stack. However, this cannot continue indefinitely, otherwise the shunt current generated by the electrolyte would become unacceptably high. However, by connecting several stacks in series (where each stack has a separate tank unit), the output voltage can be further increased. This unit, consisting of stacks and associated individual tank units, is called a battery module. The series connection of several battery modules is conventionally called a string. Therefore, the present invention relates to a battery system comprising several battery modules, wherein the battery system is configured such that the battery modules are connected in series, i.e., forming a string, during charging and discharging of the system.

[0003] The SoH (Solar Oxidation State) of a redox flow battery system can be negatively affected by various factors. Electrolyte imbalance (i.e., unequal ion concentrations in the positive and negative electrode electrolytes) can negatively impact SoH. This imbalance is typically described by the average oxidation state (AOS). An AOS deviation of +3.5 indicates this imbalance. This imbalance may be present at the start of operation or become more pronounced during operation. The latter may be caused by vanadium oxidation, other chemical side reactions, and "cross-permeation" at the stack membrane. This electrolyte imbalance is also commonly referred to as electrolyte transfer. Battery modules affected by this are referred to below as "battery modules experiencing electrolyte transfer."

[0004] Another possible cause of electrolyte imbalance is that the volume of the negative electrode electrolyte may differ from that of the positive electrode electrolyte. This discrepancy can occur in the total electrolyte volume of the battery module and / or the electrolyte volume in a single cell. In the latter case, this could be caused, for example, by air bubbles in one or more cells. Imbalances caused by differences in electrolyte volume can also worsen over time. Electrolyte imbalances based on deviations in the total electrolyte volume of the battery module cannot be detected using the method according to the invention.

[0005] Methods for monitoring the state of a redox flow battery system are known in the prior art. For example, DE 102020 123 170 A1 discloses a method in which a battery module is disconnected from a series connection and at least a portion of the electrolyte of the disconnected battery module is discharged, wherein the potential difference is repeatedly measured. The AOS is then determined based on an arbitrary step in the potential difference curve.

[0006] The purpose of this invention is to provide an alternative method for monitoring the state of a redox flow battery system, which enables the detection of electrolyte imbalances not caused by deviations in the total electrolyte volume of the battery module, and is easier to perform than methods known in the prior art.

[0007] This objective is achieved according to the invention by means of embodiments according to the independent claims. Further advantageous embodiments of the invention will be found in the dependent claims.

[0008] The invention will now be explained with reference to the accompanying drawings. The drawings show in detail: Figure 1 Battery Module Figure 2 Battery System Figure 3 The time curve of the terminal voltage during the method according to the present invention in the first embodiment; Figure 4 The second embodiment shows the time curve of the terminal voltage during the method according to the present invention.

[0009] Figure 1 A schematic representation of the battery module is shown on the left-hand side. The battery module is designated 1. The battery module includes individual cell units (designated 2) and a reservoir (designated 3). The individual cell unit 2 is a device comprising multiple redox flow cells that can be arranged in any manner. For example, it can be a single cell stack, a series connection of multiple stacks, a parallel connection of multiple stacks, or a combination of series and parallel connections of multiple stacks. The reservoir 3 is used to store electrolyte and supply electrolyte to the individual cell unit 2. For this purpose, the reservoir 3 includes at least two tanks for the negative and positive electrode electrolytes, a piping system for connecting the tanks to the individual cell unit 2, and a pump for conveying the electrolyte, one of which is designated 4. Figure 1 Two separate pumps 4 are shown. The electrolyte can be easily delivered using a dual-head pump (i.e., two pumps 4 driven by a common motor). The reservoir 3 is configured such that it can supply electrolyte to all the cells of the cell unit 2. When the pumps 4 deliver electrolyte, the electrolyte flows through all the cells of the cell unit 2.

[0010] Figure 1The battery module 1 shown includes a measuring device, designated 5, for providing a potential difference. The measuring device 5 measures the potential difference formed between a first potential in the negative electrode electrolyte and a second potential in the positive electrode electrolyte. In the measuring device 5 for providing the potential difference, the electrode for detecting the aforementioned potential is located in the corresponding cell of the cell assembly 2. The potential difference formed depends on the number of cells connected in series between the electrodes used for detecting the potential. Since redox flow battery modules typically include a measuring device capable of detecting and providing a potential difference between two external electrodes of the cell assembly 2, using this measuring device is the simplest approach. The described potential difference between the two external electrodes is referred to as the terminal voltage.

[0011] exist Figure 1 The symbol for battery module 1 is shown on the right side. This symbol will be used hereinafter. For implementing a particular embodiment of the method according to the invention, each battery module 1 includes a polarity reversal device. Figure 1 The external terminals indicated by the small circles are cross-connected with the internal terminals (i.e., the electrical connections of unit 2), such that the polarity of the external terminals is reversed under a given polarity of the unit. This polarity reversal device... Figure 1 The right side is indicated by a small rectangle, designated as 8.

[0012] Figure 2 A schematic representation of a battery system is shown. The battery system includes 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). Battery module 1 is connected in series and connected to converter 6. Figure 2 Four battery modules are shown, with dashed lines indicating any number of additional modules connected in series. Converter 6 connects the battery system to mains power or a higher-level electrical system. Control device 7 is configured to detect the operating state of converter 6 and control pump 4 in battery module 1. The operating state of converter 6 includes, for example, a "charging the battery system" state and a "discharging the battery system" state. Optionally, control device 7 can be configured to additionally detect measurements from a measuring device to provide a potential difference 5 for battery module 1.

[0013] The method according to the present invention is described below. The terminal voltage is used as the potential difference. Furthermore, a notation convention is used, according to which the terminal voltage of the battery module has a positive sign during normal operation of the battery system. If a potential difference other than the terminal voltage is used, the corresponding convention applies.

[0014] The method for monitoring the state of a redox flow battery system according to the present invention includes the following steps indicated in sequence: -S1: Identify at least one battery module 1 suspected of electrolyte transfer; -S2: When the battery system is in the "discharge" operation state, at time t1, at least one pump 4 of the identified battery module 1 is turned off; -S3: Repeatedly detect the terminal voltage value at at least one identified battery module 1 until (later) time t2, during which pump 4 remains off; -S4: Determine the AOS of at least one identified battery module 1 based on the terminal voltage value detected in step S3. The length of the time interval Δt = t2 - t1 is chosen such that at time t2, the terminal voltage of at least one identified battery module 1 is negative, but overcharging of the electrolyte in the single cell device 2 of at least one identified battery module 1 is avoided, and the battery system discharges during the time interval [t1, t2].

[0015] For a detailed description of step S4, refer to paragraphs

[0027] to

[0030] of DE 10 2020 123 170 A1. These paragraphs are considered part of this document. The terminal voltage value detected in step S3 is used as the potential difference value. Furthermore, the determination of SoH or AOS described in DE 102020 123 170 A1 analogously transitions to the range where the terminal voltage is negative. The following is in conjunction with… Figure 3 This will be explained in more detail.

[0016] The first step, identifying at least one battery module experiencing electrolyte misalignment, can be performed in any operating state of the battery system, i.e., both during charging and discharging. Conversely, other steps can only be performed during discharging. This identification step is used to check whether subsequent steps of the method according to the invention should be performed on the battery module. This means that, in a narrower sense, the method according to the invention consists of the steps mentioned after the identification step. In some of the following explanations, the term "method according to the invention" is used in this narrower sense. This is the case when it is clear from the context that one or more battery modules have been identified.

[0017] If the usable capacity of a battery module decreases, electrolyte transfer in the battery module in question is suspected. For example, this can also be detected if the battery module in question charges or discharges faster than other battery modules in the battery system (even if all battery modules discharge or charge at the same current (the series connection of the battery modules)).

[0018] refer to Figure 3 The electrochemical processes that occur in the degraded battery module in question during the execution of the method according to the invention are explained in more detail.

[0019] Figure 3The diagram shows the time curve of the terminal voltage of a battery module that experiences electrolyte shift during the execution 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 system. The terminal voltage decreases over time because the electrolyte pumped through the individual cells is partially discharged while remaining in the individual cells. When the pump is turned off at time t1, the supply of electrolyte to the individual cells stops, and therefore the electrolyte permanently located in the individual cells during this state discharges much faster because the discharge current flowing through the series-connected battery modules does not change or only undergoes a negligible change due to this process. The terminal voltage of the degraded battery module also decreases rapidly. When the terminal voltage reaches zero, the electrolyte in the individual cells is charged in the opposite direction, and the terminal voltage of the battery module in question thus becomes negative. This is a characteristic electrochemical property of vanadium electrolytes. However, a charging process with the opposite sign cannot continue indefinitely, as this would overload the electrolyte in the individual cells, thereby damaging the battery module. Therefore, the pump is restarted at a selected time t2. Fresh electrolyte now flows into the individual cells again. The supplied electrolyte has a charge state similar to that shortly before time t1, causing the terminal voltage to rise again to the (positive) value corresponding to t1. Figure 3 The curve shown indicates that the pump operates at the same flow rate as before time t1. However, this is not a necessary condition. Rather, it is only intended to indicate that the battery module in question returns to normal operation at time t2. If, for any reason, the pump rate required for normal operation of the battery module at time t2 differs from the pump rate shortly before time t1, the pump will be operated at the pump rate required at time t2. Alternatively, the battery module in question may be removed from the series connection of the battery system, for example, if AOS determines that immediate maintenance is required.

[0020] Within the range between time t1 and the zero-crossing point, the terminal voltage curve V can be observed. kl The plateau marked A can be seen in (t). In the region between the zero-crossing point and time t2, i.e., within the negative terminal voltage range, the terminal voltage curve V... kl Another platform, labeled B, is seen in (t). SoH or AOS can be determined using any platform A within the positive range of the terminal voltage in the same manner as described in DE 10 2020 123170 A1. AOS can be determined in a similar manner using any platform B within the negative range of the terminal voltage. The method according to the invention enables the determination of AOS to be performed in both the positive and negative ranges of the terminal voltage, thereby avoiding redundant determination of AOS and thus ensuring higher reliability of AOS determination and, consequently, SoH determination. For this purpose, time point t2 must be selected such that any platform B within the negative range of the terminal voltage can be detected.

[0021] Figure 4 The diagram shows the time-varying terminal voltage of a battery module during electrolyte transfer when performing a method according to another embodiment of the invention. Figure 3 In contrast to the embodiment, the pump is not activated at time t2. Instead, the polarity of the relevant battery module is reversed at time t2. This causes the sign of the current flowing through the battery module in question to reverse. Therefore, the states experienced by the battery module in time interval [t2, t3] are in the reverse order of the states experienced in time interval [t1, t2]. Thus, platforms A and B also appear in time interval [t2, t3], thereby achieving quadruple redundancy determination of SoH. At time t3, the terminal voltage is therefore positive again. At time t3, the polarity of the relevant battery module is reversed again, and the pump is activated. Alternatively, the relevant battery module can also be removed from the series connection of the battery system at time t3, for example, if AOS determines that immediate maintenance is required.

[0022] In order to execute Figure 4 The method shown requires that the battery module in question include a polarity reversal device. When the polarity reversal device is located between the measuring device used to supply the terminal voltage and the external terminals of the battery module, it generates… Figure 4 The terminal voltage curve is shown. In this arrangement, the measured terminal voltage is unaffected by polarity reversal. A polarity reversal device can also be placed between the measuring device supplying the terminal voltage and the internal terminals of the battery module. In this case, the time curve of the detected terminal voltage will show corresponding jumps at times t2 and t3.

[0023] The execution of the method according to the invention causes a change in the total voltage of the battery system, which deviates from the total voltage change during normal operation of the battery system. This is especially true when the method according to the invention is executed simultaneously on more than one battery module. To ensure normal operation of the battery system even during the execution of the method according to the invention, the battery system must be configured in a manner that allows it to compensate for the total voltage change caused by the invention.

[0024] The measuring device 5 used to provide the potential difference must be configured to detect both positive and negative potential differences and provide 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 used to provide the terminal voltage.

[0025] To improve the reliability of the SoH or AOS determination according to the present invention, the intensity of the discharge current flowing through the battery system can be reduced in the time interval [t1, t2] or [t1, t3]. This increases the width of platforms A and B, which is beneficial for evaluating the potential difference or terminal voltage curve and improving measurement accuracy. Measurement accuracy is again affected when the current intensity is very low. The inventors have recognized that for conventional battery systems, optimal measurement accuracy is achieved when the discharge current is in the range of 10% to 50% of the battery system's nominal current.

[0026] List of reference numerals 1 Battery Module 2 unit devices 3 storage 4 pumps 5. Measuring equipment for detecting potential difference 6 converter 7 Control Equipment 8. Polarity reversal device.

Claims

1. A method for monitoring the state of a vanadium-based redox flow battery system, wherein, The battery system includes 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 connected to the bidirectional converter (6), and wherein each battery module (1) includes a cell device (2) having multiple redox flow cells, a measuring device (5) for detecting potential difference, and a reservoir (3) for storing negative and positive electrolytes and for supplying electrolyte to the cell device (2), and wherein the control device (7) is configured to detect the operating state of the converter (6) and control the pump (4) in the battery module (1), characterized in that the method includes the following steps: S1: Identify at least one battery module suspected of electrolyte transfer (1). S2: When the battery system is in the "discharge" operation state, the pump (4) of the at least one identified battery module (1) is turned off at time t1. S3: Repeatedly detect the potential difference value at the at least one identified battery module (1) until (later) time t2, whereby the pump of the at least one identified battery module (1) remains closed; S4: Determine the AOS of the at least one identified battery module (1) based on the potential difference value detected in step S3. Wherein, the length of the time interval Δt = t2- t1 is selected such that at time t2, the potential difference of the at least one identified battery module (1) is negative, but the electrolyte in the single device (2) of the at least one identified battery module (1) is avoided from being overcharged, and wherein the battery system discharges in the time interval [t1, t2], and wherein the determination of the AOS in step S4 is performed redundantly based on the positive potential difference value and the negative potential difference value.

2. The method according to claim 1, wherein, Each battery module (1) includes a polarity reversal device (8), and the method further includes the following steps: S5: Reverse the polarity of at least one identified battery module (1) at time t2; S6: Repeatedly measure the potential difference at the at least one identified battery module (1) until time t3 (later), wherein the pump of the at least one identified battery module (1) remains off; S7: Determine the AOS of the at least one identified battery module (1) based on the potential difference value detected in step S6. Wherein, the length of the time interval Δt = t3- t2 is selected such that at time t3, the potential difference of the at least one identified battery module (1) is positive, but overcharging of the electrolyte in the single device (2) of the at least one identified battery module (1) is avoided, and wherein the battery system discharges within the time interval [t2, t3], and wherein the determination of the AOS in step S7 is performed redundantly based on the positive potential difference value and the negative potential difference value.

3. The method according to claim 1 or 2, wherein, The discharge current intensity during the time interval [t1, t2] is in the range of 10% to 50% of the nominal current of the battery system.

4. The method according to claim 2 or 3, wherein, The discharge current intensity during the time interval [t2, t3] is in the range of 10% to 50% of the nominal current of the battery system.

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

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