Method for restoring performance

Abstract

A method for restoring the performance of a vanadium redox flow battery module (1) in a battery system, the method comprising the following steps in the indicated sequence:-identifying at least one degraded battery module (1); closing the pump (4) of the at least one degraded battery module (1) at a time t1; -switching on the pump (4) of the at least one degraded battery module (1) at a time t2; wherein the length of the time interval [Delta] t = t2-t1 is selected such that, at time t2, the terminal voltage of the degraded battery module (1) is negative, but overcharge of the electrolyte located in the cell components (2) of the degraded battery module (1) is avoided, and wherein, in addition to the first step, these steps are carried out while the battery system is discharging.

Description

[0001] The present invention relates to a method for restoring the performance of a vanadium redox flow battery module in a battery system. Here, the battery system comprises a plurality of vanadium battery modules connected in series.

[0002] The performance of vanadium-based redox flow battery modules slowly degrades over time during operation. One of the processes responsible for this is an oxidation process at the positive electrode, whereby a thin layer of oxygen-containing functional groups can form on this electrode over time. This increases the internal resistance of the battery module, which in turn leads to a decrease in its efficiency. This phenomenon is hereinafter referred to as "degradation", and the affected battery modules are referred to as "degraded".

[0003] Methods for restoring the performance of vanadium-based battery modules are known from the prior art. For example, CN 111 261 888A discloses a method for recovering electrode materials of such battery modules. However, here, the monomer components of the battery module in question must be disassembled and completely disassembled. CN 111 509 278A discloses a method for restoring the performance of a vanadium battery module, which method can be performed while the battery module is in operation. To this end, the positive electrolyte is mixed with the negative electrolyte, and in addition the polarity of the battery module is reversed, i.e. the original positive terminal becomes the negative terminal, and vice versa. To this end, the battery module must include a device that enables mixing and polarity reversal. Such a device is generally not provided in conventional battery modules.

[0004] The object of the present invention is to provide a method for restoring the performance of vanadium-based battery modules which is cheaper than the methods known in the prior art.

[0005] This object is achieved according to the invention by the embodiments according to the independent claims. Further advantageous embodiments of the invention can be found in the dependent claims.

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

[0007] Figure 1 Battery modules;

[0008] Figure 2 Battery system;

[0009] Figure 3 Nyquist plot;

[0010] Figure 4 A time diagram of the terminal voltage during the method according to the invention in a first embodiment; and

[0011] Figure 5 Time diagram of the terminal voltage during the method according to the invention in the second exemplary embodiment.

[0012] Figure 1 A schematic representation of a vanadium-based battery module is shown on the left. The battery module is designated 1. The battery module comprises a cell assembly (designated 2), a tank device (designated 3), two pumps (one of which is designated 4) and optionally a measuring device (designated 5) for detecting the terminal voltage. The cell assembly 2 is an arrangement of a plurality of redox flow cells that can be arranged arbitrarily. For example, it can be a single cell stack, a series connection of several stacks, a parallel connection of several stacks, or a combination of a series connection and a parallel connection of several stacks. The tank device 3 is used to store electrolyte and supply electrolyte to the cell assembly 2. To this end, with a few exceptions, the tank device 3 comprises at least two tanks and a piping system for connecting the tanks to the cell assembly 2. Pump 4 is used to transport electrolyte. Figure 1 Two separate pumps 4 are shown. Likewise, the electrolyte can be delivered with a double-head pump, i.e. with two pumps driven by a common motor. In principle, there can also be more than one pump per electrolyte circuit. This can be advantageous, for example, if the battery module has a redundant structure. This means that if one pump fails, the battery module still functions normally.

[0013] exist Figure 1 On the right side of FIG. 1 , a symbolic representation of the battery module 1 is shown. This symbolic representation is used hereinafter.

[0014] Figure 2 A schematic representation of a battery system including a plurality of battery modules connected in series is shown. The battery system includes at least two battery modules (one of which is designated as 1), a bidirectional power conversion system (PCS) (designated as 6), and a control device (designated as 7). The battery modules 1 are connected in series and connected to the power conversion system 6. Figure 2 Four battery modules are shown, wherein the dashed lines connected in series indicate any number of further modules. The power conversion system 6 is used to connect the battery system to a power grid or a higher-level electrical system. Here, the control device 7 is configured to be able to detect the operating state of the power conversion system 6 and is configured to drive the pump 4 in the battery module 1. Optionally, the control device 7 can be configured to be able to additionally detect the measured values ​​of the measuring device 5 of the battery module 1.

[0015] For the following explanation, it is assumed that one or more degraded battery modules 1 have been identified that exhibit the efficiency degradation described at the beginning. The person skilled in the art is aware of several possibilities for doing this. For example, the terminal voltage of the battery module 1 and the charging or discharging current can be detected during operation of the battery system, so that the resistance of the battery module 1 can be calculated from these values. The degraded battery module 1 exhibits an increased resistance. Alternatively, the battery module 1 can be monitored by using impedance spectroscopy (EIS – “electrochemical impedance spectroscopy”). In this case, the impedance Z of the battery module is determined as a function of the angular frequency ω=2πf. It is advantageous to display Z(ω) in the form of a so-called Nyquist diagram. The real part of Z(ω) is plotted in the x-direction and the negative imaginary part of Z(ω) is plotted in the y-direction. The unit of Z(ω) is ohm. Figure 3 A typical Nyquist plot for a redox flow battery is shown in qualitative form. Two resistance values ​​can be derived from this plot: R s and R ct . R s is interpreted as the static internal resistance, which is given, for example, by the resistance of the contacts and feed lines, while R ct Describes the fraction resulting from the kinetics of charge transfer between the electrode and the electrolyte. Degraded battery module 1 exhibits R ct The value of R can be increased. A simplified impedance spectroscopy method can also be used, in which only low-frequency excitation signals are used. The simplified method can then be used to determine the total R s +R ct , which sum also increases in the case of degradation of the battery module. Another possibility for identifying a degraded battery module 1 is to monitor the state of charge (SoC) of the battery module. Degraded battery modules charge more slowly and discharge more quickly than non-degraded or less degraded battery modules. For the implementation of the method according to the invention, it is irrelevant which method is used to identify a degraded battery module.

[0016] In the following, the method according to the invention is described. For this purpose, a sign convention is used according to which the terminal voltage of the battery module has a positive sign during normal operation of the battery system.

[0017] The method according to the present invention for restoring the performance of at least one degraded battery module 1 in a battery system comprises the following steps in the indicated order:

[0018] - identifying at least one degraded battery module 1;

[0019] - switching off the pump 4 of at least one degraded battery module 1 at time t1;

[0020] - switching on the pump 4 of at least one degraded battery module 1 at a (later) time t2;

[0021] Wherein, the length of the time interval Δt=t2-t1 is selected so that at time t2, the terminal voltage of the degraded battery module 1 is negative, but overcharging of the electrolyte present in the monomer assembly of the degraded battery module 1 is avoided, and wherein, except for the first step, these steps are performed when the battery system is discharging.

[0022] The first step (i.e., identifying at least one degraded battery module) can be performed during any operating state of the battery system (i.e., when the battery system is charging and when the battery system is discharging). On the other hand, the other steps can only be performed when the battery system is discharging. The identification step is a step of checking whether the subsequent steps of the method according to the present invention should be performed on the battery module. That is, the method according to the present invention in a narrower sense consists of the steps mentioned after the identification step. In some of the following explanations, the term "method according to the present 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.

[0023] refer to Figure 4 The electrochemical processes which take place in the degraded battery module in question during the implementation of the method according to the invention are explained in more detail.

[0024] Figure 4 A time diagram of the terminal voltage of a degraded battery module during the implementation of the method according to the invention is shown. Before time t1, the degraded battery module participates in the discharge process of the battery system like any other battery module of the battery system. In this case, the terminal voltage decreases over time because the electrolyte pumped through the monomer assembly by the pump is partially discharged while flowing in the monomer assembly. When the pump is turned off at time t1, the supply of electrolyte to the monomer assembly stops, and therefore the electrolyte permanently present in the monomer assembly during this state discharges much faster, because the discharge current flowing through the battery modules connected in series does not change or only changes negligibly due to this process. The terminal voltage of the degraded battery module drops accordingly rapidly. When the terminal voltage reaches the zero line, the electrolyte present in the monomer assembly is charged in the opposite direction, and the terminal voltage of the battery module in question therefore becomes negative. This is an electrochemical characteristic of vanadium electrolytes. From this zero crossing, the process leading to the formation of harmful oxygen-containing functional groups begins to reverse, thereby at least partially restoring the performance of the degraded battery module. However, the charging process with the opposite sign cannot be continued for any length of time, since otherwise the electrolyte in the cell assembly would become overcharged, which would lead to damage to the battery module. Therefore, the pump is switched on again at the corresponding selected time t2. Fresh electrolyte now flows into the cell assembly again. This electrolyte had a charged state shortly before time t1, so that the terminal voltage rises again to the (positive) value corresponding to t1. Figure 4In the timing diagram shown, the pump is operated at the same delivery rate as before time t1. However, this is not a necessary condition. Rather, it is merely intended to indicate that the battery module in question returns to normal operation at time t2. If for some reason normal operation of the battery module requires a pump rate at time t2 that is different from the pump rate it had shortly before time t1, the pump will be operated at time t2 at the pump rate required at that time.

[0025] The effect of the step "turning off the pump 4 of at least one degraded battery module 1 at time t1" is to cause the described polarity reversal. In principle, this can also be achieved by operating the pump 4 at a very low delivery rate. Therefore, in this document, "turning off the pump" should be understood to mean an operation of the pump that causes the described polarity reversal.

[0026] The time t2 can be determined most easily by detecting the terminal voltage with the aid of a control device: if the terminal voltage is sufficiently negative but has not yet fallen below a critical limit, the pump is switched on again. However, the time t2 can also be determined without detecting the terminal voltage. This can be done by a calculation that takes into account at least the following variables: the state of charge at time t1, the magnitude of the discharge current and the volume of electrolyte in one of the cells of the cell assembly of the battery module in question. If the terminal voltage is to be used to determine t2, it must be possible to detect a voltage with a negative sign.

[0027] When the pump is turned off, the discharge current flowing through the battery system can also be reduced. This allows the time point t2 to be postponed. That is, in this case, the time interval Δt=t2-t1 becomes larger.

[0028] If necessary, the method according to the invention can be performed several times in succession until the performance of the degraded battery module is restored to a sufficient degree. When repeating, the first step does not have to be performed. However, the first step can advantageously be performed to check whether the battery module in question still exhibits any degradation. If there is no improvement or insufficient improvement even after the method according to the invention has been performed several times, the degradation is at least partly based on irreversible processes and further maintenance measures can be initiated for the battery module in question.

[0029] The inventors have found that the desired effect can be increased by extending the time span that the terminal voltage remains in the negative range. This can be advantageously achieved by subsequently operating the pump switched on at time t2 at a reduced pump rate. The term "reduced" refers to the pump rate existing before time t1. Figure 5 The terminal voltage curve corresponding to this embodiment is shown. In the time interval between t2 and t3, the pump is operated at a pump rate at which the terminal voltage remains negative. Figure 5In the case shown, the pump rate is selected so that the terminal voltage remains constant in the time interval between t2 and t3. Although this is advantageous, it is not absolutely necessary. If the pump rate is slightly different, the positive effect will be almost as significant. It is only necessary to avoid overcharging of the electrolyte in the cell assembly during the entire time interval between t2 and t3. For this reason, the pump rate does not necessarily have to be constant during the mentioned time interval. For example, it may happen that despite the reduction in the pump rate, the terminal voltage continues to decrease and will exceed the critical limit. This situation can then be avoided by further reducing the pump rate. That is, the pump rate can be controlled by the control device in the mentioned time interval to avoid overcharging. At time t3, the pump is then operated again at the pump rate of t1 so that the battery module returns to the corresponding (positive) terminal voltage. However, this is not a necessary condition. Instead, it is only intended to indicate that the battery module in question will return to normal operation at time t3. If for some reason, the normal operation of the battery module at time t3 requires a pump rate different from the pump rate it had shortly before time t1, the pump will be operated at the pump rate required at that time at time t3.

[0030] In the time interval between t2 and t3, the discharge current through the battery system will also decrease, which means that the pump rate in this time interval must or can be reduced less drastically.

[0031] It should be noted that when the pump is switched on at time t2, an inhomogeneity is formed in the cell arrangement of the battery module in question. This inhomogeneity is caused by the fact that fresh electrolyte with a different composition than the rest of the electrolyte present in the cell arrangement is supplied to a specific point in the cell arrangement. This leads to a locally different potential state and thus to the formation of a corresponding balancing current.

[0032] like Figure 5 As shown, the method for restoring the performance of at least one degraded battery module 1 in a battery system according to the present invention comprises the following steps in the indicated order:

[0033] - identifying at least one degraded battery module 1;

[0034] - switching off the pump 4 of at least one degraded battery module 1 at time t1;

[0035] - switching on the pump 4 of at least one degraded battery module 1 at a (later) time t2 and operating the pump 4 of at least one battery module 1 at a first pump rate;

[0036] - at a time t3 after t2, operating the pump 4 of at least one degraded battery module 1 at a second pump rate;

[0037] wherein the length of the time interval Δt=t2-t1 is selected so that at time t2, the terminal voltage of at least one degraded battery module 1 is negative, and wherein the first pump rate is selected so that the terminal voltage of at least one battery module 1 is negative during the time interval between t2 and t3, but avoids overcharging of the electrolyte present in the monomer assembly of at least one degraded battery module 1 in the time interval between t2 and t3, and wherein the second pump rate is selected so that the terminal voltage of at least one degraded battery module 1 becomes positive after t3, and wherein, except for the first step, these steps are performed while the battery system is discharging.

[0038] If necessary, such as Figure 5 The method according to the invention shown can also be performed several times in succession. In this case, the same applies to the method according to Figure 4 method.

[0039] Since the battery modules are connected in series in the battery system, the terminal voltages of the battery modules are added. This means that when the method according to the invention is performed, Figure 4 and Figure 5 The voltage curve shown in is also reflected in the total voltage present at the power conversion system. Therefore, the power conversion system must be configured in a way that it can cope with such voltage changes. The same applies to the power of the battery system. If the power output from the battery system is to be kept constant, this can be achieved by correspondingly increasing the discharge current in the time intervals between t1 and t2 and between t1 and t3, respectively.

[0040] In order to ensure that the battery system can continue to operate normally when the method according to the invention is performed, the total voltage applied to the power conversion system must not become negative. This limits the number of battery modules on which the method according to the invention can be performed simultaneously. In limiting cases, the number of battery modules that operate normally must be greater than the number of battery modules on which the method according to the invention is performed at a given time. In practice, it would be advantageous if the number of battery modules that operate normally is significantly greater than the number of battery modules on which the method according to the invention is performed at a given time.

[0041] In order to minimize the demands on the power conversion system, it would be advantageous to perform the method according to the invention on only one battery module at a given time. That is, if more than one battery module is degraded, the method according to the invention is performed sequentially on only one affected battery module at a time.

[0042] The method according to the invention can also be advantageously carried out preventively. That is to say, the method is carried out from time to time for each battery module of the battery system. Of course, in this case, it is not necessary to first determine whether a battery module has actually degraded to a significant extent. It is simply assumed that each battery module degrades to a certain extent after a period of time. The step "identifying at least one degraded battery module 1" then consists simply in determining at least one battery module on which the method according to the invention is to be carried out. This can be done, for example, by first determining the operating time after which the preventive execution of the method according to the invention is to be carried out on the battery module. The identification step then consists in checking, for the battery modules of the battery system, whether there are any battery modules that have been operating for a period of time longer than a predetermined operating time since the start of operation or the last execution of the method according to the invention. If this is the case, the steps of the method according to the invention that follow the identification step are carried out for the battery modules identified in this way.

[0043] By using the method according to the invention, the performance of a degraded battery module can be at least partially restored. The method can be performed during normal operation of the battery system. No additional hardware is required to perform the method, the devices present in ordinary battery modules are sufficient. Only the control device must be configured in such a way that it can perform the method steps according to the invention. For this purpose, a corresponding computer program is run in the control device, which computer program can also be stored on a computer-readable medium.

[0044] Reference numerals list

[0045] 1 Battery Module

[0046] 2 Monomer components

[0047] 3 Storage tank equipment

[0048] 4 Pumps

[0049] 5 Measuring equipment for determining terminal voltage

[0050] 6 Power conversion system

[0051] 7 Control Equipment

Claims

1. A method for restoring the performance of a vanadium redox flow battery module (1) in a battery system, wherein: The battery system comprises at least two battery modules (1) connected in series, a power conversion system (6) and a control device (7), wherein the battery modules (1) are connected to the power conversion system, and wherein each battery module (1) comprises a cell device (2), a tank device (3) for storing electrolyte and two pumps (4) for conveying electrolyte through the cell device (2), and wherein the control device (7) is adapted to detect the operating state of the power conversion system (6) and drive the pumps (4) in the battery module (1), Characterized in that the method comprises the following steps in the indicated order: - identifying at least one degraded battery module (1); - shutting down the pump (4) of the at least one degraded battery module (1) at time t1; - switching on the pump (4) of the at least one degraded battery module (1) at time t2; wherein the length of the time interval Δt=t2-t1 is selected so that at time t2, the terminal voltage of the degraded battery module (1) is negative, but overcharging of the electrolyte present in the cell assembly (2) of the degraded battery module (1) is avoided, and wherein, except for the first step, these steps are performed when the battery system is discharging.

2. The method according to claim 1, wherein: The method comprises the following steps in the order indicated: - identifying at least one degraded battery module (1); - shutting down the pump (4) of the at least one degraded battery module (1) at time t1; - switching on the pump (4) of the at least one degraded battery module (1) at time t2, and operating the pump (4) of the at least one degraded battery module (1) at a first pump rate; - operating the pump (4) of the at least one degraded battery module (1) at a second pump rate at a time t3 after t2; wherein the length of the time interval Δt=t2-t1 is selected so that at time t2, the terminal voltage of the degraded battery module (1) is negative, and wherein the first pump rate is selected so that the terminal voltage of the at least one battery module (1) is negative during the time interval between t2 and t3, but avoids overcharging of the electrolyte present in the cell assembly (2) of the at least one degraded battery module (1) in the time interval between t2 and t3, and wherein the second pump rate is selected so that the terminal voltage of the at least one degraded battery module (1) becomes positive after t3, and wherein, except for the first step, these steps are performed while the battery system is discharging.

3. A battery system, wherein: The battery system comprises at least two vanadium redox flow battery modules (1) connected in series, a power conversion system (6) and a control device (7), wherein the battery module (1) is connected to the power conversion system, and wherein each battery module (1) comprises a single device (2), a tank device (3) for storing electrolyte and two pumps (4) for transporting electrolyte through the single device (2), and wherein the control device (7) is configured to detect the operating state of the power conversion system (6), drive the pump (4) in the battery module (1), and perform the steps of the method according to any one of the preceding claims.

4. The battery system according to claim 3, wherein: The control device (7) is capable of detecting the terminal voltage of the battery module (1).

5. A computer program comprising instructions for causing the battery system according to claim 3 or 4 to execute the method steps according to claim 1 or 2.

6. A computer-readable medium having stored thereon the computer program according to claim 5.

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