Method for restoring performance

Abstract

The invention relates to 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 order indicated: - identifying at least one degraded battery module (1); - switching off the pumps (4) of the at least one degraded battery module (1) at a time t1; - switching on the pumps (4) of the at least one degraded battery module (1) at a time t2; wherein the length of the time interval ∆t= t2-t1 is selected such that, at the time t2, a terminal voltage of the degraded battery module (1) is negative, but overcharging of the electrolyte located in the cell assembly (2) of the degraded battery module (1) is avoided, and wherein these steps, with the exception of the first step, take place while the battery system is being discharged.

Description

[0001] Procedures for restoring performance

[0002] The invention relates to a method for restoring the performance of a vanadium redox flow battery module in a battery system. The battery system comprises several vanadium battery modules connected in series.

[0003] The performance of a vanadium-based redox flow battery module can slowly decrease over time during operation. One of the processes responsible for this is oxidation processes at the positive electrode, which can lead to the formation of a thin layer of oxygen-containing functional groups on this electrode over time. This increases the internal resistance of the battery module, leading to lower efficiency. This phenomenon is referred to below as "degradation," and the affected battery module is referred to as "degraded."

[0004] Methods for restoring the performance of vanadium-based battery modules are known from the prior art. For example, CN 111 261 888 A discloses a method for recycling electrode material from such a battery module. However, this requires the cell arrangement of the battery module in question to be removed and completely disassembled. CN 11 1 509 278 A discloses a method for restoring the performance of a vanadium battery module, which can be carried out while the battery module is in operation. For this purpose, the positive and negative electrolytes are mixed, and the polarity of the battery module is reversed, i.e. the originally positive connection becomes the negative connection and vice versa. For this to happen, the battery module must comprise means that enable mixing and polarity reversal. Conventional battery modules do not generally have such means.

[0005] The object of the invention is to provide a method for restoring the performance of vanadium-based battery modules that is less complex than the methods known from the prior art. This 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.

[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 Nyquist diagram;

[0010] Fig. 4 Time profile of the terminal voltage during the method according to the invention in a first embodiment; Fig. 5 Time profile of the terminal voltage during the method according to the invention in a second embodiment;

[0011] Figure 1 shows a schematic representation of a vanadium-based battery module on the left. The battery module is labeled 1. The battery module comprises a cell arrangement, labeled 2, a tank device, labeled 3, two pumps, one of which is labeled 4, and optionally a measuring device for detecting the terminal voltage, labeled 5. 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 and a pipe system connecting the tanks to the cell arrangement 2. The pumps 4 serve to pump the electrolyte. Figure 1 shows two separate pumps 4. The electrolyte could just as easily be pumped with a double-head pump, i.e., with two pumps driven by a common motor. In principle, more than one pump can be present per electrolyte circuit. This can be advantageous, for example, if the battery module is to be designed redundantly.

[0012] This means that if a pump fails, the battery module remains functional.

[0013] On the right side of Figure 1, a symbolic representation of the battery module 1 is shown. This symbolic representation will be used below.

[0014] Figure 2 shows a schematic representation of a battery system with a plurality of battery modules connected in series. 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 connected 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 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.Optionally, the control device 7 can be designed so that it can additionally record the measured values ​​of the measuring devices 5 of the battery modules 1.

[0015] For the following explanations, it is assumed that one or more degraded battery modules 1 have been identified that suffer from the deterioration in efficiency described above. Several possibilities for this are known to those skilled in the art. For example, the terminal voltage of the battery modules 1 and the charging or discharging current can be recorded during operation of the battery system, so that the resistance of the battery modules 1 can be calculated from these values. Degraded battery modules 1 have an increased resistance. Alternatively, the battery modules 1 can be monitored using impedance spectroscopy (EIS - "Electrochemical Impedance Spectroscopy"). In this case, the impedance Z of the battery modules is determined as a function of the angular frequency w=2iTf. The representation of Z(w) can advantageously be in the form of a so-called Nyquist diagram.The real part of Z(CÜ) is plotted in the x-direction, and the negative imaginary part of Z(w) is plotted in the y-direction. The unit of Z(CÜ) is ohms. Figure 3 shows a typical Nyquist diagram of a redox flow battery in qualitative form. Two resistance values ​​can be derived from the diagram: R. s and Rct. Rs is interpreted as the static internal resistance component, which is determined, for example, by the resistance of the contacts and the supply lines, while Rct describes the component resulting from the kinetics of charge transfer between electrode and electrolyte. Degraded battery modules 1 exhibit an increased Rct. A simplified impedance spectroscopy can also be used, which uses only low-frequency excitation signals. Using this simplified method, the sum R s+Rct can be determined, which is also increased in degraded battery modules. Another option for identifying degraded battery modules 1 is to monitor the state of charge (SoC) of the battery modules. 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 degraded battery modules.

[0016] The method according to the invention is described below. 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.

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

[0018] Identification of at least one degraded battery module 1 ;

[0019] Switching off the pumps 4 of the at least one degraded battery module 1 at a time t1; switching on the pumps 4 of the at least one degraded battery module 1 at a (later) time t2j, wherein the length of the time interval Δt=t2-t1 is selected such that at time t2 the terminal voltage of the degraded battery module 1 is negative, but overcharging of the electrolyte in the cell arrangement of the degraded battery module 1 is avoided, and wherein these steps, with the exception of the first step, take place while the battery system is discharging.

[0020] The first step, i.e. the identification of at least one degraded battery module, 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.

[0021] 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 4.

[0022] Figure 4 shows the temporal progression of the terminal voltage of a degraded battery module during the implementation of the method according to the invention. 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. The terminal voltage drops over time because the electrolyte pumped through the cell array by the pumps is partially discharged while it remains in the cell array. When the pumps are switched off at time t1, the supply of electrolyte to the cell array ceases, and the electrolyte permanently present in the cell array 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. From this zero crossing, the processes that led to the formation of the harmful oxygen-containing functional groups begin to reverse, resulting in the performance of the degraded battery module being at least partially restored. However, the charging process with the reversed sign cannot be continued indefinitely, 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 correspondingly selected time t2. Fresh electrolyte now flows into the cell array again.This supplied electrolyte has a charge level similar to that which existed shortly before time t1, so that the terminal voltage rises again to the (positive) value corresponding to time t1. In the curve shown in Figure 4, the pumps are operated at the same flow rate as before time t1. However, this is not a necessary condition. Rather, it is only intended to express that the battery module in question returns to normal operation at time t2. If, for whatever reason, normal operation of the battery module at time t2 requires a different pumping rate than that which existed shortly before time t1, the pumps would be operated at time t2 at the pumping rate required at that time.

[0023] The effect of the step "switching off the pumps 4 of the at least one degraded battery module 1 at a time t1" is to initiate the described polarity reversal. In principle, this can also be achieved by operating the pumps 4 at a very low delivery rate. Therefore, in this document, "switching off the pumps" is understood to mean operation of the pumps that leads to the described polarity reversal.

[0024] The easiest way to determine time t2 is to have the control device detect the terminal voltage: If the terminal voltage is sufficiently negative but has not yet fallen below the critical limit, the pumps are switched on again. However, time t2 can also be determined without detecting the terminal voltage. This can be done by a calculation that includes at least the following variables: state of charge at time ti, magnitude of the discharge current, and volume of the electrolyte in a cell of the cell arrangement of the battery module in question. If the terminal voltage is to be used to determine t2, it must be capable of detecting voltages with a negative sign.

[0025] While the pumps are off, the discharge current flowing through the battery system can also be reduced. This allows time t2 to be pushed back in time. That is, the time interval Δt=t2-t1 becomes longer.

[0026] If necessary, the method according to the invention can be carried out several times in succession until the performance of the degraded battery module has been sufficiently restored. When repeating the process, the first step does not necessarily have to be performed. However, it can advantageously be performed to check whether the battery module in question still exhibits degradation. If no or insufficient improvement is observed even after repeated execution of the method according to the invention, the degradation is at least partially due to irreversible processes, and other maintenance measures can be initiated for the battery module in question.

[0027] The inventors have recognized that the desired effect can be increased by extending the period during which the terminal voltage is in the negative range. This can advantageously be achieved by operating the pumps that are switched on at time t2 at a reduced pumping rate thereafter. The term “reduced” refers to the pumping rate that existed before time t1. Figure 5 shows the terminal voltage curve corresponding to this embodiment. In the time interval between t2 and ts, the pumps are operated at a pumping rate such that the terminal voltage remains negative. In the case shown in Figure 5, the pumping rate was selected such that the terminal voltage remains constant in the time interval between t2 and ts. This is advantageous but not absolutely necessary. The positive effect would be almost as strong if the pumping rate were slightly different.It is only necessary to avoid overcharging of the electrolyte in the cell arrangement during the entire time interval between t2 and ts. For this to happen, the pump rate does not necessarily have to be constant during the specified time interval. For example, it could happen that the terminal voltage continues to decrease despite a reduced pump rate and would exceed the critical limit. This can then be avoided by further reducing the pump rate. This means that the pump rate can be regulated by the control device during the specified time interval so that overcharging is avoided. At time ts, the pumps are then operated again at the pump rate of ti, so that the battery module returns to the corresponding (positive) terminal voltage. However, this is not a necessary condition. Rather, it is only intended to express that the battery module in question returns to normal operation at time ts.If, for any reason, the normal operation of the battery module at time ts requires a different pumping rate than that which existed shortly before time ti, the pumps would be operated at time ts with the pumping rate required at that time.

[0028] In the time interval between t2 and ts, the discharge current flowing through the battery system can also be reduced. This means that the pump rate can be reduced less during this time interval. It should be noted that when the pumps are switched on at time t2, inhomogeneities form in the cell arrangement of the battery module in question. These arise from the fact that fresh electrolyte is supplied to the cell arrangement at a specific location, which has a different composition than the rest of the electrolyte in the cell arrangement. This leads to locally different potential states, which generate corresponding compensating currents.

[0029] The method according to the invention according to Figure 5 for restoring the performance of at least one degraded battery module 1 in a battery system comprises the following steps in the specified order:

[0030] - Identification of at least one degraded battery module 1 ;

[0031] - Switching off the pumps 4 of at least one degraded battery module

[0032] 1 at a time ti;

[0033] - switching on the pumps 4 of the at least one degraded battery module 1 at a (later) time t2 and operating the pumps 4 of the at least one battery module 1 at a first pumping rate;

[0034] - Operating the pumps 4 of the at least one degraded battery module 1 at a time ts after t2 at a second pumping rate; wherein the length of the time interval Δt=t2-t1 is selected such that at time t2 the terminal voltage of the at least one degraded battery module 1 is negative, and wherein the first pumping rate is selected such that the terminal voltage of the at least one battery module 1 is negative during the time interval between t2 and ts, but overcharging of the electrolyte in the cell arrangement of the at least one degraded battery module 1 is avoided in the time interval between t2 and ts, and wherein the second pumping rate is selected such that the terminal voltage of the at least one degraded battery module 1 becomes positive after ts, and wherein these steps, with the exception of the first step, take place while the battery system is discharging.The inventive method according to Figure 5 can also be carried out several times in succession if necessary. The above statement regarding the method according to Figure 4 applies analogously.

[0035] Since the battery modules in the battery system are connected in series, the terminal voltages of the battery modules add up. This means that the voltage curve shown in Figures 4 and 5 is also reflected in the total voltage applied to the converter when the method according to the invention is implemented. Therefore, the converter must be designed to cope with this voltage variability. The same applies to the power of the battery system. If the power delivered by the battery system is to remain constant, this can be achieved by a corresponding increase in the discharge current in the time intervals between ti and t2 or ti and ts.

[0036] To ensure that the battery system continues to function smoothly while the method according to the invention is being carried out, the total voltage applied to the converter must not become negative. This limits the number of battery modules on which the method according to the invention can be carried out simultaneously. In the limiting case, the number of normally operated battery modules must be greater than the number of battery modules on which the method according to the invention is being carried out at a given time. In practice, it will be advantageous if the number of normally operated battery modules is significantly greater than the number of battery modules on which the method according to the invention is being carried out at a given time.

[0037] In order to keep the demands on the converter as low as possible, it is advantageous if the method according to the invention is only carried out on one battery module at a given time. This means that if more than one battery module is degraded, the method according to the invention is carried out successively on each affected battery module. The method according to the invention can advantageously also be carried out prophylactically. This means that the method is carried out from time to time on each battery module of the battery system. Of course, this does not first require determining whether the battery modules are actually suffering from significant degradation. It is simply assumed that each battery module degrades to a certain extent after a certain time.The step "Identification of at least one degraded battery module 1" then merely consists 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 pre-determining an operating period after which a prophylactic execution of the method according to the invention is to be carried out on a battery module. The said identification step then consists in checking the battery modules of the battery system to determine whether there are any battery modules that have been operated for longer than the pre-determined operating period since commissioning 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 following the identification step are carried out on the battery modules thus identified.

[0038] With the method according to the invention, the performance of degraded battery modules can be at least partially restored. The method can be carried out during normal operation of the battery system. No additional hardware is required for implementation; the resources available in conventional battery modules are sufficient. Only the control device must be designed to carry out the method steps according to the invention. For this purpose, a corresponding computer program runs in the control device, which can also be stored on a computer-readable medium. List of reference symbols

[0039] 1 battery module

[0040] 2 cell arrangement 3 tank device

[0041] 4 Pump

[0042] 5 Measuring device for detecting the terminal voltage

[0043] 6 inverters

[0044] 7 Control device

Claims

Patent 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, an inverter (6), and a control device (7), and wherein the battery modules (1) are connected to the inverter, and wherein each battery module (1) comprises a cell assembly (2), a tank assembly (3) for storing electrolyte, and two pumps (4) for conveying electrolyte through the cell assembly (2), and wherein the control device (7) is configured to detect an operating state of the inverter (6) and to control the pumps (4) in the battery modules (1), characterized in that the method comprises the following steps in the specified order: - Identification of at least one degraded battery module (1 ); - Switching off the pumps (4) of the at least one degraded battery module (1) at a time ti ; - Switching on the pumps (4) of the at least one degraded battery module (1) at time t2j wherein the length of the time interval At= t2-ti is chosen such that at time t2 a terminal voltage of the degraded battery module (1 ) is negative, but an overcharging of the electrolyte located in the cell arrangement (2) of the degraded battery module (1 ) is avoided, and wherein these steps except the first step are carried out while the battery system is being discharged.

2. The method of claim 1, wherein the method comprises the following steps in the specified order: - Identification of at least one degraded battery module (1 ); - Switching off the pumps (4) of the at least one degraded battery module (1) at a time ti ; - Switching on the pumps (4) of the at least one degraded battery module (1) at time t2 and operating the pumps (4) of at least one degraded battery module (1 ) with a first pump rate; - Operating the pumps (4) of the at least one degraded battery module (1) at a time ts after t2 with a second pump rate; wherein the length of the time interval At = t2 - ti is chosen such that at time t2 the terminal voltage of the degraded battery module (1) is negative, and wherein the first pump rate is chosen such that the terminal voltage of the at least one battery module (1) is negative during the time interval between t2 and ts, but an overcharging of the electrolyte in the cell arrangement (2) of the at least one degraded battery module (1) is avoided during the time interval between t2 and ts, and wherein the second pump rate is chosen such that the terminal voltage of the at least one degraded battery module (1) becomes positive after ts, and wherein these steps, with the exception of the first step, are carried out while the battery system is being discharged.A battery system, wherein the battery system comprises at least two vanadium redox flow battery modules (1) connected in series, an inverter (6), and a control unit (7), and wherein the battery modules (1) are connected to the inverter, and wherein each battery module (1) comprises a cell assembly (2), a tank assembly (3) for storing electrolyte, and two pumps (4) for conveying electrolyte through the cell assembly (2), and wherein the control unit (7) is configured to detect an operating state of the inverter (6), control the pumps (4) in the battery modules (1), and perform the steps of the method according to any one of the preceding claims. A battery system according to claim 3, wherein the control unit (7) is capable of detecting a terminal voltage of the battery modules (1). A computer program comprising commands that cause the battery system according to claim 3 or 4 to perform the method steps according to claim 1 or 2. A computer-readable medium on which the computer program according to claim 5 is stored.

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