Redox flow battery system and method of operation

Abstract

The present invention provides a method for operating a redox flow battery system, including the steps of turning off at least one pump of a target battery module (1) to stop the supply of electrolyte to a cell configuration (2) for the battery module (1); short-circuiting the target battery module (1) when the terminal voltage of the target battery module (1) falls below a predetermined value; performing measurements; turning on at least one pump of the target battery module (1); and releasing the short circuit of the target battery module (1). An intervention including these steps is executed.

Description

[Technical Field]

[0001] The present invention relates to a redox flow battery system and a method of operating a redox flow battery system, the redox flow battery system comprising a plurality of battery modules connected in series, the method relating to reducing or eliminating imbalances that occur between the series connected battery modules during charging and discharging of the battery system, maintaining battery modules, or isolating one or more battery modules in order to optimize partial load mode. [Background technology]

[0002] Redox flow battery systems and methods for reducing or eliminating imbalances that occur between series-connected battery modules during charging and discharging of the battery system are known in the prior art. For example, Patent Document 1 discloses such a system and method. Measures for reducing or eliminating the imbalance are usually referred to as "balancing." [Prior art documents] [Patent documents]

[0003] [Patent Document 1] German Patent Application Publication No. 102020108053 Summary of the Invention

[0004] It is an object of the present invention to provide a redox flow battery system and a method for operating the system that is simpler in construction than systems known in the prior art.

[0005] According to the present invention, the object is achieved by a redox flow battery system and a method according to the independent claims. Further advantageous embodiments of the invention can be found in the dependent claims.

[0006] The present invention will now be described with reference to the drawings. [Brief explanation of the drawings]

[0007] [Figure 1] 1 shows a battery module. [Figure 2] 1 shows a prior art redox flow battery system. [Figure 3] Charge-discharge cycles without balancing are shown. [Figure 4] 1 shows a charge / discharge cycle with balancing according to the prior art. [Figure 5] 1 shows a redox flow battery system according to the present invention. [Figure 6] 1 shows a charge / discharge cycle with balancing according to the present invention. [Figure 7] 1 shows a charge / discharge cycle with balancing according to the present invention. [Figure 8] 1 shows a charge / discharge cycle with balancing according to the present invention. [Figure 9] 4 shows the terminal voltage curves during balancing intervention according to the present invention; DETAILED DESCRIPTION OF THE INVENTION

[0008] FIG. 1 shows a schematic diagram of a battery module on the left side. The battery module is designated by the reference numeral 1. The battery module includes a cell configuration designated by the reference numeral 2, a tank device designated by the reference numeral 3, and a measurement device for detecting a controlled variable. The cell configuration 2 is a configuration of multiple redox flow cells that can be arranged as desired. For example, it can be a single cell stack, multiple stacks connected in series, multiple stacks connected in parallel, or multiple stacks connected in a combination of series and parallel. The tank device 3 is used to store and supply electrolyte to the cell configuration 2. With some exceptions, the tank device 3 includes at least two tanks, a piping system connecting the tanks to the cell configuration 2, and a pump for supplying the electrolyte. Here, FIG. 1 shows two independent pumps. The electrolyte can be pumped by a dual-head pump, i.e., two pumps driven by a common motor. The tank device 3 is designed to supply electrolyte to all cells in the cell configuration 2.

[0009] The battery module 1 shown in FIG. 1 comprises two measuring devices for providing a measurement variable representing a measure of the state of charge (SoC) of the associated battery module. The measuring device designated by reference number 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 (SoC) of the battery module. The measuring device designated by reference number 5 is a measuring device for providing the terminal voltage of the cell configuration 2 and consequently of the battery module 1. When charging or discharging the battery module 1, the terminal voltage differs from the open circuit voltage by the voltage drop across the internal resistance of the cell configuration 2. If the charge or discharge current and the internal resistance of the cell configuration 2 are known, the terminal voltage is a measure of the state of charge of the battery module.

[0010] The symbolic notation of the battery module 1 is shown on the right side of Figure 1. This symbolic notation will be used below.

[0011] FIG. 2 shows a schematic diagram of a battery system according to the prior art. The battery system includes at least two battery modules, one of which is designated by reference numeral 1, a bidirectional power conversion system (PCS) designated by reference numeral 6, and a control device designated by reference numeral 7. The battery modules 1 are connected in series and connected to the conversion system 6. FIG. 2 shows four battery modules, with the dashed series connection indicating any number of additional modules. The conversion system 6 serves to connect the battery system to the mains or a higher-level power system. The battery system further includes, for each battery module 1, a first switch, one of which is designated by reference numeral 8, and a second switch, one of which is designated by reference numeral 9. The first switches 8 are arranged in series with the battery modules 1, with one switch arranged before or after each battery module 1. This means that the series connection of the battery modules can be broken by each of the first switches 8. The second switches 9 are arranged to bypass each battery module 1 and its associated first switch 8. In FIG. 2, all switches 8 and 9 are shown in an open state. In practice, the switches are driven by the control device 7 so that only one switch of each pair of first and second switches is closed and one switch is open (alternately opened and closed). This means that the pair of switches has only two switch positions: in the first switch position (first switch 8 closed and second switch 9 open), the associated battery module 1 is in a series-connected battery system; and in the second switch position (first switch 8 open and second switch 9 closed), the associated battery module 1 is disconnected from the series-connected battery system by the bypass. Thus, opening the first switch 8 while the second switch 9 is closed prevents the module from discharging through the bypass. Because the bypass is a short circuit, discharging the battery module through such a bypass would still result in a very large current that could damage or even destroy the battery module.

[0012] In the battery system shown in Figure 2, which has completely identical battery modules 1, harmful imbalances cannot occur. However, actual battery modules 1 differ from one another due to manufacturing variations and aging. Furthermore, different operating conditions, such as temperature differences between individual modules, can cause them to behave differently. For these reasons, actual battery modules have different efficiency values ​​and internal resistances. For a given charge or discharge current, the higher the efficiency, the faster the target battery module reaches its end state. Because the same current flows through all battery modules 1 in the series connection shown in Figure 2, a highly efficient module will reach its end state more quickly than a less efficient module. To avoid damage, the charge or discharge process must be interrupted as soon as the module reaches its respective end state. Thus, if this effect is not compensated for, the usable storage capacity of such a battery system will decrease with each cycle ("capacity fading"). Different internal resistances of modules have a similar effect. Terminal voltages have upper and lower limits that must not be exceeded or exceeded. Even with the same efficiency, modules with high internal resistance reach the respective terminal voltage limit during charging or discharging faster than modules with low internal resistance. When the first module reaches this limit, the respective process must be stopped, which also reduces the usable capacity of the battery system. Alternatively, the system power may be reduced. In either case, these effects lead to system failure. Balancing is intended to reduce or completely eliminate these effects in order to permanently maintain a high level of usable capacity of the battery system or to eliminate the above-mentioned failures. On the other hand, successful balancing allows the use of a relatively wide range of cells in terms of efficiency and / or internal resistance, which naturally reflects in reduced manufacturing costs.

[0013] Figure 3 shows two charge / discharge cycles of two battery modules connected in series with different efficiency values. Here, the efficiency difference is chosen to be very high for illustrative purposes. In an actual battery system, the efficiency difference is much smaller. In Figure 3, the SoC curve of the battery module with the higher efficiency is shown with a solid line, and the SoC curve of the battery module with the lower efficiency is shown with a dashed line. In Figure 3, the minimum state of charge is marked with 0% and the maximum state of charge is marked with 100%.

[0014] The high-efficiency battery module reaches a 100% SoC value when the low-efficiency battery module is not fully charged. Because the same current flows through both battery modules, the charging process must end here. Now, when the battery system is discharged, this process begins when the low-efficiency battery module is not fully charged. Due to this unequal starting point for discharge and the lower side efficiencies, the low-efficiency battery module reaches a 0% SoC value when the high-efficiency battery module is not fully discharged. Due to the cumulative effect of the above, the SoC curves of the two battery modules become wider with increasing cycle count, and the usable capacity of the battery system continues to decrease.

[0015] The negative impacts mentioned above can be avoided by taking balancing measures.

[0016] FIG. 4 shows two diagrams, each corresponding to a charging cycle of two battery modules with different efficiencies connected in series. Here, balancing interventions are performed every half cycle. The illustrated SoC curves can be generated by disconnecting one or more battery modules from the series connection for a period of time, so that they do not participate in the charging or discharging of the other battery modules during that period. Ideally, the SoC curves for the disconnected modules are horizontal (self-discharge is negligible). For this purpose, first and second switches 8 and 9 are used in the battery system as shown in FIG. 2.

[0017] The high efficiency battery module is disconnected during the charge half cycle, and the low efficiency battery module is disconnected during the discharge half cycle. The difference between the two figures is due to the different disconnection times.

[0018] In the left diagram of Figure 4, the target battery module is simply disconnected until the two curves level out. For example, during charging, the higher-efficiency battery module is disconnected until the lower-efficiency battery module reaches the same state of charge as the higher-efficiency battery module currently indicates.

[0019] In the right diagram of Figure 4, the target battery module is disconnected for a longer period of time so that the two curves of the battery modules intersect. For example, in a half-cycle of charging, the high-efficiency battery module is disconnected until the low-efficiency battery module has a sufficiently high "lead" during charging so that the high-efficiency battery module catches up to the low-efficiency battery module at exactly 100% SoC. Because leveling only occurs some time after the intervention, such balancing intervention can be described as a kind of temporary overcompensation.

[0020] The inventors have recognized that this type of battery system can be constructed more simply by omitting the switch 8 for interrupting the series connection. In that configuration, the balancing intervention is performed in a different manner than known in the prior art, as will be explained in more detail below. The inventors have also recognized that a battery system constructed in accordance with the present invention allows for the disconnection of individual or multiple battery modules for maintenance purposes or for optimization of the partial load mode, as will be explained in more detail after the description of the balancing intervention.

[0021] FIG. 5 shows a battery system according to the present invention, which differs from the battery system shown in FIG. 2 in that there is no switch 8 for interrupting the series switches and the control device 7 is designed to be able to carry out balancing, maintenance or optimization interventions as described below. A battery system suitable for carrying out the method according to the present invention therefore comprises means for short-circuiting each battery module. The simplest embodiment of this means is shown in FIG. 5 and comprises a short-circuiting line and a switch 9 arranged in the short-circuiting line. The switch 9 can be a relay or a semiconductor arrangement for switching. More complex short-circuiting means are also conceivable. For example, a short-circuiting rail could be provided in parallel with the battery string, and each battery module could be connected to the short-circuiting rail by means of two switches.

[0022] The balancing intervention according to the present invention is characterized in that it is triggered only during the discharge of the battery system. As a result, in a cyclically operated battery system, all balancing interventions are performed only in the discharge half-cycle, or, if the application of the battery system allows, the battery system is switched to discharge for a short time during the charge half-cycle if a balancing intervention is required during the charge half-cycle. In the latter case, the battery system is switched back to charge after or during the balancing intervention.

[0023] FIG. 6 shows a charge / discharge cycle with balancing intervention according to the present invention. In the discharge half-cycle, the SoC curve does not differ from the SoC curve shown in FIG. 4. In the left diagram, the intervention continues until the two SoC curves level off each other. In the charge half-cycle, the intervention switches to discharge, with only the more efficient battery module actually discharging significantly. In the right diagram, the balancing intervention only occurs in the discharge half-cycle. The intervention is of the "overcompensation" type. FIG. 7 shows another charge / discharge cycle with balancing intervention according to the present invention. There is one intervention in each half-cycle. Both interventions are of the "overcompensation" type. The charge / discharge cycle is significantly prolonged due to the "overcompensation" type balancing intervention in the charge half-cycle.

[0024] Figure 8 shows another charge / discharge cycle with balancing intervention according to the invention. Contrary to Figure 7, it is the battery module on the high efficiency side that is short-circuited during the charging half-cycle, not the low efficiency side. As shown in Figure 7, the battery system is switched to discharge during the intervention, but only for a short time. Afterwards, it is switched back to charge. The period of switching to discharge can be very short; it only needs to be long enough to perform the first two steps of the balancing intervention (see next chapter). Both interventions are of the "overcompensation" type. The embodiment shown in Figure 8 is characterized by a shorter cycle time compared to the embodiment shown in Figure 7.

[0025] The balancing intervention sequence according to the present invention will be described in more detail below, assuming that the battery system is in a discharging mode and the balancing intervention is performed on at least one battery module. The balancing intervention comprises the following steps: - turning off the pump of the target battery module - short-circuiting the target battery module when the terminal voltage of the target battery module falls below a predetermined value; - Waiting step until the final equilibrium state is reached - Turning on the pump of the target battery module -Removing the short circuit of the target battery module

[0026] In the step "waiting until balancing end state is reached", the battery system is discharged as shown in Figures 6 and 7, with a discharging current flowing through all battery modules except the battery module on which the balancing intervention is performed, or in the charging half-cycle the battery modules are switched to charging after a short discharging phase as shown in Figure 8. Again, during the step "waiting until balancing end state is reached", a discharging or charging current flows through all battery modules except the battery module on which the balancing intervention is performed.

[0027] The final equilibrium state is the leveled or overcompensated state.

[0028] Since the internal resistance of a battery module is significantly greater than the resistance of the associated short-circuit line, the discharge current flows (almost entirely) through the short-circuited battery module and through the closed short-circuit line, thereby achieving the desired end state of the balancing intervention over time. The balancing intervention according to the present invention may be performed simultaneously on multiple battery modules.

[0029] The process in the battery module in which the balancing intervention according to the present invention is performed will be described in more detail with reference to Figure 9. Figure 9 shows the terminal voltage V K , as a function of time t. Before the balancing intervention, the target battery module has a terminal voltage V0. V0 depends, among other things, on the state of the target battery module. At time t0, the pump of the target battery module is turned off so that electrolyte is no longer supplied to the cell arrangement. As a result, the electrolyte in the cell arrangement drains very quickly because no "fresh" electrolyte is supplied. This leads to a very rapid decrease in the terminal voltage. When the terminal voltage reaches a predetermined threshold voltage V S If the terminal voltage drops below t1, the target battery module is short-circuited. In the illustration of FIG. 9, this occurs at time t1. This causes the terminal voltage to completely disappear. This state is maintained until the desired final state of the balancing intervention is reached. During this entire time, no significant amount of "fresh" electrolyte is supplied to the cell configuration of the target battery module. This can be ensured, for example, by locking the pump or closing a valve located in the supply line. The aforementioned measures can then be understood as being included in the step "turning off the pump of the target battery module."

[0030] During a very short time interval immediately after a short circuit, the entire remaining energy content of the cell structure of the battery module in question is converted into heat in the cell structure. The energy content must be small enough to prevent damage to the cell structure. This is because the energy content of the cell structure corresponds to the terminal voltage, and therefore a sufficiently low threshold voltage V S Therefore, the threshold voltage V Scan be determined based on energetic considerations, which must take into account at least the following variables: electrolyte volume in the cell configuration, the structure of the cell configuration (including the number of cells, electrode geometry, electrode material, and thermal coupling of the electrodes to the environment), discharge current, and state of charge. Because of the complexity of possible influencing factors, the specified threshold voltage V S It is recommended to experimentally verify the effectiveness of

[0031] The criterion "when the terminal voltage of the relevant battery module falls below a predetermined value" can also be implemented by allowing a sufficiently long time to elapse between the pump being turned off and the short circuit, i.e., by selecting a sufficiently long time interval Δt=t1-t0. Here, the selected Δt depends, inter alia, on the state of charge of the battery module in question at time t0 and the discharge current flowing through the battery module in the time interval between t0 and t1. In the battery systems investigated by the inventors, Δt was in the range of a few seconds. This also means that the method according to the present invention can be implemented without measuring or detecting the terminal voltage of the battery module in question.

[0032] The balancing intervention described according to the present invention can be advantageously combined with other known types of balancing intervention, for example, a balancing intervention in which a load is connected in parallel to the battery module in question. In this case, the balancing intervention during discharge can be realized by the intervention according to the present invention and the balancing intervention during charge by the parallel connection of the aforementioned load. In this way, switching to discharge mode can be avoided during the charge half cycle. Auxiliary systems of the battery module, such as a pump, can be considered as loads. However, this can also be an electrical resistor, so that the energy dissipated by the balancing is converted into heat.

[0033] The battery system according to the present invention is also suitable for carrying out maintenance work on at least one battery module. In this case, only the step "waiting until the balancing end state is reached" is replaced by the step "carrying out maintenance measures on the target battery module." The maintenance measures can be all possible measures to repair the target battery module after a failure or to prevent future failures. This can also include a complete replacement of the target battery module. A new battery module then takes the place of the target battery module (which is handled in the same way in the following processing steps).

[0034] Furthermore, the battery system according to the present invention is suitable for disconnecting one or more battery modules from the series connection in order to operate more efficiently in partial load mode. Disconnection reduces the internal resistance of the battery system, thereby reducing losses. This simply replaces the step "waiting until the balancing end state is reached" with the step "operating the battery system in partial load mode."

[0035] To encompass all these possibilities, the step "waiting until a balancing end state is reached" is treated as "performing measurements" in the independent claim, and is defined in the dependent claims as "waiting until a balancing end state is reached", "performing maintenance measures on the target battery module", or "operating the battery system in partial load mode".

[0036] It should be noted that the two subsequent steps "turning on the pump of the target battery module" and "opening the short circuit of the target battery module" can be started simultaneously or can be performed sequentially. The order of the steps does not matter as long as the time between the two steps is not too long.

[0037] These two steps reconnect the battery modules previously disconnected by the first two steps into a series connection. If the intervention is a balancing intervention, this naturally ensures that the reconnected battery modules have the appropriate SoC. For the two other types of intervention described, care must be taken to ensure that the connection only occurs if the SoC of the battery module to be connected roughly corresponds to the SoC of the other battery modules. This means that the SoC of the battery module to be connected should not deviate by more than 10%—preferably more than 5%—from the SoC of the other battery modules when connected. The SoC may be further adjusted by a subsequent balancing intervention.

[0038] A further embodiment of the method according to the invention comprises the following steps in the order given: - turning off the pump of the target battery module - short-circuiting the target battery module when the terminal voltage of the target battery module falls below a predetermined value; - performing measurements - Turning on the pump of the target battery module - measuring the current through the cell configuration of the target battery module; - immediately opening the short circuit of the target battery module when the measured current exceeds a predetermined threshold.

[0039] An advantageous effect of this embodiment is that the jump in voltage of the battery system when the short circuit is opened is minimized.

[0040] The method according to the invention can also be used in battery systems in which a battery module comprises a cell arrangement with several sub-cell arrangements running in parallel. For example, US Pat. No. 10,263,270 discloses such a battery system. Here, the individual sub-cell arrangements can also be individually short-circuited, either simultaneously or sequentially. Appropriate means for short-circuiting must then be provided.

[0041] In order for the method according to the invention to be carried out, the control device 7 must be designed accordingly. Here, the control device controls at least the pump, the means 9 for short-circuiting the individual battery modules, and, if necessary, other means provided for stopping the supply of electrolyte to the cell arrangement 2. If necessary, the control device 7 also detects the terminal voltage of the battery modules, i.e., is connected to a measuring device 5 for detecting the terminal voltage. Furthermore, a computer program installed in the control device is provided for carrying out the steps of the method according to the invention. The computer program according to the invention can be stored on a data carrier. [Explanation of symbols]

[0042] 1 Battery Module 2-cell configuration 3 Tank Device 4. Measuring devices for detecting OCV 5. A measuring device for detecting terminal voltage 6. Bidirectional Conversion System (PCS) 7 Control Device 8 Switch for interrupting series connection 9. Means for short-circuiting battery modules

Claims

1. A method for operating a redox flow battery system comprising at least two battery modules (1), a bidirectional conversion system (6) and a control device (7), wherein the battery modules (1) are connected in series and connected to the conversion system (6), and a means (9) for short-circuiting the battery modules (1) in question is provided for each battery module (1), each battery module (1) comprising a cell arrangement (2) having a plurality of redox flow cells, a tank device (3) for storing an electrolyte and supplying the electrolyte to the cell arrangement (2), and at least one pump for moving the electrolyte; At least one battery module (1) turning off the at least one pump of each battery module (1) to stop the supply of the electrolyte to the cell configuration (2); a step of short-circuiting each of the battery modules (1) when the terminal voltage of each of the battery modules (1) falls below a predetermined value; performing a measurement; turning on the at least one pump of each battery module (1); Opening the short circuit of each of the battery modules (1); receiving interventions, including:

2. The intervention may be turning off the at least one pump of each battery module (1) to stop the supply of the electrolyte to the cell configuration (2); a step of short-circuiting each of the battery modules (1) when the terminal voltage of each of the battery modules (1) falls below a predetermined value; performing a measurement; turning on the at least one pump of each battery module (1); measuring the current flowing through the cell configuration (2) of each of the battery modules (1); opening the short circuit of each battery module (1) as soon as the measured current exceeds a predetermined threshold; The method of claim 1 , comprising, in the order listed:

3. The method of claim 1 , wherein the measurement includes waiting until a balanced end state is reached.

4. The method of claim 1, wherein the measuring comprises performing maintenance measures on the respective battery modules (1).

5. The method of claim 1 , wherein the measuring comprises operating the redox flow battery system in a part-load mode.

6. The method described in claim 2, wherein the measurement includes a step of waiting until a final equilibrium state is reached.

7. The method described in claim 2, wherein the measurement includes a step of performing maintenance measures on each of the battery modules (1).

8. The method described in claim 2, wherein the measurement includes a step of operating the redox flow battery system in a partial load mode.

9. 9. The method of claim 1, wherein the redox flow battery system is in a discharging mode prior to the intervention.

10. 9. The method of claim 1, wherein the redox flow battery system is in a charging mode before the intervention, and the intervention comprises, as a first step, switching the redox flow battery system to a discharging mode.

11. The method described in claim 9, wherein the redox flow battery system is in a charging mode before the intervention, and the intervention comprises, as a first step, switching the redox flow battery system to a discharging mode.

12. A redox flow battery system configured to carry out the method of any one of claims 1 to 8.

13. A redox flow battery system configured to perform the method of claim 9.

14. A redox flow battery system configured to perform the method of claim 10.

15. A redox flow battery system configured to perform the method of claim 11.

16. A computer program for carrying out the steps of the method according to any one of claims 1 to 8.

17. A computer program for carrying out the steps of the method of claim 9.

18. A computer program for performing the steps of the method of claim 10.

19. A computer program for performing the steps of the method according to claim 11.

20. 17. A data carrier having stored thereon a computer program according to claim 16.

21. A data carrier on which the computer program according to claim 17 is stored.

22. A data carrier on which the computer program according to claim 18 is stored.

23. A data carrier on which the computer program according to claim 19 is stored.

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