Redox flow battery system and operating method

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, and the method relates to reducing or eliminating imbalances that occur during charging and discharging of the battery system between the battery modules connected in series in order to optimize the partial load mode, maintaining the battery modules, or disconnecting one or more battery modules.

Background Art

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

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

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

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

[0006] The present invention will be described below with reference to the drawings.

Brief Description of the Drawings

[0007]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7

Figure 8

Figure 9

DETAILED DESCRIPTION OF THE INVENTION

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

[0009] The battery module 1 shown in Figure 1 includes two measurement devices for providing measurement variables representing measured values of the state of charge (SoC) of the associated battery module. The measurement device designated by reference numeral 4 is a measurement device for providing a so-called open circuit voltage (OCV). The OCV value is a measured value of the state of charge (SoC) of the battery module. The measurement device designated by reference numeral 5 is a measurement device for providing the terminal voltage of the cell configuration 2 and, as a result, further the battery module 1. When charging or discharging the battery module 1, the terminal voltage is different from the open circuit voltage by the voltage drop across the internal resistance of the cell configuration 2. If the charging current or discharging current and the internal resistance of the cell configuration 2 are known, the terminal voltage becomes a measured value 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 is used below.

[0011] Figure 2 shows a schematic diagram of a battery system according to the prior art. The battery system includes at least two battery modules, one 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 are connected to the conversion system 6. Figure 2 shows four battery modules, and the dotted line of the series connection indicates any number of additional modules. The conversion system 6 functions to connect the battery system to a main or upper power system. The battery system further includes, for each battery module 1, a first switch designated by reference numeral 8 and a second switch designated by reference numeral 9. The first switch 8 is arranged in series with respect to the battery module 1, and one switch is arranged in front of or behind each battery module 1, respectively. This means that the series connection of the battery modules can be disconnected by each of the first switches 8. The second switch 9 is arranged to bypass each battery module 1 and the associated first switch 8. In Figure 2, all the switches 8 and 9 are shown in the open state. In practice, the switches are driven by the control device 7 such that only one of the switches of each pair of the first and second switches is closed and one switch is opened (alternately opened and closed). This means that a pair of switches has only two switch positions. In the first switch position (the first switch 8 is closed and the second switch 9 is opened), the associated battery module 1 forms a series-connected battery system, and in the second switch position (the first switch 8 is opened and the second switch 9 is closed), the associated battery module 1 is disconnected from the series-connected battery system by bypass. Therefore, by opening the first switch 8 while the second switch 9 is closed, discharge of the module through the bypass is prevented. Since the bypass is a short circuit, discharge of the battery module through such a bypass would result in a very large current that could damage or even destroy the battery module.

[0012] In the battery system shown in FIG. 2 having exactly the same battery modules 1, no harmful imbalance can occur. However, actual battery modules 1 differ from each other due to manufacturing variations and aging. Furthermore, due to different operating conditions, such as temperature differences between individual modules, they can behave in different ways. For these reasons, actual battery modules have different efficiency values and different internal resistances. For a given charging current or discharging current, the higher the efficiency, the faster the target battery module reaches its final state. In the series connection shown in FIG. 2, since the same current flows through all the battery modules 1, the high-efficiency modules reach the final state faster than the low-efficiency modules. To avoid damage, the charging or discharging process must be interrupted immediately when the modules reach their respective final states. Thus, if this effect is not compensated for, the available storage capacity of such a battery system decreases with each cycle ("capacity fading"). The different internal resistances of the modules have a similar effect. There are upper and lower limit values that the terminal voltage must not exceed or fall below. Even with the same efficiency, modules with high internal resistance reach the respective limit values of the terminal voltage faster during charging or discharging than modules with low internal resistance. When the first module reaches this limit value, the respective process must be paused, which also leads to a decrease in the available capacity of the battery system. Alternatively, the power of the system can decrease. In any case, these effects lead to system failures. Equalization is intended to reduce or completely eliminate the above-mentioned effects in order to permanently maintain the available capacity of the battery system at a high level or to eliminate the above-mentioned failures. On the other hand, if equalization is successful, the use of cells in a relatively wide range in terms of efficiency and / or internal resistance becomes possible, which is naturally reflected in the reduction of manufacturing costs.

[0013] Figure 3 shows two charge-discharge cycles of two battery modules connected in series with different efficiency values. Here, for the purpose of explanation, the difference in efficiency is selected to be very high. In an actual battery system, the difference in efficiency is even smaller. In Figure 3, the SoC curve of the battery module on the high-efficiency side is shown by a solid line, and the SoC curve of the battery module on the low-efficiency side is shown by a dashed line. In Figure 3, the minimum charge state is marked as 0%, and the maximum charge state is marked as 100%.

[0014] When the battery module on the low-efficiency side is not fully charged, the battery module on the high-efficiency side reaches the 100% SoC value. Since the same current flows through both battery modules, the charging process must end here. When the battery system is discharged here, this process starts when the battery module on the low-efficiency side is not fully charged. Due to this unequal starting point for discharge and the lower efficiency on the low side, the battery module on the low-efficiency side reaches the 0% SoC value when the battery module on the high-efficiency side is not fully discharged. Due to the cumulative effect of the above influences, the SoC curves of the two battery modules spread further with the increase in the number of cycles, and the available capacity of the battery system continues to decrease.

[0015] The above negative effects can be avoided by taking equalization measures.

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

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

[0018] In the left figure of FIG. 4, the target battery module is only disconnected until the two curves are leveled. For example, during charging, the high-efficiency battery module is disconnected until the low-efficiency side battery module reaches the same charge state as the high-efficiency side battery module is currently showing.

[0019] In the right figure of FIG. 4, the target battery module is disconnected for a longer time so that the two curves of the battery module intersect. For example, in the charging half-cycle, the high-efficiency side battery module is disconnected until the low-efficiency side battery module has a sufficiently high "lead" during charging so that the high-efficiency side battery module catches up with the low-efficiency side battery module at exactly 100% SoC value. Since leveling only occurs after a certain amount of time has passed after the intervention, such an equalization intervention can be explained as a kind of temporary overcompensation.

[0020] The inventors recognized that this type of battery system can be configured more simply by omitting the switch 8 for interrupting the series connection. In that configuration, the equalization intervention is performed in a manner different from those known in the prior art. This will be explained in more detail below. The inventors also recognized that the battery system configured according to the present invention enables the disconnection of individual or multiple battery modules for maintenance purposes or for the optimization of the partial load mode. This will be explained in more detail after the description of the equalization intervention.

[0021] FIG. 5 shows a battery system according to the present invention, which is different from the battery system shown in FIG. 2 in that there is no switch 8 for interrupting the series switch and the control device 7 is designed to be able to execute the equalization intervention, maintenance intervention or optimization intervention described below. Therefore, a battery system suitable for executing the method according to the present invention includes means for short-circuiting each battery module. The simplest embodiment of this means is shown in FIG. 5, and the embodiment includes a short-circuit line and a switch 9 arranged in the short-circuit line. The switch 9 can be a relay or a semiconductor configuration for switching. More complex short-circuit means are also conceivable. For example, a short-circuit rail is provided in parallel with the battery string, and each individual battery module can be connected to the short-circuit rail by two switches.

[0022] The equalization 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 battery system that operates in cycles, all equalization interventions are performed only in the discharge half-cycle, or if the application example of the battery system permits, the battery system is switched to discharge for a short time during the charge half-cycle when equalization intervention is required during the charge half-cycle. In the latter case, the battery system is switched back to charge after or during the equalization intervention.

[0023] FIG. 6 shows a charge-discharge cycle with equalization intervention according to the present invention. In the discharge half-cycle, the SoC curve is the same as the SoC curve shown in FIG. 4. In the left figure, the intervention continues until the two SoC curves are leveled with each other. In the charge half-cycle, it is switched to discharge during the intervention, and only the more efficient battery module actually discharges significantly. In the right figure, the equalization intervention is performed only in the discharge half-cycle. The intervention is of the "overcompensation" type. FIG. 7 shows another charge-discharge cycle with equalization intervention according to the present invention. There is one intervention in each half-cycle. Both interventions are of the "overcompensation" type. Due to the "overcompensation" type of equalization intervention in the charge half-cycle, the charge-discharge cycle is significantly extended.

[0024] Figure 8 shows another charge and discharge cycle with equalization intervention according to the present invention. Contrary to Figure 7, in the charging half-cycle, it is the battery module on the high-efficiency side, rather than the low-efficiency side, that is short-circuited. As shown in Figure 7, during the intervention, the battery system is switched to discharge, but only for a short time. Then, it is switched back to charging. The period during which it is switched to discharge can be very short. It only needs to be long enough to execute the first two steps of the equalization intervention (see the 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 sequence of the equalization intervention according to the present invention will be described in more detail below. Here, it is assumed that the battery system is in the discharge mode and the equalization intervention is performed on at least one battery module. The equalization intervention comprises the following steps. - Step of turning off the pump of the target battery module - Step of short-circuiting the target battery module when the terminal voltage of the target battery module drops below a predetermined value - Step of waiting until the final equalization state is reached - Step of turning on the pump of the target battery module - Step of opening the short circuit of the target battery module

[0026] In the step of "waiting until the final equalization state is reached", the battery system is discharged as shown in Figures 6 and 7, and a discharge current flows through all battery modules other than the battery module on which the equalization intervention is performed, or as shown in Figure 8, in the charging half-cycle, after a short discharge stage of the battery module, it is switched to charging. Also in this case, during the step of "waiting until the final equalization state is reached", a discharge or charge current flows through all battery modules other than the battery module on which the equalization intervention is performed.

[0027] The final equalization state is a levelled or overcompensated state.

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

[0029] The process in the battery module in which the equalization intervention according to the present invention is executed will be described in more detail with reference to FIG. 9. FIG. 9 shows the path of the terminal voltage V K of the target battery module as a function of time t. Before the equalization intervention, the target battery module has a terminal voltage V0. V0 depends, inter alia, on the state of change of the target battery module. At time t0, the pump of the target battery module is turned off so that the electrolyte is no longer supplied to the cell configuration. As a result, since no "new" electrolyte is supplied, the electrolyte in the cell configuration is discharged very quickly. This leads to a very rapid decrease in the terminal voltage. When the terminal voltage drops below a predetermined threshold voltage V S , the target battery module is short - circuited. In the illustration of FIG. 9, this occurs at time t1. Thereby, the terminal voltage completely disappears. This state is maintained until the desired final state of the equalization intervention is reached. Throughout this time, no significant amount of "new" electrolyte will be supplied to the cell configuration of the target battery module. This can be ensured, for example, by locking the pump or closing the valve arranged in the supply line. And the aforementioned measures can be understood as being included in the step "step of turning off the pump of the target battery module".

[0030] During a very short time interval immediately after the short - circuit, the total remaining energy content of the cell configuration of the target battery module is converted into heat of the cell configuration. The energy content must be small enough to prevent damage to the cell configuration. Since the energy content of the cell configuration corresponds to the terminal voltage, this is ensured by selecting a sufficiently low threshold voltage V S . Therefore, the threshold voltage V Scan be determined based on energetic considerations, and at least the following variables must be taken into account: the electrolyte volume in the cell configuration, the structure of the cell configuration (including the number of cells, the electrode shape, the electrode material, the thermal coupling of the electrodes to the environment), the discharge current, and the state of charge. Since the possible influencing factors are complex, the effectiveness of the specified threshold voltage V S is recommended to be verified experimentally.

[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 turning off the pump and short-circuiting, that is, by selecting a sufficiently long time interval Δt = t1 - t0. Here, the selected Δt depends, inter alia, on the state of charge of the target battery module at time t0 and the discharge current flowing through the battery module within the time interval between t0 and t1. In the battery system investigated by the inventor, Δt was in the range of several seconds. This also means that the method according to the invention can be carried out without measuring and detecting the terminal voltage of the target battery module.

[0032] The described equalization intervention according to the invention can advantageously be combined with other well-known types of equalization intervention, for example, an equalization intervention in which a load is connected in parallel to the target battery module. In this case, the equalization intervention during discharge can be achieved by the intervention according to the invention and the equalization intervention during charging by the parallel connection of the aforementioned load. Thus, the switch to the discharge mode can be avoided during the charging half-cycle. An auxiliary system of the battery module, such as a pump, can be regarded as a load. However, this may also be an electrical resistor, provided that the energy dissipated by equalization is converted into heat.

[0033] The battery system according to the present invention is also suitable for performing maintenance work on at least one battery module. In this case, only the step of "waiting until the final state of equalization is reached" is replaced by the step of "performing maintenance measures on the target battery module". The maintenance measures can be all possible measures for repairing the target battery module after a failure or preventing future failures. This may include a complete replacement of the target battery module. And the new battery module replaces the target battery module (and is treated similarly in subsequent processing steps).

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

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

[0036] It should be noted that the two subsequent steps of "turning on the pump of the target battery module" and "releasing the short circuit of the target battery module" may be started simultaneously or executed sequentially. The order of the above steps is not a problem as long as the time between the two steps is not too long.

[0037] These two steps reconnect the battery modules, which were previously disconnected by the first two steps, in series connection. If the intervention was a balancing intervention, this will of course ensure that the reconnected battery modules have an appropriate SoC. In the case of the two other types of interventions described, care must be taken to ensure that the connection is made only if the SoC of the battery modules to be connected generally corresponds to the SoC of the other battery modules. This means that the SoC of the battery modules to be connected should not deviate by more than 10% - preferably more than 5% - from the SoC of the other battery modules at the time of connection. The SoC may further be adjusted by subsequent balancing interventions.

[0038] A further embodiment of the method according to the invention comprises the following steps in the order described. - Turning off the pump of the target battery module - Short-circuiting the target battery module when the terminal voltage of the target battery module drops below a predetermined value - Performing a measurement - Turning on the pump of the target battery module - Measuring the current flowing 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 value

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

[0040] The method according to the invention can also be used in a battery system in which a battery module comprises a cell configuration with a plurality of sub-cell configurations extending in parallel. U.S. Patent No. 10,263,270, for example, discloses such a battery system. Here, the individual sub-cell configurations can also be short-circuited individually, and the short circuits can be performed simultaneously or sequentially. And appropriate means for short-circuiting must be provided.

[0041] For the method according to the invention to be carried out, the control device 7 has to be designed accordingly. Here, the control device controls at least a pump, means 9 for short - circuiting the individual battery modules, and, if necessary, other means provided for stopping the supply of electrolyte to the cell configuration 2. Optionally, the control device 7 also detects the terminal voltage of the battery module, 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 reference numerals

[0042] 1 Battery module 2 Cell configuration 3 Tank device 4 Measuring device for detecting OCV 5 Measuring device for detecting terminal voltage 6 Bidirectional conversion system (PCS) 7 Control device 8 Switch for interrupting the series connection 9 Means for short - circuiting the battery module

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, connected to the conversion system (6), and means (9) for short-circuiting the battery module (1) of interest is provided for each battery module (1), and each battery module (1) comprises a cell configuration (2) having a plurality of redox flow cells, a tank device (3) for storing an electrolyte and supplying the electrolyte to the cell configuration (2), and at least one pump for moving the electrolyte. At least one battery module (1) is turning off the at least one pump of each battery module (1) to stop the supply of the electrolyte to the cell configuration (2); short-circuiting each battery module (1) when the terminal voltage of each battery module (1) falls below a predetermined value; performing a measurement; turning on the at least one pump of each battery module (1); releasing the short-circuit of each battery module (1); A method that undergoes an intervention including.

2. The intervention is turning off the at least one pump of each battery module (1) to stop the supply of the electrolyte to the cell configuration (2); short-circuiting each battery module (1) when the terminal voltage of each battery module (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 battery module (1); releasing the short-circuit of each battery module (1) immediately when the measured current exceeds a predetermined threshold value; The method according to claim 1, comprising in the order described.

3. The method according to claim 1 or 2, wherein the measurement includes waiting until an equalization final state is reached.

4. The method according to claim 1 or 2, wherein the measurement includes performing maintenance measures on each battery module (1).

5. The method according to claim 1 or 2, wherein the measurement comprises the step of operating the redox flow battery system in a partial load mode. **Claim 6** The method according to any one of claims 1 to 5, wherein the redox flow battery system is in a discharge mode prior to the intervention. **Claim 7** The method according to any one of claims 1 to 5, wherein the redox flow battery system is in a charge mode prior to the intervention, and the intervention comprises, as a first step, switching the redox flow battery system to a discharge mode. **Claim 8** A redox flow battery system configured to perform the method according to any one of claims 1 to 7. **Claim 9** A computer program for performing the steps of the method according to any one of claims 1 to 7. **Claim 10** A data carrier storing the computer program according to claim 9.

Images