Redox flow battery and operating procedures

Abstract

Redox flow battery (1) comprising a cell arrangement (2), a control device (5) and two electrolyte circuits, each electrolyte circuit comprising a tank (3) for storing electrolyte fluid, a piping system and a pump (4) for circulating electrolyte fluid in the electrolyte circuit, and wherein the piping system includes a supply line (6.1) from the tank to the cell arrangement and a return line (6.2) from the cell arrangement to the tank, wherein the pump (4) is arranged in the supply line, and wherein each electrolyte circuit comprises an additional pipeline (7) and a shut-off valve (8), wherein the shut-off valve is arranged in the additional pipeline, and wherein the additional pipeline branches off at a first end from the pipeline system in the direction of flow of the electrolyte fluid downstream of the pump and opens at a second end into the return line or the tank, and wherein the control device is configured to control the closing states of the shut-off valves in order to improve the mixing of the electrolyte fluid stored in the tanks in predetermined operating conditions.

Description

[0001] The invention relates to a redox flow battery and a method for operating such a battery. The redox flow battery comprises means for improving the mixing of the electrolyte fluid stored in a tank.

[0002] Redox flow batteries with means for improving the mixing of the electrolyte fluid stored in a tank are known from the prior art. For example, CN 206849952 U discloses such a redox flow battery. The means for improving the mixing of the electrolyte fluid stored in a tank essentially consist of a tube that extends into the tank and has the shape of a cylindrical spiral, with the diameter of the spiral turns decreasing with the tube length. The tube has several openings. The tube is connected to the return line of the electrolyte circuit, so that the electrolyte fluid flowing back into the tank exits through the openings.The electrolyte fluid escaping through the openings can cause the electrolyte fluid in the tank to form a spiral flow, which leads to improved mixing of the electrolyte fluid stored in the tank.

[0003] The object of the invention is to provide a redox flow battery with means for improving the mixing of the electrolyte stored in a tank, wherein the means for improving the mixing of the electrolyte stored in a tank are of simpler construction than the means known from the prior art. Furthermore, the object of the invention is to provide an operating method for such a redox flow battery.

[0004] The object is achieved according to the invention by an embodiment according to the independent claims. Further advantageous embodiments of the present invention can be found in the subclaims.

[0005] The invention is explained below with reference to the figures. The figures show in detail: Fig. 1: Conventional redox flow battery with two electrolyte circuits; Fig. 2: Electrolyte circuit without special means to improve the mixing of the electrolyte liquid stored in the tank; Fig. 3: Electrolyte circuit according to the invention in a first embodiment; Fig. 4: Electrolyte circuit according to the invention in a further embodiment; Fig. 5: Electrolyte circuit according to the invention in a further embodiment; Fig. 6a: Electrolyte circuit according to the invention in a further embodiment in a first operating state; Fig. 6b: Electrolyte circuit according to Fig. 6a in a second operating state;

[0006] Fig. 1 shows a redox flow battery, designated 1. The battery comprises a cell arrangement, designated 2, and a device for storing electrolyte fluid, which comprises two tanks, one of which is designated 3. 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. To supply the cell arrangement 2 with electrolyte fluid, the redox flow battery 1 comprises two electrolyte circuits, one electrolyte circuit circulating negative electrolyte fluid and the other electrolyte circuit circulating positive electrolyte fluid.Each electrolyte circuit includes a tank 3, a piping system, and a pump for circulating the electrolyte fluid. Fig. 1, a pump is designated by 4. Each piping system comprises a first part, which extends between the tank 3 and the cell arrangement 2 and which serves to supply electrolyte fluid to the cell arrangement. These first parts of the piping systems are referred to below as supply lines. Each piping system comprises a second part, which extends between the tank 3 and the cell arrangement 2 and which serves to return electrolyte fluid from the cell arrangement 2 to the tank 3. These second parts of the piping systems are referred to below as return lines. The pumps 4 are arranged in the supply lines. The redox flow battery 1 further comprises a control device, which is designated by 5. The control device 5 serves, among other things, to control the pumps 4.

[0007] It should be noted that redox flow batteries have also become known from the prior art, in which the device for storing electrolyte fluid consists of an integral unit comprising two chambers separated by a sheet pile wall. Reference is made to WO 2022 / 183162 A1 as an example (see Fig. 1 of WO 2022 / 183162 A1: Integral unit 110, first chamber 50, second chamber 52, sheet pile wall 98). In a redox flow battery constructed in this way, the two chambers 50 and 52 represent the two tanks within the meaning of this document.

[0008] Fig. Figure 2 shows a single electrolyte circuit according to the prior art, which does not include any special means for improving the mixing of the electrolyte liquid stored in the tank 3. In addition to the designations from Fig. 1, the supply line is designated 6.1 and the return line 6.2. As is very common with conventional redox flow batteries, the supply line 6.1 opens into tank 3 at a point deep below the liquid level, while the return line 6.2 opens into tank 3 just below the liquid level. The two openings are as far apart from each other as possible. Such an arrangement makes it possible for there to be virtually no dead space in tank 3 with a sufficiently large volume flow through the piping system, i.e. that the entire electrolyte liquid stored in tank 3 takes part in the charging or discharging reaction and is thus available as capacity. However, other arrangements of the openings in the tank with similarly good properties are also possible.

[0009] During the charging or discharging process, without special means to improve the mixing of the electrolyte liquid stored in tank 3, an imperfect mixture of the electrolyte liquid stored in tank 3 occurs. This is also referred to as a stratified charge because layers form with regard to the state of charge of the electrolyte liquid in tank 3, with the state of charge being different in the different layers. Fig. This is schematically indicated by the dashed lines in Figure 2. Along the dashed lines, the charge level of the electrolyte is constant, while the charge level of the electrolyte changes perpendicular to the dashed lines. During a charging process, the charge level of the electrolyte is highest at the top right and lowest at the bottom left. During a discharging process, the situation is reversed. The arrows in Fig. 2 indicate the flow direction of the electrolyte fluid in the electrolyte circuit shown.

[0010] Such an inhomogeneous charge of the electrolyte fluid in the tank can lead to various problems. For example, when using vanadium-based electrolyte fluid, high local charge states in conjunction with high temperatures can lead to precipitation of the vanadium. At low temperatures, V 2+-crystals form. In this case, aging processes in the cell arrangement can also occur due to carbon corrosion. To avoid these negative effects, the charging process must be aborted prematurely in the event of imperfect mixing. Namely when the point in the tank with the highest state of charge has reached the maximum state of charge. However, the entire remaining volume in the tank then has a state of charge that is still below the maximum state of charge. Therefore, the theoretically possible capacity of the battery cannot be used. On the other hand, it is not easy to determine the exact point in time from a measurement point of view, since the state of charge cannot usually be measured locally for a specific point in the tank. Therefore, even larger safety margins must usually be taken into account, which further reduces the usable capacity. It is therefore advantageous if the mixing in tank 3 can be improved.

[0011] The inventors have recognized that the mixing of the electrolyte fluid in the tank can be improved by having each electrolyte circuit comprise an additional pipeline with a shut-off valve, which branches off from the piping system at a first end and flows into the electrolyte circuit at a second end. The branch of the additional pipeline from the piping system is arranged downstream of the pump in the direction of flow. The additional pipeline is used to improve the mixing of the electrolyte fluid stored in the tank under certain operating conditions. For this purpose, the closed state of the shut-off valve is changed for a predefined period of time, which leads to a change in the flow conditions in the tank during this time, resulting in better mixing in the tank.The control device 5 is designed such that it can control the closing state of the shut-off valve in order to improve the mixing of the electrolyte liquid stored in the tank in predetermined operating states.

[0012] Fig. Figure 3 shows an electrolyte circuit according to the invention in a first embodiment. The electrolyte circuit comprises an additional pipeline, designated 7. A shut-off valve, designated 8, is arranged in the additional pipeline 7 and can shut off and open the additional pipeline 7. The first end of the additional pipeline 7 branches off from the supply line 6.1. The branch is located between the pump 4 and the cell arrangement 2. The second end of the additional pipeline 7 opens into the return line 6.2.

[0013] The Fig. The arrangement shown in Figure 3 can be used to improve the mixing of the electrolyte liquid stored in the tank 3 by the following operating procedure. During normal operation of the redox flow battery, the shut-off valve 8 remains closed. This then results in the Fig. 2, since the electrolyte fluid cannot pass through the additional pipeline. Under predetermined operating conditions of the redox flow battery, the shut-off valve 8 is opened for a predefined period of time. Since the cell arrangement 2 in the electrolyte circuit represents by far the highest flow resistance for the electrolyte fluid, and the additional pipeline 7 forms a bypass around the cell arrangement 2, opening the shut-off valve 8 drastically reduces the flow resistance in the electrolyte circuit, so that the volume flow in the electrolyte circuit increases significantly while the pumping power of the pump 4 remains constant. This increase in the volume flow alone improves mixing, e.g., by mobilizing any dead spaces that may exist.Since the pumped electrolyte fluid no longer passes through the cell arrangement when the shut-off valve 8 is open, the electrolyte fluid flowing back into the tank 3 has the same charge level as the electrolyte fluid directly at the inlet opening of the supply line 6.1, which significantly improves the mixing of the electrolyte fluid near the outlet opening of the return line 6.2. If the time period during which the shut-off valve 8 is opened is kept short enough, the charging or discharging process of the redox flow battery 1 does not need to be interrupted, since the amount of electrolyte fluid stored in the cell arrangement enables short-term operation of the redox flow battery 1 without the need to supply fresh electrolyte fluid to the cell arrangement 2.The process can be repeated, whereby the intervals between the open periods of the shut-off valve 8 must be chosen to be long enough so that the cell arrangement 2 can be completely filled with fresh electrolyte fluid during these intervals. It is advantageous if the interval between two open periods of the shut-off valve 8 is longer than the respective duration of the open periods.

[0014] The Fig. 4 and Fig. 5 show embodiments of electrolyte circuits according to the invention, in which the additional pipeline 7 branches off from the supply line 6.1 with the first end and opens into the tank 3 with the second end. This means that a separate passage into the tank 3 is provided for the additional pipeline 7. The location at which the additional pipeline 7 opens into the tank 3 can be chosen arbitrarily, whereby locations near the inlet opening of the supply line 6.1 are rather disadvantageous, since then there is no significant improvement in the mixing in the tank 3. Advantageous locations are, among others, locations which tend to form dead spaces. By opening the shut-off valve 8, any dead spaces which may be present are then mobilized. Fig. 4 and Fig. 5 show two suitable locations for the outlet of the additional pipe 7 into the tank 3. In Fig. 4, the additional pipe 7 flows into the tank 3 near the surface approximately in the middle. Fig. 5, the additional pipe 7 flows into the tank 3 near the tank bottom near the tank wall.

[0015] The operating procedure for the embodiments according to Fig. 4 and Fig. 5 is analogous to the operating procedure used in connection with Fig. 3 was described.

[0016] The Fig. 6a and Fig. 6b show a further embodiment of an electrolyte circuit according to the invention. In contrast to the previously shown embodiments, the additional pipe 7 branches off with its first end from the return line 6.2, ie the branch is located in the flow direction behind the cell arrangement 2. As in the embodiments according to the Fig. 4 and Fig. 5, the additional pipeline opens into the tank 3 with its second end. In this embodiment, the additional pipeline 7 does not represent a bypass line around the cell arrangement 2 as in the previously shown embodiments. Therefore, the operation of this embodiment also differs from the operation of the previously shown embodiments.

[0017] During normal operation of the redox flow battery, the shut-off valve 8 remains open. This results in the Fig. 6a. In predefined operating conditions of the redox flow battery, the shut-off valve 8 is closed for a predefined period of time. This results in the Fig. 6b. In order to ensure that the flow conditions shown in the Fig. 6a and Fig. To establish the flow conditions shown in Figure 6b, the additional pipeline 7 must open into the tank 3 at a point that is vertically, i.e., in the direction of gravity, above the point at which the outlet of the return line 6.2 is located. Then, the backpressure in the return line 6.2 caused by the electrolyte fluid stored in the tank 3 is greater than the backpressure in the additional pipeline 7.

[0018] In contrast to the embodiments in which the additional pipeline 7 represents a bypass line around the cell arrangement 2, in the embodiment according to the Fig. 6a and Fig. 6b no mandatory upper limit for the time periods in which the shut-off valve 8 is open or closed, since the cell arrangement is always flowed through by fresh electrolyte fluid regardless of the closed state of the shut-off valve 8.

[0019] The operating method according to the invention for improving the electrolyte liquid stored in the tank 3 generally comprises the following step: - In predefined operating states of the redox flow battery, the closing state of the shut-off valve 8 is changed by the control device 5 for a predefined period of time.

[0020] The predefined operating states in which the closure state of shut-off valve 8 is changed are selected such that better mixing of the electrolyte fluid stored in the tank is advantageous in the same or subsequent operating states. For example, in vanadium-based redox flow batteries, these are the operating states in which the redox flow battery is close to its maximum possible state of charge during charging.

[0021] It should be noted that the pump performance in redox flow batteries is typically controlled to minimize the energy required to circulate the electrolyte. This promotes the formation of dead spaces in the tanks. It is therefore advantageous if the control device increases the pump performance of the associated electrolyte circuit during the predefined periods in which the shutoff valve's closing state is changed by the control device.

[0022] It should also be mentioned that the method according to the invention can also be used to improve temperature homogeneity in the tanks. Locally varying heat input from outside into the tanks or locally varying heat output to the outside can lead to uneven temperature distribution in the tanks. The improved mixing provided by the operating method according to the invention also improves temperature homogeneity. If the method according to the invention is to be used for this purpose, the predetermined operating states can include operating states in which increased temperature inhomogeneity exists. To identify these operating states, the redox flow battery can comprise measuring devices with which temperature inhomogeneity can be detected.

[0023] Redox flow batteries typically include an electrochemical measuring cell for measuring the open circuit voltage (OCV). OCV cells are often arranged hydraulically parallel to the cell array. The measured OCV value represents a measure of the state of charge of the redox flow battery. Such OCV cells can also be advantageously used in batteries according to the invention. Further details on OCV cells can be found, for example, in DE 10 2020 120 428 B3.

[0024] For various reasons, the state of charge determined using OCV cells may deviate from the actual state of charge of the battery. A so-called state observer algorithm can be used to improve the determination of the state of charge both in the tank and in the cell arrangement. For this purpose, the entire electrolyte circuit is modeled using subvolumes. In conjunction with the measured values ​​from an OCV cell and Coulomb counting, the observer algorithm then provides the states of charge of the electrolyte fluid in the individual subvolumes. Furthermore, the observer algorithm provides information about any electrolyte shifting that may be occurring. Such an observer algorithm can be advantageously used both in the inventive and in conventional redox flow batteries. List of reference symbols 1 redox flow battery 2 Cell arrangement 3 tanks 4 Pump 5 Control device 6.1 Supply line 6.2 Return line 7 Additional pipeline 8 Shut-off valve QUOTES CONTAINED IN THE DESCRIPTION

[0000] This list of documents submitted by the applicant was generated automatically and is included solely for the convenience of the reader. This list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions. Cited patent literature

[0000] CN 206849952 U

[0002] WO 2022 / 183162 A1

[0007] DE 10 2020 120 428 B3

[0023]

Claims

[1] Redox flow battery (1) comprising a cell arrangement (2), a control device (5) and two electrolyte circuits, wherein each electrolyte circuit comprises a tank (3) for storing electrolyte fluid, a piping system and a pump (4) for circulating electrolyte fluid in the electrolyte circuit, and wherein the piping system comprises a supply line (6.1) from the tank (3) to the cell arrangement (2) and a return line (6.2) from the cell arrangement (2) to the tank (3), and wherein the pump (4) is arranged in the supply line, characterized bythat each electrolyte circuit comprises an additional pipeline (7) and a shut-off valve (8), wherein the shut-off valve (8) is arranged in the additional pipeline (7), and wherein the additional pipeline (7) branches off with a first end from the pipeline system in the flow direction of the electrolyte liquid behind the pump (4) and opens with a second end into the return line (6.2) or the tank (3), and wherein the control device (5) is designed such that it can control the closing states of the shut-off valves (8) in order to improve the mixing of the electrolyte liquid stored in the tanks (3) in predetermined operating states. [2] Redox flow battery (1) according to claim 1, wherein the additional pipeline (7) branches off from the supply line (6.1) with a first end and opens into the return line (6.2) with a second end. [3] Redox flow battery (1) according to claim 1, wherein the additional pipe (7) branches off from the supply line (6.1) with a first end and opens into the tank (3) with a second end. [4] Redox flow battery (1) according to claim 1, wherein the additional pipe (7) branches off from the return line (6.2) with a first end and opens into the tank (3) with a second end, and wherein the return line (6.2) comprises an outlet opening, and wherein the location at which the second end of the additional pipe (7) is arranged is located vertically above the location at which the outlet opening is arranged. [5] Operating method for a redox flow battery (1) according to one of the preceding claims for improving the mixing of the electrolyte liquid stored in a tank (3), which comprises the following step: In predefined operating states of the redox flow battery (1), the closing state of the shut-off valve (8) is changed by the control device (5) for a predefined period of time. [6] Operating method according to claim 5 for a redox flow battery (1) according to claim 2 or 3, wherein an interval between two open periods of the shut-off valve (8) is longer than the respective duration of the open periods. [7] Operating method according to claim 5, wherein during the predefined period of time in which the closing state of the shut-off valve (8) is changed by the control device (5), the power of the pump (4) of the associated electrolyte circuit is increased by the control device (5).

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