Redox flow battery and operating procedures

Abstract

A 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 in that each electrolyte circuit comprises an additional piping line (7) and a shut-off valve (8), wherein the shut-off valve (8) is arranged in the additional piping line (7), and wherein the additional piping line (7) branches off at a first end from the supply line (6.1) in the direction of flow of the electrolyte fluid downstream of the pump (4) and at a second end into the return line (6.1).2) terminates, and wherein the control device (5) is designed to control the closing states of the shut-off valves (8) in order to improve the mixing of the electrolyte fluid stored in the tanks (3) 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 projects into the tank and has the form of a cylindrical spiral, the diameter of the spiral turns decreasing with the length of the tube. 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, resulting in improved mixing of the electrolyte fluid stored in the tank.

[0003] US patent 2014 / 0134465 A1 describes a redox flow battery featuring a cell array and two electrolyte circuits, each with its own tank. The redox flow battery is configured to generate high electrical power.

[0004] JP H04 456 7 A discloses a redox flow battery with a cell array. The cell array is connected to two tanks via piping.

[0005] The object of the invention is to provide a redox flow battery with means for improving the mixing of the electrolyte liquid stored in a tank, wherein the means for improving the mixing of the electrolyte liquid stored in a tank are simpler in design than those known from the prior art. It is also an object of the invention to provide an operating method for such a redox flow battery.

[0006] The problem is solved according to the invention by an embodiment according to the independent claims. Further advantageous embodiments of the present invention are found in the dependent claims.

[0007] The invention will be explained below with the aid of 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 fluid stored in the tank; Fig. 3: Electrolyte circuit according to the invention in a first embodiment; Fig. 4: Electrolyte circuit in a further embodiment; Fig. 5: Electrolyte circuit 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;

[0008] Fig. Figure 1 shows a redox flow battery, labeled 1. The battery comprises a cell array, labeled 2, and an electrolyte storage device, which includes two tanks, one of which is labeled 3. The cell array 2 is an arrangement of multiple redox flow cells, which can be arranged in any configuration. 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 array 2 with electrolyte, the redox flow battery 1 includes two electrolyte circuits, one circulating negative electrolyte and the other circulating positive electrolyte.Each electrolyte circuit comprises a tank 3, a piping system, and a pump for circulating the electrolyte fluid. Fig. Pump 4 is designated as pump 1. Each piping system comprises a first section, which extends between tank 3 and cell assembly 2 and serves to supply electrolyte fluid to the cell assembly. This first section of the piping system is hereinafter referred to as the supply line. Each piping system comprises a second section, which extends between tank 3 and cell assembly 2 and serves to return electrolyte fluid from cell assembly 2 to tank 3. This second section of the piping system is hereinafter referred to as the return line. The pumps 4 are arranged in the supply line. The redox flow battery 1 also includes a control unit, designated as pump 5. The control unit 5 serves, among other things, to control the pumps 4.

[0009] It should be mentioned that redox flow batteries are also 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. For an example, reference is made to WO 2022 / 183162 A1 (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.

[0010] 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 fluid stored in tank 3. In addition to the designations from Fig. 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 enters tank 3 at a point well below the liquid level, while the return line 6.2 enters tank 3 just below the liquid level. The two outlets are positioned as far apart as possible. This arrangement ensures that, with a sufficiently high flow rate through the piping system, there are virtually no dead spaces in tank 3. This means that all of the electrolyte stored in tank 3 participates in the charging and discharging reactions and is therefore available as capacity. However, other arrangements of the outlets within the tank with similarly good properties are also possible.

[0011] During the charging or discharging process, without any special measures to improve the mixing of the electrolyte fluid stored in tank 3, an imperfect mixture of the electrolyte fluid stored in tank 3 occurs. This is also referred to as stratified charging, because layers form in tank 3 with different charge levels of the electrolyte fluid, with the charge level varying in the different layers. Fig. 2. This is schematically indicated by the dashed lines. Along the dashed lines, the state of charge of the electrolyte fluid is constant, while the state of charge of the electrolyte fluid changes perpendicular to the dashed lines. During a charging process, the state of charge of the electrolyte fluid is highest in the upper right and lowest in the lower left. During a discharging process, it is the other way around. The arrows in Fig. 2 indicates the flow direction of the electrolyte fluid in the electrolyte circuit shown.

[0012] 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 levels combined with high temperatures can cause vanadium to precipitate. At low temperatures, V 2+-Crystals form. Furthermore, in this case, aging processes in the cell structure due to carbon corrosion can occur. To avoid these negative effects, the charging process must be prematurely terminated if the mixing is imperfect. Specifically, when the point in the tank with the highest state of charge reaches its maximum charge. However, at that point, the entire remaining volume in the tank has a state of charge that is still below the maximum charge. Therefore, the theoretically possible capacity of the battery cannot be utilized. On the other hand, it is not easy to determine the aforementioned point in time precisely from a measurement perspective, as the state of charge cannot usually be measured locally for a specific point in the tank. Therefore, larger safety margins must generally be factored in, which further reduces the usable capacity. Therefore, it is advantageous if the mixing in tank 3 can be improved.

[0013] The inventors recognized that the mixing of the electrolyte fluid in the tank could be improved by including an additional pipe with a shut-off valve in each electrolyte circuit. This pipe branches off from the main pipe system at one end and connects to the electrolyte circuit at the other. The branch point of the additional pipe is located downstream of the pump in the direction of flow. This additional pipe is used to improve the mixing of the electrolyte fluid stored in the tank under certain operating conditions. This is achieved by changing the closed position of the shut-off valve for a predefined period, which alters the flow conditions within the tank during this time, resulting in better mixing.The control device 5 is designed to control the closing state of the shut-off valve in order to improve the mixing of the electrolyte fluid stored in the tank under predetermined operating conditions.

[0014] Fig. Figure 3 shows an electrolyte circuit according to the invention in a first embodiment. The electrolyte circuit comprises an additional pipeline, which is 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 additional pipeline 7 branches off from the supply line 6.1 at a first end. The branch is located between the pump 4 and the cell arrangement 2. The additional pipeline 7 opens into the return line 6.2 at its second end.

[0015] The in Fig. The arrangement shown in Figure 3 can be used to improve the mixing of the electrolyte fluid stored in tank 3 by the following operating procedure. During normal operation of the redox flow battery, the shut-off valve 8 remains closed. This results in the following: Fig. The flow and mixing conditions shown in Figure 2 are not present because the electrolyte fluid cannot pass through the additional pipe. Under predefined operating conditions of the redox flow battery, the shut-off valve 8 is opened for a predefined period. Since the cell array 2 in the electrolyte circuit represents by far the highest flow resistance for the electrolyte fluid, and the additional pipe 7 forms a bypass around the cell array 2, opening the shut-off valve 8 drastically reduces the flow resistance in the electrolyte circuit. As a result, the volume flow rate in the electrolyte circuit increases significantly while the pump output of the pump 4 remains constant. This increase in volume flow rate alone improves mixing, for example, by mobilizing any dead spaces that may be present.Since the pumped electrolyte fluid no longer passes through the cell assembly when the shut-off valve 8 is open, the electrolyte fluid flowing back into tank 3 has the same state of charge as the electrolyte fluid directly at the inlet of the supply line 6.1. This significantly improves the mixing of the electrolyte fluid near the outlet of the return line 6.2. If the time during which the shut-off valve 8 is open is kept sufficiently short, the charging or discharging process of the redox flow battery 1 does not need to be interrupted, as the amount of electrolyte fluid stored in the cell assembly allows for short-term operation of the redox flow battery 1 without the need to supply fresh electrolyte fluid to the cell assembly 2.The process can be repeated, provided that the intervals between the opening periods of the shut-off valve 8 are long enough to allow the cell assembly 2 to be completely filled with fresh electrolyte fluid during these intervals. It is advantageous if the interval between two opening periods of the shut-off valve 8 is longer than the duration of the opening periods themselves.

[0016] The Fig. 4 and Fig. Figure 5 shows embodiments of electrolyte circuits in which the additional pipeline 7 branches off from the supply line 6.1 at its first end and opens into the tank 3 at its second end. This means that a separate feedthrough into the tank 3 is provided for the additional pipeline 7. The location where the additional pipeline 7 opens into the tank 3 can be chosen arbitrarily, although locations near the inlet of the supply line 6.1 are rather disadvantageous, as they do not significantly improve the mixing in the tank 3. Advantageous locations include those that tend to form dead spaces. Opening the shut-off valve 8 then mobilizes any dead spaces that may be present. Fig. 4 and Fig. Figure 5 shows two suitable locations for the outlet of the additional pipeline 7 into tank 3. Fig. 4. The additional pipeline 7 enters tank 3 near the surface, approximately in the middle. Fig. 5 The additional pipeline 7 leads into tank 3 near the bottom of the tank, close to the tank wall.

[0017] The operating procedure for the embodiments according to Fig. 4 and Fig. 5 proceeds analogously to the operating procedure which is associated with Fig. 3 was described.

[0018] The Fig. 6a and Fig. Figure 6b shows a further embodiment of an electrolyte circuit according to the invention. In contrast to the embodiments shown previously, here the additional pipe 7 branches off at its first end from the return line 6.2, i.e., the branch is located downstream of the cell arrangement 2 in the direction of flow. As in the embodiments according to the Fig. 4 and Fig. The additional pipeline 7 opens at its second end into tank 3. In this embodiment, the additional pipeline 7 does not constitute 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.

[0019] During normal operation of the redox flow battery, the shut-off valve 8 remains open. This results in the following: Fig. The flow conditions shown in Figure 6a are described below. Under 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 conditions shown in Figure 6a. Fig. Flow conditions shown in 6b. So that the conditions shown in the Fig. 6a and Fig. To achieve the flow conditions shown in Figure 6b, the additional pipe 7 must open into tank 3 at a point that is vertically (i.e., in the direction of gravity) above the point where the outlet of the return line 6.2 is located. Then the back pressure in the return line 6.2 due to the electrolyte fluid stored in tank 3 is greater than the back pressure in the additional pipe 7.

[0020] 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 there is 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 supplied with fresh electrolyte fluid regardless of the closing state of the shut-off valve 8.

[0021] The operating method according to the invention for improving the electrolyte fluid stored in tank 3 therefore generally comprises the following step: - For predefined operating states of the redox flow battery, the closing state of the shut-off valve 8 is changed by the control unit 5 for a predefined period of time.

[0022] The predefined operating states, in which the closing state of the 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 vanadium-based redox flow batteries, these are, for example, the operating states in which the redox flow battery is near its maximum possible state of charge during charging.

[0023] It should be noted that the pump output in redox flow batteries is typically regulated to minimize the energy required to circulate the electrolyte. This promotes the formation of dead spaces in the tanks. Therefore, it is advantageous if the pump output of the associated electrolyte circuit is increased by the control unit during the predefined periods when the closing state of the shut-off valve is changed.

[0024] 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 the outside into the tanks or locally varying heat dissipation to the outside can lead to uneven temperature distribution in the tanks. The improved mixing achieved 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 predefined operating conditions can include operating conditions in which increased temperature inhomogeneity is present. To identify these operating conditions, the redox flow battery can include measuring devices with which temperature inhomogeneity can be detected.

[0025] Redox flow batteries typically include an electrochemical measuring cell for measuring the so-called open-circuit voltage (OCV). OCV cells are often hydraulically arranged in 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.

[0026] For various reasons, the state of charge determined using open-circuit voltage (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 reservoir and in the cell array. For this purpose, the entire electrolyte circuit is modeled using partial volumes. In conjunction with the measured values ​​of an OCV cell and a coulomb count, the observer algorithm then provides the state of charge of the electrolyte fluid in the individual partial volumes. Furthermore, the observer algorithm provides information about any electrolyte shifting that may be present. Such an observer algorithm can be advantageously used in both the inventive and conventional redox flow batteries. Reference symbol list 1 Redox Flow Battery 2 Cell arrangement 3 Tank 4 pump 5 Control unit 6.1 6.2 Supply line Return line 7 Additional pipeline 8 shut-off valve

Claims

[1] Redox flow battery (1) comprising a cell assembly (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 comprises a supply line (6.1) from the tank (3) to the cell assembly (2) and a return line (6.2) from the cell assembly (2) to the tank (3), and wherein the pump (4) is arranged in the supply line, characterized by, that 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 at a first end from the supply line (6.1) in the direction of flow of the electrolyte fluid downstream of the pump (4) and opens at a second end into the return line (6.2), and wherein the control device (5) is configured to control the closing states of the shut-off valves (8) in order to improve the mixing of the electrolyte fluid stored in the tanks (3) under predetermined operating conditions. [2] Redox flow battery (1) comprising a cell assembly (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 comprises a supply line (6.1) from the tank (3) to the cell assembly (2) and a return line (6.2) from the cell assembly (2) to the tank (3), and wherein the pump (4) is arranged in the supply line, characterized by, that 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 from the return line (6.2) at a first end and opens into the tank (3) at a second end, and wherein the return line (6.2) comprises an outlet opening, and wherein the location where the second end of the additional pipeline (7) is arranged is located vertically above the location where the outlet opening is arranged, and wherein the control device (5) is configured to control the closing states of the shut-off valves (8) in order to improve the mixing of the electrolyte fluid stored in the tanks (3) under predetermined operating conditions. [3] Operating method for a redox flow battery (1) for improving the mixing of the electrolyte fluid stored in a tank (3), the redox flow battery (1) comprising: a cell assembly (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 comprises a supply line (6.1) from the tank (3) to the cell assembly (2) and a return line (6.2) from the cell assembly (2) to the tank (3), and wherein the pump (4) is arranged in the supply line, each electrolyte circuit comprising an additional pipe (7) and a shut-off valve (8), wherein the shut-off valve (8) is arranged in the additional pipe (7), and wherein the additional pipe (7) extends at a first end from the supply line (6.1) branches off in the direction of flow of the electrolyte fluid behind the pump (4) and opens with a second end into the return line (6.2), and wherein the control device (5) is designed to control the closing states of the shut-off valves (8) in order to improve the mixing of the electrolyte fluid stored in the tanks (3) in predetermined operating conditions, wherein the operating procedure comprises the following step: . For 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. [4] Operating method according to claim 3 for a redox flow battery (1) according to claim 1, wherein an intermediate time between two open periods of the shut-off valve (8) is longer than the respective duration of the open periods. [5] Operating method according to claim 3, wherein during the predefined time period 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).

Images