Patent Description:
As a large-capacity storage battery, a redox flow battery (which may hereinafter be referred to as "RF battery") is known, which performs charge and discharge by circulating positive and negative electrolytes through a battery cell (see, e.g., Patent Literatures <NUM> and <NUM>). The RF battery includes the battery cell, a positive electrolyte tank and a negative electrolyte tank configured to store therein a positive electrolyte and a negative electrolyte, respectively, and a positive electrolyte circulation path and a negative electrolyte circulation path each configured to allow a corresponding one of the electrolytes to circulate between a corresponding one of the tanks and the battery cell. Patent Literatures <NUM> and <NUM> each describe an RF battery that includes a communicating tube configured to allow communication between the positive electrolyte tank and the negative electrolyte tank. <CIT> relates to an inherently safe redox flow battery storage system. <CIT> relates to a redox-flow battery and method of operating thereof. <CIT>, which relates to gas management systems and methods in a redox flow battery, and <CIT>, which relates to a redox flow cell and its application, form the basis for the preamble of claim <NUM>.

A redox flow battery of the present invention is according to claim <NUM>.

When charge and discharge cycles are repeated during operation of a redox flow battery (RF battery), a phenomenon called "electrolyte crossover" may occur, in which positive and negative electrolytes are each transferred from one side to the other through a membrane interposed between a positive electrode and a negative electrode inside a battery cell. This may create a difference between the volumes of the electrolytes in a positive electrolyte tank and a negative electrolyte tank and lead to reduced battery capacity (discharge capacity).

As a solution to such problems associated with the electrolyte crossover, Patent Literatures <NUM> and <NUM> each disclose a technique in which the positive electrolyte tank and the negative electrolyte tank are connected by a communicating tube and when a difference occurs between the volumes of the electrolytes in the tanks, the levels of the electrolytes in the tanks are adjusted to the same height using the communicating tube. In the techniques disclosed in Patent Literatures <NUM> and <NUM>, however, the communicating tube is connected and positioned at a level substantially the same as, or lower than, the levels of the electrolytes in the tanks. This may cause electrolyte to leak through a connection of each tank with the communicating tube, or may cause electrolyte to flow out of the communicating tube in the event of breakage of the communicating tube.

Accordingly, the present invention aims to provide a redox flow battery that is capable not only of adjusting the volumes of the electrolytes in the positive electrolyte tank and the negative electrolyte tank, but also of preventing each electrolyte from flowing out of the tank.

The present invention provides a redox flow battery that is capable not only of adjusting the volumes of the electrolytes in the positive electrolyte tank and the negative electrolyte tank, but also of preventing each electrolyte from flowing out of the tank.

First, aspects of an embodiment of the invention of the present application will be listed.

A redox flow battery according to the present invention is according to claim <NUM>.

In the redox flow battery described above, the one and other open ends of the communicating tube are immersed in the positive electrolyte and the negative electrolyte, and the intermediate portion of the communicating tube is stretched above the levels of both the electrolytes. The communicating tube becomes a siphon by being filled with electrolyte. Then, when a difference occurs between the volumes of the electrolytes in the tanks, the levels of the electrolytes are adjusted to the same height through the communicating tube using the siphon principle. The volumes of the electrolytes are thus automatically adjusted to maintain the levels of the electrolytes in the tanks. If the communicating tube is broken, the resulting entry of air into the communicating tube terminates the siphon. As described above, the intermediate portion of the communicating tube is disposed above the levels of both the electrolytes. Therefore, even in the event of breakage, the electrolyte in the communicating tube is returned to either of the tanks by termination of the siphon. That is, even if the communicating tube is broken, the electrolyte in the communicating tube is prevented from flowing out of the tank. The redox flow battery described above is thus capable not only of adjusting the volumes of the electrolytes in the positive electrolyte tank and the negative electrolyte tank, but also of preventing each electrolyte from flowing out of the tank.

The redox flow battery includes an introducing tube configured to connect at least one of the positive electrolyte circulation path and the negative electrolyte circulation path to the communicating tube, and an open-close valve configured to open and close the introducing tube.

To serve as a siphon, the communicating tube needs to be filled with electrolyte. In this aspect of the redox flow battery, which includes the introducing tube, the electrolyte is introduced through the introducing tube into the communicating tube when the redox flow battery starts, and this allows the communicating tube to be filled with the electrolyte. Also, with the open-close valve, a siphon can be created when the open-close valve closes the introducing tube to block the flow between the circulation path and the communicating tube, with the communicating tube being filled with the electrolyte.

In another aspect of the redox flow battery, the open ends of the communicating tube may each be located on a bottom side in a corresponding one of the tanks.

Entry of a gas into the communicating tube may terminate the siphon. Gas bubbles may be produced in the vicinity of the surface of the electrolyte in each tank. Therefore, if an open end of the communicating tube is located near the surface of the electrolyte in the tank, gas bubbles are easily drawn in through the open end. In this aspect of the redox flow battery, where each open end of the communicating tube is located on the bottom side in the tank, gas bubbles are not easily drawn in through the open end. This makes it easier to keep the communicating tube in a siphon state.

In another aspect of the redox flow battery, the open ends of the communicating tube may be formed to face upward.

In this aspect of the redox flow battery, where the open ends of the communicating tube are formed to face upward, gas bubbles are not easily drawn in through the open ends. This also makes it easier to keep the communicating tube in a siphon state.

In another aspect of the redox flow battery, the communicating tube may be provided with a flow control valve.

In this aspect of the redox flow battery, where the communicating tube is provided with a flow control valve, it is possible to control, with the flow control valve, the flow rate (or the amount of transfer) of the electrolyte passing through the communicating tube when the volumes of the electrolytes in the tanks are adjusted through the communicating tube using the siphon principle.

Another aspect of the redox flow battery may include a gas vent pipe configured to allow gas bubbles to escape from inside the communicating tube, and the gas vent pipe may be connected at one end thereof to the communicating tube and connected at the other end thereof to at least one of the positive electrolyte circulation path and the negative electrolyte circulation path.

In this aspect of the redox flow battery, which includes the gas vent pipe, gas bubbles accumulated in the communicating tube can be discharged through the gas vent pipe to the circulation path. This makes it possible to keep the communicating tube in a siphon state.

Examples of a redox flow battery (RF battery) according to an embodiment of the invention of the present application will now be described with reference to the drawings. The same reference numerals in the drawings denote the same or corresponding parts. The invention of the present application is not limited to the examples described below, and is defined by the appended claims.

An RF battery according to the embodiment is typically connected through an AC/DC converter to an electric system and performs charge and discharge. The charge and discharge process involves using a positive electrolyte and a negative electrolyte each containing, as active materials, metal ions whose valences are changed by oxidation-reduction. The charge and discharge process is performed using a difference between the oxidation-reduction potential of the ions contained in the positive electrolyte and the oxidation-reduction potential of the ions contained in the negative electrolyte.

An RF battery <NUM> according to the embodiment will now be described with reference to <FIG> and <FIG>. As illustrated in <FIG>, the RF battery <NUM> of the embodiment includes a battery cell <NUM>, a positive electrolyte tank <NUM> and a negative electrolyte tank <NUM>, and a positive electrolyte circulation path <NUM> and a negative electrolyte circulation path <NUM>. A feature of the RF battery <NUM> is that it includes a communicating tube <NUM> for an adjustment which is made using the siphon principle such that electrolytes 10P and 10N stored in the positive electrolyte tank <NUM> and the negative electrolyte tank <NUM>, respectively, have the same level. Hereinafter, the configuration of the RF battery <NUM> will be described in detail.

As illustrated in <FIG>, the battery cell <NUM> includes a positive electrode <NUM>, a negative electrode <NUM>, and a membrane <NUM> interposed between the electrodes <NUM> and <NUM>, and a positive electrode cell <NUM> and a negative electrode cell <NUM> are formed with the membrane <NUM> therebetween. The membrane <NUM> is, for example, an ion-exchange membrane that allows hydrogen ions to pass therethrough. The battery cell <NUM> (i.e., the positive electrode cell <NUM> and the negative electrode cell <NUM>) has the positive electrolyte circulation path <NUM> and the negative electrolyte circulation path <NUM> connected thereto, and allows the positive electrolyte 10P and the negative electrolyte 10N to circulate therethrough. The electrolytes 10P and 10N may be ones that contain, as active materials, metal ions of the same type. For example, the electrolytes 10P and 10N may each be an electrolyte containing vanadium ions, an electrolyte containing either manganese ions or titanium ions, or an electrolyte containing both manganese ions and titanium ions.

The battery cell <NUM> may be configured either as a single cell including one battery cell <NUM>, or as a multicell including a plurality of battery cells <NUM>. In the case of a multicell, a structure called a cell stack <NUM> (see <FIG>) is used, which is formed by stacking a plurality of battery cells <NUM>. The cell stack <NUM> includes a substack <NUM> sandwiched on both sides by two end plates <NUM>, which are fastened with a fastening mechanism <NUM> (see the lower part of <FIG> illustrates a structure including a plurality of substacks <NUM>. The substacks <NUM> are each formed by sequentially stacking a cell frame <NUM>, the positive electrode <NUM>, the membrane <NUM>, and the negative electrode <NUM> in layers (see the upper part of <FIG>) and sandwiching the resulting layered body between supply/drainage plates <NUM> on both sides. The number of battery cells <NUM> stacked in layers to form the cell stack <NUM> can be appropriately determined.

As illustrated in the upper part of <FIG>, each cell frame <NUM> includes a bipolar plate <NUM> disposed between the positive electrode <NUM> and the negative electrode <NUM>, and a frame body <NUM> disposed around the bipolar plate <NUM>. The positive electrode <NUM> is disposed on one side of the bipolar plate <NUM>, and the negative electrode <NUM> is disposed on the other side of the bipolar plate <NUM>. The bipolar plate <NUM> is disposed inside the frame body <NUM>, and a recessed portion 32o is defined by the bipolar plate <NUM> and the frame body <NUM>. The recessed portion 32o is provided on each side of the bipolar plate <NUM>. The positive electrode <NUM> and the negative electrode <NUM> are housed in the respective recessed portions 32o, with the bipolar plate <NUM> interposed therebetween, and a first side of the frame body <NUM> of each cell frame <NUM> and a second side of the frame body <NUM> of an adjacent cell frame <NUM> are joined opposite each other. In the substack <NUM> (cell stack <NUM>), the positive electrode <NUM> and the negative electrode <NUM>, with the membrane <NUM> therebetween, are arranged between the bipolar plates <NUM> of adjacent cell frames <NUM> to form one battery cell <NUM>. To prevent leakage of electrolyte, the frame bodies <NUM> of adjacent cell frames <NUM> are each provided with an annular sealing member <NUM>, such as an O ring or flat gasket, therebetween.

For example, the bipolar plate <NUM> is made of plastic carbon, and the frame body <NUM> is made of plastic, such as vinyl chloride resin (PVC), polypropylene, polyethylene, fluorine resin, or epoxy resin. In this example, each cell frame <NUM> includes the bipolar plate <NUM> and the frame body <NUM> therearound that are integrally formed, for example, by injection molding.

The circulation of electrolyte into the battery cell <NUM> is made through the supply/drainage plates <NUM> (see the lower part of <FIG>) and also through liquid supply manifolds <NUM> and <NUM> and liquid discharge manifolds <NUM> and <NUM> passing through the frame body <NUM> of each cell frame <NUM> (see the upper part of <FIG>) and liquid supply slits <NUM> and <NUM> and liquid discharge slits <NUM> and <NUM> formed in the frame body <NUM>. In the case of the cell frame <NUM> (frame body <NUM>) of this example, the positive electrolyte is supplied from the liquid supply manifold <NUM> in the lower part of the frame body <NUM>, through the liquid supply slit <NUM> on the first side of the frame body <NUM>, to the positive electrode <NUM>, and then discharged through the liquid discharge slit <NUM> in the upper part of the frame body <NUM> to the liquid discharge manifold <NUM>. Similarly, the negative electrolyte is supplied from the liquid supply manifold <NUM> in the lower part of the frame body <NUM>, through the liquid supply slit <NUM> on the second side of the frame body <NUM>, to the negative electrode <NUM>, and then discharged through the liquid discharge slit <NUM> in the upper part of the frame body <NUM> to the liquid discharge manifold <NUM>. A flow-guiding portion (not shown) may be formed along the lower and upper inner edges of the frame body <NUM> having the bipolar plate <NUM> therein. The flow-guiding portions have the function of diffusing the electrolytes supplied through the liquid supply slits <NUM> and <NUM> along the lower edges of the electrodes <NUM> and <NUM>, and collecting the electrolytes discharged from the upper edges of the electrodes <NUM> and <NUM> into the liquid discharge slits <NUM> and <NUM>.

As illustrated in <FIG>, the positive electrolyte tank <NUM> and the negative electrolyte tank <NUM> store therein the positive electrolyte 10P and the negative electrolyte 10N, respectively. The tanks <NUM> and <NUM> are of the same shape and capacity. The upper part of the interior of each of the tanks <NUM> and <NUM> (i.e., upper part above the level of each of the electrolytes 10P and 10N) is a gas-phase portion. The positive electrolyte tank <NUM> has an outlet <NUM> and an inlet <NUM> connected to a supply pipe <NUM> and a return pipe <NUM>, respectively, of the positive electrolyte circulation path <NUM>. The negative electrolyte tank <NUM> has an outlet <NUM> and the inlet <NUM> connected to a supply pipe <NUM> and a return pipe <NUM>, respectively, of the negative electrolyte circulation path <NUM>. In this example, the outlets <NUM> and <NUM> and the inlets <NUM> and <NUM> are located above the levels of the electrolytes 10P and 10N in the tanks <NUM> and <NUM>, or specifically, at the tops of the tanks <NUM> and <NUM>. The outlet <NUM> and the inlet <NUM> are each provided with an open-close valve <NUM>, and the outlet <NUM> and the inlet <NUM> are each provided with an open-close valve <NUM>.

The tanks <NUM> and <NUM> have openings <NUM> and <NUM>, respectively, to which the communicating tube <NUM> is connected. The openings <NUM> and <NUM> are disposed above the levels of the electrolytes 10P and 10N in the tanks <NUM> and <NUM>. In this example, the openings <NUM> and <NUM> are located at the tops of the tanks <NUM> and <NUM>. The openings <NUM> and <NUM> are provided with open-close valves <NUM> and <NUM>, respectively.

As illustrated in <FIG>, the positive electrolyte circulation path <NUM> connects the positive electrolyte tank <NUM> to the battery cell <NUM>, whereas the negative electrolyte circulation path <NUM> connects the negative electrolyte tank <NUM> to the battery cell <NUM>, thereby allowing the electrolytes 10P and 10N to circulate between the tanks <NUM> and <NUM> and the battery cell <NUM>. The positive electrolyte circulation path <NUM> includes the supply pipe <NUM> configured to supply the positive electrolyte 10P from the positive electrolyte tank <NUM> to the positive electrode cell <NUM>, and the return pipe <NUM> configured to return the positive electrolyte 10P from the positive electrode cell <NUM> to the positive electrolyte tank <NUM>. The negative electrolyte circulation path <NUM> includes the supply pipe <NUM> configured to supply the negative electrolyte 10N from the negative electrolyte tank <NUM> to the negative electrode cell <NUM>, and the return pipe <NUM> configured to return the negative electrolyte 10N from the negative electrode cell <NUM> to the negative electrolyte tank <NUM>. The supply pipes <NUM> and <NUM> of the circulation paths <NUM> and <NUM> are connected to the outlets <NUM> and <NUM>, respectively, of the tanks <NUM> and <NUM>, and the return pipes <NUM> and <NUM> of the circulation paths <NUM> and <NUM> are connected to the inlets <NUM> and <NUM>, respectively, of the tanks <NUM> and <NUM>.

End portions of the respective supply pipes <NUM> and <NUM> are inserted through the outlets <NUM> and <NUM>, respectively, into the tanks <NUM> and <NUM>, and open ends <NUM> and <NUM> of these end portions are disposed below the levels of the electrolytes 10P and 10N in the tanks <NUM> and <NUM>. That is, the open ends <NUM> and <NUM> of the supply pipes <NUM> and <NUM> are immersed in the electrolytes 10P and 10N, respectively, which are drawn in through the open ends <NUM> and <NUM>. In this example, the open ends <NUM> and <NUM> of the supply pipes <NUM> and <NUM> are each located on the bottom side in the corresponding one of the tanks <NUM> and <NUM>. Note that "located on the bottom side in the tank" refers to being located below the level of the electrolyte 10P or 10N, that is, h/<NUM> or less from the bottom of the tank <NUM> or <NUM>, where h is a height from the bottom of the tank <NUM> or <NUM> to the surface of the electrolyte 10P or 10N.

The supply pipes <NUM> and <NUM> are provided with pumps <NUM> and <NUM>, respectively, configured to suck up the electrolytes 10P and 10N from the tanks <NUM> and <NUM> and pressure-feed them. During charge and discharge operation, the pumps <NUM> and <NUM> circulate the electrolytes 10P and 10N, respectively, through the battery cell <NUM> (i.e., the positive electrode cell <NUM> and the negative electrode cell <NUM>). In standby mode where neither charge nor discharge takes place, the pumps <NUM> and <NUM> are off and the electrolytes 10P and 10N are not circulated.

As illustrated in <FIG>, the communicating tube <NUM> is a tube immersed at one open end <NUM> thereof in the positive electrolyte 10P, stretched at an intermediate portion <NUM> thereof above the levels of the electrolytes 10P and 10N, and immersed at the other open end <NUM> thereof in the negative electrolyte 10N. The communicating tube <NUM> is configured to allow liquid-phase portions in the tanks <NUM> and <NUM> to communicate with each other. The communicating tube <NUM> is connected to the openings <NUM> and <NUM> of the tanks <NUM> and <NUM>. In this example, the communicating tube <NUM> is inserted at both end portions thereof through the openings <NUM> and <NUM> of the tanks <NUM> and <NUM> into the tanks <NUM> and <NUM>, and the open ends <NUM> and <NUM> of both the end portions are disposed below the levels of the electrolytes 10P and 10N in the tanks <NUM> and <NUM>. The intermediate portion <NUM> is placed above the tanks <NUM> and <NUM>. In this example, the open ends <NUM> and <NUM> of the communicating tube <NUM> are each located on the bottom side in a corresponding one of the tanks <NUM> and <NUM>.

The communicating tube <NUM> becomes a siphon by being filled with electrolyte. Thus, when a difference occurs between the volumes of the electrolytes 10P and 10N in the tanks <NUM> and <NUM>, the levels of the electrolytes 10P and 10N are adjusted to the same height using the siphon principle. When the tube forming the communicating tube <NUM> (or the intermediate portion <NUM> in particular) is made of a transparent material, it is possible to visually recognize from the outside that the communicating tube <NUM> is filled with electrolyte. The intermediate portion <NUM> may have a window at the top (or highest portion) thereof, and the window may be made of a transparent material. Examples of the transparent material include transparent resin, such as vinyl chloride resin, and glass.

The communicating tube <NUM> may be appropriately designed to satisfy the siphon principle. Dimensions of the communicating tube will now be described with reference primarily to <FIG>.

If a height H from the level of the electrolytes 10P and 10N in the tanks <NUM> and <NUM> to the top of the communicating tube <NUM> is too high, the transfer of electrolyte based on the siphon principle fails. The maximum height Hmax that satisfies the siphon principle is determined by the following equation:<MAT> where PO (N/m<NUM>) is pressure in the tank, ρ (kg/m<NUM>) is an electrolyte density, and g (m/s<NUM>) is the acceleration of gravity.

When the pressure P0 is equal to the atmospheric pressure (<NUM> × <NUM><NUM> N/m<NUM>) and the electrolyte density ρ is <NUM>/m<NUM>, then Hmax is <NUM>. Therefore, the installation level of the intermediate portion <NUM> (corresponding to the height H) is less than <NUM> from the level of the electrolytes 10P and 10N in the tanks <NUM> and <NUM>.

If a length L of the communicating tube <NUM> is too long, the resulting increase in frictional resistance in the communicating tube <NUM> leads to an increased flow friction loss and lowers the flow rate of the electrolyte passing through the communicating tube <NUM>. This means that it takes time to adjust the electrolyte levels. The adjustment of the electrolyte levels is preferably completed within <NUM> minutes (or <NUM> seconds). Therefore, the length L of the communicating tube <NUM> is preferably set to ensure that the flow rate of the electrolyte passing through the communicating tube <NUM> is above a certain level. For example, the length L may be <NUM>, or more preferably <NUM> or less.

An inside diameter d of the communicating tube <NUM> may also be appropriately set. For example, the inside diameter d may range from <NUM> to <NUM>, or more preferably from <NUM> to <NUM>. The flow friction loss varies depending also on the inside diameter d of the communicating tube <NUM>. That is, the smaller the inside diameter d, the greater the flow friction loss. Therefore, it is preferable to appropriately set the length L of the communicating tube <NUM> in accordance with the inside diameter d. Specifically, it is preferable to set the length L of the communicating tube <NUM> such that when a difference occurs between the volumes of the electrolytes 10P and 10N in the tanks <NUM> and <NUM>, it takes <NUM> minutes (or <NUM> seconds) or less until the levels of the electrolytes 10P and 10N reach the same height. In this case, for example, the length L of the communicating tube <NUM> may be less than or equal to <NUM> times the inside diameter d (L ≤ 100d). Specifically, the length L may be <NUM> or less if the inside diameter d is <NUM>, L may be <NUM> or less if d is <NUM>, and L may be <NUM> or less if d is <NUM>.

The RF battery <NUM> illustrated in <FIG> includes an introducing tube <NUM> configured to connect the positive electrolyte circulation path <NUM> to the communicating tube <NUM>, and an open-close valve <NUM> configured to open and close the introducing tube <NUM>. In this example, the introducing tube <NUM> is connected at one end thereof in such a manner as to branch off the supply pipe <NUM> of the positive electrolyte circulation path <NUM> and is connected at the other end thereof to the intermediate portion <NUM> of the communicating tube <NUM>. Also, in this example, the supply pipe <NUM> has an open-close valve <NUM> downstream of its connection with the introducing tube <NUM> (i.e., located closer to the battery cell <NUM> than the connection is).

The introducing tube <NUM> and the open-close valve <NUM> are used to form a siphon, when the RF battery <NUM> starts, by filling the communicating tube <NUM> with electrolyte. Specifically, when the RF battery <NUM> starts, the open-close valve <NUM> is opened to start the pump <NUM>, with the supply pipe <NUM> and the communicating tube <NUM> communicating with each other through the introducing tube <NUM>, and circulate the positive electrolyte 10P through the communicating tube <NUM>. This makes it possible to introduce the positive electrolyte 10P into the communicating tube <NUM> and fill the communicating tube <NUM> with the electrolyte. Then, with the communicating tube <NUM> being filled with the electrolyte, the open-close valve <NUM> closes the introducing tube <NUM> to block the flow between the supply pipe <NUM> and the communicating tube <NUM>. This creates a liquid-tight state in the communicating tube <NUM> and thereby forms a siphon. After the RF battery <NUM> starts, the open-close valve <NUM> is always in a closed state during the operation.

When, for example, the communicating tube <NUM> is removed from the tanks <NUM> and <NUM> for maintenance of the RF battery <NUM>, the pump <NUM> is stopped and the open-close valve <NUM> is opened. This terminates the siphon, causes the electrolyte in the communicating tube <NUM> to return to the tanks <NUM> and <NUM>, and thereby facilitates the maintenance work. After the communicating tube <NUM> is removed from the tanks <NUM> and <NUM>, the open-close valves <NUM> and <NUM> are closed to prevent air from entering the tanks <NUM> and <NUM> through the openings <NUM> and <NUM>. This inhibits oxidation of the electrolytes 10P and 10N during the maintenance work.

Although the introducing tube <NUM> is connected and attached to the positive electrolyte circulation path <NUM> (supply pipe <NUM>) in this example, the introducing tube <NUM> may be attached to the negative electrolyte circulation path <NUM> (supply pipe <NUM>) or may be attached to both the circulation paths <NUM> and <NUM>.

The RF battery <NUM> illustrated in <FIG> includes a gas-phase communicating tube <NUM> that allows the gas-phase portions in the tanks <NUM> and <NUM> to communicate with each other. With the gas-phase communicating tube <NUM>, it is possible to equalize pressures in the tanks <NUM> and <NUM>. The gas-phase communicating tube <NUM> is stretched over the tanks <NUM> and <NUM>. In this example, the gas-phase communicating tube <NUM> is connected to openings <NUM> and <NUM> at the tops of the tanks <NUM> and <NUM>. The openings <NUM> and <NUM> are provided with open-close valves <NUM> and <NUM>, respectively.

The RF battery <NUM> according to the embodiment described above has the following operational advantages.

With the communicating tube <NUM>, when a difference occurs between the volumes of the electrolytes 10P and 10N in the tanks <NUM> and <NUM> during operation of the RF battery <NUM>, the levels of the electrolytes 10P and 10N can be automatically adjusted to the same level through the communicating tube <NUM> using the siphon principle. As described above, the intermediate portion <NUM> of the communicating tube <NUM> is disposed above the levels of the electrolytes 10P and 10N. Therefore, even if the communicating tube <NUM> is broken, the electrolyte in the communicating tube <NUM> is returned to either of the tanks <NUM> and <NUM> by termination of the siphon. Thus, even in the event of breakage of the communicating tube <NUM>, it is possible to prevent the electrolyte in the communicating tube <NUM> from flowing out of the tanks <NUM> and <NUM>. Additionally, since the openings <NUM> and <NUM> of the tanks <NUM> and <NUM> to which the communicating tube <NUM> is connected are located above the levels of the electrolytes 10P and 10N, the electrolytes 10P and 10N are prevented from leaking through the openings <NUM> and <NUM>. Therefore, it is possible to effectively prevent the electrolytes 10P and 10N from flowing out of the positive electrolyte tank <NUM> and the negative electrolyte tank <NUM> while automatically adjusting the volumes of the electrolytes 10P and 10N in the tanks <NUM> and <NUM>.

With the introducing tube <NUM> and the open-close valve <NUM>, a siphon can be created by filling the communicating tube <NUM> with electrolyte when the RF battery <NUM> starts.

Since the open ends <NUM> and <NUM> of the communicating tube <NUM> are located on the bottom side in the tanks <NUM> and <NUM>, gas bubbles are not easily drawn in through the open ends <NUM> and <NUM>. This makes it easier to keep the communicating tube <NUM> in a siphon state.

As illustrated in <FIG>, the communicating tube <NUM> may be bent into a J shape at both end portions thereof to allow the open ends <NUM> and <NUM> to face upward. In this case, gas bubbles are not easily drawn in through the open ends <NUM> and <NUM>, and this makes it easier to maintain the siphon state.

Modifications of the RF battery <NUM> according to the aforementioned embodiment will now be described with reference to <FIG> and <FIG>.

The RF battery <NUM> according to a first modification illustrated in <FIG> differs from the aforementioned embodiment illustrated in <FIG> in that the communicating tube <NUM> is provided with a flow control valve <NUM>. Other configurations of the first modification are the same as those of the aforementioned embodiment.

The flow control valve <NUM> is disposed in the communicating tube <NUM> and configured to control the flow rate of the electrolyte passing through the communicating tube <NUM>. In this example, as illustrated in <FIG>, the flow control valve <NUM> is disposed in the intermediate portion <NUM> of the communicating tube <NUM>. When, in the RF battery <NUM> of the first modification, the volumes of the electrolytes 10P and 10N in the tanks <NUM> and <NUM> are adjusted through the communicating tube <NUM>, the flow rate (or the amount of transfer) of the electrolyte can be controlled by the flow control valve <NUM>. In some cases, the transfer of the electrolyte may be stopped by closing the flow control valve <NUM>. In this example, the flow control valve <NUM> is a motor-operated valve.

The RF battery <NUM> according to a second modification illustrated in <FIG> differs from the aforementioned embodiment illustrated in <FIG> in that it includes a gas vent pipe <NUM> for allowing gas bubbles to escape from inside the communicating tube <NUM>. Other configurations of the second modification are the same as those of the aforementioned embodiment.

The gas vent pipe <NUM> is used to vent, from the communicating tube <NUM>, gas bubbles accidentally drawn into the communicating tube <NUM>. The gas vent pipe <NUM> is connected at one end thereof to the communicating tube <NUM> and connected at the other end thereof to at least one of the positive electrolyte circulation path <NUM> and the negative electrolyte circulation path <NUM>. In this example, as illustrated in <FIG>, the gas vent pipe <NUM> is connected at one end thereof to the intermediate portion <NUM> in such a manner as to branch off the communicating tube <NUM> and connected at the other end thereof to the supply pipe <NUM> of the negative electrolyte circulation path <NUM> in such a manner as to join the supply pipe <NUM>. More specifically, the gas vent pipe <NUM> is connected at one end thereof to the top of the intermediate portion <NUM> and connected at the other end thereof to the supply pipe <NUM> at a location upstream of the pump <NUM> in the supply pipe <NUM> (i.e., closer to the tank <NUM> than the pump <NUM> is).

The gas vent pipe <NUM> is provided with a check valve <NUM>. The check valve <NUM> is disposed in the gas vent pipe <NUM> and configured to block the circulation from the negative electrolyte circulation path <NUM> (supply pipe <NUM>) to the communicating tube <NUM>. Additionally, in this example, an open-close valve <NUM> is provided downstream of the check valve <NUM> (i.e., closer to the supply pipe <NUM> than the check valve <NUM> is). The open-close valve <NUM> is in an open state when gas bubbles are to be removed from the communicating tube <NUM>, and is in a closed state when there is no need to remove gas bubbles from the communicating tube <NUM>.

In the RF battery <NUM> of the second modification, if gas bubbles are accidentally drawn into the communicating tube <NUM>, the gas bubbles accumulated in the communicating tube <NUM> can be vented through the gas vent pipe <NUM> to the negative electrolyte circulation path <NUM> (supply pipe <NUM>) using suction by the pump <NUM>. This allows the communicating tube <NUM> to be kept in a siphon state. With the gas vent pipe <NUM> having the check valve <NUM>, even when the pump <NUM> is stopped and the supply pipe <NUM> is emptied, the entry of gas from the supply pipe <NUM> into the communicating tube <NUM> can be blocked. Therefore, even when the pump <NUM> is stopped, the communicating tube <NUM> is kept in a siphon state and the electrolyte in the communicating tube <NUM> is not returned into either of the tanks <NUM> and <NUM>.

Additionally, by adjusting the degree of opening of the open-close valve <NUM>, it is possible to control the flow rate of the electrolyte passing through the gas vent pipe <NUM> and prevent the electrolyte from accidentally flowing out of the communicating tube <NUM> into the supply pipe <NUM>. Specifically, suction by the pump <NUM> causes the electrolyte to be fed little by little through the gas vent pipe <NUM> and the communicating tube <NUM> to the supply pipe <NUM>, so that gas bubbles accumulated in the communicating tube <NUM> are efficiently removed. When the communicating tube <NUM> is removed, for example, for maintenance of the RF battery <NUM>, the entry of air into the supply pipe <NUM> is prevented by closing the open-close valve <NUM>.

Claim 1:
A redox flow battery (<NUM>) comprising:
a battery cell (<NUM>);
a positive electrolyte tank (<NUM>) and a negative electrolyte tank (<NUM>) configured to store therein a positive electrolyte (10P) and a negative electrolyte (10N), respectively, and storing therein the positive electrolyte (10P) and the negative electrolyte (10N), respectively;
a positive electrolyte circulation path (<NUM>) and a negative electrolyte circulation path (<NUM>) each configured to allow a corresponding one of the electrolytes (10P, 10N) to circulate between a corresponding one of the tanks (<NUM>, <NUM>) and the battery cell (<NUM>); and
a communicating tube (<NUM>) including a tube immersed at one open end (<NUM>) thereof in the positive electrolyte (10P), stretched at an intermediate portion (<NUM>) thereof above levels of both the electrolytes (10P, 10N), and immersed at the other open end (<NUM>) thereof in the negative electrolyte (10N);
characterized in that the redox flow battery further comprises:
an introducing tube (<NUM>) configured to connect at least one of the positive electrolyte circulation path (<NUM>) and the negative electrolyte circulation path (<NUM>) to the communicating tube (<NUM>); and
an open-close valve (<NUM>) configured to open and close the introducing tube (<NUM>).