Patent Description:
Patent Literature (PTL) <NUM> discloses a redox flow battery that includes a cell configured to perform charge and discharge between itself and a power system, an electrolyte tank configured to store an electrolyte supplied to the cell, and a circulation mechanism disposed between the cell and the electrolyte tank and configured to circulate the electrolyte. The circulation mechanism includes a circulation pump, a pipe running from the electrolyte tank to the circulation pump, a pipe running from the circulation pump to the cell, and a pipe running from the cell to the electrolyte tank. The circulation pump is disposed to a side of the electrolyte tank. <CIT> relates to a redox flow battery. The preamble of claim <NUM> is based on this document.

A redox flow battery according to the present disclosure includes a cell, an electrolyte tank configured to store an electrolyte supplied to the cell and filled with the electrolyte, and a circulation mechanism disposed between the cell and the electrolyte tank and configured to circulate the electrolyte. The circulation mechanism includes a suction pipe configured to suck up the electrolyte from an open end thereof in the electrolyte to above an in-tank liquid level of the electrolyte in the electrolyte tank, a circulation pump disposed at an end portion of the suction pipe, an extrusion pipe running from a discharge port of the circulation pump to the cell, and a return pipe running from the cell to the electrolyte tank. An absolute value of a difference between HL1 and HL2 is greater than or equal to <NUM> times H<NUM> and both HL1 and HL2 are less than or equal to Hd, where H<NUM> is a height from an inner bottom surface of the electrolyte tank to the in-tank liquid level, HL1 is a length from the in-tank liquid level to the open end of the suction pipe, HL2 is a length from the in-tank liquid level to an open end of the return pipe in a depth direction of the electrolyte, and Hd is a distance from the in-tank liquid level to a center of a highest segment of the return pipe, the highest segment being located at the highest level of the return pipe. If the open end of the return pipe is located above the in-tank liquid level, the difference between HL1 and HL2 is HL1 and HL2 is zero.

In conventional redox flow batteries, a circulation pump is disposed to a side of an electrolyte tank to circulate an electrolyte in a cell. This means that if a pipe running from the electrolyte tank to the circulation pump is damaged, most of the electrolyte in the electrolyte tank may leak out.

Accordingly, an object of the present disclosure is to provide a redox flow battery that can prevent the electrolyte from leaking out of the electrolyte tank even if the pipe running from the electrolyte tank to the circulation pump is damaged.

In view of the problem described above, the present inventors have studied a configuration for sucking up the electrolyte to above the electrolyte tank. The configuration that sucks up the electrolyte tends to increase a suction height (also referred to as an actual suction head) by which the electrolyte is sucked up to the circulation pump and a height (actual push-up head) by which the electrolyte is pushed up from the circulation pump. As the sum of the actual suction head and the actual push-up head increases, the pump power of the circulation pump needs to be increased.

For the redox flow battery, it is also necessary to take into account the utilization ratio of the electrolyte in the electrolyte tank. The redox flow battery performs charge and discharge using changes in the valence of active material ions contained in the electrolyte. Therefore, if the height of the open end of the suction pipe for sucking up the electrolyte is equal to the height of the open end of the return pipe for returning the electrolyte back to the electrolyte tank, convection of the electrolyte is less likely to develop and this makes it difficult to effectively use active materials in the electrolyte tank. To increase the utilization ratio of the electrolyte, convection of the electrolyte in the electrolyte tank may be promoted by creating a difference between the height of the open end of the suction pipe and the height of the open end of the return pipe. However, as the lengths of the suction pipe and the return pipe increase, pressure loss associated with friction between the pipes and the electrolyte increases and this leads to increased pump power required for the circulation pump.

The present inventors have further studied the configuration for sucking up the electrolyte and have found out that by defining the relationship between the actual head and the lengths of the suction pipe and the return pipe, it is possible to reduce the size of the circulation pump included in the circulation mechanism and reduce power consumption required for operating the redox flow battery. Embodiments of the invention of the present application are listed and described below.

< <NUM> > A redox flow battery according to an embodiment includes a cell, an electrolyte tank configured to store an electrolyte supplied to the cell and filled with the electrolyte, and a circulation mechanism disposed between the cell and the electrolyte tank and configured to circulate the electrolyte. The circulation mechanism includes a suction pipe configured to suck up the electrolyte from an open end thereof in the electrolyte to above an in-tank liquid level of the electrolyte in the electrolyte tank, a circulation pump disposed at an end portion of the suction pipe, an extrusion pipe running from a discharge port of the circulation pump to the cell, and a return pipe running from the cell to the electrolyte tank. An absolute value of a difference between HL1 and HL2 is greater than or equal to <NUM> times H<NUM> and both HL1 and HL2 are less than or equal to Hd, where H<NUM> is a height from an inner bottom surface of the electrolyte tank to the in-tank liquid level, HL1 is a length from the in-tank liquid level to the open end of the suction pipe, HL2 is a length from the in-tank liquid level to an open end of the return pipe in a depth direction of the electrolyte, and Hd is a distance from the in-tank liquid level to a center of a highest segment of the return pipe, the highest segment being located at the highest level of the return pipe. If the open end of the return pipe is located above the in-tank liquid level, the difference between HL1 and HL2 is HL1 and HL2 is zero.

When the electrolyte is circulated from the electrolyte tank to the cell, the electrolyte is sucked up to above the in-tank liquid level. With this configuration, even if the suction pipe running from the electrolyte tank to the circulation pump is damaged, the electrolyte is less likely to leak out of the electrolyte tank. This is because damage to the suction pipe breaks hermeticity of the suction pipe and allows gravity to cause the electrolyte in the suction pipe to return to the electrolyte tank.

When the difference between HL1 and HL2 is small, convection of the electrolyte in the electrolyte tank is less likely to develop and the electrolyte in the electrolyte tank is not fully utilized. As a result, even when the electrolyte tank has a larger capacity, it is difficult to achieve the effect of improving the hour-rate capacity of the redox flow battery. On the other hand, when the difference between HL1 and HL2 is greater than or equal to <NUM> times H<NUM>, that is, when |HL1-HL2| ≥ <NUM><NUM> is satisfied, the distance from the open end of the return pipe to the open end of the suction pipe is long. This facilitates development of large convection in the electrolyte and improves the utilization ratio of the electrolyte in the electrolyte tank. Note that if the open end of the return pipe is located above the in-tank liquid level, HL2 is defined as zero (HL2 = <NUM>) and this gives HL1 ≥ <NUM><NUM>.

As described above, the configuration that sucks up the electrolyte tends to increase the distance Hd from the in-tank liquid level to the center of the highest segment of the return pipe located at the highest level, and this leads to increased pump power of the circulation pump. To reduce an increase in pump power, it is important to reduce friction loss in the suction pipe and the return pipe without reducing the utilization ratio of the electrolyte. Specifically, by making both HL1 and HL2 less than or equal to Hd, the pump power of the circulation pump for sucking up and circulating the electrolyte can be kept low. This makes it possible to reduce power consumption for operating the redox flow battery and achieve efficient operation of the redox flow battery.

<<NUM>> In an aspect of the redox flow battery according to the embodiment, HS may be less than or equal to <NUM> times Hd, where HS is a height from the in-tank liquid level to a center of a suction port of the circulation pump.

To suck up the electrolyte, it is necessary to consider a net positive suction head required (NPSHr) for the circulation pump and a net positive suction head available (NPSHa) which takes into account suction conditions. NPSHr is a value obtained by converting a minimum suction pressure required to avoid a decrease in pump efficiency caused by cavitation, into an electrolyte level (height) (m). NPSHr is a pump-specific value independent of liquid property or the like. In contrast, NPSHa is a head which takes into account suction conditions. NPSHa is a value which represents a margin against cavitation during suction of the electrolyte and can be determined by the following equation. To avoid the cavitation, NPSHr < NPSHa needs to be satisfied: <MAT> where.

Note that Hfs can be determined, for example, by the Darcy-Weisbach equation described below: <MAT> where.

As expressed by the derivation equation described above, NPSHa has a physical limitation and NPSHr < NPSHa may not be satisfied if HS (actual suction head) is too high. Therefore, it is preferable that the ratio of the actual suction head HS to the actual head Hd be less than or equal to <NUM>%.

<<NUM>> In another aspect of the redox flow battery according to the embodiment, the circulation pump may be a self-priming pump having a pump body including an impeller and a driving unit configured to rotate the impeller, and the pump body may be disposed above the in-tank liquid level.

The configuration described above facilitates maintenance of the circulation pump. This is because by stopping the circulation pump for maintenance of the circulation pump, the electrolyte in the suction pipe is returned to the electrolyte tank and this saves the trouble of taking the impeller out of the electrolyte. Depending on the type of circulation pump, however, the impeller may be disposed in the electrolyte while the driving unit is disposed above the in-tank liquid level of the electrolyte. Maintenance of such a circulation pump involves the trouble of taking the impeller out of the electrolyte. The electrolyte may spatter when the impeller is taken out.

<<NUM>> In an aspect of the redox flow battery according to the embodiment in which the pump body is disposed above the in-tank liquid level, the circulation pump may be provided with a priming tank disposed between the pump body and the suction pipe.

In the configuration with the priming tank, sucking the electrolyte in the priming tank with the circulation pump reduces gas-phase pressure in the priming tank and causes the electrolyte in the electrolyte tank to be sucked up into the priming tank. With this configuration, initial suction of the electrolyte stored in the electrolyte tank only involves pouring the electrolyte into the priming tank and operating the circulation pump. The initial suction operation is thus carried out easily. In the configuration without the priming tank, the electrolyte cannot be sucked up until completion of preparation which involves the trouble of filling the circulation pump and the suction pipe with the electrolyte.

<<NUM>> In another aspect of the redox flow battery according to the embodiment in which the pump body is disposed above the in-tank liquid level, the redox flow battery may include a cell chamber disposed on an upper surface of the electrolyte tank and containing the cell therein, and the pump body may be disposed in the cell chamber.

With this configuration, even if the electrolyte leaks near the pump body, the leaked electrolyte can be easily kept inside the cell chamber. This facilitates treatment of the leaked electrolyte and improves safety of the treatment.

Embodiments of a redox flow battery according to the present disclosure will now be described. Note that the invention of the present application is not limited to the configurations described in the embodiments and is defined by the claims.

Before description of a redox flow battery according to an embodiment, a basic configuration of a redox flow battery (hereinafter referred to as an RF battery <NUM>) will be described on the basis of <FIG>.

An RF battery is an electrolyte-circulating storage battery used, for example, to store electricity generated by new energy, such as solar photovoltaic energy or wind energy. A working principle of the RF battery <NUM> is described on the basis of <FIG>. The RF battery <NUM> is a battery that performs charge and discharge using a difference between the oxidation-reduction potential of active material ions (vanadium ions in <FIG>) contained in a positive electrolyte and the oxidation-reduction potential of active material ions (vanadium ions in <FIG>) contained in a negative electrolyte. The RF battery <NUM> is connected through a power converter <NUM> to a transformer facility <NUM> in a power system <NUM> and performs charge and discharge between itself and the power system <NUM>. When the power system <NUM> is a power system that performs alternating-current power transmission, the power converter <NUM> is an alternating current/direct current converter. When the power system is a power system that performs direct-current power transmission, the power converter <NUM> is a direct current/direct current converter. The RF battery <NUM> includes a cell <NUM> divided into a positive electrode cell <NUM> and a negative electrode cell <NUM> by a membrane <NUM> that allows hydrogen ions to pass therethrough.

The positive electrode cell <NUM> includes a positive electrode <NUM>. A positive electrolyte tank <NUM> that stores a positive electrolyte is connected through ducts <NUM> and <NUM> to the positive electrode cell <NUM>. The duct <NUM> is provided with a circulation pump <NUM>. These components <NUM>, <NUM>, <NUM>, and <NUM> form a positive electrolyte circulation mechanism 100P that circulates the positive electrolyte. Similarly, the negative electrode cell <NUM> includes a negative electrode <NUM>. A negative electrolyte tank <NUM> that stores a negative electrolyte is connected through ducts <NUM> and <NUM> to the negative electrode cell <NUM>. The duct <NUM> is provided with a circulation pump <NUM>. These components <NUM>, <NUM>, <NUM>, and <NUM> form a negative electrolyte circulation mechanism 100N that circulates the negative electrolyte. During charge and discharge, the electrolytes stored in the electrolyte tanks <NUM> and <NUM> are circulated in the cells <NUM> and <NUM> by the circulation pumps <NUM> and <NUM>. When no charge or discharge takes place, the circulation pumps <NUM> and <NUM> are at rest and the electrolytes do not circulate.

The cell <NUM> is typically formed inside a structure called a cell stack <NUM>, such as that illustrated in <FIG> and <FIG>. The cell stack <NUM> is formed by sandwiching a layered structure called a substack <NUM> (see <FIG>) with two end plates <NUM> and <NUM> on both sides, and then fastening the resulting structure with a fastening mechanism <NUM>. The configuration illustrated in <FIG> uses more than one substack <NUM>.

The substack <NUM> (see <FIG>) is formed by stacking a plurality of sets of a cell frame <NUM>, the positive electrode <NUM>, the membrane <NUM>, and the negative electrode <NUM> in layers and sandwiching the resulting layered body between supply/discharge plates <NUM> (see the lower part of <FIG>; not shown in <FIG>).

The cell frame <NUM> includes a frame body <NUM> having a through-window and a bipolar plate <NUM> configured to close the through-window. That is, the frame body <NUM> supports the outer periphery of the bipolar plate <NUM>. The cell frame <NUM> can be made, for example, by forming the frame body <NUM> in such a manner that it is integral with the outer periphery of the bipolar plate <NUM>. Alternatively, the cell frame <NUM> may be made by preparing the frame body <NUM> having a thin portion along the outer edge of the through-window and the bipolar plate <NUM> produced independent of the frame body <NUM>, and then fitting the outer periphery of the bipolar plate <NUM> into the thin portion of the frame body <NUM>. The positive electrode <NUM> is disposed in such a manner as to be in contact with one side of the bipolar plate <NUM> of the cell frame <NUM>, and the negative electrode <NUM> is disposed in such a manner as to be in contact with the other side of the bipolar plate <NUM>. In this configuration, one cell <NUM> is formed between the bipolar plates <NUM> fitted into adjacent cell frames <NUM>.

The circulation of the electrolyte into the cell <NUM> through the supply/discharge plates <NUM> (see <FIG>) is made by liquid supply manifolds <NUM> and <NUM> and liquid discharge manifolds <NUM> and <NUM> formed in each cell frame <NUM>. The positive electrolyte is supplied from the liquid supply manifold <NUM> through an inlet slit <NUM> (see a curved portion indicated by a solid line) formed on one side of the cell frame <NUM> (i.e., on the front side of the drawing) to the positive electrode <NUM>, and discharged through an outlet slit <NUM> (see a curved portion indicated by a solid line) formed in the upper part of the cell frame <NUM> into the liquid discharge manifold <NUM>. Similarly, the negative electrolyte is supplied from the liquid supply manifold <NUM> through an inlet slit <NUM> (see a curved portion indicated by a broken line) formed on the other side of the cell frame <NUM> (i.e., on the back side of the drawing) to the negative electrode <NUM>, and discharged through an outlet slit <NUM> (see a curved portion indicated by a broken line) formed in the upper part of the cell frame <NUM> into the liquid discharge manifold <NUM>. A ring-shaped sealing member <NUM>, such as an O-ring or flat gasket, is provided between adjacent cell frames <NUM>, and this prevents leakage of the electrolyte from the substack <NUM>.

An electrolyte may contain vanadium ions as positive and negative active materials, or may contain manganese and titanium ions as positive and negative active materials, respectively. Other electrolytes of known composition may also be used.

On the basis of the basic configuration of the RF battery <NUM> described above, the RF battery <NUM> according to an embodiment will be described on the basis of <FIG> and <FIG>. <FIG> is a schematic diagram of the RF battery <NUM>, and <FIG> is a schematic diagram illustrating the positive electrolyte circulation mechanism 100P and its neighboring region of the RF battery <NUM>.

As illustrated in <FIG>, the components of the RF battery <NUM> of the present example are in three sections. The first section is a cell chamber <NUM> that contains therein the cell stack <NUM> including the cell <NUM> and the circulation mechanisms 100P and 100N. In the present example, the cell chamber <NUM> is formed by a container. The second section is a positive tank container serving as the positive electrolyte tank <NUM>. The third section is a negative tank container serving as the negative electrolyte tank <NUM>. In the present example, the container forming the cell chamber <NUM> is disposed to extend over both the tank containers.

As containers forming the cell chamber <NUM> and the electrolyte tanks <NUM> and <NUM>, standard containers, such as maritime containers, can be used. Container sizes may be appropriately selected in accordance with the capacity or output of the RF battery <NUM>. For example, when the RF battery <NUM> has a large (or small) capacity, the electrolyte tanks <NUM> and <NUM> may be formed by large (or small) containers. Examples of the containers include international freight containers compliant with the ISO standard (e.g., ISO <NUM>-<NUM> (<NUM>)). Typically, <NUM>-foot containers and <NUM>-foot containers, and <NUM>-foot high-cube containers and <NUM>-foot high-cube containers greater in height than the <NUM>-foot and <NUM>-foot containers, can be used.

In the configuration illustrated in <FIG>, the circulation mechanism 100P (100N) includes a suction pipe <NUM>, the circulation pump <NUM> (<NUM>), an extrusion pipe <NUM>, and the return pipe <NUM>. The suction pipe <NUM> is a pipe that is positioned, at an open end thereof, in an electrolyte <NUM> and sucks up the electrolyte <NUM> to above the electrolyte tank <NUM> (<NUM>). The extrusion pipe <NUM> is a pipe that runs from the discharge port of the circulation pump <NUM> (<NUM>) to the cell <NUM>. The extrusion pipe <NUM> may correspond to the duct <NUM> (<NUM>) illustrated in <FIG>. The return pipe <NUM> is a pipe that runs from the cell <NUM> to the electrolyte tank <NUM> (<NUM>). The return pipe <NUM> may correspond to the duct <NUM> (<NUM>) illustrated in <FIG>. The return pipe <NUM> of the present example is open to the gas phase in the electrolyte tank <NUM> (<NUM>). The return pipe <NUM> is preferably spaced from the suction pipe <NUM> in the planar direction along the liquid surface in the tank. For example, the return pipe <NUM> and the suction pipe <NUM> are preferably arranged diagonally opposite each other. This is because making the pipes <NUM> and <NUM> spaced apart can promote convection of the electrolyte. The return pipe <NUM> is preferably spaced from the suction pipe <NUM> in the planar direction along the liquid surface in the tank. For example, the return pipe <NUM> and the suction pipe <NUM> are preferably arranged symmetrically with respect to the center of the liquid surface in the tank. This is because making the pipes <NUM> and <NUM> spaced apart can promote convection of the electrolyte.

As illustrated in <FIG>, the circulation pump <NUM> is a self-priming pump having a pump body <NUM> including an impeller <NUM> and a driving unit <NUM> that rotates the impeller <NUM>. The pump body <NUM> is disposed in the cell chamber <NUM> and is not immersed in the electrolyte <NUM>. The circulation pump <NUM> illustrated in <FIG> has the same configuration as the circulation pump <NUM> illustrated in <FIG>.

The circulation pump <NUM> is provided with a priming tank <NUM> disposed between the pump body <NUM> and the suction pipe <NUM>. In the configuration with the priming tank <NUM>, sucking the electrolyte <NUM> in the priming tank <NUM> with the circulation pump <NUM> reduces gas-phase pressure in the priming tank <NUM> and causes the electrolyte <NUM> in the electrolyte tank <NUM> to be sucked up into the priming tank <NUM>. With this configuration, initial suction of the electrolyte <NUM> stored in the electrolyte tank <NUM> only involves pouring the electrolyte <NUM> into the priming tank <NUM> and operating the circulation pump <NUM>. The initial suction operation is thus carried out easily. In the configuration with the priming tank <NUM>, a pipe that connects the pump body <NUM> to the priming tank <NUM> is preferably provided with a valve (not shown). For maintenance of the pump body <NUM>, closing the valve is followed by removal of the pump body <NUM> from the circulation mechanism 100P.

The RF battery <NUM> illustrated in <FIG> is configured in such a manner that the electrolyte <NUM> is sucked up to above the electrolyte tank <NUM> (<NUM>). With this configuration, even if the suction pipe <NUM> running from the electrolyte tank <NUM> (<NUM>) to the circulation pump <NUM> (<NUM>) is damaged, the electrolyte <NUM> is less likely to leak out of the electrolyte tank <NUM> (<NUM>). This is because damage to the suction pipe <NUM> breaks hermeticity of the suction pipe <NUM> and allows gravity to cause the electrolyte <NUM> in the suction pipe <NUM> to return to the electrolyte tank <NUM> (<NUM>). The pump body <NUM> of the circulation pump <NUM> (<NUM>) of the present example is not immersed in the electrolyte <NUM>, and this facilitates maintenance of the circulation pump <NUM> (<NUM>). This is because by simply stopping the circulation pump <NUM> (<NUM>), the electrolyte <NUM> in the suction pipe <NUM> is returned to the electrolyte tank <NUM> (<NUM>) and this saves the trouble of taking the impeller <NUM> (see <FIG>) out of the electrolyte <NUM>.

In the RF battery <NUM>, the pump body <NUM> is disposed in the cell chamber <NUM> formed above the electrolyte tank <NUM>. Therefore, even if the electrolyte <NUM> leaks near the pump body <NUM>, the leaked electrolyte <NUM> can be easily kept inside the cell chamber <NUM>. This facilitates treatment of the leaked electrolyte <NUM> and improves safety of the treatment.

In the RF battery <NUM> of the embodiment, the absolute value of the difference between HL1 and HL2 is greater than or equal to <NUM> times H<NUM> and both HL1 and HL2 are less than or equal to Hd, where.

When HL1 - HL2 ≥ <NUM><NUM> (HL1 ≥ <NUM><NUM> in the present example, where HL2 = <NUM>) is satisfied, the distance from the open end <NUM> of the return pipe <NUM> for discharging the electrolyte <NUM> circulated in the cell <NUM> to the open end <NUM> of the suction pipe <NUM> is long and this facilitates development of large convection in the electrolyte <NUM>. The utilization ratio of the electrolyte <NUM> in the electrolyte tank <NUM> can thus be improved. To further improve the utilization ratio of the electrolyte <NUM>, it is preferable that HL1 - HL2 ≥ <NUM><NUM> be satisfied, and that even HL1 - HL2 ≥ <NUM><NUM> or HL1 - HL2 ≥ <NUM><NUM> be satisfied.

The configuration that sucks up the electrolyte <NUM> tends to have a larger Hd, and this leads to increased pump power of the circulation pump <NUM>. To reduce an increase in pump power, it is preferable to reduce friction loss in the suction pipe <NUM> and the return pipe <NUM> without reducing the utilization ratio of the electrolyte <NUM>. Specifically, it is preferable to make HL1 relating to the length of the suction pipe <NUM> and HL2 relating to the length of the return pipe <NUM> less than or equal to the actual head Hd so as to keep the pump power of the circulation pump <NUM> for sucking up and circulating the electrolyte <NUM> low. This makes it possible to reduce power consumption for operating the RF battery <NUM> and achieve efficient operation of the RF battery <NUM>.

It is more preferable that HS be less than or equal to <NUM> times Hd (HS ≤ <NUM>d), where HS is a height (actual suction head) from the in-tank liquid level to the center of a suction port <NUM> of the circulation pump <NUM>. This is because, as described above, NPSHa has a physical limitation and if HS is too high, NPSHa may decrease and fail to satisfy NPSHr < NPSHa. To further reduce a decrease in NPSHa, it is preferable that HS ≤ <NUM>d be satisfied and it is more preferable that HS ≤ <NUM>d be satisfied.

In a second embodiment, the RF battery <NUM> is described on the basis of <FIG>, in which the submerged length HL2 of the return pipe <NUM> is longer than the submerged length HL1 of the suction pipe <NUM>. In <FIG>, components having the same functions as those in <FIG> are denoted by the same reference numerals as those in <FIG>.

In the second embodiment, satisfying HL2 - HL1 ≥ <NUM><NUM> promotes convection of the electrolyte <NUM> and increases the utilization ratio of the electrolyte <NUM>. This is because a large difference in height between the open end <NUM> of the suction pipe <NUM> and the open end <NUM> of the return pipe <NUM> facilitates development of convection in the electrolyte <NUM>. To further improve the utilization ratio of the electrolyte <NUM>, it is preferable that HL2 - HL1 ≥ <NUM><NUM> be satisfied, and that even HL2 - HL1 ≥ <NUM><NUM> or HL2 - HL1 ≥ <NUM><NUM> be satisfied.

Since the configuration that sucks up the electrolyte <NUM> tends to have a larger Hd, both HL1 and HL2 are also made less than or equal to Hd in the second embodiment.

To reduce a decrease in NPSHa, it is preferable that HS ≤ <NUM>d be satisfied and it is more preferable that HS ≤ <NUM>d or HS ≤ <NUM>d be satisfied.

The present calculation example calculates friction loss in the pipes <NUM> and <NUM> and NPSHa, in the configurations of the first and second embodiments using the circulation pump <NUM> with NPSHr = <NUM>, and examines the possibility of power reduction of the circulation pump <NUM>.

Example <NUM> shows a calculation example for the RF battery <NUM> of the first embodiment illustrated in <FIG>. Preconditions for the calculation are as follows:.

In Example <NUM>, where the liquid utilization height ratio HL1/H<NUM> ≈ <NUM>, the efficiency of utilization of active material ions in the electrolyte is fully ensured. The liquid utilization height ratio is a measure of the utilization ratio of the electrolyte. In Example <NUM>, where Hd ≥ HL1, HL2 is satisfied, the pressure loss head (suction pipe loss) of the suction pipe <NUM> is <NUM> and the pressure loss head of the extrusion pipe <NUM> and the return pipe <NUM> is <NUM>. The latter pressure loss head is larger than the former pressure loss head, because a small-diameter pipe portion in the vicinity of the cell stack <NUM> has a larger pressure loss head. Additionally, HS ≤ <NUM>d is satisfied, and NPSHa is nearly equal to <NUM> (NPSHa ≈ <NUM>) and satisfies NPSHr < NPSHa. The electrolyte can thus be circulated without problems.

Example <NUM> shows a calculation example for the RF battery <NUM> of the second embodiment illustrated in <FIG>. Preconditions in this example are the same as those in Example <NUM>, except that the submerged length HL1 of the suction pipe <NUM> is <NUM> and the submerged length HL2 of the return pipe <NUM> is <NUM>. In this case, where the liquid utilization ratio (HL2-HL1)/H<NUM> is nearly equal to <NUM> ((HL2-HL1)/H<NUM> ≈ <NUM>), the efficiency of utilization of active material ions in the electrolyte is fully ensured. In Example <NUM>, where Hd ≥ HL1, HL2 is satisfied, the pressure loss head (suction pipe loss) of the suction pipe <NUM> is <NUM> and the pressure loss head of the extrusion pipe <NUM> and the return pipe <NUM> is <NUM>. The latter pressure loss head is larger than the former pressure loss head, because a small-diameter pipe portion in the vicinity of the cell stack <NUM> has a larger pressure loss head. Additionally, HS ≤ <NUM>d is satisfied, and NPSHa is nearly equal to <NUM> (NPSHa ≈ <NUM>) and satisfies NPSHr < NPSHa. The electrolyte can thus be circulated without problems.

As in Examples <NUM> and <NUM>, the configuration that sucks up the electrolyte <NUM> tends to have a larger pressure loss head in the circulation mechanism 100P. In particular, a portion including the cell <NUM> downstream of the circulation mechanism 100P tends to have a very large pressure loss head and the pump power of the circulation pump <NUM> tends to be large. Therefore, it is of significance to reduce the pump power of the circulation pump <NUM> by ensuring the liquid utilization height ratio and satisfying Hd ≥ HL1, HL2. By satisfying Hd ≥ HL1, HL2, the amount of power required for operating the RF battery <NUM> can be made smaller than in the case of HL1, HL2, > Hd and more efficient operation of the RF battery <NUM> is ensured.

Claim 1:
A redox flow battery (<NUM>) comprising a cell (<NUM>), an electrolyte tank (<NUM>, <NUM>) configured to store an electrolyte (<NUM>) supplied to the cell (<NUM>) and filled with the electrolyte (<NUM>), and a circulation mechanism (100P, 100N) disposed between the cell (<NUM>) and the electrolyte tank (<NUM>, <NUM>) and configured to circulate the electrolyte (<NUM>),
wherein the circulation mechanism (100P, 100N) includes
a suction pipe (<NUM>) configured to suck up the electrolyte (<NUM>) from an open end (<NUM>) thereof in the electrolyte (<NUM>) to above an in-tank liquid level of the electrolyte (<NUM>) in the electrolyte tank (<NUM>, <NUM>),
a circulation pump (<NUM>, <NUM>) disposed at an end portion of the suction pipe (<NUM>),
an extrusion pipe (<NUM>) running from a discharge port (<NUM>) of the circulation pump (<NUM>, <NUM>) to the cell (<NUM>), and
a return pipe (<NUM>) running from the cell (<NUM>) to the electrolyte tank (<NUM>, <NUM>); and
characterized in that an absolute value of a difference between HL1 and HL2 is greater than or equal to <NUM> times H<NUM> and both HL1 and HL2 are less than or equal to Hd, where H<NUM> is a height from an inner bottom surface of the electrolyte tank (<NUM>, <NUM>) to the in-tank liquid level, HL1 is a length from the in-tank liquid level to the open end (<NUM>) of the suction pipe (<NUM>), HL2 is a length from the in-tank liquid level to an open end (<NUM>) of the return pipe (<NUM>) in a depth direction of the electrolyte (<NUM>), and Hd is a distance from the in-tank liquid level to a center of a highest segment of the return pipe (<NUM>), the highest segment being located at the highest level of the return pipe (<NUM>), and wherein if the open end (<NUM>) of the return pipe (<NUM>) is located above the in-tank liquid level, the difference between HL1 and HL2 is HL1 and HL2 is zero.