Water electrolysis control apparatus, water electrolysis system, and water electrolysis method

Abstract

To provide a water electrolysis control apparatus that can realize an appropriate water electrolysis.SOLUTION: A water electrolysis control apparatus 150 comprises: a voltage measurement unit 152 for measuring each terminal voltage Vs1 to Vs4 of multiple electrolysis units 1 for electrolyzing supplied water; a voltage dispersion calculation unit 153 for outputting a voltage dispersion value Val which shows a degree of dispersion of the terminal voltage; a difference calculation unit 154 for calculating each difference value DVal of the voltage dispersion values measured at multiple points in time; a voltage dispersion evaluation unit 155 for determining whether or not the difference value exceeds a prescribed threshold level Dth; and an operation command unit 156 for changing an electrolytic current I, or the discharge flow amount F or discharge pressure P of an auxiliary device 121 when the determination result of the voltage dispersion evaluation unit 155 is affirmative (DVal>Dth).SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to a water electrolysis control device, a water electrolysis system, and a water electrolysis method.

Background Art

[0002] Toward the realization of a low-carbon society, instead of thermal power generation using fossil fuels, power generation using renewable energy (renewable energy) such as sunlight and wind power is actively carried out. However, since the electric power generated by renewable energy fluctuates greatly due to natural phenomena, it is difficult to replace grid power as it is, and it also becomes a factor that destabilizes the grid. Therefore, green hydrogen has been proposed in which water is electrolyzed using the electric power generated by renewable energy to produce, store, and use hydrogen as fuel (including for power generation purposes).

[0003] The electrolysis of water is performed by a water electrolysis system including a plurality of electrolysis stacks (hereinafter also simply referred to as "stacks") in which a plurality of minimum structural units called water electrolysis cells are electrically stacked in series, a power supply device that supplies and controls power to the stack, and auxiliary machines such as a pump that supplies water to the stack and a hydrogen compressor. The method of generating green hydrogen using an electrolysis system is more costly compared to gray hydrogen produced from conventional fossil fuels. Therefore, cost reduction is important for the market expansion of green hydrogen. The cost includes the operating cost, which is the electricity cost for converting water into hydrogen, and the equipment cost of the water electrolysis system itself.

[0004] Among these, the operating cost becomes a dominant factor when the system is operating for a long time. That is, a water electrolysis system that can generate the same amount of hydrogen with less electric power is advantageous. The electric power required for the system cannot ignore the power to the auxiliary machines, but the power input to the electrolysis stack is dominant. The power input to the electrolysis stack is represented by the product of the electrolysis current that almost reflects the hydrogen production amount and the electrolysis voltage required to maintain the current. Therefore, the electric power required to generate the same amount of hydrogen depends on the reduction of the electrolysis voltage.

[0005] Therefore, reducing the electrolysis voltage leads to a reduction in the operating cost of the system. Reducing the electrolysis voltage is a differentiating factor for a water electrolysis system. On the other hand, the equipment cost is determined by the introduction costs of the electrolysis stack, power supply device, auxiliary equipment, etc. In particular, since the electrolysis stack has a shorter lifespan compared to other components, the number of replacements × unit price during a certain operating period becomes dominant. To reduce the number of replacements, it is desirable to suppress its deterioration and extend its lifespan. A system equipped with such deterioration suppression also becomes a differentiating factor.

[0006] As background art in this technical field, the abstract of Patent Document 1 below states, "[Problem] To provide a technique for suppressing deterioration of a membrane electrode assembly due to water shortage in a water electrolysis cell and reducing the efficiency of a water electrolysis apparatus in a water electrolysis apparatus. [Solution] A water electrolysis apparatus that generates oxygen and hydrogen by electrolyzing water, including a membrane electrode assembly, a water electrolysis cell that electrolyzes water, a water supply unit that supplies water to the water electrolysis cell, an impedance detection unit that detects the impedance of the membrane electrode assembly, estimating the amount of water in the water electrolysis cell using the impedance of the membrane electrode assembly, and controlling the water supply unit to adjust the amount of water supplied to the water electrolysis cell using the estimated estimated water amount."

Prior Art Documents

Patent Documents

[0007]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0008] By the way, in the above-described technology, there is a desire to achieve more appropriate water electrolysis. This invention has been made in view of the above circumstances, and an object thereof is to provide a water electrolysis control device, a water electrolysis system, and a water electrolysis method capable of realizing appropriate water electrolysis.

Means for Solving the Problems

[0009] To solve the above problems, the water electrolysis control device of the present invention is provided with water discharged from an auxiliary machine, supplied with an electrolysis current from a power supply unit, electrically connected in series, and includes a plurality of electrolysis units for electrolyzing the supplied water. It comprises a voltage measurement unit for measuring each terminal voltage, a voltage variation calculation unit for outputting a voltage variation value representing the degree of variation of the terminal voltage, a difference calculation unit for calculating a difference value of the voltage variation values measured at a plurality of time points, a voltage variation evaluation unit for determining whether the difference value exceeds a predetermined threshold value, and an operation command unit for changing the electrolysis current or the discharge flow rate or discharge pressure in the auxiliary machine when the determination result in the voltage variation evaluation unit is affirmative.

Effects of the Invention

[0010] According to the present invention, appropriate water electrolysis can be realized.

Brief Description of the Drawings

[0011]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7

Figure 8

Figure 9

Figure 10

Figure 11

[0012] [Outline of Embodiment] Normally, a water electrolysis system is configured by connecting a plurality of these constituent units with cells or stacks as the constituent units. Since the electrolysis voltage per cell is as low as 1 to 2 [V], the constituent units are electrically connected in series, and the water to be subjected to electrolysis is supplied in parallel. When the electrolysis operation is performed in this state, oxygen and hydrogen generated in the water channels of each stack appear as bubbles. When bubbles remain in the water channels, they will eventually cover the catalyst membrane (electrode) which is the site of the electrolysis reaction, reducing the electrolysis efficiency.

[0013] Furthermore, the degree of use of the catalyst is different between the part covered by the bubbles and the other parts, and the part that is severely used is promoted to deteriorate, shortening the life of the stack. This retention of bubbles and adhesion to the electrodes are particularly likely to occur in a system in which a plurality of stacks are connected in parallel from the perspective of the water flow path. Due to the temporary imbalance of bubbles in the flow path between the stacks, it becomes difficult to supply water to the flow path of the stack with a large number of bubbles. Then, since the water bypasses to the other stacks connected in parallel, it becomes easier for bubbles to accumulate in the original stack. When the retention of bubbles occurs in the flow path of a specific stack due to this vicious cycle, the adhesion of bubbles to the electrodes in that flow path also easily occurs, resulting in a decrease in the electrolysis efficiency and an acceleration of deterioration of the specific stack.

[0014] When applying the water electrolysis device described in the above-mentioned Patent Document 1, it is considered that by paying attention to the depletion status of water in the electrolytic cell, the depletion status of water can be grasped from the impedance of the electrolysis stack serving as an index. And when it is depleted, it is considered that control to increase the water flow can be executed. In this way, it is considered that the electrolysis efficiency can be improved by preventing depletion. However, Patent Document 1 does not particularly describe detecting the imbalance of bubbles in a plurality of cells or stacks. Therefore, the embodiment described later reduces the decrease in electrolysis efficiency and the deterioration of cells / stacks due to oxygen and hydrogen bubbles generated in the water passage during electrolysis, and reduces the operation cost and equipment cost.

[0015] [First Embodiment] 〈Configuration of the First Embodiment〉 FIG. 1 is a block diagram of a water electrolysis system WES1 according to the first embodiment. In FIG. 1, the water electrolysis system WES1 includes a power supply unit 120, four water electrolysis stacks 1-1 to 1-4 (electrolysis unit), auxiliary equipment 121, an auxiliary equipment control unit 124, a water electrolysis control device 150, an anode-side water supply passage 252, an anode-side drainage passage 254, and a cathode-side drainage passage 256.

[0016] In the following description, a plurality of components, information, etc. having the same or similar functions and meanings may be denoted by the same reference numeral with "-" and alphanumeric characters, for example, "water electrolysis stacks 1-1, 1-2". However, when it is not necessary to distinguish these multiple components, etc., they may be denoted by omitting "-" and alphanumeric characters, for example, "water electrolysis stack 1".

[0017] The water electrolysis stacks 1-1 to 1-4 are provided with anode terminals and cathode terminals (not labeled), and these are electrically connected in series. The power supply unit 120 supplies an electrolysis current I that follows the supplied current command value I* (control parameter) to the water electrolysis stacks 1-1 to 1-4. The auxiliary machine 121 includes a pump or the like, and supplies water to each water electrolysis stack 1 (hereinafter, may be simply referred to as stack 1) via the anode-side water supply passage 252. The stack 1 is formed by laminating one or a plurality of electrolytic cells 2 (see FIG. 2).

[0018] The water electrolysis control device 150 outputs a current command value I*, which is a command value of the electrolysis current I, to the power supply unit 120, and outputs a flow rate command value F* (control parameter), which is a command value of the discharge flow rate F of the water that the auxiliary machine 121 should discharge, to the auxiliary machine control unit 124. The auxiliary machine control unit 124 controls the auxiliary machine 121 so that the auxiliary machine 121 discharges water according to the flow rate command value F*.

[0019] Each stack 1 is connected in parallel to the anode-side water supply passage 252. The water supplied to each stack 1 branches into an anode-side drainage passage 254 and a cathode-side drainage passage 256 and is discharged. Further, the stack 1 electrolyzes a part of the supplied water into hydrogen and oxygen. The anode-side drainage passage 254 also serves as an oxygen discharge passage, and the cathode-side drainage passage 256 also serves as a hydrogen discharge passage.

[0020] The water electrolysis control device 150 includes a voltage measurement unit 152 (voltage measurement process), a voltage variation calculation unit 153 (voltage variation calculation process), a difference calculation unit 154 (difference calculation process), a voltage variation evaluation unit 155 (voltage variation evaluation process), and an operation command unit 156 (operation command process). The voltage measurement unit 152 measures stack voltages Vs1 to Vs4 (terminal voltages), which are the voltages between the anode terminals and the cathode terminals of the stacks 1-1 to 1-4.

[0021] The voltage variation calculation unit 153 calculates a voltage variation value Val based on the measured stack voltages Vs1 to Vs4. In this embodiment, the voltage variation value Val is the variance of the stack voltages Vs1 to Vs4. However, the voltage variation value Val may be a quantity other than the variance. For example, the difference between the maximum value and the minimum value among the stack voltages Vs1 to Vs4 may be used as the voltage variation value Val.

[0022] The difference calculation unit 154 stores the voltage variation value Val acquired at a predetermined reference timing as the voltage reference variation value Vals. Then, the difference calculation unit 154 subtracts the voltage reference variation value Vals from the subsequently acquired voltage variation value Val and outputs the resulting difference value DVal.

[0023] Incidentally, although details will be described later, the operation command unit 156 appropriately changes the current command value I* supplied to the power supply unit 120. After changing the current command value I*, when a predetermined standby time Ta (the first standby time, see FIG. 8) has elapsed, the operation command unit 156 supplies an initialization command CI to the difference calculation unit 154. When receiving the initialization command CI, the difference calculation unit 154 stores the voltage variation value Val at that time as a new voltage reference variation value Vals.

[0024] Also, after outputting the initialization command CI, when a predetermined standby time Tb (the second standby time, see FIG. 8) has elapsed, the operation command unit 156 outputs an evaluation command CE to the voltage variation evaluation unit 155. When receiving the evaluation command CE, the voltage variation evaluation unit 155 then sequentially determines whether the difference value DVal exceeds a predetermined threshold Dth. Then, if the determination result is affirmative, the voltage variation evaluation unit 155 supplies a determination result SJ of "1" (DVal > Dth), and if negative, "0" (DVal ≤ Dth) to the operation command unit 156 repeatedly.

[0025] The operation command unit 156 first sets a predetermined median value Ir (see FIG. 8) as the current command value I*, and supplies it to the power supply unit 120. Then, thereafter, the operation command unit 156 alternately sets "Ir - ΔI" and "Ir + ΔI" as the current command value I*. That is, the current command value I* changes in steps. Note that the "step-like" does not necessarily mean that it changes at "0 seconds", and also includes cases where the change time, that is, the ramp time, is within 10 seconds.

[0026] Here, the median value Ir is the rated current value that can be supplied to Stack 1 or a value near it. Also, the difference value ΔI with respect to the median value Ir is, for example, a value of 3 to 8 [%] of the rated current value, more preferably about 5 [%]. However, the median value Ir and the difference value ΔI are preferably set so that the average value of the current command value I* is equal to or less than the rated current value of Stack 1.

[0027] When the determination result SJ by the voltage variation evaluation unit 155 becomes "1" (DVal > Dth), the operation command unit 156 stops the output of the evaluation command CE and switches the current command value I*. On the other hand, if the determination result SJ is "0" (DVal ≤ Dth), the previous current command value I* is maintained. However, when a predetermined standby time Tc (see FIG. 8) has elapsed since the timing when the current command value I* was last switched, the operation command unit 156 switches the current command value I* regardless of the content of the determination result SJ. Thereby, the average value of the current command value I* can be suppressed to be equal to or less than the rated current value of Stack 1.

[0028] FIG. 2 is a schematic diagram of the electrolytic cell 2 applied to the water electrolysis stack 1. As described above, the water electrolysis stack 1 shown in FIG. 1 is formed by stacking one or more electrolytic cells 2 (hereinafter sometimes simply referred to as cell 2). The cell 2 includes a proton exchange membrane 201, a catalyst layer 210, a diffusion layer 220, and a water channel 240.

[0029] The catalyst layer 210 includes a cathode-side catalyst layer 211 and an anode-side catalyst layer 212. The diffusion layer 220 includes a cathode-side diffusion layer 222 and an anode-side diffusion layer 221. The water channel 240 includes a cathode-side water channel 242 and an anode-side water channel 241.

[0030] The anode-side catalyst layer 211 and the cathode-side catalyst layer 212 are joined to both sides of the proton exchange membrane 201. The anode-side water channel 241 is a water channel connected to the anode-side water supply channel 252 and the anode-side drainage channel 254 (see FIG. 1). The cathode-side water channel 242 is a water channel connected to the cathode-side drainage channel 256 (see FIG. 1).

[0031] The anode-side diffusion layer 221 is inserted between the anode-side catalyst layer 211 and the anode-side water channel 241. The cathode-side diffusion layer 222 is inserted between the cathode-side catalyst layer 212 and the cathode-side water channel 242. When water is supplied to the anode-side water channel 241 through the anode-side water supply channel 252 (see FIG. 1), a part of the supplied water is supplied to the cathode-side water channel 242 through the proton exchange membrane 201 or the like.

[0032] The power supply unit 120 passes an electrolytic current I between the anode-side catalyst layer 211 and the cathode-side diffusion layer 222. Thereby, the anode-side catalyst layer 211 decomposes water into oxygen and hydrogen ions, and the cathode-side catalyst layer 212 generates hydrogen from the hydrogen ions. The amount of hydrogen generated corresponds to the amount of electrons supplied, that is, the amount of electrons corresponding to the electrolytic current I.

[0033] FIG. 3 is a graph showing the current-voltage characteristics (I-V characteristics) of the cell 2. The horizontal axis of this figure is the electrolysis current I flowing through cell 2, and the vertical axis is the cell voltage Vc generated in cell 2. This figure shows how much cell voltage Vc is generated when the electrolysis current I is passed through cell 2. In other words, it shows the relationship regarding how much cell voltage Vc must be applied to maintain the electrolysis current I. Note that Figure 3 shows the characteristics of the cell voltage V when the electrolysis current I is kept constant for a certain period of time, and the relationship shown in Figure 3 may not hold in a transient state.

[0034] The cell voltage Vc required to maintain a predetermined electrolysis current I in cell 2 for water electrolysis is the sum of the Nernst voltage Erev, the activation overvoltage Eact, the resistance overvoltage Eohm, and the bubble overvoltage Ebb. Each curve in this figure is a curve corresponding to the voltage value obtained by adding each voltage to the sum of the voltages below.

[0035] The activation overvoltage Eact is the voltage generated in the catalyst layer 210. The resistance overvoltage Eohm is the voltage generated by the resistance component of the proton exchange membrane 201. The bubble overvoltage Ebb is the voltage generated by the bubbles adhering to the surface of the catalyst layer 210. In a situation where the electrolysis current I is small and no bubbles are generated, the bubble overvoltage Ebb becomes zero.

[0036] The power, which is the operating cost of cell 2, is the product of the electrolysis current I and the cell voltage Vc. Therefore, from the perspective of the operating cost, if the current value of the electrolysis current I is the same, the lower the required cell voltage Vc, the more preferable. However, when water electrolysis proceeds in cell 2, bubbles are generated from the surface of the catalyst layer 210, covering the catalyst layer 210, so the bubble overvoltage Ebb increases.

[0037] Bubbles are removed to some extent by the flowing water supplied. More specifically, the area to which bubbles adhere is determined by the balance between the amount of bubbles generated and the amount of bubbles removed by the water flow. Since current hardly flows through the portion of the catalyst layer 210 where bubbles are adhering, in order to maintain the current value of the electrolysis current I, it is necessary to further increase the cell voltage Vc. As a result, the bubble overvoltage Ebb becomes even higher. This bubble overvoltage Ebb is the cause of the reduction in power efficiency in the electrolysis of water.

[0038] FIG. 4 is a schematic diagram showing an example state of one of the two electrolytic cells 2-A and 2-B. As shown in the drawing, bubbles 4 are generated on the surface of the anode-side catalyst layer 211. Note that the same applies to the cathode-side catalyst layer 212 (see FIG. 2), but illustration of the cathode side is omitted. The generated bubbles 4 flow into the anode-side water channel 241 through the anode-side diffusion layer 221 and are discharged from the cells 2-A and 2-B.

[0039] Since the cells 2-A and 2-B are electrically connected in series, the same electrolysis current I is supplied. And in the example of FIG. 4, it is assumed that the oxygen bubbles 4 generated by the electrolysis reaction in each cell are sent out to the outside with the same flow rate of water. For this reason, in the example of FIG. 4, in the cells 2-A and 2-B, the generation situation of the bubbles 4 is exactly the same.

[0040] FIG. 5 is a schematic diagram showing another example state of the two electrolytic cells 2-A and 2-B. The physical configurations of the cells 2-A and 2-B are not completely identical, and slight differences occur in the shape of the water channel 240, the area of the catalyst layer 210, etc. Here, assume that the resistance of the water flow in the anode-side water channel 241 of the cell 2-B is slightly larger than that of the cell 2-A. Then, the amount of water flowing through the cell 2-A becomes slightly larger than the amount of water flowing through the cell 2-B.

[0041] As a result, while the removal of the bubbles 4 progresses in the cell 2-A, the retention of the bubbles 4 progresses in the cell 2-B, and the resistance to the flow of water in the cell 2-B increases. By repeating such an operation, as shown in FIG. 5, the amount of the bubbles 4 retained in the cell 2-B becomes significantly larger than that in the cell 2-A. As a result, the bubble overvoltage Ebb (see FIG. 3) in the cell 2-B becomes significantly higher than that in the cell 2-A, and the power efficiency in the cell 2-B decreases.

[0042] Further, when the bubbles 4 adhere to the catalyst layer 210, the effective area of the catalyst layer 210 decreases. In the cell 2-B, although the effective area of the catalyst layer 210 decreases, the catalyst layer 210 is severely used in order to maintain the flowing current. This means that the deterioration of the catalyst layer 210 is promoted.

[0043] Thus, when the cell 2 with poor power efficiency is generated due to the imbalance of the bubbles 4, the power efficiency of the entire water electrolysis system WES1 decreases, and the catalyst layer 210 is severely used, which also causes an increase in the operation cost. When the imbalance of the bubbles 4 occurs in a plurality of cells 2 through which the same electrolysis current I flows, a difference occurs in the anode-cathode voltage, that is, the cell voltage Vc, due to the influence of the bubble overvoltage Ebb (see FIG. 3).

[0044] Similarly, for the stack 1 in which a plurality of cells 2 are stacked, the imbalance of the bubbles 4 appears as a difference in the stack voltage Vs. The control content of the above-described water electrolysis control device 150 changes the generation state of the bubbles 4 by switching the value of the current command value I* when the difference value DVal becomes large, and relaxes the variation of the bubbles 4.

[0045] <Characteristics of the water electrolysis stack 1> Next, various characteristics of the water electrolysis stack 1 will be described. FIG. 6 is a diagram showing an example of the response characteristics of the stack voltage Vs when the electrolysis current I is increased stepwise. In FIG. 6, the horizontal axis represents time t, and the vertical axis represents electrolysis current I and stack voltage Vs. As shown in graph G11 of FIG. 6, when the electrolysis current I of power supply unit 120 is increased stepwise at time t102, the stack voltage Vs changes as shown in graph G12.

[0046] That is, although it depends on the configuration of stack 1, generally, the stack voltage Vs rises significantly in a short time of about several seconds and then gradually increases over a long time of about several minutes. The short-time phenomenon is mainly due to the increase in resistance overvoltage Eohm and activation overvoltage Eact (see FIG. 3) due to the increase in current. On the other hand, the increase in stack voltage Vs over a long time is mainly due to bubble overvoltage Ebb.

[0047] Graph G13 shows an example of the response characteristics of the stack voltage Vs in two water electrolysis stacks 1. Here, the two stacks 1 are referred to as water electrolysis stacks 1-a and 1-b. These are, for example, any two of the water electrolysis stacks 1-1 to 1-4 shown in FIG. 1. Since stacks 1-a and 1-b are electrically connected in series, the electrolysis current I of power supply unit 120 is commonly supplied to both of them.

[0048] The stack voltages Vs of stacks 1-a and 1-b are referred to as stack voltages Vsa and Vsb. Both stack voltages Vsa and Vsb rise significantly in a short time of about several seconds and then gradually increase over a long time of about several minutes. And regarding the latter voltage increase, stack voltage Vsb is higher than stack voltage Vsa. This is because due to the imbalance in the distribution of bubbles 4, the bubble overvoltage Ebb in stack voltage Vsb has become larger.

[0049] However, the difference that occurs in the stack voltages Vsa and Vsb over a long period of time is not necessarily only due to the bubble overvoltage Ebb. For example, factors other than the imbalance of the bubbles 4, such as variations in the dimensions of the proton exchange membrane 201, the area of the catalyst layer 210, the thickness of the catalyst layer 210, and the characteristics of the catalyst layer 210 in the stacks 1-a and 1-b, can also cause a difference in the stack voltages Vsa and Vsb. Graph G14 shows an example of the stack voltages Vsa and Vsb that vary due to factors other than the imbalance of the bubbles 4.

[0050] When the electrolysis current I rises stepwise, the voltage change due to the dimensions of the proton exchange membrane 201 corresponds to the resistance overvoltage Eohm (see FIG. 2) and appears as an instantaneous voltage change. Also, the voltage change due to the characteristics of the catalyst layer 210 corresponds to the activation overvoltage Eact and appears as a voltage change that takes at least about several seconds. On the other hand, the change due to the generation and retention of the bubbles 4 corresponds to the bubble overvoltage Ebb. This becomes determined after a time period ranging from several seconds to several tens of seconds, and sometimes several minutes.

[0051] Among the voltage changes, those corresponding to the bubble overvoltage Ebb and those due to other reasons can be discriminated by analyzing the voltage change after changing the electrolysis current I. Therefore, in the present embodiment, the voltage variation value Val is acquired and evaluated at a plurality of timings, thereby canceling out the variation in the stack voltage Vs due to factors other than the bubble overvoltage Ebb.

[0052] FIG. 7 is a diagram showing an example of the response characteristics of the stack voltage Vs when the electrolysis current I is decreased stepwise. As shown in the graph G21 of FIG. 7, when the electrolysis current I of the power supply unit 120 is decreased stepwise at time t112, the stack voltage Vs changes as shown in graph G22.

[0053] Also, the stack voltages Vsa and Vsb in the two water electrolysis stacks 1-a and 1-b are, for the same reason as the graph G13 in FIG. 6, for example, like the graph G23. Also, the stack voltages Vsa and Vsb that vary due to factors other than the imbalance of the bubbles 4 are, for the same reason as the graph G14 in FIG. 6, for example, like the graph G24.

[0054] <Explanation of various constants, etc.> For example, when the electrolysis current I is 1 [A], the cell voltage Vc is 1.5 to 2 [V]. And the bubble overvoltage Ebb is about 0.05 [V] per cell 2. The threshold value Dth may be determined according to this ratio. The waiting time Ta from when the current command value I* is switched until the initialization command CI is output is preferably in the range of 1 to 60 seconds.

[0055] More specifically, the waiting time Tb is a longer time than the waiting time Ta, preferably in the range of 5 to 1200 seconds, and more preferably in the range of 10 to 600 seconds. If the water electrolysis system WES1 is small, the waiting time Tb is about 10 seconds, and if the water electrolysis system WES1 is large, it is about 600 seconds. Appropriate values for the waiting times Ta and Tb may be determined according to the change in the stack voltage Vs after changing the current command value I* and the retention state of the bubbles 4.

[0056] After switching the current command value I* based on the imbalance of the bubbles 4, the imbalance of the bubbles 4 occurs again as time passes. Therefore, it is necessary to change the current command value I* again. In this case, as described above, when the electrolysis current I is increased or decreased at a certain timing, it is desirable to change the electrolysis current I in the opposite direction at the next timing. This is because if the variation direction is the same as the previous time, the imbalance of the bubbles 4 may not be sufficiently improved.

[0057] <Operation of the First Embodiment> Next, the operation of the first embodiment will be described. FIG. 8 is an example of a time chart showing the operation of the water electrolysis system WES1. In the illustrated example, assume that the stack voltage Vsa is the minimum value of the stack voltages Vs1 to Vs4, and the stack voltage Vsb is the maximum value. Assume that the operation of the water electrolysis system WES1 is started at time t0 in FIG. 8. After the start of the operation, the operation command unit 156 maintains the current command value I* at a predetermined median value Ir until a predetermined initial standby time Tn elapses.

[0058] Then, at time t2 when the initial standby time Tn has elapsed, the operation command unit 156 sets the current command value I* to "Ir - ΔI". Next, at time t4 (the first point in time) when the standby time Ta has elapsed from time t2, the operation command unit 156 outputs an initialization command CI to the difference calculation unit 154. As a result, the difference calculation unit 154 stores the voltage variation value Val at time t4 as the voltage reference variation value Vals.

[0059] At time t6 (the second point in time) when the standby time Tb has elapsed from time t4, the operation command unit 156 outputs an evaluation command CE to the voltage variation evaluation unit 155. As a result, the voltage variation evaluation unit 155 repeatedly determines whether "DVal > Dth" holds, and starts outputting the determination result SJ. In the illustrated example, "DVal > Dth" holds before time t6. Therefore, when the voltage variation evaluation unit 155 receives the evaluation command CE at time t6, it immediately returns a determination result SJ of "1" (DVal > Dth) to the operation command unit 156. As a result, at time t6, the operation command unit 156 sets the current command value I* to "Ir + ΔI".

[0060] Next, at time t8 when the standby time Ta has elapsed since time t6, the operation command unit 156 outputs an initialization command CI to the difference calculation unit 154. As a result, the difference calculation unit 154 stores the voltage variation value Val at time t8 as the voltage reference variation value Vals. At time t10 when the standby time Tb has elapsed since time t8, the operation command unit 156 outputs an evaluation command CE to the voltage variation evaluation unit 155. As a result, the voltage variation evaluation unit 155 repeatedly determines whether "DVal > Dth" holds after time t10 and repeatedly outputs the determination result SJ.

[0061] In the illustrated example, the state of "DVal ≤ Dth" continues after time t10. Eventually, time t12 is reached when the standby time Tc has elapsed since time t6 when the current command value I* was last switched. In this case, regardless of the value of the determination result SJ, the operation command unit 156 sets the current command value I* to "Ir - ΔI". Then, at time t14 when the standby time Ta has elapsed since time t12, the operation command unit 156 outputs an initialization command CI to the difference calculation unit 154. Thereafter, the same operations as described above are repeated.

[0062] As described above, according to the present embodiment, it is possible to reduce the variation in characteristics due to the imbalance of the bubbles 4 in the plurality of stacks 1, thereby improving the power efficiency of the water electrolysis system WES1 and reducing the operation cost of the water electrolysis system WES1. In addition, since the excessive use of the catalyst in a specific stack 1 can be suppressed, the life of the stack 1 can be extended and the equipment cost of the water electrolysis system WES1 can be reduced.

[0063] Note that in the above example, the voltage variation value Val at the timing (times t4, t8, t14) when the standby time Ta has elapsed since the timing when the current command value I* was switched is used as the voltage reference variation value Vals. However, the voltage reference variation value Vals is not limited to this. For example, the voltage variation value Val at the timing (time t0) when the water electrolysis system WES1 starts operating and starts supplying the electrolysis current I may be fixedly applied as the voltage reference variation value Vals.

[0064] Also, in the above example, the voltage measurement unit 152 (see FIG. 1) measured the stack voltages Vs1 to Vs4 for the individual water electrolysis stacks 1-1 to 1-4. However, in the case where the number of stacks 1 further increases, etc., a plurality of stacks 1 connected in series may be regarded as a "sub-group", and the terminal voltage for each of these "sub-groups" may be measured.

[0065] [Second Embodiment] Next, the water electrolysis system according to the second embodiment will be described. The configuration of the water electrolysis system according to the second embodiment is the same as that of the water electrolysis system WES1 (see FIG. 1) according to the first embodiment, except for the points described below. That is, in the second embodiment, the operation command unit 156 is different from the first embodiment in that the current command value I* is set to a constant value and the flow rate command value F* is switched as necessary.

[0066] More specifically, similar to that of the first embodiment, the voltage variation calculation unit 153 outputs a voltage variation value Val. Also, the difference calculation unit 154 subtracts the voltage reference variation value Vals from the voltage variation value Val and outputs the resulting difference value DVal. Then, when the voltage variation evaluation unit 155 receives an evaluation command CE from the operation command unit 156, it repeatedly determines whether "DVal > Dth" holds and repeatedly outputs the determination result SJ.

[0067] The operation command unit 156 first sets a predetermined median value Fr (not shown) as the flow rate command value F* and supplies it to the auxiliary control unit 124. Then, thereafter, the operation command unit 156 alternately sets "Fr - ΔF" and "Fr + ΔF" as the flow rate command value F*. That is, the flow rate command value F* changes in a stepwise manner. Here, the median value Fr is a rated flow rate that can be supplied to the water electrolysis stacks 1-1 to 1-4 or a value near it.

[0068] Further, the difference value ΔF with respect to the median value Fr is, for example, a value of 3 to 8 [%] of the rated flow rate, more preferably about 5 [%]. However, the median value Fr and the difference value ΔF may be set such that the average value of the flow rate command value F* is equal to or less than the rated flow rate. Note that, regarding the discharge flow rate F, the deviation from the rated value hardly affects the hydrogen production amount, unlike the case of the electrolysis current I.

[0069] When the operation of the water electrolysis system of the present embodiment is started, the operation command unit 156 sets the median value Fr as the flow rate command value F*. After that, when the initial standby time Tn (see FIG. 8) has elapsed, the operation command unit 156 switches the flow rate command value F* to "Fr - ΔF" in the same manner as the process at time t2 in FIG. 8. After switching the flow rate command value F*, when the standby time Ta has elapsed, the operation command unit 156 outputs an initialization command CI to the difference calculation unit 154 in the same manner as the process at time t4 in FIG. 8.

[0070] Furthermore, when the standby time Tb has elapsed after outputting the initialization command CI, the operation command unit 156 outputs an evaluation command CE to the voltage variation evaluation unit 155 in the same manner as the process at time t6 in FIG. 8. If the determination result SJ output by the voltage variation evaluation unit 155 is "1" (DVal > Dth), the operation command unit 156 switches the flow rate command value F* to "Fr + ΔF" in the same manner as the process at time t6 in FIG. 8. Also, when the standby time Tc has elapsed from the timing of the last switching of the flow rate command value F*, regardless of the value of the determination result SJ, the flow rate command value F* is switched in the same manner as the process at time t12 in FIG. 8.

[0071] Also in the present embodiment, when bubbles 4 become unbalanced in a plurality of cells 2 through which the same electrolysis current I flows, a difference occurs in the anode-cathode voltage, that is, the cell voltage Vc, due to the influence of the bubble overvoltage Ebb (see FIG. 3). Similarly, for the stack 1 in which a plurality of cells 2 are stacked, the imbalance of the bubbles 4 appears as a difference in the stack voltage Vs. On the other hand, in the present embodiment, by appropriately switching the flow rate command value F*, the discharge flow rate F of the auxiliary machine 121 is switched. Thereby, the generation state of the bubbles 4 can be changed, and the variation of the bubbles 4 can be alleviated.

[0072] FIG. 9 is a diagram showing an example of the response characteristics of the stack voltage Vs when the discharge flow rate F is increased stepwise. As shown in graph G41 of FIG. 9, when the discharge flow rate F of the auxiliary machine 121 is increased stepwise at time t122, the stack voltage Vs changes as shown in graph G42. That is, when the discharge flow rate F is increased stepwise, depending on the configuration of the water electrolysis stack 1, the stack voltage Vs drops in a few seconds and gradually rises over several minutes. The short-time phenomenon near time t122 is due to the decrease in the bubble overvoltage Ebb due to the removal of the bubbles 4. Thereafter, the reason for the gradual increase in the stack voltage Vs is due to the bubbles 4 staying again.

[0073] Graph G43 shows an example of the changes in the stack voltages Vsa and Vsb in the two stacks 1-a and 1-b, similar to graph G13 shown in FIG. 6. When the discharge flow rate F is increased stepwise, similar to the case of graph G42, the stack voltages Vsa and Vsb drop in a few seconds and gradually rise over several minutes. And regarding the latter voltage increase, the stack voltage Vsb is higher than the stack voltage Vsa. This is because the bubble overvoltage Ebb in the stack voltage Vsb has increased due to the imbalance in the distribution of the bubbles 4.

[0074] As described for graph G14 in FIG. 6, the difference that occurs in the stack voltages Vsa and Vsb is not necessarily only due to the bubble overvoltage Ebb. Graph G44 shows an example of the stack voltages Vsa and Vsb that vary due to factors not caused by the imbalance of the bubbles 4. Among the voltage changes, those corresponding to the bubble overvoltage Ebb and those due to other reasons can be discriminated by analyzing the voltage changes after changing the discharge flow rate F.

[0075] In stacks 1-a and 1-b, there are variations such as the dimensions of the proton exchange membrane 201, the area of the catalyst layer 210, the thickness of the catalyst layer 210, and the characteristics of the catalyst layer 210. The difference in the stack voltages Vsa and Vsb due to factors other than the imbalance of these bubbles 4 appears constantly in graph G44. On the other hand, the difference in the stack voltages Vsa and Vsb due to the imbalance of bubbles 4 is determined over a time period of 10 seconds to several minutes.

[0076] Therefore, in this embodiment, similar to the first embodiment, the voltage variation value Val is acquired and evaluated at multiple timings, thereby canceling out the variation in the stack voltage Vs due to factors other than the bubble overvoltage Ebb. In this embodiment, the lengths of the standby times Ta, Tb, etc. may adopt the same values as those in the first embodiment.

[0077] FIG. 10 is a diagram showing an example of the response characteristics of the stack voltage Vs when the discharge flow rate F is decreased stepwise. As shown in graph G51 of FIG. 10, when the discharge flow rate F of the auxiliary machine 121 is decreased stepwise at time t132, the stack voltage Vs changes as shown in graph G52. That is, when the discharge flow rate F is decreased stepwise, depending on the configuration of the water electrolysis stack 1, the stack voltage Vs rises in several seconds and gradually increases over several minutes.

[0078] Also, the stack voltages Vsa and Vsb in the two water electrolysis stacks 1-a and 1-b are, for example, as shown in graph G53 for the same reason as graph G43 in FIG. 9. Also, the stack voltages Vsa and Vsb that vary due to factors other than the imbalance of bubbles 4 are, for example, as shown in graph G54 for the same reason as graph G44 in FIG. 9.

[0079] In this embodiment, after switching the flow rate command value F* based on the imbalance of the bubbles 4, the imbalance of the bubbles 4 occurs again over time. Therefore, it is necessary to change the flow rate command value F* again. In this case, as in the first embodiment, when the discharge flow rate F is increased or decreased at a certain timing, it is desirable to change the discharge flow rate F in the opposite direction at the next timing. This is because if the variation direction is the same as the previous time, the imbalance of the bubbles 4 may not be sufficiently improved.

[0080] [Third Embodiment] Next, the water electrolysis system according to the third embodiment will be described. The configuration of the water electrolysis system according to the third embodiment is the same as that of the water electrolysis system WES1 (see FIG. 1) according to the first embodiment, except for the points described below. That is, in the third embodiment, the operation command unit 156 outputs a pressure command value P* (control parameter), which is not shown in the figure, instead of the flow rate command value F*. This pressure command value P* commands the discharge pressure P (not shown) of the auxiliary machine 121.

[0081] The operation command unit 156 in this embodiment executes the same processing on the pressure command value P* as the processing executed on the flow rate command value F* in the second embodiment. That is, when the operation of the water electrolysis system is started, the operation command unit 156 sets a predetermined median value Pr as the pressure command value P*. Thereafter, in the same manner as in the second embodiment, with the difference value being ΔP, the pressure command value P* is alternately switched between "Pr - ΔP" and "Pr + ΔP".

[0082] Here, the median value Pr is, for example, the rated pressure that can be applied to the water electrolysis stacks 1-1 to 1-4 or a value in the vicinity thereof. And the difference value ΔP is, for example, a value of 3 to 8 [%] of the rated pressure, more preferably about 5 [%]. The voltage response of the stack 1 when changing the pressure of the water supplied to the water electrolysis stack 1 is the same as the response to the discharge flow rate F (see FIGS. 9 and 10) described in the second embodiment.

[0083] [Fourth Embodiment] Next, a water electrolysis system according to the fourth embodiment will be described. The configuration of the water electrolysis system according to the fourth embodiment is the same as that of the water electrolysis system WES1 (see FIG. 1) according to the first embodiment, except for the points described below. That is, in the fourth embodiment, the operation command unit 156 outputs a pressure command value P* (not shown) instead of the flow rate command value F*, as in the third embodiment. Further, in the fourth embodiment, when switching the pressure command value P*, the operation command unit 156 simultaneously switches the current command value I*. The specific value of the current command value I* is the same as that in the first embodiment.

[0084] In the above-described first to third embodiments, in order to correct the imbalance of the bubbles 4, one of the control parameters of the current command value I*, the flow rate command value F*, or the pressure command value P* was switched. However, depending on the configuration of the water electrolysis stack 1 and the connection status of the water pipes, it may be difficult to obtain an effect by switching only a single control parameter. In that case, for example, as in the present embodiment, by simultaneously switching the current command value I* and the pressure command value P*, it becomes easier to reduce the imbalance of the bubbles 4. Note that, instead of the present embodiment, the same effect can be obtained by simultaneously switching the current command value I* and the flow rate command value F*.

[0085] [Configuration of Computer] FIG. 11 is a block diagram of a computer 980. The water electrolysis control device 150 shown in FIG. 1 includes one or more computers 980 shown in FIG. 11. In FIG. 11, the computer 980 includes a CPU 981, a storage unit 982, a communication I / F (interface) 983, an input / output I / F 984, and a media I / F 985. Here, the storage unit 982 includes a RAM 982a, a ROM 982b, and an SSD (Solid State Drive) 982c. The communication I / F 983 is connected to a communication circuit 986. The input / output I / F 984 is connected to an input / output device 987. The media I / F 985 reads and writes data from / to a recording medium 988.

[0086] The ROM 982b stores an IPL (Initial Program Loader) executed by the CPU and the like. The SSD 982c stores a control program, various data, and the like. The CPU 981 realizes various functions by executing a control program and the like read from the SSD 982c into the RAM 982a. The interior of the water electrolysis control device 150 shown in FIG. 1 above is mainly shown as blocks of functions realized by a control program and the like.

[0087] [Effects of Embodiment] According to each of the above-described embodiments, the water electrolysis control device 150 includes a voltage variation calculation unit 153 that outputs a voltage variation value Val representing the degree of variation in the terminal voltages (Vs1 to Vs4), a difference calculation unit (154) that calculates a difference value DVal of the voltage variation values Val measured at a plurality of time points, a voltage variation evaluation unit 155 that determines whether or not the difference value DVal exceeds a predetermined threshold value Dth, and an operation command unit 156 that changes the electrolysis current I, the discharge flow rate F, or the discharge pressure P in the auxiliary machine 121 when the determination result in the voltage variation evaluation unit 155 is affirmative (DVal > Dth).

[0088] In this way, by changing the electrolysis current I, the discharge flow rate F, or the discharge pressure P, it is possible to reduce the variation in characteristics due to the imbalance of bubbles in the plurality of electrolysis units (1), improve the power efficiency of electrolysis, and reduce the operation cost of the water electrolysis system. In addition, it is possible to prevent the excessive use of the catalyst in a specific electrolysis unit (1), extend the life of the electrolysis unit (1), and suppress the equipment cost of the water electrolysis system.

[0089] Further, the operation command unit 156 changes a control parameter (I*, F*, P*), which is any one of a current command value I* that is a command value of the electrolysis current I, a flow rate command value F* that is a command value of the discharge flow rate F, or a pressure command value P* that is a command value of the discharge pressure P, in a stepwise manner. It is more preferable that the voltage variation evaluation unit 155 sets a difference value DVal as a value obtained by subtracting a voltage variation value Val at a first time point (t4) when a predetermined first waiting time (Ta) has elapsed from the time point when the control parameter (I*, F*, P*) is changed in a stepwise manner, from a voltage variation value Val at a second time point (t6) when a second waiting time (Tb) longer than the first waiting time (Ta) has elapsed from the first time point (t4). Thereby, the difference value DVal can be obtained starting from the time point when the control parameter (I*, F*, P*) is changed in a stepwise manner.

[0090] Further, it is more preferable that the operation command unit 156 sets a difference value DVal as a value obtained by subtracting a voltage variation value Val at the timing when the power supply unit 120 starts supplying the electrolysis current I, from a subsequent voltage variation value Val. Thereby, the difference value DVal can be obtained based on the timing when the power supply unit 120 starts supplying the electrolysis current I.

[0091] Further, it is more preferable that the operation command unit 156 sets a variation range (±ΔI, ±ΔF, ±ΔP) of the control parameter (I*, F*, P*) to a value that is 6% or more and 16% or less of the rated value of the electrolysis current I, the discharge flow rate F, or the discharge pressure P, and sets the ramp time of the control parameter (I*, F*, P*) within 10 seconds. Thereby, an appropriate variation range of the control parameter (I*, F*, P*) can be realized to reduce the variation in characteristics due to unbalanced bubbles.

[0092] Further, it is more preferable that the operation instruction unit 156 sets control parameters (I*, F*, P*) so as to cause a period in which the electrolysis current I, the discharge flow rate F, or the discharge pressure P exceeds the corresponding rated value, and the average values of the electrolysis current I, the discharge flow rate F, and the discharge pressure P are each below the corresponding rated value. Thereby, while making the levels of the electrolysis current I, the discharge flow rate F, or the discharge pressure P as high as possible, the consumption of the water electrolysis system WES1 can be suppressed.

[0093] [Modification Example] The present invention is not limited to the above-described embodiments, and various modifications are possible. The above-described embodiments are examples for easy understanding and explanation of the present invention, and are not necessarily limited to those having all the configurations described. Also, a part of the configuration of one embodiment can be replaced with the configuration of another embodiment, and the configuration of another embodiment can be added to the configuration of one embodiment. Further, a part of the configuration of each embodiment can be deleted, or other configurations can be added or replaced. Also, the control lines and information lines shown in the figures indicate those considered necessary for explanation, and do not necessarily show all the control lines and information lines required on the product. In practice, it may be considered that almost all configurations are interconnected. Possible modifications to the above embodiments are, for example, as follows.

[0094] (1) Since the hardware of the water electrolysis control device 150 in the above embodiment can be realized by a general computer, a program or the like for executing the processing corresponding to each block of the water electrolysis control device 150 described above may be stored in a storage medium (a computer-readable recording medium on which the program is recorded) or distributed via a transmission path.

[0095] (2) Although the various processes in the water electrolysis control device 150 were described as software processes using a program in the above embodiment, part or all of them may be replaced with hardware processes using an ASIC (Application Specific Integrated Circuit) or an FPGA (Field Programmable Gate Array).

[0096] (3) The various processes executed in the above embodiment may be executed by a server computer via a network (not shown), and the various data stored in the above embodiment may also be stored in the server computer.

Explanation of Reference Numerals

[0097] 1 Water electrolysis stack (electrolysis unit) 120 Power supply unit 121 Auxiliary machine 150 Water electrolysis control device 152 Voltage measurement unit (voltage measurement process) 153 Voltage variation calculation unit (voltage variation calculation process) 154 Difference operation unit, difference calculation unit (difference calculation process) 155 Voltage variation evaluation unit (voltage variation evaluation process) 156 Operation command unit (operation command process) F Discharge flow rate I Electrolysis current P Discharge pressure F* Flow rate command value (control parameter) I* Current command value (control parameter) P* Pressure command value (control parameter) Ta Standby time (first standby time) Tb Standby time (second standby time) t4 Time (first point in time) t6 Time (second point in time) Dth Threshold value Val Voltage variation value DVal Difference value WES1 Water electrolysis system Vs1 to Vs4 Stack voltages (terminal voltages)

Claims

1. a voltage measuring unit that measures a terminal voltage of each of a plurality of electrolysis units that are electrically connected in series to which water discharged from an auxiliary device is supplied and to which an electrolytic current is supplied from a power supply unit and which electrolyzes the supplied water; a voltage variation calculation unit that outputs a voltage variation value that indicates a degree of variation in the terminal voltage; A difference calculation unit that calculates a difference value of the voltage variation value measured at multiple points in time; a voltage variation evaluation unit that determines whether the difference value exceeds a predetermined threshold value; and an operation command unit that changes the electrolytic current or the discharge flow rate or discharge pressure of the auxiliary device when the determination result in the voltage variation evaluation unit is positive. A water electrolysis control device comprising:

2. the operation command unit changes in a stepwise manner a control parameter which is any one of a current command value which is a command value for the electrolytic current, a flow rate command value which is a command value for the discharge flow rate, and a pressure command value which is a command value for the discharge pressure, The voltage variation evaluation unit calculates, as the difference value, a value obtained by subtracting the voltage variation value at a first time point when a predetermined first waiting time has elapsed since the time point when the control parameter is changed stepwise from the voltage variation value at a second time point when a second waiting time longer than the first waiting time has elapsed since the first time point. The water electrolysis control device according to claim 1 .

3. The operation command unit determines, as the difference value, a value obtained by subtracting the voltage variation value at the timing when the power supply unit starts supplying the electrolytic current from the voltage variation value thereafter. The water electrolysis control device according to claim 1 .

4. The operation command unit sets the fluctuation range of the control parameter to a value between 6% and 16% of the rated value of the electrolytic current, the discharge flow rate, or the discharge pressure, and sets the ramp time of the control parameter to within 10 seconds. The water electrolysis control device according to claim 2 .

5. The operation command unit sets the control parameters so that a period occurs during which the electrolytic current, the discharge flow rate, or the discharge pressure exceeds the corresponding rated value, and the average values ​​of the electrolytic current, the discharge flow rate, and the discharge pressure are equal to or lower than the corresponding rated value. The water electrolysis control device according to claim 4 .

6. A plurality of electrolysis units electrically connected in series to electrolyze supplied water; An auxiliary device that discharges water to be supplied to the electrolysis unit; a power supply unit that supplies an electrolysis current to the electrolysis unit; a water electrolysis control device, the water electrolysis control device includes a voltage measurement unit that measures a terminal voltage of each of the electrolysis units; a voltage variation calculation unit that outputs a voltage variation value that indicates a degree of variation in the terminal voltage; A difference calculation unit that calculates a difference value of the voltage variation value measured at multiple points in time; a voltage variation evaluation unit that determines whether the difference value exceeds a predetermined threshold value; and an operation command unit that changes the electrolytic current or the discharge flow rate or discharge pressure of the auxiliary device when the determination result in the voltage variation evaluation unit is positive. A water electrolysis system comprising:

7. A plurality of electrolysis units electrically connected in series to electrolyze supplied water; An auxiliary device that discharges water to be supplied to the electrolysis unit; a power supply unit that supplies an electrolysis current to the electrolysis unit; A water electrolysis method applied to a water electrolysis system including a water electrolysis control device, The water electrolysis control device, a voltage measuring step of measuring a terminal voltage in each of the electrolysis units; a voltage variation calculation step of outputting a voltage variation value representing a degree of variation in the terminal voltage; A difference calculation step of calculating a difference value of the voltage variation value measured at a plurality of points in time; a voltage variation evaluation step of determining whether the difference value exceeds a predetermined threshold value; and executing an operation command step of changing the electrolytic current or the discharge flow rate or discharge pressure of the auxiliary device when the determination result in the voltage variation evaluation step is positive. A water electrolysis method comprising:

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