Twin-stack electrolysis system and a method of operating it

Abstract

Twin-stack electrolysis system and a method of operating it An electrolysis system (1) comprising a first electrolyzer stack (2A) and a second elec trolyzer stack (2B) for producing hydrogen gas. Liquid-electrolyte is pumped by a pump (4) from the liquid-electrolyte supply (3) through a common conduit (5) to a split point (6) at which the electrolyte from the common conduit (5) is split into a first portion fed into the first stack (2A) through a first feedline (7A) and a second portion that is fed into the second stack (2B) through a second feedline (7B). The second feedline (7B) but not the first feedline (7A) comprises an adjustable throttle (8) that is configured for variably decreasing the second flow rate through the throttle (8). The first feedline (7A) is configured to have a first flow rate that is smaller than a second flow rate flow through the second feedline (7B) when the throttle (8) of the second feedline is fully open, and the throttle (8) is configured for variably decreasing the second flow rate through the throttle (8) and the second feedline (7B) to second flow rates lower than the first flow rate through the first feedline (7A).

Description

[0001] Twin-stack electrolysis system and a method of operating it

[0002] FIELD OF THE INVENTION

[0003] The present invention relates to an electrolysis system for producing hydrogen gas and a method of operating such system.

[0004] BACKGROUND OF THE INVENTION

[0005] Electrolysis systems, in which hydrogen is produced by splitting water into hydrogen gas and oxygen gas, are in practical approaches typically built up as stacks of electrolytic cells, separated by bipolar plates. In each cell, an ion conducting membrane is sandwiched between two electrodes, and a voltage is applied over the electrodes, resulting in splitting of water into oxygen and hydrogen ions, one of which is traversing the membrane, thus, separating oxygen gas from the remaining hydrogen gas. The produced hydrogen gas is collected for later use, for example in fuel cells, for combustion, or in industrial applications.

[0006] Some examples of construction of cells in electrolyzers are found in WO2022 / 156869, WO2023 / 104266, WO2024 / 078674A1, WO2024 / 149432 Al, all assigned to Stiesdal Hydrogen A / S.

[0007] Typically, if a large number of stacks are used in an electrolyzer, valves or pumps are used for each stack for regulating flow of electrolyte. Examples are disclosed in EP4001463A1 and EP4001465A1 for an alkaline electrolyzer. EP3489394B1 discloses an even more advanced flow system for a low pressure PEM electrolyzer, where the flow system comprises flow control through each individual stack.

[0008] US9957626 discloses a system with multiple stacks, each of which has a regulation valve for flow of electrolyte. It is discussed therein that, initially, the same voltage is applied to various stack for easy control. Further, currents are measured for each cell stack and input into a voltage control device, which calculates a resistance value for each stack corresponding to a minimum current value out of the measured current values. Then, the voltage control device adjusts the voltages to obtain a predetermined current value by using the resistance value. Although, this method is advantageous for optimization, unfortunately, it is a relatively complex procedure. It would be desirable to find simpler way of control.

[0009] US11326267 and the related EP3612667A1 suggest a simplification for electrolyzers comprising multiple stacks, the improvement being avoidance of valves and a symmetrical arrangement with respect to electrolyte flow, including identical pipe cross sections and pipe lengths. EP4381116A1 discloses two electrolyzer stacks in extension of each other with a water feed and a gas extraction in a manifold inserted between the two stacks. Although, such simplification may be appealing at first sight, in practice, it has turned out the flow of electrolyte is not equal in such system of two or more stacks, but differs between the stacks due to minor variations in construction tolerances as well as due to change of the stacks with time of operation, such change of performance of one stack relatively to the other is disadvantageous as the loss of efficiency in one stack is not balanced by increase of efficiency of the other stack. Accordingly, avoiding valves for such multi-stack or twin-stack arrangements is disadvantageous.

[0010] WO2023 / 272326A1 discloses two electrolyzers that are connected in parallel to a single source of electrolyte. At a split point a first portion of the electrolyte is directed into a first electrolyzer through a first feedline and a second portion of the electrolyte to a second electrolyzer through a second feedline. The feedline of the second electrolyzer is longer than the feedline of the first electrolyzer. At the more distant second electrolyzer, there is provided a valve for adjusting the feeding of electrolyte through into the second electrolyzer from the second feedline. A valve upstream of the split point can be used to regulate the flow through both electrolyzers in common and at the same time, and the second valve at the second electrolyzer can then be used to regulate the flow through the second electrolyzer relatively to the first. This system has some severe disadvantages. Due to the fact that there is not provided a valve at the first electrolyzer, the flow through the second electrolyzer will always be smaller than the flow through the first electrolyzer but can never be larger. This is especially so because the second feedline is longer than the first feedline and, thus, has a larger flow resistance, resulting in smaller flow. As the disclosure’s main objective is a regulation system for two different electrolyzers having different stacks, this appears not to be a problem if the stack in the second electrolyzer is dimensioned to use less electrolyte due to its construction. However, such regulation system is not useful for systems with two identical electrolyzers or two in parallel arranged identical stacks.

[0011] It appears that some systems have extensive flow regulation, which is advantageous when there are many stacks, but which is disadvantageous due to high complexity and costs for establishment. Other systems have no regulation, which is advantageous in design and construction but disadvantageous in practice. It would be desirable to provide an intermediate technical solution where the advantages are maintained but the disadvantages avoided. Accordingly, there is still a potential for improvements of electrolyzers with stacks arranged in parallel, especially for twin-stacks, with respect to regulation of flow of electrolyte.

[0012] DESCRIPTION / SUMMARY OF THE INVENTION

[0013] It is therefore an objective of the invention to provide an improvement in the art. In particular, it is an objective to provide a simplified electrolyte flow regulation in electrolyzers with twin-stacks. This objective and further advantages are achieved with an electrolysis system and a method of operating it, as described below and in the claims.

[0014] The electrolysis system comprises a first electrolyzer stack and a second electrolyzer stack for producing hydrogen gas. Liquid-electrolyte or water for the hydrogen production is provided by a supply of water or liquid-electrolyte and pumped by a pump from the supply through a common conduit to a split point at which the electrolyte from the common conduit is split into a first portion that is fed into the first stack through a first feedline and a second portion that is fed into the second stack through a second feedline. The second feedline but not the first feedline comprises an adjustable throttle that is configured for variably decreasing the second flow rate through the throttle. The first feedline is configured to have a first flow rate that is smaller than a second flow rate flow through the second feedline when the throttle of the second feedline is fully open, and the throttle is configured for variably decreasing the second flow rate through the throttle and the second feedline to second flow rates lower than first flow rates through the first feedline. Notice that the system described herein is adjustable to a flow through the first feedline that is smaller than the flow through the second feedline when the throttle of the second feedline is fully open. Only the decrease of flow through the throttle when partially closed adjusts the flow through the second stack to equal or less than the flow through the first stack.

[0015] As a result, a simplified electrolyte flow regulation between the first and the second feed line, and thus between the first and second stack, is achieved in electrolysis systems with twin-stacks.

[0016] Such a system is more versatile than the system disclosed in WO2023 / 272326A1, discussed in the introduction above. Whereas the system disclosed in WO2023 / 272326A1 cannot provide a flow through the second stack that is higher than through the first stack, as the second stack is at larger distance from the split point, the system described herein allows the operator to adjust the flows to be equal. Additionally, the system described herein allows the operator to freely decide which stack should have the higher flow. By addition of the function that the first feedline is configured to have a first flow rate that is smaller than a second flow rate flow through the second feedline when the throttle of the second feedline is fully open, the flow through the first and the second stack can be freely adjusted, for example to be higher in the second stack than in the first stack or adjusted to be equal for the two stacks. This is particularly useful for two stacks that are identical. The latter is also one of the aspects in which the system described herein is different from WO2023 / 272326A1, as WO2023 / 272326A1 focuses on different stacks, whereas the invention focuses on twin-stack, that is two identical stacks operating together.

[0017] The term twin-stack is used herein for an arrangement of a combination on only two stacks in an electrolysis system, where each of the stacks is fed from the same single supply of electrolyte.

[0018] It is understood that the first flow rates and the second flow rates herein for comparison are measured at the same instant in time. Thus, when it is mentioned herein that the first flow rate(s) is / re higher than the second flow rate(s), or the first flow rate(s) is / are lower than the second flow rate(s), or the two flow rates are equal, it means that the comparison is done with the respective flow rates at the same instant in time.

[0019] In more detail, the electrolysis system comprises a power supply and a supply of water or liquid-electrolyte in as far as needed for the hydrogen production, as well as a first electrolyzer stack and a second electrolyzer stack, both of which are electrically connected to the power supply and conduit-connected to the supply of water and / or electrolyte and configured for producing hydrogen gas by splitting water from the liquidelectrolyte into hydrogen and oxygen during consumption of power from the power supply. A pump is upstream-connected to the supply of water and / or electrolyte. The water or liquid-electrolyte is pumped by the pump from the supply through a common conduit to a split point at the end of the common conduit to which the pump is down- stream-connected. At the split point, the water or liquid-electrolyte from the common conduit is split into a first portion and a second portion. The first portion is fed at a first flow rate to the first stack through a first feedline, which is connecting the split-point with a first end of the first stack. The second portion is fed by a second flow rate to the second stack through a second feedline, which is connecting the split-point with a first end of the second stack. Only the second feedline but not the first feedline comprises an adjustable throttle that is configured for variably decreasing the second flow rate through the throttle and the second feedline to flow rates lower than the first flow rate through the first feedline. Furthermore, particularly for the system herein, the first feedline is configured to have a first flow rate that is smaller than the second flow rate through the second feedline when the throttle of the second feedline is fully open. Further, the throttle is configured for variably decreasing the second flow rate through the throttle and the second feedline from second flow rates that are higher than first flow rates to second flow rates that are lower than the first flow rate through the first feedline.

[0020] Such a system is particularly useful for systems that comprises only two stacks in total, which are the first and the second stack.

[0021] Advantageously, the first and second stacks are arranged in extension of each other along a line and in parallel to that line. In particular, the first and second stack can be arranged with their first ends facing each other. Such a configuration is convenient when there are only two stacks, as the supply for the liquid electrolyte is advantageously be provided between the first ends and supply the electrolyte to electrolyte inlets at the first ends.

[0022] Optionally, each of the first and second stack have a hydrogen gas outlet at their first ends and merging into a common hydrogen gas release line. In some embodiments, the common hydrogen gas release line also transports electrolyte to a gas separator in which the hydrogen gas is separated from the electrolyte and the electrolyte is recycled back to the water / liquid-electrolyte supply. The hydrogen is optionally stored in a container, for example pressurized container.

[0023] Optionally, each of the first and second stack have an oxygen gas outlet at their first ends and merging into a common oxygen gas release line. In some embodiments, the common oxygen gas release line also transports electrolyte to a gas separator in which the oxygen gas is separated from the electrolyte and the electrolyte is recycled back to the water / liquid-electrolyte supply. Optionally, the oxygen gas is released to atmosphere. Alternatively, it is collected in a storage tank.

[0024] Optionally, the electrolysis system comprises an alkaline electrolyzer with alkaline electrolyte, such as aqueous NaOH or KOH solutions.

[0025] For example, during operation of hydrogen production, a first voltage is measured over the first stack and a second voltage is measured over the second stack. On the basis of the continuous or repeated measured first and second voltages, the throttle is automatically adjusted until the first voltage relatively to the second is in agreement with a predetermined relationship between the first and second voltage.

[0026] In some embodiment, during hydrogen production, a first voltage is measured over the first stack and a second voltage over the second stack, and the throttle is adjusted until the two voltages are equal. In this case, the predetermined relationship is that the first and the second voltage are equal.

[0027] In other embodiments, the predetermined relationship is a cyclic increase and decrease of the first voltage relatively to the second voltage by cyclically closing and opening the throttle, respectively, in order to cycle the stacks through varying performance. Although, this is not optimum for a constant maximum hydrogen production, it is beneficial for the longevity of the electrolysis system.

[0028] In a simplified but highly useful embodiment, the throttle is adjusted only on the basis of the measurements of the first and second voltage. Thus, no current measurements or resistance calculation is performed, which is in contrast to the system and method as disclosed in US9957626.

[0029] SHORT DESCRIPTION OF THE DRAWINGS

[0030] The invention will be explained in more detail with reference to the drawing, where FIG. 1 illustrates a first embodiment of an electrolysis system,

[0031] FIG. 2 illustrates a second embodiment of an electrolysis system.

[0032] DETAILED DESCRIPTION / PREFERRED EMBODIMENT

[0033] FIG. 1 illustrates an electrolysis system 1 comprising a first electrolyzer stack 2A and a second electrolyzer stack 2B arranged in parallel and extension of each other. Each electrolyzer stack 2A, 2B comprises multiple electrolyzer cells 19, each cell 19 comprising an anode (not shown) and a cathode (not shown) and an ion conducting membrane (not shown) separating a cathode chamber (not shown) and an anode chamber (not shown) from each other, the anode chamber and the cathode chamber containing liquid-electrolyte, for example alkaline electrolyte. Neighbouring cells are typically separated by bipolar plates (not shown), optionally being composed of two metal sheets (not shown) between which coolant (not shown) circulates.

[0034] Some examples of construction of cells in electrolyzers are found in WO2022 / 156869, WO2023 / 104266, WO2024 / 078674A1, WO2024 / 149432 Al, all assigned to Stiesdal Hydrogen A / S. However, other configurations are possible, as also described in the prior art.

[0035] With reference to FIG. 1, liquid electrolyte and / pr water for the hydrogen production is supplied from a supply, illustrated by arrow 3, and pumped by a pump 4 through a common conduit 5. It is understood that the supply alternatively provides water for the electrolysis instead of electrolyte, if the conditions for the electrolysis only require water addition, for example if the alkalinity of the electrolyte is maintained and only water is needed for replenishing the consumption thereof.

[0036] At a split point 6 at the end of the common conduit 5, the water or liquid-electrolyte from the pump 4 is split into a first portion and a second portion. The first portion is fed to the first stack 2A through a first feedline 7A, which is connecting the split-point 6 with the first stack 2A. The second portion is fed to the second stack 2A through a second feedline 7B, which is connecting the split-point 6 with the second stack 2B. The flow of the second feedline 7B goes through an adjustable throttle 8, for example valve, which is configured for variably increasing or decreasing the flow through the throttle 8. If the flow is gradually decreased through the throttle 8 and the second feedline 7B, the flow through the second stack 2B will be less than the flow through the first stack 2A.

[0037] In the exemplified embodiment of FIG.1, the electrolysis system comprises identical stacks 2 A, 2B. Further, the feedlines 7 A, 7B have identical cross section and length, apart from the portion through the throttle 8 and the flow restrictor 9A.

[0038] A non-adjustable flow restrictor 9A is provided as part of the first feedline 7A. The flow through the second stack 2B will be larger than through the first stack 2A if the flow through the throttle 8 is adjusted to higher than the flow through the non-adjustable flow restrictor 9A. Correspondingly, the flow through the second stack 2B will be smaller than through the first stack 2A if the flow through the throttle 8 is adjusted to less than the flow by the non-adjustable flow restrictor 9A. For example, the throttle 8 is dimensioned to have a medium adjustment with a restriction equal to the non-adjustable flow restrictor 9A, in which case the flow through the first stack 2A and the second stack 2B can be regulated to be identical, when this is desired, but it can also be adjusted to not be identical, when it is so desired, where the higher flow can be selected between the first stack 2 A and the second stack 2B.

[0039] Returning to the discussion in the introduction, such a system as described has a simplicity similar to the system disclosed in WO2023 / 272326A1, but the system described herein is much more versatile due to the addition of the single simple element of the non-adjustable flow restrictor 9A. Whereas the system disclosed in WO2023 / 272326A1 cannot provide a flow through the second stack that is higher than through the first stack, as the second stack is at larger distance to the split point and has an upstream throttle, the system described herein allows the operator to adjust the flows to be equal. Additionally, the system described herein allows the operator to freely decide which stack should have the higher flow. By addition of a simple component, namely the non-adjustable flow restrictor 9A, at the right location in the conduit system, a system is provided simpler than systems with two regulation valves, which reduces cost and makes construction as well as operation easier, and which is more versatile than systems which have no valves or only a single throttle. Accordingly, a simple change has large positive effect.

[0040] For example, the non-adjustable flow restrictor 9A is a diaphragm of a throughput cross section smaller than the inner cross section of the first feedline 7A and of the second feedline 7B. However, this is not strictly necessary. If the cross sections of the first feedline 7A and the second feedline 7B are different, the throttle 8 and the non-adjustable flow restrictor 9A merely have to be dimensioned such that the flow through the second feedline 7B including the throttle 8 can be adjusted to more and can be adjusted to less than the flow through the first feedline 7A including any non-adjustable flow restrictor 9A.

[0041] This implies that the non-adjustable flow restrictor 9A can have various forms. Accordingly, the expression of the flow restrictor should be understood broadly in being a flow restriction means that causes the first flow to be smaller than the second flow when the throttle 8 is fully open.

[0042] An alternative example is illustrated in FIG. 2, which is exemplified with the same components as the system in FIG. 1 apart from a different construction of the first feedline 7A. In this case, an alternative non-adjustable flow restrictor 9B in the first feedline 7A is provided by the feedline 7A having a portion so that is length is substantially longer, for example at least 2 times or at least 3 times or at least 4 times longer, than the length of the second feedline 7B, so that the flow resistance in the substantially longer first feedline 7A, as compared to the second feedline 7B, takes the role and substitutes the non-adjustable flow restrictor 9A of FIG. 1, while providing a similar effect. Notice in relation to the discussion above regarding the system of WO2023 / 272326A1 that the system described herein has a longer first feedline 7A from the split point 6 to the first stack 2A, while the throttle is in the shorter, second feedline 2b from the split point to the second stack 7B, whereas in WO2023 / 272326A1, the throttle is provided in the longer feedline, which is the feedline to the second stack, which is a different configuration. The different configuration for the system described herein is equally simple but brings about great advantages, as discussed already above with respect to the adjustment option of both more and the option of less flow in one stack as compared to the other.

[0043] As an alternative to the extended length of the first feedline 7A, the first feedline 7A may, generally, have a smaller cross section as the second feedline 7B, which has a similar effect as the non-adjustable flow restrictor 9A in FIG. 1. Of course, two or three of the features of the non-adjustable flow restrictor 9A of FIG. 1, for example a diaphragm, the alternative flow restrictor 9B as part of the extended length of the first feedline 7A in FIG. 2, and the even further alternative flow restrictor achieved by a smaller cross section of the first feedline 7A may also be combined.

[0044] Important is the fact that the flow through the first feedline 7A is smaller than the flow through the second feedline 7B when the throttle 8 of the second feedline is fully open. Only the decrease of flow through the throttle 8 when partially closed adjusts the flow through the second stack 2B to equal or less than the flow through the first stack 2A.

[0045] Returning to FIG. 1, it is observed that a hydrogen gas release line 10 for release of hydrogen gas and an oxygen gas release line 20 for release of oxygen gas connects to both stacks 2A and 2B. For example, the hydrogen gas is released for capture in a hydrogen gas storage tank, whereas the oxygen gas is released into atmosphere.

[0046] Optionally, the gas release lines 10, 20 are also used for outlet of surplus electrolyte. For example, electrolyte dragged along by the upwards flowing gas. In such case, the hydrogen and oxygen gases, respectively, are separated from the electrolyte in corresponding gas separators, and the electrolyte is optionally recycled, all of which are principles well known in the art of electrolyzers. The specific configuration of FIG. 1 shows two stacks 2A, 2B in linear extension of each other with ends 11 A, 1 IB of the two stacks 2A, 2B facing each other. The electrolyte supplies and the gas outlets are arranged in between opposite ends 11 A, 1 IB of the two stacks 2A, 2B. This implies that each stack 2A, 2B has an inlet of electrolyte and an outlet of produced gases at the first end 11 A, 11B of the respective stack 2A, 2B. This is a simple configuration and yields advantages due to its symmetry. Even more so, it is advantageous in that the two ends 11 A, 1 IB of the respective stacks 2 A, 2B can be short circuited, as illustrated by electrical conductor 12A, 12B, 12C. From a power supply 13, current is supplied to both stacks 2A, 2B in series at the remote ends 14A, 14B of the respective stacks, the remote ends 14A, 14B, being the ends that are not facing each other.

[0047] For identical stacks 2A, 2B, the voltages over the stacks 2A, 2B, as measured by Voltmeters 15 A, 15B, can be used as a simple measure for adjustments. In particular, the system 1 may comprise a voltage control device which is functionally connected to the voltmeters 15 A, 15B and the throttle 8, which is then automatically adjusted so that the voltmeters 15 A, 15B measure voltages according to a predetermined relationship, for example identical voltages. This is a simpler way than prior art systems, like US9957626, as discussed above, in which currents are measured and resistances calculated and various voltages adjusted by varying currents. Advantageously, in the system described herein, the only adjustment feature is the throttle 8, which is adjusted by a voltage control device (not shown) until the two voltages in the voltmeters 15 A, 15B are equal.

[0048] As an alternative, the predetermined relationship is a cyclic increase and decrease of the first voltage relatively to the second voltage by cyclically closing and opening the throttle 8, respectively, in order to cycle the stacks 2A, 2B through varying performance. Although, this may not be optimum for a constant maximum hydrogen production, it is beneficial for the longevity of the electrolysis system and may lead to optimised hydrogen production when measured over longer time spans. As it appears from the above, especially for identical stacks in this symmetrical twinstack configuration with only two stacks, the system is simple in its construction with few components and is also simple to operate.

Claims

CLAIMS1. An electrolysis system (1) for hydrogen production, the system comprising- a power supply (13) and- a supply (3) of water or liquid-electrolyte as needed for the hydrogen production,- a first electrolyzer stack (2A) and a second electrolyzer stack (2B) both of which are electrically connected to the power supply (13) and conduit-connected to the supply (3) and configured for producing hydrogen gas by splitting water from the liquid-electrolyte into hydrogen and oxygen during consumption of power from the power supply (13);- a common conduit (5) with a split point (6) at the end of the common conduit (5),- a first feedline (7 A), which is connecting the split-point (6) to a first end (11 A) of the first stack (2A), and a second feedline (7B), which is connecting the split-point (6) to a first end (1 IB) of the second stack (2B), wherein the second feedline (7B) comprises an adjustable throttle (8) that is configured for variably decreasing the second flow rate through the throttle (8) and the second feedline (7B) to flow rates lower than the first flow rate through the first feedline (7 A);- a pump (4) for pumping water or liquid-electrolyte from the supply (3) through the common conduit (5) to the split point (6) for splitting the water or liquid-electrolyte into a first portion and a second portion and for feeding the first portion at a first flow rate to the first stack (2A) through the first feedline (7 A) and the second portion by a second flow rate to the second stack (2B) through the second feedline (7B) characterized in that the first feedline (7 A) is configured to have a first flow rate that is smaller than the second flow rate through the second feedline (7B) when the throttle (8) of the second feedline is fully open, and wherein the throttle (8) is configured for variably decreasing the second flow rate through the throttle (8) and the second feedline (7B) to second flow rates lower than the first flow rate through the first feedline (7 A).

2. The electrolysis system (1) of claim 1, wherein the system comprises only two stacks in total, which are the first and the second stack (2A, 2B).

3. The electrolysis system (1) of claim 2, wherein the first and second stacks (2A, 2B) are arranged in extension of each other along a line (16) and in parallel to that line (16).

4. The electrolysis system (1) of claim 3, wherein the first and second stack (2A, 2B) are arranged with their first ends (11 A, 1 IB) facing each other.

5. The electrolysis system (1) of claim 4, wherein each of the first and second stack (2A, 2B) have a hydrogen gas outlet at their first ends (11 A, 1 IB) merging into a common hydrogen gas release line (10).

6. The electrolysis system (1) of claim 5, wherein the common hydrogen gas release line (10) also transports electrolyte to a gas separator in which the hydrogen gas is separated from the electrolyte and the electrolyte is recycled back to the liquid-electrolyte supply (3).

7. The electrolysis system according to any preceding claim, wherein the electrolyte is an alkaline electrolyte based on NaOH or KOH.

8. A method of operating an electrolysis system, the system comprising- a power supply (13),- a supply (3) of water or liquid-electrolyte as needed for the hydrogen production,- a first electrolyzer stack (2A) and a second electrolyzer stack (2B) both of which are electrically connected to the power supply (13) and conduit-connected to the supply (3), for producing hydrogen gas by splitting water from the liquid-electrolyte into hydrogen and oxygen during consumption of power from the power supply (13);- providing a common conduit (5) with a split point (6) at the end of the common conduit (5),- a first feedline (7 A), which is connecting the split-point (6) to a first end (11 A) of the first stack (2A), and a second feedline (7B), which is connecting the split-point (6) to a first end (1 IB) of the second stack (2B), wherein the second feedline (7B) comprises an adjustable throttle (8);- a pump (4) upstream-connected to the supply (3) and downstream-connected to the common conduit (5);wherein the method comprises pumping water or liquid-electrolyte by the pump (4) from the supply (3) through the common conduit (5) to the split point (6) and splitting the water or liquid-electrolyte into a first portion and a second portion and feeding the first portion at a first flow rate to the first stack (2A) through the first feedline (7 A) and the second portion by a second flow rate to the second stack (2B) through the second feedline (7B) and producing hydrogen by splitting water into hydrogen and oxygen by the electrolysis system (1); characterized in that the method comprises providing the first feedline (7 A) with a first flow rate that is smaller than the second flow rate through the second feedline (7B) when the throttle (8) of the second feedline (7B) is fully open, wherein the throttle (8) is configured for variably decreasing the second flow rate through the throttle (8) and the second feedline (7B) to second flow rates lower than the first flow rate through the first feedline (7 A), and regulating the flow rate through the first feedline (7 A) and the second feedline (7B) by operating the throttle (8).

9. A method according to claim 8, wherein the method comprises during operation, measuring a first voltage over the first stack (2A) and a second voltage over the second stack (2B) continuously or repeatedly and on the basis of the continuously or repeatedly measured first and second voltage automatically adjusting the throttle (8) until the first voltage relatively to the second is in agreement with a predetermined relationship between the first and second voltage.

10. The method according to claim 9, wherein the predetermined relationship is that the first and the second voltage are equal.

11. The method according to claim 9, wherein the predetermined relationship is a cyclic increase and decrease of the first voltage relatively to the second voltage by closing and opening the throttle, respectively, in order to cycle the stacks through varying performance.

12. The method according to anyone of the claims 9-11, wherein the method comprises adjusting the throttle (8) only on the basis of the measurements of the first and second voltage.

Images