Alkaline electrolyser and a method for its operation including gas purging

Abstract

Alkaline electrolyser and a method for its operation including gas purging An alkaline electrolyser comprising a stack (17) of electrolytic cells (1) is used for producing hydrogen gas (8). Purified hydrogen gas and purified oxygen gas is used for purging the corresponding cathode and anode compartments (5, 6) for preventing buildup of dangerous gas mixtures by gas crossover during stop, before starting, or when running production low.

Description

[0001] Alkaline electrolyser and a method for its operation including gas purging

[0002] FIELD OF THE INVENTION

[0003] The present invention relates to an alkaline electrolyser for water electrolysis, for example for hydrogen production, and to a method for the operation of the electrolyser.

[0004] BACKGROUND OF THE INVENTION

[0005] Electrolysis is a promising technology for the production of hydrogen from renewable energy sources. So-called electrolysers are used for this purpose. The primary objective of an electrolyser is for production of hydrogen gas. Hydrogen is collected for later use, for example in fuel cells or industrial applications. However, due to the splitting of water in the electrolyte when applying electrical power, oxygen is also produced. The oxygen may also be collected for later use but may alternatively be discarded into atmosphere.

[0006] Several types of electrolysers exist, each exhibiting advantages and disadvantages. Alkaline electrolysis is generally seen as a mature, low-cost, robust technology, but it is disadvantaged by limited ability to operate at part load. In this context, “load” means the electric power supplied to the electrolyser to drive the splitting of water into gaseous oxygen and hydrogen. “Nominal load” means the application of a level of power to the electrolyser corresponding to the nameplate power rating of the electrolyser, and “part load” means the application of a level of power to the electrolyser that is lower than the nameplate power rating of the electrolyser.

[0007] Due to the limited ability of alkaline electrolysers to operate at part load below a certain limit, other electrolyser technologies such as PEM or SOEC are sometimes preferred over alkaline electrolysers due to their capability to operate at lower part load than alkaline electrolysers, even though this capability is often acquired at the expense of higher costs or other disadvantages, such as the need for rare metal or the need for high operational temperatures. The inability of alkaline electrolysers to operate at part load below certain limits is related to the permeability of the separators used in the electrolytic cells that are at the core of the electrolyser.

[0008] In an alkaline electrolyser, multiple electrolytic cells are connected in the so-called electrolyser stack. Each electrolytic cell comprises an anode and an associated compartment filled with electrolyte, a cathode and an associated compartment filled with electrolyte, and a separator sandwiched between the two compartments. Typically, the electrolyte is an aqueous potassium hydroxide solution of 25-35 percent concentration by weight, but other electrolyte compositions are also used.

[0009] In traditional alkaline electrolysers, each electrolytic cell has two electrode plates separated by a certain distance, and with the separator located about halfway between the two electrodes. The gap between the electrodes is filled with the electrolyte. When sufficient voltage is applied, hydrogen is released on the cathode surface and oxygen is released on the anode surface. The gap between the electrodes needs to be of sufficient width to allow the escape of hydrogen and oxygen bubbles without excessive blocking of the conductive path through the electrolyte from the anode to the cathode, and to allow electrolyte circulation without excessive pressure loss.

[0010] In later years, the configuration of traditional alkaline electrolysers has been replaced by a so-called zero-gap configuration. In the zero-gap configuration, the cell design works by pressing two porous electrodes onto either side of the separator. This achieves a gap between the two electrodes equal to the sum of the thickness of the separator, typically 0.5 mm or even less, and any additional distances from each electrode to the separator, rather than the 2-5 mm required for the traditional gap configuration. The smaller gap reduces the ohmic resistance contribution of the electrolyte to the losses in the electrolytic cells. The electrodes have pores in order to allow the escape of hydrogen and oxygen bubbles to the side of the electrode not facing the separator.

[0011] Hence, an electrolytic cell of an alkaline electrolyser is typically composed of a first solid metallic plate, an anode electrically connected to this first solid metallic plate and separated from it at sufficient distance to facilitate the escape of oxygen bubbles through the electrolyte without excessive blocking, a separator, and a cathode electrically connected to a second solid metallic plate and separated from this second solid plate at sufficient distance to facilitate the escape of hydrogen bubbles through the electrolyte without excessive blocking.

[0012] In the electrolyser stack the electrolytic cells are placed back-to-back, and the second solid metallic plate of a first electrolytic cell is the first solid metallic plate of a second electrolytic cell, thus, forming a bipolar plate.

[0013] During operation of the electrolyser, liquid water is split into gaseous oxygen and hydrogen through electrochemical reactions at the electrodes of each of the electrolytic cells in the stack.

[0014] At the cathode, the following reaction takes place:

[0015] 2H2O (1) + 2e’ H2(g) + 2OH' (aq)

[0016] At the anode, the following reaction takes place:

[0017] 2OH' (aq)1 / 2O2(g) + H2O (1) + 2e

[0018] The overall reaction is:

[0019] H2O (1) H2(g) + ' / 2O2(g)

[0020] This equation represents the splitting of water into hydrogen and oxygen gases using electrical energy. The hydrogen gas is collected at the cathode, while the oxygen gas is collected at the anode.

[0021] The electrolyte solution, which contains hydroxide ions (OH-), plays a crucial role in facilitating the movement of ions and ensuring the completion of the electrochemical circuit. The hydroxide ions created at the cathode migrate from the cathode compartment to the anode compartment through the separator, allowing the overall electrolysis process to occur. Water is consumed from the electrolyte at a rate corresponding to the gas production rate. In order to maintain an approximately constant concentration of the electrolyte an electrolyte circulation arrangement is established whereby the electrolyte is circulated through the electrolytic cells of the electrolyser stack, through two gas separation tanks for oxygen and hydrogen, respectively, replenished with fresh water, and pumped back into the electrolytic cells of the electrolyser stack.

[0022] The electrolyte circulation is enabled with conduits connecting the electrolytic cells in the stack. Typically, one electrolyte feed conduit connects all the anode compartments in the stack, and another electrolyte feed conduit connects all the cathode compartments in the stack. Other arrangements are also used, e.g., having a single feed conduit connecting both the anode compartments and the cathode compartments in the stack, or having several feed conduits each connecting only part of the compartments in the stack.

[0023] Electrolyte is returned from the electrolytic cells through two or more electrolyte return conduits connecting the electrolytic cells in the stack. Gas produced in the electrolytic cells is also removed through these electrolyte return conduits. Typically, one manifold connects all the anode compartments in the stack, and another manifold connects all the cathode compartments in the stack. During operation of the electrolyser, gaseous oxygen bubbles form at or near the anode electrode in the anode compartments of the stack, and gaseous hydrogen bubbles form at or near the cathode electrode in the cathode compartments of the stack. In each half-cell compartment, the bubbles mix with the liquid electrolyte to form an electrolyte-bubble mix that is evacuated to the electrolyte return conduit of the relevant gas, the oxygen electrolyte return conduit for the anode compartments and the hydrogen electrolyte return conduit for the cathode compartments. An example of a system with recirculation and gas separation is disclosed in US4397735.

[0024] As mentioned above, the hydroxide ions created at the cathode migrate from the cathode compartment to the anode compartment through the separator, allowing the overall electrolysis process to occur. For this transport of hydroxide ions to occur, the separator is typically porous and hydrophilic. As a consequence, gas diffusion across the separator is unavoidable. Even with the emergence of new dense separator types such as anion exchanging membranes, gas-containing electrolyte will be exchanged between the two half cells over the separator. Hence, gas crossover may be reduced but not avoided in alkaline electrolysers. This gas diffusion across the separator, normally designated crossover, occurs at all load levels. The gas crossover is mainly driven by the differences in partial pressure at the two different sides of the separator. In the anode compartment, the electrolyte is saturated with oxygen, and in the cathode compartment, the electrolyte is saturated with hydrogen. It is the differences in partial pressures resulting from the saturation that causes diffusion of the gases across the separator. Oxygen diffuses from the anode compartment to the cathode compartment, and hydrogen diffuses from the cathode compartment to the anode compartment.

[0025] Saturation of the electrolyte occurs rapidly when the production of oxygen and hydrogen commences, and following an initial build-up of saturation, the rate of crossover is fairly constant, irrespective of the electrolyser load. Additional crossover may occur as a result of supersaturation close to the electrodes, and this additional crossover may to some extent be load-dependent, but for all practical purposes, the crossover is independent of the electrolyser load.

[0026] Oxygen-hydrogen gas mixtures are not combustible if the concentration of either of the gases in the gas mixture is lower than 4 percent. If the concentration of either of the gases in the gas mixture is higher than 4 percent, the gas mixture may be combustible. Hence, a concentration of more than 4 percent of one of the gases in the gas mixture is considered to be critical.

[0027] At high load, the gas production in each electrolytic half-cell is much larger than the crossover of gas from the opposite electrolytic half-cell, and the gas contamination is well below the critical level. However, at low part load operation, the gas contamination may exceed the critical level due to the lower gas production, causing the gas mixture in the individual gas bubbles to become combustible.

[0028] Combustibility is, generally, not a problem in the electrolyte-bubble mix that occurs in an electrolytic half-cell, since the volume of each bubble is very small, typically on the order of fractions of cubic millimetres to single-digit cubic millimetres. No source of ignition exists in a bubble, and even if detonation should occur in one or few bubbles, the likelihood of chain-reaction detonations is small, since the multitude of bubbles in the electrolyte-bubble mix serves as a spring-damper system that reduces the pressure waves of any small detonations.

[0029] The situation changes when the electrolyte-bubble mix is evacuated from the halfcells through orifices to the electrolyte return conduit of the relevant gas. In the electrolyte return conduit the gas bubbles will coalesce to form large bubbles, often completely separating from the electrolyte. If the gas is contaminated to a level where it is combustible, i.e., if the concentration of oxygen in the hydrogen electrolyte return conduit is higher than 4 percent or the concentration of hydrogen in the oxygen electrolyte return conduit is higher than 4 percent, then, the risk of spontaneous combustion or even detonation is imminent.

[0030] An even worse situation may occur in the gas separation tanks. Here, the gases are separated from the electrolyte, and gas volumes may be on the order of hundreds of litres per tank, or even cubic metres. Combustion of such volumes of contaminated gas may lead to catastrophic explosion.

[0031] Consequently, it is not possible to operate an alkaline electrolyser at a part load lower than that at which the gas contamination caused by crossover reaches the critical level of 4 percent. In practice, at level of gas contamination of 2 percent is typically set as the operational limit.

[0032] The lower threshold for safe part load operation of alkaline electrolysers that will meet the operational limit of 2 percent gas contamination is often stated to be in the range of 10-25 percent, as demonstrated on p. 185 of the publication, Innovation landscape for smart electrification: Decarbonising end-use sectors with renewable power, published by the International Renewable Energy Agency (IRENA), Abu Dhabi, ISBN: 978-92-9260-532-2 (2023).

[0033] When forced to operate close to or below the lower threshold for safe part load operation of alkaline electrolysers, e.g., as a result of variations in the input power caused by the variability of renewable energy sources, such as solar or wind power, it is customary to shut down the electrolyser.

[0034] Shutdown of the electrolyser at low load has several negative effects. First, hydrogen production is lost. Second, restarting the electrolyser takes time, and the loss of hydrogen production may be sustained for a while at very considerable load levels, aggravating the loss of hydrogen production. Finally, frequent shutdowns and start-ups accelerate electrode degradation, reducing the electrolyser's expected lifespan and increasing the costs for maintenance and replacement.

[0035] Alternative strategies may be implemented to reduce the negative effects of shutdown. Such alternative strategies may involve using batteries or other energy storage devices to absorb power fluctuations, or the implementation of cascaded start-up and shutdown of a string of electrolysers.

[0036] Gas crossover and contamination cause additional problems at the restart of alkaline electrolysers after shut-down.

[0037] During operation prior to shut-down the electrolyte in each of the compartments will have been saturated with the gas produced in the respective compartment; in the anode compartments, the electrolyte is saturated with oxygen, and in the cathode compartments, the electrolyte is saturated with hydrogen. As described above, gas crossover is mainly driven by the differences in partial pressure arising from difference in gas concentration at the two different sides of the separator. The differences in partial pressure will cause diffusion of the gases across the separator.

[0038] During standstill, gas crossover, i.e., the diffusive flux of gas across the separator, with oxygen diffusing from the anode compartment to the cathode compartment and hydrogen diffusing from the cathode compartment to the anode compartment, will continue until the partial pressures of the two gases are the same at both sides of the separator.

[0039] During operation prior to shut-down, gaseous oxygen bubbles will have formed at or near the anode electrode in the anode compartments of the stack, and gaseous hydrogen bubbles will have formed at or near the cathode electrode in the cathode compartments of the stack. Some of these bubbles will remain on the electrodes after shut-down. Initially, the bubbles will contain clean or almost clean gas, but soon an equilibrium will be established where the gas mixture in each bubble will correspond to the ratio of partial pressures in the electrolyte. Consequently, once crossover has caused the partial pressures of the two gases to be the same at both sides of the separator, all bubbles will have the same mixed gas composition, which is invariably combustible.

[0040] As described above, the presence of combustible gas mixtures in small bubbles surrounded by electrolyte does not in itself pose a risk. However, once the electrolyser is restarted, the bubble formation resulting from the gas evolution of the water-splitting process will push the highly contaminated bubbles from the electrode surfaces. As a consequence, the gases initially delivered to the electrolyte return conduits will be contaminated to a level that poses imminent danger of combustion or explosion in the electrolyte return conduits and the gas separation tanks.

[0041] Typically, the problem of high contamination of the gases initially delivered to the electrolyte return conduits at startup is overcome through strong flushing of the electrolyte return conduit with nitrogen, diluting the contaminated gases to a level where combustion is no longer possible. The duration of flushing may be on the order of minutes or tens of minutes. The IRENA reference quoted above specifies the start-up time to be in the range of 1-10 minutes.

[0042] Flushing with nitrogen has several disadvantages. First, significant amounts of nitrogen are required to ensure safe operation, and this requires substantial nitrogen supply arrangements, typically, comprising batteries of high-pressure nitrogen flasks or even nitrogen generators connected through complex arrangements of pipes and valves. All of this adds both investment and operational costs to the system. Second, the hydrogen produced during startup is by default contaminated with nitrogen. In many use cases for hydrogen such contamination is not permissible, and since it is difficult to separate nitrogen from hydrogen it is often necessary to simply vent the contaminated hydrogen to the atmosphere, representing a manifest loss of economic value. US10865486 discloses a method for reduction of the partial gas pressure in the electrolytic half-cells of a high-temperature electrolyser through the flushing with gases. The actual aim of the flushing is to reduce the respective partial pressure at the flushed electrode. For example, flushing the anode leads to a reduction in the oxygen partial pressure at the anode, which reduces the corrosive properties of oxygen at the anode. Furthermore, it is noted as an advantage that the removal of the gas formed on the electrodes avoids reduction in the electrochemical performance of the electrolysis cell through removal of excess gases.

[0043] Generally, the flushing gases disclosed in US 10865486 are inert gases such as argon or nitrogen, but it is noted as an option that a part of the hydrogen generated at the cathode can be recirculated, that is to say, the hydrogen gas can again be fed in the recirculation circuit to the cathode for the flushing thereof. A similar approach can be taken for the anode. It is stated that the overall conversion rate of the electrolysis system is increased by recycling an unused reactant or a product gas mixture containing the reactant to the process inlet, which is to say, re-feeding the unused reactant to the cathode of the electrolysis cell.

[0044] The method disclosed in US10865486 has several disadvantages and appears not straightforward applicable in alkaline electrolysers. If inert gases are used for the flushing, these gases will need to be separated from the hydrogen gas before supply to the user of the gas, which is difficult and often impossible. On the other hand, if the hydrogen generated at the cathode is recirculated for the flushing of the cathode compartments, the oxygen content in the hydrogen will remain at the initial level, and no benefit is achieved regarding part load operation. The same applies on the oxygen side.

[0045] Hence, there is an unfulfilled need for alkaline electrolysers that can continue safe operation to very low levels of part load operation, and that can startup at short notice with little or no contamination with nitrogen.

[0046] The described problem with gas mixing due to gas crossover is not only appearing in electrolysers which have electrode compartments that are steadily filled with electrolyte but also for systems in which liquid electrolyte is supplied to an upper portion of the membrane only and flowing down the separator by gravity. Various systems with gravity-driven flow of electrolyte along the ion-transporting separator are disclosed in the prior art, for example in US2005 / 277015, US2013 / 240370, US2022 / 056594, W02001 / 57290, US2011 / 073491, US2005 / 183951.

[0047] There is a need for improvements with respect to safety and optimisation of the operation in electrolysers.

[0048] DESCRIPTION / SUMMARY OF THE INVENTION

[0049] It is therefore an objective of the invention to provide an improvement in the art. In particular, it is an objective to provide an alkaline electrolyser that has a high degree of gas purity for facilitating part load operation at a very low fraction of nominal load and rapid startup leading to little or no contamination of the produced hydrogen with nitrogen. Furthermore, it is an objective to provide a method for the operation of the electrolyser. These objectives and further advantages are achieved with an alkaline electrolyser for production of hydrogen gas as described below and in the claims.

[0050] In short, purified hydrogen gas and purified oxygen gas is used for purging the corresponding cathode and anode compartments in an electrolyser stack for preventing buildup of dangerous gas mixtures by gas crossover during stop, before starting, or when running production low. Particular advantages are achieved with gas recirculation systems that provide purified gas to electrolytic cells of the electrolyser stack.

[0051] The stack comprises a series of electrolytic cells. Each electrolytic cell comprises a cathode compartment and an anode compartment, separated by an ion-transporting separator that is wetted with electrolyte. Typically, the electrolyser is operated with an alkaline electrolyte based on NaOH or KOH, for example having a temperature in the range of in the range of 50-90 °C.

[0052] The stack comprises a cathode gas manifold arranged above the cells and extending along the stack for transport of received cathode gas out of the stack. Similarly, the stack comprises an anode gas manifold arranged above the cells and extending along the stack for transport of received anode gas out of the stack. For long arrangements of stacked electrolytic cells, it is possible to have several cathode gas manifolds and several anode gas manifolds, arranged serially along the arrangement, each manifold connecting the compartments of a subgroup of cells, each subgroup being a stack of cells in its own as a part of the arrangement, and the term “stack” herein should be read such that it covers either en entire arrangement of stacked cells or a stacked subgroup of cells.

[0053] Each cathode compartment comprises a cathode gas outlet at an upper part of the cathode compartment and flow-connecting the cathode compartment to the cathode gas manifold. Similarly, each anode compartment comprises an anode gas outlet at an upper part of the anode compartment and flow-connecting the anode compartment to the anode gas manifold.

[0054] In some prior art systems, electrolyte is drained through the gas manifolds together with the gas, and the corresponding manifolds are called electrolyte return manifolds, as they return the electrolyte to the electrolyte recycling system. This is not so in the system described herein, as the gas manifolds are held free from liquid electrolyte. For this reason, the cathode gas outlet for each cathode compartment is remote, and, thus, at a distance, from the liquid electrolyte in the cathode compartment for flow of cathode gas but not liquid electrolyte from the cathode compartments into the cathode gas manifold. Similarly, the anode gas outlet from each anode compartment is remote, and, thus, at a distance, from the liquid electrolyte in the anode compartment for flow of anode gas but not liquid electrolyte from the anode compartments into the anode gas manifold.

[0055] As will be explained in the following, in some embodiments the electrode compartments are at least partly filled with liquid electrolyte, however, without the liquid electrolyte flowing into the gas outlets, whereas in other embodiments, the liquid electrolyte is supplied to the separator for flowing down the separator by gravity, in which case, the electrode compartment are not filled with electrolyte but the liquid electrolyte is only kept at and in close vicinity of the separator. For example, in such embodiment, the electrode compartments are filled with gas, and the liquid electrolyte is only present on the separator and the electrodes, potentially also on metal plates pressing the electrodes onto the separator. For preventing buildup of dangerous gas mixtures, the cathode compartments are purged with purified hydrogen gas, and the anode compartments with purified oxygen gas before start of the electrolyser and / or during operation of the electrolyser, especially when the gas production rate is low.

[0056] In practical embodiments, a hydrogen recirculation system is provided and / or an oxygen recirculation system is provided.

[0057] The hydrogen recirculation system comprise a hydrogen purifier having an upstream side flow-connected to the cathode gas manifold for receiving cathode gas therefrom and for providing purified hydrogen gas by removing oxygen from the received cathode gas. The hydrogen purifier has a downstream side flow-connected, for example through a pump, to a lower part of the cathode compartments in the stack for supplying purified hydrogen gas to the cathode compartments, causing an upward flow of the purified hydrogen though the cathode compartments and into the cathode gas manifold.

[0058] The oxygen recirculation system comprising an oxygen purifier having an upstream side flow-connected to the anode gas manifold for receiving anode gas therefrom and for providing purified oxygen gas by removing hydrogen from the received anode gas. The oxygen purifier has a downstream side flow-connected, for example through a pump, to a lower part of the anode compartments in the stack for supplying purified oxygen gas to the anode compartments, causing an upward flow of the purified oxygen though the anode compartments into the anode gas manifold.

[0059] Gas purification systems for hydrogen and oxygen are typically established with a gas recombination system comprising a heterogenous catalyst such as palladium or platinum that facilitates rapid recombination of oxygen and hydrogen to form water. In practice, a gas recombination system typically comprises one or more steel tanks filled with a granular or fibrous heterogenous catalyst. Other systems for gas purification exist, including pressure swing absorption systems and other systems known from the process industry. Herein, the term “purifier” is used for a gas purification system irrespective of the actual technology applied. The feed of one type of gas, H2 or 02, to the purifier may have up to 2 percent contamination with the other type of gas. Gas purifiers may be designed for varying levels of cleanliness at the outlet; the cleanliness is above 99% but typically, a cleanliness of 99.9% or even 99.99 percent may be a target value. The gas delivered from the gas purifier is considered to be clean, since the contamination is one or more orders of magnitude lower than the contamination of the gas prior to purification.

[0060] The electrolyte is not entering the gas manifolds but have separate supply and drain manifolds, which will be explained in more detail below with reference to some examples. The system described herein implies that electrolyte and gases are kept separate, so that no liquid / gas separation is necessary in the manifolds or outside the stack, apart from optional removal of water and electrolyte which in gas form leave the electrode compartments into the gas manifolds, or this reason, a gas cleaning system is optionally installed between the gas manifold and the gas purifier. The gas cleaning system may comprise a scrubber and a demisting system, preventing electrolyte aerosol formed in the system from evaporated electrolyte from contaminating the gas purifier.

[0061] According to the needs, the electrolyser comprises only one or both of the hydrogen and oxygen recirculation systems. Each recirculation system comprises necessary devices for recirculating the gas, e.g., a gas pump or compressor, and a piping system, as well as further optional devices, including the cleaning system, mentioned above.

[0062] The recirculation system or systems is / are used to provide clean gas to the electrode compartments, diluting potentially contaminated gas with clean gas of the same type. The dilution level is set to deliver gas at a safe level of contamination irrespective of the degree of contamination delivered from the electrolytic cells to the electrolyte return conduit.

[0063] Advantageously, the electrolyser comprises both a hydrogen recirculation system and an oxygen recirculation system. However, in certain circumstances, not both but only one of them is provided. For example, if an already existing system that has been in operation is upgraded, it may desirable to provide only one of the gas recirculation system as a retrofit. For example, the already existing system may have a hydrogen purifier, making it relatively easy to establish a hydrogen recirculation system, but space constraints may make it unattractive to also establish an oxygen recirculation system with an oxygen purifier, and it is more desirable to use a different safety mechanism for the anode side of the electrolyser. Other considerations may apply for establishing an oxygen recirculation system but using a different safety mechanism for the cathode side of the electrolyser.

[0064] Advantageously, the electrolyser also comprises one or more gas analysing systems, for example one configured for measuring oxygen contamination levels in hydrogen gas flowing in or from the cathode gas manifold and, optionally, another for measuring hydrogen contamination levels in oxygen gas flowing in or from the anode gas manifold. These gas analysing systems can be used in a control system for determining the recirculation flow of the purified gases.

[0065] For example, the recirculation systems may only be in operation if the measured contamination levels of one or both of the gases, hydrogen and oxygen, are above a threshold level. Alternatively, it may be in continuous operation.

[0066] In some practical embodiments, gas recirculation is operated at variable flow rates, for example by operating recirculation pumps or compressors at variable speed, adjusted in dependence of the measured contamination levels, wherein different speeds are used for different contamination levels.

[0067] The relative amounts of the two gases that are recirculated may be identical, but they do not need to be so. For example, the diffusion of hydrogen across the separator may not be the same as the diffusion of oxygen across the separator, and therefore the gas contamination may not be the same in the anode compartments and the cathode compartments. Consequently, the typical 2 percent gas contamination operational limit may occur sooner in either the anode or the cathode compartments. In this case, it may be preferable to operate oxygen recirculation while not operating hydrogen recirculation, or to operate hydrogen recirculation while not operating oxygen recirculation. In another embodiment it may be preferable to operate both hydrogen and oxygen recirculation, but at different flow rates. An estimate of the dilution level and the amount of gas that is required to be recirculated can be determined on the basis of simple assumptions.

[0068] It is assumed that gas crossover is independent of the electrolyser load. It is also assumed that the 2 percent contamination threshold for safe operation is reached at a certain load when recirculation is not implemented. The relative load at which the threshold occurs is designated as X, where X is the load relative to nominal load.

[0069] The gas contamination at any load can now be calculated:

[0070] Contamination = 2 percent * (X / actual load in percent)

[0071] For example, it follows that for X = 20 percent the contamination level will be 20 percent at 2 percent load, 4 percent at 10 percent load, 2 percent at 20 percent load, and 0.4 percent at nominal load. Calculating the contamination level at very low part load leads to nonphysical results.

[0072] It is now assumed that a certain percentage of the gas produced at nominal load is recirculated. The recirculation percentage is designated as C, where C is the percentage of gas recirculated relative to the gas produced at nominal load.

[0073] The gas contamination at any other load can then be calculated:

[0074] Contamination = 2 percent * (X / (C + actual load in percent))

[0075] If follows that if C is set to the same value as X then the contamination at zero load will be 2 percent, and that it will be lower than the 2 percent threshold at all load values from zero up to nominal load.

[0076] For example, it follows that for C = X = 20 percent the contamination level will be 2 percent at zero load, 1.8 percent at 2 percent load, 1.3 percent at 10 percent load, 1 percent at 20 percent load, and 0.3 percent at nominal load. Consequently, the implementation of the recirculation arrangement according to the invention enables the safe operation of alkaline electrolysers through the nominal load range, from zero to 100 percent of nominal power.

[0077] The designer of an alkaline electrolyser according to the invention is not restricted to select a value of the recirculation percentage C that is equal to the value of the threshold percentage X.

[0078] The designer of the alkaline electrolyser according to the invention may elect to use a value of the recirculation percentage C that is lower than the value of the threshold percentage X.

[0079] For example, for C = 0.5 X = 10 percent the contamination level will be 4 percent at zero load, 3.3 percent at 2 percent load, 2.0 percent at 10 percent load, 1.3 percent at 20 percent load, and 0.4 percent at nominal load.

[0080] In this case operation to zero load will not be possible, but the dynamic range of the electrolyser will have been extended.

[0081] The designer of the alkaline electrolyser according to the invention may alternatively elect to use a value of the recirculation percentage C that is higher than the value of the threshold percentage X.

[0082] For example, for C = 2 X = 40 percent the contamination level will be 1.0 percent at zero load, 1.0 percent at 2 percent load, 0.8 percent at 10 percent load, 0.7 percent at 20 percent load, and 0.3 percent at nominal load.

[0083] In this case operation to zero load is not only possible but will happen at a contamination level that is even lower than the threshold percentage X.

[0084] Hence, the designer of an alkaline electrolyser according to the invention will have a hitherto unknown design flexibility available, permitting the selection of target contamination levels through the load range from zero to nominal load. For safety reasons, hydrogen recirculation and / or oxygen recirculation is advantageously done while operating the electrolyser for hydrogen production at reduced load, for example at a part load lower than 25% or lower than 10% of the nominal load. With the recirculation, it is possible to continue operating the electrolyser and produce hydrogen at such part loads without facing risk for explosion. A threshold value, for example at such 25% or 10%, can be used as a trigger for starting recirculation, but it is also possible to use the recirculation at all times and potentially use the levels of the part load as criteria for adjusting the recirculation flow rate, for example by adjusting the recirculation speed.

[0085] It is also possible, to reduce and then stop the hydrogen production, and provide a quick and safe restart by hydrogen recirculation and / or oxygen recirculation while the operation is stopped and until restart of hydrogen production by the electrolyser.

[0086] This improved dynamic range achieved with the invention has several advantages. First, the stopping of electrolysers at a lower threshold for safe part load operation, e.g., as a result of variations in the input power caused by the variability of renewable energy sources, such as solar or wind power, can be avoided with obvious economic benefits. Second, unnecessary waiting time at restart can also be avoided, enhancing the economic benefits. Third, the reduction of the el ectroly ser's expected lifespan and the increased costs for maintenance and replacement that typically result from frequent shutdowns can be reduced or eliminated.

[0087] The implementation of the recirculation arrangement according to the invention will also enable the flushing of the electrolyte return conduits, with the relevant clean gas prior to startup and restart, thereby avoiding the need for nitrogen flushing.

[0088] For example, when the hydrogen production is stopped, hydrogen recirculation and / or oxygen recirculation is continued for a predetermined time after having stopped the hydrogen production. This keeps the contamination levels down and the system ready for a quick restart within the predetermined time. If the time is longer, the recirculation is stopped. In some embodiments, hydrogen recirculation and / or oxygen recirculation is established prior to and until start of hydrogen production,.

[0089] This recirculation prior to start, such as restart, can be done for a predetermined length of time, which is determined on the basis of experience for time that is sufficiently long to avoid explosion risk

[0090] Alternatively, or in addition, the method comprises measuring oxygen contamination levels in hydrogen gas coming from the cathode manifold or in the cathode manifold and hydrogen contamination in oxygen coming from the anode manifold or in the anode manifold, and continuing the recirculation until the hydrogen and oxygen contamination levels are below predetermined levels, and only then starting hydrogen production by the electrolyser.

[0091] This new flushing arrangement achieved with the invention has several advantages. First, the need for nitrogen flushing may be significantly reduced or even eliminated, leading to obvious economic benefits. Second, contamination with nitrogen of the hydrogen produced during startup is avoided, preventing the need for venting contaminated hydrogen to the atmosphere, again leading to obvious economic benefits.

[0092] On a higher level of aggregation, the preference of other electrolyser technologies such as PEM or SOEC, due to their capability to operate at lower load level than alkaline electrolysers and in some cases their faster startup time, is no longer justified, and the higher costs and other disadvantages, such as the need for rare metal or the need for high operational temperatures, can be avoided.

[0093] When selecting the method of operation, the designer of an alkaline electrolyser according to the invention will have a range of options available for normal operation.

[0094] In preferred embodiment, at least one gas recirculation pump or compressor is operated at constant speed through the full load range, delivering an approximately constant flow rate. The flow rate of at least one gas recirculation system may be selected to cause the value of the recirculation percentage C to be lower than the value of the threshold percentage X. In this case, the electrolyser will need to be shut down when the contamination level reaches the threshold percentage X as determined either by calculation or by measurement of the contamination level in the gas from the cathode / anode gas manifold.

[0095] The flow rate of at least one gas recirculation system may alternatively be selected to cause the value of the recirculation percentage C to be equal to or higher than the value of the threshold percentage X. In this case, there is no reason to shut down the electrolyser at any load level.

[0096] In another preferred embodiment, at least one gas recirculation pump or compressor is shut down if the calculated or measured contamination level is below a certain limit. This limit may be equal to the threshold percentage X, or it may be lower than the threshold percentage X.

[0097] In yet another preferred embodiment, at least one gas recirculation pump or compressor is operated at variable speed, delivering a variable flow rate. The speed may advantageously be varied so as to keep the calculated or measured contamination level at or below a certain limit. This limit may be equal to the threshold percentage X, or it may be lower than the threshold percentage X.

[0098] Permutations of the above methods for operation of an electrolyser according to the invention may be selected to achieve particular operational targets desired by the designer.

[0099] Similarly, when selecting the method of operation, the designer of an alkaline electrolyser according to the invention will have a range of options available for startup and shutdown.

[0100] In one preferred embodiment, at least one gas recirculation pump or compressor is operated during the entire period of shutdown. In another preferred embodiment, the operation of at least one gas recirculation pump or compressor is discontinued at shutdown or at a later time determined by a time setting or by the calculated or measured level of contamination.

[0101] In yet another preferred embodiment, the flow rate of at least one gas recirculation pump or compressor is increased from zero or from an idling level at or prior to startup. The flow rate may be selected as a function of the calculated or measured level of contamination.

[0102] In yet another preferred embodiment, the electrolyser load is gradually ramped up as a function of the calculated or measured level of contamination.

[0103] Permutations of the above methods for operation of an electrolyser according to the invention at startup or shutdown may be selected to achieve particular operational targets desired by the designer.

[0104] In some embodiments, the cells are filled with electrolyte up a level below a level of the cathode gas outlet and / or the anode gas outlet in order to prevent liquid electrolyte to enter the respective gas manifold. Optionally, additional means, for example, ridges and channels, are provided in the respective compartments for preventing electrolyte from reaching the respective gas outlets.

[0105] In some embodiments, each cell comprises an electrolyte dispenser in the cell, for example in the cathode compartment and / or the anode compartment, which is arranged for dispensing liquid electrolyte to the ion-transporting separator at an upper portion of the separator. Additionally, an electrolyte drainer is arranged below the separator. Liquid electrolyte is dispensed by the electrolyte dispenser to the ion-transporting separator at an upper portion of the separator at a rate causing wetting of the separator but not filling of the cell with liquid electrolyte. After flow of the liquid electrolyte downwards along the separator by gravity, the drainer receives the dispensed liquid electrolyte from the separator, for example due to the liquid electrolyte dripping from the separator into the drainer. Optionally, the liquid electrolyte received by the drainer is recycled by an electrolyte recycling system to which the drainer is connected. In some embodiments, the dispenser comprises a spray nozzle configured for spraying liquid electrolyte into the cell for wetting the separator, for example spraying onto the separator or onto the porous electrodes. The latter is particularly of interest when the separator and the electrodes are combined, for example in a membrane electrode assembly, MEA.

[0106] In some embodiments, each of the anode and the cathode comprise a porous sheet abutting the separator and metal plates sandwich the combination of the porous sheet and the separator. The metal plates are provided with corrugations towards the porous sheet as flow guides for the liquid electrolyte and arranged inclined relatively to vertical for adding a sideways flow component to a gravity-induced flow of the electrolyte along the separator.

[0107] Typically, the electrolyser comprises an electrolyte recycling system for recycling of electrolyte for adjusting the alkalinity of the alkaline electrolyte to predetermined specifications prior to re-injection of the electrolyte through the electrolyte inlets, such as the above-mentioned dispenser. In particular, water has to be supplied to the electrolyte for the hydrogen production. In operation, electrolyte is extracted from each anode compartment and each cathode compartment through corresponding electrolyte outlets, for example the drainer as described above. The alkalinity of the alkaline electrolyte is adjusted to predetermined specifications prior to re-injecting the electrolyte into the anode and cathode compartments.

[0108] SHORT DESCRIPTION OF THE DRAWINGS

[0109] The invention will be explained in more detail with reference to the drawings, where.

[0110] - FIG. 1 A is a sketch of an electrolytic cell in a gap configuration;

[0111] - FIG. IB is a sketch of an electrolytic cell in a zero-gap configuration;

[0112] - FIG. 1 C is a sketch of an electrolyser stack comprising a series of electrolytic cells in zero-gap configuration;

[0113] - FIG. 2 is a sectional view of an electrolyser stack;

[0114] - FIG. 3 A is an exploded view of an electrolytic cell; - FIG. 3B is a sectional view of the assembled electrolytic cell;

[0115] - FIG. 3C is a view of the assembled electrolytic cell;

[0116] - FIG. 4 is a principle sketch of an H2 purification and recirculation system;

[0117] - FIG. 5 is a principle sketch of an 02 purification and recirculation system;

[0118] - FIG. 6 illustrates an embodiment of an electrolyte chamber where the gas is separated from the liquid electrolyte prior to entering the gas manifold;

[0119] - FIG. 7 illustrates a system for electrolyte supply from above the separator and flow down by gravity;

[0120] - FIG. 8 illustrates a spray system for electrolyte supply from above the separator and flow down by gravity;

[0121] - FIG. 9A illustrates an assembly with a corrugated bipolar plate;

[0122] - FIG. 9A illustrates an alternative assembly with a corrugated bipolar plate

[0123] - FIG. 10 illustrates corrugations for distribution of electrolyte.

[0124] DETAILED DESCRIPTION / PREFERRED EMBODIMENT

[0125] FIG 1 A, B, and C show principle sketches of general variants of alkaline electrolytic cells and an electrolyser stack.

[0126] FIG. 1 A is a sketch of an electrolytic cell 1 in a gap configuration, comprising a cathode 2, an anode 3, and an ion-transporting separator 4. Hydrogen gas 8 is produced at the side of the cathode 2 facing the separator 4 in the cathode compartment 5, and oxygen gas 9 is produced at the side of the anode 3 facing the separator 4 in the anode compartment 6. A power supply 7 drives the electrolytic process.

[0127] FIG. IB is a sketch of an electrolytic cell 1 in a zero-gap configuration, comprising a porous cathode 12, a porous anode 13, and an ion-transporting separator 4. The cathode compartment 5 and the anode compartment 6 are contained within metal separator plates 14, for example functioning as bipolar plates in cell stacks. Electrical connections are provided between the electrodes 12, 13 and the metal separator plates 14 with conductive elements 15, not detailed in the sketch. Hydrogen is produced at the side of the porous cathode 12 facing the ion-transporting separator 4 and is conveyed to the cathode compartment 5 through holes or pores 16 in the cathode 12, and oxygen is produced at the side of the porous anode 13 facing the separator 4, for example iontransporting membrane, and is conveyed to the anode compartment 6 through holes or pores 16 in the anode 13.

[0128] FIG. 1 C is a sketch of an electrolyser stack 17, comprising a series of electrolytic cells 1 in zero-gap configuration of the type as illustrated in FIG. IB.

[0129] Typically, in prior art alkaline electrolysers, the electrolyte leaves the electrode compartments 5, 6 for recirculation through an electrolyte return conduit which is also used as outlet for the gases from the respective electrode compartment. This implies that a water / gas separation stage is used prior to collection of hydrogen gas from the cathode in a hydrogen tank. In some systems, as disclosed in US2014 / 069807 and US2014 / 069808, the gas / liquid mix enters the manifold, and liquid is drained from the manifold by gravity through a specially designed canal in the gasket system. In other systems, there may be arranged a liquid / gas separator outside the stack. In contrast thereto, the system as explained herein uses a different approach.

[0130] In the system presented herein, the electrolyser stack comprises a hydrogen gas manifold that is connected to multiple cathode compartments and which collects and removes the hydrogen from the connected cathode compartments, however, without the electrolyte entering the hydrogen gas manifold.

[0131] Alternatively, or additionally, the electrolyser stack comprises an oxygen gas manifold that is connected to multiple anode compartments and which collects and removes the oxygen from the connected anode compartments, however, without the electrolyte entering the oxygen gas manifold.

[0132] This implies that the gas has to be separated from the electrolyte upstream of the gas manifold. Various technical solutions for this exist. For example, the electrolyte is having an upper level below the gas manifolds and, thus, cannot enter the gas manifold. Additionally, for example as disclosed in WO2022 / 156869A1, there may be arranged an inclined passage for the gas as an additional measure for preventing the electrolyte to reach the gas manifold. FIG. 6 illustrates an example showing a cathode 12, or alternatively anode 13, sandwiched between a bipolar plate 14 and an ion-transporting separator 4, sealed by a gasket 44. In the exemplified embodiment, the cathode 12 is a porous metal plate and is abutting the ion-transporting separator 4 in zero-gap configuration, electrolyte is flowing to the ion-transporting separator 4 and hydrogen gas 8 flowing away from the separator 4 through the pores in the cathode 12. The electrolyte 29 in the cathode compartment 5 has an upper level 29A below the cathode gas outlet 24A that is part of the hydrogen gas manifold that transports the cathode gas away from the cathode compartment 5. As the level 29A of the electrolyte 29 is below the cathode gas outlet 24A, there is no electrolyte 29 flowing into the cathode gas manifold. To assist in preventing liquid electrolyte 29 from entering the cathode gas outlet 24A, an inclined separating edge 45 is provided, defining a largely horizontally oriented slot for the cathode gas flow. Accordingly, there is also no need for a liquid / gas separator inside or downstream of the cathode gas manifold, which simplifies the system substantially. Alternatively, or in addition, a similar arrangement is provided for the anode 13 and the anode compartment 6 with a corresponding anode gas manifold for the oxygen gas 9.

[0133] In other embodiments, the electrolyte is provided directly onto the membrane and flowing down along the membrane by gravity. For example, the electrolyte 29 is sprayed onto the ion-transporting separator or onto the porous electrode 12 / 13 or onto a perforated metal plate which, together with the ion-transporting separator 4 sandwiches a porous layer and where the perforated metal plate in combination with the porous layer forms an assembled electrode 12 / 13. Various options exist.

[0134] FIG 2 is a sectional view of an electrolyser stack 17. Similar to the sketch in FIG. 1C, the ion-transporting separator 4 is sandwiched between a porous cathode 12 in the cathode compartment 5 and a porous anode 13 in the anode compartment 6, further delimited by liquid-tight corresponding bipolar plates 14. The stack 17 comprises multiple electrolytic cells 1 sandwiched between a first endplate 25 and a second endplate 26. The first end plate 25 and each cell 1 have a first inlet hole 21 A, which serially aligned and in combination form a first pure hydrogen supply manifold 27. The first end plate 25 and each cell 1 has a first outlet hole 22A, which serially aligned and in combination form a cathode gas manifold 28A for outlet of gas from each cathode compartment 5. The outlet hole 22A of each cell 1 is flow-connected to the corresponding cathode compartment 5 of the cell 1. Accordingly, the gas, primarily hydrogen gas, of the cathode compartment enters the respective outlet hole 22A, which is part of the cathode gas manifold 28, and the gas flows along the cathode gas manifold 28 A out of the stack 17. For flushing the cathode compartment 5 with pure hydrogen gas, the inlet hole 21 A of each cell 1 connects the pure hydrogen supply manifold 27 A with the corresponding cathode compartment 5 of the respective cell 1. Pure hydrogen gas is pumped into the pure hydrogen supply manifold 27A, flows into and through the multiple cathode compartments 5 for purging purposes, and leaves the cathode compartment 5 again through cathode gas manifold 28 A. During production of hydrogen gas by electrolysis, the produced hydrogen gas and potential oxygen gas from crossover leave the cathode compartment 5 through the first outlet hole 22A and the cathode gas manifold 28A above the ion-transporting separator 4.

[0135] Advantageously, for the purpose of flushing the entire cathode compartments 5 with pure hydrogen gas, the first inlet holes 21 A and the pure hydrogen gas manifold 27 formed by the serially aligned first inlet holes 21 A provided below the ion-transporting separator 4.

[0136] The purging with hydrogen gas is especially useful prior to starting the electrolyser and when operating the electrolyser at low hydrogen production rate, as the steady purging of the cathode compartment 5 with hydrogen gas prevents accumulation of oxygen gas at dangerous concentrations.

[0137] As an alternative or additional option, a similar arrangement is provided for flushing the anode compartments 6, with pure oxygen gas, preventing buildup of dangerous concentrations of hydrogen gas in the anode compartment 6 by crossover. For this purpose, the end plate 25 and each cell 1 have a second inlet hole 2 IB in the lower portion of the electrolytic cells 1. The aligned series of second inlet holes 21B form in combination a first pure oxygen manifold. Additionally, first end plate 25 and each cell 1 has a second outlet hole 22B in the upper portion of the electrolytic cells 1 and, in particular, above the ion-transporting separator 4. The aligned series of second outlet holes 22B form in combination an anode gas manifold for removal of produced oxygen gas from the anode compartments. Each second outlet hole 22B is connected to one of the multiple anode compartments.

[0138] Advantageously, for the purpose of flushing the entire ion-transporting separator 4 with pure oxygen gas, the second inlet holes 2 IB and the pure oxygen gas manifold formed by the serially aligned second inlet holes are provided below the ion-transporting separator 4.

[0139] Notice that any electrolyte feed manifolds and electrolyte return manifolds are not shown in the drawings for simplicity, but would be correspondingly provided, depending on the supply principles, which are discussed in more detail below.

[0140] FIG 3 A is an exploded view of an electrolytic cell 1 of the electrolyser stack of FIG. 2. Notice that the orientation of the cell 1 is different in that the first inlet holes 21 A are toward the viewer in FIG. 2 but directed remotely from the viewer in FIG. 3 A.

[0141] The cell 1 comprises a pair of solid metallic bipolar plates 14, a porous cathode 12 electrically connected to the first solid metallic plate 14 with connectors 15, a cathode gasket 18, an ion -transporting separator 4, an anode gasket 19, a porous anode 13 and a second solid metallic bipolar plate 14 electrically connected to the anode 13 by corresponding connectors 15. The solid metallic bipolar plates 14 and the gaskets 18, 19 are penetrated with first and second inlet holes 21 A, 21B forming gas supplies for purified hydrogen purge gas and purified oxygen purge gas, respectively, and first and second outlet holes 22A, 22B for the gases that leave the electrode compartments 5, 6 and which form part of the gas manifolds in the stack 17, as already explained in connection with FIG 2.

[0142] Notice that for FIG. 3, similar as for FIG. 2, the cathodic and anodic electrolyte feed manifolds along the stack 17, respectively, and the cathodic and anodic electrolyte return manifolds along the stack 17, for recirculation of electrolyte and addition of water, are not shown for simplicity in this illustration but would also be part of the stack, separate and in addition to the gas manifolds. The cathode gaskets 18 is fitted with a pure hydrogen gas inlet 23 A, connecting the first inlet hole 21 A with the cathode compartment 5 and, thus, functioning as inlet for the pure hydrogen purge gas into the cathode compartment 5 from the pure hydrogen supply manifold 27. A first cathode gas outlet 24A connects the cathode compartment 5 with the first outlet hole 22A.

[0143] Correspondingly, as an additional option or as an alternative, the anode gasket 19 is fitted with corresponding pure oxygen gas inlet 23B, connecting the second inlet hole 2 IB with the anode compartment 6 and, thus, functioning as pure oxygen purge gas inlet into the anode compartment 6 from the pure oxygen supply manifold. Correspondingly, a second anode gas outlet pathway connects the anode compartment with the second outlet hole 22B.

[0144] FIG. 3C shows the entire the assembled electrolytic cell 1, and FIG 2B is a view with a section cut through the second inlet hole 2 IB for the pure oxygen supply to the anode compartment 6 and the second outlet hole 22B which is part of the anode gas manifold. The cathode compartment 5 is contained between the right solid metallic bipolar plate 14 and the separator 4, surrounded by the cathode gasket 18, and the anode compartment 5 is contained between the left solid metallic bipolar plate 14 and the separator 4 and surrounded by the anode gasket 19.

[0145] FIG 4 illustrates an example of hydrogen recirculation in an alkaline electrolyser stack 17. The cathode gas from the cathode compartments 5, dominantly containing hydrogen gas produced by electrolysis in the stack 17, flows into and through the cathode gas manifold 28A. The cathode gas manifold 28A is flow-connected to a hydrogen purifier 33A in which potential oxygen gas is removed, typically catalytically by forming water, which is drained. The purified hydrogen gas has a purity of above 99%, typically no less than 99.9% or even at least 99.99%. It is transported 34 away from the purifier 33 for further use, for example to the end user or accumulation in a storage tank. A portion of the purified hydrogen is recycled through pump 39 and into the pure hydrogen supply manifold 27A for purging the cathode compartments 5 with the purified hydrogen in an upwards flow through the cathode compartments 5, diluting and removing potential oxygen contamination in the hydrogen gas inside the cathode compartments 5. The portion that is recirculated in this hydrogen recirculation system is advantageously regulated in response to the degree of contamination as measured by a corresponding gas analysis system 40A in the cathode gas from the cathode gas manifold 28A. For example, the speed of the pump 39 is variably regulated. Alternatively, or in addition, a valve system is employed.

[0146] FIG 5 illustrates an example of oxygen recirculation in an alkaline electrolyser stack 17. Typically, the anode gas, which contains mainly oxygen is not used for any specific consumption and therefore transported away and vented to atmosphere. However, a portion of the anode gas is purified in purifier 33B and pure oxygen fed into pure oxygen supply manifold 27B for purging the anode compartments 6 with the purified oxygen, diluting and removing potential hydrogen contamination in the oxygen gas inside the anode compartments 6. The gas from the anode compartment 6, containing the oxygen gas produced by electrolysis in the stack 17 as well as the purified oxygen purge gas flows into and through the anode gas manifold 28B and out of the stack 17. The portion that is recirculated through this oxygen recirculation system is advantageously regulated in response to the degree of contamination as measured by a corresponding gas analysis system 40B in the anode gas from the anode gas manifold 28B. For example, the speed of the pump 39 is variably regulated. Alternatively, or in addition, a valve system is employed.

[0147] Notice that only gases and no liquid electrolyte leaves the electrode compartments into the cathode gas manifold 28 A and anode gas manifold 28B. As already explained in connection with FIG. 6, gas and liquid are separated inside the electrode compartments. A further principle is explained with reference to FIG. 7.

[0148] FIG. 7 illustrates an alkaline electrolyser cell 1 in which the ion-transporting separator 4 is sandwiched between porous electrodes 12, 13, which, in turn, are sandwiched between two bipolar plates 14. For example, the bipolar plates 14 are corrugated metal plates. An example of a bipolar plate with corrugations is illustrated in FIG. 10, where the corrugations 41 are inclined and arranged for distributing electrolyte over a larger area of the electrodes 12, 13 and the separator 4 in case that some of the electrolyte flows outside the separator 4. For example, the corrugations on both sides of the bipolar plate 14, one side pressing against a cathode 12 in one cell 1 and the opposite side pressing against an anode 13 in a neighboring cell 1. Electrolyte 29 is supplied to the upper portion of the ion conduction separator 4 by an electrolyser dispenser 32 arranged above the separator 4. The electrolyte flows down to the bottom of the separator 4 by gravity and is drained into a drainer 35 for recycling. In this embodiment, the electrolyte compartment, be it the cathode compartment 5 or the anode compartment 6, is not filled with electrolyte, but there is supplied sufficient electrolyte 29 for it to keep the separator 4 wetted and with s supply sufficient for replenishing the electrolyte 29 that leaves the separator 4 on the bottom into the drainer 35, and potentially for providing sufficient cooling. As the electrode compartment is not filled with electrolyte, purified hydrogen purge gas easier flows upwards through the electrode compartment, and electrolyte is not entering the gas manifolds that are arranged at the top, which is not shown in FIG. 7 but was explained above.

[0149] FIG. 8 illustrates a different embodiment, in which the porous electrodes are composite and comprise an assembly of a porous sheet 12A, 13A and a porous metal plate 12A, 13B, the latter pressing the porous sheet 12A, 13A into a zero gap configuration with the separator 4. For example, the porous metal plate 12B, 13B is corrugated for better distribution of electrolyte over the separator 4 and the porous sheet 12A, 12B. As an option, the porous sheet 12A in the cathode compartment 5, the separator 4, and the porous sheet 13 A in the anode compartment 6 are provided as a membrane-elec- trode-assembly, MEA, and the porous metal plates 13 A, 13B are current conductors for the electrolysis and press against the MEA for good electrical contact and the benefits of a zero-gap configuration with respect to hydrogen production yield.

[0150] Typically, the electrolyte is supplied at the top of the separator 4 in order to wet the entire separator 4. However, capillary forces would also lead to wetting of the entire separator 4 if the electrolyte is applied to the upper portion of the separator 4 but not at the very top. In this connection, it is pointed out that the capillary forces, dragging electrolyte upwards against gravity, are balanced with gravity after a distance of 20-30 cm, so that capillary forces cannot be used for wetting more than a minor upper portion in large electrolyser stack where the separator 4 is much larger, for example a meter or more in height. Accordingly, the invention has particular advantage for large- scale electrolysers having separators 4 having a height of more than 0.3 meters, in particular a height larger than 0.5 m, such as at least 1 meter. As mentioned in relation to FIG. 7, the bipolar plates are optionally corrugated. FIG. 9 A illustrates an example where the bipolar plates 14 have corrugations 41 and press against the porous electrode 12 for a zero-gap configuration towards the separator 4. The porous electrode 12 is formed as a porous sheet, for example pressed non-woven or a porous polymer membrane. In FIG. 9B, the electrode 12 is a wire mesh formed by wires 42A, 42B extending in two lateral directions. Due to the corrugations 41, there are provided gaps 39 that eases the upwards transport of the purging gas.

Claims

CLAIMS1. An alkaline electrolyser for producing hydrogen gas (8) and comprising a stack (17) of electrolytic cells (1), wherein each electrolytic cell (1) contains liquid electrolyte (29) and comprises a cathode compartment (5) and an anode compartment (6) separated by an ion-transporting separator (4); wherein the stack (17) comprises a cathode gas manifold (28A) arranged above the cells (1) and extending along the stack (17) for transport of received cathode gas out of the stack (17), wherein each cathode compartment (6) comprises a cathode gas outlet (24A) at an upper part of the cathode compartment (6) and flow-connecting the cathode compartment (5) to the cathode gas manifold (28A); wherein the stack (17) comprises an anode gas manifold (28B) arranged above the cells (1) and extending along the stack (17) for transport of received anode gas out of the stack (17), wherein each anode compartment (6) comprises an anode gas outlet (24B) at an upper part of the anode compartment (6) and flow-connecting the anode compartment (6) to the anode gas manifold (28B); characterized in that the cathode gas outlet (24A) is remote from the liquid electrolyte (29) in the cathode compartment (5) for flow of cathode gas but not liquid electrolyte from the cathode compartments (5) into the cathode gas manifold (28A) and that the anode gas outlet (24B) is remote from the liquid electrolyte (29) in the anode compartment (6) for flow of anode gas but not liquid electrolyte from the anode compartments (5) into the anode gas manifold (28A); wherein the electrolyser comprises at least one of the following: a hydrogen recirculation system comprising a hydrogen purifier (33A) having an upstream side flow-connected to the cathode gas manifold (28A) and configured for receiving cathode gas therefrom and configured for providing purified hydrogen gas by removing oxygen from the received cathode gas, wherein the hydrogen purifier (33A) has a downstream side flow- connected to a lower part of the cathode compartments (5) in the stack (17) for supplying purified hydrogen gas to the cathode compartments (5), causing an upward flow of the purified hydrogen though the cathode compartments (5) and into the cathode gas manifold (28A);an oxygen recirculation system comprising an oxygen purifier (33B) having an upstream side flow-connected to the anode gas manifold (28B) and configured for receiving anode gas therefrom and configured for providing purified oxygen gas by removing hydrogen from the received anode gas, wherein the oxygen purifier (33B) has a downstream side flow-connected to a lower part of the anode compartments (6) in the stack (17) for supplying purified oxygen gas to the anode compartments (6), causing an upward flow of the purified oxygen though the anode compartments (5) into the anode gas manifold (28B).

2. The electrolyser according to claim 1, wherein the electrolyser comprises the hydrogen gas recirculation systems.

3. The electrolyser according to any preceding claim, wherein each cell (1) comprises an electrolyte dispenser (32), arranged for dispensing liquid electrolyte (29) to the ion-transporting separator (4) at an upper portion of the separator (4), and an electrolyte drainer (35) arranged below the separator (4) for receiving the dispensed liquid electrolyte (29) from the separator (4) after flow of the liquid electrolyte downwards along the separator (4) by gravity, wherein the dispenser is adjusted to dispense liquid electrolyte (29) at a rate causing wetting of the separator (4) but not filling of the cell (1) with liquid electrolyte (29).

4. The electrolyser according to claim 3, wherein the dispenser (32) comprises a spray nozzle configured for spraying liquid electrolyte (29) into the cell for wetting the separator (4).

5. The electrolyser according to claim 3 or 4, wherein the drainer (35) is connected to an electrolyte recycling system.

6. The electrolyser system according to any of the preceding claims, wherein each of the anode (12) and the cathode (13) comprise a porous sheet (12A, 13 A) abutting the separator (4), and wherein metal plates (12B, 13B, 14) sandwich the combination of the porous sheet (12A, 13A) and the separator (4), wherein the metal plates (12B, 13B, 14) are provided with corrugations (41) towards theporous sheet (12A, 13A) as flow guides for the liquid electrolyte (29) and arranged inclined relatively to vertical for adding a sideways flow component to a gravity-induced flow of the liquid electrolyte (29) along the separator (4).

7. The electrolyser system according to any of the preceding claims, wherein the hydrogen recirculation system comprises a gas analysing system (40A) for measuring oxygen contamination levels in the cathode gas from the cathode gas manifold (28A), and wherein the hydrogen recirculation system is programmed for operating at variable flow rate, adjusted in dependence of the measured oxygen contamination levels.

8. An electrolyser system according to any of the preceding claims, wherein the oxygen recirculation system comprises a gas analysing system (40B) for measuring hydrogen contamination levels in the anode gas from the anode gas manifold (28B), and wherein the oxygen recirculation system is programmed for operating at variable flow rate, adjusted in dependence of the measured hydrogen contamination levels.

9. A method of operating an electrolyser according to any preceding claim, wherein the method comprises at least one of A and B, wherein the method in A comprises providing hydrogen recirculation by the hydrogen recirculation system, causing flow of cathode gas from the cathode compartments (5), through the cathode gas outlets (24A), through the cathode gas manifolds (28), and at least a portion of the cathode gas into the hydrogen purifier (33A), providing purified hydrogen gas at a purity of at least 99% by removing oxygen from the cathode gas by the hydrogen purifier (33) and supplying at least a portion of the purified hydrogen gas to a lower part of each of the cathode compartments (5) in the stack (17), causing an upward flow of the portion of purified hydrogen gas though the cathode compartments (5) and back into the cathode gas manifold (28 A); wherein the method in B comprises providing oxygen recirculation by the oxygen recirculation system, causing flow of anode gas from the anode compartments (5), through the anode gas outlets (24B), through the anode gas manifolds (28), and at least a portion of the anode gas into the oxygen purifier (33B),providing purified oxygen gas at a purity of at least 99% by removing hydrogen from the anode gas by the oxygen purifier (33) and supplying at least a portion of the purified oxygen gas to a lower part of each of the anode compartments (5) in the stack (17), causing an upward flow of the portion of purified oxygen though the anode compartments (5) and back into the anode gas manifold (28A).

10. The method according to claim 6, wherein the electrolyser comprises the hydrogen recirculation system and the method comprises A.

11. The method according to claim 9 or 10, wherein the method comprises hydrogen or oxygen recirculating or both while operating the electrolyser for hydrogen production at a part load below a nominal load, wherein the part load is lower than 25% of the nominal load.

12. The method according to any one of the claims 9-11, wherein the method comprises at least one of hydrogen recirculation and oxygen circulation for a predetermined time after having stopped hydrogen production.

13. The method according to any of the claims 9-12, wherein the method in A comprises hydrogen recirculating before starting hydrogen production, and measuring oxygen contamination levels in hydrogen gas coming from the cathode gas manifold (28A), and continuing the hydrogen recirculation until the oxygen contamination levels are below predetermined levels, and only then starting hydrogen production by the electrolyser; and wherein the method in B and comprises oxygen recirculation before starting hydrogen production, and measuring hydrogen contamination in oxygen coming from the anode gas manifold (28B), and continuing the oxygen recirculation until the hydrogen contamination levels are below predetermined levels, and only then starting hydrogen production by the electrolyser.

14. The method according to any of the claims 9-14, wherein the method in A comprises hydrogen recirculating for a predetermined time before starting hydrogen production; and wherein the method in B comprises oxygen recirculating for a predetermined time before starting hydrogen production.

15. The method according to any one of the claims 9-14, wherein the method in A comprises measuring oxygen contamination levels in the cathode gas and operating the electrolyser with hydrogen recirculation only if the measured oxygen contamination levels are above threshold levels for oxygen contamination; wherein the method in B comprises measuring hydrogen contamination levels in the anode gas and operating the electrolyser with oxygen recirculation only if the measured hydrogen contamination levels are above threshold levels for hydrogen contamination.

16. The method according to any one of the claims 9-15, wherein the method in A comprises measuring oxygen contamination levels in the cathode gas and operating the electrolyser with the hydrogen recirculation at variable flow rates, adjusted in dependence of the measured oxygen contamination levels, wherein different flow rates are used for different oxygen contamination levels; wherein the method in B comprises measuring hydrogen contamination levels in the anode gas and operating the electrolyser with the oxygen recirculation at variable flow rates, adjusted in dependence of the measured hydrogen contamination levels, wherein different flow rates are used for different hydrogen contamination levels.

17. The method according to any one of the claims 9-16, wherein each cell (1) comprises an electrolyte dispenser (32), arranged for dispensing liquid electrolyte (29) to the ion-transporting separator (4) at an upper portion of the separator (4), and an electrolyte drainer (35) arranged below the separator (4), wherein the method comprises dispensing liquid electrolyte (29) to the ion-transporting separator (4) at an upper portion of the separator (4) at a rate causing wetting of the separator (4) but not filling of the cell (1) with liquid electrolyte (29) and after flow of the liquid electrolyte downwards along the separator (4) by gravity, receiving the dispensed liquid electrolyte (29) from the separator (4) by the drainer (35) and recycling the liquid electrolyte received by the drainer.

18. The method according to claim 17, wherein the dispenser (32) comprises a spray nozzle and the method comprises spraying liquid electrolyte (29) into the cell for wetting the separator (4).

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