Water recirculation loop for a hydrogen producing electrolysis plant

Abstract

The invention relates to a water recirculation loop (30) for a hydrogen producing electrolysis plant (30) that comprises an electrolysis stack (10). The water recirculation loop (30) comprises: at least one, preferably one, circulation pump (6); a water inlet section (9) connectable to the electrolysis stack (10), wherein the water inlet section (9) can be supplied with water by the pump (6); a water feed section (91) leading to the water inlet section (9);a water outlet section (11) connectable to the electrolysis stack (10), wherein the water at the outlet section (11) being pressurized within the electrolysis stack (10) and / or in the water feed section (91) leading to the water inlet section (9); at least one energy recovery device (8) for transferring water pressure and / or flow energy from the water outlet section (11) to said water feed section (91); and a recirculating section (31) connecting an output of the energy recovery device (8, 28) with an input port of the pump (6).

Description

[0001] WATER RECIRCULATION LOOP FOR A HYDROGEN PRODUCING ELECTROLYSIS

[0002] PLANT

[0003] TECHNICAL FIELD OF THE INVENTION

[0004] The invention relates to a water recirculation loop for a hydrogen producing electrolysis plant.

[0005] BACKGROUND OF THE INVENTION

[0006] In the electrolysis of water, an electrolysis stack is used to split (ultrapure) water into oxygen and hydrogen gas by electrolysis. The hydrogen gas can then be used as hydrogen fuel. The electrolysis stack may be powered by renewable electricity. There are two commercial electrolyser technologies, alkaline and proton exchange membrane (PEM) , as well as two developmental electrolyser technologies, solid oxide electrolyzer cell (SOEC) and anion exchange membrane (AEM) . For alkaline, PEM and AEM the water is liquid when it undergoes electrolysis, while it is steam for SOEC.

[0007] During the process huge amounts of heat are generated (200-250 kW per MW electrolyser rating) . To avoid a large temperature increase across the electrolysis (electrolyser) stack, a large flow rate is used, such as 50-100 m3 / h per MW. The water is pumped to the electrolysis stack and then to a heat exchanger for cooling before being returned to the electrolysis stack. In the case of PEM and AEM there may be a side stream loop in which an ion exchange polisher is placed. This polisher ensures that the water is continuously kept clean, so as to maintain the expected life time of the electrolysis stack. Without this, lifetime would fall, ruining the efficiency of the hydrogen plant, which is not economic. After being produced, the hydrogen is compressed in a compressor.

[0008] To better handle fluctuating power inputs (such as from wind and solar) and to save costs, the hydrogen production plant may operate with the electrolysis process under pressure. This allows the hydrogen to be produced with a high pressure, which will eliminate or minimize the need for a compressor. However, this requires that the circulation pump and the rest of the process equipment, including the polisher, must be able to handle and withstand the high pressure. Designing the components (pump, polisher, etc. ) to operate under such a high pressure (e.g. >40 bar) is complex. In particular, providing components dedicated to the higher pressure ratings is expensive and difficult.

[0009] Thus, it is an objective to simplify the production of hydrogen.

[0010] SUMMARY OF THE INVENTION

[0011] The object of the present invention is achieved by the solution provided in the enclosed independent claims . Advantageous implementations of the present invention are further defined in the dependent claims .

[0012] According to a first aspect, the invention relates to a water recirculation loop for a hydrogen producing electrolysis plant that comprises an electrolysis stack, wherein the water recirculation loop comprises: at least one, preferably one, circulation pump; a water inlet section connectable to the electrolysis stack, wherein the water inlet section can be supplied with water by the pump; a water feed section leading to the water inlet section; a water outlet section connectable to the electrolysis stack, wherein the water at the outlet section being pressurized within the electrolysis stack and / or in the water feed section leading to the water inlet section; at least one energy recovery device (such as e.g. a pressure exchange unit) for transferring water pressure and / or flow energy from the water outlet section to said water feed section; and a recirculating section connecting an output of the energy recovery device with an input port of the pump.

[0013] This achieves the advantage that the energy recovery device is placed to isolate certain parts, such as with respect to high pressure and / or high flow energy. Thus, by the water recirculation loop, the benefits of a high pressure and / or flow configuration and of a low pressure and / or flow configuration may be combined with one another. For example, the high pressure configuration allows for a lower need for compression, while the low pressure configuration allows the use of parts that do not require to withstand a high pressure. Thereby, in particular simpler (such as less durable, less rigid, less complex and / or cheaper) components can be used and / or a pressure and / or flow rate can be increased while retaining at least parts of the system designed for lower pressures and / or lower flow rates, such as a pump and / or a device for polishing water. Thus, hydrogen production can be simplified.

[0014] In the context of the present invention, "water" is to be understood as also encompassing alkaline or acidic solutions. For example, "water" in the context of the present invention may include water (H2O) with 30wt% or more of an alkaline substance, such as potassium hydroxide (KOH) .

[0015] In an embodiment, the water flow rate at the outlet section is at least 85%, preferably at least 90%, more preferred at least 95% or even 98% of the water flow rate at the water inlet section.

[0016] In an embodiment, the water pressure at the water outlet section is at least 30 bar, preferably at least 50 bar, more preferred at least 70 bar or even 100 bar.

[0017] In an embodiment, the water flow rate downstream the pump is, or can be, at least 20m3 / hour per MW electrolysis (electrolyser ) , preferably at least 100m3 / hour per MW electrolysis, more preferred more than 200m3 / hour per MW electrolysis.

[0018] The pump may be designed to not leak substances detrimental to a proton exchange membrane (PEM) electrolysis stack, such as substances detrimental to a proton transfer membrane. Therefore, the lifetime of the electrolysis stack and, thus, of the hydrogen plant can be prolonged.

[0019] The energy recovery device, such as e.g. at least one a pressure exchange unit, may be designed to not leak electrode detrimental substances detrimental to a proton exchange membrane (PEM) electrolysis stack. Therefore, the lifetime of the electrolysis stack and, thus, of the hydrogen plant can be prolonged.

[0020] The detrimental substances may be one, more or all of: metal cations, CO2, organics.

[0021] The detrimental substances may be, or may be in the form of, particles .

[0022] The pump may be designed to not leak substances detrimental to an alkaline electrolysis (AEL) stack. Therefore, the lifetime of the electrolysis stack and, thus, of the hydrogen plant can be prolonged .

[0023] The pump may be designed such that water pumped by the pump can have ASTM Type I quality at both an upstream side and a downstream side of the pump and / or to leak only substances that are not detrimental to ASTM Type I quality of water. This is particularly advantageous for the electrolysis stack and the electrolysis performed by the same. The "ASTM Type I" quality may be according to the standard "ASTM Standard D1193-99el, 2017, "Standard Specification for Reagent Water", 10.1520 / D1193- 99E01" .

[0024] In other words, the pump may be designed such that it does not contaminate water with ASTM Type I quality in such a way that the water becomes a quality not fulfilling the requirements for ASTM Type I quality as set out in said standard.

[0025] The energy recovery device may be designed to not leak substances detrimental to an electrolysis stack, such as substances detrimental to an electrode, a separator between electrodes, or a filter such as a lye filter. Therefore, the lifetime of the electrolysis stack and, thus, of the hydrogen plant can be prolonged .

[0026] These detrimental substances may be one, more or all of: multivalent cations, chloride, CO2, organics, and / or may be, or may be in the form of, particles.

[0027] In an embodiment, the energy recovery device is designed such that water traversing the energy recovery device can have ASTM Type I quality at both an upstream side and a downstream side of the energy recovery device and / or to leak only substances that are not detrimental to ASTM Type I quality of water. This is particularly advantageous for the electrolysis stack and the electrolysis performed by the same. The "ASTM Type I" quality may be according to the standard "ASTM Standard D1193-99el, 2017, "Standard Specification for Reagent Water", 10.1520 / D1193- 99E01" .

[0028] In other words, the energy recovery device may be designed such that it does not contaminate water with ASTM Type I quality in such a way that the water becomes a quality not fulfilling the requirements for ASTM Type I quality as set out in said standard.

[0029] In an embodiment, the loop is void of a pressure control valve, such as a pressure relief valve, downstream the water outlet section. Thereby, hydrogen production can be further simplified.

[0030] In an embodiment, the loop is void of turbines downstream the water outlet section. Thereby, hydrogen production can be further simplified.

[0031] In an embodiment, the number of parallelly arranged energy recovery devices is higher than the number of pumps. Additionally or alternatively, the number of concatenated energy recovery devices arranged may be higher than the number of pumps.

[0032] In an embodiment, the loop comprises a further pump, wherein, preferably, the water inlet section and / or the water feed section comprises the further pump. By the energy recovery device transferring pressure and / or flow energy to the water feed section, the further pump may be operated at a reduced power and / or may be designed more simply, e.g. having a reduced maximum capacity .

[0033] According to a second aspect, the invention relates to a hydrogen producing electrolysis plant comprising a water recirculation loop as described above.

[0034] In an embodiment, the plant comprises an electrolysis stack such as a proton exchange membrane (PEM) electrolysis stack and / or an alkaline electrolysis (AEL) stack.

[0035] In an embodiment, the energy recovery device comprises or consists of at least one pressure exchange unit.

[0036] The pressure exchange unit may be an isobaric energy recovery device (ERD) . It may use the principle of positive displacement to achieve energy transfer from a high-pressure stream to a low pressure feed stream. The pressure exchange unit may be of one or more of the following types: rotary, piston. In the rotary type, the high pressure stream comes in direct contact with low pressure feed stream. The energy transfer occurs in rotor ducts. By rotation, the ducts are first exposed to low pressure feed water, which fills the duct. The rotor continues to rotate past a sealing area and is then exposed to the high-pressure stream, which fills the duct and displaces the feed water at high pressure. The piston type pressure exchanger operates like rotary type pressure exchanger, however, instead of rotor, positive displacement pistons, housed in pressure vessels, are used. The pressure vessels exchange the functions by means of special valves. In one of the pressure vessels the first, high pressure water displaces the second, feed water at high pressure. In another pressure vessel, the second, low-pressure feed displaces the first water. In an embodiment, the energy recovery device comprises a turbo charger. The tube charger provides the advantage of being capable to handle more dissolved oxygen. Thus, at least an upstream oxygen separator may be omitted, whereby costs can be reduced. By the turbo charger, hydrogen production can be done in a particularly easy manner and by using a low cost solution using, in particular, cheaper materials.

[0037] The turbocharger is a hydraulic device that may comprise, or may consist, of a pump section and a turbine section, both of which may contain a single and / or a multi stage impeller or rotor. As liquid from a first stream flows into the turbine section of the turbocharger, the turbine rotor extracts the hydraulic energy of the first stream and converts it to mechanical energy. The pump impeller then converts the mechanical energy produced by the turbine rotor back to hydraulic energy in a second, feed stream. Thus, the turbocharger may be entirely energized by the first stream .

[0038] In an embodiment, the loop comprises a high pressure pump.

[0039] In an embodiment, the loop comprises a further water inlet section connectable to the electrolysis stack, wherein the further water inlet section can be supplied with water by the pump and is arranged to bypass the energy recovery device, and wherein the further water inlet section comprises the high pressure pump.

[0040] In an embodiment, the loop comprises an air lift pump. By the air lift pump, water flowing in the loop can be subjected to a further boost in addition to the boost provided by the energy recovery device. The air lift pump in particular provides the advantage of having no, or at least particularly less, moving parts, leading to a reduced or oven no maintenance.

[0041] The water feed section may comprise the air lift pump.

[0042] The air lift pump may comprise a gas supply port connectable to an oxygen output configured to discharge oxygen formed in the electrolysis stack.

[0043] The air lift pump may comprise a water supply port connectable to a further water outlet section configured to discharge water drained from the electrolysis stack and preferably separated, by a hydrogen separator, from hydrogen.

[0044] In an embodiment, the loop comprises an oxygen separator connectable to the electrolysis stack.

[0045] The oxygen separator provides the advantage of, in particular, enabling the use of a readily available energy recovery device, having no corrosion risk, no lubrication loss, and no overheating .

[0046] The oxygen separator comprises an oxygen output configured to discharge oxygen formed in the electrolysis stack, wherein the gas supply port of the air lift pump may connected to the oxygen output .

[0047] The oxygen separator may comprises a water output configured to discharge water drained from the electrolysis stack, wherein the water output is connected to the water outlet section. In an embodiment, the loop comprises a pressure relief valve connected to the oxygen separator.

[0048] In an embodiment, the loop comprises: a further water outlet section connectable to a hydrogen separator; and, optionally, a secondary energy recovery device, wherein the secondary energy recovery device is arranged to transfer water pressure and / or flow energy from the further water outlet section to the water feed section.

[0049] In an embodiment, the loop may comprise, instead of, or as an alternative to, the secondary energy recovery device, a booster pump. The booster pump is preferably arranged not to recover any energy, so that the booster pump may be void of any (fluid) connection to the further water outlet section. For example, the booster pump may be fluidly connected only to the energy recovery device and the water inlet section.

[0050] The loop may comprise a heat exchanger, wherein the heat exchanger is preferably arranged to be traversed by water of the water outlet section.

[0051] According to a third aspect, the invention relates to the use of a water energy recovery device in a water circulation (or recirculation) loop of a hydrogen producing electrolysis plant.

[0052] The (water) energy recovery device may comprise an output port and an input port, each connectable to a low pressure and / or low flow energy area, and an output port and an input port, each connectable to a high pressure and / or high flow energy area, wherein the low pressure and / or low flow energy area comprises a pump and / or a contaminants remover (such as an ion exchanger) , and / or wherein the high pressure and / or high flow energy area comprises an electrolysis stack. The energy recovery device is adapted to transfer pressure energy and / or flow energy from a water stream coming from the high pressure and / or high flow energy area to a water stream coming from the low pressure and / or low flow energy area.

[0053] According to a fourth aspect, the invention relates to a method for recirculating water in a hydrogen producing electrolysis plant, wherein the method comprises (the steps of) : supplying water, by using at least one pump, to a water inlet section of an electrolysis stack, wherein a water feed section leads to the water inlet section; draining water from a water outlet section of the electrolysis stack, the water at the outlet section being pressurized within the electrolysis stack and / or in the water feed section leading to the water inlet section; and transferring water pressure and / or flow energy from the water outlet section of the electrolysis stack to said water feed section using at least one energy recovery device.

[0054] BRIEF DESCRIPTION OF THE DRAWINGS

[0055] The invention will be explained in the following together with the figures .

[0056] Fig. 1 shows a schematic view of a hydrogen producing electrolysis plant according to an embodiment;

[0057] Fig. 2 shows a schematic view of a hydrogen producing electrolysis plant according to an embodiment;

[0058] Fig . 3 shows a schematic view of a hydrogen producing electrolysis plant according to an embodiment;

[0059] Fig. 4 shows a schematic view of a hydrogen producing electrolysis plant according to an embodiment;

[0060] Fig. 5 shows a schematic view of a hydrogen producing electrolysis plant according to an embodiment;

[0061] Fig. 6 shows a schematic view of a hydrogen producing electrolysis plant according to an embodiment;

[0062] Fig. 7 shows a schematic flow chart of a method for recirculating water in a hydrogen producing electrolysis plant according to an embodiment.

[0063] DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0064] Fig. 1 shows a schematic view of a hydrogen producing electrolysis plant 40 according to an embodiment.

[0065] The hydrogen producing electrolysis plant 40 comprises a water recirculation loop 30 and at least one electrolysis stack 10 (e.g. , an electrolysis cell) . The loop 30 may be connectable to a water inlet 1 into which water of a suitable quality can enter the loop 30 or plant 40. The water recirculation loop 30 is designed such that water drained water from the electrolysis stack 10 can be recirculated into the loop 30. Accordingly, by the loop 30, water drained from the electrolysis stack 10 can be recirculated into the electrolysis stack 10 in order to be again processed within the stack 10.

[0066] The loop 30 may comprise an oxygen separator 2. The oxygen separator 2 is adapted to separate oxygen from water and to release the separated oxygen, e.g. via a line 17. The oxygen separator 2 may be arranged such that water, e.g. water entered via the water inlet 1 and / or recirculated water, can be sent to the oxygen separator 2 where it can be mixed with other process streams .

[0067] The loop 30 may comprise a heat exchanger 4 that may be connected to the oxygen separator 2 via a line 3. In the heat exchanger 4, heat may be removed from the water comprising recirculated water. This heat may comprise heat that was previously transferred to the water within the electrolysis stack 10.

[0068] The loop 30 comprises a circulation pump 6 (such as a canned pump) and a water inlet section 9 that is connectable to the electrolysis stack 10. The water inlet section 9 is arranged such that it can be supplied with water by the pump 6. The water inlet section 9 is arranged downstream of the pump 6, wherein an upstream side of the pump 6 comprises an input port. The input port is arranged such that it can be supplied with recirculated water. Preferably, the pump 6 is arranged to circulate the water at least between the electrolysis stack 10 and a device downstream of the electrolysis stack 10, such as the oxygen separator 2. As shown in Fig. 1, the water inlet section 9 may be connected to an anode side 10a of the electrolysis stack 10. Alternatively, the water inlet section 9 may be connected to a cathode side 10b of the electrolysis stack 10 or both to the anode side 10a and the cathode side 10b of the electrolysis stack

[0069] 10.

[0070] In the electrolysis stack 10, water introduced via the water inlet section 9 is split, by using electricity, into oxygen (O2) and hydrogen (H2) . The hydrogen is generated on the cathode side

[0071] 10b and sent together with water (such as in the form of a solution) out of the electrolysis stack 10. For example, the hydrogen can then be released from the electrolysis stack 10, e.g. via an outlet line 13 connected to the electrolysis stack 10. The outlet line 13 may be connected to a downstream hydrogen separator 14 where the hydrogen is separated from other substances such as water. The separated hydrogen can then be released from the hydrogen separator 14, e.g. via a hydrogen outlet line 15. Hydrogen released via the hydrogen outlet line 15 may then be used as a fuel. The water separated in the hydrogen separator 14 from the hydrogen can be sent, e.g. via line 16, to the water recirculation loop 30 and, thus, recirculated into the water recirculation loop 30, such as into the oxygen separator 2.

[0072] The electrolysis stack 10 is not limited to a particular type of electrolysis stack and / or to a particular number of electrolysis stacks. Preferably, the electrolysis stack 10 comprises, or is, an electrolysis stack of the proton exchange membrane (PEM) type and / or an electrolysis stack of the Solid Oxide Electrode Cell (SOEC) type. Additionally or alternatively, the electrolysis stack 10 may comprise an electrolysis stack of the alkaline electrolysis (AEL) type.

[0073] The pump 6 is designed to be used in combination with the electrolysis stack 10. Preferably, the pump 6 is designed to not leak substances detrimental ("detrimental substances", such as metal cations, CO2, and / or organics) to the electrolysis stack 10 of the proton exchange membrane (PEM) type and / or the alkaline electrolysis (AEL) type, such as substances detrimental to a proton transfer membrane. In particular, the parts of the pump 6 which are arranged to be in contact with water that is subsequently supplied to the electrolysis stack 10 may be designed to not leak such substances, or to leak only substances which are such that the electrolysis stack 10 essentially maintains its efficiency or performance when exposed to these substances (e.g. when exposed to these substances over a period of at least 100 hours) .

[0074] Preferably, the pump 6 is designed such that water pumped by the pump 6 can have ASTM Type I quality at both an upstream side and a downstream side of the pump 6 (wherein the upstream side comprises the input port of the pump 6 and the downstream side comprises the output port of the pump 6) and / or to leak only substances that are not detrimental to ASTM Type I quality of water. For example, the pump 6 may only leak substances that are such that, when leaked into the water of ASTM Type I quality, the water comprising these substances still has ASTM Type I quality. Thereby, the pump 6 is particularly suitable to operate in combination with the electrolysis stack 10.

[0075] The requirements which qualify water as water of ASTM Type I quality are set out in the standard "ASTM Standard D1193-99el, 2017, "Standard Specification for Reagent Water", 10.1520 / D1193- 99E01" .

[0076] At least part of the water coming out of (being drained from) the electrolysis stack 10 can be recirculated into the loop 30, such as sent back to the oxygen separator 2. By connecting the water recirculation loop 30 to the electrolysis stack 10, water can circulate along a closed path, such as along a path at least defined by the oxygen separator 2, the line 3, the heat exchanger 4, the line 5, the pump 6, the line 7, the line 9 and / or a contaminants remover as described below, Accordingly, such a path may form a part of a circulation loop over the electrolysis stack 10.

[0077] The loop 30 may comprise a contaminants remover 19, such as an ion exchanger (an ion exchange polisher) , capable of removing contaminants, most often ionic species, upstream of at least the pump 6. An upstream end of the contaminants remover 19 may be connected to a line 18 into which water coming from the electrolysis stack 10 may be sent (e.g. from the oxygen separator 2) in order to be at least partly freed from contaminants. A downstream end of the contaminants remover 19 may be connected to a line 20 into which water treated by the contaminants remover 19 may be sent. The line 20 is connected directly or indirectly, e.g. via the oxygen separator 2, to the pump 6. In particular, the treated (or purified) water can be returned via line 20 to the oxygen separator 2 (e.g. for further removal of oxygen) from where the water can then be sent, e.g. via at least line 3 and preferably via line 3, the heat exchanger 4 and line 5, to the pump 6.

[0078] It may be advantageous to pressurize the whole or parts of the plant 40. In an example, (preferably only) the anode side 10a and / or the cathode side 10b may be pressurized. Additionally or alternatively, the side stream comprising at least the contaminants remover 19 and preferably the line 18 and / or line 20 may be pressurized.

[0079] The loop 30 further comprises a water outlet section 11 connectable to (a downstream side of) the electrolysis stack 10, such as to the anode side 10a and / or the cathode side 10b. In the plant 40 shown in Fig. 1, the water outlet section 11 is connected to the electrolysis stack 10, in this example to the anode side 10a. At least part of the electrolysis stack 10, such as the anode side 10a, is pressurized, so that water flowing into and along the water outlet section 11 is pressurized. The outlet section 11 is arranged such that water at the outlet section 11 may be pressurized within the electrolysis stack 10, e.g. chemically, in particular by a gas formed (e.g. during electrolysis) within the electrolysis stack 10 (such as at the electrode) and / or by a chemical reaction happening within the electrolysis stack 10 (e.g. involving a, or the, gas formed (e.g. during electrolysis) within the electrolysis stack 10) , and / or may be pressurized in a water feed section 91 upstream of the electrolysis stack 10 and leading to the water inlet section 9.

[0080] As shown in Fig. 1, the water recirculation loop 30 further comprises at least one pressure exchange unit (a pressure exchanger) 8 as an energy recovery device. The pressure exchange unit 8 is capable of transferring at least pressure (i.e. pressure energy) and can be arranged so as to isolate certain parts of the loop 30 and / or of the plant 40 with respect to high pressure .

[0081] The pressure exchange unit 8 is adapted to transfer pressure energy from a high-pressure water stream to a low-pressure water stream. The pressure exchange unit 8 may comprise a first section 8a and a second section 8b. The first section 8a may comprise an input port and an output port. The second section 8b may comprise an input port and an output port. The pressure exchange unit 8 may be adapted such that pressure energy can be transferred from the second section 8b to the first section 8a. Water can enter the first section 8a, e.g. via its input port, at a low pressure and can exit the first section 8a, e.g. via its output port, at a high pressure, wherein the pressure increase is due to the transfer of pressure energy (e.g. by using a moveable wall or diaphragm) from the second section 8b to the first section 8a. Water can enter the second section 8b, e.g. via its input port, at a high pressure and can exit the second section 8b, e.g. via its output port, at a low pressure, wherein the pressure drop is due to the transfer of pressure energy (e.g. by using a, or the, moveable wall or diaphragm) from the second section 8b to the first section 8a. Accordingly, by the pressure exchange unit 8 a pressure increase of a water flow can be effected without the use of a pump.

[0082] The pressure exchange unit 8 is designed to be used in combination with the electrolysis stack 10. Preferably, the pressure exchange unit 8 is designed to not leak electrode detrimental substances (such as detrimental to a proton exchange membrane (PEM) electrolysis stack) and / or substances detrimental to an electrolysis stack (such as of the proton exchange membrane (PEM) type and / or of the alkaline electrolysis (AEL) type) , such as substances detrimental to an electrode, a separator between electrodes (such as a proton transfer membrane) , or a filter such as a lye filter. The detrimental substances may comprise metal cations, CO2, and / or organics. In particular, the parts of the pressure exchange unit 8 which are arranged to be contacted by water traversing the pressure exchange unit 8 (i.e. water flowing from a low pressure side to a high pressure side, and from a high pressure side to a low pressure side) may be designed to not leak such substances, or to leak only substances which are such that the electrolysis stack 10 essentially maintains its efficiency or performance when exposed to these substances (e.g. when exposed to these substances over a period of at least 100 hours) . Preferably, the pressure exchange unit 8 is designed to not leak detrimental substances being one, more or all of: multivalent cations, chloride, CO2, organics.

[0083] Preferably, the pressure exchange unit 8 is designed such that water traversing the pressure exchange unit 8 can have ASTM Type I quality at both an upstream side (which may comprise the input port of the first section 8a) and a downstream side (which may comprise the output port of the first section 8a) of the pressure exchange unit 8 and / or to leak only substances that are not detrimental to ASTM Type I quality of water. For example, the pressure exchange unit 8 may only leak substances that are such that, when leaked into the water of ASTM Type I quality, the water comprising these substances still has ASTM Type I quality. Thereby, the pressure exchange unit 8 is particularly suitable to operate in combination with the electrolysis stack 10.

[0084] In the embodiment shown in Fig. 1, the loop 30 comprising the pressure exchange unit 8 is provided such that the pressure exchange unit 8 can transfer water pressure from the water outlet section 11 to the water feed section 91. The water feed section 91 may extend at least between the input and output ports of the first section 8a. Water in the water feed section 91 provided in the area of the input port may have a first pressure (i.e. may comprise a first pressure energy comprising pressure energy transferred from the pump 6) , wherein water in the water feed section 91 provided in the area of the output port may have a second pressure greater than the first pressure (e.g. greater by an amount equal to at least part of the pressure energy transferred from the second section 8b to the first section 8a) . As shown in Fig. 1, the pressure exchange unit 8 is connected to the anode side 10a and thus placed in the anode circulation loop. Alternatively, the pressure exchange unit 8 may be connected to the cathode side 10b and thus placed in the cathode circulation loop .

[0085] According to the embodiment shown in Fig 1, the circulation pump 6 may send water via the line 7 at a relatively low pressure to the pressure exchange unit 8, e.g. to the first section 8a. In the pressure exchange unit 8 the relatively low pressure water stream meets (e.g. via the diaphragm or wall, and / or within a channel, such as a channel distanced from, or eccentric with respect to, an axis and rotatable about the axis) a water stream of relatively high pressure that comes from the water outlet line 11 connected to the electrolysis stack 10, such as to the anode side 10a. Thereby, the relatively high pressure water stream coming from the water outlet line 11 transfers (e.g. by using the diaphragm or wall) at least part of its pressure energy to the relatively low pressure water stream coming from line 7. By the transfer of pressure energy, the relatively high pressure water stream is turned into a water stream of relatively low pressure sent into the line 12 connected to the second section 8b. Further, by the transfer of pressure energy to the relatively low pressure water stream coming from line 7, this water stream is turned into a water stream of relatively high pressure sent, e.g. by line 9, to, or towards, the electrolysis stack 10.

[0086] In the embodiment shown in Fig. 1, water at the input port of the first section 8a and water at the output port of the second section 8b are not exposed to high pressure (i.e. are only exposed to low pressure) . As such, at least parts provided upstream of the input port of the first section 8a and downstream of the outlet port of the second section 8b can be isolated from the high pressure. However, at least water in the electrolysis stack 10, such as at least its anode side 10a and / or cathode side, and preferably also at least in the lines 9, 11 is exposed to high pressure. This is advantageous for the electrolysis and the hydrogen production. In particular, the anode side 10a and the cathode side 10b may be operated at the same pressure to produce pressurized hydrogen at the cathode side 10b.

[0087] As shown in Fig. 2, the plant 40 may comprise a further pump 61 (such as a canned pump) . The further pump 61 may be arranged downstream of the outlet port of the first section 8a of the pressure exchange unit 8 and upstream of the electrolysis stack 10 and / or of the water inlet section 9. The further pump 61 may be arranged to overcome a pressure drop across the electrolysis stack. Preferably, the water feed section 91 comprises the further pump 61. As the water in the water feed section 91 is at least partly pressurized by the transfer of pressure by the pressure exchange unit 8, the further pump 61 can be operated with a lower power to pressurize the water to achieve a target pressure of the water entering the electrolysis stack 10, compared to a case in which the target pressure of the water entering the electrolysis stack 10 is effected by pressurization with the further pump 61 and without using the pressure exchange unit 8.

[0088] The pump 61 is designed to be used in combination with the electrolysis stack 10. Preferably, the pump 61 is designed to not leak substances detrimental ("detrimental substances", such as metal cations, CO2, and / or organics) to the electrolysis stack 10 of the proton exchange membrane (PEM) type and / or the alkaline electrolysis (AEL) type, such as substances detrimental to a proton transfer membrane. In particular, the parts of the pump 61 which are arranged to be contacted with water that is subsequently supplied to the electrolysis stack 10 may be designed to not leak such substances, or to leak only substances which are such that the electrolysis stack 10 essentially maintains its efficiency or performance when exposed to these substances (e.g. when exposed to these substances over a period of at least 100 hours) .

[0089] Preferably, the pump 61 is designed such that water pumped by the pump 6 can have ASTM Type I quality at both an upstream side and a downstream side of the pump 61 (wherein the upstream side comprises the input port of the pump 61 and the downstream side comprises the output port of the pump 61) and / or to leak only substances that are not detrimental to ASTM Type I quality of water. For example, the pump 61 may only leak substances that are such that, when leaked into the water of ASTM Type I quality, the water comprising these substances still has ASTM Type I quality. Thereby, the pump 61 is particularly suitable to operate in combination with the electrolysis stack 10.

[0090] As shown in Figs. 1 and 2, the water recirculation loop 30 comprises a recirculating section 31. The recirculating section 31 is arranged to connect an output of the pressure exchange unit 8 (such as the output port of the second section 8b) , or of any other energy recovery device, with the input port of the pump 6. Preferably, the recirculating section 31 indirectly connects the output of the pressure exchange unit 8 with the input port of the pump 6. The recirculating section 31 may comprise one or more lines (such as one or more pipes or tubes) . The recirculating section 31 may comprise at least part of the oxygen separator 2, the contaminants remover 19, and / or the heat exchanger 4. Further, the recirculating line 31 may comprise one, more or all of the lines 12, 18, 20, 3, 5. Preferably, the recirculating line 31 connects by the line 5 to the input port of the pump 6.

[0091] The water recirculating loop 30 may comprise one or more treatment devices for treating recirculated water drained from the electrolysis stack 10. As shown in Fig. 2, the one or more treatment devices may comprise a filter 32 and / or an UV lamp 33, which is / are preferably arranged upstream of the pump 6 (and more preferably upstream of the oxygen separator 2 and / or heat exchanger 4) and / or upstream of the contaminants remover 19. Preferably, the recirculating section 31 and / or the line 20 comprises the filter 32 and / or the UV lamp 33.

[0092] As shown in Fig. 2, the loop 30 and / or the plant 40 may comprise a water tank 51. The water tank 51 may be connected to a water line (e.g. the water inlet 1) via which fresh water (i.e. water that has not yet traversed the electrolysis stack 10) can be supplied to the tank 51 (where the water can temporarily stay) and thus to the loop 30 and / or the plant 40. The water tank 51 may be arranged so that water drained from the electrolysis stack 10 can be recirculated into the water tank 51 and thus into the loop 30. For example, the water tank 51 may be connected to the electrolysis stack 10, e.g. via at least line 12 and / or line 13, preferably also via the oxygen separator 2 and / or the hydrogen separator 14, wherein, optionally, the water tank 51 connects to the oxygen separator 2 by the line 18 and / or connects to the hydrogen separator 14 by the line 16. The recirculating section

[0093] 31 may comprise at least part of the water tank 51.

[0094] The loop 30, e.g. the water tank 51, may comprise an absorber 50 for absorbing carbon dioxide within the loop 30, in particular in the water tank 51. For example, the absorber 50 may be a carbon dioxide scrubber.

[0095] As shown in Figs. 1 and 2, the loop 30 may be void of a pressure relief valve downstream (of) the water outlet section 11 and preferably downstream of the pressure exchange unit 8 (such as of the second section 8b) or of any other energy recovery device. In particular, the recirculating section 31 may be void of a pressure relief valve. The provision of, for example, the pressure exchange unit 8 as an energy recovery device makes it possible that there is no need for releasing pressure, by a pressure relief valve, downstream the water outlet section 11. Instead, at least part of the pressure, which would be released by the pressure relief valve, is transferred by the pressure exchange unit 8 to the water feed section 91.

[0096] As also shown in Figs. 1 and 2, the loop 30 may be void of a turbine, or turbines, downstream (of) the water outlet section 11 and preferably downstream of the pressure exchange unit 8 (such as of the second section 8b) or of any other energy recovery device. In particular, the recirculating section 31 may be void of a turbine. The provision of, for example, the pressure exchange unit 8 as an energy recovery device makes it possible that there is no need for converting pressure energy, by a turbine, downstream the water outlet section 11 into another form of energy, such as electric energy. Instead, at least part of the pressure energy, which would be converted by the turbine, is transferred by the pressure exchange unit 8 to the water feed section 91.

[0097] The water flow rate at the water outlet section 11 may be at least 85%, preferably at least 90%, more preferred at least 95% or even 98% of the water flow rate at the water inlet section 9. In particular, the pressure exchange unit 8 may be arranged such that the water flow rate at the water outlet section 11 may be at least 85%, preferably at least 90%, more preferred at least 95% or even 98% of the water flow rate at the water inlet section 9. Accordingly, by arranging the pressure exchange unit 8, the difference between the water flow rates at the water inlet section 9 and the water outlet section 11, respectively, can be kept very low. In particular, at least the pump 6 and the pressure exchange unit 8 (and preferably also the further pump 61) may be configured such that, when the loop 30 is connected to the electrolysis stack 10, they can operate in a mode in which the water flow rate at the water outlet section 11 is at least 85%, preferably at least 90%, more preferred at least 95% or even 98% of the water flow rate at the water inlet section 9.

[0098] The water pressure at the water outlet section 11 may be at least 30 bar, preferably at least 50 bar, more preferred at least 70 bar or even 100 bar. The water outlet section 11 and / or at least part of the pressure exchange unit 8 may be designed to withstand a water pressure of at least 30 bar, preferably at least 50 bar, more preferred at least 70 bar or even 100 bar. The pressure exchange unit 8 may be configured to transfer pressure energy from water at the water outlet section 11 with a water pressure of at least 30 bar, preferably at least 50 bar, more preferred at least 70 bar or even 100 bar, to water with a lower pressure.

[0099] The water flow rate downstream the pump 6 may be at least 20m3 / hour per MW electrolysis, preferably at least 100m3 / hour per MW electrolysis, more preferred more than 200m3 / hour per MW electrolysis. In particular, the pump 6 and / or the pressure exchange unit 8 may be configured and / or arranged such that water downstream the pump 6, such as in the water inlet section 9 and / or in the water feed section 91, can have a flow rate of at least 20m3 / hour per MW electrolysis, preferably at least 100m3 / hour per MW electrolysis, more preferred more than 200m3 / hour per MW electrolysis.

[0100] The pressure exchange unit 8, or any other energy recovery device, may also be arranged / used elsewhere in the plant 40 to isolate certain parts of the plant 40 from high pressure. For example, the pressure exchange unit 8 may be arranged to transfer water pressure from a water outlet section of the line 18 (wherein the water outlet section may be connectable to the oxygen separator 2) to a water feed section comprising at least part of the line 20, wherein the water feed section may lead to a water inlet section connectable to the oxygen separator 2. Placing the pressure exchange unit 8 on lines, or streams, 18 and 20 allows the contaminant remover 19 to operate at a relatively low pressure.

[0101] Thus, by using the pressure exchange unit 8, an electrolysis system, in particular hydrogen production plant 40, can be obtained that benefits of both the high pressure (lower need for compression) and low pressure (cheaper process equipment, retained possibility of polishing) configuration.

[0102] In Figs. 3 to 6, further embodiments of a water recirculation loop and of a hydrogen producing electrolysis plant are shown. The same or equivalent components as those already explained with reference to Figs. 1 and 2 are commonly labeled, and their detailed explanation is omitted for simplicity. Fig. 3 shows an embodiment in which a turbo charger 28 is used as an energy recovery device for transferring water pressure and / or flow energy from the water outlet section 11 to the water feed section 91. As an alternative, or in addition, to the pressure exchange unit 8 in the embodiments of Figs. 1 and 2, the turbo charger 28 may be used.

[0103] Low-pressure water can be sent by the pump 6 to the turbo charger 28. The water will then pass through the tube charger 28 and will get pressurized. On the other hand, high-pressure water can be sent from the water outlet section 11 to the turbo charger 28. The turbo charger 28 will transfer pressure and / or flow energy of the water sent from the water outlet section 11 to the water sent by the pump 6. Thus, water in the water feed section 91 will have pressure energy and / or flow energy that is at least in part transferred, by the turbo charger 28, from the water in the water outlet section 11.

[0104] As shown in Fig. 3, the loop 30 may comprise a secondary energy recovery device 92, such as a pressure exchange unit and / or a turbo charger, in particular a turbo charger being smaller, or having a lower capacity, than the turbo charger 28. In other words, the maximum pressure and / or maximum flow energy, for which the secondary energy recovery device 92 is designed, may be lower than the maximum pressure and / or maximum flow energy, for which the (primary) energy recovery device 8, 28 is designed. The secondary energy recovery device 92 is arranged to transfer water pressure and / or flow energy from a further water outlet section, such as the line 16 coming from the hydrogen separator 14 and conducting water separated by the hydrogen separator 14 from hydrogen, to the water feed section 91. Thus, water in the water feed section 91 may be subjected to a first boost by the turbo charger 28 and to a second boost by the secondary energy recovery device 92 before entering the water inlet section 9 and then the electrolysis stack 10.

[0105] By the secondary energy recovery device 92 transferring water pressure and / or flow energy, the relatively high energy (pressure and / or flow energy) water stream of the further water outlet section 16 is turned into a water stream of relatively low energy. The secondary energy recovery device 92 may then send this water stream of relatively low energy into a line 93. The line 93 may be connected to the line 12.

[0106] The secondary energy recovery device 92 is only optional. Thus, in particular in the embodiment according to Figure 3, the (secondary) energy recovery device 92 may or may not be present. If the energy recovery device 92 is not present, water in the water feed section 91 may be subjected to only a single boost, namely by the (primary) energy recovery device, such as the turbo charger 28, before entering the water inlet section 9 and then the electrolysis stack 10.

[0107] In an embodiment (not shown) , the loop 30 may comprise, instead of the secondary energy recovery device 92, a booster pump. The booster pump is preferably arranged not to recover any energy, so that the booster pump may be void of any (fluid) connection to the further water outlet section, such as the line 16. In other words, the booster pump may be fluidly connected only to the energy recovery device 8, 28 and the water inlet section 9. Thus, water in the water feed section 91 may be subjected to a first boost by the energy recovery device 8, 28 and to a second boost by the booster pump before entering the water inlet section 9 and then the electrolysis stack 10. The loop 30 of Figure 2 onwards may comprise a high pressure pump 62. As shown in Figs. 3-6, the loop 30 may comprise a further water inlet section 63 connected to the electrolysis stack 10, wherein the further water inlet section 6 comprises the high pressure pump 62. The further water inlet section 63 may be arranged such that it can be supplied with water by the pump 6 and such that is bypasses the energy recovery device 8, 28. In other words, water flowing via the further water inlet section 63 into the electrolysis stack 10 does not traverse the energy recovery device 8, 28. Instead of being pressurized by at least the energy recovery device 8, 28, the water flowing in the further water inlet section 63 is pressurized by the high pressure pump 62. The water inlet section 9 and the further water inlet section 63 may be arranged to merge with one another upstream at a position of the electrolysis stack 10. From this position, the water coming from the water inlet sections 9, 63 can flow together into the electrolysis stack 10.

[0108] The loop 30 may comprise a heat exchanger 50 arranged to be traversed by water of the water outlet section 11. By the heat exchanger 50, heat may be removed from the water of the water outlet section 11, i.e. from water drained from the electrolysis stack, before entering the energy recovery device 8, 28. In other words, the heat exchanger 50 may be arranged downstream of the electrolysis stack 10 and upstream of the energy recovery device 8, 28.

[0109] Additionally, or alternatively, the heat exchanger 50 may be arranged to be traversed by water of the further water outlet section 16, as shown in Fig. 3 or Fig. 6. By the heat exchanger 50, heat may be removed from the water of the further water outlet section 16, i.e. from water separated from hydrogen in the hydrogen separator 14, e.g. before entering the optional secondary energy recovery device 92. In other words, the heat exchanger 50 may be arranged downstream of the hydrogen separator 14 and, if the optional secondary energy recovery device 92 is present, upstream of the secondary energy recovery device 92.

[0110] As shown in Fig. 4, the loop 30, such as the water feed section 91, may comprise an air lift pump 70. The air lift pump 70 comprises a gas supply port connectable to an oxygen output configured to discharge oxygen formed in the electrolysis stack 10. Thus, the air lift pump 70 may pump water in the water feed section 91 by using energy provided by oxygen formed in the electrolysis stack 10. In other words, the air lift pump 70 may give the water boosted by the energy recovery device 8, 28, such as by the pressure exchange unit 8 (see Fig. 4) , a further boost by using energy provided by the discharged oxygen. The air lift pump 70 may be arranged downstream of the energy recovery device 8, 28 and upstream of the electrolysis stack 10.

[0111] The loop 30 may comprise an oxygen separator 80 connected to the electrolysis stack 10. The oxygen separator 80 is configured to drain water from the electrolysis stack 10, which water includes oxygen formed in the electrolysis stack 10. The oxygen separator 80 comprises an oxygen output 81 configured to discharge oxygen formed in the electrolysis stack 10 and separated by the oxygen separator 80 from water. The gas supply port of the air lift pump 70 may be connected to the oxygen output 81. The oxygen separator 80 may comprises a water output 82 configured to discharge water drained from the electrolysis stack 10 and separated by the oxygen separator 80, wherein the water output 82 is connected to the water outlet section 11, from where the water can then be sent to, or into, the energy recovery device 8, 28, such as the pressure exchange unit 8.

[0112] The loop 30 may comprise a pressure relief valve 90 connected to the oxygen separator 80. By the pressure relief valve 90, oxygen separated from water but not sent into the oxygen output 81, i.e. excess oxygen, may be released, e.g. into the atmosphere.

[0113] As shown in Fig. 6, the air lift pump 70 may comprises a water supply port connected to the further water outlet section 16, so that water drained from the electrolysis stack 10 and preferably separated, by the hydrogen separator 14, from hydrogen may form a (further) water source in addition to the water coming from the water feed section 91. Water flowing in the further water outlet section 16 may traverse the heat exchanger 50 before entering the air lift pump 70.

[0114] As shown in Fig. 5, the oxygen separator 80 may also be provided such that oxygen discharged through the oxygen output 81 is not used for providing energy to operate a device in the loop 30, such as the air lift pump 70. Instead, oxygen discharged through the oxygen output 81 may be discharged out of the loop 30, e.g. into the atmosphere. In the embodiment shown in Fig. 5, the pressure relief valve 90 is omitted and the secondary energy recovery device 92 is used instead of the air lift pump 70.

[0115] Fig. 3 shows a schematic flow chart of a method 100 for recirculating water in a hydrogen producing electrolysis plant, such as the hydrogen producing electrolysis plant 40 described above. The method 100 comprises at least three, preferably consecutive steps 101-103. In step 101, water is supplied, by using at least one pump 6, to a water inlet section 9 of an electrolysis stack 10, wherein a water feed section 91 leads to the water inlet section 9.

[0116] In step 102, water is drained from a water outlet section 11 of, or connected to, the electrolysis stack 10. This water at the water outlet section 11 is pressurized within the electrolysis stack 10 and / or in the water feed section 91 leading to the water inlet section 9.

[0117] In step 103, water pressure and / or flow energy is transferred from the water outlet section 11 to said water feed section 91 using at least one energy recovery device 8, 28. In an optional step, the water comprising pressure and / or flow energy transferred from the water outlet section 11 is pressurized by a further pump 61.

[0118] The method 100 may comprise one or more further steps, such as one or more (treatment) steps for treating water, e.g. by using at least the contaminants remover 19, and / or one or more steps of sending hydrogen.

Claims

Claims1. A water recirculation loop (30) for a hydrogen producing electrolysis plant (30) that comprises an electrolysis stack(10) , the water recirculation loop (30) comprising: at least one, preferably one, circulation pump (6) , a water inlet section (9) connectable to the electrolysis stack (10) , wherein the water inlet section (9) can be supplied with water by the pump (6) , a water feed section (91) leading to the water inlet section (9) , a water outlet section (11) connectable to the electrolysis stack (10) , wherein the water at the outlet section (11) being pressurized within the electrolysis stack (10) and / or in the water feed section (91) leading to the water inlet section (9) , at least one energy recovery device (8, 28) for transferring water pressure and / or flow energy from the water outlet section(11) to said water feed section (91) , and a recirculating section (31) connecting an output of the energy recovery device (8, 28) with an input port of the pump (6) .

2. The loop (30) of claim 1, wherein the water flow rate at the outlet section (11) is at least 85%, preferably at least 90%, more preferred at least 95% or even 98% of the water flow rate at the water inlet section(9) .

3. The loop (30) of any of the preceding claims, wherein the water pressure at the water outlet section (11) is at least 30 bar, preferably at least 50 bar, more preferred atleast 70 bar or even 100 bar.

4. The loop (30) of any of the preceding claims, wherein the water flow rate downstream the pump (6) is at least 20m3 / hour per MW electrolysis, preferably at least 100m3 / hour per MW electrolysis, more preferred more than 200m3 / hour per MW electrolysis .

5. The loop (30) of any of the preceding claims, wherein the pump (6) is designed to not leak substances detrimental to a proton exchange membrane (PEM) electrolysis stack (10) , such as substances detrimental to a proton transfer membrane, and / or wherein the energy recovery device (8, 28) is designed to not leak electrode detrimental substances detrimental to a proton exchange membrane (PEM) electrolysis stack, wherein the detrimental substances are preferably one, more or all of: metal cations, CO2, organics.

6. The loop (30) of any of the preceding claims, wherein the pump (6) is designed to not leak substances detrimental to an alkaline electrolysis (AEL) stack, wherein the detrimental substances preferably are one, more or all of: multivalent cations, chloride, CO2, organics.

7. The loop (30) of any of the preceding claims,wherein the pump (6) is designed such that water pumped by the pump (6) can have ASTM Type I quality at both an upstream side and a downstream side of the pump (6) and / or to leak only substances that are not detrimental to ASTM Type I quality of water .

8. The loop (30) of any of the preceding claims, wherein the energy recovery device (8, 28) is designed to not leak substances detrimental to an electrolysis stack (10) , such as substances detrimental to an electrode, a separator between electrodes, or a filter such as a lye filter, and / or wherein the energy recovery device (8, 28) is designed such that water traversing the energy recovery device (8, 28) can have ASTM Type I quality at both an upstream side and a downstream side of the energy recovery device (8, 28) and / or to leak only substances that are not detrimental to ASTM Type I quality of water .

9. The loop (30) of any of the preceding claims, which is void of a pressure control valve, such as a pressure relief valve, downstream the water outlet section (11) , and / or which is void of turbines downstream the water outlet section (11) •10. The loop (30) of any of the preceding claims, wherein a number of parallelly arranged energy recovery devices is higher than the number of pumps.

11. The loop (30) of any of the preceding claims, comprising a further pump (61) , wherein, preferably, the water inlet section (9) and / or the water feed section (91) comprises the further pump (61) .

12. The loop (30) of any of the preceding claims, wherein the energy recovery device (8, 28) comprises a pressure exchange unit (8) and / or a turbo charger (28) .

13. The loop (30) of any of the preceding claims, comprising a high pressure pump (62) .

14. The loop (30) of claim 13, comprising a further water inlet section (63) connectable to the electrolysis stack (10) , wherein the further water inlet section (63) can be supplied with water by the pump (6) and is arranged to bypass the energy recovery device (8, 28) , and wherein the further water inlet section (63) comprises said high pressure pump ( 62 ) .

15. The loop (30) of any of the preceding claims, comprising an air lift pump (70) preferably in its water feed section ( 91 ) .

16. The loop (30) of claim 15, wherein the air lift pump (70) comprises a gas supply port connectable to an oxygen output (81) configured to discharge oxygen formed in the electrolysis stack (10) .

17. The loop (30) of any of claims 15 to 16, wherein the air lift pump (70) comprises a water supply port connectable to a further water outlet section (16) configured to discharge water drained from the electrolysis stack (10) and preferably separated, by a hydrogen separator (14) , from hydrogen .

18. The loop (30) of any of the preceding claims, comprising an oxygen separator (80) connectable to the electrolysis stack (10) , wherein the oxygen separator (80) preferably comprises an oxygen output (81) configured to discharge oxygen formed in the electrolysis stack (10) , furthermore preferably comprising a pressure relief valve (90) connected to the oxygen separator (80) .

19. The loop (30) of claim 18, wherein the gas supply port of the air lift pump (70) is connected to the oxygen output (81) .

20. The loop (30) of any of claims 18 to 19, wherein the oxygen separator (80) comprises a water output (82) configured to discharge water drained from the electrolysis stack (10) , wherein the water output (82) is connected to the water outlet section (11) .

21. The loop (30) of any of the preceding claims, comprising :- a further water outlet section (16) connectable to a hydrogen separator ( 14 ) .

22. The loop (30) of claim 21, comprising:- a secondary energy recovery device (92) , wherein the secondary energy recovery device (92) is arranged to transfer water pressure and / or flow energy from the further water outlet section (16) to the water feed section (91) .

23. The loop (30) of any of the preceding claims, comprising a heat exchanger (50) , wherein the heat exchanger (50) is preferably arranged to be traversed by water of the wateroutlet section (11) .

24. A hydrogen producing electrolysis plant (40) comprising a water recirculation loop (30) according to any of the preceding claims, and comprising an electrolysis stack (10) such as e.g. a proton exchange membrane (PEM) electrolysis stack and / or an alkaline electrolysis (AEL) stack.

25. Use of a water energy recovery device (8) in a water circulation loop (30) of a hydrogen producing electrolysis plant (40) .

26. A method (100) for recirculating water in a hydrogen producing electrolysis plant (40) , the method (100) comprising: supplying (101) water, by using at least one pump (6) , to a water inlet section (9) of an electrolysis stack (10) , wherein a water feed section (91) leads to the water inlet section (9) , draining (102) water from a water outlet section (11) of the electrolysis stack (10) , the water at the outlet section (11) being pressurized within the electrolysis stack (10) and / or in the water feed section (91) leading to the water inlet section ( 9 ) , and transferring (103) water pressure and / or flow energy from the water outlet section (11) of the electrolysis stack (10) to said water feed section (91) using at least one energy recovery device ( 8 , 28) .

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