Reversible solid oxide cell (RSOC) - electrolyzer based on solid oxide fuel cell (SOEC)

Abstract

Electrolyzer (100) which can be set up for electrolysis operation and fuel cell operation, comprising: - an electrolysis unit which is arranged for electrolysis operation, comprising: ◯ a water inlet path (10) for feeding water into an electrolysis cell (101) of the electrolyzer (100), ◯ a hydrogen outlet path (20) for removing hydrogen from the electrolysis cell (101), ◯ an air inlet path (30) for feeding air into the electrolysis cell (101) and ◯ an air outlet path (40) for removing air from the electrolytic cell (101), - a fuel cell extension (50) which is designed to enable fuel cell operation in cooperation with the electrolysis unit, comprising: o a hydrogen return line (25) for supplying hydrogen to the water inlet path (10).

Description

[0001] The invention relates to an electrolyzer having the features of the independent claim relating to an electrolyzer, a method having the features of the independent method claim, a computer program product having the features of the independent patent claim relating to a computer program product, a computer-readable data carrier having the features of the independent patent claim relating to a computer-readable data carrier, a control unit having the features of the independent patent claim relating to a control unit, and a system having the features of the independent patent claim relating to a system.

[0002] Fuel cells, particularly solid oxide electrolysis cells (SOECs), are well known. These can generate water, steam, heat, and / or electricity from hydrogen and oxygen through chemical reactions. Their counterpart, the electrolyzer, is also well known. This can generate hydrogen and oxygen from water using power, particularly electricity.

[0003] However, known systems and methods have disadvantages. For example, a fuel cell can only be used for one fuel cell operation. An electrolyzer can only be used for one electrolysis operation. However, an electrolyzer and / or a fuel cell (each) require resources, weight, space (especially volume), costs, manufacturing effort, maintenance effort, a single control unit, and / or safety precautions. Furthermore, certain systems, (industrial) operations, power plants, and the like may or could use both an electrolyzer and a (separate) fuel cell.

[0004] It is therefore an object of the present invention to at least partially overcome at least one of the disadvantages described above. In particular, the object of the invention is to provide a resource-saving, lightweight, space-saving, cost-effective, easy-to-manufacture, easy-to-maintain, and / or less complex device. Furthermore, it may be an object to reduce the number of control units.

[0005] The above object is achieved by an electrolyzer having the features of the independent claim relating to an electrolyzer, a method having the features of the independent method claim, a computer program product having the features of the independent patent claim relating to a computer program product, a computer-readable data carrier having the features of the independent patent claim relating to a computer-readable data carrier, a control unit having the features of the independent patent claim relating to a control unit, and a system having the features of the independent patent claim relating to a system. Further features and details of the invention emerge from the subclaims, the description, and the drawings.Features and details described in connection with the electrolyzer according to the invention naturally also apply in connection with the method according to the invention and / or in connection with the computer program product according to the invention and / or in connection with the computer-readable data carrier according to the invention and / or in connection with the control unit according to the invention and / or in connection with the system according to the invention and vice versa, so that with regard to the disclosure of the individual aspects of the invention, reference is or can always be made to each other. In particular, advantages described in the context of the first, second, third, fourth, fifth and / or sixth aspect also apply to the first, second, third, fourth, fifth and / or sixth aspect.

[0006] The above object is achieved according to a first aspect by an electrolyzer which can be set up, in particular set up, for electrolysis operation and fuel cell operation, comprising: • an electrolysis unit which is set up for electrolysis operation, comprising: ◯ a water inlet path for feeding water (and / or steam) into an electrolysis cell of the electrolyzer, ◯ a hydrogen exit path for removing hydrogen from the electrolysis cell, ◯ an air inlet path for feeding air into the electrolysis cell and ◯ an air outlet path for removing air from the electrolysis cell, • a fuel cell extension which is designed to enable fuel cell operation in conjunction with the electrolysis unit, comprising: o a hydrogen return line for supplying hydrogen, in particular from the electrolysis unit, e.g. a hydrogen tank of the electrolysis unit, into the water inlet path.

[0007] The electrolyzer can thus be designed to be changeable, in particular reversibly switchable. Accordingly, the electrolyzer can switch or be switched between electrolysis mode and fuel cell mode, in particular by controlling it with a control unit. In electrolysis mode, (classical) electrolysis can be operated, whereby in particular water (or water vapor) is converted into hydrogen and oxygen using energy (electricity). The electrolyzer, in particular the electrolysis unit, can be configured for this purpose. In fuel cell mode, the electrolyzer can be operated like a fuel cell. Hydrogen and oxygen (or air) can be the reactants. The products generated from these can be water, heat and / or electricity. The electrolysis unit and the fuel cell extension can interact. It can therefore be provided that the fuel cell extension can be added or removed.Switching can be carried out by a control unit, in particular a single and / or common control unit.

[0008] Particularly preferred is that an (existing) electrolyzer can be configured for both electrolysis and fuel cell operation with one or (only) a few modifications. Preferably, one or more of the following modifications are implemented: - a fuel cell extension, in particular comprising a hydrogen return line and / or a water return line, - a (modified) inverter, in particular replacing a direct current source with a (bidirectional) alternating current converter and / or - a (modified) control unit and / or software for the (existing) control unit.

[0009] This enables electrolysis and fuel cell operation with minimal changes.

[0010] The electrolyzer can have an electrolysis unit. The electrolysis unit can have the components and / or features necessary for electrolysis operation. The electrolyzer or the electrolysis unit can have an electrolysis cell in which a chemical reaction preferably takes place, particularly depending on whether electrolysis operation or fuel cell operation is carried out. The electrolysis cell can have a (reversible) solid oxide cell (RSOC). The electrolyzer can therefore be operated as a solid oxide fuel cell (SOFC) in fuel cell operation. In electrolysis operation, the electrolysis cell can convert electrical energy into chemical energy. In fuel cell operation, the electrolysis cell can convert chemical energy into electrical energy.

[0011] Within the scope of the invention, adjacent or connected components can be connected by lines, e.g., pipes. This allows a volume flow to be enabled. Heat exchangers can preferably only allow an exchange of heat; in particular, a first volume flow can cool or heat a second volume flow. The exchange of heat can increase efficiency, for example, by a warmer volume flow from the electrolysis cell heating a volume flow introduced into the electrolysis cell. Preferably, (each) heat exchanger comprises a first inlet connected to a first outlet and a (separate) second inlet connected to a second outlet.

[0012] The electrolysis unit may have a water inlet path for feeding water (which, in particular, has been converted to water vapor upon entering the electrolysis cell) into an electrolysis cell of the electrolyzer. The water inlet path may have the following features: The water inlet path can be connected to a water electrode inlet of the electrolysis cell. A volume flow from the water inlet path can be fed into this inlet. During electrolysis operation, water can be fed into the water inlet path via a water inlet, e.g., from a water tank or a water pipe. The water inlet can be connected to a water treatment unit, which can be configured, in particular, to purify water. The water treatment unit can be connected to a water pump, which can be controlled, in particular, by the control unit to adjust a volume flow. The water pump can be connected to a first water heat exchanger (e.g., a first inlet of the first water heat exchanger), which is preferably configured for heat transfer from the water inlet path to the hydrogen outlet path and / or vice versa. This can increase efficiency.The water pump can be configured, particularly in electrolysis operation, to adjust the steam utilization and / or the water flow. The first water heat exchanger (e.g., a first outlet of the first water heat exchanger) can be connected to a water evaporator, which is preferably configured to at least partially heat and / or evaporate water. The water evaporator can be connected to a water path connection point, which can preferably be configured for at least partial introduction of a hydrogen outlet path into the water inlet path. As a result, a volume flow encompassed by the hydrogen outlet path can be at least partially guided into the water inlet path; for example, hydrogen, water, and / or water vapor, in particular at a comparatively higher temperature, can be fed in this way. The water path connection point can be connected to a second water heat exchanger (e.g.,a first inlet of the second water heat exchanger), which is preferably configured for heat transfer from the water inlet path to the hydrogen outlet path and / or vice versa. This can increase the efficiency. The second water heat exchanger (e.g., a first outlet of the second water heat exchanger) can be connected to a water heater, which is configured, in particular, for heating, compressing, drying, and / or increasing the temperature (of the volume flow flowing through it). The water heater can be connected to a water electrode inlet, which is configured, in particular, for feeding the volume flow (from the water inlet path) into the electrolysis cell. Thus, water (e.g., a volume flow of water) can be fed in or guided via the water inlet path, in particular from the water inlet to the water electrode inlet.

[0013] The electrolysis cell may comprise a hydrogen electrode, which is preferably connected to the water electrode inlet and the hydrogen electrode outlet (shown particularly at the bottom of the figures). The electrolysis cell may comprise an air electrode, which is preferably connected to the air electrode inlet and the air electrode outlet (shown particularly at the top of the figures). The hydrogen electrode and the air electrode are configured for a chemical reaction. The electrolysis cell may thus be configured for electrolysis operation and / or fuel cell operation.

[0014] The electrolysis cell may comprise an inverter (or rectifier), in particular a bidirectional one. This inverter may be configured to supply a (direct) current and / or a (direct) voltage to the electrolysis cell during electrolysis operation, preferably to enable electrolysis within the electrolysis cell. The inverter may therefore comprise a rectifier or be configured to function as a rectifier. In addition, the inverter may preferably be configured to tap or discharge (in particular "pull") a current and / or a voltage during fuel cell operation. The inverter may be configured to operate as a direct current / alternating current converter during fuel cell operation.Preferably, the inverter can be controlled by the control unit (see below) to preferably enable both electrolysis operation and fuel cell operation, in particular by switching.

[0015] The electrolysis unit can have a hydrogen outlet path for removing (a volume flow of) hydrogen (and / or water vapor) (e.g., during electrolysis operation) and / or water (and / or hydrogen) (e.g., during fuel cell operation) from the electrolysis cell. The hydrogen outlet path can have the following features: The hydrogen outlet path can be connected to the hydrogen electrode outlet, preferably to remove a volume flow that arises, in particular, at the hydrogen electrode. The hydrogen outlet path can lead from the hydrogen electrode outlet to the second water heat exchanger. In other words, the hydrogen electrode outlet can be connected to the second water heat exchanger (e.g., to a second inlet of the second water heat exchanger), whereby heat can preferably be transferred from the hydrogen outlet path to the water inlet path. This can increase efficiency.The second water heat exchanger can be connected to the first water heat exchanger. A second outlet of the second water heat exchanger can be connected to a second inlet of the first water heat exchanger. The second inlet of the first water heat exchanger can be connected to a second outlet of the first water heat exchanger. The first heat exchanger, in particular the second outlet of the first heat exchanger, can be connected to a compressor, which is designed in particular to compress hydrogen and / or water. This can produce a condensate, in particular comprising water. This can be returned to the water treatment unit, preferably to optimize the utilization of the water and / or the thermal energy or temperature.The first water heat exchanger, in particular its second outlet, can be connected to a hydrogen path connection point, which can in particular be connected to the compressor. Between the first heat exchanger and the compressor, preferably at the hydrogen path connection point, the volume flow (coming from the first water heat exchanger) can be at least partially branched off and preferably fed to a hydrogen blower. In other words, the hydrogen path connection point can be connected to the hydrogen blower (in particular, the hydrogen recirculation blower). The hydrogen blower can be controlled by the control unit to preferably adjust a volume flow (of hydrogen) through the hydrogen blower. It can be provided that the hydrogen blower can also be operated in fuel cell mode. This can improve efficiency.This allows the volume flow and / or concentration of hydrogen, which is preferably supplied to the electrolysis cell (via the water electrode inlet), to be adjusted, particularly during electrolysis operation and / or fuel cell operation. Furthermore, the formation of condensate can be controlled, particularly during electrolysis operation and / or fuel cell operation. In fuel cell operation, the hydrogen blower can adjust the fuel utilization of the electrolysis cell and / or the fuel utilization of the electrolyzer (e.g., the entire system). The hydrogen blower can be connected to a second air heat exchanger, in particular to a second inlet thereof. The second air heat exchanger can transfer heat from the air outlet path to the volume flow of hydrogen. This can increase efficiency.A hydrogen blower valve can be arranged between the hydrogen blower and the second air heat exchanger, which can preferably be opened or closed (at least partially) by actuating it with the control unit. The second air heat exchanger, in particular a second outlet thereof, can be connected to the water path connection point, whereby hydrogen can preferably be fed into the water inlet path. A volume flow of fed-in hydrogen can be adjusted by the hydrogen blower valve and / or the hydrogen blower, in particular via the control unit. The compressor can form a condensate, in particular water, particularly during operation, e.g., in electrolysis mode and / or fuel cell mode, which condensate can preferably be drained away. The compressor can be connected to the water treatment unit. This can reduce the (overall) water requirement.Furthermore, the water (condensate) from the compressor and / or the first water heat exchanger can be (relatively) pure, which can reduce the effort required for cleaning and / or the (energy) costs. The compressor can be connected to a hydrogen tank, which can preferably be configured to store hydrogen. The hydrogen tank can be connected to a hydrogen outlet, which can preferably be configured to discharge hydrogen from the hydrogen outlet path. This allows hydrogen to be supplied, for example, to another system such as an (external) fuel cell. The control unit can control the hydrogen tank in order to discharge hydrogen from the hydrogen tank, for example via the hydrogen outlet and / or a hydrogen return line. Thus, water and / or hydrogen (e.g.a volume flow of water and / or hydrogen) via the hydrogen outlet path, in particular from the hydrogen electrode outlet to the hydrogen outlet.

[0016] The electrolysis unit may include an air inlet path for feeding air into the electrolysis cell. The air inlet path may have the following features: The air inlet path can have an air inlet, in particular can begin there. The air inlet can have an (air) filter, preferably to clean incoming or fed-in air. The air inlet can be connected to an air blower, which can preferably adjust the volume flow, in particular in the air inlet path or through the air blower. For this purpose, the air blower can preferably be controlled by the control unit. The air blower can preferably be configured in electrolysis mode to effect discharge of products of the electrochemical reaction and / or temperature control (in particular cooling and / or heating) of the electrolysis cell. The air blower can preferably be configured in fuel cell mode to provide air and / or oxygen for the electrochemical reaction in the electrolysis cell and / or to enable cooling of the electrolysis cell.The air blower can be connected to a first air heat exchanger, in particular a first inlet thereof. The first inlet of the first air heat exchanger can be connected to a first outlet of the first air heat exchanger. Heat can be transferred from the air outlet path to the air inlet path through the first air heat exchanger. This can increase efficiency. The first air heat exchanger, in particular its first outlet, can be connected to an air heater, which is preferably configured for tempering, in particular cooling and / or heating, the air or the volume flow of air. The air heater can be connected to the electrolysis cell, in particular an air electrode inlet of the electrolysis cell. Thus, air (e.g., a volume flow of air) can be fed in or guided via the air inlet path, in particular from the air inlet to the air electrode inlet.

[0017] The electrolysis unit may have an air outlet path for removing (a volume flow of) air / oxygen (e.g., during electrolysis operation), oxygen-enriched air, pure O2, and / or air / oxygen (e.g., during fuel cell operation) from the electrolysis cell. The air outlet path may have the following features: The air electrode outlet can be connected to a first air heat exchanger, in particular a second inlet of the first air heat exchanger. The second inlet can be connected to a second outlet of the first air heat exchanger. Advantageously, heat can be transferred from the air outlet path to the air inlet path by the first air heat exchanger. This can improve efficiency. The first air heat exchanger, in particular a second outlet of the first air heat exchanger, can be connected to a second air heat exchanger, in particular a first inlet of the second air heat exchanger. The first inlet of the second air heat exchanger can be connected to a first outlet of the second air heat exchanger. Advantageously, heat can be transferred from the air outlet path to the air inlet path by the second air heat exchanger. This can (further) improve efficiency.The second air heat exchanger, in particular its first outlet, can be connected to an air outlet, which is preferably configured to discharge air from the air outlet path or the electrolysis cell. Thus, air (e.g., a volume flow of air and / or oxygen-enriched air or pure air) can be discharged via the air outlet path, in particular from the air electrode outlet to the air outlet.

[0018] The electrolyzer has a fuel cell extension configured to enable fuel cell operation in conjunction with the electrolysis unit. The fuel cell extension may include a hydrogen return line for supplying hydrogen to the water inlet path. Advantageously, hydrogen, in particular from the hydrogen tank, can be (at least partially) returned to the water inlet path via the hydrogen return line. This advantageously enables fuel cell operation (at all). The hydrogen can then react, in particular, with air or oxygen from the air inlet path in the electrolysis cell (as in a fuel cell), during fuel cell operation to generate electricity, heat, and / or water.

[0019] Within the scope of the invention, it may be advantageous that the hydrogen return line is connected to a hydrogen tank, wherein the hydrogen tank is arranged in the hydrogen outlet path and / or that the hydrogen return line has a hydrogen metering valve, in particular a needle metering valve.

[0020] Hydrogen can be fed from the hydrogen tank into the water inlet path. Alternatively or additionally, the hydrogen return line can have a hydrogen metering valve, which can be designed in particular as a needle metering valve. The control unit can actuate the hydrogen metering valve, in particular the needle metering valve. This makes it possible to adjust the volume flow of hydrogen fed in via the hydrogen return line. Opening it allows hydrogen to be fed into the water inlet path, preferably during fuel cell operation. Closing it prevents hydrogen from being fed into the water inlet path, preferably during electrolysis operation.

[0021] Within the scope of the invention, it is conceivable that the hydrogen return line is connected to a water path connection point or a hydrogen return valve, thereby enabling hydrogen to be supplied to a water electrode inlet of the electrolysis cell.

[0022] This allows hydrogen to be added upstream of the water electrode inlet.

[0023] The hydrogen tank can be connected to the water path connection point. Preferably, the hydrogen tank can be connected to the hydrogen return line. The hydrogen return line can be connected to the water path connection point. Thus, hydrogen can be fed from the hydrogen tank into the water inlet path, particularly immediately upstream of the second water heat exchanger. This can enable rapid switching between electrolysis operation and fuel cell operation. As a result, the first water heat exchanger and / or the water evaporator can be bypassed. This can enable faster start-up of fuel cell operation. Furthermore, control can be facilitated and / or wear (e.g., of components not subject to flow) can be reduced.

[0024] Alternatively or additionally, it can be provided that the hydrogen return line is connected to a hydrogen return valve. The hydrogen return valve can be connected to the water pump and / or the first water heat exchanger. Preferably, the hydrogen return valve is arranged between the water pump and the first water heat exchanger. As a result, the fed-in water and / or the fed-in hydrogen can advantageously be heated by the first water heat exchanger and / or the water evaporator, which can preferably increase efficiency. The first water heat exchanger, particularly during fuel cell operation, can extract heat from the hydrogen output path, whereby water vapor in particular is condensed at least partially, preferably as completely as possible. In other words, (more) condensate can thus be formed. The hydrogen return valve can be designed as a T-piece or as a three-way valve.The control unit can control the hydrogen recirculation valve to adjust its preference. This allows the volume flow of hydrogen fed through the hydrogen recirculation line to be adjusted. Opening the valve allows hydrogen to be fed into the water inlet path, preferably during fuel cell operation. Closing the valve prevents hydrogen from being fed into the water inlet path, preferably during electrolysis operation.

[0025] It can be provided within the scope of the invention that the fuel cell extension comprises a water return valve and a water return line connected thereto for at least partially returning water to the water inlet path.

[0026] The water return line can be configured to carry water and / or hydrogen. This can be routed to another point of the water inlet path, preferably further upstream (e.g., toward the water inlet).

[0027] This allows water to be at least partially recirculated to a point upstream of the water recirculation valve. Preferably, the water and / or hydrogen (particularly in fuel cell operation) can be (re)heated in the first water heat exchanger. This can then be at least partially recirculated, thereby increasing, in particular, the inlet temperature at the first water heat exchanger. This allows for greater heating. This can increase efficiency.

[0028] The water return valve can be controlled by the control unit to set its preferred flow rate. The volume flow of water and / or hydrogen being returned can be adjusted. This can be done during electrolysis operation and / or fuel cell operation. This allows efficiency to be optimized in each case. Opening the valve allows water and / or hydrogen to be returned to the water inlet path, preferably during fuel cell operation. Closing the valve prevents water and / or hydrogen from being returned to the water inlet path, preferably during electrolysis operation. The water return valve can be designed as a T-piece or as a three-way valve.

[0029] Particularly preferably, the water return line can be used to at least partially and / or completely remove water from the water inlet path or the first water heat exchanger during fuel cell operation, in particular during and / or shortly before switching to fuel cell operation. For this purpose, the control unit can control the water return valve during and / or shortly before switching to fuel cell operation. This can thereby be adjusted to open a connection to the water return line. It can preferably be provided alternatively or additionally to close the water return valve, in particular when the water inlet path or the first water heat exchanger is emptied (i.e. water has been drained away). This can be carried out by the control unit controlling the water return valve, whereby the water return valve is advantageously adjusted.

[0030] It is further conceivable that the water inlet path comprises a water evaporator and a water treatment unit, in particular one located upstream thereof, wherein the water return line is connected to the water treatment unit, and wherein the water return valve is arranged downstream or upstream of the water evaporator, wherein in particular a water cooling unit is arranged between the water return valve and the water treatment unit.

[0031] The water evaporator can adjust the temperature, particularly during water electrolysis operation, or the volume flow of water flowing through the water evaporator. The control unit can control the water evaporator to adjust the temperature. It can be provided that the control unit controls the water evaporator during fuel cell operation to switch it off. This can save energy and / or improve efficiency.

[0032] The water treatment unit can be configured to purify water. The water treatment unit can be located upstream (in the water inlet path) of the water evaporator. The water return line can be connected to the water treatment unit, allowing, in particular, recirculated water and / or hydrogen to be (re)purified. Furthermore, the water pump can thereby (re)adjust or readjust the volume flow.

[0033] The water recirculation valve can be located downstream of the water evaporator. Thus, the water recirculation valve can be positioned downstream of the water evaporator in the flow direction. This allows the water and / or hydrogen to be (at least partially) heated and / or evaporated by the water evaporator.

[0034] The water return valve can be arranged upstream of the water evaporator. Thus, the water return valve can be arranged upstream of the water evaporator in the flow direction. The water and / or hydrogen can be returned before being passed through the water evaporator and, in particular, heated and / or evaporated. Preferably, the water return valve can be arranged downstream of a bypass valve. Accordingly, both water return and bypassing of the water evaporator can occur.

[0035] It can be provided that a water cooling unit is arranged between the water return valve and the water treatment unit. This allows recirculated water and / or hydrogen to be cooled, in particular before it is fed to the water treatment unit. Preferably, particularly in fuel cell operation, in combination with a water return line arranged at a water return valve downstream of the water evaporator, water can be recirculated through the water return line and subsequently cooled before this water cools the hydrogen in the first water heat exchanger. Thus, the water inlet path (at least its front part) and the water return line can be used as a cooling circuit.

[0036] It is also conceivable that the water inlet path has a bridging line for bridging a water evaporator which is arranged in the water inlet path, whereby in particular hydrogen can be guided past the water evaporator, wherein preferably the bridging line is connected to a bridging valve which is arranged upstream of the water evaporator.

[0037] The bypass line can be connected to the water path connection point. This can enable at least partial bypassing of the water evaporator. This can thus be bypassed, particularly during fuel cell operation. This can protect it from wear. The bypass valve can therefore be controlled by the control unit during fuel cell operation or when switching to fuel cell operation. This can adjust the bypass valve to preferably enable bypassing of the water evaporator. The bypass valve can be designed as a T-piece or as a 3-way valve. During electrolysis operation or when switching to electrolysis operation, the control unit can control the bypass valve, whereby it is preferably adjusted. This allows a volume flow to flow (again) through the water evaporator.

[0038] Within the scope of the invention, it is optionally possible for the electrolyzer to have an inverter which is configured for electrolysis operation and fuel cell operation, wherein in particular the inverter is designed to be switchable in order to switch between electrolysis operation and fuel cell operation.

[0039] A control unit can control the inverter to switch the electrolyzer between electrolysis mode and fuel cell mode. Preferably, the control unit can control the inverter when switching to electrolysis mode, whereupon the inverter is advantageously configured as a rectifier in electrolysis mode and / or supplies electrical energy, power, current, and / or voltage to the electrolysis cell. Preferably, the control unit can control the inverter when switching to fuel cell mode, whereupon the inverter is advantageously configured as a direct current / alternating current converter in fuel cell mode and / or draws or taps electrical energy, power, current, and / or voltage from the electrolysis cell.

[0040] The above object is achieved according to a second aspect by a method according to the invention for an electrolyzer for switching between an electrolysis operation and a fuel cell operation, comprising: - providing an electrolyser according to the first aspect, - controlling, in particular by a control unit, the fuel cell extension in order to carry out a change from electrolysis operation to fuel cell operation or from fuel cell operation to electrolysis operation and - Operating, in particular by means of a control unit, the electrolyzer in electrolysis mode or in fuel cell mode.

[0041] Preferably, the method can be computer-implemented and / or carried out by a control unit, in particular where technically feasible. The method can be carried out (at least partially) repeatedly. It can be provided to switch between electrolysis operation and fuel cell operation multiple times and / or repeatedly. This results in a flexible selection of the respective operating mode. For example, the selection can be made dependent on whether external energy, in particular voltage and / or current, is available, for example because a solar module can be operated. If this is not the case, fuel cell operation can be selected instead in order to preferentially generate energy or current.

[0042] This results in the same advantages with respect to a method according to the invention according to the second aspect as have already been described with respect to an electrolyzer according to the invention according to the first aspect.

[0043] Furthermore, it can be provided within the scope of the invention that the control has at least one of the following features: - Controlling a water return valve, - Controlling a water cooling unit, in particular switching the water cooling unit on or off, - Controlling a hydrogen dosing valve, in particular to adjust a volume flow of recirculated hydrogen, - controlling a hydrogen recirculation valve, in particular opening or closing it, to allow or prevent a volume flow of recirculated hydrogen into the water inlet path and / or - Controlling a bypass valve, in particular to enable or prevent bypassing of a water evaporator.

[0044] When switching from electrolysis operation to fuel cell operation, it can be provided that the activation of a water return valve comprises opening the water return line, whereby said line returns, in particular, water and / or hydrogen, for example, to the water treatment unit. Particularly preferably, the water return line can be used to at least partially and / or completely remove water from the water inlet path or the first water heat exchanger during fuel cell operation, in particular during and / or shortly before switching to fuel cell operation. For this purpose, the control unit can activate the water return valve during and / or shortly before switching to fuel cell operation.

[0045] When switching from electrolysis operation to fuel cell operation, it can be provided that the control of a water cooling unit particularly includes switching the water cooling unit on or off. The water cooling unit can preferably be switched on to enable cooling. This allows a temperature difference in the first water heat exchanger to be increased, which can result in increased cooling capacity. As a result, water and / or hydrogen, which flows through the first water heat exchanger, in particular in the water inlet path, can cool the water and / or hydrogen flowing through the first heat exchanger in the hydrogen outlet path.

[0046] When switching from electrolysis operation to fuel cell operation, it can be provided that the control of a hydrogen metering valve, in particular the control to adjust a volume flow of recirculated hydrogen, comprises. In this case, the hydrogen metering valve can preferably be opened (at least partially). A greater opening can increase the volume flow, as a result of which more hydrogen can be made available at the electrolysis cell (in particular at the water electrode inlet). This can increase the performance of the electrolyzer in fuel cell operation, in which it preferably functions as a fuel cell. In this case, a greater closing or reduced opening can reduce the volume flow, as a result of which less hydrogen can be made available at the electrolysis cell (in particular at the water electrode inlet).This can reduce the performance of the electrolyzer in fuel cell mode, in which it preferably functions as a fuel cell.

[0047] When switching from electrolysis operation to fuel cell operation, it can be provided that the control of a hydrogen recirculation valve comprises, in particular, opening or closing it in order to enable or prevent a volume flow of recirculated hydrogen into the water inlet path. In this case, the hydrogen recirculation valve can preferably be opened (at least partially). A greater opening can increase the volume flow, as a result of which more hydrogen can be made available at the electrolysis cell (in particular at the water electrode inlet). This can increase the performance of the electrolyzer in fuel cell operation, in which it preferably functions as a fuel cell. A greater closing or reduced opening can reduce the volume flow, as a result of which less hydrogen can be made available at the electrolysis cell (in particular at the water electrode inlet).This can reduce the performance of the electrolyzer in fuel cell mode, in which it preferably functions as a fuel cell.

[0048] When switching from electrolysis operation to fuel cell operation, it can be provided that a bypass valve is activated, in particular to enable or prevent bypassing of a water evaporator, whereby the bypass valve is preferably adjusted. Preferably, the bypass valve can open the bypass line during fuel cell operation, thereby bypassing the water evaporator.

[0049] When changing from electrolysis operation to fuel cell operation, it may be provided that the control of a water evaporator includes switching it off.

[0050] When switching from fuel cell operation to electrolysis operation, it can be provided that the control of a water return valve comprises closing the water return line, whereby said water return line in particular does not (or no longer) return water and / or hydrogen, for example to the water treatment unit. Particularly preferably, the water treatment unit and / or water pump can be used to feed water at least partially and / or completely into the water inlet path or the first water heat exchanger during electrolysis operation, in particular during and / or shortly before switching to electrolysis operation. For this purpose, the control unit can control the water treatment unit and / or water pump during and / or shortly before switching to electrolysis operation.

[0051] When switching from fuel cell operation to electrolysis operation, it can be provided that the control of a water cooling unit includes, in particular, switching the water cooling unit on or off. Preferably, the water cooling unit can be switched off to prevent cooling, which can, in particular, save power.

[0052] When switching from fuel cell operation to electrolysis operation, it can be provided that the control of a hydrogen metering valve, in particular the control to adjust a volume flow of recirculated hydrogen, comprises. In this case, the hydrogen metering valve can preferably be closed (at least partially). A stronger closure can reduce the volume flow, whereby less hydrogen can be provided at the electrolysis cell (in particular at the water electrode inlet). This can increase the performance of the electrolyzer in electrolysis operation, in which it preferably functions as an electrolyzer. Preferably, the hydrogen metering valve can be closed completely.

[0053] When switching from fuel cell operation to electrolysis operation, it can be provided that the control of a hydrogen recirculation valve comprises, in particular, opening or closing it to enable or prevent a volume flow of recirculated hydrogen into the water inlet path. In this case, the hydrogen recirculation valve can preferably be (at least partially) closed. A stronger closure can reduce the volume flow, whereby less hydrogen can be provided at the electrolysis cell (in particular at the water electrode inlet). This can increase the performance of the electrolyzer in electrolysis operation, in which it preferably functions as an electrolyzer. Preferably, the hydrogen recirculation valve can be closed completely or almost completely.

[0054] When switching from fuel cell operation to electrolysis operation, it can be provided that a bypass valve is activated, in particular to enable or prevent bypassing of a water evaporator, whereby the bypass valve is preferably adjusted. Preferably, the bypass valve can close the bypass line during electrolysis operation, preventing the water evaporator from being bypassed.

[0055] When changing from fuel cell operation to electrolysis operation, it may be provided that the control of a water evaporator includes switching it on.

[0056] The above object is achieved according to a third aspect by a computer program product according to the invention, comprising instructions which, when the computer program product is executed by a computer, cause the computer to implement the method according to the second aspect.

[0057] This results in the same advantages with regard to a computer program product according to the invention as have already been described with regard to an electrolyzer according to the invention according to the first aspect and / or a method according to the invention according to the second aspect.

[0058] The above object is achieved according to a fourth aspect by a computer-readable data carrier according to the invention in which instructions are stored which, when executed by a computer, cause the computer to carry out the method according to the second aspect.

[0059] This results in the same advantages with regard to a computer-readable data carrier according to the invention as have already been described with regard to an electrolyzer according to the invention according to the first aspect and / or a method according to the invention according to the second aspect and / or a computer program product according to the invention according to the third aspect.

[0060] The above object is achieved according to a fifth aspect by a control unit according to the invention, comprising a computing unit and a memory unit in which instructions are stored which, when at least partially executed by the computing unit, carry out a method according to the second aspect.

[0061] The control unit can be connected to the water treatment unit for controlling and / or regulating, in particular via a data connection (for data exchange), or to the water pump, the first water heat exchanger, the water evaporator, the second water heat exchanger, the water heater, the water return valve, the water cooling unit, the compressor, the hydrogen heat exchanger, the hydrogen metering valve, the hydrogen return valve, the bypass valve, the air blower, the first air heat exchanger, the air heater, the second air heat exchanger, the fuel cell extension, the electrolysis cell, and / or the inverter. Thus, control can occur, for example, when switching from electrolysis mode to fuel cell mode or vice versa. The control unit can control the electrolyzer, in particular in electrolysis mode or fuel cell mode, in order to operate the electrolyzer.

[0062] This results in the same advantages with regard to a control unit according to the invention as have already been described with regard to an electrolyzer according to the invention according to the first aspect and / or a method according to the invention according to the second aspect and / or a computer program product according to the invention according to the third aspect and / or a computer-readable data carrier according to the invention according to the fourth aspect.

[0063] The above object is achieved according to a sixth aspect by a system according to the invention, comprising an electrolyzer according to the first aspect and / or a control unit according to the fifth aspect.

[0064] The system may comprise a motor vehicle, ship structure, aircraft, (residential) building, industrial building, storage facility, and / or power plant that has and / or is connected to the electrolyzer to realize the above advantages. An example may be a power plant, e.g., a solar power plant, that produces electricity during the day that could be at least partially used for electrolysis (e.g., in the event of a surplus), but cannot produce electricity at night. Thus, an electrolyzer could be operated during the day and a fuel cell at night, for example, with the hydrogen and / or oxygen produced by the electrolyzer during the day. A combined solution can be provided by the electrolyzer according to the invention, which can be operated both in electrolysis mode and in fuel cell mode.This allows resources, weight, space (especially volume), costs, manufacturing effort, maintenance effort, control unit(s) and / or safety precautions to be optimized, in particular reduced.

[0065] This results in the same advantages with regard to a system according to the invention as have already been described with regard to an electrolyzer according to the invention according to the first aspect and / or a method according to the invention according to the second aspect and / or a computer program product according to the invention according to the third aspect and / or a computer-readable data carrier according to the invention according to the fourth aspect and / or a control unit according to the invention according to the fifth aspect.

[0066] Further advantages, features, and details of the invention will become apparent from the following description, which describes several embodiments of the invention in detail with reference to the drawings. The features mentioned in the claims and in the description may be essential to the invention individually or in any combination. Each of these schematically shows: Fig. 1 an electrolyzer, Fig. 2 an electrolyzer with hydrogen return line to the water path connection point, Fig. 3 an electrolyzer with hydrogen return line to the hydrogen return valve, Fig. 4 a system and Fig. 5 a procedure.

[0067] In the following figures, identical reference numerals are used for the same technical features, even for different embodiments.

[0068] Fig. 1 shows an electrolyzer 100 which can be set up for electrolysis operation and fuel cell operation, comprising: • an electrolysis unit which is set up for electrolysis operation, comprising: o a water inlet path 10 for feeding water into an electrolysis cell 101 of the electrolyzer 100, in particular via a water electrode inlet 101.1, o a hydrogen outlet path 20 for removing hydrogen from the electrolysis cell 101, in particular via a hydrogen electrode outlet 101.2, ◯ an air inlet path 30 for feeding air into the electrolysis cell 101, in particular via an air electrode inlet 101.3 and ◯ an air outlet path 40 for discharging air from the electrolysis cell 101, in particular via an air electrode outlet 101.4.

[0069] The electrolysis cell 101 can have a hydrogen electrode 102, at which hydrogen is formed, preferably during electrolysis operation. The electrolysis cell 101 can have an air electrode 103, at which oxygen and / or air is formed or removed, preferably during electrolysis operation. The electrolysis cell 101 is connected to an inverter 60, which preferably supplies a current or voltage to the electrolysis cell during electrolysis operation or taps or draws a current or voltage during fuel cell operation.

[0070] The water inlet path 10 can have the following features: The water inlet path 10 can be connected to a water electrode inlet 101.1 of the electrolysis cell 101. A volume flow from the water inlet path 10 can be fed into this. During electrolysis operation, water can be fed into the water inlet path 10 via a water inlet 11, e.g., from a water tank or a water pipe. The water inlet 11 can be connected to a water treatment unit 12, which can be configured, in particular, to purify water. The water treatment unit 12 can be connected to a water pump 13, which can be controlled, in particular, by a control unit FCCU to adjust a volume flow. The water pump 13 can be connected to a first water heat exchanger 14 (e.g., a first inlet of the first water heat exchanger), which is preferably configured for heat transfer from the water inlet path 10 to the hydrogen outlet path 20 and / or vice versa.

[0071] The first water heat exchanger 14 (e.g., a first outlet of the first water heat exchanger) can be connected to a water evaporator 15, which is preferably configured for at least partial heating and / or evaporation of water. The water evaporator 15 can be connected to a water path connection point 16, which can preferably be configured for at least partial introduction of a hydrogen outlet path 20 into the water inlet path 10. As a result, a volume flow encompassed by the hydrogen outlet path 20 can be at least partially guided into the water inlet path 10; for example, hydrogen, water, and / or water vapor, in particular at a comparatively higher temperature, can be fed in this way. The water path connection point 16 can be connected to a second water heat exchanger 17 (e.g.,a first inlet of the second water heat exchanger), which is preferably configured for heat transfer from the water inlet path 10 to the hydrogen outlet path 20 and / or vice versa. This can increase efficiency. The second water heat exchanger 17 (e.g., a first outlet of the second water heat exchanger) can be connected to a water heater 18, which is configured, in particular, for heating or increasing the temperature (of the volume flow flowing through it). The water heater 18 can be connected to a water electrode inlet 101.1, which is configured, in particular, for feeding the volume flow (from the water inlet path 10) into the electrolysis cell 101. Thus, water (e.g., a volume flow of water) can be fed in or guided via the water inlet path 10, in particular from the water inlet 11 to the water electrode inlet 101.1.

[0072] The electrolysis unit may have a hydrogen outlet path 20 for discharging (a volume flow of) hydrogen (e.g., during electrolysis operation) and / or water (e.g., during fuel cell operation) from the electrolysis cell 101. The hydrogen outlet path 20 may have the following features: The hydrogen outlet path 20 can be connected to the hydrogen electrode outlet 101.2 in order to preferably discharge a volume flow that arises in particular at the hydrogen electrode 102. The hydrogen outlet path 20 can lead from the hydrogen electrode outlet 101.2 to the second water heat exchanger 17. In other words, the hydrogen electrode outlet 101.2 can be connected to the second water heat exchanger 17 (e.g., to a second inlet of the second water heat exchanger), whereby heat can preferably be transferred from the hydrogen outlet path 20 to the water inlet path 10. This can increase efficiency. The second water heat exchanger 17 can be connected to the first water heat exchanger 14. A second outlet of the second water heat exchanger 17 can be connected to a second inlet of the first water heat exchanger 14.The second inlet of the first water heat exchanger 14 can be connected to a second outlet of the first water heat exchanger 14. The first water heat exchanger 14, in particular the second outlet of the first water heat exchanger 14, can be connected to a compressor 21, which is designed in particular to compress hydrogen and / or water. In the process, a condensate, in particular comprising water, can be produced. This can be returned to the water treatment unit 12 in order to preferably optimize the utilization of the water and / or the thermal energy or temperature. The first water heat exchanger 14, in particular its second outlet, can be connected to a hydrogen path connection point, which can in particular be connected to the compressor.Between the first water heat exchanger 14 and the compressor 21, preferably at the hydrogen path connection point, the volume flow (coming from the first water heat exchanger) can be at least partially branched off and preferably fed to a hydrogen blower 24. In other words, the hydrogen path connection point can be connected to the hydrogen blower 24. The hydrogen blower 24 can be controlled by the control unit FCCU to preferably adjust a volume flow (of hydrogen) through the hydrogen blower 24. This allows the volume flow and / or concentration of hydrogen, which is preferably supplied to the electrolysis cell 101 (via the water electrode inlet 101.1), to be adjusted, particularly during electrolysis operation. Furthermore, the formation of condensate can be controlled, particularly during electrolysis operation and / or fuel cell operation.During fuel cell operation, the hydrogen blower 24 can adjust the fuel utilization of the electrolysis cell 101 and / or the fuel utilization of the electrolyzer 100 (e.g., the entire system). The hydrogen blower 24 can be connected to a second air heat exchanger 41, in particular a second inlet thereof. The second air heat exchanger 41 can transfer heat from the air outlet path 40 to the volume flow of hydrogen. This can increase efficiency. A hydrogen blower valve can be arranged between the hydrogen blower 24 and the second air heat exchanger 41, which can preferably be opened or closed (at least partially) by controlling it with the control unit. The second air heat exchanger 41, in particular a second outlet thereof, can be connected to the water path connection point 16, whereby hydrogen can preferably be fed into the water inlet path 10.A volume flow of fed-in hydrogen can be adjusted by the hydrogen blower valve and / or the hydrogen blower 24, in particular via the FCCU control unit. The compressor 21 can, in particular during operation, e.g., in electrolysis mode and / or fuel cell mode, form a condensate, in particular water, which can preferably be drained away. The compressor 21 can be connected to the water treatment unit 12. This can reduce the (overall) water requirement. Furthermore, the water (condensate) from the compressor 21 and / or the first water heat exchanger 14 can be (relatively) pure, which can reduce the effort required for cleaning and / or the (energy) costs. The compressor 21 can be connected to a hydrogen tank 22, which can preferably be configured to store hydrogen.The hydrogen tank 22 can be connected to a hydrogen outlet 23, which can preferably be configured to discharge hydrogen from the hydrogen outlet path 20. This allows hydrogen to be supplied, for example, to another system, such as an (external) fuel cell. The control unit FCCU can control the hydrogen tank 22 to discharge hydrogen from the hydrogen tank, for example, via the hydrogen outlet 23 and / or a hydrogen return line 25. Thus, water and / or hydrogen (e.g., a volume flow of water and / or hydrogen) can be discharged via the hydrogen outlet path 20, in particular from the hydrogen electrode outlet 101.2 to the hydrogen outlet 23.

[0073] The electrolysis unit may have an air inlet path 30 for feeding air into the electrolysis cell 101. The air inlet path 30 may have the following features: The air inlet path 30 can have an air inlet 31, in particular can begin there. The air inlet 31 can have an (air) filter, preferably to clean incoming or fed-in air. The air inlet 31 can be connected to an air blower 32, which can preferably adjust the volume flow, in particular in the air inlet path 30 or through the air blower 32. For this purpose, the air blower 32 can preferably be controlled by the FCCU control unit. The air blower 32 can preferably be configured during electrolysis operation to effect discharge of products of the electrochemical reaction and / or temperature control (in particular cooling and / or heating) of the electrolysis cell 101. The air blower 32 can preferably be configured during fuel cell operation to provide air and / or oxygen for the electrochemical reaction in the electrolysis cell 101 and / or to enable cooling of the electrolysis cell 101.The air blower 32 can be connected to a first air heat exchanger 33, in particular a first inlet thereof. The first inlet of the first air heat exchanger 33 can be connected to a first outlet of the first air heat exchanger 33. Heat can be transferred from the air outlet path 40 to the air inlet path 30 through the first air heat exchanger 33. This can increase efficiency. The first air heat exchanger 33, in particular its first outlet, can be connected to an air heater 34, which is preferably configured for tempering, in particular cooling and / or heating, the air or the volume flow of air. The air heater 34 can be connected to the electrolysis cell 101, in particular an air electrode inlet 101.3 of the electrolysis cell 101. Thus, air (e.g. a volume flow of air) can be fed in or guided via the air inlet path 30, in particular from the air inlet 31 to the air electrode inlet 101.3.

[0074] The electrolysis unit may have an air outlet path 40 for discharging (a volume flow of) air / oxygen (e.g., during electrolysis operation) and / or air / oxygen (e.g., during fuel cell operation) from the electrolysis cell 101. The air outlet path 40 may have the following features: The air electrode outlet 101.4 can be connected to a first air heat exchanger 33, in particular a second inlet of the first air heat exchanger 33. The second inlet can be connected to a second outlet of the first air heat exchanger 33. Advantageously, heat can be transferred from the air outlet path 40 to the air inlet path 30 through the first air heat exchanger 33. This can improve efficiency. The first air heat exchanger 33, in particular a second outlet of the first air heat exchanger 33, can be connected to a second air heat exchanger 41, in particular a first inlet of the second air heat exchanger 41. The first inlet of the second air heat exchanger 41 can be connected to a first outlet of the second air heat exchanger 41. Advantageously, heat can be transferred from the air outlet path 40 to the air inlet path 30 through the second air heat exchanger 41.This can (further) improve efficiency. The second air heat exchanger 41, in particular its first outlet, can be connected to an air outlet 42, which is preferably configured to discharge air from the air outlet path 40 or the electrolysis cell 101. Thus, air (e.g., a volume flow of air) can be discharged via the air outlet path 40, in particular from the air electrode outlet 101.4 to the air outlet 42.

[0075] The Fig. 2 and Fig. 3 show an electrolyzer 100, in particular based on Fig. 1. The electrolyzer 100 (in each case) has a fuel cell extension 50, which, in cooperation with the electrolysis unit, is configured to enable fuel cell operation. The fuel cell extension 50 can have a hydrogen return line 25 for supplying hydrogen to the water inlet path 10. Advantageously, hydrogen, in particular from the hydrogen tank 22, can be (at least partially) returned to the water inlet path 10 via the hydrogen return line 25. This advantageously enables fuel cell operation (at all). The hydrogen can then react, in particular, with air or oxygen from the air inlet path 30 in the electrolysis cell 101 (as in a fuel cell) during fuel cell operation to generate electricity, heat, and / or water.Furthermore, the hydrogen return line 25 can be connected to a hydrogen metering valve 26, wherein the FCCU control unit can control the hydrogen metering valve 26 to preferably adjust the volume flow of (recirculated) hydrogen (by adjusting the hydrogen metering valve 26). Water and / or hydrogen can be returned from a water return valve 19.1 via a water return line 19, which is connected to the water treatment unit 12. The water return valve 19.1 can be opened for fuel cell operation by the FCCU control unit controlling the water return valve 19.1. During electrolysis operation, the water return valve 19.1 can be closed by the FCCU control unit controlling the water return valve 19.1.

[0076] Fig. 2 has a water cooling unit 19.2, which is connected to the water treatment unit 12 and the water return valve 19.1. The water return valve 19.1 is arranged upstream of the water path connection point 16. The water return valve 19.1 is arranged downstream of the water evaporator 15. The hydrogen return line 25 is connected to the water path connection point 16. This allows hydrogen, particularly during fuel cell operation, to be fed into the water inlet path 10 directly after the water return valve 19.1 (particularly downstream). The returned water can be used to cool the volume flow from the hydrogen electrode outlet 101.2, particularly in the first water heat exchanger 14.

[0077] Fig. 3 has a hydrogen recirculation valve 27, which is arranged between the water pump 13 and the first water heat exchanger 14. The hydrogen recirculation valve 27 is connected to the hydrogen recirculation line 25 and / or the hydrogen metering valve 26. Hydrogen can be fed between the water pump 13 and the first water heat exchanger 14 through the hydrogen recirculation valve 27, particularly for fuel cell operation. The water recirculation valve 19.1 is arranged between the first water heat exchanger 14 and the water evaporator 15 and / or the bypass valve 28.1. Accordingly, a portion of the fed-in hydrogen can also be recirculated via the water recirculation line 19. This can allow further degrees of freedom during operation, particularly during control and / or regulation by the FCCU control unit. A bypass line 28 is connected to a bypass valve 28.1 and the water path connection point 16.The bypass valve 28.1 is arranged between the water return valve 19.1 and the water evaporator 15. The water evaporator 15 can be bypassed via the bypass line 28, particularly during fuel cell operation. In other words, part or all of the volume flow can flow through the bypass line 28, and preferably not through the water evaporator 15.

[0078] Fig. Figure 4 shows a system 200 comprising an electrolyzer 100 and a control unit FCCU. The control unit may have a computing unit CU and a memory unit MU.

[0079] Fig. 5 shows a method comprising: - Providing 110 an electrolyzer 100 according to one of the preceding claims, - controlling 120, in particular by a control unit FCCU, the fuel cell extension 50 in order to carry out a change from an electrolysis operation to a fuel cell operation or from a fuel cell operation to an electrolysis operation and - Operating 130, in particular by a control unit FCCU, the electrolyzer 100 in electrolysis mode or in fuel cell mode. List of reference symbols 10 Water entrance path 11 Water inlet 12 Water treatment unit 13 Water pump 14 first water heat exchanger 15 water evaporators 16 Water path connection point 17 second water heat exchanger 18 water heaters 19 Water return line 19.1 Water return valve 19.2 Water cooling unit 20 Hydrogen exit path 21 Compressor 22 hydrogen tank 23 Hydrogen outlet 24 hydrogen blowers 25 Hydrogen return line 26 Hydrogen dosing valve 27 Hydrogen return valve 28 Bridging line 28.1 Bypass valve 30 Air inlet path 31 Air inlet (filter) 32 air blowers 33 first air heat exchanger 34 air heaters 40 Air outlet path 41 second air heat exchanger 42 Air outlet 50 Fuel cell expansion 60 inverters 100 electrolyzer 101 Electrolysis cell 101.1 Water electrode input 101.2 Hydrogen electrode output 101.3 Air electrode input 101.4 Air electrode output 102 Hydrogen electrode 103 Air electrode 200 systems

Claims

[1] Electrolyzer (100) which can be set up for electrolysis operation and fuel cell operation, comprising: - an electrolysis unit which is arranged for electrolysis operation, comprising: ◯ a water inlet path (10) for feeding water into an electrolysis cell (101) of the electrolyzer (100), ◯ a hydrogen outlet path (20) for removing hydrogen from the electrolysis cell (101), ◯ an air inlet path (30) for feeding air into the electrolysis cell (101) and ◯ an air outlet path (40) for removing air from the electrolytic cell (101), - a fuel cell extension (50) which is designed to enable fuel cell operation in cooperation with the electrolysis unit, comprising: o a hydrogen return line (25) for supplying hydrogen to the water inlet path (10). [2] Electrolyzer (100) according to claim 1, characterized by , that the hydrogen return line (25) is connected to a hydrogen tank (22), wherein the hydrogen tank (22) is arranged in the hydrogen output path (20), and / or that the hydrogen return line (25) has a hydrogen metering valve (26), in particular a needle metering valve. [3] Electrolyzer (100) according to claim 1 or 2, characterized by , that the hydrogen return line (25) is connected to a water path connection point (16) or a hydrogen return valve (27), whereby a supply of hydrogen into a Water electrode input (101.1) of the electrolysis cell (101) is enabled. [4] Electrolyzer (100) according to one of the preceding claims, characterized bythat the fuel cell extension (50) comprises a water return valve (19.1) and a water return line (19) connected thereto for at least partially returning water to the water inlet path (10). [5] Electrolyzer (100) according to claim 4, characterized by , that the water inlet path (10) has a water evaporator (15) and a water treatment unit (12), in particular located upstream thereof, wherein the water return line (19) is connected to the water treatment unit (12), and wherein the water return valve (19.1) is arranged downstream or upstream of the water evaporator (15), wherein in particular a water cooling unit (19.2) is arranged between the water return valve (19.1) and the water treatment unit (12). [6] Electrolyzer (100) according to one of the preceding claims, characterized byin that the water inlet path (10) has a bridging line (28) for bridging a water evaporator (15) which is arranged in the water inlet path (10), whereby in particular hydrogen can be guided past the water evaporator (15), wherein preferably the bridging line (28) is connected to a bridging valve (28.1) which is arranged upstream of the water evaporator (15). [7] Electrolyzer (100) according to one of the preceding claims, characterized by in that the electrolyzer (100) has an inverter (60), in particular a bidirectional inverter, which is designed for electrolysis operation and fuel cell operation, wherein in particular the inverter (60) is designed to be switchable in order to switch between electrolysis operation and fuel cell operation. [8] A method for an electrolyzer (100) for switching between electrolysis operation and fuel cell operation, comprising: - Providing (110) an electrolyzer (100) according to one of the preceding claims, - controlling (120), in particular by a control unit (FCCU), the fuel cell extension (50) in order to carry out a change from an electrolysis operation to a fuel cell operation or from a fuel cell operation to an electrolysis operation and - Operating (130), in particular by a control unit (FCCU), the electrolyzer (100) in electrolysis mode or in fuel cell mode. [9] Method according to the preceding claim, characterized by that the control (120) has at least one of the following features: - Control of a water return valve (19.1), - controlling a water cooling unit (19.2), in particular switching the water cooling unit (19.2) on or off, - controlling a hydrogen metering valve (26), in particular to adjust a volume flow of recirculated hydrogen, - controlling a hydrogen recirculation valve (27), in particular opening or closing, in order to enable or prevent a volume flow of recirculated hydrogen into the water inlet path (10) and / or - Controlling a bypass valve (28.1), in particular to enable or prevent bridging of a water evaporator (15). [10] A computer program product comprising instructions which, when the computer program product is executed by a computer, cause the computer to implement the method according to one of the preceding claims 8 or 9. [11] Computer-readable data carrier in which instructions are stored which, when executed by a computer, cause the computer to carry out the method according to one of the preceding claims 8 or 9. [12] Control unit (FCCU), comprising a computing unit (CU) and a memory unit (MU) in which instructions are stored which, when at least partially executed by the computing unit (CU), carry out a method according to one of the preceding claims 8 or 9. [13] System (200) comprising an electrolyzer (100) according to any one of the preceding claims 1 to 7 and / or a control unit (FCCU) according to the preceding claim.

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