Steam electrolysis system and corresponding method for hydrogen production

Abstract

The present invention relates to a steam electrolysis system for producing hydrogen. The system comprises at least one feed gas arrangement with at least one steam electrolysis cell and at least one electric steam generator, at least one feed gas supply path for supplying a flow of feed gas containing at least steam from the at least one feed gas arrangement to the at least one steam electrolysis cell, at least one gas transfer device for removing hydrogen from the at least one steam electrolysis cell, and at least one external power source for operating the system. The at least one external power supply is electrically coupled to the at least one electric steam generator of the feed gas arrangement and to a minimum of one steam electrolysis cell. The at least one steam electrolysis cell and the at least one electric steam generator are electrically connected in parallel.

Description

[Technical field]

[0001] The present invention relates to a steam electrolyser system for producing hydrogen and a method for operating the steam electrolyser system. [Background technology]

[0002] A promising method for large-scale hydrogen production is high-temperature electrolysis with reactors deploying steam electrolysis cells such as solid-oxide electrolyser cells (SOE) or reversible steam electrolysis cells that can operate as electrolysers or fuel cells such as reversible solid oxide cells (rSOC). High-temperature electrolysis is more efficient than low-temperature electrolysis because part of the energy is provided as heat, which is usually cheaper than electricity. This heat can be provided from external heat dissipation sources such as incinerators, catalytic or chemical reactors, or nuclear reactors, or by Joule heat generated by the electrolysis reactor itself, and can be quantified as useful heat in the endothermic steam conversion reaction.

[0003] A steam electrolysis cell basically consists of a gas-impermeable electrolyte, a porous steam-receiving electrode, and a counter electrode with opposite polarity. Depending on the type of ions that can pass through the electrolyte, which in the case of solid oxide cells can be oxygen ions or protons, and can be facilitated by the electrocatalytic activity of the electrodes for the relevant electrochemical reaction, molecular hydrogen is formed at either the steam electrode or the counter electrode.

[0004] Solid oxide cells known today constitute a significant amount of ceramic and are therefore vulnerable to thermomechanical stresses. It is therefore preferable to have a uniform temperature distribution to guarantee the life of the electrolysis cell. This is achieved by passing an electric current through the cell so that the heat required for the endothermic electrolysis reaction is supplied by the internal electrical resistance (i.e. cell ohmic losses), the activation polarization and the concentration polarization associated with the transport of oxygen ions and electrons, or by controlled heating (if the cell operates in endothermic mode), controlled cooling (cells in exothermic mode), or by Joule heating generated in the electrolysis cell due to the concentration polarization and the internal electrical resistance associated with the transport of oxygen ions and electrons (i.e. cell ohmic losses). This operating point, where the electrolysis efficiency is 100%, is called the thermal neutral voltage.

[0005] As a specific example of a steam electrolyser, a known method for controlling the temperature of a solid oxide electrolyser (SOE) cell is by introducing air into the anode flow side and adjusting the air flow rate and air temperature on the counter electrode side (Chem. Eng. Trans., Vol. 61, 2017, p. 1069). A specific design using air control is described in Canadian Patent Application Publication No. 2626751, where the air required for the SOE stack is supplied by a gas turbine, the air is preheated using incident heat, and the SOE exhaust is recycled. Similarly, European Patent Application Publication No. 2674515 proposes controlling the temperature by integrating a feedback loop from the stack exhaust to a heat exchanger and a thermal regulation unit upstream of the SOE stack. Alternatively, US Patent Application Publication No. 20100200422 proposes using steam instead of air to control the SOE stack temperature. US Patent Application Publication No. 20170279134 proposes controlling the temperature of the SOE stack by integrating a heat exchanger in the interconnect with a separate conduit for the heat transfer fluid. In all of these cases, active heating and cooling increases the complexity of the system.

[0006] The concept proposed in US Patent Application Publication No. 201690244890 is rather simple, in that when the SOE stack operates in endothermic mode, additional heat is provided by redirecting a portion of the electrical energy from an intermittent / fluctating energy source to a resistive heater.

[0007] Moreover, there are several patents that require dynamic operation, which in principle is attractive for operation with intermittent power. US Patent No. 8,231,774 proposes dissipating excess heat when the SOE stack operates in exothermic mode and using its stored heat when the stack operates in endothermic mode. More specifically, phase change materials can be used for temperature control in dynamically operating SOE systems. US Patent Application Publication No. 20140329161 proposes interposing plates in an SOE stack containing such materials, while Dillig et al. propose applying so-called heat pipes (Fuel Cells, Vol. 14, 2014, p. 479). In all of these cases, temporary temperature deviations can be met, but this cannot be a solution if these deviations last too long. Thus, frequent cycling between exothermic and endothermic modes is necessary, and in this case it is not clear whether this can be balanced with the intermittent and unpredictable nature of renewable power (e.g., wind, solar).

[0008] WO2007048997 describes a system including two solid oxide electrolyzer stacks, one operating with high efficiency at a relatively high temperature and the other at a lower temperature. If the operating conditions are such that the first stack operates in endothermic mode (low current) at high temperature and therefore requires the input of thermal energy, this heat is provided by the second stack, which due to its higher internal resistance at the lower operating temperature still operates in exothermic mode even at low current. The advantage is that for both stacks, small thermal gradients can be realized, while temperature control in endothermic mode based on an external heat supply can be suppressed or avoided. The disadvantage is that the system relies on at least two stacks, with one stack (the one operating at the lower temperature) operating with a lower electrical efficiency.

[0009] Several patents disclose methods of operating SOE stacks at constant voltage. EP 3221494 discloses methods of operating SOE or SOFC stacks in potentiostatic or galvanostatic modes. More specifically, US 8163158 proposes controlling the stack voltage to approach the thermal neutral voltage under varying power input by varying the steam concentration in the hydrogen-steam supply while the total flow is preferably kept constant. The reactant concentration increases when more power is available and decreases when the available power is low. The patent further teaches that the hydrogen preferably comes from a recirculation loop, and that more hydrogen gas needs to be recirculated when the availability of electricity is low. This results in two disadvantages. First, the recirculation loop increases the complexity of the system, thus making it more expensive and less easy to control. Second, the recirculation of more hydrogen, which requires more power, results in an efficiency penalty to maintain the thermal neutral voltage when availability is lower. Summary of the Invention [Problem to be solved by the invention]

[0010] It is therefore at least one object of the present invention to overcome the shortcomings of the prior art. In particular, it is an object to provide a steam electrolyser system for producing hydrogen that is less complex than those known in the prior art and offers a simpler approach to temperature control. [Means for solving the problem]

[0011] The object is solved by the subject matter of the independent claims, and preferred embodiments are described in connection with the dependent claims.

[0012] A first aspect of the present invention relates to a steam electrolysis system for producing hydrogen. The system may also be suitable for producing oxygen and / or carbon monoxide and / or hydrogen. The system comprises at least one steam electrolysis cell comprising a positive electrode, a negative electrode and a gas impermeable electrolyte. The positive electrode is electrically connected to the negative electrode, and the negative electrode is electrically connected to the positive electrode. The electrolyte is arranged between the positive electrode and the negative electrode. The system comprises at least one feed gas arrangement comprising at least one electric steam generator, and at least one feed gas supply path for supplying a flow of feed gas comprising at least steam from the at least one feed gas arrangement to the at least one steam electrolysis cell. The system also comprises at least one gas transfer device for removing hydrogen from the at least one steam electrolysis cell, and at least one external power source for operating the system. The at least one external power supply is electrically coupled to the at least one electric steam generator of the feed gas arrangement and to a minimum of one steam electrolysis cell. The at least one steam electrolysis cell and the at least one electric steam generator are electrically connected in parallel.

[0013] Due to the parallel arrangement of the at least one steam electrolysis cell and the at least one electric steam generator, both devices can beneficially adjust their operating points independently without additional control, thus providing a simple and reliable system without the need for further control elements.

[0014] The system can include an auxiliary gas heater that can be configured in parallel with the at least one steam electrolysis cell and the at least one electric steam generator. The auxiliary gas heater can be an air preheater.

[0015] Such an arrangement is particularly advantageous when the high temperature steam electrolyser operates with electrically heated air.

[0016] The at least one steam electrolysis cell can be a solid oxide electrolysis cell or a reversible solid oxide cell (rSOC). The rSOC can operate as a solid oxide electrolyzer (SOE) or a solid-oxide fuel cell (SOFC). In a broader sense, the at least one steam electrolysis cell can be a ceramic oxide electrolysis cell, a phosphorous acid electrolysis cell, or a molten carbonate electrolysis cell.

[0017] At least one gas transfer device can be arranged upstream or downstream of the electrolytic cell. The gas transfer device, e.g. a blower, allows extraction of product gas from the cell by creating an overpressure upstream of the electrolytic cell or a negative pressure downstream of the electrolytic cell.

[0018] The system may also include more than one gas transfer device, for example one gas transfer device per electrode.

[0019] The electrolyser system may also include a gas separation device as described in more detail below.

[0020] Manufacturing limitations may constrain the size of a single cell, and therefore systems having two or more steam electrolysis cells per repeating element are preferred. The cells may be arranged in a stack, preferably with single planar cells placed in series within the stack or assembly. The stack may contain 10-200 repeating elements, preferably 50-100.

[0021] This stack arrangement allows for large scale production of hydrogen. In the case of a stack arrangement, at least one electric steam generator is preferably electrically connected in parallel with the stack arrangement.

[0022] The gas impermeable electrolyte may be an oxygen-ion conducting electrolyte. In the case of an oxygen-ion conducting electrolyte, the hydrogen-generating electrode produces hydrogen by removing oxygen ions from the steam supplied to it. This oxygen removal requires the presence of electrons, and therefore the hydrogen-generating electrode is the cathode. These oxygen ions then pass through the electrolyte and finally reach the counter electrode. If no gas is supplied to this counter electrode, oxygen gas is formed, which must be extracted from the reactor, for example by a gas transfer device. In other cases, a sweep gas is used to remove the oxygen gas. This sweep gas is a gas that does not react with oxygen gas, and can be, for example, air or another inert gas, such as nitrogen. The counter electrode can also be supplied with one or more reactants that react with the oxygen ions. Instead of supplying the hydrogen-forming electrode with pure steam, other compounds, such as carbon dioxide, can also be supplied to the hydrogen-forming electrode, where the oxygen ions are also removed from these molecules. In the case of CO2, this typically results in carbon monoxide.

[0023] The electrolyte may be a proton conducting electrolyte. In the case of proton conducting electrolytes, steam is fed to the counter electrode side. Protons are removed from the steam, which generates electrons. These protons then pass through the electrolyte. Upon reaching the hydrogen forming electrode, the protons recombine with the help of electrons into hydrogen gas, which must be extracted from the reactor or can be removed by a sweep gas that does not react with hydrogen. Instead of feeding steam to the counter electrode, other compounds that can act as proton donors can be co-fed as well. In all cases, the hydrogen generating electrode is the cathode, but in the case of oxygen ion conducting electrolytes, steam (and other oxygen ion donating gases) is fed to the cathode, whereas in the case of proton conducting electrolytes, steam (and other proton donating gases) is fed to the anode.

[0024] The oxygen ion conducting electrolyte may include zirconia and ceria based materials. Preferably, the electrolyte has a fluorite crystal structure, with MX2 being the predominant crystal type. It comprises a cubic unit cell with cations occupying face-centered positions and anions at tetrahedral positions. The cubic structure may be stabilized at room temperature by introducing acceptor based cations into the lattice, thereby introducing oxygen vacancies. The dopant cation may be yttrium (Y 3+ ), Erbium (Er 3+ ), Gadolinium (Gd 3+ ), Europium (Eu 3+ ), Dysprosium (Dy 3+ ), Scandium (Sc 3+ ), Calcium (Ca 2+ ), Ytterbium (Yb 3+ ) and magnesium (Mg 2+ ) Yttria stabilized electrolytes are preferred as they can operate in a high temperature range such as 600-1000° C. Doping can range from 5-10 mol %, preferably 8 mol %, with yttria doped zirconia being particularly preferred.

[0025] Advantageously, nickel (Ni)-based composites with yttria (YO)-stabilized zirconia (ZrO) (YSZ) and yttrium-doped barium cerate zirconate (BCZY), known for oxygen ion and proton conduction, are used as hydrogen electrodes. They are less cost intensive compared to platinum and exhibit reasonable electrochemical activity, chemical stability, and thermal expansion coefficients that are consistent with the other components of the cell.

[0026] The proton conducting electrolyte is preferably a perovskite-based oxide having a preferred formula of ABO3, where the A-sites are typically occupied by larger cations than the B-sites and similar in size to the O-site anions. The A-sites can be occupied by alkaline earth elements such as barium, strontium or calcium. The B-sites can be occupied by tetravalent elements such as zirconium or cerium. The compounds can be doped with trivalent elements such as yttrium, neodymium, samarium, ytterbium, indium, europium and gadolinium to enhance proton conductivity.

[0027] The counter electrode can be an air electrode that transports oxygen ions / electrons. The air electrode can be a composite of a perovskite material, such as strontium-doped lanthanum manganite (LSM), strontium-doped lanthanum cobaltite (LSC), or strontium-doped lanthanum cobalt ferrite (LSCF), and an oxygen ion or proton conducting material, such as yttria (Y2O3) stabilized zirconia (ZrO2) (YSZ), gadolinia (Gd) doped ceria (GDC), and yttrium (Y) doped barium zirconate cerate (BCZY).

[0028] Advantageously, the at least one electric steam generator is an electric steam boiler or an electrode steam boiler.

[0029] Typically, the term electric steam boiler can be used for steam generators that rely on sending power through resistance wires, while the term electrode steam boiler can be used for steam generators that rely on sending power through the water itself with positive and negative electrodes immersed in the water. Steam boilers can potentially accumulate steam produced in excess of the steam required by the process.

[0030] The characteristics of the at least one steam electrolysis cell combined with the resistive characteristics of the at least one electric steam boiler or electrode steam boiler allow for a simple and reliable system for operating the system without additional controls.

[0031] The system may comprise at least one gas separation device for the separation and / or purification of the different product gases. The at least one gas separation device may be based on any of the known gas separation techniques. For example, the techniques for separating gases include adsorption and / or absorption techniques. The techniques may involve temperature cycling (e.g., temperature swing adsorption) or pressure cycling (e.g., pressure swing adsorption). The techniques may be used to remove gaseous compounds or to reuse the adsorbent or absorbent. It may also be appropriate to remove gaseous contaminants, such as traces of undesirable gases, from gas streams where the adsorbent / absorbent is disposed of upon saturation.

[0032] The at least one gas separation device may also comprise a cryogenic separation element, such as a condenser, in which one of the compounds in the mixture of gaseous compounds undergoes a phase change while the remaining compounds remain in their gas phase.

[0033] Additionally, at least one gas separation device can also include a concentration-driven or electrically-driven membrane, which allows the passage of one gas compound, either in a gaseous state or as ions incorporated in the membrane material, while the other gas compound cannot pass through the membrane.

[0034] The system may comprise two or more gas separation devices, preferably two, preferably one for each electrode site.

[0035] The at least one gas separation device has the advantage of separating the product gas from the unreacted feed gas and allowing the recirculation of the unreacted feed gas back into the system, thus making the feed gas consumption more efficient. Additionally, the device allows for purification of the product gas.

[0036] Additionally, the system may include at least one steam condenser in fluid communication with the at least one electrolysis cell, preferably by at least one gas separation device. The steam condenser may be an additional part of the gas separation device or may be the gas separation device itself. For example, the steam condenser may be an efficient way to separate the converted gas from the unconverted steam, and may condense the steam back to the electric steam generator. The heat recovered from the steam condensation may be used to heat the water supplied to the electric steam generator, thus reducing the power demand of the electric steam generator.

[0037] Advantageously, at least one feed gas device comprises a co-feed gas supply. The co-feed gas supply can provide co-gas for reaction and / or auxiliary gas for system maintenance. For example, auxiliary gas such as hydrogen or any other reducing gas can be added for maintenance of each electrode, since it can help prevent oxidation of the electrode.

[0038] The auxiliary or co-gas can be a pure gas or a mixture of gases. The gas supply can be connected or connectable to one or more gas sources. The gas source can be a gas bottle filled with a reactant gas or a reactant gas mixture, a reactor for a chemical reaction, a gas station, or a gas tank, where gas is produced through a chemical reaction.

[0039] In general, the system according to the invention can be used to produce fuels such as hydrogen (H2), methane (CH4), methanol (CH3OH), dimethyl ether (CH3-O-CH3) or other fuels and / or ammonia (NH3) based on electrolysis. The conversion depends on the initial gas used and the operation mode of the cell. In SOE mode, for example, hydrogen (H2) can be produced from water (H2O) and carbon monoxide (CO) from carbon dioxide (CO2).

[0040] Advantageously, the power of the power source is variable power. When natural sources such as wind or sun are used for power generation, variable power can occur. However, in conventional SOE systems, operation with variable power requires additional control to prevent thermal runaway of the SOE. Due to the arrangement of the present invention, in particular the parallel connection of at least one electrolysis cell and at least one electric steam generator, no additional control is required and the system can safely operate with variable power. Thus, the system can be operated using renewable power generation sources.

[0041] The nominally available power sharing between the at least one electric steam generator and the at least one steam electrolysis cell may be in the range of 10:90 to 50:50, preferably 17:83. The exact power sharing between electrolysis (EL), steam generation (Ev), and auxiliary gas heater (Aux.Heat) may depend on the nominal operating point selected.

[0042] For example, the table below shows three examples corresponding to 60%, 70% and 80% steam conversion (SC). In the calculations, it was assumed that the SOE operates at isothermal conditions, the heat exchange network approach temperature is 100°C and the steam is delivered at 120°C. The conditions are chosen to be slightly above the point where the SOE is at thermal neutral voltage, which is considered beneficial to minimize the temperature gradient across the electrochemical cells to compensate for the heat losses that are inevitably present in the electrochemical reactor. In this case, auxiliary heating is used to bring the system to operating temperature and keep it in heat balance.

[0043] [Table 1]

[0044] Additionally, the system may include a supervisory device. The supervisory device may include elements that allow for monitoring of the system. It may include sensors for controlling or monitoring voltage or temperature to control the feed gas supply or exhaust gas. It may include a blower for moving gas through the system. It may include electronic elements to allow for remote control. It may include a computing system.

[0045] Preferably, the supervisory device is electrically connected or connectable to a non-variable power source. The supervisory device should preferably be prioritized with respect to power supply to ensure optimal operating conditions. Power can be provided from the electric utility or from backup power. This allows operation when variable power is temporarily unavailable and also allows the system to be kept in standby mode.

[0046] The system may further comprise power limiters, preferably arranged in at least one electric steam generator. These power limiters may further prevent overheating and thus avoid system failure. In particular, they may avoid system overloading, which may damage the equipment. The power limiters may be typically installed in the electric steam generator and / or in the solid oxide electrolyzer using power limiting techniques such as forward charging, constant current source or folded back characteristics. The power limiters do not limit the invention in any way, since the power between the power limiters and the electric steam generator and / or the steam electrolysis cell may be adjusted independently or freely, respectively.

[0047] Advantageously, the at least one steam electrolysis cell can or does operate in thermoneutral conditions. This arrangement can provide for the operation of the at least one electrolysis cell at a fixed potential. The lower voltage limit at which potentiostatic operation is possible is related to the Nernst potential, while the upper limit depends on what the materials used in the electrolysis cell can withstand. The best method is to operate the electrolysis cell at or near the thermoneutral voltage in order to balance the heat consumption of the electrolysis reaction with the Joule heat generated by the internal resistance of the electrolysis cell and the power passing through it.

[0048] The term "thermoneutral" refers to the operating condition where the heat generated in the electrolyzer due to internal electrical losses is equal to the heat required for the decomposition of steam. It usually refers to the cell voltage defined by dH / 2F, where dH is the molar enthalpy of the water splitting reaction and F is the Faraday constant. The cell voltage for the co-electrolysis of CO2 and HO in one gas mixture is usually in the range of 1.2-1.5 V.

[0049] Additionally, the system may include a recycle system for recirculating unconverted feed gas. The recycle system may include a condenser as described above. The recycle system may be connected or connectable to at least one gas separation device, such as a gas separation device as described above. The recycle system may also be part of a co-feed system, in which unconverted gas is re-fed to the system.

[0050] The system may further comprise an additional auxiliary water heater for pre-heating or evaporation of water. Preferably, the heat may come from internal heat recovery. The auxiliary heat source may act as a backup in case of any failure, ensuring stable operating conditions.

[0051] A second aspect of the invention refers to a method of operating a steam electrolysis system, the method comprising: providing a steam electrolysis system as previously described; - providing power to at least one electric steam generator of the steam electrolysis system and at least one supply gas device; applying a voltage to at least one steam electrolysis cell; - supplying a feed gas comprising at least steam from at least one feed gas device to one of the electrodes of at least one steam electrolysis cell; - enabling at least partial consumption of the feed gas in at least one steam electrolysis cell to produce hydrogen; - extracting hydrogen from the at least one electrolysis cell.

[0052] The step of applying a voltage to the at least one electrolysis cell can be performed before or after supplying the feed gas, but is preferably before supplying the feed gas. The voltage is preferably maintained upon removing the feed gas. The voltage may be in the range of 1.2 to 1.5 V per stack repeat element. The at least one steam electrolysis cell may be of the type described above.

[0053] The method allows for the following operation when a minimum of one electrolysis cell and at least one electric steam generator are electrically connected in parallel.

[0054] Initially, the at least one steam electrolysis cell, e.g., a solid oxide electrolysis (SOE) cell, does not receive steam and therefore has a high internal resistance. As a result, no current flows and is supplied to the at least one electric steam generator. The at least one electric steam generator starts producing steam at full power, which is then supplied to the at least one electrolysis cell. This reduces the internal resistance of the electrolyzer and therefore current flows to the electrolysis cell. Due to the power consumption of the electrolysis cell, less power is available to the at least one electric steam generator. Thus, less steam is produced and supplied to the electrolysis cell than in the first case. As less steam is available in the electrolysis cell, its internal resistance increases, thereby reducing the current flow through the electrolysis cell and more power is supplied to the at least one electric steam generator. This cycle is repeated until the at least one electric steam generator and the at least one electrolysis cell balance the amount of power each of them needs to receive. Thus, the system can regulate itself and does not require any additional control.

[0055] The feed gas may contain additional gases or gas mixtures as previously described. The system advantageously operates off of fluctuating power, thus allowing for the use of renewable sources of electricity that may be less stable in their power supply than traditional methods, such as from nuclear power plants.

[0056] Between 10% and 50% of the nominally available power may be provided to at least one electric steam generator, and between 50% and 90% of the nominally available power may be provided to at least one steam electrolysis cell.

[0057] The electrical operation of the supervisory device is preferably prioritized to allow complete control of the system.

[0058] The at least one steam electrolysis cell may be operated in a thermoneutral condition as described above.

[0059] Advantageously, the unconverted feed gas is recycled by recirculation to at least one electrolysis cell to avoid losses of unconverted gas and to allow efficient operation of the system.

[0060] In view of the present invention, it should be recognized that the resistance characteristics of an electrolytic cell are not constant, but rather depend on the operating conditions of the system, such as operating temperature, current density, steam utilization, and the like.

[0061] The following figures are presented to further illustrate the present invention and should not be taken as limiting. [Brief description of the drawings]

[0062] [Figure 1] FIG. 1 shows the connections of a basic steam electrolyser system according to the present invention. [Diagram 2] FIG. 1 shows a parallel connection of a steam electrolysis cell and an electric steam generator. [Diagram 3] FIG. 1 shows a parallel connection of an adjustable resistance steam electrolysis cell and an electric steam generator. [Figure 4] FIG. 1 shows SOE area specific resistivity as a function of vapor flow. [Diagram 5] FIG. 13 shows ASR as a function of SOE current density for steam feed flows of 2, 4, 8 or 12 Nml / min.cm2. [Figure 6] FIG. 1 illustrates the distribution of available power between an electric steam generator and an electrolysis cell. [Figure 7] FIG. 1 illustrates the distribution of available power between the electric steam generator and the electrolysis cell and auxiliary heating. [Figure 8] FIG. 1 shows the time evolution of the power sharing of the electric steam generator and the electrolysis cell. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0063] Like reference numbers refer to like elements of the present invention. Figure 1 shows the flow connections of a basic steam electrolyser system 1. The system comprises a blower 11 supplying air to the counter electrode and generating an overpressure upstream of the electrolyser, an auxiliary heat source 12 for preheating the air, a steam electrolysis cell in the form of a solid oxide electrolysis cell 13 receiving DC current from an AC-DC converter, an electric steam generator 14 and, if the system receives AC power, an AC-DC converter 15.

[0064] 2 shows the parallel electrical connection of an electric steam generator 14 and a solid oxide electrolysis cell 13. The fluctuating power is shown by the circled waveform.

[0065] In FIG. 3 an arrangement is shown in which an auxiliary heat source 12 for preheating the air is also arranged in parallel to the electrolysis cell 13 and the electric steam generator 14. This may be a preferred arrangement in which the high temperature steam electrolyzer operates with electrically heated air. The fluctuating power is shown as a circled waveform. The fluctuating power may be an AC current, which may come for example from the grid, wind, or hydro, or solar, or a DC current, for example PV. The electrolyzer operates at a fixed DC voltage, for example 400V. In the case of an AC power source, an AC-DC converter (not shown) may generate the fixed DC voltage required for the electrolyzer, for example 100V. Furthermore, AC-DC or DC-DC conversion (not shown) may be deployed to the steam generator and / or the electric air preheater. The electrolysis cell, the steam generator and the auxiliary air preheater may operate at a fixed voltage, but power fluctuations may result in current fluctuations.

[0066] Figure 4 shows how the total internal resistance of an electrolytic cell (expressed as area specific resistance ASR) can depend on the feed steam flow. It can be observed that the ASR generally decreases as the steam flow increases.

[0067] Figure 5 shows how the total internal resistance (expressed as ASR) of the electrolytic cell can depend on the current density. The ASR as a function of SOE current density is 2 (D), 4 (C), 8 (B) and 12 (A) Nml.min -1 .cm -2 5. It can be observed that the ASR generally increases with current density. In FIG. 5, a constant flow of hydrogen was added to the steam feed stream to maintain the steam electrode.

[0068] FIG. 6 shows the distribution of available power between the electric steam generator 14 and the electrolysis cell 13. Curve 41 shows the electric steam generator power divided by the total power (P ev / P tot ) is shown. The nominal power line is shown by 42. For the calculations of this graph and the following graphs 7-8, data from Figure 4 was used. Figure 4 shows how the ASR of an electrolytic cell varies with its operating conditions.

[0069] Figure 7 shows the distribution of available power between the electric steam generator 14 and the electrolysis cell 13, and the auxiliary electric heating 12, which is an electric air preheater (see Figure 3) arranged in parallel with the electrolyser and steam generator. Curve 51 shows the electric steam generator power divided by the total power (P ev / P tot ) which is different from curve 41.

[0070] FIG. 8 shows an example of the time evolution of the power sharing of the electric steam generator 14 and the electrolysis cell 13 with a given time variation of the power. The curve 71 shows the electric steam generator power divided by the total power (P ev / P tot ) Curve 72 shows the total power.

[0071] Interpreting curve 71, at low power, the electric steam generator is favored due to the higher initial resistance of the electrolytic cell at low steam flow. At nominal power, the power share settles down to 20% for the electric steam generator and 80% for the electrolytic cell because the internal resistance of the electrolytic cell decreases with steam flow. This graph simply shows the time variation of the power, but the same principles of operation apply to fluctuating power from renewable sources.

Claims

1. A steam electrolysis system for generating hydrogen, comprising: - at least one steam electrolysis cell comprising a positive electrode, a negative electrode and a gas-impermeable electrolyte, wherein the positive electrode is electrically connected to the negative electrode, the negative electrode is electrically connected to the positive electrode, and the electrolyte is disposed between the positive electrode and the negative electrode; - at least one supply gas device comprising at least one electrical steam generator; - at least one supply gas supply path for supplying a flow of supply gas containing at least steam from the at least one supply gas device to the at least one steam electrolysis cell; - at least one gas transfer device for removing hydrogen from the at least one steam electrolysis cell; - at least one external power source for operating the system ; The at least one external power source is electrically coupled to the at least one electrical steam generator of the supply gas device and the at least one steam electrolysis cell, and the at least one steam electrolysis cell and the at least one electrical steam generator are electrically connected in parallel.

2. The system according to claim 1, wherein an auxiliary gas heater is arranged in parallel with the at least one steam electrolysis cell and the at least one electrical steam generator.

3. The system according to claim 1, wherein the at least one electrical steam generator is an electrical steam boiler or an electrode steam boiler.

4. The system according to claim 1, further comprising at least one gas separation device for separating and / or purifying different product gases.

5. The system according to claim 1, further comprising at least one steam condenser fluidly connected to the at least one steam electrolysis cell.

6. The system according to claim 1, wherein the at least one supply gas device comprises a co-supply gas supply section.

7. The system according to claim 1, wherein the power of the power source is variable power.

8. The system according to claim 1, further comprising a monitoring device.

9. The system according to claim 8, wherein the monitoring device is electrically connected or connectable to a non-variable power source.

10. The system according to claim 1, further comprising a recycling system for recycling unreacted supply gas.

11. The system according to claim 1, further comprising an auxiliary water heater for preheating or evaporating water.

12. A method of operating a steam electrolysis system, comprising: - providing a steam electrolysis system according to claims 1 to 11; - supplying power to at least one electric steam generator of the steam electrolysis system and the at least one supply gas device; - applying a voltage to the at least one steam electrolysis cell; - supplying a supply gas containing at least steam from the at least one supply gas device to one of the electrodes of the at least one steam electrolysis cell; - enabling at least partial consumption of the supply gas in the at least one steam electrolysis cell to produce hydrogen; - extracting hydrogen from the at least one steam electrolysis cell. A method comprising the steps of:

13. The method according to claim 12, wherein the system operates with variable power.

14. The method according to claim 12, wherein the at least one steam electrolysis cell operates under thermoneutral conditions.

15. The method according to claim 12, wherein unreacted supply gas is recycled to the at least one steam electrolysis cell by recirculation.

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