Electrolytic system

Abstract

The electrolytic system 20 comprises an electrolytic cell 16 having at least one stack 10 of electrolytic cells 11, at least one fuel inlet 22 for fuel for at least one stack 10, at least two off-gas outlets 18, 24 for off-gas from at least one stack 10, where at least one of the at least two off-gas outlets is an anode-side off-gas outlet 18 for discharging anode off-gas from at least one stack 10, and at least one of the at least two off-gas outlets is a cathode-side off-gas outlet 24 for discharging cathode off-gas from at least one stack 10, means 142 for exchanging heat from cathode-side off-gas to fuel for at least one stack 10, and means 132 for exchanging heat from anode-side off-gas to fuel for at least one stack 10, wherein the stack 10 is configured to output anode-side off-gas under normal operating conditions, and at least 50% by weight of the anode-side off-gas is oxygen produced by the electrolytic cell 16. The present invention further relates to a method for operating an electrolytic system 20, comprising the steps of: calculating the fluid pressure on one side of at least one electrolytic cell 11 or at least one stack 10; and controlling the pressure on the other side of at least one electrolytic cell 11 or at least one stack 10 such that the pressure on the cathode side of the electrolytic cell 11 or each electrolytic cell 11 or stack 10 or each stack 10 is higher than the pressure on the anode side of the electrolytic cell 11 or each electrolytic cell 11 or stack 10 or each stack 10.

Description

Technical Field

[0001] The present invention relates to an electrolysis system, and more particularly to an electrolysis system capable of achieving high energy efficiency.

Background Art

[0002] An electrolysis system has one or more electrolytic cells, each of which may comprise one or more stacks of electrolytic cells (also known as regenerative fuel cells). The electrolytic cell is used to split a source fluid (the "fuel"), which is usually steam or carbon dioxide, into its components, and for this purpose requires a power source to supply current and voltage across / through at least one stack of the electrolytic cells. As an example, the electrolytic cell can be used to produce hydrogen and oxygen from water, or carbon monoxide and oxygen from carbon dioxide, respectively, by electrolysis.

[0003] The collection or use of the generated oxygen is important because it can be utilized in industrial and medical applications, among many other uses. The collection of carbon monoxide, and subsequent storage and / or distribution, is also important because it is useful in many chemical processes. Furthermore, the collection of hydrogen, and subsequent storage and / or distribution, is important because hydrogen is a fuel commodity that can serve in the competition for decarbonization and achieving net-zero targets. The generated hydrogen can be used as a fuel for combustion or as a fuel in a fuel cell system to achieve an electrolysis reaction in the fuel cell and recombine hydrogen with oxygen, resulting in electrical and heat outputs. Hydrogen can further have many other uses.

[0004] It is also important to combine the electrolytic cell with a green energy source, as this significantly contributes to the green credentials of the electrolytic cell, particularly in the use of the electrolytic cell in hydrogen capture, and thus can accelerate the achievement of net-zero and decarbonization targets.

[0005] Given the importance of electrolytic cells in meeting net-zero and decarbonization targets, any improvement in the efficiency or lifespan of electrolytic cells across industries is considered important and valuable. [Overview of the project] [Problems that the invention aims to solve]

[0006] In a typical electrolysis system, oxygen is produced at the anode side of the electrolytic cell and is a product of the electrolysis of both water and carbon dioxide. In a typical electrolytic cell, this oxygen is pushed out of the cell by a "sweep gas." The sweep gas is typically pumped through the electrolytic cell via a fan, but the energy required to power the fan reduces the overall efficiency of the system. This can be called a "parasitic load."

[0007] Furthermore, it is advantageous to raise the temperature of the sweep gas to the stack's operating temperature before passing it through to the anode side of the electrolytic cell. This is to avoid lowering the stack's temperature and reducing its operating efficiency. Most electrolytic cell stacks have a narrow optimal operating temperature range for optimal efficiency, and therefore, it is typically desirable to operate at a substantially constant temperature within that range. Moreover, temperature deltas (variations / differences) across the entire stack or electrolytic cells can create thermal stress within the stack or cells, which can then lead to premature failure of the stack or cells, or leakage within the stack due to the thermal expansion properties of the materials involved. However, heating the sweep gas in this way also increases the energy load on the electrolytic system, i.e., an additional "parasitic load."

[0008] Furthermore, sweep gases, particularly in medium- or high-temperature electrolytic cells, i.e., electrolytic cells with operating temperatures exceeding 400°C, can introduce contaminants into the stack or, otherwise, cause stack degradation through the interaction of the sweep flow gas with the system's materials. These contaminants or degradation / interactions can shorten the stack's operational lifespan through anode damage or through the sweep flow pipeline manifold or piping. In fact, systems with such sweep flows always encounter gradual degradation of the anode or sweep flow pipeline manifold or piping during electrolytic cell use, but contaminants such as dust particles or other trace elements of toxic or reactive substances can exacerbate the rate of degradation.

[0009] Therefore, an object of the present invention is to provide an electrolytic system having a longer service life, preferably an electrolytic system in which parasitic loads associated with sweep gas are reduced, removed, or avoided. [Means for solving the problem]

[0010] According to a first aspect of the present invention, an electrolytic system comprising an electrolytic cell having at least one stack of electrolytic cells, wherein the electrolytic cell is At least one fuel inlet for fuel for at least one stack, At least two off-gas outlets for off-gas from at least one stack, wherein at least one of the at least two off-gas outlets is an anode-side off-gas outlet for discharging anode off-gas from at least one stack, and at least one of the at least two off-gas outlets is a cathode-side off-gas outlet for discharging cathode off-gas from at least one stack, An electrolytic system comprising means for exchanging heat from cathode-side off-gas to fuel for at least one stack and means for exchanging heat from anode-side off-gas to fuel for at least one stack, An electrolytic system is provided in which the stack is configured to output an anode-side off-gas under normal operating conditions, and at least 50% by weight of the anode-side off-gas is oxygen produced by the electrolytic cell.

[0011] In some embodiments, the means for exchanging heat is at least one heat exchanger. Other well-known mechanisms for heat exchange include condensers, evaporators, and boilers, in which heat can be exchanged by any one or more of convection, conduction, thermal radiation, and evaporative cooling.

[0012] In some embodiments, a single means for heat exchange, having separate fluid paths for the cathode off-gas, anode off-gas, and fuel (to avoid mixing them), may be used to achieve heat exchange of both the cathode off-gas and anode off-gas to the fuel. However, more generally, at least two separate heat exchange means are provided. For example, at least one heat exchanger may be provided for heat exchange of the cathode off-gas to the fuel, and at least one further heat exchanger may be provided for heat exchange of the anode off-gas to the fuel.

[0013] In some embodiments, the sweep flow gas is used to push oxygen from the anode side of the electrolytic cell, and the sweep flow gas is less than 50% by weight of the anode side off-gas. The sweep flow gas may be an externally supplied sweep flow gas, or it may be recirculation of at least a portion of the anode side off-gas, or a recirculated gas within the electrolytic system, such as oxygen produced by the electrolytic cell.

[0014] In some embodiments, the stack is configured to generate an anode-side off-gas in which, under normal operating conditions, the weight ratio of oxygen produced by the electrolytic cell is greater than the proportion of sweep flow gas.

[0015] In some embodiments, the anode-side off-gas has a weight ratio exceeding 50%, 60%, 70%, 80%, or 90% of the oxygen produced by the electrolytic cell. Thus, the anode-side off-gas contains sweep flow gas at a weight ratio of 50%, 40%, 30%, 20%, or 10% or less, respectively. Therefore, the amount of sweep flow gas entering the stack or the anode side of each stack is reduced in order to extract the anode-side off-gas from the stack or from any of the electrolytic cells within that stack. Compared to systems of the prior art, the sweep flow gas can be reduced to up to 50% of the flow from the stack. More preferably, the sweep flow gas can be reduced to 40%, 30%, 20% or less, most preferably 10% or less, of the flow from the stack.

[0016] In some embodiments, the sweep gas is air, nitrogen, carbon dioxide, or oxygen. Oxygen has the advantage of allowing high-purity oxygen, potentially higher than 85% by weight, potentially higher than 90% by weight, even more potentially higher than 95% by weight, or even more than 99% by weight, to be discharged from the electrolysis system as an anode-side off-gas.

[0017] In some embodiments, the stack is configured to operate without a sweep gas flow for extracting anode-side off-gas from the stack or from any of the electrolytic cells within the stack. In some embodiments, this configuration is used selectively, for example, under normal operating conditions, i.e., steady-state conditions, rather than during startup. During startup, heated sweep gas can be used to assist the startup process.

[0018] In some embodiments, the stack is configured with a control system for operating the stack at substantially a thermally neutral voltage.

[0019] In some embodiments, the control system is configured to operate the stack on one side of the thermal neutral voltage. This allows for an offset to be introduced to bring the entire fuel cell stack to thermal equilibrium.

[0020] In some embodiments, the control system operates the stack over a given period of time with a voltage that can fluctuate on both sides of the thermal neutral voltage and through the thermal neutral voltage.

[0021] This invention eliminates or reduces reliance on sweep gas for collecting oxygen generated on the anode side of an electrolytic cell. This extends the service life of each electrolytic cell because less gas passes through the anode side of each electrolytic cell. Furthermore, the anode off-gas becomes a gas of higher purity (i.e., a higher oxygen content), and typically even a pure oxygen stream. Therefore, the anode off-gas has higher commercial value.

[0022] Furthermore, considering the resulting enclosure of such a sweep flow circuit, the possibility of external contaminants entering the sweep flow circuit is lower or zero, thus reducing the causes of wear, corrosion, or other deterioration on the anode side of the electrolytic cell stack, particularly the anode of the electrolytic cell and the manifold or piping of the sweep flow circuit.

[0023] Furthermore, since the anode-off gas is essentially pure oxygen from the anode side of the stack, and pure oxygen in the case of a zero-sweep gas solution, the commercial value of the generated gas can be higher from the moment it is extracted. Also, since the anode-off gas can already be sufficiently pure for such commercial use, it requires little to no downstream processing before being stored or distributed for downstream use.

[0024] Furthermore, since the electrolysis cell operates at or near the thermal neutral voltage, there is no need, or substantially no need, to provide or operate an auxiliary heater in the electrolyzer to maintain the operating temperature within the stack(s). Since electric heaters are relatively inefficient, avoiding their use increases the efficiency of the electrolysis system by removing the parasitic load of the heaters.

[0025] Thermal neutrality is achieved by controlling the voltage (or current, i.e., the supplied power) supplied to the electrolysis cell (or its stack). By operating thermally neutrally, the voltage drop is sufficient not only to drive the reaction but also to maintain a constant stack temperature. Thus, the electrolysis system can maintain the temperature of the incoming fuel (usually steam or carbon dioxide) at the optimal operating temperature of the stack through a heat exchanger.

[0026] In some embodiments of the present invention, each electrolysis cell or stack of electrolysis cells operates substantially thermally neutrally and at a constant current (i.e., with a constant current).

[0027] In some embodiments, the continuous operation of the electrolysis system under normal operating conditions is achieved without an auxiliary electric heater in either the manifold or piping of the sweep flow pipeline or the fuel flow pipeline within the electrolyzer, or within the electrolysis system. However, in some embodiments, such an auxiliary heater is provided, for example, to assist in starting the electrolysis system. Such an auxiliary electric heater is preferably disconnected, turned off, or rendered ineffective thereafter from a pipeline or the like during the normal operating state of the electrolyzer, stack(s), or electrolysis cell.

[0028] In some embodiments, normal operating conditions include operating the stack within a target range of operating temperature for a given stack chemistry, i.e., not being started or stopped. Parameters of normal operating conditions may additionally or alternatively include any one or more of the target ranges of operating gas pressure, sweep flow rate, voltage, current, power consumption, and electrical resistance for a given stack chemistry, a given stack size, or both, while not starting or stopped.

[0029] In some embodiments, the electrolytic cell is a solid oxide electrolytic cell.

[0030] In some embodiments, the electrolytic cell comprises two or more stacks. The stacks may be electrically connected in parallel or in series.

[0031] In some embodiments, the means for exchanging heat comprises one or more heat exchangers.

[0032] In some embodiments, two heat exchangers are arranged in series along a fuel supply pipeline. These two heat exchangers may be coupled to each other and thus essentially form a single heat exchanger in which heat is exchanged from two outlets to inlet fuel.

[0033] In some embodiments, the means for exchanging heat comprises two heat exchangers arranged in parallel across separate fuel supply lines. These two heat exchangers may be coupled to each other and thus essentially form a single heat exchanger in which heat is exchanged from two outlets to inlet fuel.

[0034] In some embodiments, additional switching means are provided to switch the arrangement of two heat exchangers across a fuel supply line(s) from a series configuration to a parallel configuration and vice versa. This is particularly applicable to transient operating conditions of the electrolytic cell, such as starting, stopping, warming up, cooling, and temperature changes (for example, one arrangement may be thermodynamically preferable to the other). Having switching means allows for the selection of the most efficient arrangement under given operating conditions. For example, it may be advantageous to arrange the heat exchangers in series during starting or warming up to quickly heat both heat exchangers before switching to parallel during normal operation for steady-state operation, and / or for more uniform heating. This is particularly useful in zero-sweep flow conditions (in which case the heat exchanger between the oxygen outlet and fuel inlet is inactive until the electrolytic cell is heated).

[0035] According to one embodiment, an electrolytic system is provided comprising a fuel inlet and at least one electrolytic cell. There are two outlets from the electrolytic cell, which exchange heat with the fuel inlet via a first heat exchanger and a second heat exchanger. The system further comprises switching means (in the form of one or more valves) adapted to switch the inlet fluid flow so that the heat exchangers are arranged either in series or in parallel.

[0036] The switching mechanism may comprise a series of valves and / or additional piping. For example, the valves may include two-way or three-way valves.

[0037] Both supply lines can be connected to the same stack. These supply lines can be joined before the fuel inlet, or they can supply fuel to separate fuel inlets.

[0038] In some embodiments, the fuel is water. Typically, the fuel is steam, and the external steam source may be supplied from a water source. When supplied from a liquid water source, the electrolytic system may include a steam generator for converting liquid water into steam.

[0039] In some embodiments, the fuel is carbon dioxide. In other embodiments, the fuel is a mixture of water (steam) and carbon dioxide.

[0040] The present invention is particularly applicable to electrolytic cells in medium-temperature and high-temperature electrolytic cell sectors (>400°C and >750°C, respectively). The present invention may also have specific applications at lower electrolytic cell temperatures, although operating at temperatures above 100°C and therefore operating with steam rather than water. This is because the heat exchanger can preheat the fuel before it enters at least one stack. However, water-based electrolytic cells (with operating temperatures below 100°C) can also benefit from the present invention. Similarly, carbon dioxide electrolytic cells can also benefit from the present invention.

[0041] In some embodiments, at least one electrolytic cell is a solid oxide electrolytic cell, i.e., the electrolytic cell or the electrochemically active region of each electrolytic cell is a solid oxide. Solid oxide electrolytic cells (SOECs) typically operate in the temperature range of 400°C to 900°C (or 400°C to 700°C for some chemical properties), or more specifically, in the temperature range of 450°C to 650°C. Such electrolytic cells operating at operating temperatures of 400°C to 750°C are sometimes called medium-temperature solid oxide electrolytic cells, or IT-SOECs.

[0042] The advantage of steam-based electrolytic cells is that steam electrolysis (particularly medium and high-temperature steam electrolysis above 400°C) efficiently produces hydrogen because the high-temperature environment can reduce the power requirements for electrolysis of water molecules from steam compared to the electrolysis of liquid water. Furthermore, higher temperatures can relatively increase the reaction activity in the electrolytic cell compared to the reaction activity of liquid water. Thus, the present invention is very suitable for use with solid oxide electrolytic cells (or SOECs) operating at temperatures above 400°C (commonly known as medium-temperature SOECs, or high-temperature SOECs when above 750°C).

[0043] There are many possible forms of SOECs that utilize different electrochemically active electrolyte chemistry properties. For example, three well-known electrolyte materials are yttria-stabilized zirconia (YSZ), scandia-stabilized zirconia (ScSZ), and gadolinium-doped ceria (GDC or CGO).

[0044] Due to the operating temperature of SOECs (typically exceeding 400°C), water passing through the electrolytic cell will vaporize into high-temperature steam if it is not supplied at such a temperature and in such a state. However, heat exchangers typically preheat the steam to that state.

[0045] In some embodiments, the electrolytic system further comprises a hydrogen / vapor separator positioned after the cathode-side off-gas outlet. The hydrogen / vapor separator may be located between the cathode-side off-gas outlet and means for exchanging heat from the cathode-side off-gas to, for example, fuel. The hydrogen / vapor separator may be used to separate the cathode off-gas into (a) vapor for recirculation to the electrolytic cell (which may be done via a vapor recirculation pipeline) and (b) hydrogen for passing through its heat exchanger. Using the hydrogen / vapor separator and its heat exchanger, the hydrogen can be substantially dried before being sent for subsequent collection or distribution. This results in a hydrogen supply of increased value because the hydrogen is already purer and substantially or largely separated from the vapor.

[0046] In some embodiments, the hydrogen / vapor separator is a cooling / reheating loop, which, for example, uses one or more regenerative heat exchangers to cool the humid hydrogen to a temperature below the dew point of water to condense the vapor, and then reheat it. In some embodiments, this occurs before the heat exchange means. In other embodiments, this occurs after the heat exchange means. In some embodiments, the heat exchange means provides this function in addition to heat exchange to the fuel.

[0047] In some embodiments, the hydrogen / vapor separator is a membrane filter. In some embodiments, the membrane filter operates with a pressure delta on both sides of the membrane. For example, the mixture side (vapor and hydrogen) may be at a pressure substantially corresponding to the pressure of the electrolytic cell or stack, while the other side through which hydrogen can pass may be at a reduced pressure, e.g., ambient pressure or a pressure lowered by a pump. In some embodiments, the pressure delta is provided by the electrolytic system operating at a pressure higher than the ambient pressure around the electrolytic system or on the hydrogen outlet side of the membrane filter. For example, the electrolytic system may operate at a pressure greater than 2 bar (or 1 bar g), while the hydrogen outlet side of the membrane filter may be at ambient pressure (e.g., about 1 bar or 0 bar g).

[0048] According to a further aspect of the present invention, an electrolytic system is provided comprising an electrolytic cell having at least one stack of electrolytic cells, the electrolytic cell comprising at least one fuel inlet for fuel for at least one stack, and at least two off-gas outlets for off-gas from at least one stack, wherein at least one of the at least two off-gas outlets is an anode-side off-gas outlet for discharging anode off-gas from at least one stack, and at least one of the at least two off-gas outlets is a cathode-side off-gas outlet for discharging cathode off-gas from at least one stack, and the electrolytic system further comprises a hydrogen / vapor separator positioned between the cathode-side off-gas outlet and a heat exchanger for exchanging heat from the cathode-side off-gas, wherein the hydrogen / vapor separator is a membrane filter operating with a pressure delta on both sides of the membrane.

[0049] In some embodiments, the electrolytic cell further comprises means for exchanging heat from the cathode-side off-gas to the fuel for at least one stack, and means for exchanging heat from the anode-side off-gas to the fuel for at least one stack.

[0050] In some embodiments, the stack is configured with a control system for operating the stack at substantially a thermally neutral voltage.

[0051] In some embodiments, the stack is configured to output an anode off-gas under normal operating conditions, and at least 50% by weight of the anode off-gas is oxygen produced by the electrolytic cell.

[0052] The characteristics of the first embodiment may also be present in this further embodiment.

[0053] In some embodiments, these electrolytic systems are (a) To extract anode-side off-gas from the stack or from any of the electrolytic cells within the stack, an external sweep flow gas supply exceeding 10% by weight is configured to produce oxygen with a volume purity exceeding 90% by weight by operating at normal operating temperatures without entering the stack or the anode side of each stack, or (b) configured to operate at normal operating temperature without an external sweep gas flow for extracting anode-side off-gas from the stack or any of the electrolytic cells within the stack. It may be either one or both of the above. Therefore, the system may be a system according to the above-described embodiment.

[0054] In some embodiments of the electrolytic system of any of these aspects, the membrane filter is a palladium-hydrogen membrane that operates at temperatures exceeding 400°C. Therefore, the membrane filter is suitable for use with the SOEC electrolytic system.

[0055] According to a preferred embodiment, the electrolytic system is configured to operate at a pressure exceeding 2 barg. The increased pressure relative to the ambient pressure allows the membrane filter to operate efficiently without an external pump, thereby avoiding further parasitic loads on the system.

[0056] A further aspect of the present invention provides a method for discharging substantially pure oxygen off-gas from an electrolytic cell of an electrolytic system, comprising the step of generating a pressure difference between the inside of the electrolytic cell and an inlet or outlet for the off-gas from the electrolytic cell, wherein the pressure difference is generated by the production of oxygen through electrolysis by the electrolytic cell.

[0057] In some embodiments, the method operates without an external sweep gas, or with a sweep gas that is substantially pure oxygen, and as a result, the purity of the oxygen produced determines the purity of the oxygen off-gas. Since electrolytic processes on fuels such as water and carbon dioxide produce substantially pure oxygen at the anode side of the electrolytic cells in the electrolytic cell, the oxygen off-gas can be substantially pure oxygen.

[0058] In some embodiments, the method includes the step of controlling oxygen discharge by a back pressure regulator. In other embodiments, the oxygen generation rate by the electrolytic cell is such that the electrolytic cell self-regulates to maintain a pressure difference or accumulates enough pressure to self-discharge from the electrolytic cell when sufficient oxygen has been produced.

[0059] A further aspect of the present invention provides a method for generating substantially pure oxygen off-gas from an electrolytic cell of an electrolytic system, comprising the steps of generating a pressure difference between the inside of the electrolytic cell and an inlet or outlet for substantially pure oxygen off-gas from the electrolytic cell, and controlling the discharge of oxygen with a back pressure regulator. With a back pressure regulator, the back pressure regulator can be opened to allow the discharge of oxygen that has passed through the back pressure regulator when sufficient pressure has accumulated or when sufficient pressure is maintained by the rate of oxygen generation by the electrolytic cell.

[0060] For each method, in some embodiments, the weight purity of the substantially pure oxygen off-gas is at least 90%. More preferably, the weight purity is at least 95%, or even more preferably at least 99%.

[0061] Typically, the substantially pure oxygen off-gas from the electrolytic cell is discharged at the anode side of the electrolytic cell.

[0062] In some embodiments, the oxygen off-gas has a weight ratio exceeding 50%, 60%, 70%, 80%, or 90% of the oxygen produced by the electrolytic cell. Thus, the anode-side off-gas has sweep flow gas in proportions of 50% by weight, 40% by weight, 30% by weight, 20% by weight, or 10% by weight or less, respectively, and therefore the sweep flow gas entering the electrolytic cell for the extraction of oxygen off-gas from the electrolytic cell is reduced to a maximum of 50%, more preferably 40% or less, 30% or less, 20% or less, and most preferably 10% or less of the flow from the stack compared to a conventional system.

[0063] In some embodiments, the sweep gas is air, nitrogen, carbon dioxide, or oxygen. Oxygen has the advantage of allowing high-purity oxygen to be discharged from the electrolysis system as an oxygen off-gas, potentially higher than 85% by weight, potentially higher than 90% by weight, even more potentially higher than 95% by weight, or even more than 99% by weight, even when the amount of sweep gas passing through the electrolysis cell exceeds 50% of the amount discharged from the anode side of the electrolysis cell.

[0064] The electrolysis system may include two or more electrolytic cells.

[0065] In some embodiments, the electrolytic cell or each electrolytic cell comprises a stack of at least one electrolytic cell.

[0066] In some embodiments, the electrolytic cell or each electrolytic cell has at least one fuel inlet for fuel for at least one stack. In typical embodiments, the fuel is either water or vapor, or carbon dioxide. Neither an inlet nor an outlet for substantially pure oxygen off-gas from the electrolytic cell is connected to the fuel inlet.

[0067] In some embodiments, the electrolytic cell or each electrolytic cell does not have an inlet for flushing a substantially pure oxygen off-gas, resulting in a pressure difference between the inside of the electrolytic cell and the outlet for the substantially pure oxygen off-gas from the electrolytic cell.

[0068] In some embodiments, the electrolytic cell or each electrolytic cell comprises at least two off-gas outlets for off-gas from at least one stack. At least one of the at least two off-gas outlets is an anode-side off-gas outlet for discharging anode off-gas from at least one stack. At least one of the at least two off-gas outlets is a cathode-side off-gas outlet for discharging cathode off-gas from at least one stack. The anode-side off-gas outlet for discharging anode off-gas from at least one stack may be configured to connect to an outlet for substantially pure oxygen off-gas, where the anode off-gas is substantially pure oxygen off-gas.

[0069] In some embodiments, the electrolytic system is an electrolytic system according to a first embodiment or further embodiment of the present invention, or any of the various embodiments thereof.

[0070] In some embodiments, a return loop, which may be a continuously open loop (or may be selectively openable and closed), is provided between the sweep inlet of at least one stack and the anode-side off-gas outlet of that stack. This allows the oxygen produced by the stack to circulate within the loop without being discharged from the electrolysis system. This helps to avoid oxygen stagnation, which could otherwise lead to combustion due to reaction with leaked hydrogen near the stagnation point.

[0071] In some embodiments, the back pressure regulator is a pressure relief valve.

[0072] In some embodiments, the back pressure regulator is located on the outlet side of the electrolytic cell.

[0073] The pressure difference is typically generated by the production of oxygen through electrolysis by the stack or the electrolytic cells within each stack. However, in some embodiments, the pressure difference may be initially generated or assisted by an auxiliary sweep gas to be maintained between the inside of the electrolytic cell and an inlet or outlet for a substantially pure oxygen off-gas from the electrolytic cell.

[0074] In some embodiments, the electrolytic cell pressure is 0.1 barg to 10 barg, preferably 0.1 barg to 5 barg, and more preferably 0.2 barg to 1 barg. This can be the pressure on both the anode and cathode sides of the electrolytic cell.

[0075] In some embodiments, the pressures on the anode and cathode sides of the electrolytic cells within the electrolytic cell are substantially balanced, preferably with a dispersion not exceeding + / - 10% of the anode barg pressure. In some embodiments, the dispersion does not exceed 0.1 barg. This equilibrium, or substantially equilibrium, results in less stress and strain on the electrochemically active layer of the electrolytic cells and on the interconnections between adjacent electrolytic cells.

[0076] According to another embodiment, there is a method for operating an electrolytic system comprising an electrolytic cell having an electrolytic cell having at least one stack of electrolytic cells, each having an anode side and a cathode side, the method comprising the steps of: calculating the fluid pressure on one side of at least one electrolytic cell or at least one stack; and controlling the fluid pressure on the other side of at least one electrolytic cell or at least one stack such that the pressure on the cathode side of the electrolytic cell or each electrolytic cell or stack or each stack is higher than the pressure on the anode side of the electrolytic cell or each electrolytic cell or stack or each stack.

[0077] In some embodiments, the pressure on the anode (oxygen) side of an electrolytic cell or each electrolytic cell or stack or each stack is controlled to be higher than the pressure on the cathode (fuel) side of an electrolytic cell or each electrolytic cell or stack or each stack. Controlling the anode pressure to be higher than the cathode pressure means that if a leak occurs between the anode and cathode sides, the pressure difference will feed oxygen and / or sweep gas into the fuel (cathode) volume rather than hydrogen (produced on the fuel side of the electrolytic cell) into the oxygen (anode) volume. This prevents the undesirable accumulation of flammable gases (e.g., hydrogen) in the high-oxygen environment of the electrolytic cell, thus enabling safe operation in the event of a leak. Furthermore, if combustion of oxygen and hydrogen occurs due to a cross-leak, the combustion process generates vapor, which can accelerate the deterioration of the air-side components of the electrolytic cell; therefore, it is beneficial to allow this combustion to occur in the fuel volume rather than the air volume. Typically, the air is dried before being supplied to the electrolytic cell to mitigate the effects of humidity. Similarly, reducing or eliminating sweep flow reduces the anode's exposure to moisture.

[0078] A further advantage is that oxygen (or nitrogen, if air or nitrogen is used as the sweep gas) is a larger molecule than hydrogen, and therefore the higher pressure on the oxygen side reduces the leakage rate. It should be noted that many applications of hydrogen, particularly ammonia production, also utilize nitrogen, although nitrogen leaking to the fuel side may require subsequent separation. It should be understood that this concept applies to all electrolytic cells and operating methods (e.g., with or without sweep flow) regardless of the oxygen concentration of the anode gas (as long as the oxygen concentration on the anode side is higher than 0%).

[0079] In some embodiments, the pressure difference between the cathode and anode sides of the electrolytic cell is controlled to be as low as reasonably possible while still maintaining a higher pressure on the cathode side than on the anode side. Optionally, the anode pressure is 50 mbar or less, optionally 40 mbar or less, optionally 30 mbar or less, optionally 20 mbar or less, or optionally 10 mbar or less than the cathode pressure. In this way, the anode pressure is actively controlled to always be higher than the cathode pressure.

[0080] In the above embodiments, a pressure control device or system communicating with one or more pressure calculating devices or systems controls the pressure on one side of at least one electrolytic cell or stack such that the pressure on the cathode side is higher than the pressure on the anode side. The pressure calculating device or system can calculate the pressure on one or both sides of the cathode and anode sides of at least one electrolytic cell or stack, or the pressure on one or both of the fluid inlet and fluid outlet for each cell or each of its stacks. The pressure calculating device or system may be a pressure sensor or a fluid flow sensor. The pressure difference may be controlled by a back pressure regulator. In some embodiments, it may be preferable to calculate the pressure on the cathode (fuel) side and control the pressure on the anode (oxygen) side. This is because the fuel pressure may be determined by an external system and therefore may not be directly or completely controllable by the electrolytic system.

[0081] In some embodiments, the method includes a starting or transition cycle in which air, carbon dioxide, or nitrogen is used as a substitute for fuel, passed through the cathode side of the electrolytic cell, when the electrolytic cell has an internal temperature below 100°C. This is because vapors can condense below 100°C, particularly in pressurized vessels, and the resulting water or moisture can damage the cathode of the electrolytic cell.

[0082] Air / carbon dioxide / nitrogen may be preheated, for example, in another industrial process (and therefore waste heat), or by another industrial process or heat source (e.g., via a heat exchanger), or by an electric heater, or by one or more of these heat sources in an electrolytic system.

[0083] In some embodiments, the method operates without an external sweep flow, and therefore a sweep inlet is not required in the electrolytic system. However, in some embodiments, the method includes the step of recirculating a portion of the generated oxygen as a sweep gas within the electrolytic system (or within the electrolytic cell or stack). For this purpose, an internal manifold or piping may also be used, but the aforementioned loop may be provided.

[0084] In some embodiments, the sweep flow, whether oxygen recirculation or external sweep flow gas, is driven through the electrolytic cell or stack by one or more pumps. In some embodiments, the pumps are thermal pumps.

[0085] In some embodiments, a removable preheater is provided in the stack, electrolytic cell, or electrolytic system. The removable preheater may be an electric heater or a heat exchange heater. The preheater may be provided on the anode side of the electrolytic cell or stack, but it is preferable that it be provided only on the cathode side of the electrolytic cell or stack.

[0086] In some embodiments, the electrolytic system further comprises a hydrogen / vapor separator positioned between a cathode-side off-gas outlet and a heat exchanger for exchanging heat from the cathode-side off-gas. In a preferred embodiment, the hydrogen / vapor separator is a membrane filter. The membrane filter may operate with a pressure delta on both sides of the membrane.

[0087] Next, these and other features of the present invention will be described in more detail, merely as examples, with reference to the accompanying drawings. [Brief explanation of the drawing]

[0088] [Figure 1] This is a schematic diagram of a typical conventional electrolytic cell, several of which may be potentially modified as described below, and which can be stacked in one or more stacks of the electrolytic cells of the electrolytic system of the present invention. [Figure 2] This is a schematic diagram of a simplified first embodiment of the present invention, in which a first heat exchanger and a second heat exchanger are arranged in series to preheat fuel for an electrolytic cell. [Figure 3] This is a schematic diagram of a simplified second embodiment of the present invention, in which a first heat exchanger and a second heat exchanger are arranged in parallel. [Figure 4] This is a schematic diagram of a simplified third embodiment of the present invention, in which the heat exchangers are arranged in series as in Figure 2, but the hydrogen / vapor separator is positioned between the off-gas outlet of the electrolytic cell and the second heat exchanger. [Figure 5] This is a schematic diagram of a simplified fourth embodiment of the present invention, in which the heat exchangers are arranged in parallel as in Figure 3, but in some embodiments, an air supply section and an air inlet for the electrolytic cell are provided, which may be required to assist in starting the electrolytic cell. [Figure 6] This is a schematic diagram of a simplified fifth embodiment of the present invention, in which the heat exchangers are arranged in parallel as in Figure 2, but which also includes a starting heater for fuel, which may be similarly required to assist in starting the electrolytic cell in some embodiments. [Figure 7] This is a schematic diagram of a sixth embodiment of the electrolysis system according to the present invention, in which an air supply unit is optionally provided to connect to the sweep inlet of the electrolytic cell for the anode-side sweep flow, along with a sweep flow circuit that connects the off-gas outlet on the anode side and the sweep inlet of the electrolytic cell. [Figure 8] This is a schematic diagram of a seventh embodiment of the electrolysis system according to the present invention, showing an alternative configuration for the sweep flow on the anode side of the electrolytic cell. [Figure 9]This figure shows a simplified eighth embodiment of the present invention, in which the first and second heat exchangers can be switched between a parallel arrangement (Figure 9) and a series arrangement (Figure 10). [Figure 10] This figure shows a simplified eighth embodiment of the present invention, in which the first and second heat exchangers can be switched between a parallel arrangement (Figure 9) and a series arrangement (Figure 10). [Modes for carrying out the invention]

[0089] Referring first to Figure 1, the basic structure and operation of a conventional electrolytic cell 11 are shown by referring to one fuel cell 11 in a stack 10 of cells 11 within an electrolytic cell 16.

[0090] The electrolytic cell 11 comprises an anode 60, a cathode 62, and an electrolyte 54. Such a structure of the electrolytic cell 11 is well known in the art.

[0091] The "fuel" for the electrolytic cell 16 (usually water or carbon dioxide, but in this embodiment water in the form of vapor 43) passes over the cathode 62, and the sweep gas 42 passes over the anode 60. To supply power to the electrolytic cell 11, a current / voltage is applied across the electrolytic cell 11 via the electrical terminals / connections 66, 68 on the anode and cathode sides of the electrolytic cell 11. As a result, an electrolytic reaction occurs through the electrolyte 54, and oxygen ions move through the electrolyte 54 from the cathode 62 to the anode 60, thereby decomposing some of the vapor 43 into hydrogen on the cathode side and oxygen on the anode side of the electrolytic cell 11. When carbon dioxide is used as fuel, the fuel is partially decomposed into carbon monoxide and oxygen, where oxygen is produced on the anode side and carbon monoxide is produced on the cathode side.

[0092] In this conventional electrolytic cell 11, oxygen (first off-gas) is extracted by a sweep gas 42, which generates an airflow or sweep flow through the electrolytic cell 11 to discharge oxygen (which is then mixed with the sweep gas) from the off-gas outlet 38 on the anode side of the electrolytic cell 11. Since the sweep gas 42 is usually air, the gas mixture discharged from the off-gas outlet 38 is oxygen-enriched air 64. However, in some conventional systems, a different sweep gas 42 is used; for example, the sweep gas 42 may be nitrogen, and therefore the oxygen is pushed out from the anode side by being mixed with nitrogen.

[0093] Hydrogen (or a second off-gas) on the cathode side can be extracted from the electrolytic cell 11 and discharged from the electrolytic cell 11 through another off-gas outlet 44 of the electrolytic cell 11. This second off-gas 63 is typically a mixture of hydrogen (or carbon monoxide if the fuel is carbon dioxide) and residual fuel 43 (i.e., residual vapor 43 or carbon dioxide). Since the conversion of fuel 43 usually occurs only for a portion of the supplied fuel 43, in the case of vapor 43, the generated hydrogen is discharged from the cathode side through the other off-gas outlet 44 as "wet" hydrogen 63.

[0094] Furthermore, it is known that it is desirable to supply the fuel 43 and sweep gas 42 to the inlets 32, 22 of the cell 11 at a temperature close to or at the operating temperature of the cell 11. In the case of a vapor electrolytic cell 16, this will be a temperature above 100°C. In the case of a pressurized vapor electrolytic cell 16, and in practice, in the case of most vapor electrolytic cells 16, including the electrolytic cells of a solid oxide electrolytic cell (SOEC) system 20, this will typically be a temperature well above 100°C. In the case of an SOEC system 20, for example, the temperature will typically exceed 400°C. For this reason, due to the high operating temperature of the electrolytic cell 11 for the vapor electrolytic cell 16 (i.e., above 100°C), and in the case of a solid oxide electrolytic cell (SOEC) 11, which is usually above 400°C, the heat of the off-gas 63, 64 from the off-gas outlets 38, 44 should not be wasted in the off-gas collection (or forward delivery) system. Therefore, that heat is typically usefully used by the electrolytic system 20 to supply at least some heat for steam generation, sweep gas heating, or steam superheating on the input side of the stack 10.

[0095] Such operating characteristics of the electrolytic cell 11 and its stack 10 are well known in the art. However, in a preferred embodiment of the present invention, no externally supplied sweep gas is supplied during normal operation of the electrolytic cell 16. As a result, all such "recovered" heat is transferred to the steam 43. This can increase the efficiency of the system 20, especially since there is no sweep gas 42 to heat. This removes parasitic loads from the electrolytic system 20. Furthermore, deterioration of the anode side and the sweep gas manifold and piping is reduced by the decrease in the amount of fluid flowing through them.

[0096] A schematic diagram of a first such embodiment is shown in Figure 2. As shown in the figure, an electrolytic system 20 is provided, comprising an electrolytic cell 16 having a stack 10 of electrolytic cells 11. In this embodiment, there are eight electrolytic cells 11 in the stack 10, but a typical stack 10 may have more or fewer electrolytic cells 11. For example, a typical commercial stack 10 may have 100 electrolytic cells 11.

[0097] The electrolytic cell 16 may also have two or more stacks 10 of electrolytic cells 11.

[0098] Typically, the cells 11 are arranged in series via the stack 10, as shown in the illustration. However, the electrolytic cells 11 may be arranged in parallel. Similarly, if two or more stacks 10 are provided, the stacks 10 may be in parallel or in series.

[0099] In this example, the electrolytic cell 16 has a single fluid inlet 22. This inlet is for fuel 43 (usually steam or carbon dioxide). To illustrate this, steam will be described as fuel 43.

[0100] Steam is supplied from a steam source or steam generator (not shown). The steam source or steam generator supplies steam 43 to the electrolytic system 20 via a supply line 124. Two heat exchangers 132, 142 are provided along the supply line 124. The first heat exchanger 132 performs initial recirculation heating for the steam 43, and the second heat exchanger 142 performs final recirculation heating for the steam 43. These two heat recirculation processes remove heat from the off-gases 63, 64 of the electrolytic cell 16 and exchange it for the supply steam 43 for the electrolytic cell 16. However, it should be understood that the two heat exchangers 132, 142 can be formed as a single unit that takes in two “high temperature” inputs and transfers the combined heat to the steam 43.

[0101] As shown in Figure 2, two heat exchangers 132 and 142 are provided, arranged in series. The first heat exchanger is the anode-side heat exchanger 132, which exchanges heat from the anode off-gas 64 for steam 43 from a steam source or steam generator. The anode off-gas 64 can be substantially pure oxygen, but can be oxygen-enriched air or nitrogen if a sweep gas is also used. However, ideally, the oxygen is at least 90% pure oxygen when the system 20 is operating in steady state, and the present invention is ideally operated without an externally supplied sweep gas, and therefore with only one fluid input 22. In such a configuration, heat from both off-gas is exchanged for a single fuel input, and therefore the energy requirements for raising this fuel to a given temperature are reduced.

[0102] The anode off gas 64 is sent from the anode off gas outlet 18 of the stack 10 along the pipeline 106 to the heat exchanger 132. The anode off gas 64, which has cooled somewhat at this point, can then be collected for use elsewhere.

[0103] The second heat exchanger 142 is a cathode-side heat exchanger 142, which exchanges heat from the cathode-off gas 63 for steam that is preheated at that point from the first heat exchanger 132, thereby superheating the steam 43 before it enters the electrolytic cell 16. However, it is also possible to reverse the order of the first heat exchanger 132 and the second heat exchanger 142.

[0104] The cathode-off gas 63 may be moist hydrogen (i.e., hydrogen mixed with water vapor) when the fuel is water vapor, but may be carbon monoxide mixed with carbon dioxide when the fuel is carbon dioxide.

[0105] The anode-off gas 63 is sent from the cathode-off gas outlet 24 of the stack 10 to the heat exchanger 142 via another pipeline 126. The cathode-off gas 63, which has now cooled somewhat, can then be collected for use elsewhere.

[0106] In this simplified example of an electrolytic system 20, preferably, an electrolytic cell 16 having only one fluid inlet 22 for fuel 43 (vapor) is provided, as shown in the figure. In some embodiments of the present invention, since an externally supplied sweep gas is not required, no sweep gas inlet is provided. Thus, the stack / electrolytic cell has, in addition to its single fluid inlet 22, an input section (terminal) for power (for applying current throughout the electrolytic cells 11 in the stack 10) and fluid outlets 18, 24 for off-gas 63, 64. Thus, although this is a very simple configuration externally, internal manifolds and piping may exist for supplying and extracting fuel and off-gas from the individual cells 11 and / or the stack 10.

[0107] When heat exchangers are in series, the temperature delta (difference) across the two fluids passing through the heat exchangers (i.e., off-gas and steam) is smaller than in the arrangement in Figure 3 (where a parallel configuration is provided), so each heat exchanger may not operate at its maximum heat transfer efficiency.

[0108] The extraction of hydrogen produced by the electrolytic cell 11 in Figure 2 may be conventional in that, unlike the extraction of oxygen, it is entrained in the residual vapor passing through the cell 11 in the stack 10. As for oxygen, it can be extracted as long as it is sufficiently abundant on the anode side of the stack 10. However, during normal use, the electrolytic cell 16 operates continuously, and therefore oxygen is continuously produced. Thus, the electrolytic cell 16 can continuously self-pressurize on the anode side of the electrolytic cell 11. Thus, the electrolytic cell 16 can generate its own extraction force (or fluid flow) as its molar volume and pressure exceed the threshold of the outlet valve or accumulate relative to the downstream pressure of the collection chamber or pipeline.

[0109] In operation, the stack typically attempts to avoid large pressure differences on either side of the cell, so thresholds can be appropriately set, or the cathode pressure can be controlled to balance the pressures on either side of the cell. In one embodiment, the pressure difference can be controlled (and kept as small as possible) by using a common regulator system on both sides. However, controlling the pressure difference to be positive on the oxygen (O2) side to ensure that O2 leaks into the fuel is more advantageous than the reverse. This is more advantageous for stack life (less degradation) and applies to all electrolytic cells and operating methods (e.g., with or without sweep flow). Optionally, the anode (O2) side pressure is 50 mbar or less, optionally 40 mbar or less, optionally 30 mbar or less, optionally 20 mbar or less, and optionally 10 mbar or less than the cathode (fuel) side pressure. In this way, the anode pressure is actively controlled to always be higher than the cathode pressure.

[0110] In some embodiments, the stack may be heated for startup using an external system that heats the inlet flow. This may be a flow of steam, nitrogen, hydrogen, or air, or a combination thereof. The external heat may be waste from another industrial process or from an alternative heat source such as an electric heater.

[0111] If oxygen is present in the starting gas stream (i.e., air), and the cathode contains nickel, nickel oxidation of the cathode can be avoided by applying a voltage across the entire stack. This is effective when the starting gas is a mixture of vapor and air.

[0112] A modified version of this system 20 is shown in Figure 3. As described above, the two heat exchangers 132 and 142 are arranged in parallel instead of in series. To achieve this, two pipelines 124 for steam 43 are provided upstream of the heat exchangers 132 and 142. The outputs from these heat exchangers 132 and 142 are then reconnected to a single continuous pipeline of these pipelines 124, supplying heated steam 43 to the electrolytic cell 16 at a common inlet 22. However, it should be noted that it is also possible to provide separate inlets 22 for each of these steam flows.

[0113] The piping in this alternative configuration is more complex, resulting in higher capital investment in the architecture. However, the temperature delta across both heat exchangers can be higher, thus potentially increasing their heat transfer efficiency. Over the lifespan of the electrolytic system, this efficiency improvement can make the entire system more cost-effective.

[0114] In this case as well, an external system may be required to start the system, and depending on the configuration of the starting fluid, nickel oxidation protection may be desirable.

[0115] Further variations of this system 20 are shown in Figures 9 and 10. In Figure 9, as in Figure 3, both heat exchangers 132 and 142 are shown arranged in parallel, with valves 200a and 200b (first or upstream as specified herein) configured to be open and valves 202a and 202b (second or downstream as specified herein) configured to be closed. To this end, the first pipeline or supply line 124 for steam 43 is divided into two pipelines, each of which connects to one of the heat exchangers 132 and 142, and then reconnects to a single continuous pipeline of the first pipeline or supply line 124 to supply heated steam to the electrolytic cell at a common inlet 22.

[0116] However, valves 200a, 200b, 202a, and 202b can be switched to the configuration shown in Figure 10, in which the first valves 200a and 202a are opened and the second valves 200b and 202b are closed, in order to arrange heat exchangers 132 and 142 in series. Opening and closing the valves in the reverse order has the same effect, but the flow proceeds to the heat exchangers in the reverse order, i.e., to heat exchanger 142 before heat exchanger 132.

[0117] The valve configuration can be simplified by replacing each set of four valves, 200a, 202a, 202b, and 202a, with a single three-way valve. This is because the valves operate in pairs consisting of similar numbers 200a, 200b (etc.) or similar letters 200a, 202a (etc.). Such a configuration requires fewer parts, but the parts may be more expensive.

[0118] Referring now to Figure 4, a further modification of the system 20 in Figure 2 is shown. In this modification, the heat exchangers 132 and 142 are arranged in series, as in Figure 2, and have the same advantages and disadvantages as described above. However, a hydrogen / steam separator 28 is added and positioned between the cathode-off gas outlet 24 of the electrolytic cell and the second heat exchanger 142. The hydrogen / steam separator 28 can be used to separate the cathode-off gas into (a) steam for recirculation to the electrolytic cell 16 via the steam recirculation pipeline 30 and (b) hydrogen for passing through the second heat exchanger 142 to heat the steam supplied from the outside as described above.

[0119] If the hydrogen is cooled to a temperature below the dew point of the vapor, it can be substantially dried at this point by the hydrogen / vapor separator 28 and potentially further by the second heat exchanger 142, and then sent for subsequent collection or distribution.

[0120] This modified form, which includes a hydrogen / steam separator 28, results in a hydrogen supply of increased value because the hydrogen is already purer and substantially or largely separated from the steam.

[0121] A hydrogen / steam separator 28 is a separator 28 for separating at least a portion of hydrogen from a humid hydrogen stream. There are various forms of hydrogen / steam separators 28 known in the art. For example, the hydrogen / steam separator 28 may be a cooling / reheating loop that condenses the steam by cooling the humid hydrogen to a temperature below the dew point of water, for example using a regenerative heat exchanger, and then reheating it, as described with respect to Figure 7 below. However, this can also be achieved by a membrane filter. For example, membrane filters are known that, at appropriate pressures, allow hydrogen to pass through the membrane but not water. These pressures may, as an example, be a pressure delta on both sides of the membrane. Such membrane filters are well known in the art. Membrane filters may include membranes that function at temperatures related to a steam electrolytic cell 16, particularly an SOEC electrolytic cell operating at temperatures of 400°C to 750°C. For example, a palladium-based H2 membrane can operate at 400°C to 500°C and can produce dry hydrogen by separating hydrogen from a mixture of hydrogen and steam. In other embodiments, electrochemical films can be used, and some ceramic films can function at higher temperatures.

[0122] Although not shown, a hydrogen / steam separator 28 can similarly be added to the parallel heat exchanger system 20 in Figure 3. It has the same advantages.

[0123] Therefore, in these embodiments, the high-temperature membrane is advantageously used to separate hydrogen from the vapor, and thus can replace or reduce the requirement for a downstream condensation system for hydrogen in or before the downstream collection or distribution piping.

[0124] A typical such membrane generally requires a pressure delta of 5 to 10 bar to generate the driving force for moving hydrogen protons / atoms through the membrane. These pressures can be achieved by operating the electrolysis system at such high pressures and using atmospheric conditions outside the membrane, or by using a suction pump outside the membrane, although the use of a suction pump may introduce new parasitic loads into the system 20.

[0125] Next, referring to Figure 5, further modifications are shown. This modification, based on the parallel heat exchanger system of Figure 3, shows a starting sweep gas system. Such a starting sweep gas system can similarly be added to systems using series heat exchangers (or a combination of the two).

[0126] This starting sweep gas system may be any selectively operable or disengaged component of the electrolytic system 20. Typically, the starting sweep gas system may be used for starting or other transient operations of the electrolytic system 20. For example, if the heat of the vapor 43 from the vapor source or vapor generator is insufficient to start or maintain operation of the electrolytic cell 16, or if a faster start or higher heat maintenance is desired. To this end, a supply of hot air or hot nitrogen (which may be heated air or waste from another industrial process) is supplied from its source 80 to the air inlet 32 ​​on the anode side of the electrolytic cell 16. This is done to preheat the electrolytic cell 16 or to assist in heating the electrolytic cell 16 in order to achieve or maintain an operating temperature during starting or transient conditions. During this process, the electrolytic cell 16 may begin electrolysis of the vapor 43 flowing in through the cathode side of the electrolytic cell 16, and thus the air or nitrogen may act as a temporary sweep stream of the generated oxygen 64. However, this can often slow down the starting process, as it can even cool the incoming steam, and therefore, more generally, the exhaust is not recovered initially.

[0127] In some embodiments, the exhaust can still be recirculated through the first pipeline 106 to the first heat exchanger 132. However, typically, this air or nitrogen is discharged through a separate vent 40. This may also be done towards the end of the start-up cycle if it does not meet the target purity requirements of the oxygen collection system.

[0128] As described above, this air source may be a selectively operable or detachable component of system 20, thereby allowing system 20 to continue operating without its air source and to return the generated oxygen to a target purity for collection.

[0129] The starting sweep gas system may be an electric or heat exchanger-based heater. The starting sweep gas system may be mounted inside or to the side of the electrolytic cell 16, upstream thereof, or even inside or to the side of the stack 10. The starting sweep gas system may be permanent or disengaged. For example, the starting sweep gas system may be disengaged or turned off when the stack achieves steady-state conditions or when the stack is hot enough to self-sustain. In this embodiment, the starting sweep gas system is located on the anode side of the electrolytic cell 16. Preferably, this embodiment is considered only when additional heating or heating using steam is not feasible or feasible on the cathode (fuel) side of the electrolytic cell 16 alone.

[0130] If it is necessary to raise the temperature of the fluids flowing through pipelines between the various components of system 20, it will be understood that other forms of heaters (e.g., indirect electric heaters, or additional heat exchangers connected to an additional waste heat source, etc.) may be provided instead or in addition.

[0131] Furthermore, sensors and variable flow valves or stop valves may be provided along the pipeline or on each component to control these systems. These may enable flow control, heater control, and bypass control for heat exchangers or heaters that may be installed. Flow control of fuel, recirculated steam, or off-gas around the system 20 may also be performed.

[0132] Therefore, the flow rate and / or temperature of the input fluid(s) at the stack inlet(s) and from the stack outlet(s) can be controlled as needed.

[0133] In some embodiments, a low-flow sweep supply may be permanently implemented on the anode side. This requires a sweep inlet, which may be provided to ensure oxygen flow from the anode side of cell 11. This may be necessary, depending on the chemical properties of the anode or piping and manifold, for example, if oxidation problems could otherwise occur, or to avoid pressure buildup or fluid stagnation. Pressure buildup or fluid stagnation could create an explosion hazard if hydrogen leaks within stack 10. In such situations, the oxygen produced by the electrolytic cell forms the majority of the anode off-gas. For example, the amount of air for this purpose dilutes the oxygen produced by the electrolytic cell by 10% or less, thereby maintaining the oxygen's purity at least 90%. In some embodiments, oxygen levels exceeding 50% are clearly beneficial compared to the prior art (where the ratio of sweep flow to generated oxygen is conventionally at least 7:1 or 8:1, and therefore the oxygen level when using air for the sweep flow never exceeds 40% to 45% by weight in the prior art, considering that air starts at 20% oxygen).

[0134] While a purity of at least 90% by weight is the preferred target oxygen weight percentage for the anode off-gas, please understand that 50%, 60%, 70%, or 80% are all more advantageous than conventional techniques and therefore can all be the target oxygen weight percentage for the anode off-gas.

[0135] Referring now to Figure 6, a fifth embodiment of the present invention is shown, in which heat exchangers 132, 142 are arranged in series, as in Figures 2 and 4. Furthermore, a separator 28 similar to that in Figure 4 is provided, which in this embodiment is a hydrogen membrane 28 for filtering out most of the hydrogen from the steam / hydrogen mixture. The filtered hydrogen can pass through the second heat exchanger 142, as in the embodiment of Figure 4. However, when the hydrogen separator 28 cannot completely remove all the hydrogen from the mixture, the steam that is still mixed with some hydrogen can either be recirculated to the first pipeline 124, or it can be recirculated directly to the electrolytic cell 16 via such a separate port 120, or both, rather than having a separate port 120 for returning to the electrolytic cell 16.

[0136] Furthermore, in this embodiment, an additional heater 136 is provided in the steam supply pipeline 124. This may be an additional heat exchanger connected to an alternative heat source, such as an electric heater or waste heat from another industrial process, as described above. This heater 136 may preheat the steam 43 before the first heat exchanger 132 and the second heat exchanger 142. However, the heater 136 may be positioned between the heat exchangers 132, 142, or similarly after the heat exchangers 132, 142. The heater 136 can assist in starting, similar to the air heating in Figure 5, or reduce the heating requirements from the first heat exchanger 132 and the second heat exchanger 142 during steady-state operation. Placing an additional heater in the cathode-side pipeline 124 is preferable to flowing heated air or fluid to the anode side.

[0137] The temporary heaters in Figures 5 and 6 may be removable. The temporary heaters can then be moved between stacks of a given system or different systems. This can reduce the capital expenditure of a given piece of equipment and may be beneficial for the anticipated staggered start-up of a given system, for example, for start-ups after maintenance or replacement of a stack in a given system where only a partial shutdown of the entire system 20 is required.

[0138] In some embodiments, during startup, the steam (fuel) is completely replaced with air, carbon dioxide, or nitrogen, and as a result, air, carbon dioxide, or nitrogen, rather than steam 43, enters the electrolytic cell 16 through the inlet 22. This is advantageous until the stack reaches a temperature above 100°C. This is because, before reaching that temperature, water can flow into the stack as a liquid, either directly into the stack or through steam condensation within the stack, which can severely damage the stack. Thereafter, steam (or a mixture of steam and air, carbon dioxide, or nitrogen) may be used for the continuous heating of the stack.

[0139] It will be understood that these first five embodiments significantly reduce the number of required subsystems, which are, for example, few for electric inline heaters, few for high-temperature air supply subsystems, and a minimum for cooling systems, and potentially not even require a steam generator (assuming steam from an external industrial process is available). This significantly reduces the cost and complexity of the electrolytic systems of the present invention compared to electrolytic systems of the prior art. The oxygen produced is of higher purity than that of the prior art, and the oxygen can be used to heat the incoming fuel and recover heat before oxygen storage or distribution, where all residual heat is discarded.

[0140] Similarly, the heat from the hydrogen and steam output is recycled.

[0141] Referring now to Figure 7, a further embodiment of the electrolytic system 20 having an electrolytic cell 16 is shown. The electrolytic cell 16 may comprise one or more stacks 10 of electrolytic cells 11, similar to the previous embodiment. In this case as well, the parasitic load associated with sweep gas is removed or reduced. In this embodiment, the electrolytic cell 16 is operated at a high pressure, for example, a pressure greater than 1.5 barg (1.5 bar higher than atmospheric pressure), and the electrolytic system 20 utilizes the resulting pressure difference to extract off-gas at least on the anode side of the electrolytic cells 11. This will be described in more detail below. However, a further advantage of operating the electrolytic cell 16 at a pressure higher than atmospheric pressure is that the off-gas (hydrogen 63 and oxygen 64) can be generated at a pressure higher than atmospheric pressure and, consequently, extracted at a pressure higher than atmospheric pressure. This reduces the amount of downstream compression required for the extracted gases 63, 64 before storage. These gases 63, 64 are conventionally stored at high pressure to reduce their volume.

[0142] Higher pressure relative to atmospheric pressure can also facilitate the separation of component gases from off-gas mixtures 63 and 64, as described above with respect to, for example, Figures 4 and 6.

[0143] The electrolytic cell 16 in Figure 7 includes an optional air or sweep inlet 32 ​​and a vapor or water inlet 22. The electrolytic cell 16 further has a first side outlet 18 for outputting high-temperature oxygen 64 (depending on the form of sweep gas 42 used by the system 20, either only the generated oxygen, or oxygen-enriched air, or oxygen-enriched nitrogen). Typically, this first side outlet 18 is located on the anode side of the electrolytic cell 16 and is connected to the anode side outlet 38 of the electrolytic cell 11 or its stack 10.

[0144] For clarity, the stacks 10 and cells 11 are not shown in this figure. However, the electrolytic cell 16 typically has one or more stacks 10 of electrolytic cell cells 11, much like those in Figures 2 to 5. For each embodiment, one or more stacks 10 may be used, and each stack 10 may have fewer or more fuel cells 11 than those shown (for example, stacks of at least 100 cells are well known in the art).

[0145] The electrolytic cell 16 in Figure 7 also has a second side outlet 24 for outputting high-temperature humid hydrogen 63. Typically, this second side outlet 24 is located on the cathode side of the electrolytic cell 16 and is connected to the cathode side outlet 44 of the electrolytic cell 11 or its stack 10.

[0146] In this embodiment, an optional air or sweep inlet 32 ​​is provided, because it may be beneficial to introduce air (or nitrogen) from an external source during transient operation (such as starting, stopping, purging, or changes in operating conditions) to generate / maintain high pressure and / or high temperature within the electrolytic cell 16. This is similar to that shown in Figure 5. As shown in Figure 7, an air, oxygen, or nitrogen supply source 80 is provided. This can be pressurized or pumped to flow sweep gas 42 toward the electrolytic cell 16 through the first pipeline 102. The sweep gas 42 passes through the first shut-off valve 98 and the first pressure regulator 100 in the first pipeline 102, and also through the first heat exchanger 104 toward the electrolytic cell 16. The first shut-off valve allows the air supply source 80 to be isolated from the system 20.

[0147] Along the first pipeline 102, a heater, or a pair of heaters 158 in parallel as shown, is provided to preheat the sweep gas 42 before it is introduced into the electrolytic cells 11 of the electrolytic cell 16. A first temperature sensor 110 allows control of this heating. The sweep gas enters the electrolytic cell 16 at the inlet 32 ​​and exits the electrolytic cell 16 at the first side outlet 18, controlled by the second pressure regulator 105. A second pressure sensor 108 allows control of this process. As noted above, the external air supply source 80 is not mandatory and may be merely for transient operation as described above, or for any auxiliary sweep flow operation. However, in some embodiments, as described below, the external air supply source 80 may be turned off completely and may even be disconnected from the system 20.

[0148] The high pressure in the electrolytic cell 16 works in conjunction with a second pressure regulator 105 on a second pipeline 106 that extends away from the first side outlet 18. In this configuration, the second pressure regulator 105 can be set to flow pressure requirements that allow for a pressure difference on both sides of the pressure regulator 105 (i.e., between the inside of the electrolytic cell 16 (anode side) and the downstream side (storage / discharge) of the pressure regulator 105). This pressure difference allows a pressure-driven flow of oxygen-enriched air, or even just the generated oxygen, to exit the electrolytic cell 16. Furthermore, the pressure regulator 105 can be closed when the pressure drops below a threshold, ensuring that the pressure inside the electrolytic cell 16 is reliably maintained on the anode side.

[0149] Since oxygen generation continues during the use of the electrolytic cell 16, the pressure regulator can allow a steady flow of oxygen to flow out of the electrolytic cell during steady-state operation of the electrolytic cell. As a result, a sweep gas 42 is not required to discharge the oxygen produced from electrolysis from the anode side of the electrolytic cell.

[0150] The heat in the discharged oxygen 64 can also be recovered here by the second heat exchanger 112. Advantageously, when sweep gas 42 is used, the first heat exchanger 104 can use the heat of the oxygen off-gas 64 to preheat the sweep gas 42 before it enters the electrolytic cell 16 by connecting its secondary flow line to the second heat exchanger 112. However, when the sweep gas is turned off or removed, the second heat exchanger 112 may instead be connected to the fuel supply line, i.e., the third pipeline 124.

[0151] The pressure of the generated oxygen can be increased relative to the ambient pressure by the second pressure regulator 105, so that the collected oxygen off-gas 64 can also be normally pressurized and stored or collected in that form for use elsewhere. However, the pressure of the oxygen may be further pressurized by downstream equipment before storage or use. For this purpose, an oxygen collection area 114 is provided. Alternatively, or additionally, a portion of the collected oxygen off-gas 64 can be mixed and diluted with additional air (shown here as a separate air source 80, but potentially from the same air source 80). This off-gas can then be stored or utilized elsewhere as pressurized oxygen-enriched air 116.

[0152] While maintaining pressure is often useful or beneficial, it will be understood that in some embodiments, if the pressure is not important (or undesirable) for downstream use, oxygen may be discharged from the pressure regulator to atmospheric pressure.

[0153] A check valve 118 can be provided to prevent backflow.

[0154] In this configuration, the electrolytic cell 16 is operated at a high pressure configuration. Although operation at a pressure of 1.5 barg is shown above, operation at pressures of 0.5 barg to 5 barg (gauge pressure) is also possible. The higher the pressure, the more complex / expensive the sealing required to maintain that high pressure in the system becomes; therefore, a pressure of 0.5 barg to 3 barg is preferable to strike a good trade-off between cost and benefit. 1.5 barg or less is more preferable, and 0.5 barg to 1 barg achieves even greater value.

[0155] Since pressurized steam is supplied directly to the electrolytic cell 16 via the third pipeline 124 and recirculated via the fourth pipeline 126, the hydrogen (cathode) side of the electrolytic system is also pressurized. In a typical embodiment, the anode pressure is controlled to match the hydrogen / steam / cathode pressure, so that the pressure is substantially equal across the electrolyte 54 of the electrolytic cell 26. This is preferable because many designs of the electrolytic cell 11 are relatively susceptible to damage when exposed to excessive internal stress or strain. Therefore, a second pressure regulator 105 may be set to match the inflow steam pressure. To this end, a second shut-off valve 128 and a third pressure regulator 130 may be provided on the steam supply line 160 between the steam supply source 43 and the third pipeline 124, and a fourth pressure regulator 152 may be provided on the fourth pipeline. The second shut-off valve 128 can control whether additional steam 43 is added to the system 20. The third pressure regulator 130 and the fourth pressure regulator 152 can be monitored or operated together with the second pressure regulator 105 to maintain a consistent pressure across the electrolytic cell 11.

[0156] In normal use, a steam or water source 43 supplies pressurized steam to the electrolytic cell 16 via a third pipeline 124. An additional pressure sensor 134 is also provided to monitor the pressure in the third pipeline 124 for the control system. The steam in the third pipeline then passes through a second heat exchanger 132 (i.e., the second side of the heat exchanger 132 in the second pipeline 106) to preheat or superheat the steam before it enters the electrolytic cell 16, utilizing the heat of the oxygen off-gas 63. Pressure regulators 105, 130, 152 may control the pressure on both sides of the electrolytic cell, or may be controlled by sensors and the control system.

[0157] In some embodiments, an additional heater 136 may also be provided, along with a third temperature and / or pressure sensor 138, as shown here in Figure 7. The steam temperature can be further increased as needed. Pressure regulators 105, 130, and 152 can also check the overall system pressure, as the gas must flow into the electrolytic cell 16 at a temperature close to the target operating temperature of the electrolytic cell 26 and at an appropriate pressure that matches the oxygen / sweep gas pressure on the anode side.

[0158] Therefore, the steam 43 then enters the electrolytic cell 16 and, as described with respect to Figure 1, passes through the electrolyte 54 of the electrolytic cell 11 of the electrolytic cell 16 and is at least partially split into hydrogen and oxygen. The humid hydrogen then exits the electrolytic cell 16 through the second side outlet 24 and enters the fourth pipeline 126. A fourth heat exchanger 142 is provided along the fourth pipeline 126. This fourth heat exchanger 142 works in conjunction with a loop within the fourth pipeline 126, and as described below, the unsplit steam can be recirculated through the fourth pipeline 126 to the electrolytic cell via the third pipeline 124.

[0159] A fourth pressure sensor 140 may be provided between the second pressure regulator 105 and the fourth pipeline 126 to help match the pressure on both sides of the electrolytic cell 11 (or to ensure there is no excessive pressure difference). An excessive pressure difference could cause damage to the electrolytic cell or lead to leakage of the electrolyte or other parts of the electrolytic cell 16, which could lead to combustion of hydrogen and oxygen or other inefficiencies.

[0160] A fourth pipeline 126 is provided to take in moist hydrogen 63 through a fourth heat exchanger (so that the moist hydrogen is slightly cooled), and then through a fifth heat exchanger 150, which may be a regenerative heat exchanger. This may form part of a compressor 26 for compressing the hydrogen ( schematically shown as an ellipse in this figure), for example, a metal hydride or an adsorption / desorption compressor, but may be added to a separate compressor 26 for hydrogen.

[0161] In this embodiment, the humid hydrogen can exit the fifth heat exchanger 150 at an even lower temperature (below the dew point of vapor), where the water in the humid hydrogen can condense from the mixture and be collected in the condensation container 144. Thus, the humid hydrogen 63 is partially dried by this process after exiting the fifth heat exchanger 150, and the dried hydrogen can be collected or stored in the collector 90.

[0162] The condensed water can be recirculated through the fifth heat exchanger 150 and the fourth heat exchanger 142, reheated, and returned to steam for reintroduction into the third pipeline 124. A pump 146 is provided in the water line to supply water 38 through these heat exchangers 142, 150, so that the steam can flow into the third pipeline at a sufficiently high pressure.

[0163] Alternatively, for example, if a steam source is readily available and a simpler design is desired, the collected water may be discarded.

[0164] As the condensed water and moist hydrogen pass through the fourth heat exchanger 142 and the fifth heat exchanger 150, both heat exchangers cool the moist hydrogen to condense the water, reheat the water back into steam, and recirculate it through the electrolytic cell 16. Thus, these are regenerative heat exchangers for the recirculation of water through the electrolytic cell 16, thereby reducing the amount of external water or steam 43 required for the electrolysis system 20. Depending on the off-gas temperature, three or more such heat exchangers may be required, or it may even be possible to achieve regeneration with a single heat exchanger.

[0165] The recirculated water (in this case, steam) flows into and passes through the third pipeline 124, then through the third heat exchanger 132 and any heater 136, and then enters the electrolytic cell again as steam for the electrolytic cell, or mixed with additional steam from the steam source 43.

[0166] The fifth pressure regulator 152 may be located after the fourth heat exchanger 142 in the fourth pipeline 126 and, in conjunction with the fourth pressure sensor 140 and control system, can help monitor the system 20 and help ensure the safety and operational integrity of the system 20.

[0167] Because this system operates at high pressure (higher than atmospheric pressure, preferably 0.5 to 5 bar higher than atmospheric pressure), pressurized off-gas is generated, further reducing the demands on the compressor 26 for hydrogen.

[0168] Next, referring to Figure 8, a modified configuration of Figure 7 is shown. In this modification, much of the hydrogen side is the same as in Figure 7. Therefore, it will not be described in further detail. However, an additional pressure sensor 134 is moved to a position after the third temperature sensor 138 to measure the pressure of the steam after it has passed through the third heat exchanger 132 and an additional (optional) heater 136 in the third pipeline 124. This can assist the control system in identifying the appropriate pressure in the second pressure regulator 105 to match the pressure across the electrolytic cell 16.

[0169] Furthermore, the second pipeline 106 on the oxygen side is almost identical. However, its sweep gas circuit is modified to allow for the recirculation of a portion of the anode off-gas (oxygen). This can completely replace the external sweep gas supply 42, as it generates a recirculated sweep gas flow using the generated oxygen and residual air from the initial use of sweep gas 43 during system startup.

[0170] As described below, this recirculation of a portion of the anode off-gas can be powered by a thermal pump. In some embodiments, no external sweep gas is required at all for steady-state operation.

[0171] In this embodiment, the second pipeline 106 branches off before the third heat exchanger 132 to form a loop 156 that returns to the air or sweep inlet 32 ​​of the electrolytic cell 16. An electric heater 158 is provided to power a heat pump or thermal pump to influence the sweep flow, but the gas used for the sweep flow is, in this case, oxygen from the electrolytic cell 16. The heater 158 may instead be located on the opposite side of the heat exchanger, for example, the second heat exchanger 112. In that case as well, the second pressure regulator 105 operates to influence the outflow of oxygen 64 from the electrolytic cell 16 to the storage or external outlet channels 114, 116 by the pressure difference on both sides of the regulator 105, as described above with respect to Figure 7. However, the oxygen in the electrolytic cell continues to flow through the electrolytic cell across the loop 156.

[0172] To prevent reverse reversal of oxygen recirculation, a second check valve 154 can be installed within this loop 156. Furthermore, an additional pressure sensor 160 can be provided for loop 156 and for the control system.

[0173] Figure 8 further shows an optional additional loop 162 for circulating oxygen back from behind the third heat exchanger 132. This may be provided instead of, or in addition to, the loop before the third heat exchanger 132. This alternative or additional loop may be desirable or better suited to balance the pressure and thermal requirements within the electrolytic cell 16 and across the system 20, depending on the performance of the heater 158 or thermal pump.

[0174] The thin dashed lines in Figures 7 and 8 indicate control lines between the relevant sensors and valves or regulators, where the value / position of one may affect or control the other. Such control functions may be performed similarly by each element connected to a central control system.

[0175] It is also conceivable that there may be embodiments in which a second pressure regulator is not required, or in which the system 20 is operated at ambient pressure. The pressure or sweep flow may be maintained purely by the oxygen production rate. However, the regulator is typically preferred because it still serves the purpose of balancing the pressure across the stack, thereby reducing undesirable consequences of pressure delta, such as internal leakage, even when operated under atmospheric conditions.

[0176] In all of these embodiments, the electrolytic system is intended to operate with maximum electrical and thermal efficiency by operating at a voltage close to the thermal neutral voltage. In practice, the system operates so that the outlet temperature is higher than the inlet temperature in order to allow for the elimination of inline heaters. This requires operation at a voltage slightly higher than the thermal neutral voltage. For example, the outlet temperature may be about 30°C higher than the inlet temperature to improve efficiency and potentially achieve optimal efficiency. This is the temperature under the assumption that the heat exchanger is more than about 90% efficient. Under such conditions, no additional or auxiliary heating of the stack fuel or the stack itself is required. Furthermore, by reducing the anode-side airflow to zero (or as close to zero as is permissible considering safety requirements that would result from flow stagnation or oxidation of components), the oxygen produced becomes purer and therefore more intrinsically valuable. Keeping the pressure well above atmospheric pressure helps to improve the efficiency of the hydrogen separation membrane 28.

[0177] Balancing the pressure across the electrolytic cells is also beneficial in maintaining a long service life for the stack, considering that it reduces stress on the electrochemically active materials of the electrolytic cell and on the interfaces between adjacent cells within the stack.

[0178] In the above, various embodiments of the present invention have been described merely as examples, with reference to the accompanying drawings. Detailed modifications to the present invention may be made within the scope of the attached claims. For example, features described as part of one embodiment can be used in another embodiment to produce yet another embodiment. Thus, the present invention is intended to encompass modified and altered forms as those within the scope of the attached claims. For example, although the above description focuses primarily on H2O electrolysis, it is applicable to CO2 electrolysis in which CO is produced at the cathode and O2 is produced at the anode. In such an embodiment, CO2 replaces the vapor input and CO replaces the H2 output.

Claims

1. A method for discharging substantially pure oxygen off-gas from the electrolytic cell of an electrolytic system, The step includes generating a pressure difference between the inside of the electrolytic cell and the inlet or outlet for the off-gas from the electrolytic cell, The aforementioned pressure difference is generated by the production of oxygen through electrolysis by the electrolytic cell, and the method of discharge is as follows.

2. The method according to claim 1, wherein the sweep gas is operated without using an external sweep gas, or using a sweep gas that is substantially pure oxygen.

3. The method according to claim 1 or 2, wherein at least 50% by weight of the substantially pure oxygen off-gas is oxygen produced by the electrolytic cell.

4. The method according to any one of claims 1 to 3, further comprising the step of controlling the discharge of the oxygen by a back pressure regulator.

5. The method according to any one of claims 1 to 4, wherein the weight purity of the substantially pure oxygen off-gas is at least 85%.

6. The electrolytic cell or each electrolytic cell comprises at least two off-gas outlets for off-gas from the at least one stack, wherein at least one of the at least two off-gas outlets is an anode-side off-gas outlet for discharging anode off-gas from the at least one stack, and at least one of the at least two off-gas outlets is a cathode-side off-gas outlet for discharging cathode off-gas from the at least one stack, wherein the anode off-gas outlet for discharging anode off-gas from the at least one stack is configured to be connected to the outlet for substantially pure oxygen off-gas, and the anode off-gas is substantially pure oxygen off-gas, according to any one of claims 1 to 5.

7. The method according to any one of claims 1 to 6, wherein the electrolytic cell or each electrolytic cell does not have an inlet for pushing the substantially pure oxygen off-gas, and the pressure difference is generated between the inside of the electrolytic cell and the outlet from the electrolytic cell for the substantially pure oxygen off-gas.

8. The method according to any one of claims 1 to 7, wherein a return loop is provided between the sweep inlet of at least one stack and the anode-side off-gas outlet of the stack.

9. The method according to any one of claims 1 to 8, wherein the pressure of the electrolytic cell is 0.1 barg to 10 barg, preferably 0.1 barg to 5 barg, and more preferably 0.2 barg to 1 barg.

10. The method according to any one of claims 1 to 9, wherein the pressures on the anode and cathode sides of the electrolytic cells in the electrolytic cell are substantially balanced.

11. The method according to any one of claims 1 to 10, comprising a starting or transition cycle in which, when the electrolytic cell has an internal temperature of less than 100°C, air, carbon dioxide, or nitrogen is supplied to the cathode side of the electrolytic cell instead of fuel.

12. The method according to any one of claims 1 to 11, wherein the system operates without an external sweep flow, and therefore a sweep inlet is not required for the electrolytic system.

13. The method according to any one of claims 1 to 12, wherein the electrolysis system further comprises a hydrogen / vapor separator positioned between the cathode-side off-gas outlet and a heat exchanger for exchanging heat from the cathode-side off-gas.

14. The method according to claim 13, wherein the hydrogen / vapor separator is a membrane filter.

15. The method according to claim 14, wherein the membrane filter operates with a pressure delta on both sides of the membrane.

16. A step of calculating the pressure on one side of the electrolytic cell in the electrolytic cell, A step of controlling the pressure on the other side of the electrolytic cell so that the pressure on the anode side of the electrolytic cell in the electrolytic cell is higher than the pressure on the cathode side of the electrolytic cell. The method according to any one of claims 1 to 15, including the method described in any one of claims 1 to 15.

17. A method for operating an electrolytic system comprising an electrolytic cell having at least one stack of electrolytic cells, each having an anode side and a cathode side, A step of calculating the fluid pressure on one side of at least one of the electrolytic cells or at least one stack, An operating method comprising the step of controlling the fluid pressure on at least one of the electrolytic cells or the other side of at least one stack such that the pressure on the cathode side of the electrolytic cell or each electrolytic cell or the stack or each stack is higher than the pressure on the anode side of the electrolytic cell or each electrolytic cell or the stack or each stack.

18. The method according to claim 16 or claim 17, wherein the pressure difference between the anode side and the cathode side of the electrolytic cell or electrolytic cell in the stack is 50 mbar or less, arbitrarily 40 mbar or less, arbitrarily 30 mbar or less, arbitrarily 20 mbar or less, and arbitrarily 10 mbar or less.

19. The method according to any one of claims 16 to 18, wherein the pressure on the anode side of the electrolytic cell or stack in the electrolytic cell is controlled by a back pressure regulator.

20. An electrolytic system configured to perform the method according to any one of claims 1 to 19.

21. The electrolytic system comprises an electrolytic cell having at least one stack of electrolytic cells, each having an anode side and a cathode side, wherein the electrolytic cell is At least one fuel inlet for fuel for the at least one stack, At least two off-gas outlets for off-gas from the at least one stack, wherein at least one of the at least two off-gas outlets is an anode-side off-gas outlet for discharging anode off-gas from the at least one stack, and at least one of the at least two off-gas outlets is a cathode-side off-gas outlet for discharging cathode off-gas from the at least one stack, A pressure calculating device for calculating the fluid pressure on at least one side of the electrolytic cell or stack, A pressure control device for controlling the fluid pressure on the other side of the electrolytic cell or stack, and An electrolytic system comprising, An electrolytic system in which the pressure control device is configured to control the pressure on the cathode side of the electrolytic cell or each electrolytic cell or the stack or each stack to be higher than the pressure on the anode side of the electrolytic cell or each electrolytic cell or the stack or each stack.

22. An electrolytic system comprising an electrolytic cell having at least one stack of electrolytic cells, wherein the electrolytic cell is At least one fuel inlet for fuel for the at least one stack, At least two off-gas outlets for off-gas from the at least one stack, wherein at least one of the at least two off-gas outlets is an anode-side off-gas outlet for discharging anode off-gas from the at least one stack, and at least one of the at least two off-gas outlets is a cathode-side off-gas outlet for discharging cathode off-gas from the at least one stack, means for exchanging heat from the cathode-side off-gas to the fuel for the at least one stack and means for exchanging heat from the anode-side off-gas to the fuel for the at least one stack An electrolytic system comprising, An electrolytic system in which the stack is configured to output the anode-side off-gas under normal operating conditions, and at least 50% by weight of the anode-side off-gas is oxygen produced by the electrolytic cell.

23. The electrolytic system according to claim 22, further comprising a hydrogen / vapor separator positioned after the cathode-side off-gas outlet.

24. The electrolytic system according to claim 23, wherein the hydrogen / vapor separator is located between the cathode-side off-gas outlet and the heat exchange means from the cathode-side off-gas to the fuel.

25. The electrolytic system according to claim 23 or 24, wherein the hydrogen / vapor separator is a cooling / reheating loop that cools wet hydrogen to a temperature below the dew point of water to condense the vapor, and then reheats it.

26. The electrolytic system according to claim 25, wherein the cooling / reheating loop uses one or more regenerative heat exchangers.

27. The electrolytic system according to claim 23 or 24, wherein the hydrogen / vapor separator is a membrane filter.

28. The electrolytic system according to claim 27, wherein the membrane filter operates with a pressure delta on both sides of the membrane.

29. An electrolytic system comprising an electrolytic cell having at least one stack of electrolytic cells, wherein the electrolytic cell is At least one fuel inlet for fuel for the at least one stack, At least two off-gas outlets for off-gas from the at least one stack, wherein at least one of the at least two off-gas outlets is an anode-side off-gas outlet for discharging anode off-gas from the at least one stack, and at least one of the at least two off-gas outlets is a cathode-side off-gas outlet for discharging cathode off-gas from the at least one stack. An electrolytic system comprising, An electrolytic system further comprising a hydrogen / vapor separator positioned between the cathode-side off-gas outlet and a heat exchanger for exchanging heat from the cathode-side off-gas, the hydrogen / vapor separator being a membrane filter operating with a pressure delta on both sides of the membrane.

30. The electrolytic system according to claim 29, further comprising means for exchanging heat from the cathode-side off-gas to the fuel for the at least one stack, and means for exchanging heat from the anode-side off-gas to the fuel for the at least one stack.

31. The electrolytic system according to claim 29 or 30, wherein the stack is configured to output the anode-side off-gas under normal operating conditions, and at least 50% by weight of the anode-side off-gas is oxygen produced by the electrolytic cell.

32. The electrolytic system according to any one of claims 25 to 31, wherein a sweep flow gas is used to push oxygen from the anode side of the electrolytic cell, and the sweep flow gas is less than 50% by weight of the anode side off-gas.

33. The electrolytic system according to any one of claims 25 to 31, wherein the stack is configured to operate without a sweep gas flow for extracting the anode-side off-gas from the stack or from any of the electrolytic cells within the stack.

34. The electrolytic system according to any one of claims 25 to 33, wherein the stack is configured with a control system to operate the stack substantially at a thermally neutral voltage.

35. The electrolytic system according to claim 34, wherein the control system is configured to operate the stack on one side of the thermal neutral voltage.

36. The electrolytic system according to any one of claims 25 to 35, wherein each electrolytic cell or stack of electrolytic cells is configured to operate substantially thermally and with constant current.

37. The electrolytic system according to any one of claims 25 to 36, wherein the continuous operation of the electrolytic system under normal operating conditions is achieved without the use of an auxiliary electric heater in the manifold or piping of the sweep flow pipeline or fuel flow pipeline within the electrolytic cell, or within the electrolytic system itself.

38. The electrolytic system according to any one of claims 25 to 37, wherein the electrolytic cell is a solid oxide electrolytic cell.

39. The electrolytic system according to any one of claims 25 to 38, wherein the electrolytic cell comprises two or more stacks.

40. The electrolytic system according to any one of claims 25 to 39, wherein the heat exchange means comprises one or more heat exchangers.

41. The electrolytic system according to claim 40, wherein two heat exchangers are arranged in series along a fuel supply pipeline.

42. The electrolytic system according to claim 40, wherein the heat exchange means comprises two heat exchangers arranged in parallel across a separate fuel supply line.

43. The electrolytic system according to any one of claims 40 to 42, wherein the two heat exchangers are coupled to each other and thus essentially form a single heat exchanger in which heat is exchanged from the two outlets to the inlet fuel.

44. The electrolytic system according to claims 40 to 43, further comprising a switching means for switching the two heat exchangers from a series configuration to a parallel configuration and vice versa.

45. The electrolytic system according to any one of claims 25 to 44, wherein the electrolytic cell is configured to operate at a temperature exceeding 400°C.

46. The electrolytic system according to any one of claims 25 to 45, as dependent on claim 27 or claim 29, wherein the membrane filter is a palladium-hydrogen membrane that operates at temperatures exceeding 400°C.

47. An electrolytic system according to any one of claims 25 to 46, configured to operate at a pressure exceeding 2 barg.

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