Electrolyser system

Abstract

An electrolyser system (20) comprising an electrolyser (16) with at least one stack (10) of electrolyser cells (11), at least one fuel inlet (22) for a fuel for the at least one stack (10), at least two off-gas outlets (18, 24) for off-gases from the at least one stack (10), at least one of the at least two off-gas outlets being an anode side off-gas outlet (18) for venting an anode off-gas from the at least one stack (10) and at least one of the at least two off-gas outlets being a cathode side off-gas outlet (24) for venting a cathode off-gas from the at least one stack (10) and means (142) for exchanging heat from the cathode side off-gas to a fuel for the at least one stack (10), and means (132) for exchanging heat from the anode side off-gas to a fuel for the at least one stack (10) wherein the stack (10) is configured to output, under normal operating conditions, the anode side off-gas, wherein at least 50% by weight of the anode side off-gas is oxygen produced by the electrolyser (16). The present invention also concerns a method of operating an electrolyser system (20) comprising calculating the pressure of a fluid on one side of at least one of the electrolyser cells (11) or the at least one stack (10); and controlling the pressure of a fluid on the other side of the at least one of the electrolyser cells (11) or the at least one stack (10) so that the pressure on the cathode side of the or each of the electrolyser cells (11), or the or each if the stacks (10) is greater than the pressure on the anode side of the or each or each of the cells (11) or the or each of the stacks (10).

Description

[0001] Electrolyser System

[0002] The present invention relates to an electrolyser system, and in particular an electrolyser system in which high energy efficiencies are achievable.

[0003] An electrolyser system will have one or more electrolysers, each of which may comprise one or more stacks of electrolyser cells - also known as regenerative fuel cells. The electrolysers are used to split a source fluid (the “fuel”) - usually steam or carbon dioxide - into its constituent parts and for that purpose it requires a source of electricity for supplying an electric current and voltage across / through the at least one stack of electrolyser cells. As an example, electrolysers can be used to produce hydrogen and oxygen from water, or carbon monoxide and oxygen from carbon dioxide, each by way of electrolysis.

[0004] The collection or use of the produced oxygen is important as it can be utilised in industry and medical applications, amongst many other uses. The collection and the subsequent storage and / or distribution of carbon monoxide is also important as it is useful for numerous chemical processes. The collection and the subsequent storage and / or distribution of hydrogen is also important as it is a fuel commodity that can help in the race for decarbonisation and for achieving net zero targets. The produced hydrogen can be utilised as a fuel for combustion, or as a fuel for a use in a fuel cell system for achieving an electrolytic reaction in the fuel cell to recombine the hydrogen with oxygen, with resultant electrical and heat outputs. The hydrogen can also have many other uses.

[0005] Combining electrolysers with green energy sources is also important as that can greatly contribute towards the green credentials of electrolysers, and particularly in their use in hydrogen capture, thus accelerating the achievement of net zero and decarbonisation targets.

[0006] Given the importance of electrolysers for meeting net zero and decarbonisation targets, and in industry in general, any improvement in the efficiency or service life of an electrolyser is considered important and valuable.

[0007] In typical electrolyser systems, oxygen is produced at the anode side of the electrolyser, and it is a product of the electrolysis of both water and carbon dioxide. In typical electrolysers, this oxygen is forced out of the electrolyser by way of a ‘sweep gas’. The sweep gas is typically pumped through the electrolyser via a fan, but the energy required to power the fan reduces the overall efficiency of the system. It can be termed a ‘parasitic load’. It is also advantageous to bring the sweep gas up to the operational temperature of the stack before passing it through the anode side of the electrolyser. This is to avoid dropping the temperature of the stack, and diminishing its operational efficiency. Most electrolyser stacks have a narrow optimum operational temperature range at which they have an optimum efficiency, and thus they typically are desired to operate at a substantially constant temperature within that temperature range. Furthermore, temperature deltas (variations / differences) across the stack or across an electrolyser cell, can create thermal stresses within the stack or cell, which in turn can lead to premature failure of the stack or cell, or leaks in the stack due to thermal expansion characteristics of the materials involved. The provision of such heating for the sweep gas, however, also increases the energy load on the electrolyser system - a further “parasitic load”.

[0008] Additionally, the sweep gas can introduce contaminants into the stack, or otherwise cause degradation to the stack through the interaction of the sweep-flow gas with the materials of the system, especially in the case of intermediate or high temperature electrolysers - i.e. those with an operational temperature in excess of 400 degrees C. These contaminants or degradations / interactions can reduce the operational service life of the stack through damage to the anode, or through the manifolds or pipework of the sweep-flow pipelines. Indeed, any system with such a sweep-flow will always encounter a gradual degradation of the anode or the manifolds or pipework of the sweep-flow pipelines during use of the electrolyser, but contaminants such as dirt particulates or trace elements of other toxic or reactive substances can worsen that rate of degradation.

[0009] The present invention therefore seeks to provide an electrolyser system with a longer service life, and preferably one where the parasitic loads associated with the sweep gas are reduced, removed, or avoided.

[0010] Statements of Invention

[0011] According to a first aspect of the present invention there is provided an electrolyser system comprising an electrolyser with at least one stack of electrolyser cells, the electrolyser comprising: at least one fuel inlet for a fuel for the at least one stack; at least two off-gas outlets for off-gases from the at least one stack, at least one of the at least two off-gas outlets being an anode side off-gas outlet for venting an anode off-gas from the at least one stack and at least one of the at least two off-gas outlets being a cathode side off-gas outlet for venting a cathode off-gas from the at least one stack; and means for exchanging heat from the cathode side off-gas to a fuel for the at least one stack, and means for exchanging heat from the anode side off-gas to a fuel for the at least one stack; wherein the stack is configured to output, under normal operating conditions, the anode side off-gas, wherein at least 50% by weight of the anode side off-gas is oxygen produced by the electrolyser.

[0012] In some embodiments, the means for exchanging heat is at least one heat exchanger. Other known mechanisms for heat exchange include condensers, evaporators and boilers, and the heat may exchange through any one or more of convection, conduction, thermal radiation, and evaporative cooling.

[0013] In some embodiments, a single means for heat exchange - with separate fluid pathways for the cathode side off-gas, the anode side off-gas and the fuel (to avoid mixing thereof) - might be used for achieving the heat exchange for both the cathode side off-gas and the anode side off-gas to the fuel. However, more usually there are at least two separate means for heat exchange provided. For example, at least one heat exchanger may be provided for the cathode side off-gas to the fuel and at least one further heat exchanger may be provided for the anode side off-gas to the fuel.

[0014] In some embodiments, a sweep flow gas is used for sweeping the oxygen from anode sides of the electrolyser cells, and the sweep flow gas will be less than 50% by weight of the anode side off-gas. The sweep flow gas may be an externally sourced sweep flow gas, or it may be a recirculating gas within the electrolyser system - such as a recirculation of at least part of the anode side off-gas, or the oxygen produced by the electrolyser.

[0015] In some embodiments, the stack is configured to produce, under normal operating conditions, the anode side off-gas having a greater proportion by weight of oxygen produced by the electrolyser to a proportion of the sweep flow gas.

[0016] In some embodiments, the anode-side off gas has a proportion by weight in excess of 50, 60, 70, 80 or 90% of oxygen produced by the electrolyser. Thus, the anode side off-gas has no more than a 50, 40, 30, 20 or 10% by weight, respectively, proportion of sweep flow gas within it. As such, the sweep flow gas entering the anode side of the or each stack for extraction of the anode side off-gas from the stack, or from any of the electrolyser cells within that stack, is reduced. Versus the prior art systems, it may be reduced to at most 50% of the flow from the stack. More preferably it may be reduced to no more than 40, 30, 20% and most preferably no more than 10% of the flow from the stack.

[0017] In some embodiments, the sweep gas is air, nitrogen, carbon dioxide or oxygen. Oxygen has the benefit in allowing highly pure oxygen to be vented from the electrolyser system as the anode side off-gas - potentially greater than 85% by weight, potentially greater than 90% by weight, and even potentially greater than 95% or even 99% by weight.

[0018] In some embodiments, the stack is configured to operate without any flow of sweep gas for extraction of the anode side off-gas from the stack, or from any of the electrolyser cells within that stack. In some embodiments this configuration is selectively utilised - for example it may be utilised when under normal operating conditions, i.e. steady state conditions, rather than during start-up. During the latter, a heated sweep gas may be utilised to assist with the startup process.

[0019] In some embodiments, the stack is configured with a control system to operate the stack substantially at a thermoneutral voltage.

[0020] In some embodiments, the control system is configured to operate the stack at one side of a thermoneutral voltage. This might then permit an offset to be provided for balancing heat across the fuel cell stack.

[0021] In some embodiments, the control system operates the stack with a voltage that can fluctuate either side of and through the thermoneutral voltage over a given period of time.

[0022] The present invention eliminates or reduces the reliance upon a sweep gas for the collection of the oxygen produced on the anode side of the electrolyser cells. This provides a longer service life for each electrolyser cell, as a lower volume of gas passes through the anode side of each electrolyser cell, and furthermore, the anode off-gases will be of a higher purity (i.e. a higher percentage of oxygen) - typically even a pure oxygen flow. The anode off-gas is thus of a higher commercial value.

[0023] Further, given the resulting closed nature of such a sweep-flow circuit, there is a lower or zero possibility of external contaminants entering the sweep-flow circuit, thus reducing wear, corrosion or other causes for degradation of the anode side of the electrolyser stack, and in particular the anodes of the electrolyser cells and any manifolds or pipework of the sweepflow circuit. Yet further, as the anode off-gas is substantially pure oxygen from the anode side of the stack - pure oxygen in the case of a zero sweep gas solution, the commercial value of the produced gas can be higher from the moment it is extracted. Also, it will require less or no downstream processing before it is stored or distributed for downstream use, as it can already be at an adequately pure condition for such commercial use.

[0024] Yet further, as the electrolyser cells operate at or close to a thermoneutral voltage, that eliminates or substantially eliminates the need to provide or operate supplementary heaters within the electrolyser for maintaining operating temperature in the stack(s). As electrical heaters are relatively inefficient, avoiding their use increases the efficiency of the electrolyser system by removing the parasitic load of those heaters.

[0025] Thermoneutrality is achieved by controlling the voltage (or the current, i.e. the supplied electrical power) fed to the electrolyser cells (or the stack thereof). By operating with thermoneutrally, the voltage drop is sufficient to drive the reaction but also to maintain a constant stack temperature, and thus the system, through the heat exchangers, can maintain the temperature of the incoming fuel (usually steam or carbon dioxide) at the optimal operating temperature for the stack.

[0026] In some embodiments of the present invention, the electrolyser cells, or each stack of electrolyser cells, operates substantially thermoneutrally, and galvanostatically (i.e. with a constant current).

[0027] In some embodiments, the sustained operation of the electrolyser system at normal operational conditions is achieved with no supplementary electrical heaters within the manifolds or pipework of either any sweep-flow pipelines or any fuel flow pipelines within the electrolyser, or within the electrolyser system. However, in some embodiments, such supplementary heaters are provided - for example to assist with start-up of the electrolyser system. It is preferred that such supplementary electrical heaters are subsequently either disconnected from the pipelines or the like, or are turned off or disabled, during normal operational conditions of the electrolyser, the stack(s) or the electrolyser cells.

[0028] In some embodiments, normal operational conditions include operating the stack within a target range of operational temperatures for a given stack chemistry - i.e. not during start up or shut down. The parameters for normal operational conditions may also or instead include any one or more of a target range of operational gas pressures, sweep-flow volumes, electrical voltages, electrical currents, electrical power draw and electrical resistance, for a given stack chemistry or a given stack size, or both, again while not during start up or shut down.

[0029] In some embodiments, the electrolyser cells are solid oxide electrolyser cells.

[0030] In some embodiments, the electrolyser comprises more than one stack. 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, there are two heat exchangers arranged in series along a fuel supply pipeline. These two heat exchangers may be coupled to one-another thus essentially forming a single heat exchanger where heat is exchanged from the two outlets to the inlet fuel.

[0033] In some embodiments, means for exchanging heat comprises two heat exchangers arranged in parallel across separate fuel supply lines. These two heat exchangers may be coupled to one-another thus essentially forming a single heat exchanger where heat is exchanged from the two outlets to the inlet fuel.

[0034] In some embodiments, there are additional switching means arranged to switch the arrangement of the two heat exchangers across the fuel supply line(s) from a series configuration to a parallel configuration and vice versa. This has a particular application to transient operating conditions for the electrolyser, such as start up, shutdown, warm-up, cooldown, and temperature changes etc., for example where one arrangement may be thermodynamically more preferable to the other. Having a switching means affords the choice of the most efficient arrangement at a given operating condition. For example, it may be advantageous to have the heat exchangers arranged in series during startup or warmup in order to heat up both heat exchangers quickly, before switching to parallel during normal operations for steady state operation, and / or for more even heating. This is particularly useful for cases with zero sweep flow where the heat exchanger between the oxygen outlet and the fuel inlet would otherwise be non-operational until the electrolyser had heated up.

[0035] According to one embodiment there is provided an electrolyser system comprising a fuel inlet, and at least one electrolyser. There are two outlets from the electrolyser which exchange heat with the fuel inlet via a first 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 either in a series or parallel arrangement.

[0036] The switching means may comprise a series of valves and / or additional pipework. For example, the valves may comprise two or three way valves.

[0037] Both supply lines may connect into the same stack. They may join before a or the fuel inlet or they may feed separate fuel inlets.

[0038] In some embodiments, the fuel is water. Usually it is steam - it may be provided from an external steam or water source. If from a liquid water source, the electrolyser system may comprise a steam generator to convert 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 has particular application to electrolysers in the intermediate and high temperature electrolyser cell sectors (>400 degrees C and >750 degrees C, respectively). It can also have particular application with lower temperature electrolyser cell temperatures, albeit with operational temperatures above 100 degrees C, and thus operating with steam rather than water. This is since the heat exchangers can pre-heat the fuel before it enters the at least one stack. However, water based electrolysers (with operational temperatures below 100 degrees C) may also benefit from the present invention. Likewise, carbon dioxide electrolysers can also benefit from the present invention.

[0041] In some embodiments, the at least one electrolyser cell is a solid oxide electrolyser cell, i.e. the electrochemically active region of the or each electrolyser cell is a solid oxide. A solid oxide electrolyser cell (SOEC) typically operates in the 400-900 degrees C range, or for some chemistries, 400 to 700 degrees C, or more particularly in the 450-650 degrees C temperature range. Such electrolyser cells that operate at working temperatures of between 400 and 750 degrees C may be referred to as intermediate-temperature solid oxide electrolyser cells, or IT- SOECs.

[0042] An advantage of steam-based electrolysers is that steam electrolysis - particularly intermediate and high temperature steam electrolysis at temperatures above 400 degrees C - efficiently produces hydrogen as the high temperature environment can reduce the electric power requirements for the electrolysis of the water molecules from steam compared to electrolysis of liquid water. Additionally, the higher temperature can relatively increase the reaction activity with the electrolyser versus that of liquid water. The present invention is thus highly suitable for use with solid oxide electrolyser cells (or SOECs) operating at temperatures above 400 degrees C (generally known as intermediate temperature SOECs, or instead high temperature SOECs if above 750 degrees C.

[0043] There are many possible forms of SOEC, using different electrochemically active electrolyte chemistries. 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 working temperature of a SOEC (usually in excess of 400 degrees C), the water passing through the electrolyser cell will be vaporised into high temperature steam if not provided at such temperatures and in such a state. Usually, however, the heat exchangers will pre-heat the steam into such a condition.

[0045] In some embodiments, the electrolyser system further comprises a hydrogen / steam separator positioned after the cathode side off-gas outlet. It may be between the cathode side off-gas outlet and the means for exchanging heat from the cathode side off-gas, e.g. to the fuel. The hydrogen / steam separator can be utilized to separate the cathode off-gases into a) steam for recycling back into the electrolyser, which may be via a steam recirculation pipeline, and b) hydrogen for passing through that heat exchanger. With a hydrogen / steam separator, and that heat exchanger, the hydrogen can be substantially dried before it is passed along for subsequent collection or distribution. This then provides a hydrogen supply with an increased value as it is already purer, having been substantially or mostly separated from the steam.

[0046] In some embodiments, the hydrogen / steam separator is a cooling / reheating loop, which condenses out the steam by cooling the wet hydrogen to a temperature below the dew point of the water, and then reheating it, using, for example, one or more regenerative heat exchangers. In some embodiments, this happens before the means for heat exchange. In other embodiments, it happens after the means for heat exchange. In some embodiments, the means for heat exchange provides this function in addition to the heat exchange to the fuel.

[0047] In some embodiments, the hydrogen / steam separator is a membrane filter. In some embodiments, the membrane filter operates with a pressure delta across the membrane. For example, the mixture side (steam and hydrogen) may be at a pressure corresponding to substantially that of the electrolyser or stack, and the other side - to which the hydrogen can pass - can be at a reduced pressure - for example ambient pressure or a lowered pressure by virtue of a pump. In some embodiments, the pressure delta is provided by the electrolyser system operating at an elevated pressure versus ambient pressure around the electrolyser system, or at a hydrogen outlet side of the membrane filter. For example, the electrolyser system may operate at a pressure in excess of 2bar (or 1 barg), and the hydrogen outlet side of the membrane filter may be at ambient pressure - for example at approximately 1 bar or Obarg.

[0048] According to a further aspect of the present invention there is provided an electrolyser system comprising an electrolyser with at least one stack of electrolyser cells, the electrolyser comprising: at least one fuel inlet for a fuel for the at least one stack; at least two off-gas outlets for off-gases from the at least one stack, at least one of the at least two off-gas outlets being an anode side off-gas outlet for venting an anode off-gas from the at least one stack and at least one of the at least two off-gas outlets being a cathode side off-gas outlet for venting a cathode off-gas from the at least one stack; andthe electrolyser system further comprises a hydrogen / steam separator positioned between the cathode side off-gas outlet and the heat exchanger for exchanging heat from the cathode side off-gas, wherein the hydrogen / steam separator is a membrane filter that operates with a pressure delta across the membrane.

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

[0050] In some embodiments, the stack is configured with a control system to operate the stack substantially at a thermoneutral voltage.

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

[0052] Features of the first aspect may be likewise present on this further aspect.

[0053] In some embodiments, these electrolyser systems may be configured either or both: a) to produce oxygen with a volumetric purity by weight in excess of 90% by operating while at normal operational temperatures without any more than a 10% by weight external sweep flow gas supply entering the anode side of the or each stack for extraction of the anode side off-gas from the stack, or from any of the electrolyser cells within that stack, or b) to operate while at normal operational temperatures without any flow of an external sweep gas for extraction of the anode side off-gas from the stack, or from any of the electrolyser cells within that stack. The system may thus be in accordance with the preceding aspect.

[0054] In some embodiments of the electrolyser system of either aspect, the membrane filter is a palladium hydrogen membrane that operates at a temperature in excess of 400 degrees C. It is thus then suitable for use with an SOEC electrolyser system.

[0055] In accordance with a preferred embodiment, the electrolyser system is configured to operate at a pressure in excess of 2 barg. The elevated pressure versus ambient pressures enable a membrane filter to operate efficiently - without an external pump, thus avoiding a further parasitic load on the system.

[0056] According to a further aspect of the present invention there is provided a method of venting a substantially pure oxygen off-gas from an electrolyser of an electrolyser system, the method comprising: generating a pressure differential between the interior of the electrolyser and an inlet or outlet for the off-gas from the electrolyser; wherein the pressure differential is generated by the production of oxygen through electrolysis by the electrolyser.

[0057] In some embodiments, the method operates without any external sweep gas, or with a sweep gas that is substantially pure oxygen, whereby the purity of the produced oxygen dictates the purity of the oxygen off-gas. As the electrolysis process on fuels such as water and carbon dioxide produce substantially pure oxygen on an anode side of electrolyser cells within the electrolyser, the oxygen off-gas can then be substantially pure oxygen.

[0058] In some embodiments, the method comprises controlling the venting of the oxygen by way of a back pressure regulator. In other embodiments, the rate of production of oxygen by the electrolyser is such that the electrolyser self regulates to maintain the pressure differential, or such that when sufficient oxygen is generated it builds up enough pressure to self-vent from the electrolyser.

[0059] According to a further aspect of the present invention there is provided a method of generating a substantially pure oxygen off-gas from an electrolyser of an electrolyser system, the method comprising: generating a pressure differential between the interior of the electrolyser and an inlet or outlet for the substantially pure oxygen off-gas from the electrolyser; wherein the method comprises controlling the venting of the oxygen by way of a back pressure regulator. With the back pressure regulator, when sufficient pressure builds up, or if sufficient pressure is maintained by the rate of production of oxygen by the electrolyser, the back pressure regulator can open to enable venting of the oxygen past the back pressure regulator.

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

[0061] Typically, the substantially pure oxygen off-gas from the electrolyser is vented on an anodeside of the electrolyser.

[0062] In some embodiments, the oxygen off-gas has a proportion by weight in excess of 50, 60, 70, 80 or 90% of oxygen produced by the electrolyser. Thus the anode side off-gas has no more than a 50, 40, 30, 20 or 10% by weight, respectively, proportion of sweep flow gas within it, and as such any sweep flow gas entering the electrolyser for extraction of the oxygen off-gas from the electrolyser is reduced, versus the prior art systems, to at most 50% of the flow from the stack, and more preferably no more than 40, 30, 20% and most preferably no more than 10%.

[0063] In some embodiments, the sweep gas is air, nitrogen, carbon dioxide or oxygen. Oxygen has the benefit in allowing highly pure oxygen to be vented from the electrolyser system as the oxygen off-gas - potentially greater than 85% by weight, potentially greater than 90% by weight, and even potentially greater than 95% or even 99% by weight, even if the sweep gas volume through the electrolyser exceeds 50% of the outflow from the anode side of the electrolyser.

[0064] The electrolyser system can comprise more than one electrolyser.

[0065] In some embodiments, the or each electrolyser comprises at least one stack of electrolyser cells.

[0066] In some embodiments, the or each electrolyser comprises at least one fuel inlet for a fuel for the at least one stack. The fuel in typical embodiments is either water, or steam, or carbon dioxide. Neither the inlet nor the outlet for the substantially pure oxygen off-gas from the electrolyser is connected to the fuel inlet. In some embodiments, the or each electrolyser does not comprise an inlet for sweeping the substantially pure oxygen off-gas, and the pressure differential is between the interior of the electrolyser and an outlet for the substantially pure oxygen off-gas from the electrolyser.

[0067] In some embodiments, the or each electrolyser comprises at least two off-gas outlets for offgases from the at least one stack thereof. At least one of the at least two off-gas outlets is an anode side off-gas outlet for venting an anode off-gas from the at least one stack. At least one of the at least two off-gas outlets is a cathode side off-gas outlet for venting a cathode off-gas from the at least one stack. The anode side off-gas outlet for venting an anode off-gas from the at least one stack can be configured for connection to, or is connected to, the outlet for the substantially pure oxygen off-gas, and the anode off-gas is the substantially pure oxygen offgas.

[0068] In some embodiments, the electrolyser system is the electrolyser system of the first or further aspects of the present invention, or any of their various embodiments.

[0069] In some embodiments, a return loop - which may be a continuously open loop (or it may be selectively openable and closeable), is provided between a sweep-flow inlet of the at least one stack and the anode side off-gas outlet of that stack. This allows oxygen produced by the stack to circulate around the loop, even if not vented from the electrolyser system. This helps to avoid stagnation of the oxygen, which could otherwise lead to combustion with any leaking hydrogen in the vicinity of that stagnation.

[0070] In some embodiments, the back pressure regulator is a pressure release valve.

[0071] In some embodiments, the back pressure regulator is on an outlet side of the electrolyser.

[0072] The pressure differential is typically generated by the production of oxygen through electrolysis by the electrolyser in the or each stack thereof. However, in some embodiments, the pressure differential is initially generated, or it may be helped to be maintained between the interior of the electrolyser and the inlet or outlet for the substantially pure oxygen off-gas from the electrolyser, by a supplemental sweep gas.

[0073] In some embodiments, the pressure of the electrolyser is 0.1 to 10 barg, preferably 0.1 to 5 barg, and more preferably 0.2 to 1 barg. This may be the pressure on both the anode side and the cathode side of the electrolyser. In some embodiments, the pressures on each of anode and cathode sides of electrolyser cells in the electrolyser are substantially balanced - preferably with a variance not exceeding + / - 10%, of the barg pressure of the anode side. In some embodiments, the variance does not exceed 0.1 barg. By being balanced or substantially so, there are fewer stresses and strains on electrochemically active layers of the electrolyser cells, and on the interconnections, between adjacent electrolyser cells.

[0074] According to another embodiment, there is provided a method of operating an electrolyser system comprising an electrolyser with at least one stack of electrolyser cells, the cells each having an anode side and a cathode side, the method comprising: calculating the pressure of a fluid on one side of at least one of the electrolyser cells or the at least one stack; and controlling the pressure of a fluid on the other side of the at least one of the electrolyser cells or the at least one stack so that the pressure on the cathode side of the or each electrolyser cell, or the or each stack is greater than the pressure on the anode side of the or each electrolyser cell or the or each stack.

[0075] In some embodiments, the pressure on the anode (oxygen) side of the or each electrolyser cell or the or each stack is controlled to be greater than the pressure on the cathode (fuel) side of the or each electrolyser cell or the or each stack. Controlling the pressure on the anode side to be greater than on the cathode side means that, in the event of a leak between the anode side and cathode side, the pressure differential forces oxygen and / or sweep gas into the fuel (cathode) volume rather than hydrogen (produced on the fuel side of the electrolyser cell) into the oxygen (anode) volume. This allows for a safer operation in the event of a leak, as it prevents an undesireable build-up of combustible gases (e.g. hydrogen) in the electrolyser’s high percentage oxygen environment. Additionally, in the event of combustion of oxygen and hydrogen due to a cross over leak, allowing this combustion to occur in the fuel volume rather than the air volume is beneficial, as the combustion process generates steam, which can accelerate degradation of air-side components of the electrolyser cell. Typically, air is dried before being fed into an electrolyser to ameliorate the effects of humidity. Similarly, the reduction or elimination of a sweep flow reduces the exposure the anode has to humidity.

[0076] An additional advantage is that oxygen (or nitrogen if using air or nitrogen as a sweep gas) is a larger molecule than hydrogen so the pressure being higher at the oxygen side reduces the leak rate. It should be noted that nitrogen leaking into the fuel side may require subsequent separation, but many uses of hydrogen also utilise nitrogen, notably ammonia production. It should be appreciated that this concept applies to all electrolysers and methods of operation (e.g. with or without a sweep flow) irrespective of the oxygen concentration of the anode side gas (so long as the oxygen concentration at the anode side is greater than 0%).

[0077] In some embodiments, the pressure differential between the cathode and anode sides of the electrolyser cell is controlled to be as low as reasonably possible while still maintaining a greater pressure on the cathode side than the anode side. Optionally, the anode side pressure is no more than 50mbar greater than the cathode side pressure, optionally no more than 40mbar, optionally nor more than 30mbar, optionally no more than 20mbar, optionally no more than 10mbar. In this way, the pressure on the anode side is actively controlled to always be greater than the pressure on the cathode side.

[0078] In the above embodiment, a pressure controlling device or system in communication with one or more pressure calculating device or system will control the pressure on one side of at least one of the electrolyser cells or stacks so that the pressure on the cathode side is greater than the pressure on the anode side. The pressure calculating device or system may calculate the pressure on one or both of the cathode and anode sides of at least one of the electrolyser cells or stacks, or for each cell or each stack thereof, at one or both of the fluid inlet and outlet. The pressure calculating device or system may be a pressure sensor or a fluid flow rate sensor. The pressure differential may be controlled by a back pressure regulator. In some implementations, it may be preferable to calculate the pressure on the cathode (fuel) side and to control the pressure on the anode (oxygen) side. This is because the fuel pressure may be defined by an external system and thus it might not be directly or entirely controllable by the electrolyser system.

[0079] In some embodiments, the method comprises a start-up or transition cycle in which when the electrolyser has an internal temperature below 100 degrees C, air, carbon dioxide or nitrogen, is used as a replacement through a cathode side of the electrolyser, in place of a fuel. This is because steam can condense below 100 degrees C, especially in a pressurised vessel, and the resulting water or moisture can be damaging to a cathode of an electrolyser.

[0080] The air / carbon dioxide / nitrogen may be preheated - for example in another industrial process (and thus be waste heat), or by another industrial process or heat source (for example via a heat exchanger) or by an electrical heater, or it may be heated by one or more of these sources of heat within the electrolyser system. In some embodiments, the method operates with no external sweep-flow, whereby no sweepflow inlet is required for the electrolyser system. However, in some embodiments the method comprises recirculating a portion of the produced oxygen within the electrolyser system (or even within the electrolyser or stack) as a sweep gas. For that purpose, the previously mentioned loop may be provided, although internal manifolds or pipework can also be used for this purpose.

[0081] In some embodiments, the sweep flow - be it a recirculation of the oxygen, or an external sweep flow gas - is driven through the electrolyser or the stack by one or more pumps. In some embodiments the pump is a thermal pump.

[0082] In some embodiments, a removable pre-heater is provided for the stack or electrolyser or electrolyser system. The removable preheater may be an electric heater or a heat exchanger heater. It may be provided on the anode side of the electrolyser or stack, although it is preferably only provided on the cathode side of the electrolyser or stack.

[0083] In some embodiments, the electrolyser system further comprises a hydrogen / steam separator positioned between the cathode side off-gas outlet and the heat exchanger for exchanging heat from the cathode side off-gas. In preferred embodiments, the hydrogen / steam separator is a membrane filter. It may operate with a pressure delta across the membrane.

[0084] Brief Description of Drawings

[0085] These and other features of the present invention will now be described in further detail, purely by way of example, with reference to the accompanying drawings in which:

[0086] Figure 1 schematically shows a typical prior art electrolyser cell, multiples of which may be stacked in one or more stacks of an electrolyser of the electrolyser system of the present invention, albeit potentially modified as discussed below;

[0087] Figure 2 schematically shows a simplified first embodiment of the present invention in which first and second heat exchangers are arranged in series for preheating the fuel for the electrolyser;

[0088] Figure 3 schematically shows a simplified second embodiment of the present invention in which the first and second heat exchangers are instead arranged in parallel; Figure 4 schematically shows a simplified third embodiment of the present invention in which the heat exchangers are once again arranged in series, like in Figure 2, but with a hydrogen / steam separator positioned between the electrolyser’s off-gas outlet and the second heat exchanger;

[0089] Figure 5 schematically shows a simplified fourth embodiment of the present invention in which the heat exchangers are instead arranged in parallel, like in Figure 3, but with an air supply and air inlet for the electrolyser, which may be required in some embodiments for assisting with start-up of the electrolyser;

[0090] Figure 6 schematically shows a simplified fifth embodiment of the present invention in which the heat exchangers are again arranged in parallel, like in Figure 2, but with a start-up heater for the fuel, which may similarly be required in some embodiments for assisting with start-up of the electrolyser;

[0091] Figure 7 schematically shows a sixth embodiment of an electrolyser system in accordance with the present invention, in which an air supply is optionally provided to connect to a sweepflow inlet of the electrolyser for a sweep-flow of an anode side of the electrolyser, along with a sweep-flow circuit that links between the anode side’s off-gas outlet and the sweep-flow inlet; and

[0092] Figure 8 schematically shows a seventh embodiment of an electrolyser system in accordance with the present invention, showing an alternative configuration for the sweep-flow on the anode side of the electrolyser;

[0093] Figures 9 and 10 show a simplified eighth embodiment of the present invention in which the first and second heat exchangers can be switched between parallel (Fig. 9) and series (Fig. 10) arrangements

[0094] Referring first to Figure 1 , the basic structure and operation of a prior-art electrolyser cell 11 is shown by reference to one fuel cell 11 of a stack 10 of cells 11 within an electrolyser 16.

[0095] The electrolyser cell 11 comprises an anode 60, a cathode 62 and an electrolyte 54. Such a structure for an electrolyser cell 11 is well known in the art. The “fuel” for the electrolyser 16 - usually water or carbon dioxide - hereinafter water in the form of steam 43 for this embodiment, is passed over the cathode 62 and a sweep gas 42 is passed over the anode 60. To power the electrolyser cell 11 , an electric current / voltage is applied across the electrolyser cell 11 via electric terminals / connections 66, 68 at the anode and cathode sides of the electrolyser cell 11 . As a consequence, an electrolytic reaction occurs across the electrolyte 54, with oxygen ions passing across the electrolyte 54 from the cathode 62 to the anode 60, whereby some of the steam 43 is broken down into hydrogen on the cathode side of the electrolyser cell 11 and oxygen at the anode side. With carbon dioxide as the fuel, the fuel is instead partially broken down into carbon monoxide and oxygen, where the oxygen again is produced at the anode side and the carbon monoxide is produced on the cathode side.

[0096] In this prior art electrolyser cell 11 , the oxygen (a first off-gas) is extracted by the sweep gas 42, which produces an air flow or sweep-flow through the electrolyser cell 11 for venting the oxygen - now mixed with the sweep gas, out of an off-gas outlet 38 on the anode side of the electrolyser cell 11 . As the sweep gas 42 is usually air, the gas mixture vented from the offgas outlet 38 will be an oxygen-enriched air 64. In some prior art systems, however, a different sweep gas 42 is used - for example the sweep gas 42 may be nitrogen, and the oxygen is thus then swept from the anode side by being mixed instead with the nitrogen.

[0097] The hydrogen (or second off-gas) on the cathode side can instead be extracted and vented out of the electrolyser cell 11 via another off-gas outlet 44 of the electrolyser cell 11. This second off-gas 63 will typically be a mixture of the hydrogen (or the carbon monoxide when the fuel is instead carbon dioxide) with the remaining fuel 43 (i.e. the remaining steam 43 or carbon dioxide). As the conversion of the fuel 43 is usually only in respect of a proportion of the supplied fuel 43, in the case of steam 43 the produced hydrogen is vented from the cathode side, via the other off-gas outlet 44, as ‘wet’ hydrogen 63.

[0098] Furthermore, it is known that it is desirable to provide the fuel 43 and the sweep gas 42 to inlets 32, 22 of the cells 11 at a temperature close to or at the operational temperature of the cells 11. In the case of a steam electrolyser 16, this will be a temperature in excess of 100 degrees C. In the case of a pressurised steam electrolyser 16, and indeed for most steam electrolysers 16, including those of solid oxide electrolyser cell (SOEC) systems 20, this will typically be a temperature well above 100 degrees C. For SOEC systems 20, for example, the temperature is typically in excess of 400 degrees C. For this purpose, due to the high operating temperature of the electrolyser cell 11 (i.e. above 100 degrees C) for a steam electrolyser 16, and in the case of a solid oxide electrolyser cell (SOEC) 11 , usually in excess of 400 degrees C, the heat of the off-gases 63, 64 from the off-gas outlets 38, 44 would not want to be wasted in an off-gas collection (or onward delivery) system. As such, that heat is usually usefully used by the electrolyser system 20 to provide at least some heat for steam generation, sweep-gas heating or for steam super-heating, on an input side of the stack 10.

[0099] Such operational characteristics of electrolyser cells 11 and stacks 10 thereof are well known in the art. However, with preferred embodiments of the present invention, during normal operational use of the electrolysers 16, no externally supplied sweep gas is provided. As a result, such “recovered” heat can all be transferred to the steam 43. This can improve the efficiency of the system 20, not least since there is no sweep gas 42 to heat. That removes that parasitic load from the electrolyser system 20. Furthermore, there will then be a reduced degradation of the anode side and sweep gas manifold and pipework due to the reduced volume of fluid flow therethrough.

[0100] A schematic view of a first such embodiment is shown in Figure 2. As shown, in simplified form, there is provided an electrolyser system 20 comprising an electrolyser 16 that comprises a stack 10 of electrolyser cells 11 . In this embodiment, there are 8 electrolyser cells 11 in the stack 10, although a typical stack 10 can have a greater or smaller number of electrolyser cells 11. For example, a typical commercial stack 10 may have 100 electrolyser cells 11.

[0101] It is also possible for the electrolyser 16 to have more than one stack 10 of electrolyser cells 11.

[0102] Typically, the cells 11 are arranged in series through the stack 10, as shown. However, electrolyser cells 11 can be arranged in parallel. Likewise, if more than one stack 10 is provided they can be in parallel or in series.

[0103] The electrolyser 16 in this example has a single fluid inlet 22. This inlet is for the fuel 43 - usually steam or carbon dioxide. For the purposes of this discussion, we will discuss steam as the fuel 43.

[0104] The steam is provided from a source of steam, or a steam generator (not shown). It feeds steam 43 to the electrolyser system 20 via a supply line 124. Along that supply line 124 two heat exchangers 132, 142 are provided. A first heat exchanger 132 provides initial recycled heating for the steam 43, and the second 142 provides final recycled heating for the steam 43. These two heat recycle processes take heat from the off-gases 63, 64 of the electrolyser 16 and exchange it into the supplied steam 43 for the electrolyser 16. However, it should be appreciated that the two heat exchangers 132, 142 may be formed as one unit, which takes two ‘hot’ inputs and transfers the combined heat to the steam 43.

[0105] As shown in Figure 2, there are two heat exchangers 132, 142 and they are arranged in series. The first heat exchanger is an anode side heat exchanger 132, and it exchanges heat from the anode off-gas 64 into the steam 43 from the steam source or steam generator. The anode off-gas 64 may be substantially pure oxygen, although it might be oxygen enriched air or nitrogen if a sweep gas is also being used. Ideally, however, the oxygen is at least 90% pure oxygen when the system 20 is operating at a steady state operation, as the present invention is ideally operated without an externally sourced sweep gas - and hence just the one fluid input 22. With such an arrangement, the heat from both off-gasses is exchanged with a single fuel input thus reducing the energy requirements to bring this fuel up to temperature.

[0106] That anode off-gas 64 is passed from an anode off-gas outlet 18 of the stack 10 along a pipeline 106 to the heat exchanger 132. Thereafter, the now somewhat cooled anode off-gas 64 can then be collected for use elsewhere.

[0107] The second heat exchanger 142 is a cathode side heat exchanger 142 and it exchanges heat from the cathode off-gas 63 into the now pre-heated steam from the first heat exchanger 132 for superheating the steam 43 prior to its entry into the electrolyser 16. It is possible, however, for the order of the first and second heat exchangers 132, 142 to be reversed.

[0108] The cathode off-gas 63 may be wet hydrogen, i.e. hydrogen mixed with steam, when the fuel is steam, although it might be carbon monoxide mixed with carbon dioxide if the fuel is instead carbon dioxide.

[0109] That anode off-gas 63 is passed from a cathode off-gas outlet 24 of the stack 10 along another pipeline 126 to the second heat exchanger 142. Thereafter, the now somewhat cooled cathode off-gas 63 can then be collected for use elsewhere.

[0110] This simplified example of an electrolyser system 20 is preferably provided, as shown, with an electrolyser 16 having only the one fluid inlet 22 - for the fuel 43 (steam). No sweep gas inlet is provided, as no externally sourced sweep gas is necessary in some embodiments of the present invention. Thus, otherthan that one fluid inlet 22, the stack / electrolyserwill have inputs (terminals) for power (for applying a current across the electrolyser cells 11 in the stack 10), plus the fluid outlets 18, 24 for the off-gases 63, 64. As such this is a very simple arrangement externally, although there can be internal manifolding and pipework for delivering and extracting the fuel and the off-gases from the individual cells 11 and / or stacks 10.

[0111] With the heat exchangers in series, each heat exchanger might not be working at their highest heat transfer efficiency as the temperature delta (difference) across the two fluids passing through the heat exchanger (i.e. the off-gas and the steam) will be smaller versus the arrangement in Figure 3 (where a parallel configuration is provided).

[0112] The extraction of the hydrogen produced by the electrolyser cells 11 in Figure 2, unlike the extraction of the oxygen, can be conventional, in that it becomes entrained within any remaining steam that passes through the cells 11 in the stack 10. As for the oxygen, that can be extracted as and when it is plentiful enough on the anode side of the stack 10. However, during normal use the electrolyser 16 will run continuously, so the oxygen will be being produced continuously. Therefore, it can continuously self-pressurise on the anode side of the electrolyser cells 11 . Thus, it can create its own extraction force (or fluid flow) as the molar volume, and pressure thereof, builds up above a threshold of an exit valve, or relative to a downstream pressure of a collection chamber or pipeline.

[0113] Operationally, a stack will usually want to avoid large pressure differentials across the cells, so the threshold can be set appropriately, or the cathode side’s pressure can be controlled to balance the pressures across the cells. In one embodiment, the pressure differential can be controlled (and is kept as small as possible) by use of a common regulator system on both sides. However, it is advantageous to control the pressure differential to be positive on the oxygen (O2) side to ensure any O2 leaks to the fuel rather than the other way around. This is better for the stack life (less degradation) and applies to all electrolysers and methods of operation (e.g. with or without a sweep flow). Optionally, the anode (O2) side pressure is no more than 50mbar greater than the cathode (fuel) side pressure, optionally no more than 40mbar, optionally nor more than 30mbar, optionally no more than 20mbar, optionally no more than 10mbar. In this way, the pressure on the anode side is actively controlled to always be greater than the pressure on the cathode side.

[0114] In some embodiments, for start-up, the stack can be heated using an external system that heats the inlet stream. This might be any one of, or a combination of, a stream of steam, nitrogen, hydrogen or air. The external heat might be a waste product of another industrial process, or from an alternative heat source such as an electric heater. Where oxygen is present in this start-up gas flow (i.e. for air), and when the cathode comprises nickel, nickel oxidation of the cathode can be avoided by applying a voltage across the stack. This is effective where the start-up gas comprises a mixture of steam with the air.

[0115] A variant of this system 20 is shown in Figure 3. As mentioned above, instead of the two heat exchangers 132, 142 being arranged in series, they are instead arranged in parallel. For this purpose, two pipelines 124 for the steam 43 are provided upstream of the heat exchangers 132, 142. The outputs from those heat exchangers 132, 142 then reconnect to a singular continuance of those pipelines 124 to supply the now heated steam 43 to the electrolyser 16 at a common inlet 22. It is to be appreciated, however, that separate inlets 22 for each of these streams of steam is a possible instead.

[0116] The pipework for this alternative configuration is more complicated, and as a result, the capital investment in the architecture would be more costly. However, the temperature deltas across both heat exchangers can be higher, thus potentially increasing their heat transfer efficiency. Over the life of the electrolyser system, this improvement in efficiency could make the system overall more cost efficient.

[0117] Again, an external system may be required for system start-up, and nickel oxidation protection may be desirable, dependent upon the make-up of the start-up fluid.

[0118] A further variant of this system is shown in Figures 9 and 10. In Figure 9, as in Figure 3, both heat exchangers 132, 142 are shown arranged in parallel, with (herein first or upstream) valves 200a, 200b being configured as open and (herein second or downstream) valves 202a, 202b being configured as closed. For this purpose, a first pipeline, or supply line 124 for the steam 43 splits into two pipelines, with each of these two pipelines connecting to one of the heat exchangers 132, 142 before reconnecting to a singular continuance of the first pipeline or supply line 124 to supply now heated steam to the electrolyser at a common inlet 22.

[0119] The valves 200a, 200b, 202a, 202b can however be switched to the configuration seen in Figure 10, where the first valves 200a, 202a are open and the second valves 200b, 202b are closed, in order to arrange the heat exchangers 132, 142 in series. The inverse opening I closing of valves will have the same effect, but the flow then going to the heat exchangers in a reverse order, i.e. to heat exchanger 142 before heat exchanger 132.

[0120] The valve arrangement may be simplified by replacing each set of four valves 200a, 202a, 202b and 202a with a single three-way valve. This is as the valves operate in pairs made from similar numbers 200a, 200b (etc.), or similar letters 200a, 202a (etc.). Such an arrangement requires fewer parts, but the parts may be more expensive.

[0121] Referring next to Figure 4, a further variant of the system 20 of Figure 2 is shown. In this variant, the heat exchangers 132, 142 are once again arranged in series, like in Figure 2, and with the same advantages and disadvantages as mentioned above. However, a hydrogen / steam separator 28 is additionally positioned between the electrolyser’s cathode offgas outlet 24 and the second heat exchanger 142. The hydrogen / steam separator 28 can be utilized to separate the cathode off-gases into a) steam for recycling back into the electrolyser 16 via a steam recirculation pipeline 30 and b) hydrogen for passing through the second heat exchanger 142 for heating the externally sourced steam as before.

[0122] The hydrogen - now substantially dried by the hydrogen / steam separator 28 - and potentially also by the second heat exchanger 142, if it is reduced in temperature below its steam’s dew point, can then be passed along for subsequent collection or distribution.

[0123] This modified embodiment with the hydrogen / steam separator 28 provides a hydrogen supply with an increased value as it is already more pure, having been substantially or mostly separated from the steam.

[0124] The hydrogen / steam separator 28 is a separator 28 for separating at least part of the hydrogen out of the wet hydrogen stream. There are various forms of hydrogen / steam separator 28 known in the art. For example, this may be a cooling / reheating loop, which condenses out the steam by cooling the wet hydrogen to a temperature below the dew point of the water, and then reheating it, using, for example, regenerative heat exchangers, as discussed with reference to Figure 7 below. However, it might also be achieved with a membrane filter. For example, there are known membrane filters that, at suitable pressures, can allow the hydrogen to pass through the membrane, whereas the water does not. These pressures may be pressure deltas either side of the membrane, as an example. Such membrane filters are known in the art. They can include membranes that function at the temperatures involved with steam electrolysers 16 - particularly SOEC electrolysers operating at temperatures of between 400 and 750 degrees C. For example, palladium based H2 membranes can operate at 400-500 degrees C, and they can generate dry hydrogen by separating hydrogen from a mixture of hydrogen and steam. In other embodiments, electrochemical membrane may be used, and some ceramic membranes can even function at higher temperatures. Although not shown, a hydrogen / steam separator 28 can likewise be added to the parallel heat exchanger system 20 of Figure 3. With the same benefits.

[0125] In these embodiments, therefore, a high temperature membrane may be beneficially used to separate hydrogen from the steam, thus replacing or reducing the demands on a downstream condensation system for the hydrogen at or before any downstream collection or distribution pipework.

[0126] Typical such membranes commonly require a pressure delta of between 5 and 10 bar for creating a driving force for moving the hydrogen protons / atoms through the membrane. These pressures may be provided by operating the electrolyser system at such elevated pressures, and using atmospheric conditions for the outer side of the membrane, or by using a suction pump on the outer side of the membrane, although the latter could be introducing a fresh parasitic load on the system 20.

[0127] Referring next to Figure 5, a further variant is shown. In this variant, which is based on the parallel heat exchanger system of Figure 3, a start-up sweep-gas system is shown. Such a start-up sweep-gas system can similarly be added to a system using heat exchangers in series (or combinations of the two).

[0128] This start-up sweep-gas system can be an optional, selectively operational or disengageable component of the electrolyser system 20. Typically, it may be used for start-up or other transient operations of the electrolyser system 20. For example, where the heat of the steam 43 from the source of steam or steam generator is inadequate for starting up or maintaining operation of the electrolyser 16, or where a faster start up, or a higher heat maintenance, is desired. For this purpose, a supply of hot air or hot nitrogen (which may be atmospheric air that has been heated, or a waste product from another industrial process) is fed from that source 80 into an air inlet 32 on the anode side of the electrolyser 16. This will be for preheating the electrolyser 16, or for assisting with the heating of the electrolyser 16, for achieving or maintaining operational temperatures during start-up or transient conditions. During this process, the electrolyser 16 may start electrolysing the incoming steam 43 through cathode side of the electrolyser 16, and thus the air or nitrogen might serve as a temporary sweep flow for the produced oxygen 64. However, this can often slow down the start-up process as it may even cool the incoming steam, so more commonly the exhaust is not initially recovered.

[0129] In some embodiments the exhaust can still cycle through the first pipeline 106 to the first heat exchanger 132. However, typically this air or nitrogen will be vented out through a separate vent 40. This might even be towards the end of the start-up cycle if it will not meet the target purity requirements of the oxygen collection system.

[0130] As indicated above, this air source can be selectively operational or a disengageable component of the system 20, whereby the continued operation of the system 20 can be without that air source, whereupon the oxygen produced can revert to the target purity for collection.

[0131] The start-up sweep-gas system may be an electrical or heat exchanger based heater. It can be fitted inside the electrolyser 16, or to a side of it, upstream of it, or even inside or to a side of the stack 10. It can be permanent or disengageable. For example, it might be disengaged or turned off when the stack achieves steady state conditions, or when it is hot enough to selfsustain. In this embodiment, it is provided for the anode side of the electrolyser 16. Preferably, it would only be considered where adding heating, or heating up using the steam, is impractical or not feasible just on the cathode (fuel) side of the electrolyser 16.

[0132] It will be appreciated that other forms of heater (e.g. indirect electric or additional heat exchangers connected to additional waste heat sources or the like), can instead or also be provided where required to increase the temperature of the fluids through pipelines between the various components of the system 20.

[0133] Furthermore, for controlling these systems, sensors and variable flow valves or stop valves, may be provided along the pipelines or on the respective components. These can allow a control of flows, a control of heaters, and a control of any bypasses for the heat exchangers or heaters that may be provided. There can even be a control of the flow rate of the fuel, the recycled steam, or the off-gases in general around the system 20.

[0134] Flow rates and / or temperatures of the input fluid(s) at the stack inlet(s) and from the stack outlets can thus be controlled as necessary.

[0135] In some embodiments, a low flow-volume sweep flow supply might be implemented permanently on the anode side. This would require a sweep flow inlet. This might be provided to guarantee flow of the oxygen out of the anode side of the cells 11 . It may be necessary, dependent upon the chemistry of the anode or the pipework and manifolds - for example if oxidation issues might otherwise arise, or to avoid pressure build-ups or fluid stagnation. The latter can create explosion risks in the event of hydrogen leaks within the stack 10. In such circumstances, the oxygen produced by the electrolyser would form the majority of the anode off-gas. For example, any volume of air for this purpose would dilute the oxygen produced by the electrolyser by no more than 10% whereby the oxygen remains 90% pure at least. In some embodiments more than 50% oxygen is clearly beneficial versus the prior art, where conventionally at least a 7:1 or 8:1 ratio of sweep flow to oxygen produced is utilised, and as such the oxygen level when using air for the sweep-flow never exceeds 40-45% by weight in the prior art (remembering that air starts at 20% oxygen).

[0136] It is to be appreciated that at least 90% purity by weight is a preferred target oxygen weight percentage for the anode side off-gas, although 50%, 60%, 70% or 80% would all be better than the prior art, so any of them could be a target oxygen weight percentage for the anode side off-gas.

[0137] Referring next to Figure 6, a fifth embodiment of the present invention is shown in which the heat exchangers 132, 142 are once again arranged in series, like in Figures 2 and 4. Furthermore, a separator 28, like that of Figure 4, is provided, which in this embodiment is a hydrogen membrane 28 for filtering much of the hydrogen out of the steam / hydrogen mixture. The filtered out hydrogen can pass through the second heat exchanger 142, as per the embodiment of Figure 4. However, the steam - still mixed with some hydrogen when the hydrogen separator 28 is unable to fully remove all the hydrogen from the mixture, can either or both recirculate into the first pipeline 124, rather than having a separate port 120 for returning into the electrolyser 16, or it can recirculate directly into the electrolyser via such a separate port 120.

[0138] Additionally, in this embodiment an additional heater 136 is provided in the steam supply pipeline 124. This can be an electric heater or a further heat exchanger connected to an alternative heat source, such as a waste heat from another industrial process, as discussed above. This heater 136 can preheat the steam 43 in advance of the first and second heat exchangers 132, 142. However, it can equally be positioned between the heat exchangers 132, 142 or after the heat exchangers 132, 142. It can assist with start-up, much like the air heating in Figure 5, or it can reduce the heating requirements from the first and second heat exchangers 132, 142 during steady state operation. Locating the additional heater in the cathode side pipeline 124 is more preferred than flowing heated air or fluids through the anode side.

[0139] The temporary heaters of Figures 5 and 6 can even be removable. It is then able to be moved between stacks in a given system, or in different systems. This can reduce the capital expense of a given installation, and can be beneficial due to the likely staggered start-ups of any given system - for example following servicing or replacement of stacks in a given system, where only partial shut downs of the overall system 20 are necessary.

[0140] In some embodiments, during start-up, the steam (fuel) is entirely replaced with air or carbon dioxide or nitrogen, such that air or carbon dioxide or nitrogen is passed into the electrolyser 16 through the inlet 22, rather than steam 43. This is advantageous until stack achieves a temperature in excess of 100 C, as before then water - as a liquid - could enter the stack, either as water directly entering the stack of steam condensing within the stack, which could significantly damage the stack. Thereafter, steam might be used for the ongoing heating of the stack (or a mixture of steam with the air, carbon dioxide or nitrogen).

[0141] It will be appreciated that these first five embodiments greatly reduce the number of subsystems required, including few if any electric inline heaters, few if any hot air delivery subsystems, minimal if any cooling systems and potentially not even a steam generator, (assuming steam from an external industrial process is available). This significantly reduces the cost and complexity of the electrolyser system versus prior art electrolyser systems. The produced oxygen will be at a higher purity percentage than in the prior art and it can be used to heat the incoming fuel to recover the heat before storage or distribution of the oxygen - whereat any remaining heat is wasted.

[0142] Similarly heat in the hydrogen and steam output is recycled.

[0143] Referring next to Figure 7, a further embodiment of electrolyser system 20, again with an electrolyser 16, is shown. That electrolyser 16 can comprise one or more stacks 10 of electrolyser cells 11 , similar to that of the previous embodiments. Again, the parasitic load associated with the sweep gas is removed or reduced. In this embodiment the electrolyser 16 is operated at an elevated pressure - for example at a pressure in excess of 1.5 barg (1.5 bar above atmospheric pressure), and the electrolyser system 20 utilises a resulting pressure differential to extract off-gas on at least the anode side of the electrolyser cells 11 . This will be further described below. However, a further advantage of operating the electrolyser 16 at an elevated pressure versus atmospheric pressure is that the off-gases (hydrogen 63 and oxygen 64) are produced at an elevated pressure versus atmospheric pressure, and thus can be extracted at an elevated pressure versus atmospheric pressure. This reduces the amount of downstream compression required for the extracted gases 63, 64 prior to storage - it is conventional for such gases 63, 64 to be stored at high pressure to reduce their volume. The elevated pressure versus atmospheric pressures can also facilitate separation of the component gases from the off-gas mixtures 63, 64, for example as discussed previously with reference to Figures 4 and 6.

[0144] The electrolyser 16 of Figure 7 is fitted with an optional air or sweep-flow inlet 32 and a steam or water inlet 22. It also has a first side outlet 18 for output of hot oxygen 64 (either just the produced oxygen or oxygen enriched air or oxygen enriched nitrogen, dependent upon the form of any sweep gas 42 used by the system 20. Typically, this first side outlet 18 is on the anode side of the electrolyser 16 - connected to anode side outlets 38 of the electrolyser cells 11 or stacks 10 thereof.

[0145] For clarity, the stack 10 and cells 11 are not shown in this figure. However, the electrolyser 16 will typically have one or more stacks 10 of electrolyser cells 11 , much like those of Figures 2 to 5. For each embodiment, more than one stack 10 may be used and each stack 10 may have fewer or more fuel cells 11 than that shown - stacks of at least 100 cells, for example, are known in the art.

[0146] The electrolyser 16 of Figure 7 also has a second side outlet 24 for output of hot wet hydrogen 63. Typically, this second side outlet 24 is on the cathode side of the electrolyser 16 - connected to cathode side outlets 44 of the electrolyser cells 11 or stacks 10 thereof.

[0147] The optional air or sweep-flow inlet 32 is provided in this embodiment since during transient operations (such as start-up, shutdown, purging, or a change to operating conditions) it may be beneficial to introduce an external source of air (or nitrogen) to generate / maintain the elevated pressure and / or temperature within the electrolyser 16. This is similar to that shown in Figure 5. As shown in Fig. 7, an air, oxygen or nitrogen source 80, is provided. It can be pressurised or pumped to flow sweep gas 42 towards the electrolyser 16 through a first pipeline 102. That sweep gas 42 passes through a first block valve 98 and a first pressure regulator 100 in the first pipeline 102 and also through a first heat exchanger 104 towards the electrolyser 16. The first block valve allows the air supply 80 to be isolated from the system 20.

[0148] Along the first pipeline 102, a heater, or as shown a pair of heaters 158 in parallel, is provided to pre-heat the sweep gas 42 prior to it being introduced into the electrolyser cells 11 of the electrolyser 16. A first temperature sensor 110 enables control of this heating. The sweep gas enters the electrolyser 16 at the inlet 32 and exits the electrolyser 16 at the first side outlet 18, controlled by a second pressure regulator 105. A second pressure sensor 108 enables control of this process. As noted above, the external source of air 80 is not essential, and can be simply for transient operations, as mentioned above, or for optional supplementary sweep flow operations. However, as will be discussed below, it can be turned off entirely, and even disconnected from the system 20, in some embodiments.

[0149] The elevated pressure of the electrolyser 16 works in conjunction with the second pressure regulator 105 on a second pipeline 106 that extends away from the first side outlet 18. With this arrangement, the second pressure regulator 105 can be set with a pressure requirement for flow that enables a pressure difference either side of the pressure regulator 105 (i.e. between the interior (anode side) of the electrolyser 16 and the downstream (storage I venting) side of the pressure regulator 105. This pressure difference enables a pressure-driven flow of oxygen enriched air - or even just the produced oxygen - to exit the electrolyser 16. It also ensures that the pressure within the electrolyser 16 is maintained on the anode side as the pressure regulator 105 can be closed when the pressure drops below a threshold.

[0150] As the production of oxygen will continue during the use of the electrolyser 16, during steady state operation of the electrolyser the pressure regulator can permit a steady stream of that oxygen to flow out of the electrolyser. As a result, a sweep gas 42 is not required for expelling the oxygen generated from the electrolysis from the anode side of the electrolyser.

[0151] The heat in the vented oxygen 64 can also still be recovered - here by a second heat exchanger 112. Advantageously, if a sweep gas 42 is used, the first heat exchanger 104 may have its secondary flow line connected to the second heat exchanger 112 so that the heat of that oxygen off-gas 64 can be used to pre-heat the sweep gas 42 before it enters the electrolyser cell 16. However, when the sweep gas is turned off or removed, the second heat exchanger 112 may instead be connected to the fuel’s supply line - a third pipeline 124.

[0152] As the pressure of the produced oxygen can be allowed to be elevated versus ambient pressures by the second pressure regulator 105, the collected oxygen off-gas 64 will likewise usually be pressurised, and it can be stored or collected in that form for use elsewhere. However, it might also be further pressurised by downstream equipment before storage or use. For that purpose an oxygen collection area 114 is provided. Alternatively, or additionally, part of the collected oxygen off-gas 64 can be mixed with additional air (also potentially from the same air source 80 - albeit here shown as a separate air source 80) to dilute it. It then can be stored or utilised elsewhere as pressurised oxygen enriched air 116. Although maintaining the pressure is often useful or valuable, it will be appreciated that in some embodiments, where pressure is not important (or undesirable) for the downstream use, the oxygen can be allowed to vent from the pressure regulator to atmospheric pressure.

[0153] To avoid backflow, a non-return valve 118 can be provided.

[0154] With this arrangement, the electrolyser 16 is operated in an elevated pressurise configuration.

[0155] I .5 barg was mentioned above, but it might equally be operated for example at pressures from 0.5 to 5 barg (gauge pressure). Higher pressures require more complex I expensive sealing to maintain that elevated pressure in the system, so a pressure of 0.5 to 3 barg is preferred for providing an appropriate trade-off between cost and benefit. Up to 1.5 barg is more preferred, and 0.5 barg up to 1 barg still achieves worthwhile benefits.

[0156] The hydrogen (cathode) side of the electrolyser system is likewise pressurised as pressurized steam is directly fed into the electrolyser 16 via the third pipeline 124 and recirculated via a fourth pipeline 126. In typical embodiments, the pressure on the anode side is controlled to match that of the hydrogen / steam / cathode side so that the pressures are substantially equal across the electrolytes 54 of the electrolyser cells 26. This is preferred as many designs of electrolyser cell 11 are relatively easy to damage if exposed to excessive internal stresses or strains. The second pressure regulator 105 may thus be set to match the incoming steam pressure. For that purpose, a second block valve 128 and a third pressure regulator 130 may be provided on a steam feed line 160 between a steam source 43 and the third pipeline 124, and a fourth pressure regulator 152 may be provided on the fourth pipeline. The second block valve 128 can then control whether additional steam 43 is added to the system 20. The third and fourth pressure regulators 130, 152 can be monitored or operated in conjunction with the second pressure regulator 105 to maintain matched pressures across the electrolyser cells

[0157] I I .

[0158] In normal use, the steam or water source 43 provides pressurised steam to the electrolyser 16 via the third pipeline 124. A further pressure sensor 134 is also supplied to monitor the pressure in the third pipeline 124 for a control system. The steam in the third pipeline then passes through the second heat exchanger 132 (i.e. the second side of the heat exchanger 132 in the second pipeline 106) for utilising the heat of the oxygen off-gas 63 to pre-heat or superheat the steam before its entry into the electrolyser 16. The pressure regulators 105, 130, 152 may control the pressures to each side of the electrolyser cells, or can be controlled by the sensors and control system. In some embodiments - as shown here in Figure 7 - an additional heater 136 can also be provided, along with a third temperature and / or pressure sensor 138. It can be to further top up the temperature of the steam if needed. They can also check the pressures across the system, as the steam should enter the electrolyser 16 at close to the target operating temperature of the electrolyser cells 26 and at an appropriate pressure for being matched by the pressure of the oxygen / sweep gas on the anode side.

[0159] The steam 43 thus then enters the electrolyser 16 and is at least partially split into hydrogen and oxygen across the electrolyte 54 of the electrolyser cells 11 of the electrolyser 16, as discussed with reference to Figure 1. The wet hydrogen then exits the electrolyser 16 at the second side outlet 24 to enter the fourth pipeline 126, along which a fourth heat exchanger 142 is provided. This fourth heat exchanger 142 links across a loop in the fourth pipeline 126, through which the non-split steam can recirculated to the electrolyser, via the third pipeline 124, as will be described below.

[0160] A fourth pressure sensor 140 can be provided between the second pressure regulator 105 and the fourth pipeline 126 to help with the pressure matching (or to ensure there is not an excessive pressure difference) across the electrolyser cells 11. Excessive pressure differences can cause damage to the electrolyser cells, or create leaks across the electrolyte or elsewhere within the electrolyser 16, which leaks can lead to combustion between the hydrogen and the oxygen, or other inefficiencies.

[0161] The fourth pipeline 126 is provided to take the wet hydrogen 63 through the fourth heat exchanger, such that the wet hydrogen will be 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 (shown schematically as the oval in this drawing), for example a metal hydride or adsorption-desorption compressor, for compressing the hydrogen, although it could be additional to a separate compressor 26 for the hydrogen.

[0162] In this embodiment the wet hydrogen exits the fifth heat exchanger 150 at a further lowered temperature (at a temperature below the dew point of the steam), such that the water of the wet hydrogen can be condensed out of the mixture and be collected in a condenser vessel 144. The wet hydrogen 63 is thus somewhat dried by this process after it exits that fifth heat exchanger 150 and the dried hydrogen can be collected or stored at a collector 90.

[0163] The condensed water instead can be recirculated back through the fifth heat exchanger 150 and the fourth heat exchanger 142 to reheat it and to convert it back into steam for reintroduction into the third pipeline 124. A pump 146 is provided in the water line to feed the water 38 through these heat exchangers 142, 150 so that the steam can renter the third pipeline at a suitably elevated pressure.

[0164] Alternatively, the collected water may be discarded, for example if a source of steam is readily available and a simpler design is desired.

[0165] As the condensed water as well as the wet hydrogen are passed through the fourth and fifth heat exchangers 142, 150, the heat exchangers both cool the wet hydrogen to condense out the water, and reheat the water back up again into steam for recirculation through the electrolyser 16. They are thus regenerative heat exchangers for recirculation of the water through the electrolyser 16, thus reducing the amount of external water or steam 43 required for the electrolyser system 20. Dependent upon the temperature of the off-gas, more than two such heat exchangers may be required, or it might even be possible to achieve the regeneration with a single heat exchanger.

[0166] The recirculated water (now steam) flows into and through the third pipeline 124 and passes through the third heat exchanger 132 and the optional heater 136 before re-entering the electrolyser, either as the steam for the electrolyser or mixed with additional steam from the source of steam 43.

[0167] A fifth pressure regulator 152 may be provided after the fourth heat exchanger 142 in the fourth pipeline 126 and it may link with the fourth pressure sensor 140 and the control system for helping to monitor the system 20 to help to ensure the safety and operational integrity of the system 20.

[0168] As this system operates at an elevated pressure (higher than atmosphere, and preferably 0.5 to 5 bar above atmospheric pressure), pressurised off-gasses are generated leading to a lower demand on the compressor 26 for the hydrogen.

[0169] Referring next to Figure 8, a variant of the arrangement in Figure 7 is provided. In this variant, much of the hydrogen side is the same as in Figure 7. It will thus not be further discussed in detail. However, the further pressure sensor 134 is moved to a position located after the third temperature sensor 138 so as to measure the pressure of the steam after it has passed through the third heat exchanger 132 and the additional (optional) heater 136 in the third pipeline 124. This may assist the control system in identifying the appropriate pressure for the second pressure regulator 105 for matching pressures across the electrolyser 16. Further, the second pipeline 106 in the oxygen side is largely the same. However, its sweep gas circuit is modified to enable recirculation of a portion of the anode off-gas (oxygen). This can entirely replace the external sweep gas supply 42 as it provides a recirculating sweep gas flow using the produced oxygen, and any remaining air from any initial use of sweep gas 43 during the start-up of the system.

[0170] As will be explained 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 whatsoever is required for steady-state operations.

[0171] In this embodiment, the second pipeline 106 branches off prior to the third heat exchanger 132 to provide a loop 156 back to the air or sweep-flow inlet 32 of the electrolyser 16. An electrical heater 158 is provided to power a heat or thermal pump to affect a sweep-flow, but the gas used for the sweep-flow is now the oxygen from the electrolyser 16. The heater 158 may instead be a heat exchanger, for example the opposite side of the second heat exchanger 112. The second pressure regulator 105 still operates to affect the outflow of oxygen 64 from the electrolyser 16 to the storage or external delivery channels 114, 116 due to the pressure differential across that regulator 105, as discussed above with reference to Figure 7. However, the oxygen in the electrolyser keeps sweeping through the electrolyser around the loop 156.

[0172] To prevent a reverse of the recirculation of the oxygen, a second non-return valve 154 can be fitted in this loop 156. Furthermore, an additional pressure sensor 160 can be provided for the loop 156, again for the control system.

[0173] Figure 8 also shows an optional further loop 162 for looping back the oxygen from after the third heat exchanger 132. This may be instead of or in addition to the loop before that third heat exchanger 132. This alternative or additional loop may be desirable or more appropriate for balancing the pressure and heat requirements in the electrolyser 16 and across the system 20, dependent upon the performance of the heater 158 or thermal pump.

[0174] Thin dot-dashed lines on Figures 7 and 8 indicate control lines between related sensors and valves or regulators, where the value I position of one may affect or control another. Such control functionality could equally be performed by each element being connected to a central control system. It is also envisioned that there may be embodiments where the second pressure regulator is not required, or where the system 20 is operated at ambient pressures. The pressure or sweep flow may be maintained purely by the oxygen production rate. However, the regulator is typically preferred as it would still serve the purpose of balancing pressure across the stack, thereby reducing the undesirable consequences of pressure deltas, such as internal leaks, even if operated at atmospheric conditions.

[0175] With all of these embodiments it is envisioned that the electrolyser systems will aim to operate at maximum electrical and thermal efficiencies by operating close to a thermoneutral voltage. In practice, the system is operated so that the outlet temperature is hotter than the inlet temperature so as to enable to removal of in-line heaters. This requires operating slightly above thermoneutral voltage. For example, the outlet temperature may be approximately 30 degrees C hotter than the inlet temperature for improving and potentially obtaining optimal, efficiency. This is under the assumption that the heat exchangers are about 90+% efficient. Under such conditions, no additional or supplementary heating is required for the fuel for the stack or for the stack itself. Furthermore, by reducing air flow on the anode side to zero, (or as close to zero as permitted given any safety requirements triggered by flow stagnation or component oxidation) the produced oxygen will be purer and thus more inherently valuable. Elevating the pressure well above atmospheric pressures assists with improving the efficiencies in the hydrogen separation membrane 28.

[0176] Balancing the pressure across the electrolyser cells also is beneficial as it maintains a long service life for the stack, given the reduced stresses on the electrochemically active materials of the electrolyser, and the interfaces between neighbouring cells in the stack.

[0177] Various embodiments of the present invention have thus been described above, purely byway of example, with reference to the accompanying drawings. Modifications in detail may be made to the invention within the scope of the claims as appended hereto. For instance, features described as part of one embodiment can be used on another embodiment to yield a still further embodiment. Thus, it is intended that the present invention cover such modifications and variations as come within the scope of the appended claims. For example, while the description is predominantly focused on H2O electrolysis, it could be applied to CO2 electrolysis where CO is produced at the cathode and O2 at the anode. In such an implementation, CO2 would replace the steam input and CO would replace the H2 output.

Claims

Claims1 . A method of venting a substantially pure oxygen off-gas from an electrolyser of an electrolyser system, the method comprising: generating a pressure differential between the interior of the electrolyser and an inlet or outlet for the off-gas from the electrolyser; wherein the pressure differential is generated by the production of oxygen through electrolysis by the electrolyser.

2. The method of claim 1 , wherein the method operates without any external sweep gas, or with a sweep gas that is substantially pure oxygen.

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

4. The method of any preceding claim, wherein the method comprises controlling the venting of the oxygen by way of a back pressure regulator.

5. The method of any preceding claim, wherein the substantially pure oxygen off-gas is at least 85% pure by weight.

6. The method of any preceding claim, wherein the or each electrolyser comprises at least two off-gas outlets for off-gases from the at least one stack thereof, at least one of the at least two off-gas outlets being an anode side off-gas outlet for venting an anode off-gas from the at least one stack and at least one of the at least two off-gas outlets being a cathode side off-gas outlet for venting a cathode off-gas from the at least one stack, wherein the anode side off-gas outlet for venting an anode off-gas from the at least one stack is configured for connection to, or is connected to, the outlet for the substantially pure oxygen off-gas, and the anode off-gas is the substantially pure oxygen off-gas.

7. The method of any preceding claim, wherein the or each electrolyser does not comprise an inlet for sweeping the substantially pure oxygen off-gas, and the pressure differential is between the interior of the electrolyser and an outlet for the substantially pure oxygen off-gas from the electrolyser.

8. The method of any preceding claim, wherein a return loop is provided between a sweep-flow inlet of the at least one stack and the anode side off-gas outlet of that stack.

9. The method of any preceding claim, wherein the pressure of the electrolyser is 0.1 to 10 barg, preferably 0.1 to 5 barg, and more preferably 0.2 to 1 barg.

10. The method of any preceding claim, wherein the pressures on each of anode and cathode sides of electrolyser cells in the electrolyser are substantially balanced.

11. The method of any preceding claim, wherein the method comprises a start-up or transition cycle in which when the electrolyser has an internal temperature below 100 degrees C, and air, carbon dioxide or nitrogen, is fed to a cathode side of the electrolyser in place of a fuel.

12. The method of any preceding claim, wherein the method operates with no external sweep-flow, whereby no sweep-flow inlet is required for the electrolyser system.

13. The method of any preceding claim, wherein the electrolyser system further comprises a hydrogen / steam separator positioned between the cathode side off-gas outlet and the heat exchanger for exchanging heat from the cathode side off-gas.

14. The method of claim 13, wherein the hydrogen / steam separator is a membrane filter.

15. The method of claim 14, wherein the membrane filter operates with a pressure delta across the membrane.

16. The method of any preceding claim, comprising: calculating the pressure on one side of the electrolyser cell in the electrolyser; and controlling the pressure on the other side of the electrolyser cell so that the pressure on an anode side of electrolyser cells in the electrolyser is greater than the pressure on a cathode side of the electrolyser cells.

17. A method of operating an electrolyser system comprising an electrolyser with at least one stack of electrolyser cells, the cells each having an anode side and a cathode side, the method comprising: calculating the pressure of a fluid on one side of at least one of the electrolyser cells or the at least one stack; and controlling the pressure of a fluid on the other side of the at least one of the electrolyser cells or the at least one stack so that the pressure on the cathode side of the or eachelectrolyser cell, or the or each stack is greater than the pressure on the anode side of the or each cell or the or each stack.

18. The method of claim 16 or 17, wherein the difference in pressure between the anode and cathode sides of an electrolyser cell in the electrolyser cells or stacks is no more that 50mbar, optionally no more than 40mbar, optionally nor more than 30mbar, optionally no more than 20mbar, optionally no more than 10mbar.

19. The method of any of claims 16 to 18, wherein the pressure of the anode side of the electrolyser cells or stacks in the electrolyser is controlled by a back pressure regulator.

20. An electrolyser system configured to carry out the method according to any preceding claim.21 . An electrolyser system comprising an electrolyser with at least one stack of electrolyser cells, the cells each having an anode side and a cathode side, the electrolyser comprising: at least one fuel inlet for a fuel for the at least one stack; at least two off-gas outlets for off-gases from the at least one stack, at least one of the at least two off-gas outlets being an anode side off-gas outlet for venting an anode off-gas from the at least one stack and at least one of the at least two off-gas outlets being a cathode side off-gas outlet for venting a cathode off-gas from the at least one stack; a pressure calculating device for calculating the pressure of a fluid on one side of at least one of the electrolyser cells or stacks; a pressure controlling device for controlling the pressure of a fluid on the other side of the at least one of the electrolyser cells or stacks; wherein the pressure controlling device is configured to control the pressure on the cathode side of the or each of the electrolyser cells or the or each of the stacks to be greater than the pressure on the anode side of the or each of the electrolyser cells or the or each of the stacks.

22. An electrolyser system comprising an electrolyser with at least one stack of electrolyser cells, the electrolyser comprising: at least one fuel inlet for a fuel for the at least one stack; at least two off-gas outlets for off-gases from the at least one stack, at least one of the at least two off-gas outlets being an anode side off-gas outlet for venting an anode off-gas from the at least one stack and at least one of the at least two off-gas outlets being a cathode side off-gas outlet for venting a cathode off-gas from the at least one stack; andmeans for exchanging heat from the cathode side off-gas to a fuel for the at least one stack, and means for exchanging heat from the anode side off-gas to a fuel for the at least one stack; wherein the stack is configured to output, under normal operating conditions, the anode side off-gas, wherein at least 50% by weight of the anode side off-gas is oxygen produced by the electrolyser.

23. The electrolyser system of claim 22, further comprising a hydrogen / steam separator positioned after the cathode side off-gas outlet.

24. The electrolyser system of claim 23, wherein the hydrogen / steam separator is between the cathode side off-gas outlet and the means for exchanging heat from the cathode side offgas to the fuel.

25. The electrolyser system of claim 23 or claim 24, wherein the hydrogen / steam separator is a cooling / reheating loop, which condenses out the steam by cooling the wet hydrogen to a temperature below the dew point of the water, and then reheating it.

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

27. The electrolyser system of claim 23 or claim 24, wherein the hydrogen / steam separator is a membrane filter.

28. The electrolyser system of claim 27, wherein the membrane filter operates with a pressure delta across the membrane.

29. An electrolyser system comprising an electrolyser with at least one stack of electrolyser cells, the electrolyser comprising: at least one fuel inlet for a fuel for the at least one stack; at least two off-gas outlets for off-gases from the at least one stack, at least one of the at least two off-gas outlets being an anode side off-gas outlet for venting an anode off-gas from the at least one stack and at least one of the at least two off-gas outlets being a cathode side off-gas outlet for venting a cathode off-gas from the at least one stack; and the electrolyser system further comprises a hydrogen / steam separator positioned between the cathode side off-gas outlet and the heat exchanger for exchanging heat from thecathode side off-gas, wherein the hydrogen / steam separator is a membrane filter that operates with a pressure delta across the membrane.

30. The electrolyser system of claim 29, wherein the electrolyser further comprises means for exchanging heat from the cathode side off-gas to a fuel for the at least one stack, and means for exchanging heat from the anode side off-gas to a fuel for the at least one stack.

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

32. The electrolyser system of any one of claims 25 to 31 , wherein a sweep flow gas is used for sweeping the oxygen from anode sides of the electrolyser cells, and the sweep flow gas will be less than 50% by weight of the anode side off-gas.

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

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

35. The electrolyser system of claim 34, wherein the control system is configured to operate the stack at one side of a thermoneutral voltage.

36. The electrolyser system of any one of claims 25 to 35, wherein the electrolyser cells, or each stack of electrolyser cells, is configured to operate substantially thermoneutrally, and galvanostatically.

37. The electrolyser system of any one of claims 25 to 36, wherein the sustained operation of the electrolyser system at normal operational conditions is achieved with no supplementary electrical heaters within the manifolds or pipework of either any sweep-flow pipelines or any fuel flow pipelines within the electrolyser, or within the electrolyser system.

38. The electrolyser system of any one of claims 25 to 37, wherein the electrolyser cells are solid oxide electrolyser cells.

39. The electrolyser system of any one of claims 25 to 38, wherein the electrolyser comprises more than one stack.

40. The electrolyser system of any one of claims 25 to 39, wherein the means for exchanging heat comprises one or more heat exchangers.41 . The electrolyser system of claim 40, wherein there are two heat exchangers arranged in series along a fuel supply pipeline.

42. The electrolyser system of claim 40, wherein the means for exchanging heat comprises two heat exchangers arranged in parallel across separate fuel supply lines.

43. The electrolyser system of any one of claims 40 to 42, wherein the two heat exchangers are coupled to one-another thus essentially forming a single heat exchanger where heat is exchanged from the two outlets to the inlet fuel.

44. The electrolyser system of claims 40 to 43, wherein the system further comprises switching means for switching the two heat exchangers from a series configuration to a parallel configuration and vice versa.

45. The electrolyser system of any one claims 25 to 44, wherein the electrolyser is configured to operate at a temperature in excess of 400 degrees C.

46. The electrolyser system of any one of the preceding claims, when dependent upon claim 27 or claim 29, wherein the membrane filter is a palladium hydrogen membrane that operates at a temperature in excess of 400 degrees C.

47. The electrolyser system of any one of claims 25 to 46, configured to operate at a pressure in excess of 2 barg.

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