Pressure control for electrolyser system

Abstract

An electrolyser system and method of operating an electrolyser system The electrolyser system comprising a plurality of electrolyser cells contained within a pressure vessel containing a pressurizing fluid, a first fluid pathway for collecting a working fluid from a first side of said cells, and a second fluid pathway for collecting a working fluid from a second side of said cells. Pressures in the system are controlled such that the pressure (330) of pressurizing fluid in the pressure vessel is greater than the pressure (331, 332) of the working fluids in both of said first and second pathways.

Description

[0001] PRESSURE CONTROL

[0002] Field of the Invention

[0003] The present invention relates to a pressurised system, and in particular a pressurised electrochemical cell system configured to operate at an elevated (above ambient) pressure, such as a pressurisable electrolyser system or a pressurisable fuel cell system, and method of operation thereof. The pressurised system may comprise at least one stack of electrochemical cell units -typically electrolyser cell units, which may include cell units of solid oxide or molten carbonate electrolyser cells. The present invention more specifically relates to the integration of solid oxide electrolyser cell (SOEC) units within an enclosure to form the pressurisable electrochemical cell system, and control of relative pressures therein. The solid oxide electrolyser cell (SOEC) units may include metal-supported solid oxide electrolyser cell (MS-SOEC) units.

[0004] Background to the Invention

[0005] Electrochemical cell units are commonly referred to as fuel cell units or electrolyser cell units, and in some instances their names are interchangeable as some fuel cell units can work as electrolyser cell units and some electrolyser cell units can operate as fuel cell units, each either as a producer of electricity or in a regenerative mode - electrolyzing a fluid to electrochemically split it into two or more component parts. For example, some fuel cell units can produce electricity by using an electrochemical conversion process that oxidises fuel to produce electricity. Some fuel cell units can also, or instead, operate as regenerative fuel cells (or reverse fuel cells) units, often known as electrolyser cell units, for example to separate hydrogen and oxygen from water, carbon monoxide and oxygen from carbon dioxide, or nitrogen monoxide and oxygen from nitrogen dioxide. In the case of separating hydrogen and oxygen from water, care must be taken to ensure a potentially dangerous mixture of hydrogen does not result.

[0006] Electrochemical cell units may be tubular or planar in configuration. Planar cell units may be arranged overlying one another in a stack arrangement, for example 100-400 cell units in a stack, with the individual fuel cell units arranged, for example, electrically in series. Tubular cell units may be arranged in groups, stacks or coils thereof.

[0007] A solid oxide fuel cell (SOFC) unit that produces electricity is based upon a solid oxide electrolyte that conducts negative oxygen ions from an oxygen electrode to a fuel electrode located on opposite sides of the electrolyte. For this, a fuel, or reformed fuel, contacts the fuel electrode and an oxidant, such as air or an oxygen rich fluid, contacts the air electrode. A solid oxide electrolyser cell (SOEC) may have the same structure as an SOFC but is essentially that SOFC operating in reverse, or in its regenerative mode, to achieve the electrolysis of fuel, for example water and / or carbon dioxide, by input of electrical energy and using the solid oxide electrolyte to produce hydrogen gas and / or carbon monoxide and oxygen. Conventional ceramic-supported (e.g. fuel electrode-supported) SOFCs and SOECs have low mechanical strength and are vulnerable to fracture. Hence, metal-supported SOFCs and SOECs have been developed which have the active cell component layer supported on a metal substrate. In these cells, the ceramic layers can be very thin since they only perform an electrochemical function: that is to say, the ceramic layers are not self-supporting but rather are thin coatings / films laiddown on and supported by the metal substrate. Such metal-supported SOEC stacks are more robust, lower cost, have better thermal properties than ceramic-supported SOECs and can be manufactured using conventional metal welding techniques.

[0008] Electrolyser cell stacks commonly operate at elevated temperatures. For example, intermediate or high temperature electrolysers (such as SOEC and alkaline electrolyte cells) have operational temperatures in excess of 400 °C, typically 450 °C to 700 °C for an intermediate temperature electrolyser such as one based on an MS-SOEC, and above 700 °C for high temperature electrolysers.

[0009] Electrolyser cell stack(s) can be assembled within a housing, and can be combined with heat exchangers and fluid delivery pipework, amongst other equipment, to form an electrolyser system, and the electrolyser system will be configured to receive supply input gas to and exhaust off-gas from the electrolyser cell stack(s) at the required operational temperatures. It is often desirable for outputs of an electrolyser system to be provided or operated at elevated pressures, and therefore the electrochemical cell units may themselves be designed to operate with pressurized fluids, ie they are designed to withstand elevated internal pressures, ie they must be of sufficient structural integrity to contain pressurized fluids. An alternative approach is to locate the cell units, or stack of cell units in a pressurized environment, in order to balance the internal and external pressures. For example the cells can be contained in or integrated within an enclosure or vessel designed to be pressurised. In this way the (stacks of) cells may not have to have any enhanced structural integrity as the pressure inside of them is balanced by the pressure of the vessel in which they are located.

[0010] In some electrochemical cell systems, for example where the electrochemical cell units are solid oxide electrolyser cell (SOEC) units, or more specifically metal-supported solid oxide electrolyser cell (MS-SOEC) units, at least some of the components may have ceramic or relatively brittle components within them. Such components can be relatively fragile, or temperamental when exposed to bending forces or unexpected (orout-of-the-ordinary) mechanical strains or stresses. Pressure imbalances across surfaces of the cell units, or forceimbalances either side of the components can lead to such bending forces or unexpected (or out-of-the-ordinary) mechanical strains or stresses. These in turn can lead to cracking of the ceramic components, or other failures or defects within the cell units.

[0011] Aspects of the present invention seek to provide a pressurised electrolyser system and method which at least mitigates such issues or problems.

[0012] SUMMARY OF THE INVENTION

[0013] According to a first aspect of the present invention there is provided a method of operating an electrolyser apparatus comprising a plurality of electrolyser cells contained within a pressure vessel containing a pressurizing fluid; a first (for example oxygen side, also referred to as air side) fluid pathway for collecting a working fluid from a first side of said cells; and a second (for example fuel side) fluid pathway for collecting a working fluid from a second side of said cells; said method comprising controlling the pressure in the system such that the pressure of fluid in the vessel is greater than the pressure of either of the working fluids in said first and second pathways.In this way a pressure differential is set up between the fluids in and produced by the electrolyser and the pressurizing fluid around them.

[0014] As explained above, in such a system including a pressure vessel, it would be expected that the pressurising fluid would be set at the same pressure as the working fluids in the cells, which is in turn typically dictated by the pressure at which it is desired to produce electrolysis products. This would balance the pressures and result in substantially zero stress / strain on the walls or structural elements of the cells or collection / exhaust pathways.

[0015] However the present inventors have found that a differential can be advantageous in the case of a leak or other failure, whereby working fluids could potentially enter the pressure vessel, presenting a possible safety risk. According to aspects of the present invention, because of the differential, in the case of any leak, pressurising fluid (which can be chosen to be inert) will enter the fluid pathway or pathways, driven by the pressure difference, thereby preventing any potentially undesirable mixture of exhaust products.

[0016] The present inventors have further determined that at the pressure differentials which reliably provide such an advantage, the resulting mechanical stresses or strains tend to be tolerable for a typical arrangement of cells and fluid pathway components, ie it does not require any additional mechanical reinforcement and hence cost or weight / volume.

[0017] The first fluid pathway is typically an air side pathway, or exhaust manifold, for collecting oxygen produced by the cells at and transporting it to the exterior of the pressure vessel, byway of an outlet port at the wall of the pressure vessel. In some cases, the first fluid pathway may include a pathway through the cells, and may additionally comprise an inlet manifold for providing input fluid (eg a sweep fluid such as air) from outside the vessel to said cells via an inlet port at the pressure vessel wall. Similarly the second fluid pathway is typically a fuel side pathway, or exhaust manifold, for collecting hydrogen produced by the cells and transporting it to the exterior of the pressure vessel byway of an outlet port at the wall of the pressure vessel, and may include a pathway through the cells, and may additionally comprise an inlet manifold for providing fuel (eg steam) from outside of the vessel to said cells via an inlet port at the pressure vessel wall.

[0018] In some examples, the cells are arranged in one or more arrays, or stacks, with each stack having at least one inlet, and two outlets, such that fuel (eg steam) is provided to the stack inlet (eg via an inlet manifold), and distributed to multiple cells within the stack. Electrolysed products (eg oxygen and hydrogen) from the cells within the stack are then channelled to respective stack outlets (eg to an exhaust manifold).

[0019] Typically the pressure vessel will have an inlet and an outlet for pressurizing fluid, although the flow rate through the pressure vessel, and the pressure drop across the vessel may be low. In some examples, the pressure of fluid in the system is controlled such that pressure of the fluid in the vessel (preferably at any and all points in the vessel) is greater than the pressure of either of the working fluids at all points along their respective fluid pathways.

[0020] As explained in greater detail below, in operation there will tend to be a variation in pressure along the various fluid paths, for example through the cells (eg from stack inlet to stack outlet(s)) and along a fluid pathway, and to some extent across the vessel, ie from a fluid pathinlet to the fluid path outlet. By ensuring that there is a positive pressure differential (ie the pressure of the pressurizing fluid is greater than the pressure of the working fluid) at all points, risk mitigation is provided regardless of the location of a potential failure.

[0021] In some examples where the first fluid pathway is an air side pathway (ie the working fluid in at least a portion of the first fluid pathway contains oxygen at elevated concentrations), the pressure in the system is controlled such that pressure in said first fluid pathway is greater than the pressure in said second fluid pathway.

[0022] Such an example may be found in a system where water (or steam) undergoes electrolysis to produce hydrogen and oxygen. In this case the first pathway is for exhaustingthe air side off gas (ie oxygen) and the second pathway is typically for exhausting the fuel side off gas (ie hydrogen). By arranging for a positive pressure differential between the first and second pathways in this manner, if a leak, or some malfunction results is crossover (ie fluid communication between the two pathways) then the oxygen containing fluid will tend to pass into the other (hydrogen) pathway, rather than vice versa.

[0023] This feature may be provided independently, and as such, in one aspect of the invention there is provided a method of operating an electrolyser apparatus comprising a plurality of electrolyser cells, a first fluid pathway for collecting oxygen produced from a first side of said cells; and a second fluid pathway for collecting hydrogen produced from a second side of said cells; said method comprising controlling the pressure in the system such that the pressure in said first fluid pathway is greater than the pressure in said second fluid pathway.

[0024] This can be advantageous because it typically results in a mixture with a low oxygen component or concentration, reducing the risk of a reaction (oxidation). This is preferable to the converse situation of a small amount of reactant (eg hydrogen) entering an oxygen rich environment, which gives a greater likelihood of oxidation, which could be a safety risk.

[0025] In some examples, the vessel has an inlet and an outlet, providing a fluid path for pressurizing fluid to enter and exit the vessel. In such examples, the pressure in the system is preferably controlled such that the pressure difference between vessel fluid pressure at the vessel outlet and the pressure of working fluid at the inlet port of the first fluid path is greater than 100 mbar, more preferably greater than 150 mbar and more preferably still greater than 200 mbar.

[0026] Preferably the differential is less than 300 mbar, and more preferably less than 250 mbar.

[0027] In some examples, the pressure in the system is controlled such that the pressure difference between the pressure of fluid at the stack outlet of the first fluid pathway and the pressure of fluid at the stack inlet of the second fluid pathway is greater than 20 mbar, more preferably greater than 50 mbar and more preferably still greater than 100 mbar. Preferably the differential is less than 200 mbar, and more preferably less than 150 mbar.

[0028] In embodiments the system comprises one or more pressure regulating valves in said first and second pathways, and controlling pressures comprises inputting a control or desired pressure signal to said one or more pressure regulating valves. Similarly a pressure regulating valve may be included in the fluid path of the pressurizing fluid, and the pressure in the vessel may be controlled by providing a control or desired pressure signal to said valve.

[0029] In embodiments the method further comprises determining appropriate control or desired pressures and / or pressure signals to apply to one or move regulating valves, to control thevarious pressures in the system. The method may further comprise measuring pressures in the system, and adjusting said desired or control pressure signals in response to said measured pressures. Determining and / or adjusting in this way may include using models of said system, and may include calculating or interpolating pressure values within the system.

[0030] According to a further aspect of the present invention there is provided an electrolyser apparatus comprising a plurality of electrolyser cells contained within a pressure vessel, and further comprising a first (for example air side) fluid pathway for collecting a working fluid from a first side of said cells; and a second (for example fuel side) fluid pathway for collecting a working fluid from a second side of said cells, and one or more pressure controllers for controlling the pressures of the fluids in said vessel and in said first and second pathways, and wherein said one or more pressure controllers is adapted to maintain the pressure of fluid in said vessel to be greater than the pressure of either of the working fluids in said first and second pathways.

[0031] According to an aspect there is provided an electrolyser apparatus comprising a controller configured to perform the methods discussed herein.

[0032] According to an aspect there is provided a controller configured to perform the methods discussed herein.

[0033] According to an aspect there is provided a computer program comprising instructions which, when the program is executed by a computer, cause the computer to carry out the steps of the method according to the aspects above. In particular, when the program is executed by a computer, cause the computer to: control pressures in the system such that a pressure of pressurizing fluid in the pressure vessel is greater than a pressure of the working fluids in both of said first and second pathways.

[0034] According to an aspect there is provided a non-transitory computer-readable medium with instructions stored thereon, that when executed by a processor, perform (or cause the processor to perform) the steps of the method of according to the aspects above. In particular, when executed by a processor, perform (orcause the processor to perform): controlling the pressures in the system such that a pressure of pressurizing fluid in the pressure vessel is greater than a pressure of the working fluids in both of said first and second pathways.

[0035] Aspects and examples of the invention described above may be advantageous in the case of fluid streams suffering cross flow or cross contamination, ie a fluid in one flow path or stream entering another path or stream which is unintended. Typically this is because of a leak or fault, ie a mechanical failure of a component resulting in bulk flow of fluid, and pressures and control methods associated with the system are configured accordingly. There may however also be cases where such unintended flow is caused by diffusion of fluid through materials eg gaskets or active ceramic components. Some embodiments may therefore be tailored for such diffusion additionally or instead.

[0036] Particular and preferred aspects of the invention are set out in the accompanying independent claims. Combinations of features from the dependent claims may be combined with features of the independent claims as desired and appropriate and not merely as explicitly set out in the claims. The term "comprising" as used herein to specify the inclusion of components also includes examples in which no further components are present.

[0037] An enabling disclosure of the present invention, to one of ordinary skill in the art, is provided herein. Reference now will be made in detail to examples of the invention, one or moreexamples of which are set forth below. Each example is provided by way of explanation of the invention, and not limitation of the invention.

[0038] Brief Description of the Drawings

[0039] Features of the present invention will now be described in further detail, by way of various embodiments, and just byway of example, with reference to the accompanying drawings (which drawings are not to scale), in which:

[0040] Fig. 1 shows an electrolyser system;

[0041] Fig. 2 is a schematic representation of fluid flows through an electrolyser system;

[0042] Fig. 3 illustrates pressure drops across flow pathways through an electrolyser system; and Fig. 4 is a simplified control device for controlling an electrolyser system.

[0043] Detailed Description

[0044] Fig. 1 is a simplified schematic of an electrolyser system 100 including a plurality of electrolyser stacks 10 in an enclosure 105. The enclosure 105 may be a pressure vessel 105 configured to withstand a pressure difference between its interior and exterior. The pressure vessel, in operation, is filled with a pressurizing fluid via inlet 131, and pressurizing fluid may pass through, and leave the vessel at outlet 133. The vessel, or pressurizing fluid may be a relatively inert gas to avoid corrosion of the components within the enclosure, and to minimized the possibility of a reaction between the vessel fluid and any leakage of product (e.g. hydrogen, carbon monoxide, nitrogen or oxygen) or first and second fluids from the manifolded passages elsewhere within the enclosure. For example, the vessel fluid may be air or nitrogen The pressure of fluid in the vessel may be controlled by a regulator at or downstream of the outlet 133. A gradual sweep flow of pressurizing fluid is useful for removing / preventing build up of contaminants that may enter the pressure vessel.

[0045] Two stacks 10 are shown in Fig. 1 , but it will be understood that any number of stacks 10 may be present, including fewer and greater than four, for example 1 , 2, 10, 12 and so forth.

[0046] Each stack 10 comprises a stack of electrolyser cell units 180. A typical stack may have 100 to 500 electrolyser cell units.

[0047] The general form of electrolyser cell units and stacks of cells is well known in the art, but is described here briefly for reference: each cell unit typically comprises an active layer or surface, separating first and second fluid volumes on either side. Fuel (eg steam) is passed through one of the volumes across one side of the active surface, and is (at least partially) converted to one electrolysis product (eg hydrogen) while a second electrolysis product (eg oxygen) is produced at the other side of the active surface, with the application of electrical energy (eg a current passed through the active layer). Cells are typically arranged together in stacks which provide a physical support for arranging the stacks together, and channel input and output fluids through the cells appropriately.

[0048] Each stack 10 has a first fluid outlet 115, and a second fluid outlet 121 , which collect the products of the electrolysis process respectively and output them. A first manifold 117 is connected to outlet 115 and forms a first fluid pathway transporting off gas (eg oxygen) to a port 119 (shown dashed line) in the wall of the pressure vessel, fordownstream processing. Similarlya second manifold 123 is connected to outlet 121 and forms a second fluid pathway transporting off gas (eg oxygen) to a port 125 (shown dashed line) in the wall of the pressure vessel, fordownstream processing.

[0049] Fuel (eg steam) is provided to the stack(s) at stack inlet 145 by inlet manifold 141, which enters the vessel at port 143, and optionally a sweep gas such as air may be provided to the stack(s) by manifold 147 entering the pressure vessel at port 149.

[0050] Thus a first fluid pathway is defined, which takes off gas from one side of the cells, to the exterior of the pressure vessel, and a second fluid pathway similarly takes off gas from the other side of the cells outside of the pressure vessel. The fluid pathways may, in embodiments, be considered to extend upstream to the inlet ports and manifolds (if present). The fluids in the respective pathways are (in the absence of any leaks or crossover) kept separate from one another and from the pressurizing fluid. It should be noted that the electrolysis products are not usually pure, eg hydrogen is usually mixed with unreacted fuel / steam, and oxygen is commonly mixed with a sweep gas such as air.

[0051] A heat exchanger unit 150 is provided downstream of the pressure vessel to extract heat from the off gas flows, and a control valve arrangement 160 still further downstream to control the pressures of the respective fluid pathways. The valve arrangement may control the pressure of the fluids upstream by back pressure regulation for example, maintaining a desired or control pressure (set by a user or control algorithm input for example) immediately upstream of the arrangement by allowing any excess pressure to be vented. The electrolyser system 100 typically operates at an elevated temperature, for example 400-700 °C for cell units based on a solid oxide electrolyte.

[0052] Figure 2 is a schematic representation of the system of Figure 1 modelling the three fluid streams / pathways as a pressure resistance network. Shown are pressurizing fluid stream 230, air side stream 231 , which ultimately outputs oxygen, and fuel side fluid stream 232, which ultimately outputs hydrogen. The pressure vessel is shown schematically as box 205.

[0053] Considering stream 230, the pressure at the input to the vessel is shown at point 241 , and the pressure at the output of the vessel is shown at point 242, with resistance of fluid passing through the vessel shown schematically by resistance 251. Pressure sensors may be provided to determine pressures 241 and 242. Resistance 252 corresponds to the heat exchangers(s) and 253 illustrates a back pressure valve, to control the pressure of the fluid immediately upstream of it. Pressures immediately downstream of components 252 and 253 are represented as 243 and 244 respectively.

[0054] Streams (and associated fluid pathways) 231 and 232 can be understood in a corresponding fashion. It will be appreciated from reference to Figure 1 however, that the resistance 251 (corresponding to passage of pressurizing fluid through the vessel and around the stack or stacks) is somewhat different in nature to resistances 261 and 271 , which represent fluid resistance from a passage including an inlet manifold, fluid conduits through stacks of cells and an outlet manifold.A controller 290 may receive signals 291 representing various measure pressures along the fluid pathways and produce control signals to control valve 253 and its equivalent in the other two pathways, to set the various pressures of the system.

[0055] Figure 3 illustrates examples of pressure profiles of the three fluid streams during normal operation of an electrolyser system (ie steady state, as opposed to a warm up or cool down operating mode) with pressure illustrated on the vertical y axis, and a representation of distance or extent along the fluid path is depicted by the horizontal y axis. Line 330 represents pressurizing fluid path, line 331 represents the air or oxygen fluid path, and line 332 represents the fuel, or hydrogen fluid path.

[0056] Each path can be seen having a portion 391 upstream of the vessel, a portion 396 passing through the vessel, and a portion 395 downstream of the vessel. The vessel itself is depicted as having an inlet manifold portion 392, and stack or cell portion 393, and an outlet manifold portion 394. Atypical pressure profile is shown solid line for each path, but dashed lines illustrate a range of possible values or profiles which should also be considered in order to maintain desired pressure differentials across a range of possible circumstances, as will be explained further below.

[0057] All three fluid paths show a drop in pressure from upstream to downstream as would be expected. Pressurizing fluid 330 shows only a very small pressure drop, due to the relatively large volume of the vessel and the low flow rate typically employed in use. As can be seen, each of the three paths typically exhibit different pressure profiles ie the overall pressure drop is different, and the drop across different portions of the flow path is different eg fuel path 332 experiences a greater pressure drop across the stack (393) than the air path 331.

[0058] As noted above, it has been found to be desirable to arrange the system to have specified pressure drops between the three fluid streams or paths (in use), particularly for the pressurizing fluid stream to have a greater pressure than the air or fuel streams, and for the air stream to have a greater pressure than the fuel stream.

[0059] In Figure 3 it can be seen that stream 330 advantageously has a pressure greater than either of the other two streams at all points in (and at the boundaries of) the vessel. This ensures that any cross flow (eg because of a leak) between the pressurizing fluid and either or both of the other two streams at any point will result in pressurizing fluid (which can be an inert gas such as nitrogen) entering those streams, and notvice versa. This prevents any build up of fuel or air / oxygen in the pressure vessel.

[0060] Since the lowest pressure of stream 330 is at the vessel outlet, and the highest pressure of stream 331 (which is the highest pressure of the two other streams) is at the pressure inlet, these two pressures represent the smallest differential, and are typically used to control the system pressures, ie to maintain a desired pressure differential between these two points, as shown by arrow A1 in Figure 3 (note that for A1, A2 and A3 in Figure 3, the horizontal component of the arrow represent the physical location of the two points, and the vertical component the actual difference in pressure). The desired differential is determined to be sufficient to cause a potential flow in the desired direction between streams (or at least to prevent an opposite flow), but sufficiently low to minimize mechanical stress or strain on components. Additionally the desired differential should take into account potential variations (during steady state) ofpressure drops across the various components. In other words the system should be designed to accommodate the situation where the pressure drop of path 330 is as shown by the lower, dashed line. A suitable differential has been determined to be 150-250 mbar in certain examples.

[0061] Because the pressure drop across path 330 is typically low, even in a “worst case” scenario (ie the maximum foreseeable pressure drop across path 330), the difference between the pressurizing fluid at the vessel inlet and the air stream at the vessel inlet A2 may be used as a proxy for A1 in some examples.

[0062] As further noted above, it has been found desirable for air stream 331 to be at a higher pressure than fuel stream 332, to ensure that any cross flow between these two streams because of a fault or leak is from the oxygen / air pathway 331 to the fuel / hydrogen pathway 332 (minimising the risk of a potentially dangerous mixture forming. The risk of cross flow between these two streams is typically only present in the stacks or cells 393, where the flow paths come into close contact, eg separated only by the electrochemical layers of the cells, or by gaskets designed to separate the flow paths in the stack, as opposed to the manifold regions 392 and 394 where the two flows are typically in separate conduits, and those conduits are physically spaced apart (and typically have pressurizing fluid therebetween). Therefore controlling the relative pressures of these two streams is preferably performed by reference to pressure drop A3 in Figure 3. This is the difference between fluid pathway 331 at the stack outlet and fluid pathway 332 at the stack inlet. A suitable differential has been determined to be 50-150 mbar in certain embodiments. It should be noted that these two points will typically be inside of a pressure vessel, and therefore it may be difficult and / or costly to measure with the use of sensors. However, the pressure of pathways 331 and 332 may be measured at the input and / or output of the vessel, and the pressures along the flowpath inside the vessel can be calculated or estimated by interpolation or modelling techniques, based on an understanding of the geometry and arrangement of the flow paths.

[0063] Figure 4 shows a control device 400 (which may be an example of controller 290) for controlling an electrolyser system comprising electrolyser cell units arranged in stacks. The control device 400 comprises an input device 402 for receiving input from pressure sensors 404 so as to determine, for one, each, or a plurality of the pressures 241 to 244 of each of the fluids of figure 2. The control device 400 thus comprises a pressure monitoring system 406-408 for determining respective pressures (which may be in response to signals 291) and / or a control system 410 for determining adjustments to respective pressures and resulting signals to control valves (valve 253 and its equivalent in the other two pathways) an inlet temperature at the fluid inlet and an outlet temperature monitoring. These systems may utilize sensors and data transmission devices or wiring. The control device receives sensor data relating to each of these measurements. The control device may control the system by controlling valves for supply to and exhaust from each fluid volume. A suitably programmed processor 412 and associated memory 414 is provided for processing such inputs.

[0064] During normal operation the control device is adapted to control pressures in the system such that the pressure of pressurizing fluid in the pressure vessel is greater than the pressure of the working fluids in both of said first and second pathways according to the methods herein.In such a way, the control device 400 is adapted to control an electrolyser system for example that shown in Figures 1 and 2 as described with respect to Figure 3.

[0065] The present invention has therefore been described with reference to various embodiments. The present invention is not limited to only the above examples. Other examples will be readily apparent to one of ordinary skill in the art without departing from the scope of the appended claims.

[0066] These and other features of the present invention have been described above purely byway of example. Modifications in detail may be made to the invention within the scope of the claims.

Claims

CLAIMS1. A method of operating an electrolyser system comprisinga plurality of electrolyser cells contained within a pressure vessel containing a pressurizing fluid;a first fluid pathway for collecting a working fluid from a first side of said cells; and a second fluid pathway for collecting a working fluid from a second side of said cells; whereinsaid method comprises controlling pressures in the system such that the pressure of pressurizing fluid in the pressure vessel is greater than the pressure of the working fluids in both of said first and second pathways.

2. A method according to claim 1 , wherein the pressures in the system are controlled such that pressure of the pressurizing fluid in the pressure vessel is greater than the pressure of both of the working fluids at all points along their respective fluid pathways.

3. A method according to claim 1 or claim 2, wherein the pressures in the system are controlled such that the pressure of pressurizing fluid at all points in the pressure vessel is controlled to be greater than the pressure of both of the working fluids at any point along their respective fluid pathways.

4. A method according to any preceding claim, wherein the working fluid in the first fluid pathway contains at least 30% oxygen by weight and wherein the pressure in the system are controlled such that pressure in said first fluid pathway is greater than the pressure in said second fluid pathway.

5. A method according to any preceding claim, wherein the pressure vessel has an inlet and an outlet for pressurizing fluid, and the first fluid pathway extends from an inlet port at the pressure vessel wall, via the electrolysis cells to an outlet port at the pressure vessel wall; andwherein pressure in the system is controlled such that such that the pressure difference between the pressure of the pressurizing fluid at the vessel outlet and the pressure of working fluid at the inlet port of the first fluid path is greater than 100 mbar, more preferably greater than 150 mbar and more preferably still greater than 200 mbar.

6. A method according claim 5, wherein pressure in the system is controlled such that such that the pressure difference between the pressure of the pressurizing fluid at the vessel outlet and the pressure of working fluid at the inlet port of the first fluid path is less than 200 mbar, more preferably less than 150 mbar.

7. A method according to any preceding claim, wherein the plurality of electrolyser cells are arranged in a stack having at least one stack inlet and at least one stack outlet, with the first and second fluid pathways including a path through said stack from inlet to outlet; andwherein the pressure in the system is controlled such that the pressure difference between the pressure of fluid of the first fluid pathway at the stack outlet, and the pressure of fluid of the second fluid pathway at the stack inlet is greater than 20 mbar, more preferably greater than 50 mbar and more preferably still greater than 100 mbar.

8. A method according to claim 7, wherein the pressure in the system is controlled such that the pressure difference between the pressure of fluid of the first fluid pathway at the stack outlet and the pressure of fluid of the second fluid pathway at the stack inlet is less than 200 mbar, and more preferably less than 150 mbar.

9. A method according to any preceding claim, wherein the system comprises one or more pressure regulating valves in said first and second pathways, and controlling pressures in said pathways comprises inputting a desired pressure value to said one or more pressure regulating valves.

10. A method according to any preceding claim, wherein the system comprises one or more pressure regulating valves in the fluid path of said pressurizing fluid, and controlling pressures in the pressure vessel comprises inputting a desired pressure value to said one or more pressure regulating valves.

11. A method according to any preceding claim, wherein controlling the pressures further comprises determining desired pressure values for said first and second pathways, based on measured or calculated pressure drops across said first and second pathways.

12. A method according to any preceding claim, performed in steady state operation of said electrolyser system.

13. An electrolyser apparatus comprisinga plurality of electrolyser cells contained within a pressure vessel, and further comprisinga first fluid pathway for collecting a working fluid from a first side of said cells; and a second fluid pathway for collecting a working fluid from a second side of said cells; andone or more pressure controllers for controlling the pressures of the fluids in said vessel and in said first and second pathways, and wherein said one or more pressure controllers is adapted to maintain the pressure of pressurizing fluid in said pressure vessel to be greater than the pressure of either of the working fluids in said first and second pathways.

14. A controller configured to control an electrolyser system according to the method of any one of claims 1 to 12.

15. A computer program comprising instructions which, when the program is executed by a computer, cause the computer to carry out the method according to any one of claims 1 to 12.

16. A non-transitory computer-readable medium with instructions stored thereon, that when executed by a processor, perform the method according to any one of claims 1 to

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