Method of controlling an electrolyser cell stack

Abstract

A method of controlling an electrolyser cell stack with a system having a fluid temperature control system is defined. The method inlcudes: a current control system, a voltage monitoring system and mo

Description

The present invention relates to a method of controlling an electrolyser cell stack, and in particular a method of controlling the operational state of an electrolyser cell stack of an electrolyser for extending the life of the stack and for detecting thermal runaways or failures within the stack. An electrolyser may comprise one or more stacks of electrolyser cells - commonly known as electrolyser cells or regenerative fuel cells. The electrolyser is used to split a source fluid 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. The collection or use of the produced oxygen is important as it can be utilised in industry and medical applications, amongst many other uses. Carbon monoxide is also useful for numerous chemical processes. The collection and the subsequent storage and / or distribution of the produced hydrogen is also important as it is a fuel that can help in the race for decarbonisation and 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 a resultant electrical and heat output. The hydrogen can also have many other uses. Combining electrolysis with green energy sources is also important as that can greatly contribute towards the green credentials of electrolysis, and particularly hydrogen capture, thus accelerating the achievement of net zero and decarbonisation targets. 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. One area where electrolysers are known to have inefficiencies is towards the end of the serviceable life of a stack of electrolyser cells. When a stack starts to fail or when it degrades, the operational efficiency of the electrolyser will diminish. For example, electrical resistance of the stack can increase, leading to higher voltage requirements for the same electrical current flow. However, degradation itself is not a problem that can readily be avoided, and instead it is accepted as inevitable. Compensating for degradation is thus an important operational consideration when maintaining a useful operational capability from the electrolyser. However, it is also important to manage the operational state of an electrolyser for minimising degradation and thus to reduce service or downtime costs of the electrolyser. One known way of managing the operational state of an electrolyser is to operate the electrolyser at a fixed voltage. This can have advantages as some degradation and failure modes are related to, or driven by, over-potentials. Allowing the voltage to increase can thus create an increased risk of a new failure mode kicking in, either in the short term, or earlier later on in the electrolyser’s service life. However, a drawback with this strategy is that the current will slowly decrease with time as the stacks degrade, reducing the hydrogen production rate of the system. This has resulted in a need to compensate for lost production capacity over time by introducing additional electrolyser cell modules or stack, or downtime and stack replacement, all of which increase costs and disruption, and some of which require infrastructure changes or more space. Other prior art teachings, such as prior art documents JP2020128576 and WO2020201485, consider the relationship between temperature control and voltage control in an electrolyser cell for increasing service life of an electrolyser cell. WO2018033948 further considers current control, adopting either a constant current or a constant voltage alongside temperature control. The present invention seeks to take these concepts further. The present invention thus seeks to improve or extend the service life of a stack, and to maintain operability of an electrolyser for an extended period of time. The present invention also seeks to identify when a service life ending stack failure has occurred. According to the present invention there is provided a method of controlling an electrolyser cell stack of an electrolyser in an electrolyser system; the electrolyser cell stack comprising a fluid inlet and one or more fluid outlets; and the electrolyser system comprising: a fluid temperature control system for controlling temperature of a fluid entering the electrolyser cell stack at the fluid inlet; a current control system for controlling a current supply to the electrolyser cell stack; a voltage monitoring system for determining a stack operating voltage across the electrolyser cell stack; an inlet temperature monitoring and / or control system for determining an inlet temperature at the fluid inlet; and an outlet temperature monitoring and / or control system for determining an outlet temperature at at least one of the one or more fluid outlets; wherein the method comprises: controlling the current supply to the electrolyser cell stack to a fixed input current; calculating a temperature delta by subtracting a determined inlet temperature from a determined outlet temperature; and i) if the absolute value of the temperature delta is greater than a threshold value, the method comprises adjusting the inlet temperature using the following equation: Tnew — Told + (dT X TA) where: Tnew is the adjusted target input temperature, Toid is the current (target) input temperature, dT is an adjustment factor that is less than one and greater than zero and TA is the calculated temperature delta; ii) if the absolute value of the temperature delta is lower than a threshold value, the method comprises: determining the operating voltage across the electrolyser cell stack; and determining a stack operation status dependent upon whether the determined operating voltage is below a voltage threshold. In some embodiments, the electrolyser cell stack comprises at least one fluid inlet for fuel and at least one fluid inlet for a sweep gas. In some embodiments, the electrolyser cell stack comprises at least one cathode side fluid outlet and at least one anode side fluid outlet. In some embodiments, the electrolyser system comprises fluids for entering the electrolyser cell stack at the fluid inlets, the fluid temperature control system controlling the temperature of the fluids for entering the electrolyser cell stack. In some embodiments, the current control system provides or controls an input current for powering the electrolyser cell stack. In some embodiments, the voltage monitoring system comprises a voltage sensor for detecting a stack operating voltage across the electrolyser cell stack. In some embodiments, the outlet temperature monitoring and / or control system comprises a temperature sensor for detecting an output temperature at at least one of the fluid outlets. In some embodiments, the method comprises: setting a target input temperature for at least one fluid inlet; controlling a fluid temperature for the fluid for entering the electrolyser cell stack at that at least one fluid inlet to target that target input temperature; supplying an input current to the electrolyser cell stack as a constant current to operate the electrolyser cell stack galvanostatically; and detecting or determining an operating voltage across the electrolyser cell stack with the voltage monitoring system. In other embodiments, the temperatures may vary from targets, or the targets may be temperature ranges, or the temperature is set by the supply temperature and any heating therefor, and the method seeks to maintain a fixed current, a fixed voltage and a fixed temperature delta, adjusting the inlet temperature, the target or the temperature range when needed in accordance with the method of the present invention. In some embodiments, the outlet temperature monitoring and / or control system detects output temperatures at each fluid outlet. In some embodiments, the inlet temperature monitoring and / or control system detects the input temperatures at the or each fluid inlet. In some embodiments, a variable temperature delta threshold can be set. For example, if an electrolyser has multiple stacks, each may have a different temperature delta threshold. In some embodiments, the method comprises setting maximum and minimum stack voltage thresholds for the or each stack. In some embodiments, a different dT is applied to each stack of an electrolyser or electrolyser system. This may be advantageous to enable the system to be thermoneutral, rather than merely each stack. Similarly, the dTs may be set to provide a “bias” towards exothermic operation of the stack or each of the (or some of the) stacks. This could be to generate excess heat for compensating for heat losses in the system’s heat exchangers or other inefficiencies in the system. In some embodiments, if the absolute value (i.e. modulus) of the temperature delta is less than the temperature delta threshold, the method may comprise checking a detected operating voltage against a maximum and / or minimum stack voltage threshold, and if either stack voltage threshold is exceeded, the method comprises issuing a warning or alarm, or shutting down the stack by reducing or turning off the input current. With the present invention, in some embodiments, if the determined stack operation status is a potential runaway, the method comprises either or both issuing a warning or shutting down the stack. In some embodiments, the voltage threshold is related to an optimal voltage for achieving thermoneutral conditions at a given temperature. In some embodiments, the voltage threshold is the optimal voltage minus a predefined delta. In some embodiments, if either or both stack voltage threshold is exceeded, the method comprises attempting to operate the stack potentiostatically at the threshold. In some embodiments, the method comprises setting a maximum inlet temperature threshold, and switching the operating mode from galvanostatic to potentiostatic when the inlet temperature reaches said maximum inlet temperature threshold In some embodiments, the electrolyser cell stack is a solid oxide electrolyser cell stack. In some embodiments, the electrolyser comprises more than one stack. The stacks may be electrically connected in parallel or in series. In some embodiments, the method is applied to each stack separately. Alternatively, the method is applied to the stacks collectively, with the output temperature being at the fluid outlets of the collective of stacks, and the input temperature being either the target input temperature for the collective of stacks or a measured temperature for the collective of stacks. In some embodiments, the method aims to operate the stack in a thermoneutral state such that the temperature within the stack remains substantially constant. With the present invention, when the absolute value (i.e. modulus) of the temperature delta is more than the temperature delta threshold, an adjustment to the target input temperature is applied such that when the temperature delta is positive, the adjustment to the target input temperature is also positive, and such that when the temperature delta is negative, the adjustment to the target input temperature is also negative. This is counterintuitive as typically in the prior art a heating stack wants to be cooled, whereas a cooling stack wants to be heated. However, the present invention relies upon the resistance response to stack temperature, and that resistance’s interaction with the voltage across the stack under galvanostatic conditions: as the input temperature increases, the stack warms. This reduces the electrical resistance of the stack, which in turn reduces the voltage across the stack. That reduction of voltage, in galvanostatic conditions (i.e. with a constant current), reduces the power draw of the stack, and thus, if now lower than the thermoneutral voltage, due to the endothermic nature of electrolysis, the stack actually avoids heating up by as much as the increased input temperature, and can even cool. Conversely, as the input temperature reduces, the stack cools. This increases the electrical resistance of the stack, which in turn increases the voltage across the stack. That increase of voltage, in galvanostatic conditions (i.e. with a constant current), increases the power draw of the stack, and thus, when it exceeds the thermoneutral voltage, due to the now exothermic nature of the configuration (due to the increase in power input from the constant current source), the stack actually avoids cooling down by as much as the reduced input temperature, and can even heat up. The present invention thus provides a regulated internal temperature control for the stack using external fluid temperature control, galvanostatic conditions across the stack, and the power draw characteristics of the stack arising from fluctuations in voltage either side of a thermoneutral voltage for the stack. In addition, the present invention allows for the inevitable degradation of an electrolyser cell to be compensated for. As a cell degrades, its resistance increases, and thus a power draw will increase (to maintain a constant current). The invention allows for the increased power draw to be compensated for by increasing the input temperature, and ultimately an increase in the operating temperature of the stack. The stack can thus be controlled to maintain thermoneutral voltage conditions alongside galvanostatic (constant current) conditions, even as the stack degrades (at least up to a maximum temperature where potentiostatic operation may be preferable). The present invention also provides a “runaway” protection, when leaks start to form, whereby, in the case of steam electrolysis, hydrogen may mix with oxygen. Although initially in small amounts - for example in the output side of the stack, such leaks can lead to combustion of the hydrogen with the oxygen either in the hot air or in the oxygen-enriched sweep flow. That combustion ultimately can cause alternative heating of the stack, but initially it creates shorts across the stack or faster degradation of the electrolyte or anode or cathode. In other words, leaks which cause internal combustion may produce balancing temperature changes (the leak reducing the temperature, but the subsequent combustion increasing the temperature); this makes such leaks difficult to detect. Leaks will provide an additional heat source whereby formed hydrogen fuel reacts with oxygen to form steam, releasing heat in the process. It is desirable to monitor the magnitude of the internal leaks within the stack such that a shutdown can be preventatively initiated in case this increases above a certain maximum value. This prevents thermal runaways from happening and allows recovery and repair of stacks. Developing leaks will effectively lead to a lower stack voltage as the control system works to maintain thermoneutrality. This means that stack voltage can be used as an approximate gauge for stack internal leak rates. This can be very practical as an additional monitoring mechanism as no additional measurement systems are required -stack voltage is already being monitored. With a small leak, or a failure of the cell, a significant temperature delta might not arise. Indeed, initially, the failure might only be identified by the voltage across the stack changing. If this changes considerably, a warning or alarm, or even a shutdown of the electrolyser system can be instigated. The present invention thus allows the system to avoid missing the detection of leaks that present no output temperature change. The present invention also enables such degradation to be compensated for by operating the electrolyser system with an altered input temperature, and a matching output temperature for ensuring both galvanostatic and thermoneutral conditions remain observed, thus extending the life of the electrolyser cell stack, and likewise maximising the performance and efficiency of the stack near its end of life. This also ensures that the electrolyser is used in a manner that maximises the amount of hydrogen produced for a given electrical input for a longer period of time as galvanostatic and thermoneutral conditions are considered optimum operating conditions for efficient electrolysis of water (or carbon dioxide). The method of the invention typically runs over set time intervals, so that temperature deltas over that given period of time are taken into account, thus avoiding false detections from momentary or periodic spikes. Therefore, in some embodiments detecting or determining the operating voltage across the electrolyser cell stack with a voltage sensor, and detecting or determining the output temperature at the fluid outlets, occurs periodically at predetermined time intervals. For example time intervals of 10 minutes, 30 minutes, 1 hour or shorter or longer time intervals. In particular, should numerous consecutive issues be detected - i.e. a number of datapoints or readings showing a change, the present invention can be triggered to react by issuing a warning or starting a shut down. In such a way, a moving average of a series of samples taken over a specified time period may be used to determine a temperature measurement avoid the impact of outlying or erroneous data points. The same or a similar process may also apply to the voltage measurement. The stack voltage may be measured directly or inferred from a voltage measurement across a single cell, or sample of cells. Similarly, the inlet / outlet temperature may be measured directly or inferred from a sensor measurement up / downstream of the inlet / outlet. Typically any temperature changes are small, so threshold temperature deltas may be as small as 1, 2 or 5 degrees C. Therefore, in some embodiments the temperature delta threshold is set at a value in the range of between 1 and 5 degrees C, inclusive. In some embodiments, the temperature delta may include an offset (for example 1, 5 or 10 degrees C) so as to allow the stack to generate some excess heat. This may allow for the removal or reduction in use of an electric heater in the system. In some embodiments, the method also monitors the output fluids - for example to inspect hydrogen or oxygen output volumes from the stack. As such, the method the may monitor the output fluids to inspect target gas output volumes or percentages from the stack. If there is a leak and resulting combustion, volumes of hydrogen and oxygen would decrease. The moisture content of the oxygen side output might also increase. These can also provide quantitative indicators of failure. The method can be configured - by adjusting the voltage deltas from the known ideal thermoneutral voltage for a given temperature - to trigger an alarm or a warning or a shutdown procedure in the event of a failure equivalent to between 5 and 10 % of the hydrogen leaking across to the oxygen. Depending on the cost of repair or replacement of the cells, and also the risk of catastrophic failure which may occur by not repairing larger leaks, a repair cost versus efficiency reduction calculation can be undertaken. This can be to determine at what point it is best to replace or repair a stack versus continuing to operate the electrolyser stack at a reduced efficiency. After all, the target is to maximise the amount of hydrogen produced by a stack versus the cost of maintenance and operation thereof (termed a ‘levelized’ cost of hydrogen). The present invention has particular application to electrolysers in the intermediate and high temperature electrolyser cell sectors - and also lower temperature electrolyser cell temperatures albeit with operational temperatures above 100 degrees C, and thus operating with steam rather than water. However, water-based electrolysers (with operational temperatures below 100 degrees C) may also benefit from the present invention. In some embodiments, the at least one electrolyser cell is a solid oxide electrolyser cell, i.e. the electrochemically active region 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 may be referred to as an intermediate-temperature solid oxide electrolyser cell, or IT-SOEC. 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. 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 orCGO). Due to the 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. In some embodiments the electrolyser cell system instead comprises a high temperature electrolyser cell with an operational stack temperature between 750 degrees C and 1100 degrees C In summary, the present invention allows the gradual increase of the temperature of the stack over its life while maintaining galvanostatic and thermoneutral conditions so as to control stack performance (i.e. current density), and this is achieved through control of an input fluid’s temperature when required. The present invention also provides a control device for controlling an electrolyser cell stack of an electrolyser in an electrolyser system, the control device comprising: a fluid temperature control system for controlling temperature of a fluid entering the electrolyser cell stack at a fluid inlet of the electrolyser cell stack; a current control system for controlling a current supply to the electrolyser cell stack; a voltage monitoring system for determining a stack operating voltage across the electrolyser cell stack; an inlet temperature monitoring and / or control system for determining an inlet temperature at the fluid inlet; and an outlet temperature monitoring and / or control system for determining an outlet temperature at at least one fluid outlet of the electrolyser cell stack; wherein the control device is adapted to: control the current supply to the electrolyser cell stack to a fixed input current; and calculate a temperature delta by subtracting the determined inlet temperature from the determined outlet temperature; and wherein: i) the control device is further adapted to adjust the inlet temperature if the absolute value of the temperature delta is greater than a threshold value using the following equation: Tnew — Told + (dT X T△) where Tnew is the adjusted target input temperature, Toid is the current (target) input temperature, dT is an adjustment factor that is less than one and greater than zero and TA is the calculated temperature delta; and ii) the control device is further adapted to determine the operating voltage across the electrolyser cell stack and determine a stack operation status dependent upon whether the determined operating voltage is below a voltage threshold if the absolute value of the temperature delta is lower than a threshold value. The control device can be further configured to carry out the method previously described, and can be implemented with or within any electrolyser system described herein. Brief Description of Drawings The present invention will now be described in further detail, purely by way of example, with reference to the accompanying drawings in which: Figure 1 schematically shows a typical electrolyser cell, multiples of which may be stacked in a stack; and Figure 2 shows a schematic of an example of an electrolyser system with external stream flow paths for anode and cathode sides of the stack with heat exchangers and heaters for input fluid temperature control. Figure 3 is a flow diagram showing the general steps of the present invention. Figure 4 shows a schematic of a control device for controlling an electrolyser cell stack of an electrolyser in an electrolyser system. Referring first to Figure 1, the basic structure and operation of a typical electrolyser cell 11 within an electrolyser 10 of an electrolyser system 20 is shown by reference to one fuel / electrolyser cell 11 of a stack 12. It should be noted that other ancillary components related to the electrolyser cell 11 are included in an electrolyser system 20. These usually include heat exchangers, heaters, valves and sensors. Figure 2 schematically shows an example of an electrolyser system 20, similar to that known from WO2023 / 012456, the entire contents of which are incorporated herein by way of reference. This shows example ancillary components, such as heat exchangers, heaters and sensors. These will be discussed in more detail below. Figure 2 also shows that the electrolyser system 20 comprises an electrolysis stack 12 which comprises a stack of electrolyser cells 11. As shown, this example has five such cells, although generally a stack will have tens or even hundreds of cells in parallel, and multiple stacks 12 may be provided, in electrically connected in series or in parallel. Referring now back to Figure 1, the electrolyser cell 11 comprises an anode 33, a cathode 34 and an electrolyte 35. Such a structure for an electrolyser cell 11 is well known in the art. In this example, the electrolysis of water will be discussed, although other gases can similarly be electrolysed to split them into component parts, such as carbon dioxide into oxygen and carbon monoxide. Water - here in the form of steam 43 from a water source - which may be a steam source if less internal heating of the water is desired in the electrolyser system 20 - is passed over the cathode 34 via inlet 41 and hot air 42 is passed over the anode 33 via inlet 40. To power the electrolyser cell, an electric current is applied across the electrolyser cell 11 via electric terminals / connections 36, 37 at the anode and cathode sides of the electrolyser cell 11. These terminals may be positioned adjacent to one-another on one side or end of the stack 12 of cells 11, for example by having stacked cells in parallel and extending one terminal to the other end of the stack 12 using a bus bar, as known in the art. As a consequence of the electrical current, an electrolytic reaction occurs across the electrolyte 35, with oxygen ions passing across the electrolyte 35 from the cathode 34 to the anode 33, and some of the steam braking down into hydrogen on the cathode side of the electrolyser cell 11 and oxygen is produced at the anode side. The oxygen can be extracted via an air flow or sweep flow provided by the hot air 42, thus venting it out of an off-gas outlet 38 on the anode side of the electrolyser cell 11. That output is generally oxygen enriched air (with the oxygen enriching the hot gas flow). The hydrogen can instead be extracted and vented out of another off-gas outlet 39 on the cathode side of the electrolyser cell 11. This off gas typically will be mixed with the remaining steam, as the splitting of the steam into oxygen and hydrogen is usually only in respect of a proportion of the supplied steam. The hydrogen is thus vented as ‘wet’ hydrogen on the cathode side. Thus, the steam exiting the cathode side is hydrogen enriched, and the air exiting the anode side is oxygen enriched. Those off-gases will usually be at a similar temperature to the operational temperature of the electrolyser cell 11. However, the specific delta from the input temperature will depend upon the amount of electrical power supplied to the electrolyser, and the internal resistance of the cells. Such operational characteristics of electrolyser cells, including SOECs, are well known in the art. Due to the high operating temperature of the electrolyser cell 11 (i.e. above 100 degrees C for a steam electrolyser, and in the case of a solid oxide electrolyser cell (SOEC) it is usually in excess of 400 degrees C), the heat of the off-gases from the off-gas outlets 38, 39 is able to be usefully used by the electrolyser system 20, rather than being wasted, for example to provide at least some of the heat for the steam generation on the input side of the stack, and likewise for heating the hot air entering the electrolyser, in advance of the inlets 40, 41. As shown in Figure 2, this may be achieved via heat exchangers 22, 50, 62. Heaters 36, 52, 58 can also be provided where required to increase the temperature of the fluid through flow lines 54, 56, 18, 24, 28 between components of the system 20, when needed. For controlling this, sensors 64, 66, 68, 70, 72, 74, 76 may be provided along the flow lines 54, 56, 18, 24, 28. These can also allow a control of flow rates of the respective fluids through the heat exchangers 22, 50, 62 or through the heaters 36, 52, 58 - for example for controlling the flow rate of the air 42, or the water 16, or the off-gases -herein from stack outlets 46, 48, which are connected to the outlets 38, 39 of the cells 11. Flow rates and or temperatures of the input fluids at the two stack inlets 14, 44, which are connected to the inlets 40, 41 of the cells 11, can thus be controlled as necessary, as discussed below. A flow line or flow path represents a flow that passes through one or more pipe or line from one position to another. Therefore, it defines a fluid connection or fluid communication between points. In Figure 2 there is a sweep gas supply flow path 18. It is a sweep gas supply pipe or line 18 and provides fluid communication from a sweep gas supply 16 (usually air) to an anode side stack inlet 14. The stack 12 also comprises an anode side stack outlet 46, wherein an anode side exhaust product (offgas - usually oxygen enriched air) is expelled for utilization in any one or more of various possible purposes - usually outside of the system 20, although its heat can be usefully utilized in the system 20, as discussed below. Also entering the electrolyser system 20 is an external fluid stream 26. The external fluid stream 26 is a stream that is produced separate from the process of the electrolyser system 10. The external fluid stream 26 can be a hot flow of fluid or gas - e.g. an external heat source. Whilst it is expected that the external fluid stream 26 will be an exhaust gas from another process and may be of low-grade heat (e.g. around 200°C), different sources can be utilised to provide the external fluid stream 26 with higher or lower grades of heat. The external fluid stream 26 forms an external stream flow path 28. The external stream flow path 28 passes through a first heat exchanger 62. The first heat exchanger 62 is also arranged across the sweep gas supply flow path 18 between the sweep gas supply 16 and the anode inlet 14. Therefore, the external stream flow path 28 and the sweep gas supply flow path 18 both pass through the first heat exchanger 62 and exchange heat in the first heat exchanger 62. It is also shown in Figure 2 that an anode outlet flow path 24 from the anode side stack outlet 46 passes through a second heat exchanger 22 before passing out of the first exhaust 88 (e.g. for collection for subsequent use elsewhere). This second heat exchanger 22 is also arranged across the sweep gas supply flow path 18. The second heat exchanger 22 in this embodiment is arranged across the sweep gas supply flow path 18 between the first heat exchanger 62 and the anode inlet 14. Therefore, the anode outlet flow path 24 and the sweep gas supply flow path 18 exchange heat in the second heat exchanger 32. During operation of the system 10, the exhaust gas may contain high-grade heat (e.g. around 550°C for an SOEC electrolyser system). Therefore, the heat from the exhaust gas can be used to heat the sweep gas supply flow path 18 in the second heat exchanger 32. This is beneficial to provide sweep gas at the anode inlet 14 of high temperature to increase efficiency of the electrolysis reaction in the stack 12. A first heater 36 is provided in the sweep gas supply flow path 18. In this embodiment this is positioned between the second heat exchanger 32 and the anode inlet 14. The first heater 36 is used for heating the sweep gas supply 16 in the sweep gas supply flow path 18 when needed. The first heater 36 can be an electric heater or a combustion heater as required. The first heater 34 is also referred to as a trim heater as it provides small amounts of heat for fine-tuning the temperature at the anode inlet 14 to ensure a consistent and efficient electrolyser reaction in the stack 12. A bypass flow path 30 is shown connecting to the sweep gas supply flow path 18. The bypass flow path 30 is connected at its first end 78 at a point on the sweep gas supply flow path 18 between the sweep gas supply 16 and the first heat exchanger 62. Therefore, the first end 78 is upstream of the first heat exchanger 62. The bypass flow path 30 is connected at its second end 80 at a point between the second heat exchanger 22 and the first heater 62. Therefore, the second end is downstream of the second heat exchanger 22. The bypass flow path 36 bypasses the first and second heat exchanger 30, 32. This can allow for the sweep gas supply flow path 18 to avoid any heat exchange with the heat exchangers. This can be beneficial where the heat energy in the external stream flow path 28 and / or the anode outlet flow path 24 is of a level that is not required for the sweep gas supply flow path 18. Such situations can include start-up where the flow paths might be cold, or during cool down, where transfer of heat to the sweep gas supply flow path 18 is no longer required and the sweep gas supply 16 is being used to cool the system 10. The bypass flow path 36 is operable by a control valve 82 or the like. The control valve can be a mechanical or electrical control valve set to a certain setting to open or close, or can be manually or automatically operated by a controller, which may include a processor and a memory that can be programmed to operate the control valve in certain situations as required. For example, one or more sensor 66, 72, 74, 76 may be provided on one or more of these flow lines to allow control of the flows and temperatures, the controller being connected to these sensors and potentially to the heaters and further flow valves or bypasses. Whilst the above are presented as shown in Figure 1, it is highlighted that certain variations can be made. In particular, the presence of the first heat exchanger 62, the second heat exchanger 22, the first heater 36 and the bypass flow path 30 can be varied as required for the function of the system 10. For instance, a second heat exchanger 22 may not be required in some situations, such as where the external fluid stream 26 is high grade heat. Additional heaters can also be supplied, such as the illustrated third heater 58 on the anode outlet flow path 24, which can also be controlled to operate when required. Referring next to a cathode side of system, a water supply 84 is provided and is connected via a water or steam supply flow path 56 to a cathode side stack inlet 44 of the electrolysis stack 12. The cathode side stack inlet 44 connects to the steam inlet 41 on the cathode side of each electrolyser cell 11 of the electrolysis stack 12. The water supply flow path 56 provides a flow, pipe or line for the water - or steam - to pass into the electrolyser 10. The electrolysis stack 12 also comprises a cathode outlet 48 that is the outlet from the cathode of the electrolysis stack 12, and is connected to the steam outlet 39 of each electrolyser cell 11. The cathode outlet 48 is connected to a second exhaust 86 by a cathode outlet flow path 54. Wet hydrogen is the exhaust gas on this side of the electrolyser 10, and it can be collected for downstream processing and use. A third heat exchanger 50 is provided in the water supply flow path 56 between the water supply 42 and the cathode inlet 44. The cathode outlet flow path 54 is also connected across the third heat exchanger 50 between the cathode outlet 48 and the second exhaust 86 of the cathode outlet flow path 54. Therefore, heat is exchanged between the cathode outlet flow path 54 and the water supply flow path 56. The product, e.g. wet hydrogen, produced in the stack 12 and output at the cathode outlet 48 can be of high heat energy (e.g. around 550° for an SOEC system). Therefore, this heat energy can be used to heat the water supply flow path 56. This can be beneficial as it is preferred that steam is supplied for the electrolyser reaction. Therefore, heat energy can be transferred from the cathode outlet flow path 54 to the water supply flow path 56 in the third heat exchanger 50. This heat exchanger 50 can be referred to as a recuperating heat exchanger. This can also be beneficial as the product, e.g., wet hydrogen, at the second exhaust 86 is preferred to be lower in heat energy for exporting to other processes and storage. Therefore, the transfer of heat from the product is beneficial. A second heater 52 is provided on the water supply flow path 56 for heating the flow path. The second heater 52 is positioned between the third heat exchanger 50 and the cathode inlet 44. Therefore, the second heater 52 is downstream of the water supply 84. The second heater 52 is similar to the first heater 36 in that it can be an electric heater or a combustion heater as required. The second heater 52 provides additional heating to the water supply flow path 56 as required. Sensors 64, 68, 70 and a control system can again be provided for this purpose. In situations where there is high heat transfer at the third heat exchanger 50, the amount of heat energy supplied at the second heater 52 can be low. Whereas where there is low heat transfer at the third heat exchanger 50, e.g. during start-up, the heat energy supplied at the second heater 52 can be high. This can be of benefit during start-up where the system 10 is not yet hot. The second heater 52 is also referred to as a trim heater for the same reasons as the first heater 34 as it provides small amounts of heat for fine-tuning the temperature at the cathode inlet 44 to ensure a consistent and efficient electrolyser reaction in the stack 12, although both can be used for start-up heating as well. As discussed above, therefore, it is possible to control the input temperature of the fluids entering the electrolyser 10, and in particular, the electrolyser cell stack 12 and thus the electrolyser cells 11 therein. Referring now back to Figure 1, and Figure 3, the concepts behind the present invention will now be further discussed. As mentioned above, the off-gases will usually be at a similar temperature to the operational temperature of the electrolyser cell 11. However, the specific delta from the input temperature will depend upon the amount of electrical power supplied to the electrolyser, and the internal resistance of the cells. With the present invention, the power is supplied to the stack with a constant current. The stack is thus operated in galvanostatic conditions. The electrical resistance of the stack thus controls the voltage applied across the stack, and there is thus a variable power draw from the stack as the resistance changes. Operational efficiency can be best improved by reducing the amount of external heat supplied to the system via the fluid temperature control system. Where that external heat is provided for free - for example as a waste product of another industrial process, then that external heat can be usefully used without cost - i.e. it provides financial efficiencies. However, if that external heat has an associated cost, then operational efficiencies would be better improved instead. The present invention achieves this by using both galvanostatic conditions within the stack, i.e. a constant electrical current (constant amps), and by adopting thermoneutral voltages across the stack to avoid heat wastage in the stack, as at a thermoneutral condition the electrolyser is in an adiabatic state, i.e. it is balanced energetically which effectively means no heat is consumed or released. For example, Energy in and out are defined as: Energy input at thermoneutral = Pstack = Vstack ’ I / mol\ I ■ cell count Energy out at thermoneutral = AHfH2 ■ rhH2,prod I--- I — AHfH2 —----- where: mH2,Prod is the produced molar flow of H2 from the stack. n is the number of electrons transferred per unit mole of reactant (in this case n=2) F is Faraday’s constant. △Hf,H2 is the enthalpy of hydrogen formation (LHV) from steam (not water). It is a weak function of temperature. It follows that the thermoneutral voltage (Vtn) of the stack is: _ VTN, stack _ AHfH2 TN cell count nF This is the ideal, adiabatic case which assumes no heat losses or leaks. In reality, neither are true and the real-world energy balance is instead: Ileaktot I' cell count Qheat release [W] ^stack ' I + AHf H2 —I AHf H2 — Qheat loss [W] where: Ileak tot is the total leak with the stack including both physical and electronic leak. Both mechanisms will lead to release of chemically stored energy. Qheat loss is the non-adiabatic losses to the surroundings (conduction via metalwork, insulation etc.). The present inventors realised that Ileaktot and Qheat loss are not controlled, and are difficult to measure. Furthermore, they will change over time as the stack degrades and it is therefore very difficult to predict what the real thermoneutral voltage will need to be. It is for these reasons that it is difficult to solve equation above for Vstack such that Qheat release-0. Furthermore, it is not practical to continuously use a both a constant voltage and a constant current as to do so creates other inefficiencies, again as the electrical resistance of the stack varies over time - due to fluctuations in the temperature of the stack and due to degradation of the stack over time. In the prior art, it is known to use either constant voltages or constant currents on a stack, and then to control of the current or the voltage, respectively, to maintain the stack at a substantially thermoneutral condition. This then allows the stack to avoid overcooling or overheating, as when under voltage (for a galvanostatic stack - i.e. constant current), the stack shows endothermic characteristics, and it thus cools the fluid (and likewise the operational temperature of the stack), whereby the fluid output temperature is lower than the fluid input temperature, whereas when over voltage (for a galvanostatic stack - i.e. constant current), the stack shows exothermic characteristics, and it thus heats the fluid (and likewise the operational temperature of the stack), whereby the fluid output temperature is higher than the fluid input temperature. The present invention still uses a galvanostatic condition for the stack, but fluctuates as necessary between thermoneutral conditions, over voltage conditions and under voltage conditions, in response to input fluid temperature control that aims to maintain a fluid input temperature equal to a fluid output temperature. This is done since the present inventors realised that it is relatively straightforward to measure temperature at inlet and outlet of the stack and thus to instead control the thermoneutral voltage automatically to a voltage that satisfies the equation above without knowing what Qheatloss and Ileak.tot really are. At this condition Qheatloss=0 and the fluid input temperature (Tstkln) = the fluid output temperature (TstkOut). In other words, the present invention seeks to minimize the temperature delta between the fluid output temperature from the stack and the fluid input temperature for the stack, aiming for a zero delta. This then allows extended use of a galvanostatic condition for the stack, alongside a thermoneutral voltage condition, even while the stack, or one or more of the cells therein, degrades, albeit recognizing that there will come a point at which the degradation is too great and an alarm or shut-down of the stack becomes necessary. Referring to Figure 3, the method of the present invention operates by checking the fluid output temperature (TstkOut) from a stack and comparing it to the fluid input temperature. Usually this is the actual input temperature (Tstkln) taken from a sensor at the input end of the stack, although it might be the target input temperature (TstklnEst). As shown in Figure 3, in this example, the actual input temperature (Tstkln) is taken to be the same as the target input temperature (TstklnEst) as the fluid temperature control system should in practice be able to match the actual temperature to the target temperature. However, this will not always be the case - especially if the source fuel (usually steam or carbon dioxide), or even a sweep gas, has a variable temperature. This detection of the temperatures may be done over / after a set period of time, i.e. periodically - for example every 10 minutes, every 30 minutes, or every hour, or sometime longer or shorter than these periods (in other words there is a dwell period). This is to avoid one-off measurement reactions as might otherwise occur due to incidental peaks or troughs in the data. Indeed, the present invention can run almost continuously - in practice this may involve generating a moving average over a given time period or even over a few hours, to reduce the impact of spurious measurements. The temperature delta (TstkOut-Tstkln) is thus calculated and compared against a temperature delta threshold (TstkDeltaDvt). If the absolute value (i.e. modulus) of the temperature delta is greater than the temperature delta threshold (i.e. |TstkOut-Tstkln| >TstkDeltaDvt), then the present invention adjusts the target input temperature using the following equation, with the aim of maintaining a minimum temperature delta: Tnew — Told + (dT X TA) where: Tnew is the adjusted target input temperature (i.e. it becomes Tstkln), Toid is the current - now old - target input temperature, dT is an adjustment factor that is less than 1 and greater than zero, and TA is the calculated temperature delta (TstkOut-Tstkln). The method then loops back to the next detection cycle (and dwell time). If the absolute value (i.e. modulus) of the temperature delta is instead less than the temperature delta threshold (i.e. |TstkOut-Tstkln| <TstkDeltaDvt), then the present invention instead checks the detected operating voltage against a minimum stack voltage threshold. This is set as a given delta from predetermined optimal voltages for achieving thermoneutral conditions at the given temperature, or more generically at a chosen temperature. Whether the stack is operating below or within this voltage range enables the diagnosis of a stack operating status. This is signified in Figure 3 by VPwRDcStkTN - VPwRDcStkDvt <VPwRDcStkAct. VPwRDcStkTN is the thermoneutral voltage under adiabatic conditions (“voltage-power” “direct-current” for “stack” in “thermal neutral” condition), and is usually around 1.28V per cell for hydrogen from water. VPwRDcStkDvt is the maximum allowable deviation from VPwRDcStkTN (“voltage-power” “direct-current” for “stack” “deviation”). This can be different for the increases in voltage versus the drops in voltage (+ and -). VPwRDcStkAct is the actual measured voltage (“voltage-power” “direct-current” for “stack” “actual”). In other words, the actual voltage (VPwRDcStkAct) must stay above an ideal value (VPwRDcStkTN) but a certain amount of deviation (VPwRDcStkDvt) is permitted. In one example, the allowable deviation is up to 30mV. This threshold serves as protection against runaways (e.g. progressive leak developments) and other issues (e.g. faulty signals or cells, or faulty fluid flow passageways etc.). If the determined stack voltage threshold drops below a minimum threshold (‘No’ as shown in Figure 3) this is indicative of a potential runaway leak and the system can be made to issue a warning or an alarm, or it can even start to shut down the stack - for example by reducing or turning off the input current. Otherwise the loop returns to the next reading. Figure 4 next shows a control device 400 for controlling an electrolyser cell stack of an electrolyser in an electrolyser system. The control device 400 comprises an input device 402 for receiving input from sensors 404 so as to determine: a stack operating voltage across the electrolyser cell stack; an inlet temperature at the fluid inlet; and an outlet temperature at a fluid outlet. The control device thus comprises a voltage monitoring system 406 for determining a stack operating voltage across the electrolyser cell stack, an inlet temperature monitoring and / or control system 408 for determining an inlet temperature at the fluid inlet and an outlet temperature monitoring and / or control system 410 for determining an outlet temperature at at least one fluid outlet of the electrolyser cell stack. These systems may utilize sensors and data transmission devices or wiring. The control device receives sensor data relating to each of these measurements. A suitably programmed processor 412 and associated memory 414 is provided for processing such inputs. The control device’s inlet temperature monitoring and / or control system 408 may comprise an output device for controlling the temperature of a fluid entering the electrolyser cell stack at a fluid inlet. A current control system 416 is also provided for controlling a current supply to the electrolyser cell stack. As mentioned above, during normal operation this is adapted to provide a constant current to the electrolyser cell stack, but the current may be reduced or turned off to automatically shut down the stack. In such a way, the controller 400 is adapted to control an electrolyser cell stack of an electrolyser in an electrolyser system in accordance with the method described above with reference to Figure 3. Figures 2 and 3 show the method operating on a single electrolyser cell stack, but it could be one of a plurality of stacks in the system, and each stack may independently operate this method of control, each with temperature sensors at the inputs and the outputs thereof. As discussed previously, the present invention has particular application to electrolysers in the intermediate and high temperature electrolyser cell sectors. In some embodiments the at least one electrolyser cell in the stack is a solid oxide electrolyser cell, i.e. the electrochemically active region 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 may be referred to as an intermediate-temperature solid oxide electrolyser cell, or IT-SOEC. The present invention also preferably operates on an electrolyser that is converting steam into hydrogen and oxygen. 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. Furthermore, these temperatures produce low resistance in the cells. 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. Again as previously discussed, 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). Due to the temperature of a SOEC (usually in excess of 400 degrees C), any liquid water passing through the electrolyser cell will be vaporised into high temperature steam, although typically the fluid temperature control system for controlling the temperature of the fluids for entering the electrolyser cell stack will ensure that any liquid water is already converted into superheated steam prior to it entering the stack. In some embodiments the electrolyser cell system instead comprises a high temperature electrolyser cell with an operational stack temperature between 750 degrees C and 1100 degrees C As mentioned briefly above, instead of steam and water, the fuel may be carbon dioxide, for creating carbon monoxide and oxygen instead of hydrogen and oxygen. FURTHER STATEMENTS OF INVENTION Clause 1. A method of controlling an electrolyser cell stack of an electrolyser in an electrolyser system; the electrolyser cell stack comprising a fluid inlet and one or more fluid outlets; and the electrolyser system comprising: a fluid temperature control system for controlling temperature of a fluid entering the electrolyser cell stack at the fluid inlet; a current control system for controlling a current supply to the electrolyser cell stack; a voltage monitoring system for determining a stack operating voltage across the electrolyser cell stack; an inlet temperature monitoring and / or control system for determining an inlet temperature at the fluid inlet; and an outlet temperature monitoring and / or control system for determining an outlet temperature at at least one of the one or more fluid outlets; wherein the method comprises: controlling the current supply to the electrolyser cell stack to a fixed input current; calculating a temperature delta by subtracting a determined inlet temperature from a determined outlet temperature; and i) if the absolute value of the temperature delta is greater than a threshold value, the method comprises adjusting the inlet temperature using the following equation: Tnew — Told + (dT X TA) where: Tnew is the adjusted target input temperature, Toid is the current (target) input temperature, dT is an adjustment factor that is less than one and greater than zero and TA is the calculated temperature delta; ii) if the absolute value of the temperature delta is lower than a threshold value, the method comprises: determining the operating voltage across the electrolyser cell stack; and determining a stack operation status dependent upon whether the determined operating voltage is below a voltage threshold. Clause 2. The method of clause 1, wherein the electrolyser comprises more than one stack. Clause 3. The method of clause 2, wherein the method is applied to each stack separately. Clause 4. The method any one of the preceding clauses, wherein determining the operating voltage across the electrolyser cell stack with a voltage sensor, and determining the output temperature at the fluid outlets, occurs periodically at predetermined time intervals. Clause 5. The method of any one of the preceding clauses, wherein the temperature delta threshold is set at a value in the range of between 1 and 5 degrees C, inclusive. Clause 6. The method of any one of the preceding clauses, wherein the temperature delta comprises an offset. Clause 7. The method of any one of the preceding clauses, wherein the method also monitors the output fluids to inspect target gas output volumes or percentages from the stack. Clause 8. The method of any one of the preceding clauses, wherein the electrolyser cell stack is an intermediate or high temperature electrolyser cell stack. Clause 9. The method of any one of the preceding clauses, wherein the electrolyser cell stack is a solid oxide electrolyser cell stack. Clause 10. The method of any one of the preceding clauses, wherein the electrolyser is a steam electrolyser. ClauseH. The method of any one of the preceding clauses, wherein if the determined stack operation status is a potential runaway, the method comprises either or both issuing a warning or shutting down the stack. Clause 12. The method of any one of the preceding clauses, wherein the voltage threshold is related to an optimal voltage for achieving thermoneutral conditions at a given temperature. Clause 13. The method of clause 12, wherein the voltage threshold is the optimal voltage minus a predefined delta. Clause 14. The method of any one of the preceding clauses, further comprising setting a maximum inlet temperature threshold, and switching the operating mode from galvanostatic to potentiostatic when the inlet temperature reaches said maximum inlet temperature threshold. Clause 15. A control device for controlling an electrolyser cell stack of an electrolyser in an electrolyser system, the control device comprising: a fluid temperature control system for controlling temperature of a fluid entering the electrolyser cell stack at a fluid inlet of the electrolyser cell stack; a current control system for controlling a current supply to the electrolyser cell stack; a voltage monitoring system for determining a stack operating voltage across the electrolyser cell stack; an inlet temperature monitoring and / or control system for determining an inlet temperature at the fluid inlet; and an outlet temperature monitoring and / or control system for determining an outlet temperature at at least one fluid outlet of the electrolyser cell stack; wherein the control device is adapted to: control the current supply to the electrolyser cell stack to a fixed input current; and calculate a temperature delta by subtracting the determined inlet temperature from the determined outlet temperature; and wherein: i) the control device is further adapted to adjust the inlet temperature if the absolute value of the temperature delta is greater than a threshold value using the following equation: Tnew — Told + (dT X TA) where: Tnew is the adjusted target input temperature, Toid is the current (target) input temperature, dT is an adjustment factor that is less than one and greater than zero and TA is the calculated temperature delta; and ii) the control device is further adapted to determine the operating voltage across the electrolyser cell stack and determine a stack operation status dependent upon whether the determined operating voltage is below a voltage threshold if the absolute value of the temperature delta is lower than a threshold value. Clause 16. The control device of clause 15, further configured to carry out the method of any one of clauses 1 to 14. The present invention has been described above purely by way of example. Modifications in detail may be made to the invention within the scope of the claims as appended hereto.

Claims

1. A method of controlling an electrolyser cell stack of an electrolyser in an electrolyser system;the electrolyser cell stack comprising a fluid inlet and one or more fluid outlets; andthe electrolyser system comprising:a fluid temperature control system for controlling temperature of a fluid entering the electrolyser cell stack at the fluid inlet;a current control system for controlling a current supply to the electrolyser cell stack;a voltage monitoring system for determining a stack operating voltage across the electrolyser cell stack;an inlet temperature monitoring and / or control system for determining an inlet temperature at the fluid inlet; andan outlet temperature monitoring and / or control system for determining an outlet temperature at at least one of the one or more fluid outlets;wherein the method comprises;setting a target input temperature for at least one fluid inlet;setting a maximum inlet temperature threshold;controlling a fluid temperature for the fluid for entering the electrolyser cell stack at that at least one fluid inlet to target that target input temperature;supplying an input current to the electrolyser cell stack as a constant current to operate the electrolyser cell stack at a fixed current;detecting or determining an operating voltage across the electrolyser cell stack with the voltage monitoring system and;switching the operating mode from a fixed current mode to a fixed voltage mode when the inlet temperature reaches the maximum inlet temperature threshold.

2. The method of claim 1, wherein the electrolyser comprises more than one stack.

3. The method of claim 2, wherein the method is applied to each stack separately.

4. The method of any preceding claim, wherein the method further comprisesmaintaining the fixed current at the operating voltage by adjusting the input temperature for the fluid inlet until the maximum inlet temperature threshold is reached.

5. The method of any preceding claim, wherein the stack is operated in a thermoneutral state6. The method of any preceding claim, wherein the method further comprises determining a stack operation status dependent upon whether the operating voltage is below a voltage threshold.

7. The method of claim 6, wherein the voltage threshold is related to an optimal voltage for achieving thermoneutral conditions at a given temperature.

8. The method of claim 7, wherein the voltage threshold is the optimal voltage minus a predefined delta.

9. The method of any one of claim 6 to 8, wherein if the determined stack operation status is a potential runaway, the method comprises either or both issuing a warning or shutting down the stack.

10. The method of any one of the preceding claims, wherein the electrolyser cell stack is an intermediate or high temperature electrolyser cell stack.

11. The method of any one of the preceding claims, wherein the electrolyser cell stack is a solid oxide electrolyser cell stack.

12. The method of any one of the preceding claims, wherein the electrolyser is a steam electrolyser.

13. A control device for controlling an electrolyser cell stack of an electrolyser in an electrolyser system, the control device comprising:a fluid temperature control system for controlling temperature of a fluid entering the electrolyser cell stack at a fluid inlet of the electrolyser cell stack;a current control system for controlling a current supply to the electrolyser cell stack;a voltage monitoring system for determining a stack operating voltage across the electrolyser cell stack;an inlet temperature monitoring and / or control system for determining an inlet temperature at the fluid inlet; andan outlet temperature monitoring and / or control system for determining an outlet temperature at at least one fluid outlet of the electrolyser cell stack;5 wherein the control device is configured to carry out the method of any one ofclaims 1 to 12.A

Images