Method of control for use with an electrolyser cell stack

Abstract

A method for establishing a voltage set point for one or more electrolysers is disclosed, the method comprising: (1) receiving a temperature delta between an inlet fluid temperature and an outlet fluid temperature; (2) if the received temperature delta is beyond a first threshold: perturbing an operating voltage of the stack by a first voltage delta to a first voltage level; and (3) observing any change in the temperature delta between the inlet fluid temperature and the outlet fluid temperature; (4) wherein if the observed temperature delta is beyond a second threshold, perturbing the voltage from the first voltage by a further voltage delta, wherein the value and direction of the further voltage delta depends on change to the temperature delta.

Description

[0001] Method of control for use with an electrolyser cell stack

[0002] The present invention relates to a method of controlling an electrolyser cell stack, and in particular a method for use in controlling the operational state of an electrolyser cell stack of an electrolyser for extending the life of the stack.

[0003] 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, also known in the art as a fuel, 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. Herein, the water or carbon dioxide may be referred to as a “fuel”, as known in the art. Other “fuels” can also be electrolysed to produce off-gases formed from constituent parts of the fuel, and the term “fuel” in this context does not necessarily require it to be combustible.

[0004] The collection or use of oxygen as a first product of the electrolysis can be 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 hydrogen as a second product of the electrolysis can also be important as it is a highly calorific fuel for combustion, or as a fuel for electrochemical reaction within a fuel cell, 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.

[0005] 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.

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

[0007] 15446194-1Another 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.

[0008] 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.

[0009] Other prior art teachings, such as patent publications JP P2020-128576A, to Toshiba Energy System & Solution Corp, and WO 2020 / 201485 A1, to Dynelectro APS, consider the relationship between temperature control and voltage control in an electrolyser cell for increasing service life of an electrolyser cell. Patent publication WO 2018 / 033948 A1, to Toshiba KK, further considers current control, adopting either a constant current ora constant voltage alongside temperature control.

[0010] 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.

[0011] Various aspects of the present invention make slight changes (termed perturbations) in the stack operating voltage (and therefore to the voltage across each cell within the stack) to try to match the fluid outlet temperature of the stack to the fluid inlet temperature, i.e. to achieve

[0012] 15446194-1a thermoneutral voltage condition, or to aim for a specific offset between them - a target endothermic or exothermic condition. Based on the temperature response to the perturbation (an observation), a further perturbation (or series of perturbations) can be made so that the system converges to the desired state (e.g. thermoneutral operation). This method allows for fast and efficient control of an electrolyser without relying on detailed models or a priori information of the electrolyser. This is particularly advantageous as the performance of the electrolyser changes over its lifetime, for example due to degradation.

[0013] The present inventors have noted that within the photo voltaic (PV Solar) sector, power outputs can be maximised using perturb and observe algorithms. See, for example, US9595830, WO2011049732, CN102354110 and CN101694943. Within an electrolyser, however, a reduction in the power usage for a given electrolyser output volume is desired.

[0014] According to an aspect of the present invention, there is provided a method for establishing a voltage set point for one or more electrolysers, the method comprising:

[0015] (1) receiving a temperature delta between an inlet fluid temperature and an outlet fluid temperature;

[0016] (2) if the received temperature delta is beyond a first threshold:

[0017] perturbing an operating voltage of the stack by a first voltage delta to a first voltage level; and (3) observing any change in the temperature delta between the inlet fluid temperature and the outlet fluid temperature;

[0018] (4) wherein if the observed temperature delta is beyond a second threshold, perturbing the voltage from the first voltage by a further voltage delta, wherein the value and direction of the further voltage delta depends on a change to the temperature delta.

[0019] With this method an optimal voltage set point can be continuously established for one or more electrolysers, e.g. a stack thereof. This can be an optimal or improved operating voltage for the stack at its current state of deterioration, which can vary over time.

[0020] In some embodiments, if the received temperature delta is within the first threshold, the method comprises maintaining the operating voltage as a new operating voltage for the electrolyser, and returning to (1).

[0021] In some embodiments, if the observed temperature delta is within the second threshold, the method comprises maintaining the operating voltage as a new operating voltage for the electrolyser, and returning to (3).

[0022] 15446194-1In some embodiments, the value and direction of the further voltage delta that depends on the value and direction of the change to the temperature delta, depends upon whether the change in the temperature delta suggests that the perturbance of the operating voltage is in the right direction and / or of an appropriate magnitude. For example, in some embodiments, if the observed temperature delta is beyond the second threshold:

[0023] (4)(a) if the observed temperature delta has the same sign as before, but a lower magnitude, the method comprises introducing a further voltage delta to the operating voltage, the further voltage delta being the same magnitude as a last-introduced voltage delta, and returning to (3); and / or

[0024] (4)(b) if the observed temperature delta has an opposing sign to before, the method comprises introducing a further voltage delta to the operating voltage, the further voltage delta being in the opposite direction to the last-introduced voltage delta and having a smaller absolute magnitude than a last-introduced voltage delta, and returning to (3).

[0025] In such embodiments, option (4)(a) may suggest that the perturbance was in the right direction and of a suitable magnitude to move closer to the target status, whereby a further similar perturbance is appropriate to target the target status. Option (4)(b), however, instead suggests that the perturbance had too great a magnitude, and thus a correction in the other direction, albeit of a smaller magnitude, is needed.

[0026] In some embodiments, in (4)(b) the absolute value of the further voltage delta is less than 3 / 4 of the absolute value of the first voltage delta, or optionally less than 2 / 3, or less than 1 , of the absolute value of the first voltage delta.

[0027] In some embodiments, the method comprises a dwell period or delay following a voltage perturbation and / or determination that the temperature delta is within the first or second threshold. In some embodiments, the dwell period is less than 2 hours.

[0028] In some embodiments, the method is carried out as a continuous iterative process.

[0029] In some embodiments, the first threshold is the same as the second threshold.

[0030] In some embodiments, the first threshold is larger than the second threshold.

[0031] 15446194-1The one or more electrolysers, or the stack thereof, can be provided with one or more inlet fluids at an inlet fluid temperature, and exhaust one or more outlet fluids at one or more an outlet at a fluid temperature. The electrolysers (cells) or the stacks thereof are electrically powered to electrolyse at least a portion of the one or more inlet fluids.

[0032] The method provides an iterative process for establishing and maintaining a sufficiently thermoneutral voltage condition even as the stack or cells deteriorate over time, an accuracy thereof being determined by the first threshold. This may have, by virtue of the first threshold, a variance from a perfect thermoneutral voltage not exceeding that first threshold, whereby it will be no more offset form a thermoneutral condition than a predetermined endothermic or exothermic condition that meets that first threshold.

[0033] In some embodiments, the method is used within the operational control of an electrolyser cell stack.

[0034] In some embodiments, the electrolyser cell stack comprising one or more electrolyser cells operating at an operating voltage. The stack or cells thereof have an outlet fluid at the outlet fluid temperature and an inlet fluid at the inlet fluid temperature.

[0035] In some embodiments, the method comprises:

[0036] the step (1) of receiving the temperature delta, and the step (2)(a) of determining if the temperature delta is within the first threshold, and if so, maintaining the operating voltage, and returning to step (1).

[0037] If instead the temperature delta is beyond the first threshold, the perturbing of the operating voltage of step (2) may involve introducing a first voltage delta to the operating voltage to provide a revised operating voltage for the stack and the observing any change of step (3) may involve, after a period of time, receiving a revised temperature delta between the outlet temperature and the inlet temperature.

[0038] In some embodiments, step (4) may comprise determining whether the revised temperature delta is within the first threshold, and if so, the method comprises maintaining the revised operating voltage as a new operating temperature for the stack, and returning to step (1). However, if instead the temperature delta is still beyond the first threshold, but by less than before, the method step (4) may comprise introducing a further voltage delta to the operating

[0039] 15446194-1voltage, the further voltage delta being the same magnitude as a last-introduced voltage delta, and then returning to step (3).

[0040] Step (4) may also comprise determining whether the temperature delta is instead still beyond the first threshold but by more than before, and if so, introducing a further voltage delta to the operating voltage, the further voltage delta being in the opposite direction to the last-introduced voltage delta and having a smaller absolute magnitude than a last-introduced voltage delta, and then returning to step (3).

[0041] This process can be a continuous iterative process. Through this iterative process, a sufficiently thermoneutral voltage condition can be achieved and maintained, even as the stack or the cells therein deteriorate, an accuracy thereof being determined by the first threshold. This may have, by virtue of the first threshold, a variance from a perfect thermoneutral voltage not exceeding that first threshold, whereby it will be no more offset form a thermoneutral condition than a predetermined endothermic or exothermic condition that meets that first threshold.

[0042] The above aspects of the present invention are beneficial since electrolysers have a theoretical optimal thermoneutral voltage, but this varies on a stack-by-stack basis, and over the life cycle of the stack due to the effects of degradation. The thermoneutral voltage also has a weak temperature dependence, which introduces feedback into the control. These methods, in particular, allow substantially continuous ‘fine tuning’ of an operational stack voltage for a stack with no a priori knowledge about the condition of the stack.

[0043] In each method, an absolute value of the or each temperature delta is generally used to compare against the threshold.

[0044] In some embodiments, the temperature delta is an average of the difference between the outlet temperature and the inlet temperature over the course of multiple temperature readings.

[0045] In some embodiments, the temperature delta is the difference between an average of the outlet temperature and an average of the inlet temperature taken from multiple stacks.

[0046] In some embodiments, the temperature delta includes an offset.

[0047] 15446194-1In some embodiments, the sign of the first voltage delta is based on the temperature delta, preferably based on the sign of the temperature delta.

[0048] In some embodiments, the first voltage delta is predetermined from previous calibration of the electrolyser cell stack and / or is selected based on the initially obtained temperature delta.

[0049] In some embodiments, when the process returns to step (1), there is a dwell period or delay in the temperature delta being received again in step (1). This is to wait for a steady state to return after each perturbation. However because the perturbations (i.e. operation voltage changes or steps may be very small (e.g. less than 1% of an absolute value of the voltage being changed), the dynamic transient time back to a steady state (or to a substantially steady state) can also be very small (e.g. measured in seconds or minutes).

[0050] In some embodiments, the electrolyser is a sub-stack of one or more electrolyser cells within a stack, or a plurality of sub-stacks of one or more electrolyser cells within one or more stacks. Usually the cells are arranged in series.

[0051] In some embodiments, the electrolyser is an intermediate or high temperature electrolyser cell stack.

[0052] In some embodiments, the electrolyser is a solid oxide electrolyser cell stack.

[0053] In some embodiments, the electrolyser is a steam electrolyser.

[0054] In some embodiments, the method further comprises control circuitry for determining if a determined stack operation status is a potential runaway. In some embodiments, in the event the determined stack operation status is a potential runaway, the method further comprises either or both issuing a warning or shutting down the stack. For example, it may turn off the voltage or current / power to the stack. The control circuitry may determine a potential runaway if the temperature delta keeps increasing irrespective of the iterative operation of the method defined above.

[0055] In some embodiments, the method is carried out on an electrolyser system comprising a plurality of electrolyser cell stacks arranged in parallel, each electrolyser cell stack comprising a plurality of electrolyser cells arranged in series.

[0056] 15446194-1In some embodiments, the method is performed independently for each electrolyser cell stack, or for sub-stacks within each cell stack. In some embodiments, the method is performed independently for a plurality of subsets of electrolyser cell stacks in the electrolyser system - for example on pairs of stacks.

[0057] In some embodiments, the temperature deltas are determined based on averages obtained from two or more of the plurality of electrolyser cell stacks.

[0058] In some embodiments, the method is carried out by an analogue controller, for example comprising comparators, transistors and other electrical components.

[0059] In some embodiments, the method is carried out by a digital controller, for example additionally comprising a processor, associated memory and other electrical components.

[0060] According to a further aspect, there may be provided a method used within the operational control of an electrolyser cell stack, the electrolyser cell stack comprising one or more electrolyser cells, the stack or cells operating at an operating voltage, with an outlet fluid at an outlet temperature and an inlet fluid at an inlet temperature, the method comprising:

[0061] (1) receiving a temperature delta (i.e. difference) between the outlet temperature and the inlet temperature;

[0062] (2)(a) if the temperature delta is within a first threshold, maintaining the operating voltage, and returning to step (1);

[0063] (2b) if instead the temperature delta is beyond a first threshold, introducing a first voltage delta to the operating voltage to provide a revised operating voltage for the stack; and then

[0064] (3) after a period of time receiving a revised temperature delta between the outlet temperature and the inlet temperature;

[0065] (4)(a) if the revised temperature delta is within the first threshold, maintaining the revised operating voltage as a new operating temperature for the stack, and returning to (1);

[0066] (4)(b) if the temperature delta is instead still beyond the first threshold but by less than before, introducing a further voltage delta to the operating voltage, the further voltage delta being the same magnitude as a last-introduced voltage delta, and returning to (3);

[0067] (4)(c) if the temperature delta is instead still beyond the first threshold but by more than before, introducing a further voltage delta to the operating voltage, the further voltage delta being in the opposite direction to the last-introduced voltage delta and having a smaller absolute magnitude than a last-introduced voltage delta, and returning to (3).

[0068] 15446194-1This process can be a continuous iterative process. Through this iterative process, a sufficiently thermoneutral voltage condition can be achieved and maintained, even as the stack or the cells therein deteriorate, an accuracy thereof being determined by the first threshold. This may have, by virtue of the first threshold, a variance from a perfect thermoneutral voltage not exceeding that first threshold, whereby it will be no more offset form a thermoneutral condition than a predetermined endothermic or exothermic condition that meets that first threshold.

[0069] This aspect may be combined with any of the preceding or following aspects.

[0070] The present invention also provides a control device configured to carry out the method described above. For example, there is provided a control device for controlling an electrolyser cell stack of an electrolyser in an electrolyser system, the control device comprising:

[0071] a voltage control system for controlling an operating voltage across the electrolyser cell stack;

[0072] an inlet temperature monitoring system for determining an inlet temperature at the fluid inlet; and

[0073] an outlet temperature monitoring system for determining an outlet temperature at at least one fluid outlet of the electrolyser cell stack;

[0074] wherein the control device is adapted to perform the method as defined above.

[0075] The present invention also provides an electrolyser system operating using such a control device to perform such a method, and as such the method can be implemented with or within any electrolyser system described herein.

[0076] The present invention also provides a computer program comprising instructions which, when the program is executed by a processor, cause the processor to carry out the steps of the method described above.

[0077] The present invention also provides a non-transitory computer-readable medium with instructions stored thereon, that when executed by a processor, perform the steps of the method described above.

[0078] 15446194-1Brief Description of Drawings

[0079] The present invention will now be described in further detail, purely by way of example, with reference to the accompanying drawings in which:

[0080] Figure 1 schematically shows a typical electrolyser cell, multiples of which may be stacked in a stack within an electrolyser;

[0081] Figure 2 shows a schematic of part of an electrolyser system showing fluid flow paths for anode and cathode sides of a stack within the electrolyser system, with heat exchangers and heaters for inlet fluid temperature control, such as may be used alongside the present invention.

[0082] Figure 3 is a flow diagram showing the general steps of an embodiment of the present invention.

[0083] Figures 4 and 5 show the impact of perturbations in the operating voltage to the temperature of the stack; and

[0084] Figure 6 shows a schematic of a control device for controlling an electrolyser cell stack of an electrolyser in an electrolyser system, such as may be used with the present invention.

[0085] 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, an example of which will be discussed below with reference to Figure 2.

[0086] Referring first to Figure 1, however, the electrolyser cell 11 comprises an air electrode 33 (also called an oxygen electrode), a fuel electrode 34 and an electrolyte 35. Such a structure for an electrolyser cell 11 is well known in the art. However, we will briefly discuss its function below. In this example, the electrolysis of water will be discussed, although other fluids, including gases such as carbon dioxide or nitrous oxide, can similarly be electrolysed to split them into component parts, such as carbon dioxide into oxygen and carbon monoxide.

[0087] 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 fuel electrode 34 via inlet 41 and hot air 42 is passed over the air electrode 33 via inlet 40. To power the electrolyser cell, an electric current is applied across the electrolyser cell 11 via

[0088] 15446194-1electric terminals / connections 36, 37 at the air and fuel 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.

[0089] 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 fuel electrode 34 to the air electrode 33, and some of the steam braking down into hydrogen on the fuel electrode side of the electrolyser cell 11 and oxygen is produced at the air electrode side.

[0090] 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 air electrode side of the electrolyser cell 11. That output is generally oxygen enriched air (with the oxygen enriching the hot gas flow). It is also possible, however, to use no sweep flow and to use the extracted oxygen to self-vent from the system.

[0091] The hydrogen can instead be extracted and vented out of another off-gas outlet 39 on the fuel electrode 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 fuel electrode side. Thus, the steam exiting the fuel electrode side is hydrogen enriched, and the air or gas exiting the air electrode side is oxygen enriched.

[0092] 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 inlet temperature will depend upon the amount of electrical power supplied to the electrolyser, and the internal resistance of the cells.

[0093] 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.

[0094] 15446194-1As 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.

[0095] 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.

[0096] The electrolyser system 20 in this example has five electrolyser cells. However, generally a stack will have tens or even hundreds of cells in series, and multiple stacks 12 may be provided for an electrolyser system, each electrically connected in series or in parallel. For example, a single stack array module may comprise 24 stacks, all arranged in parallel, each stack comprising, in some embodiments, up to 400 individual cells, the individual cells being arranged in series within each stack.

[0097] It is to be appreciated that a single electrolyser plant may have dozens of stack array modules arranged in series or in parallel. It is also to be appreciated that the size of the stacks and the arrangement of the stacks and modules - i.e. whether arranged in series or in parallel, or in combinations thereof, may be configured as necessary for the available electrical voltage or power range in the power supply.

[0098] Operationally, a stack will usually want to avoid large pressure differentials across the cells, so a threshold pressure differential between first and second fluid volumes defined by the air electrode and the fuel electrode, and their connected fluid pathways, can be set appropriately, or the first fluid volume’s pressure can be controlled to balance the pressures across the cells. In one example, the pressure differential can be controlled (and is kept as small as possible) by use of a common regulator system on both sides. However, in operation it is sometimes advantageous to control the pressure differential to be positive on second fluid volume side to ensure oxygen would tend to leak to the steam side, rather than the other way around. This is better for the stack life as it is likely to suffer from less degradation.

[0099] 15446194-1In steady state operation, the first and second fluid volume off-gases will usually be at a similar temperature to the operational temperature of the electrolyser cell. However, a specific delta from the input temperature can be controlled by the heat exchanges or by varying the amount of electrical power supplied to the electrolyser.

[0100] Operation of the stack of Figure 2 will now be discussed in further detail.

[0101] The stack 12 comprises an air electrode side stack outlet 46, wherein an air electrode side exhaust product (off gas - 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.

[0102] 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.

[0103] 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.

[0104] 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 air electrode 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.

[0105] It is also shown in Figure 2 that an air electrode outlet flow path 24 from the air electrode 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 air electrode inlet 14. Therefore, the air electrode outlet flow path 24 and the sweep gas supply flow path 18 exchange heat in the second heat exchanger 22.

[0106] 15446194-1During 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 22. This is beneficial to provide sweep gas at the air electrode inlet 14 of high temperature to increase efficiency of the electrolysis reaction in the stack 12.

[0107] 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 22 and the air electrode inlet 14. The first heater 36 is used for heating the sweep gas from 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.

[0108] The first heater 36 is also referred to as a trim heater as it typically only provides relatively small amounts of heat for fine-tuning (trimming) the temperature at the air electrode inlet 14 to ensure a consistent and efficient electrolyser reaction in the stack 12. In an optimized system, the first heater provides no heat to the system as it requires additional power to provide that heat, which is an inefficiency on the system.

[0109] A bypass flow path 30 may also be provided. This 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 36. Therefore, the second end is downstream of the second heat exchanger 22.

[0110] The bypass flow path 30 bypasses the first and second heat exchanger 62, 22. 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 air electrode outlet flow path 24 is of a level that is not (yet) required or appropriate for the sweep gas supply flow path 18. Such situations can include start-up where those flow paths might be cold, or during cool down, where transfer of the outlet and external 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.

[0111] 15446194-1The bypass flow path 30 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.

[0112] For example, one or more sensor 66, 72, 74, 76 may be provided on one or more of these flow lines to allow detection and subsequent control of the flows and temperatures, the controller being connected to these sensors and potentially to the heaters and further flow valves or bypasses.

[0113] Whilst the above system and flow processes are presented as shown in Figure 2, 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 modifying the functionality 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 air electrode outlet flow path 24, which can also be controlled to operate when required. As with the first heater, however, it is preferred that this would remain unpowered during normal use to avoid an electrical inefficiency within the system.

[0114] Referring next to a fuel electrode side of the system, a water supply 84 is provided and is connected via a water or steam supply flow path 56 to a fuel electrode side stack inlet 44 of the electrolysis stack 12. The fuel electrode side stack inlet 44 connects to the steam inlet 41 on the fuel electrode 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.

[0115] The electrolysis stack 12 also comprises a fuel electrode outlet 48 that is the outlet from the fuel electrode of the electrolysis stack 12, and is connected to the steam outlet 39 of each electrolyser cell 11. The fuel electrode outlet 48 is connected to a second exhaust 86 by a fuel electrode 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.

[0116] A third heat exchanger 50 is provided in the water supply flow path 56 between the water supply 84 and the fuel electrode inlet 44. The fuel electrode outlet flow path 54 is also

[0117] 15446194-1connected across the third heat exchanger 50 between the fuel electrode outlet 48 and the second exhaust 86 of the fuel electrode outlet flow path 54. Therefore, heat is exchanged between the fuel electrode outlet flow path 54 and the water supply flow path 56.

[0118] The product, e.g. wet hydrogen, produced in the stack 12 and output at the fuel electrode 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 preheated steam, rather than water or low-temperature steam, is supplied for the electrolyser reaction. Therefore, heat energy can be transferred from the fuel electrode 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.

[0119] 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 produced outlet flow is beneficial.

[0120] A second heater 52 is shown to be 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 fuel electrode 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 or if required. Sensors 64, 68, 70 and a control system can again be provided for this purpose.

[0121] 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.

[0122] 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 with the other heaters, it is preferred that this would remain unpowered during normal use to avoid an electrical inefficiency within the system.

[0123] 15446194-1As discussed above, therefore, it is possible to control the inlet temperature of the fluids entering the electrolyser 10, and in particular, the electrolyser cell stack 12 and thus the electrolyser cells 11 therein. It is well known that there is a relationship between the operating temperature of a cell, or a stack of cells, and the quantity of desired output gases, such as the output of Hydrogen. In this regard, there is an optimum temperature range for a given electrolyser, usually based upon the electrolyser’s electrolyte chemistry. Thus, controlling the inlet temperature is necessary or useful to bring the temperature of the system into at least the region that provides the stack’s best efficiency.

[0124] Referring now back to Figure 1, and Figure 3, the concepts behind the present invention will now be discussed.

[0125] As mentioned above, the off-gases will usually be at a similar temperature to the operational temperature of the electrolyser cells 11 within the stacks. However, a specific delta from the inlet temperature will depend upon the amount of electrical power supplied to the electrolyser, which is predominantly dependent upon the operating voltage, and the internal resistance of the cells. At thermoneutral conditions (where inlet and outlet temperatures substantially match), the rate of production (e.g. hydrogen) is proportional to the current supplied. As discussed below, a ‘thermoneutral voltage’ does not vary significantly depending on which temperature the stack is operating. However, the resistance of the cells is strongly temperature dependent. Thus, at a high temperature, the resistance of the cells is low, but the thermoneutral voltage is similar to that at a much lower temperature; this means that a higher current can be supplied when maintaining the thermoneutral voltage.

[0126] Operational efficiency can be best improved by reducing the amount of external heat supplied to the system via the fluid temperature control system, and particularly the heaters. 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 through using a less costly energy source. The present invention can generally achieve this 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.

[0127] 15446194-1For example, Power in and out are defined as:

[0128] Power input at thermoneutral = Pstack=Vstack’ I

[0129] / mol\ I ■ cell count

[0130] Power out at thermoneutral = AHfH2■ rhH2,prod I - J=AHfH2- - -

[0131]

[0132] where:

[0133] mH2 prodis the produced molar flow of H2 from the stack.

[0134] n is the number of electrons transferred per unit mole of reactant (in this case n=2)

[0135] F is Faraday’s constant.

[0136] AHf,H2 is the enthalpy of hydrogen formation (LHV) from steam (not water). It is a weak function of temperature.

[0137] It follows that the thermoneutral voltage (VTN) of the stack is:

[0138] ,T> VTN> stack > AHfH2

[0139] VTM — —

[0140] cell count nF

[0141] 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:

[0142] ' cell count

[0143] Qheat release [W] = - - - Qheatloss [W]

[0144]

[0145]

[0146] where:

[0147] ILEAK.TOT is the total leak with the stack including both physical and electronic leak.

[0148] Both mechanisms will lead to a release of chemically stored energy.

[0149] Qheat loss is the non-adiabatic losses to the surroundings (conduction via metalwork, insulation etc.).

[0150] The present inventors realised that ILEAK.TOT 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 reiease=0.

[0151] Furthermore, it is not practical to use a fixed voltage as the desired thermoneutral point (at a value assumed to be generally optimal, based on theoretical calculations of an ideal case) as that will lead to inefficiencies 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.

[0152] 15446194-1In the prior art, it is known to use either constant voltages or constant currents on a stack, and then to control the other of the current or the voltage, respectively, to try to maintain the stack at a substantially thermoneutral condition. This then allows the stack to avoid overcooling or overheating, as when at an under voltage condition, the stack shows endothermic characteristics, and it thus cools the fluid (and likewise the operational temperature of the stack), whereby the fluid outlet temperature is lower than the fluid inlet temperature, whereas when at an over voltage condition, the stack shows exothermic characteristics, and it thus heats the fluid (and likewise the operational temperature of the stack), whereby the fluid outlet temperature is higher than the fluid inlet temperature.

[0153] The present invention provides a novel approach that uses stack operating voltage control that aims to maintain a fluid outlet temperature relative (e.g. equal) to a fluid inlet temperature. It is relatively straightforward to measure temperature at inlet and outlet of the stack and, thus, it becomes straightforward to determine whether any steps taken to control or to minimize the difference between them is working. For example, at a target thermoneutral condition where QHEATLOSS=0, the fluid inlet temperature (Tstkln) = the fluid outlet temperature (TstkOut).

[0154] In other words, the present invention seeks to minimize the temperature delta between the fluid outlet temperature from the stack and the fluid inlet temperature for the stack, aiming for a substantially zero delta, and this can be achieved, in accordance with the present invention by providing substantially continuous control of the voltage across the stack. This control is an iterative process as defined in the appended claims. This then allows extended use of the stack, as the negative impact of periodic over and under voltage can be minimized due to the continuous maintenance of a substantially thermoneutral voltage condition, even while the stack, or one or more of the cells therein, degrades. Minimizing the time period away from optimum operating conditions also improves lifetime efficiency. It is to be recognized, however, that there will come a point at which the degradation is too great, and the thermoneutral temperature for a given operating condition (i.e. production rate) is too high for safe operation. In such a case, control of the voltage to thermoneutral condition would still be required, albeit with an additional control to ensure that the current level does not make the temperature exceed safe operation. If such control is not feasible or practical, an alarm or shut-down of the stack can then become necessary.

[0155] 15446194-1The method of the present invention operates without any reference to the actual voltage supplied to the stack, or the actual resistance or the actual current applied across the stack. Instead it operates by checking the fluid outlet temperature (TstkOut) from a stack and comparing it to the fluid inlet temperature for that stack. Usually this is the actual inlet temperature (Tstkln) taken from a sensor at the input end of the stack. Typically, the inlet temperature is the inlet temperature for the source fuel (water / steam, carbon dioxide, etc.), as in some embodiments the sweep gas is not continuously supplied. The outlet temperature typically is similarly for the outlet flow which is rich in the source fuel, such as at the wet hydrogen outlet, again as the outflow from the other outlet might not be continuous. However, it is possible for the inlet and the outlet temperatures to be measured on either side of the electrolyser - the anode side or the cathode side - when the flows therethrough are substantially continuous and at a generally steady flow rate.

[0156] The actual inlet temperature (Tstkln) may be taken to be the same as the target inlet 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 initial temperature upon entering the system.

[0157] This detection of the temperatures may be done over / after a set period of time, i.e. periodically - for example every 30 seconds, every minute, every 5 minutes, every 10 minutes, every 30 minutes, or every hour, or sometime intermittently over longer or shorter time periods (in other words there is a dwell period between detections). Indeed, the present invention can run almost continuously - in practice this may involve generating periodic moving averages (of temperatures or temperature deltas) over given time periods to reduce the impact of spurious measurements or temporary spikes in temperature.

[0158] An example of a process leading to the control of the stack operating voltage in accordance with the present invention is shown in the flowchart of Figure 3.

[0159] The following description refers to “steps”. However, this is merely for ease of terminology, and cannot be taken to mean that the different functions are entirely distinct from each other in the way they are embodied in practice. Moreover, where, as in a couple of instances, the same text or similar text is present in two boxes, this does not mean the functions are, in practice, in any way different, or carried out based on different software or coding. Thus, the flowchart is presented as it is for improved clarity.

[0160] 15446194-1The control of such a process may be by way of a digital controller or an analogue controller.

[0161] Figure 3 does not show the steps of obtaining the outlet and inlet temperatures of the stack. These are, however, obtained prior to this process and at various times during it. In that they are only needed in the process when the inlet and outlet temperatures are compared, they may be obtained specifically in time for the relevant steps. Alternatively, they may be obtained based on a clock timing that is entirely independent of where the process has reached. The process then uses the current temperatures or temperature deltas (whether individual or averages or otherwise as may be useful) at the time it is needed, with some such temperature measurements being made or even temperature deltas determined, but never being used.

[0162] Figure 3 shows an example ‘perturb and observe’ method for controlling the voltage of an electrolyser stack to operate at an optimum condition. In the example of Figure 3, this is thermoneutral, defined by the outlet fluid temperature being equal to the inlet temperature.

[0163] In this control method, the electrolyser stack is already running in a (relatively) steady state, at an operating voltage that may be based on a suitable voltage for a previous operation of the same stack, a previous operation of other stacks of the same age, or a previous operation of other stacks of the same or similar configuration, or on a default voltage based on a standard, or even ideal, per-cell operating voltage. This may be, for example, a voltage in the order of 512V for a stack comprising 400 cells in series, or based upon a voltage of 1.28V per cell. It is to be noted though that different voltages per cell or different voltages per stack may be appropriate or necessary dependent upon the design or structure of the cell or the stack, and dependent upon the chemistries used within the electrochemically active regions of the cells or the stack.

[0164] Step 202 is the step of comparing the outlet temperature with the inlet temperature to provide a temperature delta (Temp A), that is a difference between the two temperatures.

[0165] From step 202, the process proceeds to step 204. Step 204 determines whether the absolute value of the temperature delta (|Temp A|) is greater than a first threshold value. The first threshold value is a temperature delta threshold value which, ideally, will be zero or near zero (e.g. 0.5°C).

[0166] 15446194-1If the answer to step 204 is no (i.e. the absolute value of the temperature delta (|Temp A|) is not greater than the first threshold value, whereby the outlet and inlet temperatures are sufficiently balanced), then the process returns to step 202 (i.e. it reverts to comparing the the outlet temperature with the inlet temperature). This return to step 202 may be subject to a time delay or a dwell time, for instance any one of 1 minute, 2, 5 or 10 minutes, or shorter or longer if so desired. Such a delay may be determined based on how stable the stack temperatures tend to be in practice, and thus may vary between electrolyte chemistries, stack sizes or operational temperatures or between one source fluid and another. In one example, the dwell time is below 2 hours. Multiple factors affect the time for transients to die down, such as the size of the step in voltage. Bigger steps take longer to settle. As described herein, the step size is reduced once the right direction has been determined. In such a case, the dwell time could similarly be reduced for each iteration as the voltage converges to thermoneutral. In other words, the dwell time between perturbations is proportional to the magnitude of the voltage perturbation.

[0167] This dwell time could also be determined rather than pre-set by checking that the transient is substantially over by calculating X points of dv / dtfor stack and dT / dtfor inlet and outlet. Once the gradient has reduced below a threshold level, it can be determined that the transient behavior is over. Passing this threshold level may be observed, or predicted from a series of measurements. Knowledge from previous systems could indicate how many measurement “X value” to check to ensure that the transient is substantially over.

[0168] Steps 202 and 204 can be termed an ‘observe’ stage.

[0169] If the answer to 204 is instead yes (i.e. the absolute value of the temperature delta (|Temp A|) is greater than the first threshold value, whereby the outlet and inlet temperatures are insufficiently balanced), then the system realises that the outlet temperatures need modifying, so the process proceeds to step 205 instead of back to step 202.

[0170] In step 205, the direction of the voltage change is determined by determining whether the outlet temperature is greater than the inlet voltage. If the outlet temperature is too high (relative to the inlet temperature), then the operating voltage is reduced (step 206), but, if the outlet temperature is too low (relative to the inlet temperature), then the operating voltage is increased (step 207).

[0171] 15446194-1The magnitude of the first voltage delta is ideally estimated by the system, or a predefined amount, which may be determined based on prior calibration data for the specific form of the stack. However, it could be based simply on an ideal model or on the results of voltage changes during processes performed on other stacks, and / or selected based on the initially obtained temperature delta (e.g. from a look-up table populated, for example, according to operation of this same stack or other similar stacks).

[0172] The first voltage delta will also depend upon the number of cells in the stack, since the cells are usually arranged in series within a stack. As such, the first voltage delta for a stack with a first number of cells would be halved for a stack with half that number of cells. For example, for a 400-cell stack, the first voltage delta might be 1V (0.0025V per cell), but for a 200-cell stack, the first voltage delta might be 0.5V (still 0.0025V per cell). It is to be understood, however, that the first voltage delta may depend on the operating voltage and the maximum voltage that the stack can operate on. The first voltage delta, for example, should be chosen so that it will not drive the operating voltage too close to a known unsafe operating voltage. For example, for a cell that might have a standard expected operating voltage of 1.28V, there may be an assigned maximum operating voltage of 1.4V per cell, so the first voltage delta must keep the operating voltage below that maximum voltage per cell. Indeed, there may be an additional safety factor factored into the determination of the first voltage delta to ensure that the operating voltage can never intentionally arrive at the maximum operating voltage (in this case 1 ,4V per cell).

[0173] Steps 205 and 206 / 207 can be termed a ‘perturb’ stage. This is as the voltage is perturbed by the first voltage delta to a first voltage level.

[0174] The system or the user then, following another dwell period, observes for any change in the temperature delta between the inlet fluid temperature and the outlet fluid temperature at step 208.

[0175] If an observed change in the temperature delta takes the temperature delta beyond a second threshold 210, the voltage is changed again to perturb the voltage from the first voltage by a further voltage delta. In one example, the second threshold is the same as the first threshold. However, the second threshold may be smaller than the first threshold to enable finer control once the algorithm has been ‘triggered’ by the temperature drifting by a larger amount.

[0176] 15446194-1The value and direction of the further voltage delta depends on the value and direction of the change to the temperature delta.

[0177] As with step 206 above, if the outlet temperature is greater than the inlet temperature, (yes at step 212), it is known that a voltage decrease is required. If the sign of the temperature difference has switched (i.e. the outlet temperature was not greater than the inlet temperature before - ‘no’ at step 214), it can be determined that the voltage has passed the thermoneutral value and a smaller perturbation is required for convergence (step 216). As such the new voltage perturbation (A Vn) is smaller than the old voltage perturbation (A Vo). Conversely, if the sign of the temperature difference is the same as before (yes to step 214), the voltage has not passed the thermoneutral level, so the same voltage perturbation is applied at step 215.

[0178] As with step 207 above, if the outlet temperature is lower than the inlet temperature (‘no’ at step 212), a voltage increase is required. The magnitude of this perturbation depends on whether the voltage has passed through the thermoneutral point - determined at step 218. The voltage is increased by the same amount as before if the temperature difference has not changed sign (step 219), or changed by a smaller amount if the temperature difference has changed sign (step 220). As with step 216, at step 220, the new voltage perturbation (A Vn) is smaller than the old voltage perturbation (A Vo).

[0179] The absolute value of this lower voltage delta A may, for example, be no more than 3 / 4 or 2 / 3 or 14 of the absolute value of the last-used voltage delta A, and may preferably be just less than 14. The selection of the proportion (e.g. 3 / 4 / , 2 / 3, 14, less than 14,) can help tune the speed at which the threshold temperature delta is reached.

[0180] Following this perturbation stage, the process returns to ‘observation’ step 208 following a dwell time.

[0181] In some situations, the voltage perturbation may be going the wrong way, in which case it needs to be reversed. In some embodiments, this may be achieved by a one-off voltage delta that is a larger amount than the previous voltage delta to correct the previous change. More likely, however, the voltage change is an overshoot, over-correcting the temperature delta, rather than a change of voltage in the wrong direction, so the reversal in the voltage delta needs to be by a lesser voltage delta than the previous voltage delta. Applying a voltage direction in the wrong direction can be avoided with knowledge of the relative (i.e. not

[0182] 15446194-1absolute) temperature delta. If the outlet is hotter than the inlet, the stack is exothermic and the voltage is reduced; if the outlet is cooler than the inlet, the stack is endothermic and the voltage is increased.

[0183] Via this process and, especially via the repetition of steps 216 and / or 220, an iterative process of the voltage delta A applied to the stack operating voltage getting smaller and smaller will cause the stack operating voltage itself to get closer and closer to the ideal or best stack operating voltage for the stack in its present condition, which ideal or best stack operating voltage will vary over time due to a gradual degradation of the cells within the stack. As a result, the stack inlet and outlet temperatures converge again, having previously diverged as detected in step 204.

[0184] Typically, the operation is such that the stack outlet temperature converges towards the stack inlet temperature. The stack inlet temperature may instead be controlled using the heat exchangers as previously discussed with reference to Figure 2.

[0185] Ultimately the convergence of the temperatures can be such that the difference between the inlet and outlet temperatures is no greater than the first threshold as used in step 204 and 210. Furthermore, if over time the stack again starts to diverge its inlet and outlet temperatures beyond the first threshold, this process will act to again correct that, bringing the temperature delta back below that threshold again.

[0186] The process of Figure 3, and thus the maintenance of the stack operating voltage at an optimized or improved operational voltage, even as that optimized or improved operational voltage varies over time, can continue for as long as the stack is in use - commonly 40,000 hours or more, or until some other process detects a stack failure or some other issue requiring a stack shut-down or voltage reduction. It should be appreciated however that the stack may transition to a different operating condition (i.e. different rate of production). During such transitions, the voltage may be intentionally deviated from the thermoneutral voltage, meaning that the voltage control method will be suspended during such transitions.

[0187] Due to the constant but slow degradation of the cells within the stack, there is no one single optimized or improved (or ideal) voltage for the stack throughout its useful life. Whilst the optimized operating voltage may stay constant for an initial part of a stack’s life, the stack will ultimately begin to show symptoms of its degradation and the optimal operating voltage for a given inlet temperature or a given operational current, will begin to change, with the changes

[0188] 15446194-1potentially accelerating the closer the stack gets towards an end of its useful life. The frequency of performing step 202 may reflect this variable rate of change. However, as there will be a control system running for the electrolyser system in any event, this process may run constantly and at consistent intervals, throughout the working life of the stack.

[0189] Whilst the above description and Figure 3 refer specifically to determining if the absolute value of the temperature delta (|Temp A|) is greater than the first threshold value and / or than the previous absolute value, this is in effect the same as determining if it is less than those values, requiring just the Y and N labels to be swapped around. Moreover, the question in steps 204 and 210 may be whether the value is greater than or equal to (or less than or equal to) the threshold value.

[0190] Whilst the above description and Figure 3 refer specifically to single determined temperature deltas, each comparison could involve averages, such as an average of two or more temperature deltas (or of their absolute values) or a temperature delta of averages of the temperatures, or combinations thereof, of other approaches, to reduce the effect of transients or spurious readings.

[0191] In some circumstances there may be benefits in operating the stack in a (slightly) exo- or endothermic mode of operation, whereby matching the outlet temperature with the inlet temperature might not lead to the best results. Thermoneutral operation may be desired in order to maximise efficiency, but this may lead to a system that is difficult to control. Operating the system with a non-zero offset value damps the system easing control. The offset value may be set as low as possible to achieve the desired damping effect while being as close as practical to thermoneutral for sake of efficiency. Accordingly, a specific offset in the temperatures may be introduced. In some embodiments, the offset value is between 2 and 25 °C. In some embodiments, the offset value is at least 3 °C, more preferably at least 4 °C, more preferably at least 5 °C, more preferably at least 8 °C. In some embodiments, the offset value is at most 20 °C, preferably at most 18 °C, more preferably at most 15 °C. This offset can be introduced in the step or steps of comparing the temperatures (i.e. steps 202 and / or 208), e.g. the temperature delta is the inlet temperature minus the outlet temperature plus or minus (as appropriate according to whether the operation is intended to be exothermic or endothermic) the offset. Alternatively, the offset can be taken into account within the first threshold in steps 204 and 210 (or the first threshold in step 204 and the second threshold in step 210, if used), which are then not zero or not near-zero, but based on a specific offset, towards which the process aims. With this variation, steps 204 and 210 determine whether

[0192] 15446194-1the absolute value of the temperature delta is, in effect, significantly greater than or less than the offset (for which purpose, the first threshold has both an upper and a lower boundary), and steps 204 and 210 additionally determine whether the absolute value of the temperature delta is less than the lower threshold boundary. If it is, then the process proceeds from step 204 to step 206, as before, but, in the case of step 210, if the absolute value of the temperature delta is less than the lower threshold bound, step 212 would be varied to use “<” rather than “>”, but the no and yes determinations lead to the same steps 214 and 216 as before.

[0193] In a particularly advantageous implementation, the offset value is greater than zero (i.e., endothermic operation, where the inlet is hotter than the outlet). Operating the stack endothermically results in a negative feedback loop wherein as the cells / stack cool, resistance of the stack increases. This increases the amount of resistive heating in the stack as a result the current flowing through the stack, in turn returning the stack toward a target temperature.

[0194] During operation of the stack, the operating voltage may also be checked against a minimum stack operating voltage threshold. This is set as a given delta from a predetermined optimal voltage for its given temperature. Whether the stack is operating below or within this voltage range enables a diagnosis of a stack operating status.

[0195] 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.).

[0196] If the determined stack operating voltage drops below a minimum threshold, 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 and voltage.

[0197] The stack operating voltage is applied across all of the cells of the stack, which are arranged in series. Thus, the voltage adjustment across the whole stack changes the voltage across each cell by a much lower amount, typically or at least on average, 1 / N of the voltage change across the stack, where N is the number of cells in the stack.

[0198] In some embodiments, the method is applied to a subset of the cells within a stack, or across multiple stacks. This can allow either fine tuning of the individual subsets of cells for improving

[0199] 15446194-1the overall efficiency of the system or fine tuning of a larger array of cells for reducing the complexity of the system.

[0200] Referring next to Figures 4 and 5, examples of a temperature response to changes in the operating voltage are shown.

[0201] Referring first to Figure 4, a positive voltage delta is applied to a stack, raising the voltage per stack from 1.28V to 1.283V - an increase of 0.003V per cell. This is shown in the lefthand graph. Noting that the corresponding time codes in the right-hand graph match the time codes of the left-hand graph, it can be seen from the right-hand graph that the outlet temperature - which was already hotter than the input temperature, signifying an exothermic state for the stack, starts to increase in response to the voltage change, and that change tends towards an increased outlet temperature, and thus a new temperature delta that is larger than at the start. This tells us that the voltage change that was applied is in the wrong direction for achieving a thermoneutral state for the stack. The next voltage change would thus want to be in the opposite direction.

[0202] In this embodiment, the time from t1 to t4 is less than or equal to 2 hours.

[0203] Referring then instead to Figure 5, the voltage is instead dropped - from 1.28V per cell to 1.277V per cell - a drop of 0.004V per cell. In this example, the cell is again running in an exothermic state, but due to the voltage drop the temperature of the outlet starts to drop towards a new outlet temperature, reducing the temperature delta between the input and the output to a temperature delta of only about 2 degrees C. This could be considered, in some circumstances, sufficiently thermoneutral.

[0204] Had the voltage been dropped further, the temperature of the outlet would have dropped further. Had the voltage been dropped less, the temperature of the outlet would have dropped less.

[0205] Figure 6 next shows a control device 400 for controlling an electrolyser cell stack of an electrolyser in an electrolyser system.

[0206] 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

[0207] 15446194-1thus 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.

[0208] 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, for example through control of the system shown in Figure 2. A current and voltage, or power control system 416 is also provided for controlling a current supply and a voltage 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, for providing galvanostatic operations within the stacks, and the voltage can be controlled in line with the present invention, but the current and voltage may be reduced or turned off to automatically shut down the stack if an irresolvable issue with the system is detected, or if otherwise required.

[0209] 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.

[0210] 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 the operating voltage for each stack may be independently controlled, as per the above described method. Alternatively, individual stacks may be divided up into sub-stacks of smaller number of cells in series (e.g. a stack of 400 cells divided into 4 sub-stacks, each of 100 cells), and an operating voltage across each substack may be controlled (or varied) as per the above described method.

[0211] As another option, the operating voltage across two or more stacks in parallel or in series, for example, pairs of stacks, may be controlled jointly based on this method, for example based on the average temperature delta for each stack, or based on a temperature delta of the average inlet temperature and the average outlet temperature, or based on such averages from less than each stack (for instance if the temperature sensors for one or more stacks are

[0212] 15446194-1not working or not working properly). For example, multiple pairs of stacks may be provided, each controlled independently by the present invention.

[0213] 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.

[0214] 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.

[0215] 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).

[0216] 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.

[0217] 15446194-1In 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

[0218] As mentioned above, instead of steam and water, the fuel may be carbon dioxide, for creating carbon monoxide and oxygen instead of hydrogen and oxygen, or some other compound that may be desired to be split into constituent parts, such as nitrous oxide into nitrogen and oxygen.

[0219] 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.

[0220] 15446194-1

Claims

CLAIMS:

1. A method for establishing a voltage set point for one or more electrolysers, the method comprising:(1) receiving a temperature delta between an inlet fluid temperature and an outlet fluid temperature;(2) if the received temperature delta is beyond a first threshold:perturbing an operating voltage of the stack by a first voltage delta to a first voltage level; and(3) observing any change in the temperature delta between the inlet fluid temperature and the outlet fluid temperature;(4) wherein if the observed temperature delta is beyond a second threshold, perturbing the voltage from the first voltage by a further voltage delta, wherein the value and direction of the further voltage delta depends on change to the temperature delta.

2. The method of claim 1 wherein if the received temperature delta is within the first threshold, maintaining the operating voltage as a new operating voltage for the electrolyser, and returning to (1).

3. The method of claim 1 or 2 wherein if the observed temperature delta is within the second threshold, maintaining the operating voltage as a new operating voltage for the electrolyser, and returning to (3).

4. The method of any one of the preceding claims wherein if the observed temperature delta is beyond the second threshold:(4)(a) if the observed temperature delta has the same sign as before, but a lower magnitude, introducing a further voltage delta to the operating voltage, the further voltage delta being the same magnitude as a last-introduced voltage delta, and returning to (3);(4)(b) if the observed temperature delta has an opposing sign to before, introducing a further voltage delta to the operating voltage, the further voltage delta being in the opposite direction to the last-introduced voltage delta and having a smaller absolute magnitude than a last-introduced voltage delta, and returning to (3).

5. The method of claim 4, wherein in (4)(b) the absolute value of the further voltage delta is less than 3 / 4 of the absolute value of the first voltage delta, or optionally less than 2 / 3, or less than 1 , of the absolute value of the first voltage delta.15446194-16. The method of any preceding claim further comprising a dwell period or delay following a voltage perturbation and / or determination that the temperature delta is within the first or second threshold, preferably the dwell period is less than 2 hours.

7. The method of any preceding claim, carried out as a continuous iterative process.

8. The method of any preceding claim wherein the first threshold is the same as the second threshold.

9. The method of any of claim 1 to 7 wherein the first threshold is larger than the second threshold.

10. The method of any one of the preceding claims, wherein the one or more electrolysers, or the stack thereof, are provided with one or more inlet fluids at an inlet fluid temperature, and exhaust one or more outlet fluids at one or more an outlet at a fluid temperature.

11. The method of any one of the preceding claims, wherein the temperature delta is an average of the difference between the outlet temperature and the inlet temperature over the course of multiple temperature readings.

12. The method of any one of the preceding claims, wherein the temperature delta is the difference between an average of the outlet temperature and an average of the inlet temperature taken from multiple stacks.

13. The method of any preceding claim wherein the temperature delta includes an offset.

14. The method of any one of the preceding claims, wherein the sign of the first voltage delta is based on the temperature delta, preferably based on the sign of the temperature delta.

15. The method of any one of the preceding claims, wherein the first voltage delta is predetermined from previous calibration of the electrolyser cell stack.

16. The method of any one of claims 1 to 14, wherein the first voltage delta is selected based on the initially obtained temperature delta.15446194-117. The method of any one of the preceding claims, wherein the electrolyser is an intermediate or high temperature electrolyser cell stack.

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

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

20. The method of any one of the preceding claims, wherein the method is carried out on an electrolyser system comprising a plurality of electrolyser cell stacks arranged in parallel, each electrolyser cell stack comprising a plurality of electrolyser cells arranged in series.

21. The method of claim 20, wherein the method is performed independently for each electrolyser or independently for a plurality of subsets of electrolyser cell stacks in the electrolyser system.

22. A control device for controlling an electrolyser cell stack of an electrolyser in an electrolyser system, the control device comprising:a voltage control system for controlling an operating voltage across the electrolyser cell stack;an inlet temperature monitoring system for determining an inlet temperature at the fluid inlet; andan outlet temperature monitoring system for determining an outlet temperature at at least one fluid outlet of the electrolyser cell stack;wherein the control device is adapted to perform the method according to any one of the preceding claims.

23. An electrolyser system comprising the control device of claim 22.

24. A computer program comprising instructions which, when the program is executed by a processor, cause the processor to carry out the steps of the method of any one of claims 1 to 21.

25. A non-transitory computer-readable medium with instructions stored thereon, that when executed by a processor, perform the steps of the method of any one of claims 1 to 21.15446194-1

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