Method for transitioning load in an electrolyser

Abstract

The present invention provides a method of changing the electrolytic conversion rate within at least on electrolyser cell stack of an electrolyser system having a fluid inlet, fluid outlet and a power control system, and using said power control system to change the voltage across the electrolyser cell stack, allowing the fluid outlet temperature to change in response to the voltage change, setting an inlet temperature to substantially match the new outlet temperature, and then allowing the voltage to revert to a substantially thermoneutral value, such that the electrolyser cell stack is operating at a changed stack temperature and changed electrolytic conversion rate.

Description

[0001] Method for transitioning load in an electrolyser

[0002] The present invention relates to a method for transitioning load in an electrolyser comprising an electrolyser cell stack, and in particular a method for transitioning from one state of operation to another - for example, changing the electrolytic conversion rate within an electrolyser cell 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 into its constituent parts and to output those constituent parts in the off-gases from the electrolyser. For that purpose it requires a source of electricity for supplying an electric current and voltage through / across the at least one stack of electrolyser cells. As an example, electrolysers can be used to produce hydrogen and oxygen from water, or carbon monoxide and oxygen from carbon dioxide, each by way of electrolysis.

[0004] The collection, storage or distribution 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, and thus too will be collected, stored or distributed for downstream or later use. The collection, storage or distribution of the produced hydrogen is also important as it is a fuel that can help in the race for decarbonisation and for helping to achieve net zero targets. For example, the produced hydrogen can be utilised as a fuel for combustion, or as a fuel for a use in a fuel cell system for achieving an electrolytic reaction in the fuel cell to recombine the hydrogen with oxygen, with resultant electrical and heat outputs. The hydrogen can also have many other uses.

[0005] Given the various uses of the off-gases, the off-gas demands from an operator of the electrolyser can change. For example, lower rates of production of the oxygen, hydrogen or carbon monoxide might be required by a downstream service, or increases may be required at peak times.

[0006] It would also be desirable to maintain continuity of operation of the electrolyserfor longer periods of time since unnecessary shut-downs and start-ups of the electrolyser leads to production inefficiencies, and potentially premature degradation of the electrolyser.

[0007] In the prior art it is known that when a cell or a stack starts to fail, or when it otherwise degrades, the operational efficiency of the electrolyser will diminish. In a degrading stack, the electrical resistance of the stack typically increases, leading to higher voltage requirements for the same electrical current flow. However, operating a stack with an over-potential (i.e. too high a voltage) is known to drive degradation and can cause certain failure modes to be more prevalent. A preferred method of managing the operational state of an electrolyser is therefore instead to operate the electrolyser at a fixed voltage - ideally at or close to a thermoneutral voltage. Ideally this can be a galvanostatic thermoneutral condition - i.e. with a constant current too. However, a drawback of a fixed voltage is that the current will slowly decrease with time as the electrolyser, the stacks or the cells degrade (and thus their electrical resistance increases). The reduced current reduces the off-gas (e.g. hydrogen and oxygen) production rate of the system. This, however, is a slow change. Nevertheless, after a long enough period of time such rates may have dropped significantly. This can result in an operational need to compensate for lost production capacity. This may be achieved by introducing additional electrolyser cell modules or stacks, with resulting downtime during stack reconfiguration or stack replacement, all of which increase operational costs and operational disruption. The present invention also seeks to provide a way of at least partially alleviating the disruption.

[0008] In addition to operating in a galvanostatic thermoneutral condition, other modes of operating an electrolyser are discussed in the prior art. Prior art documents JP2020128576 and W02020201485 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.

[0009] The present invention seeks to incorporate the known operational characteristics of an electrolyser into a novel mode of use whereby the inventors have taken these concepts further. They use the operational characteristics of a stack to facilitate rapid adjustments to, and potentially compensation for the demands from, the electrolyser, while still improving, maintaining or extending the service life and operational capabilities of the electrolyser, and in particular of the stacks and the cells within the stacks. In particular, the present invention intends to maintain efficient operability of the electrolyser for extended periods of time, requiring less downtime than with previous electrolysers.

[0010] STATEMENTS OF INVENTION

[0011] According to the present invention there is provided a method of controlling an electrolyser system to change an electrolytic conversion rate within at least one electrolyser cell stack of the electrolyser system, the electrolyser system comprising at least one electrolyser, the at least one electrolyser comprising the at least one electrolyser cell stack, and the at least one electrolyser cell stack comprising a plurality of electrolyser cells, at least one fluid inlet to the electrolyser cells and at least one fluid outlet from the electrolyser cells, wherein the electrolyser system further comprises: a power control system for changing the electrolytic conversion rate within the at least one electrolyser cell stack, the method comprises: i) using the power control system to change the voltage across the at least one electrolyser cell stack; ii) allowing a fluid outlet temperature to change in response to said change in voltage; iii) setting a fluid inlet temperature to substantially match said changed fluid outlet temperature; and iv) allowing the voltage to revert towards its initial value so as to normalise the electrolyser cell stack at a substantially isothermal operation with the at least one electrolyser cell stack at a changed stack temperature, and thus a changed stack resistance and thus a changed stack current and a changed electrolytic conversion rate.

[0012] In some embodiments the electrolyser system further comprises a processor and memory circuit for storing a plurality of preset current, voltage or power delivery changes for the control of the at least one electrolyser cell stack, and when the method uses the power control system to change the voltage across the at least one electrolyser cell stack, the power control system changes the current, the voltage or the power to one of the plurality of preset current, voltage or power delivery changes (i.e. a “preset change”).

[0013] In some embodiments the preset change is chosen for a target temperature change of the at least one electrolyser cell stack. In some embodiments there are a plurality of temperature changes in the memory. Preset changes may be for targeting these temperature changes.

[0014] The power control system may control the current, which in turn controls the voltage / power delivery - for example the current or voltage delivered through or across the cells / the stack.

[0015] In some embodiments the electrolyser system further comprises at least one heat exchanger for exchanging heat from off-gas exiting the at least one fluid outlet to fluid entering the at least one fluid inlet. This can help with, or provide, the setting of the fluid inlet temperature to substantially match the changed fluid outlet temperature.

[0016] The change in the voltage across the at least one electrolyser cell stack resulting from using the power control system is usually only temporary - i.e. for a few seconds, minutes or in some circumstances for only a few hours, as the voltage re-regulates back to (or substantially back to - see below as the “normal” voltage may be slightly above thermoneutral) the thermoneutral voltage. This can be a quick return to the thermoneutral voltage if it is a singular adjustment, or it may be slower if subsequent adjustments are deemed appropriate or necessary before the thermoneutral voltage is resumed.

[0017] The change in voltage is typically achieved by causing a change in the current applied through the stack, which in turn results in a change in voltage across the stack if the power delivery remains either constant or not proportionally different relative to the change in current. This is due to the fixed relationship between current, voltage and power.

[0018] The change in voltage results in either an exothermic or endothermic operating mode. The operating mode, however, remains dependent upon the power, voltage and current applied to the electrolyser cell stack.

[0019] A change of operating mode results in a change in an off-gas temperature of the off-gas exiting the at least one fluid outlet. That in turn should also change, or cause a change in, an infeed temperature of the fluid entering the at least on fluid inlet, for example due to the operation of the at least one heat exchanger.

[0020] In some embodiments the memory circuit also stores information about current set points and / or input temperature set points. In some embodiments the memory circuit stores additional target temperatures and operation temperature thresholds for the electrolyser cell stack or electrolyser cells, and conditional voltage or current set points for avoiding crossing those thresholds.

[0021] In some embodiments the preset changes, or target stack temperatures associated therewith, or target stack currents associated therewith, are modified within the memory over time to reflect expected, detected or approximated states of deterioration of the stack. The present invention thus can allow for (i.e. compensates for) any inevitable degradation of an electrolyser cell; as a cell degrades, its resistance increases for any given temperature, and thus at a fixed voltage, a power draw will decrease, due to the drop in current draw (as the voltage is the controlled value - at the thermoneutral voltage) as such, the conversion rate drops. The invention, by allowing the stored presets to update over time, allows for the changes that occur due to degradation. In some embodiments the preset changes, or target stack temperatures associated therewith are generated by a predictive model. The predictive model may be used to predict the temperature at which thermoneutral operation may be achieved, taking into account factors including but not limited to electrochemical stack performance, stack age and stack resistance. The use of a predictive model allows for a faster response time when changing set points.

[0022] In some embodiments the system comprises a current control system.

[0023] In some embodiments changing the voltage comprises controlling the current applied through the stack.

[0024] In some embodiments, as the outlet temperature rises, an inlet temperature set point for the inlet temperature is set to match instantaneous outlet temperatures. This allows the system to adjust the temperature of the inlet fluid via control of the heat exchanger(s), or ancillary trim heaters, coolers or heat exchanger / heater bypasses. This can increase the rate of change of the inlet temperature, thus transitioning from one power load to the target power load more quickly.

[0025] In some embodiments, when the power control system changes the voltage across the at least one electrolyser cell stack, a stack inlet temperature set point is set at an estimated or preset temperature corresponding to a final target temperature for the changed stack temperature. Adjusting the stack inlet temperature set point to that target temperature allows the stack inlet temperature to be changed towards that final temperature more quickly. This also accelerates the change in the stack’s internal temperature and thus the overall stack resistance. That in turn enables more (or less) current to flow for the same voltage set point. This acts to reduce both the change in voltage and the change in outlet temperature during the load transition before steady state operation resumes.

[0026] Typically the electrolyser system, or the electrolyser cell stack thereof, will be operating at a thermoneutral operating point before the voltage change is made (i.e. a thermoneutral voltage so as to be substantially isothermic - inlet temperature = outlet temperature). However, it should be appreciated that the stack may be operating under normal conditions such that the outlet temperature is higher than the inlet temperature by 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 of, or a reduction in the use of, an electric heater in the system. The present inventors, in particular, have realised that running slightly above thermoneutral voltage allows the outlet stream temperature to be raised slightly, improving its ability to raise the temperature of the inlet stream to an appropriate temperature via a heat exchanger.

[0027] In preferred embodiments, the normal operating mode of the electrolyser system is a substantially isothermic condition - any one of many possible thermoneutral operating points, as dictated by the respective operating temperature of the stack, and thus the current passing through the stack.

[0028] The voltage for most if not all thermoneutral operating points is substantially the same. Typically this is a cell voltage of 1.28V for a solid oxide electrolyser cell.

[0029] In some embodiments, the change in voltage may be up to a 25% change from a thermoneutral voltage for the cells in a stack - i.e. from 0.96V to 1 ,6V for cells having a thermoneutral voltage of 1.28V. Preferably between 1.0V and 1.4V. Different stack or electrolyser configurations and different electrolyser chemistries, will have different operational limits, and can be specified by cell manufacturers. It is to be appreciated that excessive over-potentials (too high a voltage change) can rapidly damage the stack or cells. Similarly, excessive under potentials can prevent operation of the stack, and may also cause damage to the cells or the stack or equipment connected thereto. In some embodiments, if the voltage drops much below 1V the stack will not operate as an electrolyser because it has no over-potential relative to its open circuit voltage and so no current will flow. If the voltage reduces (or is removed) then the electrolyser will revert to operating as a fuel cell and will generate its own voltage potential. This may not be intrinsically damaging, but undesirable nonetheless.

[0030] In some embodiments, upon normalisation of the electrolyser cell stack, the substantially isothermal operation of the at least one electrolyser cell stack is a galvanostatic thermoneutral condition for the electrolyser cell stack - i.e. with a substantially fixed current and a substantially fixed voltage - although the current can be allowed to drift over time due to deterioration of the stack, and some variation in the voltage (and current) may occur due to power fluctuations, or fluid inflow variations.

[0031] Operating the or each electrolyser cell stack at or close to a thermoneutral voltage is advantageous because it avoids both the need to add significant heat to the system to support the electrolysis process or to reject significant heat from the system. This is because cells operating at the thermoneutral voltage are approximately isothermal (the inlet temperature is approximately equal to the outlet temperature). This brings two important benefits: a) Stack airflow is not required to facilitate heat transfer to / from the stack, and so is only acting to ‘sweep’ the oxygen produced in the electrolysis process out of the stack. This means that airflow can be reduced significantly compared to a stack operating in an exothermic or endothermic condition, which increases the thermal efficiency of the system as the airflow is a primary source of heat loss. Zero airflow could also be possible - particularly in thermoneutral operation, allowing pure oxygen to be generated by the system. b) The lack of thermal gradient across the cells means that a much more uniform current density is achieved across the electrochemically active area of the cells. The cell temperature is thus able to be controlled within a narrow range and this gives the ability to adjust the operating temperature within the IOM (installation, operational and maintenance) temperature limits. In exothermic or endothermic operation, the cell temperature gradient can significantly limit the ability to vary the operating temperature whilst maintaining both inlet and outlet temperatures within the specified limits.

[0032] Thermoneutral operation can be achieved by setting (or allowing a normalisation of) the stack operating voltage to maintain the individual cell voltages around their theoretical thermoneutral voltage. For normal operating temperatures, this is normally around 1.28 Volts per cell.

[0033] As discussed previously, the present invention mostly operates its stack at a thermoneutral voltage. However, by adjusting that voltage, the temperature of the stack can be changed and as a result of a change in the stack temperature, the stack can be made to operate with higher or lower electrolytic conversion rates.

[0034] With the present invention, therefore, for example if the user demands a drop in the electrolytic conversion rates, the targeted change can be a drop in voltage, so the power control system causes an initial drop in voltage. A drop in voltage precipitates a drop in the temperature of the stack as the stack becomes endothermic due to the “under potential” state of the cells. Due to the endothermic nature of the stack, the stack consumes heat, thus lowering the output temperature, thus in turn reducing the amount of heat available for the heat exchanger, and thus in turn dropping the fluid input temperature, and thus the temperature of the stack progressively (but rapidly) cools. That cooling of the stack increases the resistance of the stack and thus the current drops too. As the temperature stabilises at a new thermoneutral state, the current stabilises and the voltage returns towards its initial thermoneutral level by setting the inlet temperature to the (lower) outlet temperature. The lowering of the current causes the rate of conversion of “fuel” (such as steam or carbon dioxide) into oxygen and hydrogen (in the case of steam) or oxygen and carbon monoxide (in the case if carbon dioxide) to lower as well. In addition, there can be a change (drop) in the reaction activity of electrochemically active layers (i.e. the electrolyte in each cell) as their temperature reduces, thus further reducing the electrolytic conversion rate within the at least one electrolyser cell stack.

[0035] If the user instead demands an increase in the electrolytic conversion rates, the targeted change can be an increase of the voltage across the cells or stack, so the power control system causes an initial increase in voltage. That “over potential” will instead make the temperature of the stack increase as the stack becomes exothermic: Due to the exothermic nature of the stack, the stack emits heat, thus raising the output temperature, thus in turn increasing the amount of heat available for the heat exchanger, and thus in turn raising the fluid input temperature, and thus the temperature of the stack progressively (but rapidly) increases. That reduces the resistance of the stack and thus the current increases too. The inlet temperature is set to the (higher) outlet temperature to move to the new thermoneutral state. This may be achieved solely be heat exchangers, or additionally by using in-line heaters. As the temperature stabilises at to a new thermoneutral state, the current stabilises and the voltage returns towards its initial thermoneutral level. That increase in the current causes the electrolytic conversion rate within the at least one electrolyser cell stack to increase as well. Furthermore, there will be a partial increase in the electrolytic conversion rate within the at least one electrolyser cell stack due to the higher reaction activity of the electrochemically active layers (the electrolyte) of the electrolyser cells (due to them being hotter). The voltage is then allowed to return to the thermoneutral level, but at an increased current (i.e. an increased power) corresponding to the desired, targeted or demanded change in the electrolytic conversion rate of the cell or stack - achieved via an increased temperature of the cell or stack.

[0036] The current, voltage or power control can thus trigger rapid changes to the electrolytic conversion rate within the at least one electrolyser cell stack.

[0037] With the present invention, the voltage can be initially perturbed away from a thermoneutral level, which causes a change in stack temperature. Cell resistance is negatively correlated to temperature, so this causes a change in resistance and thus current. As the temperature stabilises to a new thermoneutral level, the voltage also returns to the thermoneutral level, but at a changed current (and thus power). The correspond changed electrolytic conversion rate of the cell at the changed temperature may be specifically targeted by memorised characteristics of the stack.

[0038] As mentioned above, returning to a new thermoneutral state is achieved by setting the inlet temperature to that of the (changed) outlet temperature, and this modified inlet temperature can be achieved by heat exchangers, in-line heaters or a combination of the two.

[0039] It should be noted, however, that there are upper and lower temperature limits for the electrolyser cells above or below which the efficiencies always diminish. The pre-set current, power or voltage changes in the memory are thus preferably predetermined or calculated for a particular form of stack such that they will not cause the stack to reach those upper or lower limits.

[0040] Furthermore there are operational thresholds beyond which a rate of degradation (or a greater risk of degradation) increases. The pre-set current, voltage or power changes in the memory are thus preferably predetermined or calculated for a particular form of stack such that they will not cause the stack to excessively degrade.

[0041] In some embodiments, to avoid crossing operational temperature thresholds for the stack, current, voltage or power changes may be done in stages. For example, the initial current, voltage or power change may involve a lesser change initially - for example reducing (or increasing) the current in an initial step to a point where an operational inlet or outlet temperature threshold, and / or an internal cell temperature threshold, is potentially reached but not crossed, and then a subsequent step changes the current, voltage or power some more - towards the intended current, voltage or power set point for achieving the desired change in operational output (i.e. hydrogen output). The subsequent change may be a gradual change to the target current, or one or multiple further changes. In this manner, the inlet and outlet (and stack) temperature changes are controlled, thus avoiding the operational thresholds. These steps can be carried out as often as necessary to achieve the target temperature and current for the power load required.

[0042] Usually, a stack would be operated at an optimum temperature for achieving maximum conversion rates for the fuel, but with the present invention, it is possible rapidly to change the conversion rate - should the need arise - whether for short term operation, or for longer term operation. Thus, in some embodiments, changes in the operational point of an electrolyser cell stack may be temporary (i.e. for a relatively brief period of time - such as a few hours). One such use for this would be to provide an increase in operational output (a higher electrolytic conversion rate) from one or more electrolyser cell stack in an electrolyser system to compensate for downtime on another electrolyser cell stack in the electrolyser system - such as if the downed electrolyser cell stack is needing to be serviced, repaired or expanded to compensate for degradation or damage.

[0043] Changing a target voltage causes either an exothermic or endothermic operating mode and as such can rapidly change the operating temperature of the stack. In such a way, the stack conditions (e.g. away from thermoneutrality to exothermic or endothermic) are used to change the outlet temperature and thermoneutrality is regained by matching the inlet temperature to this. In contrast, changing the operating temperature purely by means of the inlet gas temperature whilst maintaining a thermoneutral voltage is slow because the thermal inertia of the system acts to maintain the previous operating state, and the electrolysis process does not influence the heat balance in the system.. Further, the recovery of the operating voltage for reverting to an thermoneutral operating condition can also be relatively quick and can be controlled by the system because the electrolysis process itself acts to restore the heat balance at the new operating point. In such a way, the change in set point may be in the order of a few minutes. This time varies depending on the size of the change and arrangement of the system (for example, efficacy of heat exchangers I in-line heaters).

[0044] This method may also be used for long term power control, where the changes occur over the course of 1000s of hours. In such a scenario, the changes would likely be a series of many small step changes resulting in a large change over a long period (e.g. a step change in a certain period, for example every hour, day, week, month etc.).

[0045] The present invention can thus rapidly change the operational performance of an electrolyser - much quicker than by use of just externally controlling the operating temperature of the stack using external coolers or heaters. The latter is slow due to thermal inertia of the system being high. However, by having the cells themselves provide the heating / cooling, whereby the fluid within the cells changes temperature very rapidly (due to the exothermic or endothermic operating mode of those cells), the present invention can instigate a much more rapid change of the electrolytic conversion rate within an electrolyser cell stack.

[0046] Other approaches used in the prior art for changing the electrolytic conversion rate within an electrolyser cell stack have included reducing the steam (or carbon dioxide) utilisation - i.e. reducing the volume of steam (or carbon dioxide) entering the stack. Moderating the amount of available fuel can likewise have a rapid effect on the electrolytic conversion rate. However, this can result in ‘fuel starvation’ which has a significantly detrimental effect on the service life off the stack, and as such is undesirable except in emergencies. The present invention thus maintains the service life of the stack, while also achieving a rapid change in the electrolytic conversion rate of the stack - much more quickly than would be achievable through controlling the fluid temperatures via additional heat exchangers, additional or alternative fluid flows via valves, or using heater / cooler circuits.

[0047] In addition, there is less thermal shock to the stack as the stack itself (i.e. the cells) is controlling the temperature change.

[0048] In some embodiments, the electrolyser system comprises an inlet temperature monitoring and / or control system for monitoring and / or controlling an inlet temperature, for example at the at least one fluid inlet.

[0049] In some embodiments the inlet temperature monitoring and / or control system comprises a sensor for detecting the temperature at the at least one fluid inlet and the inlet temperature monitoring and / or control system is connected with a (or the) processor for feedback on the inlet temperature. This can be used to avoid crossing temperature thresholds for the stack.

[0050] In some embodiments, the electrolyser system comprises an outlet temperature monitoring and / or control system for monitoring and / or controlling an outlet temperature, for example at the at least one fluid outlet.

[0051] In some embodiments the outlet temperature monitoring and / or control system comprises a sensor for detecting the temperature at the at least one fluid outlet and the outlet temperature monitoring and / or control system is connected with a (or the) processor for feedback on the outlet temperature. This can be used to avoid crossing temperature thresholds for the stack.

[0052] 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.

[0053] 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, a current detection circuit is provided to detect (and optionally control) an input current for powering the electrolyser cell stack. If the detected current exceeds a threshold, a (or the) processor may further adjust the voltage to reduce or increase the current as necessary.

[0054] In some embodiments, a voltage detection circuit is provided to detect a voltage across the electrolyser cell stack. If the detected voltage exceeds a threshold, a (or the) processor may further adjust the voltage to reduce or increase the voltage as necessary.

[0055] In some embodiments, an inlet temperature detection circuit and / or an outlet temperature detection circuit is provided to detect an inlet temperature and / or an outlet temperatures from the fluids entering or exiting the respective inlet(s) or outlet(s) of the electrolyser cell stack. If the detected temperature exceeds a threshold, a (or the) processor may further adjust the voltage to reduce or increase the voltage as necessary.

[0056] In some embodiments, the method comprises setting a maximum stack voltage and / or a minimum stack voltage for the or each stack. In such a way, operational thresholds of the electrolyser cell stack are imposed, for example to avoid excessive deterioration of the stack.

[0057] In some embodiments, the magnitude, direction and / or duration of the change in voltage is dependent on a target change in stack temperature for achieving thermoneutral conditions at target given temperature.

[0058] In some embodiments, the electrolyser comprises more than one electrolyser cell stack. The electrolyser cell stacks may be electrically connected in parallel or in series.

[0059] In some embodiments, the method is applied to each electrolyser cell stack separately. Alternatively, the method is applied to multiple stacks collectively, with a common voltage applied to each stack of a collective group of stacks.

[0060] In some embodiments, the or each electrolyser cell stack is a solid oxide electrolyser cell stack. In other words, the electrochemically active region of the electrolyser cell is a solid oxide. A solid oxide electrolyser cell (SOEC) typically operates in the 400-900 degrees C range. These include intermediate and high temperature SOECs. For some chemistries, the cells are just operating between 400 and 700 degrees C, or more particularly in the 450-650 degrees C temperature range. Such electrolyser cells may be referred to as intermediate temperature solid oxide electrolyser cells, or IT-SOECs.

[0061] Typically the present invention uses water in the form of steam as the “fuel”. As such the or each electrolyser is used to electrolyse water in the form of steam. 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 700 degrees C. (Sometimes the cut-off between intermediate temperature and high temperature SOECs is 750 degrees C).

[0062] 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).

[0063] Due to the operating temperature of a SOEC (usually in excess of 400 degrees C), any water passing through the electrolyser cell will be vaporised into high temperature steam. More usually this occurs outside the cells - i.e. in or prior to pipework connecting to the at least one fluid inlet.

[0064] 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.

[0065] The present invention will now be described, purely by way of example, with reference to the accompanying drawings, in which:

[0066] Figure 1 schematically shows a typical electrolyser cell, multiples of which may be stacked in a stack;

[0067] Figure 2 shows a schematic of an example of an electrolyser system with a single stack and 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 schematic diagram showing an embodiment of the present invention, showing the elements used for controlling an electrolyser system;

[0068] Figure 4 shows how galvanostatic thermoneutral (GTN) strategy for electrolyser stack control can utilise an initially exothermic operating condition with an outlet temperature above an inlet temperature to cause a voltage to decrease when the current is constant (galvanostatic), with the stack achieving the galvanostatic thermoneutral state when the temperatures equalise and the voltage reaches a thermoneutral voltage;

[0069] Figure 5 shows how galvanostatic thermoneutral (GTN) strategy for electrolyser stack control can utilise an initially endothermic operating condition with an outlet temperature below an inlet temperature to cause a voltage to increase when the current is constant (galvanostatic), with the stack achieving the galvanostatic thermoneutral state when the inlet and outlet temperatures equalise and the voltage reaches a thermoneutral voltage;

[0070] Figure 6 shows how a galvanostatic thermoneutral (GTN) strategy can utilise a voltage change (a drop) to trigger an endothermic operating condition for achieving a lower power operating point for the stack upon a return to galvanostatic thermoneutral conditions, including a thermoneutral voltage, a constant current and substantially equalised inlet and outlet temperatures.

[0071] Figure 7 shows how a galvanostatic thermoneutral (GTN) strategy can utilise a voltage change (an increase) to trigger an exothermic operating condition for achieving a higher power operating point for the stack upon a return to galvanostatic thermoneutral conditions, including a thermoneutral voltage, a constant current and substantially equalised inlet and outlet temperatures; and

[0072] Figure 8 shows how a galvanostatic thermoneutral (GTN) strategy can be combined with conventional inlet temperature control so as to utilise a voltage change (a drop in this example) to trigger an endothermic operating condition, along with setting a target inlet temperature and controlling inlet fluid accordingly for achieving that temperature, for achieving the lower power operating point for the stack more quickly than in Figure 6, upon a return to galvanostatic thermoneutral conditions, including a thermoneutral voltage, a constant current and equalised inlet and outlet temperatures. 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 electrolyser cell stack 12 - hereinafter a stack. 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.

[0073] Figure 2 schematically shows an example of an electrolyser system 20, similar to that known from W02023 / 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.

[0074] Figure 2 also shows that the electrolyser system 20 comprises a stack 12 which comprises a stack of electrolyser cells 11 , hereinafter referred to as cells. As shown, this example has five such cells, although generally a stack will have tens or even hundreds of cells - usually all in parallel, although some may use cells in series or combinations of the two. In some electrolysers, multiple stacks 12 may be provided, electrically connected in series or in parallel.

[0075] Figure 3 instead shows a stack 12 of twenty-one electrolyser cells 11 in parallel, with power terminals 120 at top and bottom ends of the stack for application of a voltage across the stack 12 - and thus current through the cells 11. The application of electrical power through an electrolyser 10 in this manner is well known in the art.

[0076] 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.

[0077] 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. Via the terminals 36, 37, a voltage can be applied across the stack 12 - and thus current through the cells 11. The application of electrical power through an electrolyser 10 in this manner is well known in the art.

[0078] 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.

[0079] 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.

[0080] 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.

[0081] Such operational characteristics of electrolyser cells, including SOECs, are well known in the art.

[0082] 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.

[0083] 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. Bypasses 90 for the heat exchangers 22, 50, 62 might also be provided to reduce heating of the inflowing fluid, such as the one shown for heat exchanger 50 in Figure 2. In some embodiments, a heat exchanger can even be used to cool the fluid using a colder fluid source or fan.

[0084] For controlling the temperatures and flow of fluid around the electrolyser system 20, 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 or bypasses 90 - 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.

[0085] 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.

[0086] 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.

[0087] The stack 12 also comprises an anode side stack outlet 46, wherein an anode 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.

[0088] 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.

[0089] 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.

[0090] 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.

[0091] 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.

[0092] During operation of the system 10, the exhaust gas (also known as off-gas) may contain highgrade 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.

[0093] 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.

[0094] 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.

[0095] 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.

[0096] 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.

[0097] 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.

[0098] A similar control valve may be provided for the bypass 90 for the heat exchanger 50, likewise connected to a or the controller.

[0099] In a preferred embodiment the controller is a single controller for the electrolyser system and thus also connected to a power, current or voltage control system 108, as shown in Figure 3.

[0100] Returning to Figure 2, the control valve may be connected to, for example, one or more sensors 66, 72, 74, 76 provided on one or more of the flow lines between the components of the system to allow control of the flows, and thus the fluid temperatures. The controller is then connected to these sensors and potentially to the heaters and further flow valves or bypasses.

[0101] Whilst the above 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 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.

[0102] Referring next to a cathode side of the system 20, 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.

[0103] A third heat exchanger 50 is provided in the water supply flow path 56 between the water supply 42 and the cathode inlet 44. This has the bypass 90 provided for it. 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, although the amount of transfer might be controlled by the bypass 90.

[0104] 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.

[0105] 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.

[0106] 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.

[0107] 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.

[0108] 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.

[0109] Referring now back to Figure 1 , and Figure 3, the concepts behind the present invention will now be further discussed.

[0110] As mentioned above, the off-gases from the stack will usually be at a similar temperature to the operational temperature of the electrolyser cell 11 and also to the fluids entering the stack. However, the specific difference or delta between the inlet temperature and the outlet 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 generally supplied to the stack by controlling a constant current and thus producing a constant voltage (or vice versa). The stack is thus generally - or usually - operated in galvanostatic thermoneutral conditions during normal operations of the electrolyser. However, the voltage across the stack and cells, the current through the stack and cells, the temperature of the stack and cells, and thus the electrical resistance of the stack and cells, can be controlled or varied by an operator or the system, and may be monitored, detected or estimated. Typically, the control will be by way of a processor 110 and memory circuit 112, as shown in Figure 3. In this embodiment, the processor 110 and memory circuit 112 are connected to a power, current or voltage control system 108 of the stack 12, and the power, current or voltage control system 108 is connected to the stack to control the power supplied to the stack 12. For this purpose, the system 20 includes a voltage interface 100, which may monitor, control or vary the voltage across the stack 12. This embodiment also has a current interface 102 which may likewise monitor, control or vary the current across the stack 12. Usually this serves to maintain a constant current. However, it can also be used by the power, current or voltage control system to allow changes in current, or to set a particular new current. This embodiment also includes an outlet temperature interface 104 and an inlet temperature interface 106, which can also be provided for the power, current or voltage control system 108 to ensure that operational temperature thresholds are not exceeded, or to simply monitor temperatures. In this example, these outlet and inlet temperature interfaces 104, 106 are connected to outlet and inlet ports 116, 114, respectively, for monitoring the temperature of fluids passing through the ports 116, 114.

[0111] With this power, current or voltage control system, the stack can be operated without a fixed voltage value to move between different operating conditions. Control of the current and the inlet temperature (the latter being by way of heat exchangers, bypasses and trim heaters or coolers, as shown or discussed with reference to Figure 2) is also possible.

[0112] In the prior art, it is known to use either constant voltages or constant currents on a stack, and then to control the current or the voltage, respectively, to maintain the stack at a substantially thermoneutral condition. Indeed, for much of the service life of a stack, the stack operates at substantially galvanostatic thermoneutral conditions since the voltage and the current can largely remain constant and any changes are small or gradual. However, being able to control the current or the voltage (or both), provides requisite control to allow the stack to compensate for degradation of the stack or cells within the stack - compensating, for example, by reducing the current or increasing the temperature. The present invention still uses a galvanostatic thermoneutral condition for the stack 12 during normal steady state operations, but it also utilizes exothermic and endothermic conditions for changing the power level of the stack quickly. This is achieved by the temporary provision of a higher or lower voltage. This voltage deviation can be controlled directly or via a change to the current. These over- or under- potentials across the stack 12 temporarily divert the stack from steady state thermoneutral conditions before the stack normalizes back into a galvanostatic thermoneutral (substantially isothermal) condition albeit at a different stack temperature, and thus at a different current and with a different electrolytic conversion rate, and thus different off-gas output rates.

[0113] The present invention is particularly applicable to solid oxide electrolysers due to the high temperatures involved, and thus due to the possibility of variances in temperature from one side of a cell to the other arising from exothermic and endothermic conditions.

[0114] For solid oxide electrolysers, the thermoneutral voltage is commonly 1.28V per cell.

[0115] In some embodiments the present invention also utilises inlet temperature control (for example by controlling the input fluid temperatures) to further accelerate a return of the stack 12 to galvanostatic thermoneutral conditions, while still adopting the different stack temperature, and thus the different current and the different electrolytic conversion rate, and thus the different offgas output rates. Referring next to Figure 4, galvanostatic thermoneutral strategy (GTN strategy) will be discussed. The principles behind this are utilised by the present invention in achieving its novel approach for controlling an electrolyser system to change an electrolytic conversion rate within at least one electrolyser cell stack of the electrolyser system.

[0116] GTN strategy utilises an outlet temperature (e.g. of the stack, the cell or the electrolyser) as a feedback control to adjust an inlet temperature (e.g. of the stack, the cell or the electrolyser), while maintaining a constant current operation. In ideal conditions, the inlet temperature setpoint is equal to the measured outlet temperature. These temperatures can be measured, detected or estimated temperatures of inlet and outlet ports of the stack 12 or the electrolyser 10, or measured, detected or estimated temperatures of the fluid(s) exiting those ports.

[0117] As discussed in respect of Figure 2, sensors may be used for detecting, determining, measuring or estimating these temperatures.

[0118] In ideal conditions, which in practice are only likely to be approximated, in a steady state operation the cells 11 will be running in an isothermal condition and cell voltage will be at or close to a thermoneutral voltage for the cell 11 .

[0119] Figure 4 shows a stack moving from an exothermic operating condition into this target isothermal condition. In this first example, the stack 12 exhibits the exothermic operating condition as its initial condition on the left - the x axis is time. In that exothermic operating condition, an outlet temperature (middle line vertically) is above an inlet temperature (bottom line vertically). Using the outlet temperature as a set point for the inlet temperature (i.e. a target to which the inlet temperature is attempting to achieve, for example by using a heat exchanger and potentially trim heaters), the two temperatures eventually equalize - i.e. over time the inlet temperature will rise and approach and eventually equal the inlet temperature. However, the outlet temperature also rises over time since the input temperature is higher (thus warming the stack overall) and since the stack is operating in an exothermic operating condition whereby the output heats up versus the input.

[0120] As the outlet temperature rises, the set-point for the inlet temperature also rises. The stack is thus heating up. The resulting increasing stack temperature in turn causes the area specific resistance (ASR) of the cells 11 within the stack 12 to reduce (i.e. the electrical resistance reduces). This is due to the underlying physics. As such, as the stack temperature rises the cell voltage required to maintain a constant current decreases,. The voltage (and thus the degree of over-potential) is thus also dropping (see the top line) and the result is that degree of exotherm reduces, as that is linked to the current (fixed) and the voltage (varying): in other words, as the degree of over-potential reduces, the exotherm reduces. The result of this is that the outlet temperature increases more slowly than the inlet temperature, and thus, eventually, the outlet stack temperature and the inlet stack temperature converge to the same temperature (isothermal). In this example this occurs as the voltage reaches the thermoneutral voltage for the cells. In typical solid oxide electrolyser cells, this thermoneutral voltage is about 1.28V per cell - usually irrespective of the final (substantially common) temperature of the inlet and the outlet. This is thereafter a steady state operating point, where inlet and outlet temperatures are the same and cell voltage has reduced to a steady state thermoneutral voltage.

[0121] Figure 5 instead shows the GTN strategy’s response to the alternative initial condition in which the stack 12 is operating in an endothermic operating condition with the outlet temperature (middle line) below the inlet temperature (top line). As with Figure 4, this configuration uses the outlet temperature as the set point for the inlet temperature, so the inlet temperature instead now falls. The outlet temperature also now falls due to both the lower inlet temperature and the endothermic nature of the stack 12. The decreasing stack temperature in turn causes the ASR of the cells 11 to increase meaning that as temperature decreases the cell voltage required to maintain a constant current increases. The degree of endotherm thus also reduces as the degree of under-potential is reduced. Therefore the rate of change in temperature of the outlet is lower than the inlet, and thus eventually the stack stack temperatures converge. Again, in this example this occurs as the stack voltage converges to the thermoneutral voltage for that particular cell type - again 1.28V in this example. Thus again the steady state operating point is achieved, where the inlet and outlet temperatures are the same and cell voltage is thermoneutral.

[0122] In these examples an idealised case has been depicted where the stack is able to operate with 100% Faradaic efficiency during the conversion process. In practice there will always be some losses, which manifest as heat release, so isothermal stack boundary conditions will be achieved at a cell voltage slightly below the theoretical thermoneutral voltage for the operating temperature. Operation at the theoretical thermoneutral voltage therefore tends to give outlet temperatures slightly higher than inlet temperatures (in some examples up to 4 degrees C).

[0123] These principles have been adopted by the present inventors to allow higher or lower operating points to be achieved more quickly than using conventional inlet temperature control. These higher or lower operating points allow a stack to produce more or less off-gas from input fuel (steam or carbon dioxide usually) as each operating point has a different electrolytic conversion rate. This is useful since during system operation, there will be situations where the operating current and the corresponding hydrogen production rate needs to be changed. This may be because the system is harvesting available power to generate hydrogen from steam, and the amount of available power changes, or because the amount of hydrogen demanded by a downstream consumer has changed, or due to a need to compensate for downtime on other stacks 12 within the system - for example due to system repair, servicing or upgrades.

[0124] For all of these situations it is an advantage if the transition from one operating point to another can be made quickly. The present invention enables the change to be made more quickly than if relying just upon the rate at which the system’s operating temperature can be changed based purely on controlling input temperatures via the heat exchangers, trim heaters or heat exchanger bypasses or fluid coolers.

[0125] The present inventors realised that by controlling the electrolyser system or the electrolyser or the stack between a galvanostatic thermoneutral condition (isothermal) and one or more endothermal or exothermal conditions, rapid load point changes can be achieved. Controlling the voltage can trigger current changes (or vice versa), which in turn trigger endothermal or exothermal conditions and thus a rapid change in the outlet temperature. The inlet temperature is then allowed (or set) to change to substantially match the outlet fluid temperature. This can be achieved by passing outlet fluids through heat exchangers that are connected to the inlet fluids and / or by trim heaters.

[0126] The rapid change in temperature in the stack causes the stack’s ASR to change. That causes either the current to change further (if a current control system allows that) or the voltage changes further - albeit back towards its initial value. As a constant current is ultimately the target - as galvanostatic thermoneutral conditions are the target, it will advantageously be the voltage that changes at this time. Both may change in some embodiments. The current defines the Hydrogen production rate, so if more or less hydrogen production is required, the current demand is modified and the new demand is met by a corresponding change in voltage.

[0127] This can be thought of as having an independent variable being the current, and the voltage is the dependent variable (because its value changes with the current and power). As an alternative, it can be the voltage that is the independent variable, and the current is dependent thereto. The delivered power, however, controls the relative values of the current and voltage. With the present invention, however, the inlet and outlet temperatures are caused to substantially match and that forces the voltage to return to a thermoneutral condition - with the thermoneutral voltage and a given current level for that temperature.

[0128] The power supplied to the stack can thus be controlled by varying the current or the voltage (either directly or via a change in voltage or current, respectively) to trigger the chain reaction above, before the voltage is brought back towards a steady state thermoneutral operating point, with the stack at a different temperature, and thus with a different current level (and in a galvanostatic condition). Therefore, the stack will now be operating at a new power level, with a different electrolytic conversion rate, yet still in a galvanostatic thermoneutral condition. Indeed, the initial voltage drop can be targeting a target new current - that final different current level.

[0129] In the case where the system needs to move to a lower power operating point with a lower hydrogen production rate, the current demand from the power supply can be reduced. To achieve this, a downward ramp is applied to the voltage across the stack by the system’s (or the electrolyser’s or the stack’s) power, current or voltage control system 108 - see Figure 3. This drop is shown in Figure 6:

[0130] Figure 6 shows that the system 20 was initially operating at a galvanostatic thermoneutral condition - i.e. a steady state condition with substantially isothermal operation at 540 degrees C for the inlet and outlet temperatures. See point 1 in Figure 6 and the y axis scale. The reduction in current set point causes a corresponding drop in voltage, moving the stack from a thermoneutral condition to an endothermic operating mode.

[0131] As the cell voltage is now below the thermoneutral voltage, the stack is put into an endothermic operating mode and so the outlet temperature drops rapidly. This achieves a minimum temperature at point 4 in Figure 6. This minimum temperature is usually below the final steady state operating temperature for the stack when steady state operation at the reduced power level is achieved as the endothermic condition is at its most strong in the initial phase - up to around point 3, whereby the electrolysis process is drawing the most heat energy from the system at this early stage, and this is before the inlet temperature has dropped down to its final operational temperature. The lowest outlet temperature occurs after then - at point 4. It should be noted that there is a lag time between the cell operation change (i.e. the change to endothermic state) and the change in temperature at the fluid at the inlet; this is due to the thermal inertia of the system. Figure 6 also shows that the voltage recovers and ultimately returns to the original voltage, so thermoneutral operation is restored at the new operating point - i.e. the electrolyser cell stack normalises again into a thermoneutral condition that provides substantially isothermal operation of the stack again. During that voltage recovery, however, the current is not reinstated to the previous level. Instead it maintains the new set point - dictating the new power level.

[0132] For this embodiment, the electrolyser is a solid oxide electrolyser and the thermoneutral voltage is 1.28V per cell, as also shown in Figure 6 by the alternative y axis scale. The system then needs to be dropped to a lower power level, so there is a request to change the current set point at point 2. This is achieved by causing the voltage to ramp down to point 3. In this example it is dropped to 1.2V per cell. However, for smaller drops in power, the voltage drop may be less and for higher power drops the drop in voltage may be more. Preset voltage drops may be configured into the processor and memory for achieving target power level drops - i.e. for target final currents (or current set-points). As the voltage always wants to return to the thermoneutral voltage - 1.28V for this embodiment, it is the current that changes to reflect the change in power).

[0133] In this embodiment, therefore, the voltage to which the power is reduced is chosen to satisfy the intended new current set-point, and may be a preset voltage stored in the memory 112 of a processor or computer 110 (see Figure 3) or a current known by the operator to achieve that changed power level.

[0134] Returning to point 3 of Figure 6, as the outlet temperature reduces, the GTN control strategy discussed above with reference to Figure 5 reduces the set point for the inlet temperature to match the instantaneous outlet temperature. The set point for the inlet temperature thus varies (drops) over time.

[0135] As the inlet temperature for the stack 12 decreases, the operating temperature off the cells 11 in the stack 12 also reduces, which in turn increases the electrical resistance of the cells 11. This causes the voltage to rise as the system tries to achieve or maintain the set point current.

[0136] As the voltage has been rising again in order to return to the thermoneutral voltage, this progressively reduces the level of endotherm. As a result, the difference between the inlet and outlet temperatures will eventually start to decrease - from point 4 in Figure 6. As a consequence, the inlet and outlet temperatures converge again at point 5, wherafter a steady state thermoneutral operation is restored at the new operating power point - here having a lowered temperature of 530 degrees C. This is illustrated at point 6, where the voltage is stable at 1.28 V and the isothermal temperatures occur across the stack (as the inlet and outlet temperatures are equal).

[0137] As illustrated in Figure 6, reducing the power load through the stack 12 can cause a rapid drop in outlet temperature. The size of the load step that can be made may be limited by the minimum stack operating temperature. For solid oxide electrolysers this may be around 400 degrees C to 450 degrees C, although it will depend upon the chemistries involved in the solid oxide electrolyser cell. To avoid dropping the temperature bellow such a minimum stack operating temperature, for larger drops in load this may need to be done in stages. For example, the voltage and current drop may involve a lesser current drop initially - reducing the current in an initial step to a point where a minimum stack outlet temperature is potentially reached but not crossed, and then reduce current more - towards the intended set point - only gradually or in further steps, while the bulk stack temperature reduces, until the set point is achieved. The voltage can then have enough time to recover enough to prevent too great an endothermic load on the cells, thus keeping the stack outlet temperature above a lower threshold.

[0138] The minimum load point for thermoneutral operation may be defined by the load at which the uniform stack temperature is reduced to the minimum stack outlet temperature. However, in practice it is unlikely that this load point would be used often. More normal would be for the system 20 to adopt a more efficient load point.

[0139] Figure 7 considers the opposite situation, where the system 20 needs to move to a higher power operating point with an increased hydrogen production rate. For this the current demand from the power supply can be increased using a relatively fast ramp rate. Initially the higher current is achieved by increasing the voltage applied to the stack from the steady state thermoneutral condition at point 1 to an overpotential and thus exothermic operating mode at point 2. As with Figure 6, the voltage later returns to its thermoneutral operation (shown at point 6), but this is at the new operating point - i.e. with revised inlet and outlet temperatures of 540 degrees C from the initial 530 degrees C.

[0140] This voltage increase has thus, in this example, reinstated the initial condition of Figure 6.

[0141] Still referring to Figure 7, a request to change the current set point causes the voltage to ramp up to point 3 where the voltage has increased to the required level to satisfy the new current set-point. As the cell voltage is now above the thermoneutral voltage, the stack is in an exothermic operating mode and so the outlet temperature rises rapidly towards a maximum point at 4 as the electrolysis process rejects heat energy to the system, thus increasing the cell temperature and the outlet temperature.

[0142] As the outlet temperature rises, the GTN control strategy increases the set point for the inlet temperature to match the instantaneous outlet temperature, so the inlet temperature also starts to rise.

[0143] The increasing cell temperature after point 3 reduces the electrical resistance of the cells and so the voltage starts to fall again in order to maintain the current set-point - this being the converse to that of Figure 6.

[0144] The voltage reduction back towards thermoneutral voltage progressively reduces the level of exotherm in the cells. As a result, the difference between the inlet and outlet temperatures starts to decrease again around point 4 prior to the temperatures converging again at point 5, whereafter there is once again a steady state galvanostatic thermoneutral operation - see point 6.

[0145] Just as the dropping temperature in Figure 6 should not cross a minimum temperature threshold, an increasing temperature should not cross an upper threshold - solid oxide electrolyser cells, for example rarely work above 700 degrees C - and commonly the limit is 650 degrees C, although the actual temperature threshold will vary between different electrochemistries on the cells. This means that in practice the size of the load step that can be made may be limited by the maximum stack operating temperature limit. This means that large steps in load may need to increase current in a smaller initial step, during which to the maximum stack outlet temperature is potentially reached but not crossed, and then there can be subsequent gradual or further steps as the bulk stack temperature rises. The avoidance of crossing the temperature threshold is thus again achieved. This can be important as subjecting the cells to excessive over potential I voltages, even for only a limited period of time, can be detrimental to stack durability.

[0146] It should also be noted that the maximum (or minimum) voltage is not necessarily a constant value, but may be a function of the operating temperature: higher operating temperatures usually act to reduce the tolerance of the stack to high voltages. Having the monitoring of the inlet and outlet temperatures - as shown in Figure 4 - can thus be used by the power, current or voltage control system to allow the processor 110 to control the changes to the voltage - and potentially also the current if needed, based upon stored set point data and thresholds in the memory 112.

[0147] Further, it can be that the maximum load point for thermoneutral operation may be defined by the load at which a uniform stack temperature and the thermoneutral voltage coincide with the safe operating envelope of the stack.

[0148] Referring finally to Figure 8, operation of a further embodiment of the present invention is illustrated.

[0149] The inventors noted that an undesirable characteristic of the response of the system 20 to the change in the current demand set point is that the outlet temperature can significantly overshoot (or undershoot) the target stack temperature. For example, in Figure 6, the outlet temperature falls to as low as 525 degrees C before increasing back up to the target temperature of 530, i.e. it undershoots, or shoots to under, the target temperature). As for the example of Figure 7, the outlet temperature instead overshoots the target temperature - shooting beyond the target temperature of 540 degrees C to 545 degrees C. Such over or undershooting of the target can create a problem towards upper and lower operational thresholds, as the stack temperature must not exceed those thresholds - as discussed above.

[0150] The present inventors devised a modification to the strategy of Figures 6 and 7 to reduce incidences of (or the extent of) over and undershooting of target temperatures. The modification is to employ a stack temperature feed forward control, which can reduce these temperature deviations.

[0151] It is established from conventional thermoneutral operations that adjusting the stack operating temperature alone is an effective way to transition between power load set points. The adoption of the GTN strategy as discussed with reference to Figures 6 and 7 achieves this transition more quickly by perturbing the voltage away from thermoneutral voltage, and thus generating endothermic or exothermic conditions within the cells to speed up the temperature changes. However, both approaches still converge to the same final operating temperature point for a given current set point (at thermoneutral voltages). It follows that temperature deviations during the transition can be reduced by combining the conventional approach with the present approach. In this modification, rather than the voltage and the current being adjusted first and waiting for the changing stack temperature set points to return the voltage back to the GTN point, a stack temperature set point is immediately increased (or decreased), in parallel with the current and voltage set point change, to an estimated temperature corresponding to the final target temperature for the desired power load set point. Adjusting the stack inlet temperature set point to that target temperature allows the stack inlet temperature to change to that final temperature more quickly. This also accelerates the change in the stack’s internal temperature and thus the overall stack resistance. That in turn enables more (or less) current to flow for the same voltage set point. This acts to reduce both the change in voltage and the change in outlet temperature during the load transition before steady state operation resumes.

[0152] Referring to Figure 8, the process of Figure 6 is overlaid (dotted lines) on the modified process for reference. This allows a comparison of the respective voltage and temperature lines of this modification (in solid lines).

[0153] At point 1 the system 20 is operating at a thermoneutral voltage and the inlet and outlet temperatures are equal and in a steady state. At point 2, a request to change the current set point is placed, causing the inlet temperature set point (heavy broken lines) to change to the estimated feed-forward value at point 3. As can be seen, this aligns with the final temperature at point 8, and can be a value stored in the memory 112 for automated processing, can be in a look up table, or can be in the knowledge of the operator. This causes the inlet temperature immediately to start to drop - much faster than in Figure 6, as in Figure 6 it reduces in response to the dropping outlet temperature instead.

[0154] At the same time, the voltage (top line) is ramped down to point 4, where the voltage has dropped to the required level to satisfy the new current set-point. This won’t be as low as in Figure 6 as the inlet temperature is already reducing, and thus the electrical resistance of the cells is increasing already.

[0155] As the cell voltage is now below the thermoneutral voltage, the stack 12 is put into an endothermic operating mode and so the outlet temperature drops as before, but this is now more rapid in response to the also reducing inlet temperature (as the inlet temperature set point was also reduced).

[0156] In this example, the reducing outlet temperature crosses the inlet temperature set-point at point

[0157] 5. At that point it is optional but preferred that the GTN control strategy resumes and the inlet temperature set point is again defined by the instantaneous outlet temperature. This helps to continue the cooling of the inlet temperature in a faster manner.

[0158] The outlet temperature continues to reduce after point 5 until it achieves its lowest temperature at point 6.

[0159] All the time the reducing cell temperature will be increasing the electrical resistance of the cells, and so the voltage starts to rise again after its initial drop in voltage in order to maintain the current set-point. Galvanostatic operation is thus maintained once the target current is achieved.

[0160] The voltage rises back towards its original thermoneutral voltage to progressively reduce the level of endotherm. As a result, the difference between the inlet and outlet temperatures decreased again from point 6, and converge to a substantially common temperature - the target temperature - at point 7. As can be seen from the broken lines, point 7 occurs much sooner than the corresponding point 6 in Figure 6. Thereafter, as with Figure 6, a steady state thermoneutral operation is restored, but at the new power load operating point. This is shown at point 8 in Figure 8.

[0161] 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.

[0162] 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.

[0163] 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).

[0164] 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.

[0165] 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

[0166] 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.

[0167] The present invention has therefore been described above purely by way of example with reference to the accompanying drawings. Modifications in detail may be made to the invention within the scope of the claims as appended hereto.

Claims

CLAIMS:1 . A method of controlling an electrolyser system to change an electrolytic conversion rate within at least one electrolyser cell stack of the electrolyser system, the electrolyser system comprising at least one electrolyser, the at least one electrolyser comprising the at least one electrolyser cell stack, and the at least one electrolyser cell stack comprising a plurality of electrolyser cells, at least one fluid inlet to the electrolyser cells and at least one fluid outlet from the electrolyser cells, wherein the electrolyser system further comprises: a power control system for changing the electrolytic conversion rate within the at least one electrolyser cell stack, the method comprises: i) using the power control system to change the voltage across the at least one electrolyser cell stack; ii) allowing a fluid outlet temperature to change in response to said change in voltage; iii) setting a fluid inlet temperature to substantially match said changed fluid outlet temperature; and iv) allowing the voltage to revert towards its initial value so as to normalise the electrolyser cell stack at a substantially isothermal operation with the at least one electrolyser cell stack at a changed stack temperature, and thus a changed stack resistance and thus a changed stack current and a changed electrolytic conversion rate.

2. The method of claim 1 , wherein the electrolyser system further comprises a processor and memory circuit for storing a plurality of preset current, voltage or power delivery changes for the control of the at least one electrolyser cell stack, and when the method uses the power control system to change the voltage across the at least one electrolyser cell stack, the power control system changes the current, the voltage or the power to one of the plurality of preset current, voltage or power delivery changes.

3. The method of claim 2, wherein the preset change is chosen for a target temperature change of the at least one electrolyser cell stack.

4. The method of claim 3, wherein there are a plurality of temperature changes in the memory.

5. The method of any one of claims 2 to 4, wherein the memory circuit also stores information about current set points and / or input temperature set points.

6. The method of any one of claims 2 to 5, wherein the memory circuit stores additional target temperatures and operation temperature thresholds for the electrolyser cell stack or electrolyser cells, and conditional voltage or current set points for avoiding crossing those thresholds.

7. The method of any one of claims 2 to 6, wherein the preset changes, or target stack temperatures associated therewith, or target stack currents associated therewith, are modified within the memory over time to reflect expected, detected or approximated states of deterioration of the stack.

8. The method of any one of the preceding claims, wherein, as the outlet temperature rises, an inlet temperature set point for the inlet temperature is set to match instantaneous outlet temperatures.

9. The method of any one of the preceding claims, wherein, when the power control system changes the voltage across the at least one electrolyser cell stack, a stack inlet temperature set point is set at an estimated or preset temperature corresponding to a final target temperature for the changed stack temperature.

10. The method of any one of the preceding claims, wherein the electrolyser system, or the electrolyser cell stack thereof, is operating at a thermoneutral operating point before the voltage change is made.11 . The method of any one of the preceding claims, wherein a normal operating mode of the electrolyser system is a substantially isothermic condition.

12. The method of any one of the preceding claims, wherein a normal operating mode of the electrolyser system involves a cell voltage of 1.28V and the cells are solid oxide electrolyser cells.

13. The method of any one of the preceding claims, wherein upon normalisation of the electrolyser cell stack, the substantially isothermal operation of the at least one electrolyser cell stack is a galvanostatic thermoneutral condition for the electrolyser cell stack.

14. The method of any one of the preceding claims, wherein if a user demands a drop in the electrolytic conversion rates, the power control system causes an initial drop in voltage.

15. The method of any one of the preceding claims, wherein if a user demands an increase in the electrolytic conversion rates, the power control system causes an initial increase in voltage.

16. The method of any one of the preceding claims, wherein current, voltage or power changes are done in stages.

17. The method of claim 16, wherein an initial current, voltage or power change involves a lesser change initially to change the current or voltage in an initial step to a point where an operational inlet or outlet temperature threshold and / or an internal cell temperature threshold is not crossed, and then a subsequent step changes the current, voltage or power some more towards the intended current, voltage or power set point.

18. The method of any one of the preceding claims, wherein the electrolyser system comprises an inlet temperature monitoring and / or control system for monitoring and / or controlling an inlet temperature.

19. The method of any one of the preceding claims, wherein the electrolyser system comprises an outlet temperature monitoring and / or control system for monitoring and / or controlling an outlet temperature.

20. The method of any one of the preceding claims, wherein a current detection circuit is provided to detect and optionally control an input current for powering the electrolyser cell stack.

21. The method of any one of the preceding claims, wherein a voltage detection circuit is provided to detect a voltage across the electrolyser cell stack.

22. The method of any one of the preceding claims, wherein an inlet temperature detection circuit and / or an outlet temperature detection circuit is provided to detect an inlet temperatureand / or an outlet temperatures from the fluids entering or exiting the respective inlet(s) or outlet(s) of the electrolyser cell stack.

23. The method of any one of the preceding claims, comprising setting a maximum stack voltage and / or a minimum stack voltage for the or each stack.

24. The method of any one of the preceding claims, wherein the method is applied to each electrolyser cell stack of the electrolyser separately.

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

26. The method of any one of the preceding claims, wherein the or each electrolyser is used to electrolyse water in the form of steam.

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