Warm up method for an electrolyser system

Abstract

A method of warm up of an electrolyser system comprising one or more stacks of electrolyser cells, each of the one or more stacks with fuel and oxygen volumes. The method comprising heating the one or more stacks to raise the one or more stacks to a first threshold temperature T1. A heat transfer fluid is provided to the fuel volume of each of the one or more stacks when the temperature is above first threshold temperature T1. The temperature of the heat transfer fluid is incrementally increased to further heat the one or more stacks above the first threshold temperature T1. When a second threshold temperature, T2, is reached, fuel is provided to the fuel volume of each of the one or more stacks and electrical current to each of the one or more stacks to generate product via electrolysis.

Description

[0001] WARM UP METHOD FOR AN ELECTROLYSER SYSTEM

[0002] Field of the Invention

[0003] The present invention relates to warm up methods for electrolyser systems. The electrolyser system comprises at least one stack of electrolyser cell units, which may include cell units of solid oxide or alkaline electrolyser cells. The present invention more specifically relates to solid oxide electrolyser cell (SOEC) units, and these may include metal-supported solid oxide electrolyser cell (MS-SOEC) units.

[0004] Background to the Invention

[0005] Some fuel cell units can produce electricity by using an electrochemical conversion process that oxidises fuel to produce electricity. Some fuel cell units can also, or instead, operate as regenerative fuel cells (or reverse fuel cells) units, often known as electrolyser cell units, for example to separate hydrogen and oxygen from water, carbon monoxide and oxygen from carbon dioxide, or nitrogen monoxide and oxygen from nitrogen dioxide. They may be tubular or planar in configuration. Planar fuel cell units may be arranged overlying one another in a stack arrangement, for example 100-400 cell units in a stack, with the individual fuel cell units arranged, for example, electrically in series. Tubular cell units may be arranges in groups or stacks thereof.

[0006] A solid oxide fuel cell (SOFC) unit that produces electricity is based upon a solid oxide electrolyte that conducts negative oxygen ions from an oxygen electrode to a fuel electrode located on opposite sides of the electrolyte. For this, a fuel, or reformed fuel, contacts the fuel electrode and an oxidant, such as air or an oxygen rich fluid, contacts the air electrode. A solid oxide electrolyser cell (SOEC) may have the same structure as an SOFC but is essentially that SOFC operating in reverse, or in a regenerative mode, to achieve the electrolysis of fuel, for example water and / or carbon dioxide, by input of electrical energy and using the solid oxide electrolyte to produce hydrogen gas and / or carbon monoxide and oxygen. Conventional ceramic-supported (e.g. fuel electrode-supported) SOFCs and SOECs have low mechanical strength and are vulnerable to fracture. Hence, metal-supported SOFCs and SOECs have been developed which have the active cell component layer supported on a metal substrate. In these cells, the ceramic layers can be very thin since they only perform an electrochemical function: that is to say, the ceramic layers are not self-supporting but rather are thin coatings / films laid down on and supported by the metal substrate. Such metal supported SOEC stacks are more robust, lower cost, have better thermal properties than ceramic-supported SOECs and can be manufactured using conventional metal welding techniques.

[0007] Electrolyser cell stacks operate at elevated temperatures; intermediate or high temperature electrolysers (such as SOEC and alkaline electrolyte) have operational temperatures in excess of 400 °C, typically 450 °C to 700 °C for an intermediate temperature electrolyser such as one based on an MS-SOEC. Electrolyser cell stack(s) are placed in electrolyser systems and the electrolyser system supplies input gas to and exhausts off-gas from the electrolyser cell stack(s) at the required operational temperatures. Electrolyser systems, and stack(s) therein, need to be heated from ambient or standby temperatures to enable operation. It is advantageous to reduce warm up time to reduce system downtime and maximise efficiency, both in utilisation of the system and in reducing resource usage during warm up. However, cell stacks may be sensitive to thermal shock and / or thermal imbalances therein.

[0008] The present invention seeks to provide warm up methods for electrolyser systems which reduce the warm up time while heating stack(s) within said systems in a balanced manner.

[0009] SUMMARY OF THE INVENTION

[0010] In a first aspect there is provided a method of warm up (which may also be referred to as start up) of an electrolyser system comprising one or more stacks of electrolyser cells, each of the one or more stacks with fuel and oxygen volumes. The method comprises: heating the one or more stacks to raise the one or more stacks to a first threshold temperature Tl; providing a heat transfer fluid to the fuel volume of each of the one or more stacks when the temperature is above first threshold temperature Tl; increasing the temperature of the heat transfer fluid to further heat the one or more stacks above the first threshold temperature Tl; and when a second threshold temperature, T2, is reached, providing fuel to the fuel volume of each of the one or more stacks and electrical current to each of the one or more stacks to generate product via electrolysis (i.e., an electrolysis reaction).

[0011] The method allows an electrolyser system - and the stacks therein - to be heated to an operational temperature. Provision of the heat transfer fluid enables the stack(s) to be heated with low latency while also minimising thermal stresses and imbalances in the stacks because the heat is provided thereto and at a temperature that is incrementally increased. Flexibility in heating the system is provided in heating the stack(s) to the first threshold temperature, Tl, before use of the heat transfer fluid. The first threshold temperature may be at least 100 °C and less than 300 °C. The former to allow the heat transfer fluid to comprise water without condensation in the system, and the latter to promote timely warm up using the heat transfer fluid. The second threshold temperature, T2, may be at least 400 °C, but is preferably relatively low to enable electrolysis to begin early in the method, thereby producing reduced products of the electrolysis which in turn reduce potential for oxidation of components by fuel (e.g., steam). The second threshold temperature, T2, may be between 400 and 600 °C, optionally between 450 and 550 °C. Further, the stack(s) can be operated in exothermic conditions such that the electrolysis reaction (and provision of current thereto) further assists warmup to a desired steady state operating temperature. The temperature of the heat transfer fluid may be incrementally increased during the further heating of the one or more stacks above the first threshold temperature Tl (i.e., increased in increments, e.g., of between 20 and 100°C)

[0012] The electrolyser cells may be based on a solid oxide electrolyte and so are solid oxide electrolyser cells (SOEC). The electrolyser cells may be metal-supported electrolyser cells (e.g., MS-SOEC), which aids stability of said cells. When the second threshold temperature is reached, the flow of heat transfer fluid may be stopped (unless the heat transfer fluid is the fuel used in the electrolysis reaction). The temperatures referred to may be a temperature of the stack(s). The temperature of off-gas (from the fuel or oxygen volumes of the stack(s)) may be used as the temperature of the stack(s) or the temperature of the stack(s) derived therefrom. The stack(s) may be arranged in a common enclosure or vessel, preferably a pressure vessel, and references herein to each may be used interchangeably. Heating the stack(s) may also comprise heating the enclosure or vessel. It may be assumed that the temperature within the enclosure or vessel is the same as that of the stack(s) and the temperature of either may be measured. References to heating the stack(s) may be replaced with references to heating the enclosure or vessel and vice versa.

[0013] References to supply of a fluid or gas to the stack(s) may involve providing said fluid / gas to one or both of the fuel and oxygen volumes of the cell units unless otherwise specified. For example, the heat transfer fluid may be provided to either or both of the fuel and oxygen volumes, but preferably to the fuel volume because larger flow rates are typically achievable (little or no flow rate of fluid is provided to the oxygen volumes during steady state operation) and fuel may be used as the heat transfer fluid, which the fuel volume is adapted to be exposed to.

[0014] Preferably, the first threshold temperature, Tl, is between 100 °C and 300 °C. Optionally, the first threshold temperature is at most 300 °C, optionally at most 250 °C, optionally at most 200 °C, optionally at most 150 °C, optionally at most 125 °C. Preferably, the first threshold temperature, Tl, is between 125 °C and 175 °C., optionally between 125 °C and 150 °C. Optionally, the first threshold temperature is between 150 and 200 °C. A lower first threshold temperature reduced warm up time by allowing the heat transfer fluid to be provided from a lower temperature.

[0015] Preferably, heating the one or more stacks to raise the one or more stacks to the first threshold temperature comprises supplying a non-condensing gas to the one or more stacks (e.g., directly or via the vessel) to raise the one or more stacks to the first threshold temperature Tl. The noncondensing gas may itself supply heat to the stack(s) to provide said heating and / or the noncondensing fluid may act as a purge gas to remove contaminants in which case the heat may be provided by other means. Said non-condensing gas may heat the at least one stack to the first threshold temperature, Tl.

[0016] Preferably, the non-condensing gas is supplied to one or both of the fuel volume and the oxygen volume of each of the one or more stacks. Optionally, the non-condensing gas is supplied to the oxygen volume only. Optionally, the non-condensing gas is supplied to the oxygen and fuel volume, in such case the non-condensing gas may first be supplied to the fuel volume and recycled for supply to oxygen volume. Optionally, the non-condensing gas is supplied to the fuel volume only.

[0017] Preferably, the method comprises recirculating the non-condensing gas through the fuel volume and / or oxygen volume via a recirculation loop in which heat is transferred to the non-condensing gas. In this case, non-condensing gas is recirculated from an outlet of the fuel or oxygen volume to an inlet of the same volume during warm up, typically with heat added to the non-condensing gas through the recirculation. Preferably, the non-condensing gas comprises or consists of nitrogen or dry air. Both gasses are readily available in a typical system.

[0018] Preferably, supplying the non-condensing gas to the one or more stacks comprises attaching a temporary heater to the electrolyser system to supply the non-condensing gas (e.g., to heat the one or more stacks to Tl, optionally to between 200 and 300°C, optionally to T2, optionally to T3) and removing the temporary heater once Tl or T2 or T3 is reached. The temporary heater may supply the non-condensing gas to the enclosure or vessel directly or via one or more supply lines to fuel and / or oxidant volumes. The temporary heater may be sized to have a capacity to provide a fast warmup, said capacity being not needed for steady state operation, and therefore may result in reduced system cost and relatively fast warm up.

[0019] Preferably, when the first temperature threshold Tl is reached, the method comprises stopping the supply of non-condensing gas to the one or more stacks. In some cases, the supply of noncondensing gas may continue to one of the fuel and oxygen volumes above Tl. Preferably, supply of non-condensing gas to the fuel volume is stopped at Tl and supply of the heat transfer fluid to the fuel volume is started, meanwhile supply of non-condensing gas to the oxygen volume is continued. This reduces warm-up time by supply of heat to the stack(s) (and enclosure) using the oxygen volume, reduced thermal stress in the stack(s), and allows for the respective volumes to be pressurised (while balancing pressure difference between the two volumes) which enables faster warmup by the greater heat carrying capacity of pressurised fluid.

[0020] Preferably, heating the one or more stacks to raise the one or more stacks to the first threshold temperature comprises using resistive heating. Resistive heating may comprise use of a heat trace outside the at least one stack, or around the enclosure, or around pipes leading to the enclosure which heat fluid flowing through for delivery to the stack(s) or enclosure. Resistive heating by passing a current through the stack(s) may be used. Such heating may be from a temperature at which the electrolyser cells become conductive, e.g., above 350 °C for a ceria-based electrolyte, optionally above 400 °C.

[0021] Preferably T2 is at least 350 °C, optionally at least 400 °C, optionally at least 450 °C. Preferably T2 is less than 700 °C. T2 may be between 350 °C and 700 °C, The electrolyte of the electrolyser cell units may comprise ceria in which case T2 may be at most 600 °C, optionally at most 550 °C.

[0022] Preferably the heat transfer fluid comprises a fuel (i.e., a fuel for electrolysis), preferably comprising or consisting of steam. Steam has a relatively high heat capacity and so promotes fast warm up. Preferably the fuel used as heat transfer fluid is also - or comprises - the fuel used in steady state operation.

[0023] Preferably, when the temperature of the at least one stack is between a third threshold temperature T3 and the second threshold temperature T2, where T3 is greater than Tl and less than T2, the method comprises supplying fuel to the fuel volume of each of the one or more stacks, and operating the at least one stack in an electrolysis mode at an exothermic voltage. The fuel may comprise or consist of steam or carbon dioxide, and advantageously may be the same as the heat transfer fluid, especially in the case of the heat transfer fluid being steam which is a fuel for electrolysis and has a relatively high heat capacity. An exothermic voltage is a voltage greater than the thermoneutral voltage. As T3 is less than the operating temperature T2, the current supplied in said operation is lower than a steady state current supply, for example less than 50% of the steady state current supply, optionally less than 25% of the steady state current supply. At this stage an oxidant or nitrogen may be supplied to the oxygen volume to continue to heat the oxygen volume (and stack(s)) by transfer of heat from the oxidant or nitrogen to (surrounds of) said volume.

[0024] Alternatively, when the temperature of the at least one stack is between a third threshold temperature T3 and the second threshold temperature T2, where T3 is greater than T1 and less than T2, the method may comprise operating the at least one stack in a fuel cell mode. The fuel cell reaction is exothermic thereby suppling heat to increase the temperature of the stack(s). Fuel cell mode may be realised by supplying a fuel (e.g., containing hydrogen, a hydrocarbon, or ammonia) to the fuel volume and oxidant (e.g., oxygen or air) to the oxygen volume of the cell units, and generating power by the stack(s). Product of the fuel cell reaction released into the fuel volume is hydrogen, which provides a reducing atmosphere, in turn promoting reliability of the stack(s).

[0025] Preferably, when the temperature of the at least one stack is between the third threshold temperature T3 and the second threshold temperature T2, where T3 is greater than T1 and less than T2, the heat transfer fluid comprises a reducing gas. Preferably, the method further comprising supplying an oxidant or nitrogen to the oxygen volume of each of the one or more stacks. The reducing gas may comprise hydrogen, and the reducing gas prevents oxidation of components in the fuel volume, for example it may prevent oxidation of a metal (or metal oxide) in the fuel electrode. The oxidant may be the same as the non-condensing gas and may be oxygen or air. Power may be generated by the stack(s) if the reducing gas can be used by the stack(s) as a fuel in fuel cell operation. The fluids supplied to the fuel and oxygen volumes may be pressurised.

[0026] Preferably, the reducing gas comprises hydrogen gas. Preferably, the hydrogen gas is supplied from a tank in the electrolyser system that is fillable by previous operation of the electrolyser system. Alternatively or additionally, a stack or group of stacks running in a product generating mode may supply hydrogen gas to the one or more stacks subject to the warm up method. This means that supply of hydrogen gas is built in to the electrolyser system, thereby reducing the need for supply from external sources. The reducing gas may additionally or alternatively comprise carbon monoxide, which may act (on said components of the stack) as reducing gas itself or may reduce steam to hydrogen, thereby supplying hydrogen as a reducing gas.

[0027] Preferably, the reducing gas has the same composition as the non-condensing gas.

[0028] Preferably, the heat transfer fluid supplied to the fuel volume of the at least one stack comprises between 1 and 25mol% of reducing gas in steam, optionally between 5 and 25mol% of reducing gas in steam, optionally between 8 and 15mol% of reducing gas in steam, optionally between 1 and 5mol% of reducing gas in steam.

[0029] Preferably, the third threshold temperature, T3, is between 300 °C and 500 °C, optionally between 300 °C and 400 °C, optionally between 320 °C and 360 °C. Optionally, third threshold temperature is at least 300 °C, optionally at least 400 °C, optionally at least 440 °C, optionally at least 470 °C. Optionally, the third threshold temperature is at most 490 °C. Note that T3 is less than T2. Preferably, a purge gas (e.g., comprising or consisting of N2) is supplied to the fuel volume (optionally also the oxygen volume) at a temperature less than or equal to Tl, before supply of the heat transfer fluid, optionally from or at an ambient temperature, optionally once Tl is reached and after stopping flow of the non-condensing gas in the fuel volume and before starting flow of the heat transfer fluid.

[0030] The fluid(s) supplied to the fuel and / or oxidant volumes are heated to provide heat to the stack(s), heat may be transferred to said fluids using respective trim heaters (i.e., heaters configured for providing heat to the input fluid(s) during steady state operation / generation in electrolysis.

[0031] Preferably, the one or more stacks are arranged in a vessel enclosing a vessel volume. Preferably, the one or more stacks comprise a plurality of stacks, optionally at least 6 stacks, optionally at least 10 stacks, optionally at least 20 stacks.

[0032] Preferably, the vessel volume is in fluidic communication with one or more of the fuel volume and the oxygen volume of each of the one or more stacks during the method. In such cases, preferably the vessel volume is fluidically connected with one of fuel and oxygen volumes at outlet, optionally also inlet, of each stack. Preferably, the vessel volume is in fluidic communication with the oxygen volume of each of the one or more stacks. In other words, the oxygen volume of the plurality of stacks is open manifolded (i.e., not manifolded, and the oxygen outlet of each stack exhausts fluid from the oxygen volume into the vessel volume.

[0033] Alternatively, the fuel and oxygen volumes of each stack is fully manifolded and so neither volume is in fluidic communication with the vessel volume. In such cases, a vessel fluid is supplied to the vessel volume. The vessel fluid may comprise a purge gas. The vessel fluid may comprise or consist of nitrogen or air. The vessel may be provided with a vessel volume inlet and a vessel volume exhaust for supply to and exhaust from the vessel volume. The vessel fluid may supply heat to the vessel and stack(s) in the warm up method. The vessel volume may be pressurised to balance the fluid pressure within the fuel and oxygen volumes of the stack(s).

[0034] Preferably, the fuel volume of each of the one or more stacks manifolded. Preferably, fuel flow in plurality of stacks is in parallel.

[0035] Preferably, the vessel is a pressure vessel configured to withstand a pressure difference between the inside of the pressure vessel and an ambient pressure, wherein the method further comprises regulating (e.g., back pressure regulating) the pressure of at least one or both of the fuel volume and the oxygen volume (optionally also regulating pressure in the vessel volume independently or via fluidic communication between the vessel volume and one of the fuel and oxygen volumes) of each of the one or more stacks at a pressure greater than ambient pressure when the temperature of the one or more stacks is greater than the first threshold temperature, Tl. Alternatively, the pressure regulation may be when the temperature of the one or more stacks is greater than the third threshold temperature, T3, for example if there is no fluid supplied to the oxygen volume (through the inlet or by electrolysis).

[0036] For pressure regulation at a temperature above Tl, the heat transfer fluid may be used in the fuel volume and fluid supplied to the oxygen volume is used in said volume. For pressure regulation at a temperature above T3, the reducing gas may be also used in the fuel volume. Pressure regulation may be executed when fluids are supplied to or generated by electrolysis and released in to each of the fuel and oxygen volumes. For pressure regulation at a temperature below Tl, or if the pressure vessel has an inlet to the and outlet from the vessel volume, non-condensing gas may be used to pressurise the vessel volume.

[0037] The non-condensing gas may be supplied to the fuel and oxygen volumes (and vessel volume directly or indirectly, via open manifolding of one of the fuel and oxygen volumes) in order to pressurise the volumes from below Tl - i.e., to pressurise while heating the one or more stacks to Tl.

[0038] Once pressurised, the pressure is maintained or increased as the temperature increases including when switching from supply of one fluid to supply of a different fluid to the respective volumes (and once electrolysis may begin, by switching to using oxygen produced by the electrolysis reaction to pressurise the oxygen volume.

[0039] Preferably, the pressure of the at least one or both of the fuel volume and the oxygen volume is at least 0.5barg, optionally at least lbarg, optionally at least 1.5barg, optionally between 1.5barg and 3barg, optionally at most 10 barg, optionally at most 5barg.

[0040] Preferably, regulating comprises regulating the pressure of the fuel volume and the oxygen volume to balance pressure between the fuel volume and the oxygen volume (and optionally also the vessel volume either directly because the vessel volume is not in fluid communication with the fuel or oxygen volume or indirectly via fluidic communication between the vessel volume and one of the fuel and oxygen volume). The pressure may be balanced such that a difference in pressure between the volumes is at most 0.5 barg, optionally at most 0.2 barg, optionally at most 0.1 barg, optionally at most 0.05 barg. Balancing the pressure comprises controlling any pressure imbalance [i.e., to reduce or eliminate a pressure difference] across the cells by supplying one or more of fluid to the fuel or oxygen volume - or regulating the exhaust from said volumes.

[0041] Preferably, the electrolyser system comprises a controller configured to perform the methods discussed herein.

[0042] According to an aspect of the invention there is provided a controller configured to perform the methods discussed herein.

[0043] According to an aspect of the invention there is provided a computer program comprising instructions which, when the program is executed by a computer, cause the computer to carry out the steps of the method according to the aspects above. In particular, when the program is executed by a computer, cause the computer to heat the one or more stacks to raise the one or more stacks to a first threshold temperature Tl; provide a heat transfer fluid to the fuel volume of each of the one or more stacks when the temperature is above first threshold temperature Tl; (incrementally) increase the temperature of the heat transfer fluid to further heat the one or more stacks above the first threshold temperature Tl; and when a second threshold temperature, T2, is reached, provide fuel to the fuel volume of each of the one or more stacks and electrical current to each of the one or more stacks to generate product via electrolysis reaction. According to an aspect of the invention there is provided a non-transitory computer-readable medium with instructions stored thereon, that when executed by a processor, perform (or cause the processor to perform) the steps of the method of according to the aspects above. In particular, when executed by a processor, perform (or cause the processor to perform): heating the one or more stacks to raise the one or more stacks to a first threshold temperature Tl; providing a heat transfer fluid to the fuel volume of each of the one or more stacks when the temperature is above first threshold temperature Tl; (incrementally) increasing the temperature of the heat transfer fluid to further heat the one or more stacks above the first threshold temperature Tl; and when a second threshold temperature, T2, is reached, providing fuel to the fuel volume of each of the one or more stacks and electrical current to each of the one or more stacks to generate product via electrolysis reaction.

[0044] Particular and preferred aspects of the invention are set out in the accompanying independent claims. Combinations of features from the dependent claims may be combined with features of the independent claims as desired and appropriate and not merely as explicitly set out in the claims. The term "comprising" as used herein to specify the inclusion of components also includes examples in which no further components are present.

[0045] An enabling disclosure of the present invention, to one of ordinary skill in the art, is provided herein. Reference now will be made in detail to examples of the invention, one or more examples of which are set forth below. Each example is provided by way of explanation of the invention, and not limitation of the invention.

[0046] Brief Description of the Drawings

[0047] Features of the present invention will now be described in further detail, by way of various embodiments, and just by way of example, with reference to the accompanying drawings (which drawings are not to scale), in which:

[0048] Figure 1 is a simplified schematic view of an electrolyser system.

[0049] Figure 2 is a flow diagram depicting a method of warm up of an electrolyser system.

[0050] Figure 3 depicts examples of a warm up strategy for an electrolyser system.

[0051] Figure 4 depicts examples of a warm up strategy for an electrolyser system.

[0052] Figure 5 depicts examples of a warm up strategy for an electrolyser system.

[0053] Figure 6 is a simplified schematic view of an electrolyser system.

[0054] Figure 7 is a simplified schematic view of an electrolyser system.

[0055] Figure 8 is a simplified control device for controlling an electrolyser system. In the following figures and description like reference numerals will be used for like elements in different figures.

[0056] Detailed Description

[0057] Fig. la is a simplified schematic of an electrolyser system 100 including a plurality of electrolyser stacks 10 in a vessel 105. The vessel 105 may be a pressure vessel 105 configured to withstand a pressure difference between its interior and exterior. Four stacks 10 are shown in Fig. la, but it will be understood that any number of stacks may be present, including fewer and greater than 4, for example 1, 2, 10, 12 and so forth. The stacks 110 comprise a stack of electrolyser cell units. A typical stack may have 100 to 500 electrolyser cells units. The cell units are in fluidic communication with - and separate - a first fluid volume (typically for fuel) and a second fluid volume (typically for oxygen as a product of electrolysis).

[0058] Each stack 110 has a first fluid inlet 115 and a first fluid outlet 116 each of which is in fluidic communication with the first fluid volume of each cell unit. Supply to the first fluid volume is via the first fluid inlet 115 and exhaust from the first fluid volume is by the first fluid outlet 116. First fluid is provided to the first fluid volume of the stacks 110 from a first fluid source 143 via respective first fluid inlets 115. In the example of Fig. la, the first fluid is fully manifolded, that is there is a manifold providing fluidic communication between the first fluid source 143, external to the vessel 105, and the first fluid inlets 115 of the stacks 110. Likewise, there is a manifold providing fluidic communication between the first fluid outlets 116 of the stacks 110 and a first fluid volume off-gas collection 163, external to the vessel 105.

[0059] Each stack 110 may have a second fluid inlet 122 and has a second fluid outlet 123 which where present is in fluidic communication with the second fluid volume of each cell unit. Exhaust from the second fluid volume is by the second fluid outlet 123, optional supply to the second fluid volume is via the optional second fluid inlet 122. Optional second fluid is provided to the second fluid volume of the stacks 110 from a second fluid source 144 via respective second fluid inlets 122. In the example of Fig. la, the second fluid is open manifolded, that is there is a vessel inlet 120 to the vessel 105 for delivery of second fluid to the interior of the vessel 105 from the second fluid source, but there is no branched manifold to deliver the second fluid to respective second fluid inlets 122 of each stack 110. Likewise exhaust from the second fluid volume of the stacks 110 is open manifolded, that is there is a vessel exhaust 121 from the vessel 105 for exhaust of second fluid volume off-gas from the interior of the vessel 105 to the second fluid off-gas collection 164, but there is no branched manifold between the respective second fluid outlets 123 of each stack 110 and the vessel exhaust 121.

[0060] In some cases the second fluid may be partially manifolded - i.e., there may exist a manifold between the vessel inlet 120 and the respective second fluid inlets 122 of the stacks 110 or there may exist a manifold between the respective second fluid outlets 123 of the stacks 110 and the vessel exhaust 121.

[0061] In some cases both the inlet and exhaust of the second fluid may be manifolded, as depicted in Fig. lb, to ensure that any sweep gas supplied to the second fluid volume is effective and product released in to the second fluid volume is removed therefrom (particularly during start up, shut down and standby modes). In Fig. lb the electrolyser system 101, which is similar to the electrolyser system 100 of Fig. la, but includes inlet and exhaust manifolds for the second fluid which communicate with the second fluid source 144 and second fluid collection 164, respectively. Further, electrolyser system 101 has a vessel inlet 120 and a vessel exhaust 121 for a vessel fluid supplied to the vessel volume from vessel volume supply 165 (optionally via heater 152, which may be similar to heaters 150,151) and exhausted from the vessel volume to vessel volume collection 166. As such, the vessel volume is not in fluid communication with either the first and / or second fluid volumes. When the stack(s) are operated at elevated pressures, the vessel fluid in the vessel volume may be regulated to balance the pressure in the vessel volume with that in the first and / or second fluid volumes - in a similar manner to described elsewhere for balancing of pressure between the first and second fluid volumes. The vessel fluid may a relatively inert gas, and may be air or nitrogen.

[0062] However it is sometimes preferable for the second fluid to be open (as in Fig. la) or partially manifolded such that there exists fluidic communication between the second fluid volume of the stacks and the interior of the vessel 105 for pressure equalisation therebetween - in turn simplifying control strategies and pipework required for the respective fluids.

[0063] The first fluid volume is typically a fuel volume and, during steady operation, is for fuel - usually steam and / or carbon dioxide. In cases where the electrolyser cell units in the stacks 110 comprise an oxygen ion conducting electrolyte (e.g., solid oxide electrolyte cell units), the first fluid volume off-gas exhausted to the first fluid off-gas collection 163 comprises hydrogen and / or carbon monoxide (dependent on the fuel used) generated in the electrolysis reaction and unspent fuel, and the second fluid volume off-gas exhausted to the second fluid off-gas collection 164 comprises oxygen generated in the electrolysis reaction. The second fluid off-gas may be pure oxygen or oxygen enriched sweep gas. The second fluid volume off-gas may comprise at least 50% oxygen generated by the cells. The second fluid volume off-gas 164 may be substantially pure oxygen, although it might be oxygen enriched air or nitrogen if a sweep gas is also being used. Ideally, however, the oxygen is at least 90% pure oxygen when the system 20 is operating at a steady state operation, as the present invention is ideally operated without an externally sourced sweep gas - and hence just the one fluid input 115 to each stack.

[0064] Other than the fluid inlet(s) and outlets, the stack / electrolyser / vessel will have inputs (terminals, not shown) for power (for applying a current across the electrolyser cells in the stacks 110).

[0065] Operationally, a stack will usually want to avoid large pressure differentials across the cells, so a threshold pressure differential between the first and second fluid volumes can be set appropriately, or the first fluid side's pressure can be controlled to balance the pressures across the cells. In one example, the pressure differential can be controlled (and is kept as small as possible) by use of a common regulator system on both sides. However, in operation it is sometimes advantageous to control the pressure differential to be positive on second fluid volume side to ensure 02 leaks to fuel rather than the other way around. This is better for the stack life (less degradation). The electrolyser system 100 typically operates at an elevated temperature, for example 400-700 °C for cell units based on a solid oxide electrolyte. It will be appreciated that heat in the first fluid off-gas and the second fluid off-gas will typically be exchanged with (transferred to) the first fluid and, if present, second fluid prior to their delivery to the stacks 110, typically prior to their entry to within the boundary of the vessel 205 using one or more heat exchangers (but typically two or more heat exchangers such that heat is recovered from the first fluid volume off-gas and the second fluid volume off-gas). In steady state operation said heat exchange may be sufficient to maintain an operating temperature of the electrolyser system 100 alongside temperature control by varying power applied to the electrolyser cells. Heaters 150 and / or 151 (e.g., an electric heater, e.g., a trim heater) may be provided in the input stream of the first and / or second fluid, respectively, to provide additional heat to said fluid and for flexibility in the system. Said heaters may be sized for steady state requirements only, or may also be used for warm-up.

[0066] In steady state operation, the first and second fluid volume off-gases will usually be at a similar temperature to the operational temperature of the electrolyser cell. However, a specific delta from the input temperature will depend upon the amount of electrical power supplied to the electrolyser, and the internal resistance of the cells. The power is supplied to the stack(s) with a constant current. The stack is thus operated in galvanostatic conditions. The electrical resistance of the stack thus controls the voltage applied across the stack, and there is thus a variable power draw from the stack as the resistance changes. Alternatively the power supplied to the stack(s) is controlled potentiostatically.

[0067] Operational efficiency can be best improved by reducing the amount of external heat supplied to the system via the fluid temperature control system - i.e., via the heaters. Where that external heat is provided for free - for example as a waste product of another industrial process, then that external heat can be usefully used without cost - i.e. it provides financial efficiencies. However, if that external heat has an associated cost, then operational efficiencies would be better improved instead. At steady state this is achieved by using both galvanostatic conditions within the stack(s), i.e. a constant electrical current (constant amps), and by adopting thermoneutral voltages across the stack to avoid heat wastage in the stack(s), as at a thermoneutral condition the electrolyser is in an adiabatic state, i.e. it is balanced energetically which effectively means no heat is consumed or released.

[0068] In the prior art, it is known to use either constant voltages or constant currents on a stack, and then to control of the current or the voltage, respectively, to maintain the stack at a substantially thermoneutral condition. This then allows the stack to avoid overcooling or overheating, as when under voltage (for a galvanostatic stack - i.e. constant current), the stack shows endothermic characteristics, and it thus cools the fluid (and likewise the operational temperature of the stack), whereby the fluid output temperature is lower than the fluid input temperature, whereas when over voltage (for a galvanostatic stack - i.e. constant current), the stack shows exothermic characteristics, and it thus heats the fluid (and likewise the operational temperature of the stack), whereby the fluid output temperature is higher than the fluid input temperature. At steady state, the present invention uses a galvanostatic condition for the stack, but fluctuates as necessary between thermoneutral conditions, over voltage conditions and under voltage conditions, in response to input fluid temperature control that aims to maintain a fluid input temperature equal to a fluid output temperature. This is done since the present inventors realised that it is relatively straightforward to measure temperature at inlet and outlet of the stack and thus to instead control the thermoneutral voltage automatically. In other words, at steady state the temperature delta between the fluid output temperature from the stack and the fluid input temperature for the stack is minimised, aiming for a zero delta. This then allows extended use of a galvanostatic condition for the stack, alongside a thermoneutral voltage condition, even while the stack, or one or more of the cells therein, degrades.

[0069] Galvanostatic and thermoneutral (and galvanostatic thermoneutral) conditions such as those discussed above apply in steady state when the electrolyser system is at an operational temperature. Other steady state conditions or operating strategies, such as endothermic or exothermic may also be used depending upon characteristics of the cell units and / or of input and exhaust fluids available or desired, respectively.

[0070] Other strategies are required in order to raise the electrolyser system to the operational temperature. Further, in cases where the heaters 150 and / or 151 are present, they may only have the capacity for provision of trim heating at operational temperatures, e.g., to raise the heat of the input stream by 10-40 °C. Such heaters are unable to provide sufficient heat to the electrolyser system to warm the same from ambient to operational temperatures in a reasonable timeframe. However, provision of larger capacity heaters for heaters 150 and / or 151, to provide warm up heat, would result in oversized heaters for steady state operation. Nonetheless, in each of the examples subsequently discussed, the heaters 150 and / or 151 - if in the form of trim heaters - may be used to assist warm up of the system when efficient to do so.

[0071] Fig. 2 is a warm up method 200 for the electrolyser systems of Fig. 1. At step 205, the method comprises heating the one or more stacks 110 to raise the one or more stacks to a first threshold temperature Tl. T1 may be between 100 °C and 300 °C. In an example T1 is between 125 °C and 175 °C. In another example, Tl is between 150 and 200°C. Tl is at least 100 °C such that fluid containing steam or water vapour provided to the system above Tl does not condense in the system.

[0072] At step 210, the method comprises providing a heat transfer fluid to the fuel volume of each of the one or more stacks when the temperature is above first threshold temperature Tl. Said heat transfer fluid having a temperature the same as or greater than Tl.

[0073] At step 215, the method comprises incrementally increasing the temperature of the heat transfer fluid to further heat the one or more stacks above the first threshold temperature Tl. In other words, the heat transfer fluid heats the one or more stacks towards an operational temperature.

[0074] At step 220, the method comprises when a second threshold temperature, T2, is reached, providing fuel to the fuel volume of each of the one or more stacks and electrical current to each of the one or more stacks to generate product via electrolysis reaction. At such time, the flow of heat transfer fluid may be stopped if the heat transfer fluid is different to the fuel. The electrolyser cells in the stack(s) may be solid oxide electrolyser cells (SOEC) and T2 is a temperature at which the electrolysis reaction can begin, or a temperature at which steady state operation or rated operating performance is achieved. T2 may be at least 350 °C. T2 may be between 350 °C and 700 °C, optionally at least 400 °C, optionally at least 450 °C. The electrolyte of the electrolyser cell units may comprise ceria in which case T2 may be optionally at most 600 °C, optionally at most 550 °C. T2 may be between 450 and 550 °C.

[0075] The temperatures referred to above are cast as temperature of the stack(s) 110. However, the temperature may be measured in one or both of the outlet streams from the stack(s) and inside or outside the vessel, or may be a temperature of the interior of the vessel. In each case, the temperature of the stack(s) can be derived from the measured temperature - and in many cases may be assumed to be identical.

[0076] Figs. 3-5 depict example warm up strategies according to the method of Fig. 2 for the electrolyser system described with reference to Fig. 1. In each Figure a temperature profile is shown and provision of fluids or heat sources is shown by the boxes below the profile. The temperature profile is simplified and may in practice constitute steps with transitions therebetween. Time is on the x-axis. The scale of the time axis is not equivalent from one example to the next and so threshold temperatures T1 and T2 are shown on the temperature plots and threshold temperatures Tl, T2 and T3 are indicated on the time axis of each subfigure. Fig. 3 also shows an example voltage profile during the warm-up strategies of Figs. 3-5 as an example that a voltage across the stack may be applied or generated from T3, and voltage may be increased from T2 as in the example plot, or voltage may be increased before T3 if fuel consumption (for electrolysis or fuel cell operation) is increased.

[0077] The electrolyser system of Fig. 1 is simplified and does not show heat exchangers which, during warm up, may be used to exchange heat between the off-gasses of the first and second fluid volume and gasses input to the first and / or second fluid volume. This improves efficiency of the system by recovering heat from the off-gasses and transferring said heat to the input gasses. Additional heat sources are used to further raise the temperature of the gasses input to the first and / or second fluid volume to heat said volumes in the strategies described herein. Heat from these sources is transferred to the fluid(s) supplied to the first and / or second fluid volumes to heat the stack(s) as described herein. The electrolyser system may also comprise one or more recirculation loops to recirculate first fluid or second fluid from the respective outlet of the vessel to the corresponding inlet of the vessel, optionally while adding further heat in the recirculation loop. This improves efficiency of the warm-up process since thermal mass of the stack(s) and vessel may mean that said stack(s) and / or vessel are not heated to the temperature of the fluid during its first pass through the stack(s) and / or vessel. Further, this reduces the total volume of fluids required for the warm up method, reducing cost, space usage and so forth..

[0078] Fig. 3a is a first example warm-up strategy. At T<T1 a non-condensing (NC) fluid is provided to the first and second fluid volumes to heat the stack(s). The non-condensing fluid may be dry air, oxygen, or nitrogen. In some cases, dry air is provided to both first and second fluid volumes. In some cases, dry air is provided to the second fluid volume and nitrogen is provided to the first fluid volume. The non-condensing fluid may provide a dual purpose of a purge gas (to remove any contaminants or unwanted fluids in the volumes) and to transfer heat to the stack(s).

[0079] Once T1 is reached, flow of non-condensing fluid to the first fluid volume is stopped and a heat transfer fluid, in this case fuel, is provided to the first fluid volume to heat the stack(s). Provision of dry air, oxygen, or nitrogen - or humid air - to the second fluid volume continues. Said fluids to the first and second fluid volumes incrementally increase the temperature of the stack(s) to T2. The fuel provided to the first fluid volume may comprise or consist of steam or carbon dioxide, or a mixture thereof. Preferably it is steam. The fuel may be provided from the fuel source at elevated pressure. From Tl, the pressure in the electrolyser system is gradually increased to a working pressure by supplying the pressurised fuel to the first fluid volume and by supplying a pressurised gas to the second fluid volume. Use of pressurised fluids, especially steam, during warm up increases the speed at which heat is transferred to the stack(s) and system thereby decreasing warm-up time. The respective pressures of the first and second fluid volumes is regulated to keep the pressure difference between said volumes less than a threshold (e.g., 20- 600 mbar, optionally 20-500 mbar). The working pressure may be 1-10 barg, optionally 1.5-5 barg.

[0080] Once T2 is reached, the supply of second fluid to the second fluid volume is stopped or significantly reduced since it is no longer needed to warm up the stack(s). Electrical current is supplied to the stack(s) along with fuel, and oxygen generated in the electrolysis reaction is released in to the second fluid volume. Said product is used to maintain the pressure in the second fluid volume at the working pressure.

[0081] Above Tl and before T2 is reached, hydrogen may be provided to the first fluid volume from a third threshold temperature T3. The third threshold temperature T3 is between 300 °C and 500 °C, optionally between 300 °C and 400 °C, optionally between 320 °C and 360 °C. In such cases, hydrogen is used to provide a reducing environment in the first fluid volume. In some cases the hydrogen is supplied from outwith the stack(s). Said hydrogen supply may be from an external source or may be from a storage tank of the system which was filled by previous operation of the electrolyser system. Further, the hydrogen supply may be from a stack or collection of stacks (e.g., in a vessel) which are already at operating temperature and generating hydrogen through electrolysis of steam, a portion of said hydrogen routed to the stack or collection of stacks (e.g., in a vessel) for start-up of said stack or collection of stacks. The fluid supplied to the first fluid volume may comprise between 1 and 10 mol% hydrogen. In other cases, the stack may be supplied with electrical power from T3 to produce hydrogen by electrolysis. Between T3 and T2 (T2>T3), the reaction rate and efficiency of the electrolysis reaction may be low but may be large enough to produce a reducing environment. If the electrolyser cells comprise a ceria-based electrolyte, then the cell unit is operable from a relatively low temperature and T3 can be relatively low. The stack(s) may be powered with an overvoltage (e.g., a voltage which is larger than the open circuit voltage of the stack(s)) to drive exothermic operation of the stacks thereby increasing the temperature of the system at a greater rate (said overvoltage is not shown in the trace in Fig. 3). As will be understood, exothermic operation (in an electrolysis mode) comprises driving the stack(s) at an exothermic voltage (i.e., each cell unit in the stack(s) is driven to a voltage greater than the thermoneutral voltage, and so resistive heating in the cell / stack transfers heat to said cell / stack(s)).

[0082] As temperature of the system rises between T3 and T2 in the presence of fuel and electrical power, the electrolysis reaction begins. Power may be incrementally increased with temperature and supply of fluid to the second fluid volume incrementally decreased - because product of the electrolysis reaction (e.g., oxygen) is released by the electrolyser cell units into the second fluid volume and pressurisation of the second fluid volume is by said product. Supply (flow rate) to the second fluid volume via second fluid inlet 122 - and the vessel volume by second fluid inlet 120 - is incrementally reduced so that at T2 the second fluid off-gas (at vessel exhaust 121) may comprise at least 50% oxygen generated by the cells, optionally at least 90% oxygen generated by the cells, substantially pure oxygen generated by the cells.

[0083] Fig. 3b is a further example warm-up strategy, similar to Fig. 3a wherein only relevant differences will be described. In this case the heat transfer fluid provided to the first fluid volume from T1 is hydrogen. This creates a reducing atmosphere in the first fluid volume that may improve lifetime of the system. Hydrogen is a fuel used by electrochemical cells in fuel cell mode. The electrolyser cells in the stack(s) may be capable of operation in fuel cell mode, in which case the fluid provided to the second fluid volume between T1 and T2 comprises oxygen and the cells use the hydrogen and oxygen to generate power, which may be consumed by the system for further warming. The fuel cell reaction is typically exothermic thereby contributing to heating the stack(s) and system. Power produced by the system may be introduced and incrementally increased from 300 °C to T2. Fuel for the electrolysis reaction is introduced at T2 and supply of the heat transfer fluid (hydrogen) is stopped.

[0084] Fig. 3c is a further example warm-up strategy, similar to Fig. 3a wherein only relevant differences will be described. In this example no hydrogen supply is used and no non-condensing fluid is supplied to the first fluid volume below Tl. Fuel (e.g., steam) is used as the heat transfer fluid from Tl. At Tl, supply to the second fluid volume is stopped and heating between Tl and T2 is via the heat transfer fluid supplied to the first fluid volume. This may result in overall system cost savings since heating requirements on the fluid supplied to the second fluid volume is lower and no non-condensing fluid is needed for supply to the first fluid volume. Fig. 3c depicts fluid supply to the second fluid volume and pressurisation from a fourth threshold temperature T4. T4 may be between 300 and 500 °C, optionally between 350 and 450 °C. Supply of the second fluid to the second fluid volume (in addition to the heat transfer fluid to the first fluid volume) is used to pressurise the system from T4. Once the system is pressurised, and optionally once power is supplied to the stack(s) so that product of the electrolysis reaction is released into the second fluid volume, then supply of second fluid to the second fluid volume is reduced or stopped. Alternatively, once Tl is reached the fluid supply to the second fluid volume may continue in parallel with supply to the first fluid volume for sufficient time to pressurise the system before reducing or stopping supply of fluid to the second fluid volume. This may further reduce overall system cost by obviating the need to supply fluid to the second fluid volume at elevated temperature (T4) to pressurise said volume. In cases where second fluid is supplied to the second fluid volume during steady state operation, second fluid may be supplied from T2.

[0085] Fig. 3d is a further example warm-up strategy, similar to Fig. 3a wherein only relevant differences will be described. In this case, pressurisation of the stack(s) is from ambient by use of pressurised non-condensing fluid in the first fluid volume. Such early pressurisation leads to faster warm-up times. In this case non-fluid heating is used to heat the stack(s) from ambient to T2. Such nonfluid heating may be a heat trace around the vessel, pipes leading to the vessel through which an electrical current is passed to generate heat. Alternatively or additionally, the non-fluid heating may comprise passing an electrical current through the stack(s) and using the resistive properties of the same (typically above T3). The generated heat may radiate through the system, but typically will also be transferred around the system by flow of fluid(s) in the first and / or second fluid volume. Alternatively or additionally, the non-fluid heating may comprise inductively heating the system, particularly the stack(s). Such non-fluid heating may be combined with any other example described herein, typically to improve warm up time of the system.

[0086] Fig. 3e is a further example warm-up strategy, similar to Fig. 3a - and Fig. 3c in that no reducing fluid (hydrogen) is supplied to the first fluid volume - wherein only relevant differences will be described. In this case non-fluid heating is used from T3 by supplying power to the stack(s) from T3. Fuel is also supplied, and product of the electrolysis reaction released in to the first fluid volume (e.g., hydrogen when the fuel comprises steam) creates a reducing environment in said volume. Operation of the stack(s) in exothermic condition further heats the stack(s) towards T2.

[0087] Fig. 4a is a further example warm-up strategy, similar to Fig. 3c wherein only relevant differences will be described. In this case a non-condensing fluid is supplied to the first fluid volume below Tl. The non-condensing fluid (e.g., nitrogen) is a purge gas to ensure cleanliness and safety of the system, configured to sweep contaminants out of the first fluid volume and supply / exhaust of the same. The non-condensing gas may also supply heat to the stack(s) to contribute to heating the stack(s) to Tl. Supply of non-condensing gas to the first fluid volume below Tl may be combined with any other example described herein. Further in this case, hydrogen is supplied to the first fluid volume from T3 to provide a reducing environment therein, as described with reference to Fig. 3a.

[0088] Fig. 4b is a further example warm-up strategy, similar to Fig. 4a wherein only relevant differences will be described. In this case hydrogen is not supplied to the first fluid volume from T3 - instead no hydrogen is present or hydrogen is produced from T4 as described with reference to Fig. 3c. In this case, the fluid supplied to the second fluid volume is continued until after Tl is reached so that the first and second fluid volumes are pressurised by the fuel and the second fluid, respectively, before flow to the second fluid volume is reduced or stopped (supply to and exhaust from the second fluid volume may be capped off, or valves closed, to "lock in" fluid in the second fluid volume at pressure). This allows for warm-up to continue with pressurised fuel provided to the first fluid volume with reduces warm-up time. As in other examples, power may be supplied to the stack(s) from T3 such that product of the electrolysis reaction released into the first fluid volume creates a reducing atmosphere therein - and product released in to the second fluid volume can be used for (continuing) pressurisation thereof.

[0089] Fig. 4c is a further example warm-up strategy, similar to Fig. 4a wherein only relevant differences will be described. Fig. 4b is also similar to Fig. 3c in that heating requirements for fluid delivered to the second fluid volume are low because that fluid is not used for heating the stack(s) above Tl, optionally is not used for any heating of the stack(s). In the case of Fig. 4c, the non-condensing fluid is used (optionally fluid supply to the second fluid volume is also used) to heat the stack(s) to Tl. Thereafter, heat transfer fluid (fuel) delivered to the first fluid volume is used to heat the stack(s) to T2. The electrolysis reaction may begin at T3, and so product of the electrolysis reaction is released in to the second fluid volume. Said product may be used to pressurise the second fluid volume while that pressure is balanced in the first fluid volume by pressurising the fuel therein. Pressurisation expedites warm up, and operation of the stack(s) in an exothermic mode, as discussed elsewhere, may be used to further expedite warm up.

[0090] Fig. 5a is a further example warm-up strategy, similar to Fig. 3d wherein only relevant differences will be described. In this case, no non-fluid heating is shown but may be included - especially exothermic operation of the stack(s) once T3 is reached. In this case, the non-condensing fluid supplied to the first fluid volume is not pressurised so pressurisation begins at Tl using the fuel supplied to the first fluid volume and second fluid supplied to the second fluid volume. This may reduce system cost and complexity by reducing the number or volume of pressurised fluids required for start-up.

[0091] Fig. 5b is a further example warm-up strategy, similar to Fig. 5a wherein only relevant differences will be described. In this case, system costs are reduced further by omission of a hydrogen supply. A reducing environment is provided once power is supplied to stack(s), similar to the example of Fig. 3c.

[0092] Fig. 5c is a further example warm-up strategy, similar to Fig. 5a wherein only relevant differences will be described. In this case the heat transfer fluid, supplied from Tl, is hydrogen, which also creates a reducing environment in the first fluid volume. The hydrogen is pressurised. Fuel is not supplied until T3, optionally not until T2, is reached. Electrical power is supplied to the stack(s) when fuel is provided to the first fluid volume.

[0093] Fig. 5d is a further example warm-up strategy, similar to Fig. 5a wherein only relevant differences will be described. In this case pressurisation is achieved from ambient temperature. To enable this, the non-condensing fluid is pressurised, which may not be a significant cost since the noncondensing fluid may itself be supplied to the system in a compressed state. It will be appreciated that pressurisation at temperatures below Tl, where non-condensing fluid is supplied to the fuel volume, may be combined with other examples described herein. Such early pressurisation expedites warm up. As discussed with reference to Fig. 5a, no non-fluid heating is shown but it is preferable that exothermic electrolysis operation of the stack(s) is executed to expedite warming of the stacks and system once T3 is reached (albeit at a lower current relative to the current in the product generating mode from and above T2 (e.g., 10-50%, optionally 10-30% of the steady state current from and above T2) due to lower conductivity of the cell units below T2). It is preferable that the same non-condensing fluid (e.g., air or nitrogen) is supplied to the first and second volumes while raising the temperature to Tl. Said non-condensing fluid may continue to be supplied to the second volume above Tl, preferably until T2. From T3, hydrogen is preferably supplied to the first volume as a reducing gas, and preferably from this temperature exothermic electrolysis operation of the stack(s) is started. Alternatively, from T3 hydrogen is supplied to the first volume (e.g., between 3 and 10 mol% H2 in heat transfer fluid e.g., fuel, where fuel is preferably steam), and from a further temperature threshold (e.g., 30 to 100 °C, preferably 40 to 75 °C above T3), exothermic electrolysis operation of the stack(s) is started, at which point the flow rate of hydrogen may be reduced below the rate (or mol%) introduced at T3 (e.g., reduced to between 0 and 3 mol% H2 in heat transfer fluid / fuel, preferably between 0.5 and 2 mol%).

[0094] It will be appreciated that aspects of each of the above examples may be combined. Table 1 below provides a summary of the method where at each of the temperatures, the listed gas is supplied to or present in one or both of the first and second fluid volumes and warm up heating is by one or more of the methods listed in the "Warm up heating by" column for that temperature:

[0095] Table 1 Temperature T4 is not depicted in Table 1, but can be considered as an additional temperature threshold for commencing pressurisation (as per Fig. 3c) which may be applied to any of the examples above and in Table 1. In many cases, the temperature at which the stack(s) are pressurised, T4 in the terminology of Fig. 3c, will coincide with one of the other temperature thresholds. It is preferable to pressurise throughout the warm up method, (i.e., from the step of heating to Tl), as such pressurisaion expedites warm up of the stack(s) and system.

[0096] In cases where the vessel volume is not in fluidic communication with one of the first and second fluid volumes, a non-condensing fluid or a fluid of similar composition to that supplied to the second fluid volume (e.g., nitrogen or air), may be supplied to the vessel volume. It is preferable that the vessel volume is supplied with nitrogen. Such supply may be to assist with warm up of the stack(s) and system, and / or it may be to balance pressure between the vessel volume and the first and second fluid volumes. In other words, the supply to the vessel volume may be pressurised in balance with the supply to the first and second fluid volumes (e.g., pressure differences between said volumes of less than 0.5 bar, preferably less than 0.2 bar).

[0097] It will be appreciated that the examples discuss pressurisation of the stack(s) in the warm up strategy because electrolysis stacks are advantageously operated at elevated pressure such that product gases are themselves pressurised which leads to cheaper (further pressurisation, if needed, and) storage or transportation. However, in some cases, the warm up methods described herein may operate at substantially atmospheric pressure (pressures in the volumes balanced thereto), for example when pressurised fluids are not available (e.g., all fluids or merely those used at relatively low temperatures) or if the apparatus is not arranged for pressurised fluids.

[0098] The warm up strategies herein may also be described in the following terms and combined with any of the above examples.

[0099] In some cases a pressurised supply to the second fluid volume may not be available - this lowers system cost and complexity. In such cases pressurisation of the stack(s) is from T3 or T2, by supply of fuel to the first fluid volume and by release of oxygen as a product of the electrolysis reaction into the second fluid volume. As the oxygen is released in to the second fluid volume, that oxygen can be used to pressurise said second fluid volume and the first fluid volume is pressurised to balance the pressure in the second fluid volume.

[0100] In some cases a limited pressurised supply to the second fluid volume is available - this lowers system cost and may be used to facilitate faster warm up of the stack(s). In this case, fuel or other fluid supplied to the first fluid volume is primarily used to warm the stack(s) from Tl to T2. Above T3, optionally above Tl, a pressurised fluid is supplied to the second fluid volume which enables supply of pressurised fuel or other fluid to the first fluid volume - to balance pressure between the first and second fluid volumes. This enables a greater supply of heat to the stacks via the pressurised fuel or other fluid than with unpressurised fuel / fluid. In a variation of this case, a pressurised supply (of non-condensing fluid) to the second fluid volume is available from below Tl, eg from ambient temperature, which may be used alongside a pressurised fluid (a noncondensing fluid) supplied to the first fluid volume to heat the stack(s) to Tl and that supply to the second fluid volume may be continued above Tl, to T3 or T2, to balance pressure of pressurised fluid supplied to the first fluid volume.

[0101] Fig. 6 is an example warm-up configuration of the electrolyser system of Fig. 1 in which, for warmup, the first and second fluid volumes are in fluidic communication. This configuration may be in place from ambient temperature to Tl. A non-condensing fluid is supplied to the first fluid volume by a non-condensing fluid source 644 and the exhaust from the first fluid volume is routed to the inlet 120 for the second fluid volume. Dashed lines indicate warm-up specific routing of fluids. The non-condensing fluid source 644 may be the same as the second fluid source 144, if present, and if the sweep gas at steady state provided by the second fluid source 144 is a non-condensing fluid. The non-condensing fluid may be dry air, oxygen or nitrogen. Noncondensing fluid from the source 644 is heated by heater 150 in this example, but it will be understood that it may be heated by another heat source for example heater 151, by heat exchange, or by a dedicated heater. Heated non-condensing fluid is passed through the first fluid volume in the stack(s) 110 and related manifolds in vessel 105. Upon exit from the vessel, the non-condensing fluid is routed from the first fluid volume to an inlet to the second fluid volume - and not to the first fluid volume off-gas collection 163 - such routing may be via a valve. In the example of Fig. 6, the non-condensing fluid may be further heated by the heater 151 (if present) prior to entry to the second fluid volume, in this case via the vessel inlet 120 and second fluid inlets 122 of the stacks 110 before being routed out of the second fluid volume of the stacks at outlet 123 and out of the vessel at vessel exhaust 121 (and which may be routed differently than to the collection 164 if the second fluid collection 164).

[0102] Fig. 7 is an example warm-up configuration of the electrolyser system of Fig. 1 in which a temporary heater 751 is used to heat the stack(s) to Tl. This configuration may be in place from ambient temperature to Tl, which in this case may be up to 300 °C. The temporary heater 751 is supplied with non-condensing fluid from a source 744 (which may be the same as source 144) and an exhaust of said heater is routed to the second fluid volume of the stack(s) preferably via the vessel volume. In some cases, the exhaust from the temporary heater is routed into the vessel volume via the vessel inlet 120 (which is also used for sweep gas from source 144). In other cases, a dedicated second vessel inlet 720 is used to route exhaust from the temporary heater into the vessel volume and the second fluid volume of the stack(s). The latter is preferable since the second vessel inlet 720 can be larger than the vessel inlet 120 to allow a greater flow rate of fluid into the vessel volume and to heat the same more quickly. Once Tl is reached, the temporary heater 751 and source 744 (if used - if different to source 144) are removed and the second vessel inlet 720 (or entry via vessel inlet 120) is blocked off. Residual heat is retained in the vessel during this process, and the warm up may continue using the first and / or second fluid volumes. It will be appreciated that similar temporary heater configuration(s) to those of Figs. 6 and 7 may be utilised for the electrolyser system of Fig. 2.

[0103] Figure 8 next shows a control device 400 for controlling an electrolyser cell stack of an electrolyser in an electrolyser system. The control device 400 comprises an input device 402 for receiving input from sensors 404 so as to determine at least one of: a stack operating voltage across the electrolyser stack(s); a stack operating current through the electrolyser stack(s); an inlet temperature at the fluid inlet; and an outlet temperature at a fluid outlet; a vessel volume temperature. The control device thus comprises a voltage monitoring system 406 for determining a stack operating voltage across the electrolyser stack(s), an inlet temperature monitoring and / or control system 408 for determining an inlet temperature at the fluid inlet and an outlet temperature monitoring and / or control system 410 for determining an outlet temperature at the at least one fluid outlet of the electrolyser stack(s). These systems may utilize sensors and data transmission devices or wiring. The control device receives sensor data relating to each of these measurements. The control device may control the system by controlling valves for supply to and exhaust from each fluid volume and power supplies for non-fluid heating. A suitably programmed processor 412 and associated memory 414 is provided for processing such inputs.

[0104] The control device's inlet temperature monitoring and / or control system 408 may comprise an output device for controlling the identity and temperature of a fluid entering the electrolyser stack(s) at a fluid inlet. A current control system 416 is also provided for controlling a current supply to (and in some cases from) the electrolyser stack. As mentioned above, during normal operation this is adapted to provide a constant current to the electrolyser cell stack, but the current may be reduced or turned off to automatically shut down the stack or increased to warmup the electrolyser stack(s).

[0105] The controller may also control non-fluid heat sources (heaters and such like described above), recirculation loops, and temporary warm up configurations as described above.

[0106] In such a way, the controller 400 is adapted to control an electrolyser stack(s) and the electrolyser system 100 in accordance with the methods described above with reference to Figs. 2-5.

[0107] The present invention is not limited to the above examples only, and other examples will be readily apparent to one of ordinary skill in the art without departing from the scope of the appended claims.

[0108] These and other features of the present invention have been described above purely by way of example. Modifications in detail may be made to the invention within the scope of the claims.

Claims

Claims1. A method of warm up of an electrolyser system comprising one or more stacks of electrolyser cells, each of the one or more stacks with fuel and oxygen volumes, the method comprising: heating the one or more stacks to raise the one or more stacks to a first threshold temperature Tl; providing a heat transfer fluid to the fuel volume of each of the one or more stacks when the temperature is above first threshold temperature Tl; increasing the temperature of the heat transfer fluid to further heat the one or more stacks above the first threshold temperature Tl; and when a second threshold temperature, T2, is reached, providing fuel to the fuel volume of each of the one or more stacks and electrical current to each of the one or more stacks to generate product via electrolysis.

2. The method of claim 1, wherein the first threshold temperature, Tl is between 100 °C and 300 °C.

3. The method of claim 1 or 2, wherein heating the one or more stacks to raise the one or more stacks to the first threshold temperature comprises supplying a non-condensing gas to the one or more stacks to raise the one or more stacks to the first threshold temperature Tl.

4. The method of the preceding claim, wherein the non-condensing gas is supplied to one or both of the fuel volume and the oxygen volume of each of the one or more stacks.

5. The method of claim 4, further comprising recirculating the non-condensing gas through the fuel volume and / or oxygen volume via a recirculation loop in which heat is transferred to the non-condensing gas.

6. The method of any one of claims 3 to 5, wherein the non-condensing gas comprises or consists of nitrogen or dry air.

7. The method of any one of claims 3 to 6, wherein supplying the non-condensing gas to the one or more stacks comprises attaching a temporary heater to the electrolyser system to supply the non-condensing gas.

8. The method of any one of claims 3 to 7, wherein when the first temperature threshold Tl is reached, the method comprises stopping the supply of non-condensing gas to the one or more stacks.

9. The method of any preceding claim, wherein the second threshold temperature, T2 is between 350 °C and 700 °C.

10. The method of any preceding claim, wherein the heat transfer fluid comprises a fuel, preferably comprising steam.

11. The method of any preceding claim, wherein when the temperature of the one or more stacks is between a third threshold temperature T3 and the second threshold temperature T2, where T3 is greater than Tl and less than T2, the heat transfer fluid comprises a reducing gas.

12. The method of claim 11, further comprising supplying an oxidant or nitrogen to the oxygen volume of each of the one or more stacks.

13. The method of claim 11 or 12, wherein when the temperature of the at least one stack is between the third threshold temperature T3 and the second threshold temperature T2,the method comprises supplying fuel to the fuel volume of each of the one or more stacks, and operating the at least one stack in an electrolysis mode at an exothermic voltage.

14. The method of claim 11 or 12, wherein when the temperature of the at least one stack is between a third threshold temperature T3 and the second threshold temperature T2, where T3 is greater than T1 and less than T2, the method comprises operating the one or more stacks in a fuel cell mode.

15. The method of any one of claims 11 to 14, wherein the reducing gas comprises hydrogen gas.

16. The method of claim 15, wherein the hydrogen gas is supplied from a tank in the electrolyser system that is fillable by previous operation of the electrolyser system.

17. The method of any one of claims 11 to 16, wherein the reducing gas has the same composition as the non-condensing gas.

18. The method of any one of claims 11 to 17, wherein the heat transfer fluid supplied to the fuel volume of the one or more stacks comprises between 1 and 25mol% of reducing gas in steam.

19. The method of any one of claims 11 to 18, wherein the third threshold temperature is between 300 °C and 500 °C.

20. The method of any preceding claim, wherein the one or more stacks comprise a plurality of stacks, optionally at least six stacks.

21. The method of any of the preceding claims, wherein the one or more stacks are arranged in a vessel enclosing a vessel volume.

22. The method of claim 21, wherein the vessel volume is in fluidic communication with one or more of the fuel volume and the oxygen volume of each of the one or more stacks during the method.

23. The method of claim 22, wherein the fuel volume of each of the one or more stacks is manifolded.

24. The method of any one of claims 21 to 23, wherein the vessel is a pressure vessel configured to withstand a pressure difference between the inside of the pressure vessel and an ambient pressure, wherein the method further comprises regulating the pressure of at least one or both of the fuel volume and the oxygen volume of each of the one or more stacks at a pressure greater than ambient pressure when the temperature of the one or more stacks is greater than the first threshold temperature.

25. The method of claim 24, wherein the pressure of the at least one or both of the fuel volume and the oxygen volume is at least 0.5barg.

26. The method of claim 24 or 25, wherein regulating comprises regulating the pressure of the fuel volume and the oxygen volume to balance pressure between the fuel volume and the oxygen volume.

27. A controller configured to perform the method of any one of the preceding claims or an electrolyser system comprising a controller configured to perform the method of any one of the preceding claims.

28. A computer program comprising instructions which, when the program is executed by a computer, cause the computer to carry out the steps of the method of any one of the claims 1 to 26.

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

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