AU2024211141A1

AU2024211141A1 - Electrolyser System for an Intermittent Electricity Supply

Info

Publication number: AU2024211141A1
Application number: AU2024211141A
Authority: AU
Inventors: Hacib BEN AISSA
Original assignee: Ceres Intellectual Property Co Ltd
Current assignee: Ceres Intellectual Property Co Ltd
Priority date: 2023-01-27
Filing date: 2024-01-26
Publication date: 2025-07-10
Also published as: TW202446994A; GB2637456A; WO2024157023A1; KR20250143144A; CN120569516A; GB202301212D0; JP2026505263A; EP4655430A1

Abstract

The invention provides an electrolyser system (10) comprising a heat storage unit (14) and an electrolyser (16). The heat storage unit (14) comprises at least one heat source infeed. The electrolyser (16) comprises at least one electrolyser cell (20), a steam inlet and at least one off-gas outlet. The off-gas outlet is connected to the heat source infeed to heat the heat storage unit (14). The heat storage unit (14) is configured to use its stored heat to produce steam for feeding into the steam inlet and for generating electrical power, either one at a time or both at the same time. The invention also provides a system comprising an intermittent or variable electricity source (12) and an electrolyser system (10) as defined above. The intermittent or variable electricity source (12) can be configured to power the electrolyser (16) and to heat the heat storage unit (14) via a heating element, either both at the same time or individually.

Description

Electrolyser System for an Intermittent Electricity Supply

The present invention relates to an electrolyser system, and preferably a solid oxide electrolyser system, arranged to accommodate an intermittent or variable electricity supply, such as typical renewable power supplies, including, but not limited to, hydroelectric power, tidal power, solar power, wind power or wave power.

Due to the intermittent or variable nature of the sources of power for intermittent electricity supplies, for example due to a lack of sunlight at night for solar power, or periods of still for wind power or wave power, or due to the slow cycle of tides for tidal power, or due to periods of drought for hydroelectric power, there can be variations, reductions and even drop-offs in power output from such electricity supplies.

Grid electricity may also be considered as an intermittent or variable electricity supply, for example if other electricity users (e.g. homes, hospitals etc.) are prioritised during periods of high demand by restricting or stopping supply to industrial users (such as electrolysers). Similarly, a grid may be considered intermittent or variable if demand regularly outstrips supply and ‘blackouts’ or power cuts I supply issues occur due to inadequate and/or unreliable infrastructure.

As a consequence, where those energy supplies are providing electricity for powering electrolysis devices - for example for harvesting hydrogen and oxygen from water - where there is no alternative power supply in order to maintain the operations of the electrolyser devices, the electrolyser devices will need to be taken offline.

In the case of certain electrolyser cells, in particular medium to high temperature electrolyser cells, and most particularly electrolyser cells with operating temperatures in excess of 400 degrees C, such as SOECs, such downtimes can create an extended period of inoperability, or output inefficiencies. This is because many forms of electrolyser cell have an extended start-up procedure for protecting or activating the components of the electrolyser cell. For example, an SOEC typically will not function below an operating temperature of 400 degrees C, and to achieve optimum efficiency, some SOEC designs require an operating temperature above that at which electrolysis commences - for example 450 degrees C or even 500 degrees C. Therefore, the power supply for such electrolysis devices needs to be maintained in order for such devices to provide optimum efficiencies, or even to enable continued operability. It is also important for some electrolysis devices to maintain its power supply for long enough to enable a proper shut-down procedure to protect the components thereof - particularly the electrochemically active components thereof, due to the wide variance between its operational temperatures and ambient temperatures.

It would therefore be useful to be able to compensate for such drop offs of power from intermittent or variable electricity sources such as typical renewable power supplies, including, but not limited to, solar, wind, tidal, hydroelectric and wave power.

Commonly this is done by providing a variety of sources of power - for example using a combination of solar and wind power, or other alternative energy supplies such as tidal, wave or hydroelectric, or by supplementing the power supply with more conventional ‘on demand’ power sources such as gas, coal, wood pellets, geothermal or nuclear power. Another approach is to store up the natural source of power, such as a reservoir for hydroelectric power. For solar and wind power, however, the accepted approach is to use energy storage devices such as batteries - most commonly lithium-ion batteries due to their acceptable storage capacity per unit weight. However, additional approaches for smoothing out power drop-outs would be beneficial as lithium for such batteries is scarce, and the mining and refining of lithium is energy and carbon intensive.

Statements of Invention

According to a first aspect of the present invention there is provided an electrolyser system comprising: a heat storage unit; and an electrolyser; the heat storage unit comprising at least one heat source infeed; the electrolyser comprising at least one electrolyser cell, a steam inlet and at least one off-gas outlet, the off-gas outlet being connected to the heat source infeed to heat the heat storage unit; wherein the heat storage unit is configured to use its stored heat to produce steam for feeding into the steam inlet and for generating electrical power, either one at a time or both at the same time. With this arrangement, waste heat generated as a result of the electrolytic reaction of the electrolyser, or from elsewhere, such as from an external industrial process in a nearby industrial plant, can be stored in the heat storage unit and recycled to generate steam and/or to produce electrical power, selectively, either both at the same time or individually. This enables a smoothing out of supplies of steam and/or electricity to the electrolyser when required by the electrolyser due to, for example, fluctuations in the supply of electricity, or the supply of electricity to external equipment or to a power grid when not required by the electrolyser.

Typically the at least one electrolyser cell is part of a stack of electrolyser cells.

The or each electrolyser cell may comprise an anode, a cathode and an electrolyte. Some stacks may additionally comprise dummy cells, as known in the art for improving temperature distribution to active electrolyser cells within a stack.

In some embodiments, heat from the heat storage unit can be selectively used to produce hot air for feeding to the electrolyser or external equipment. This can be in addition to steam and electricity (via a steam cycle/turbine, for example). The present invention can then compensate for fluctuations in delivery to the electrolyser of any of a) heat (for maintaining desired operating temperatures of the electrolyser cell(s)) - either as steam or hot air feeds to the electrolyser, or both, and b) electricity - for at least partially powering the electrolyser if required due to fluctuations in its normal power supply.

In some embodiments, therefore, at least part of the generated electrical power is arranged to be selectively delivered to the electrolyser cell to at least partially power the electrolyser. For example, in some embodiments the electrolyser system is configured to selectively connect at least part of the electrical power generated by the electrolyser system to the electrolyser, so that the electrolyser can selectively be at least partially powered by that generated electrical power. This allows intermittency in its normal power supply, such as from alternative-energy power sources such as typical renewable power supplies, including, but not limited to, solar, wind or wave power, and the like, to be compensated for when needed.

In some embodiments, the heat storage unit comprises a hot air (or nitrogen) outlet. In some embodiments the electrolyser has a hot air (or nitrogen) infeed.

In some embodiments, the hot air outlet of the heat storage unit is connected to the hot air infeed of the electrolyser, for selective delivery of hot air (or nitrogen) to the electrolyser during use of the electrolyser system. The input gases to the electrolyser are typically just steam (or water) and air (or nitrogen).

With the present invention, a hot air stream may selectively exit the hot air (or nitrogen) outlet of the heat storage unit for delivery to the electrolyser, or additionally/alternatively to external equipment that requires hot air (or nitrogen), or the heat thereof.

In some embodiments, the heat storage unit comprises a steam outlet. A stream of steam may thus selectively exit the steam outlet during use of the heat storage unit for delivery to the electrolyser cell/electrolyser or to a steam turbine, or to alternative external equipment that requires steam (or the heat thereof).

In some embodiments, the system comprises a steam supply line for feeding steam from the heat storage unit, or a steam turbine connected to the heat storage unit, to the steam inlet of the electrolyser.

In some embodiments, the steam supply line connects between the heat storage unit and the steam turbine and between the steam turbine and the steam inlet, and in some embodiments it may have a bypass for selectively bypassing the steam turbine.

In some embodiments, the heat from the heat storage unit is configured to power a steam cycle for producing electricity (electrical power) for external distribution - e.g. to a power grid or external equipment, or for at least partially powering the electrolyser. The steam cycle may occur within the heat storage unit or outside it.

In some embodiments, the system further comprises a steam turbine for the selective generation of electrical power using heat or steam from the heat storage unit.

In some embodiments, the heat storage unit comprises a water inlet and/or an air inlet. These can be to supplement or add water and/or air into the heat storage unit. For example, water can be provided for conversion to steam, and air can be provided for preheating prior to passing into the electrolyser for a sweep flow for the anode side thereof.

In some embodiments, the heat storage unit is configured to vaporise water within the heat storage unit to produce steam that can be used for external purposes. The water may be fed into the heat storage unit via the water inlet. In some embodiments, the off-gas outlet connects via a supply pipe to the heat source infeed of the heat storage unit.

The electrolyser system can be configured to receive steam or water into the steam inlet of the electrolyser from one or more external industrial system. Such a steam or water supply may be intermittent, and steam may thus be fed additionally from the heat storage unit, or a steam cycle connected thereto, to compensate for intermittencies in the steam supply to the electrolyser.

Since high pressure steam can damage elements of the electrolyser system, particularly the electrolyser or electrolyser cell, or some types of external equipment, the electrolyser system is preferably configured to deliver lower pressure steam to the steam inlet of the electrolyser than to the steam turbine. In some embodiments, the steam cycle, and usually a steam turbine thereof, is used to reduce the pressure of the steam from its pressure as delivered to the steam cycle or turbine to a delivery pressure as it exits the steam cycle or turbine for delivery to the steam inlet of the electrolyser (or to external equipment).

The steam generated by the heat of the heat storage unit may be superheated steam - potentially at temperatures exceeding 400 degrees C. Passing superheated steam through a steam cycle (or a steam turbine) can potentially damage elements of that equipment. However, the steam entering the steam inlet of the electrolyser is required to be at such high temperatures (for example in excess of 400 degrees C) to maintain the desired temperatures of, and thus the efficiency of, the electrolyser. Therefore, in some embodiments, the electrolyser system comprises a steam cycle or a steam turbine.

Furthermore, in some embodiments the electrolyser system also comprises a regenerative heat exchanger. The regenerative heat exchanger is beneficially arranged to exchange heat between an outlet of the steam cycle or steam turbine and the steam inlet of the electrolyser. In some embodiments the regenerative heat exchanger is both between the heat storage unit and the steam cycle or steam turbine, and between the steam cycle or steam turbine and the steam inlet of the electrolyser. Such a regenerative heat exchanger is thus configured to cool the steam before entering the steam cycle or steam turbine, and to reheat the steam after exiting the steam cycle or steam turbine, but before the steam passes to the steam inlet of electrolyser. In some embodiments the electrolyser system comprises a control system for controlling the distribution of steam through the system, the control system being configured to selectively feed steam either to a steam cycle or steam turbine or to the steam inlet of the electrolyser, or to both.

In some embodiments, the or each electrolyser cell has an operational stack temperature in excess of 400 degrees C.

In some embodiments the at least one electrolyser cell is a solid oxide electrolyser cell, i.e. the electrochemically active region is a solid oxide. A solid oxide electrolyser cell (SOEC) typically operates in the 400-650 degrees C range, or more particularly in the 520-620 degrees C temperature range. Such electrolyser cells may be referred to as an intermediate-temperature solid oxide electrolyser cell, or IT-SOEC.

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).

Ideally the or each electrolyser cell is an intermediate temperature solid oxide electrolyser cell or IT-SOEC with an operational stack temperature of between 400 degrees C and 700 degrees C. However, in some embodiments the electrolyser cell system comprises a high temperature electrolyser cell with an operational stack temperature between 700 degrees C and 1100 degrees C.

In some embodiments, the heat storage unit is a molten-salt technology unit. It may use various eutectic mixtures of different salts (e.g., sodium nitrate, potassium nitrate and/or calcium nitrate).

In some embodiments, the heat storage unit is a latent heat storage unit or a thermochemical heat storage unit or a hot silicon technology unit or a steam accumulator unit or a molten aluminium technology unit, such as one provided by the company Azelio ®.

In some embodiments, the electrolyser system comprises connections for connecting to an intermittent or variable electricity source. An intermittent or variable electricity source is one where ‘blackouts’ or power cuts or continuity of supply issues occur due to inadequate and/or unreliable infrastructure, rather than simply one with an alternating current (i.e. not simply an AC supply). For example, intermittent does not necessarily mean “dropping to zero”; it instead refers to where the available supply intermittently drops below a required or target threshold for operation to a sub-optimal or insufficient supply.

In some embodiments, the heat storage unit is connected to an external heat source to increase the amount of heat stored therein. This allows increased compensation for downtime in the electricity supply for the electrolyser.

The present invention also provides a system comprising an intermittent or variable electricity source and an electrolyser system as defined above. This system can be configured to utilise excess heat stored in the heat storage unit to compensate for power fluctuations or drops from the intermittent or variable electricity source. This can be by selectively powering a steam turbine to generate electrical power for at least partially powering the electrolyser.

In some embodiments, the intermittent or variable electricity source is configured to power the electrolyser and to heat the heat storage unit via a heating element, either both at the same time or individually.

In some embodiments, the intermittent or variable electricity source is configured to power the electrolyser, to heat or generate steam for the stack and/or to generate hot air for the stack, either both at the same time or individually.

The present invention also provides a method of operating an electrolyser system, the electrolyser system comprising: a heat storage unit; and an electrolyser; the heat storage unit comprising at least one heat source infeed; and the electrolyser comprising at least one electrolyser cell, a steam inlet and at least one off-gas outlet, the off-gas outlet being connected to the heat source infeed; the method comprising: using off-gas from the off-gas outlet to heat the heat storage unit; and using heat stored in the heat storage unit to produce steam for feeding into the steam inlet and for generating electrical power, either one at a time or both at the same time. In a preferred aspect the present invention provides a method of operating an electrolyser system or an electrical power supply system as defined above.

In some embodiments, the method comprises supplying additional heat to the heat storage unit from one or more external industrial process. Using this method, waste heat generated as a result of the electrolytic reaction of the electrolyser, or from elsewhere, such as from an industrial plant, can be stored in the heat storage unit and recycled to generate steam and/or to produce electrical power, selectively, either both at the same time or individually. This enables a smoothing out of supplies of steam and/or electricity to the electrolyser when required by the electrolyser due to, for example, fluctuations in the intermittent supply of electricity, or electricity to external equipment or a power grid when not required by the electrolyser.

In some embodiments, the method comprises selectively using heat from the heat storage unit to produce hot air for feeding to the electrolyser or external equipment.

In some embodiments, the method comprises using said stored heat to compensate for fluctuations in delivery to the electrolyser of at least one of a) heat either as steam or hot air feeds to the electrolyser, or both, and b) electricity.

The heat may be used at least in part for maintaining desired operating temperatures of the electrolyser, if required. The electricity may be used at least in part for at least partially powering the electrolyser, if required.

In some embodiments, the method comprises selectively delivering at least part of the generated electrical power to the electrolyser cell to at least partially power the electrolyser.

In some embodiments, a hot air (or nitrogen) stream selectively exits a hot air (or nitrogen) outlet of the heat storage unit for delivery to the electrolyser.

In some embodiments, the method comprises controlling a stream of steam that selectively exits a steam outlet of the heat storage unit for delivery to each of the electrolyser and a steam turbine for generating electrical power, either individually or both at the same time. A control system may be provided to provide this control. In some embodiments, steam is selectively fed through a steam supply line from the heat storage unit, or a steam turbine connected to the heat storage unit, to the steam inlet of the electrolyser.

In some embodiments, heat from the heat storage unit powers a steam cycle for producing electricity either for external distribution to a power grid or external equipment or for at least partially powering the electrolyser.

In some embodiments, the heat storage unit vaporises water within the heat storage unit to produce steam.

In some embodiments, the off-gas outlet provides off-gas from the electrolyser and a gas supply line feeds that off-gas through the heat source infeed of the heat storage unit for heating the heat storage unit.

In some embodiments the off-gas is either or both hydrogen-enriched steam and oxygen- enriched air.

In some embodiments the heat storage unit has outlets forthat orthose off-gas for further collection or distribution thereof after passing through the heat storage unit.

In some embodiments, the electrolyser system receives steam into the steam inlet from one or more external industrial process and steam is selectively fed to the steam inlet additionally from the heat storage unit, or a steam cycle connected thereto, to compensate for intermittencies in the steam supply to the electrolyser from the external industrial process.

In some embodiments the method comprises reducing the pressure of steam from an inlet pressure as delivered to the steam cycle or turbine, to a delivery pressure as it exits the steam cycle or turbine. In some embodiments, a steam cycle or a steam turbine of the electrolyser system reduces the pressure of steam from its pressure as delivered to the steam cycle or turbine to a delivery pressure as it exits the steam cycle or turbine for delivery to the steam inlet of the electrolyser or to external equipment at that reduced pressure.

In some embodiments, the electrolyser system comprises a steam cycle or a steam turbine. In some embodiments the electrolyser system further comprises a regenerative heat exchanger, the regenerative heat exchanger exchanging heat between an outlet of the steam cycle or steam turbine and the steam inlet of the electrolyser. The regenerative heat exchanger may be both between the heat storage unit and the steam cycle or steam turbine, and between the steam cycle or steam turbine and the steam inlet of the electrolyser, the regenerative heat exchanger cooling steam before the steam enters the steam cycle or steam turbine, and reheating the steam after exiting the steam cycle or steam turbine before the steam passes to the steam inlet of electrolyser.

In some embodiments, the method utilises a control system for controlling the distribution of steam through the system, selectively feeding steam either to a steam cycle or steam turbine or to the steam inlet of the electrolyser, or to both.

In some embodiments the at least one electrolyser cell operates with an operational stack temperature of between 400 degrees C and 700 degrees C, preferably in the 400- 650 degrees C range, during its electrolysis process, or more particularly in the 520-620 degrees C temperature range. However, in some embodiments the at least one electrolyser cell operates with an operational stack temperature of between 700 degrees C and 1100 degrees C.

In some embodiments, the method comprises connecting the electrolyser system to an intermittent or variable electricity source, and the electrical power generated using the stored heat of the heat storage unit is used to compensate for power fluctuations in the intermittent or variable electricity source. The intermittent or variable electricity source may be one or more power supply such as typical renewable power supplies, including, but not limited to, solar or wind or wave or tidal or hydroelectric power.

In some embodiments, the heat storage unit is additionally connected to an external heat source, which provides heat thereto to increase the amount of heat stored therein. This allows increased compensation for downtime in the electricity supply for the electrolyser.

Brief Description of Drawings

The present invention will now be described in further detail, purely by way of example, with reference to the accompanying drawings in which: Figure 1 schematically shows a first embodiment of an electrolyser system in accordance with the present invention;

Figure 2 a schematically shows a second embodiment of an electrolyser system in accordance with the present invention; and

Figure 3 schematically shows a typical electrolyser cell, multiples of which may be stacked in an electrolyser cell stack, for use as an electrolyser in the electrolyser system.

The present invention relates to coupling an electrolyser to a heat storage device which can selectively provide steam and/or electricity to the electrolyser when power for such inputs is unavailable, variable, unreliable or insufficient (e.g. due to intermittency or variability of supply). Figures 1 and 2 show different system configurations for coupling a heat storage device with an electrolyser so as to enable extended or continual operation with an intermittent or variable electricity source.

Referring first to Figure 1 , an electrolyser system 10 is shown. It uses electricity generated by an intermittent or variable electricity source 12 to power an electrolyser 16 to generate hydrogen and oxygen from water. These gases exit the electrolyser 16 in a hot condition (i.e. above ambient temperatures (or greater than 20 degrees C)). This is typically a temperature that is at or just below the operating temperature of the electrolyser. In the case of an electrolyser using solid oxide electrolyser cells - also known as an SOEC - this may be in excess of 400 degrees C.

The intermittent or variable electricity source may utilise external power sources, and commonly renewable energy supplies such as wind turbines 46, solar panels 48 or hydroelectric power 50.

The heat of the gases exiting the electrolyser - also known as off-gases - can be used to heat up a heat storage unit 14 by passing the gases into or through the heat storage unit 14. The stored heat can be returned to the electrolyser 16, as or when required, to maintain an operating temperature for the electrolyser 16. For this purpose, the heat storage unit can have a steam outlet 34 and a hot air outlet 30, which can connect via a steam supply line 36 and a hot air supply line 52 to a steam inlet 22 and a hot air infeed 32 of the electrolyser 16. In this embodiment, superheated steam exiting the heat storage unit 14 passes first through a regenerative heat exchanger 40 before then passing through a steam turbine 38 of a steam cycle so as to generate electricity. That electricity can be redirected to feed into the electrolyser 16 or can be utilised elsewhere by connecting external equipment to a power-take off 58. The outlet of the steam turbine 38 is recuperated 36 back at a lower pressure into the steam inlet 22. This steam is heated by the regenerative heat exchanger 40 which recovered heat from the superheated steam exiting the heat storage unit 14 to increase the temperature of the steam 36 entering the electrolyser 16.

With the present invention, the energy in the form of heat in the by-products of the electrolytic reaction of the electrolyser 16 is stored in the heat storage unit. This heat can then be used to provide electricity and/or steam to the electrolyser each either exclusively or simultaneously. In such a way, intermittent power flow, or power drop-outs, from the intermittent or variable electricity source can be smoothed out by reusing stored energy.

The electrolyser system 10 comprises a main supply of electricity from one or more sources. In the present example this may be renewable energy sources such as solar panels 48, wind turbines 46 and hydroelectric power 50. Commonly, only one of these would be present. The electrolyser 16 comprises at least one stack 28 of solid oxide electrolyser cells (SOEC) for carrying out the electrolytic reaction. This reaction converts water (in the form of steam) into hydrogen and oxygen. The outflowing gases from the anode and cathode then heat the heat storage unit 14 for storing at least some of the excess heat contained within the off-gasses released by the SOEC.

The heat storage unit 14 can then provide steam to a steam turbine 38 which generates electricity. The steam turbine also provides lower pressure steam for distribution back into the electrolyser 16.

In one example, the heat storage unit utilizes molten salt to store excess heat. The operating temperatures of molten salt storage systems are typically 150 degrees Celsius to 565 degrees Celsius. This range significantly overlaps with the operating temperature of low and intermediate temperature SOECs and as such is well suited to be thermally coupled with an SOEC as described above. In use, the excess heat is used to heat molten salt, and a steam generator is used to extract the heat when required. As described above, this steam can drive a turbine to generate electricity, supply the SOEC with steam, or both simultaneously. Alternatively, the heat storage unit could be metal-hydride or metal-carbonate based storage system. Such systems have the potential to have a higher heat storage density than molten salts but are more complex and expensive. Metal-hydride and metal- carbonate energy storage utilize a thermochemical reaction to store excess energy, which can then be delivered as steam and/or electricity as discussed above. These technologies can also be designed to have operating temperatures which overlap with those of SOECs so are well suited to be thermally coupled with an SOEC as discussed above.

Other heat storage units operating on similar principles which would be appropriate in the present context include a latent heat storage unit unit, a hot silicon technology unit, a steam accumulator unit or a molten aluminium technology unit.

In many of these cases, it is possible to design a heat storage unit having a higher power density than those of electrical batteries. Furthermore, storing excess power in the form of heat is beneficial as this excess energy may be required in the form of heat later to provide steam and/or to maintain the temperature of the SOEC stack.

The regenerative heat exchanger 40 assists in reducing heat losses when the steam produced by the heat storage passes through the steam turbine. It thus, in essence cools the hot steam released by the heat storage unit and then reheats the steam after it has passed through the steam turbine 38 so that it is at an appropriate temperature for introduction into the electrolyser 16 as a source of water for the electrolyser. It is important to note that the regenerative heat exchanger 40 shown is not necessary, but rather is a convenient way of ensuring the steam re-entering the SOEC is at an appropriate temperature. This could instead be achieved by a heater, and/or a heat exchanger with another (hot) stream of the system.

Referring next to Figure 3, the basic structure and operation of the electrolyser 16 is shown by reference to one electrolyser cell 20 of the stack 28. These electrolyser cells can also be known as regenerative fuel cells.

The electrolyser cell 20 comprises an anode 60, a cathode 62 and an electrolyte 64, as well known in the art. Water - here in the form of steam - is passed over the cathode 62 and hot air (optional - but useful as an extraction flow) is passed over the anode 60. Further, an electric current/voltage is applied across the electrolyser cell 20 via electric terminals 66, 68. As a consequence, an electrolytic reaction occurs across the electrolyte, with oxygen ions passing across the electrolyte from the cathode to the anode. Thus some of the steam is broken down into hydrogen on the cathode side of the electrolyser cell 20 and oxygen is generated at the anode side.

The oxygen can be extracted using a sweep flow. For example this may be via an air flow through an inlet side for venting the off-gas out through the off-gas outlet 36 on the anode side.

The hydrogen can instead be extracted and vented out of the other off-gas outlet 24 on the cathode side by the remaining flow of steam (only a part of the steam is converted into hydrogen and oxygen). Thus, the steam exiting the cathode side is ‘hydrogen enriched’, and the air exiting the anode side is ‘oxygen enriched’. These off-gasses will be at a similar temperature to the operating temperature of the electrolyser cell. In the case of an SOEC, this is usually in excess of 400 degrees C.

Such operational characteristics of a SOEC are well known in the art, but are beneficial for the present invention as the heat in the off-gases is able to be usefully used by the heat storage unit 14, rather than being wasted.

The off-gasses from the SOEC are passed through the heat storage unit via the heat source infeed and the excess heat from the off-gasses is stored in the heat storage unit 14.

Referring now to Figure 2, a second embodiment of the present invention is shown. This Figure shares many of the same elements as those shown in Figure 1 , and the same reference numerals have been used. Detailed description of the shared elements has been omitted for brevity.

As can be seen, this again shows an electrolyser system 10 with an intermittent or variable electricity source 12 such as wind turbines, solar panels or hydroelectric power, and a heat storage unit 14 with two heat source in-feeds 18 - one for hot oxygen- enriched air and one for hot hydrogen enriched steam. It also comprises the electrolyser 16, which again typically comprises at least one stack 28 of electrolyser cells 20 (not shown). The electrolyser 16 again has a steam inlet 22 and two off-gas outlets 24, 26 - for delivering off-gases - typically oxygen enriched air and hydrogen enriched steam - to the heat storage unit 14. In this embodiment the heat storage unit 14 is also shown to have exit ports 102, 104 for the off-gases once their heat has been extracted into the heat storage unit 14, for delivery of those gases elsewhere, or into bulk storage. Similar exit ports may also be present in the first embodiment, but are not shown for simplicity. However, in place of the turbine 38 and the regenerative heat exchanger 40, a steam cycle 70 is provided that is driven by hot gas or hot fluid exiting the heat storage unit 14. This steam cycle 70 - or even the steam turbine 38 of the previous embodiment, can instead be integrated into the heat storage unit 14 if preferred.

In this embodiment, hot fluid vents from the heat storage unit 14 via an outlet 74 and enters the steam cycle 70 via an inlet 76. Within the steam cycle 70 there can be a steam generator and steam turbine, or some other electrical energy producer that uses the heat of the steam or heat storage unit to power the electrical generation. The steam, once utilised by the steam cycle 70 can then vent from the steam cycle 70 via a steam vents 78, while generated electrical power exits the steam cycle 70 via an exit terminal 80.

As shown in Figure 2, there can be a bypass 72 for the steam cycle to allow the heat from the heat storage unit to directly pass to a steam supply line 36 for the SOEC. The hot fluid exiting the heat storage unit 14 will then typically be steam. This can negate the need for the steam cycle to have a steam generator in it. It can instead heat or generate steam by heating instead a separate water supply (e.g. via a heat exchanger or a heating loop).

An optional external steam supply 82 is also shown in Figure 2. The external steam supply 82 may be from an external industrial process (not shown). This might instead be a water supply for heating by the steam (or heated fluid) passing through the steam supply line 36. With steam coming from the steam supply line 36, the external steam supply, if intermittent or absent, can be compensated for by the team from the steam supply line 36.

Steam thus passes towards the electrolyser 16 along the steam supply line 36, and optionally from the external steam supply 82. It then passes through a heat exchanger 84 towards the electrolyser 16. In this embodiment, via the heat exchanger 84, the steam is heated - as it will have been cooled by the steam cycle. The heat exchanger in turn cools hydrogen-enriched steam venting from the electrolyser 16 from the first off-gas outlet 24 of the electrolyser 16. As it is preferred that the steam entering the electrolyser is at or close to the operating temperature of the electrolyser 16, a supplemental heater 90 might also be connected across a steam line 94. This first supplementary heater 90 can be powered by the intermittent or variable electricity source 12, or even by the electrical power exiting the steam cycle 70. In this example, a circuit connects from the exit terminal 80 thereof back to the intermittent or variable electricity source’s circuit 96.

That circuit 96 is also shown to be feeding power to the electrolyser 16 via the electrolyser power supply line 98. However, it also extends to a second supplementary heater 88, and a third supplementary heater 92, for providing the optional functionality discussed below. It can also feed elsewhere, if desired via the power take off 58

These circuits are only shown schematically for ease of reference.

The second supplementary heater 88 is for heating a hot air supply prior to entry into the anode side of the electrolyser cells within the electrolyser 16 at a hot air infeed 32 thereof. This is optional, and is required only if the hot air (which might instead be hot nitrogen in some embodiments, or any other suitable sweep gas, dependent upon whether suitable sources are available from nearby industrial processes or storage sources) is insufficiently heated already. A control system may control these supplementary heaters 88, 90, 92.

The hot air is provided from a mix of ambient air and optional pre-heated air from the heat storage unit 14. For the latter, the heat storage unit 14 can have an air inlet 44, as per the previous embodiment. The air inlet allows air to be heated in the heat storage unit before it is then passed along a hot air supply line 52 towards a second heat exchanger 86 or the second supplementary heater 88 (or both). A bypass line 96 is shown for bypassing the second heat exchanger 86, in case that is not required. A similar bypass might be provided likewise for the first heat exchanger 84.

In this embodiment, the second heat exchanger 86 uses the hot oxygen enriched air exiting the second off-gas outlet 26 of the electrolyser 16 to heat up ambient air before the ambient air enters the electrolyser. The second supplementary heater 88 tops up that heating if required. However, if the ambient air is mixed with the hot air from the heat storage unit, then the second heat exchanger 86 might instead be redundant. It might even serve to heat up the off-gas if the heat storage unit becomes hotter than the electrolyser.

For heating the heat storage unit, a third supplementary heater 92 is provided (alongside, perhaps, an external heat supply - for example from nearby industrial processes). This can also be driven by the intermittent or variable electricity source, or the power from the steam cycle, or both.

The provision of the heat exchangers, and the supplemental heaters, allows a wide range of heat sources, steam sources and power sources - intermittent, variable or constant - to be accommodated and any drop-outs, variations or reductions in supply can be compensated for, if needs be, by the stored heat in the heat storage unit 14 via the power and steam exiting the steam cycle 70.

The present invention has therefore been described above with reference to the drawings and the claims. It will be apparent to those of ordinary skill in the art that various modifications and variations can be made without departing from the scope of the invention as defined by the appended claims. For instance, features described as part of one embodiment can be used on another embodiment to yield a still further embodiment. Thus, it is intended that the present invention cover such modifications and variations as come within the scope of the appended claims.

Reference numerals in the claims are purely for illustrative purposes and do not limit the scope of the claims.

Claims

1 . An electrolyser system comprising: a heat storage unit; and an electrolyser; the heat storage unit comprising at least one heat source infeed; the electrolyser comprising at least one electrolyser cell, a steam inlet and at least one off-gas outlet, the off-gas outlet being connected to the heat source infeed to heat the heat storage unit; wherein the heat storage unit is configured to use its stored heat to produce steam for feeding into the steam inlet and for generating electrical power, either one at a time or both at the same time.

2. The electrolyser system of claim 1 , wherein the heat storage unit comprises a hot air outlet, and the electrolyser has a hot air infeed, the hot air outlet of the heat storage unit being connected to the hot air infeed of the electrolyser for selective delivery of hot air to the electrolyser during use of the electrolyser system.

3. The electrolyser system of any one of the preceding claims, wherein the heat storage unit comprises a steam outlet.

4. The electrolyser system of any one of the preceding claims, further comprising a steam supply line for feeding steam from the heat storage unit, or a steam turbine connected to the heat storage unit, to the steam inlet of the electrolyser.

5. The electrolyser system of claim 4, wherein the steam supply line connects between the heat storage unit and the steam turbine and between the steam turbine and the steam inlet, and it has a bypass for selectively bypassing the steam turbine.

6. The electrolyser system of any one of the preceding claims, further comprising a steam turbine for the selective generation of electrical power using heat or steam from the heat storage unit.

7. The electrolyser system of any one of the preceding claims, wherein the heat storage unit comprises a water inlet and/or an air inlet.

8. The electrolyser system of any one of the preceding claims, wherein the heat storage unit is configured to vaporise water within the heat storage unit to produce steam.

9. The electrolyser system of any one of the preceding claims, wherein the off-gas outlet connects via a supply pipe to the heat source infeed of the heat storage unit.

10. The electrolyser system of any one of the preceding claims, further comprising: a steam cycle or a steam turbine;

11. The electrolyser system of claim 10 further comprising: a regenerative heat exchanger; the regenerative heat exchanger arranged to exchange heat between an outlet of the steam cycle or steam turbine and the steam inlet of the electrolyser.

12. The electrolyser system of any one of the preceding claims, wherein the at least one electrolyser cell is a solid oxide electrolyser cell.

13. The electrolyser system of any one of the preceding claims, wherein the or each electrolyser cell has an operational stack temperature in excess of 400 degrees C

14. The electrolyser system of any one of the preceding claims, wherein the heat storage unit is a molten-salt heat storage unit.

15. The electrolyser system of any of claims 1 to 13 wherein the heat storage unit is a thermochemical heat storage unit,

16. The electrolyser system of claim 1 , wherein the at least one electrolyser cell is part of a stack of electrolyser cells.

17. A system comprising an intermittent or variable electricity source and an electrolyser system according to any one of the preceding claims.

18. The system of claim 17, wherein the intermittent or variable electricity source is configured to power the electrolyser and to heat the heat storage unit via a heating element, either both at the same time or individually.

19. The system of claim 17 or claim 18, wherein the intermittent or variable electricity source is configured to power the electrolyser, to heat or generate steam for the stack and/or to generate hot air for the stack, either both at the same time or individually.

20. A method of operating an electrolyser system, the electrolyser system comprising: a heat storage unit; and an electrolyser; the heat storage unit comprising at least one heat source infeed; and the electrolyser comprising at least one electrolyser cell, a steam inlet and at least one off-gas outlet, the off-gas outlet being connected to the heat source infeed; the method comprising: using off-gas from the off-gas outlet to heat the heat storage unit; and using heat stored in the heat storage unit to produce steam for feeding into the steam inlet and for generating electrical power, either one at a time or both at the same time.

21. The method of claim 20, wherein the electrolyser system is in accordance with any one of claims 1 to 16.

22. The method of claim 20 or claim 21 , comprising supplying additional heat to the heat storage unit from one or more external industrial process.

23. The method of any one of claims 20 to 22, comprising selectively using heat from the heat storage unit to produce hot air for feeding to the electrolyser or external equipment.

24. The method of any one of claims 20 to 22, comprising using said stored heat to compensate for fluctuations in delivery to the electrolyser of at least one of: a) heat, either as steam or hot air feeds to the electrolyser, or both, and b) electricity, for at least partially powering the electrolyser.

25. The method of any one of claims 20 to 24, comprising selectively delivering at least part of the generated electrical power is selectively delivered to the electrolyser cell to at least partially power the electrolyser.

26. The method of any one of claims 21 to 25, comprising controlling a stream of steam that selectively exits a steam outlet of the heat storage unit for delivery to each of the electrolyser and a steam turbine for generating electrical power, either individually or both at the same time.

27. The method of any one of claims 20 to 26, wherein the off-gas outlet provides offgas from the electrolyser and a gas supply line feeds that off-gas through the heat source infeed of the heat storage unit for heating the heat storage unit.

28. The method of any one of claims 20 to 27, comprising reducing the pressure of steam from an inlet pressure as delivered to the steam cycle or turbine, to a delivery pressure as it exits the steam cycle or turbine.

29. The method of any one of claims 20 to 28, wherein the electrolyser system comprises a steam cycle or a steam turbine and a regenerative heat exchanger, the regenerative heat exchanger exchanging heat between an outlet of the steam cycle or steam turbine and the steam inlet of the electrolyser.

30. The method of any one of claims 20 to 29, wherein the electrolyser system is connected to an intermittent or variable electricity source, and the electrical power generated using the stored heat of the heat storage unit is used to compensate for power fluctuations in the intermittent or variable electricity source.