Power plant arrangement

Abstract

A method of operating a power plant arrangement (12), the power plant arrangement (12) comprising at least one electrical generator (14) and first and second electrolysis stacks (42a, 42b). The method comprises: operating the first electrolysis stack (42a) to consume electrical power at a first power level corresponding to a first efficiency level for the first electrolysis stack (42a); and operating the second electrolysis stack (42b) to consume electrical power at a second power level corresponding to a second efficiency level for the second electrolysis stack (42b), the second efficiency level being lower than the first efficiency level. At least a portion of the electrical power consumed by each of the first and second electrolysis stacks (42a, 42b) is produced by the at least one electrical generator (14).

Description

[0001] Power plant arrangement

[0002] Technical field

[0003] The invention relates to power plant arrangements incorporating conversion of electrical power to fluids such as hydrogen gas, and in particular to wind power plant arrangements incorporating such conversion.

[0004] Background to the invention

[0005] Some power plant arrangements have begun to adopt the production of hydrogen and other fluid fuels or feedstocks within or alongside a power plant, using electrical power produced by generators of the plant. For the case of power plants in which electrical power is produced by renewable energy sources, such as a wind power plant, the hydrogen that is produced may be regarded as ‘green hydrogen’.

[0006] Such arrangements typically involve incorporating an electrolysis system into the power plant arrangement, which acts to consume electrical power supplied by wind turbines, or other power producing assets of the plant, to decompose an input of water into flows of hydrogen and oxygen. The hydrogen is typically the desired product as it can be used as feedstock in various ways, while the oxygen may be regarded as a byproduct. The electrolysis system may be incorporated into a power plant itself, and may even be directly coupled to a wind turbine generator, or alternatively may form part of a wider power plant arrangement in which the electrolysis system is separate from and electrically connected to a power plant, for example by a private transmission line.

[0007] An electrolysis system may include a set of electrolysis cells arranged in a stack, which may be referred to as an ‘electrolysis stack’. A stack may have a power rating of between 200 kW and 5 MW, for example, although this may vary. Each cell is typically an electrochemical cell comprising a pair of electrodes, namely an anode and a cathode. The cell acts to force a chemical reaction when a voltage is applied to the electrodes that decomposes a reactant into constituent products, for example to split water into hydrogen and oxygen. In principle, increasing the electrical power supplied to the stack increases the rate at which hydrogen is produced. An electrolysis system may incorporate multiple electrolysis stacks or modules that can be driven in parallel to increase the maximum output of the system and to enable larger power gradients. It is against this background that the invention has been devised.

[0008] Summary of the invention

[0009] An aspect of the invention provides a method of operating a power plant arrangement. The power plant arrangement comprises at least one electrical generator and first and second electrolysis stacks. The first and second electrolysis stacks may each be configured to consume an electrical power input to produce a flow of fluid such as hydrogen gas.

[0010] The method comprises operating the first electrolysis stack to consume electrical power at a first power level corresponding to a first efficiency level for the first electrolysis stack, and operating the second electrolysis stack to consume electrical power at a second power level corresponding to a second efficiency level for the second electrolysis stack. The second efficiency level is lower than the first efficiency level, for example at least 1 % lower and optionally lower by 5%, 10% or more. At least a portion of the electrical power consumed by each of the first and second electrolysis stacks is produced by the at least one electrical generator.

[0011] The method may be used when a total electrical power input for the first and second stacks is less than a combined capacity of the stacks, meaning that at least one of the stacks may be operated at a power level that is below its maximum capacity. The combined electrical power input for the first and second stacks may be determined on this basis, to enable the first electrolysis stack to operate below its maximum capacity and therefore at a higher efficiency level.

[0012] By operating the first and second stacks at power levels corresponding to different efficiency levels, the method exploits a non-linear efficiency flow relationship for the stacks, in that the efficiency of the first stack increases by more than the efficiency of the second stack decreases compared to a scenario in which the first and second stacks operate at the same efficiency level and receive the same overall input electrical power. In turn, the combined or average efficiency of the stacks increases compared to operating them at the same efficiency level. This means that the stacks can produce a greater quantity of fluid such as hydrogen gas for a given electrical power input, or conversely that a given quantity of fluid can be produced while consuming less electrical power. The second electrolysis stack may therefore be treated as a sacrificial stack, in that it is operated at a lower efficiency level to improve the overall combined efficiency of the first and second stacks collectively. The power plant arrangement may be embodied as a power plant that comprises the at least one electrical generator, the first electrolysis stack and the second electrolysis stack, for example. Alternatively, the power plant arrangement may comprise a power plant that includes the at least one electrical generator, while the first electrolysis stack and the second electrolysis stack are separate from the power plant whilst being part of the wider power plant arrangement. In this case, the first and second electrolysis stacks may be electrically connected to the power plant by a transmission line, for example. In either case, the power plant may be a wind power plant. The at least one electrical generator may comprise one or more wind turbines.

[0013] The method may comprise dividing an electrical power input unevenly between the first and second electrolysis stacks. The electrical power input may be received from the at least one electrical generator. Dividing the electrical power input unevenly may comprise allocating different levels of power to the first and second electrolysis stacks. Dividing the electrical power input unevenly may comprise allocating power disproportionately relative to respective capacities of the first and second electrolysis stacks, in which case the first and second power levels could be the same in absolute terms if the first and second stacks have different capacities.

[0014] The first power level may be lower than the second power level.

[0015] The method may comprise altering one of the first and second power levels while holding the other of the first and second power levels steady if an electrical power input varies. This may allow the updated level of electrical power input to be allocated between the stacks in a manner that enhances the combined efficiency of the stacks.

[0016] The second power level may correspond to a maximum capacity of the second electrolysis stack.

[0017] The first power level may correspond to a minimum output of the first electrolysis stack.

[0018] The first and second electrolysis stacks may be of a similar capacity, in which case the first and second power levels may be different.

[0019] The method may comprise operating the second electrolysis stack for accelerated degradation, to extend an operating life of the first electrolysis stack. The method may comprise determining the first and second power levels to control degradation of the first electrolysis stack and / or the second electrolysis stack.

[0020] The method may comprise determining the first and second power levels in accordance with at least one of: historical operation of each of the first and second electrolysis stacks; and a degradation state of each of the first and second electrolysis stacks. The second electrolysis stack may be more degraded than the first electrolysis stack. In other words, for a given pair of electrolysis stacks, the stack of the pair that is more degraded may be selected as the second electrolysis stack.

[0021] The method may comprise sacrificing performance of the second electrolysis stack to improve performance of the first electrolysis stack.

[0022] The first and second electrolysis stacks may be independently operable.

[0023] The first and second electrolysis stacks may be electrically separate and / or physically separate from one another. For example, the first and second electrolysis stacks may reside in separate electrolysis modules. Alternatively, the first and second electrolysis stacks may reside in the same electrolysis module while being electrically separate to support independent operation.

[0024] The first efficiency level may represent a proportion of electrical power supplied to the first electrolysis stack that is converted to an output fluid. The second efficiency level may represent a proportion of electrical power supplied to the second electrolysis stack that is converted to an output fluid.

[0025] The first electrolysis stack may form part of a first group of electrolysis stacks of the power plant arrangement, in which case each stack of the first group may be operated at the first power level corresponding to the first efficiency level. Relatedly, the second electrolysis stack may form part of a second group of electrolysis stacks of the power plant arrangement, in which case each stack of the second group may be operated at the second power level corresponding to the second efficiency level. So, the power plant arrangement may include more than two electrolysis stacks, one or more of which may be operated at a lower efficiency level than one or more of the other stacks. The power plant arrangement may therefore include more than one stack that is treated as a sacrificial stack, and / or more than one stack that is operated at a higher efficiency level. All of the variants and optional features noted above are applicable to arrangements having more than two electrolysis stacks. Another aspect of the invention provides a control system for a power plant arrangement. The power plant arrangement comprises at least one electrical generator and first and second electrolysis stacks. The control system is configured to: operate the first electrolysis stack to consume electrical power at a first power level corresponding to a first efficiency level for the first electrolysis stack; and operate the second electrolysis stack to consume electrical power at a second power level corresponding to a second efficiency level for the second electrolysis stack. The second efficiency level is lower than the first efficiency level. At least a portion of the electrical power consumed by each of the first and second electrolysis stacks is produced by the at least one electrical generator.

[0026] The invention also extends to a power plant arrangement comprising the control system of the above aspect.

[0027] It will be appreciated that preferred and / or optional features of each aspect of the invention may be incorporated alone or in appropriate combination in the other aspects of the invention also.

[0028] Brief description of the drawings

[0029] So that it may be more fully understood, the invention will now be described, by way of example only, with reference to the remaining drawings, in which like features are assigned like reference numbers, and in which:

[0030] Figure 1 is a graph showing a relationship between flow rate and efficiency for an electrolysis stack;

[0031] Figure 2 is a schematic view of a power system including a wind power plant;

[0032] Figure 3 is a graph showing a relationship between current density and degradation for an electrolysis stack;

[0033] Figure 4a represents a conventional approach for operating a pair of electrolysis stacks of the system shown in Figure 2; and

[0034] Figure 4b represents an approach according to an embodiment of the invention for operating a pair of electrolysis stacks of the system shown in Figure 2. Detailed description of embodiments of the invention

[0035] In general terms, embodiments of the invention relate to power plant arrangements incorporating means for converting electrical power into hydrogen gas or another fluid medium that can be used as a chemical feedstock or energy storage. Such arrangements may be referred to as ‘power-to-X’ or ‘P2X’ arrangements.

[0036] More specifically, embodiments of the invention relate to P2X arrangements in which multiple electrolysis stacks are used to produce the fluid medium, which may be gaseous or liquid. Each electrolysis stack may be configured to consume electrical power to decompose water into a flow of hydrogen and a byproduct flow of oxygen, for example, the hydrogen being the wanted fluid fuel representing chemical feedstock or energy storage.

[0037] Embodiments of the invention are not limited to any particular use of the produced hydrogen or other fluid medium, and are applicable to a range of use cases, including both off-grid PtX arrangements and on-grid PtX arrangements, for example. In off-grid arrangements, produced hydrogen may be conveyed out to users. For on-grid arrangements, electricity market prices may influence trade-offs between hydrogen production and electricity export to grid, and power levels chosen to operate electrolysis stacks (or modules) may be determined based on a combination of the efficiency characteristics of the stacks (or modules) and the other uses of available power at the PtX plant, such as electricity export.

[0038] Each electrolysis stack may form part of a respective electrolysis module, each module optionally having additional electrolysis stacks. The electrolysis stacks may alternatively form part of the same electrolysis module, whilst being operable individually independently of one another. In this respect, an electrolysis module may comprise one or more electrolysis stacks, and optionally associated auxiliary equipment for ‘Balance of Stack’. An electrolysis module may have a power rating of between 10 to 20 MW, for example, although this may vary. In turn, the electrolysis stacks, or modules, may form part of an electrolysis system.

[0039] Equipment for ‘Balance of Stack’ (BoS) may include equipment providing for: a flow loop for the electrolysis system; lye circulation for the electrolysis stacks; gas circulation for the electrolysis stacks; gas / liquid pressure balancing for the electrolysis stacks; pressure equalization of cells within the electrolysis stacks; pH and KOH (potassium hydroxide) balancing for the electrolysis stacks; and any other equipment that may be used to produce the raw hydrogen and oxygen gas. The efficiency of an electrolysis stack may be defined by the proportion of input electrical power received by the stack that is converted to hydrogen, or other wanted product, and therefore by the ratio of the input power to the output flow rate of hydrogen. Electrolysis stacks typically have a non-linear efficiency-flow dependency, meaning that the efficiency varies with the output flow rate in a non-linear manner. More specifically, the efficiency of an electrolysis stack reduces as the output flow rate increases.

[0040] In this respect, Figure 1 shows a graph indicating the relationship between the efficiency of conversion of electrical power to hydrogen and the output flow rate of hydrogen for an electrolysis stack. The graph makes clear that, as noted above, this relationship curves in a convex manner. An electrolysis stack may be regarded as a flexible load with a varying efficiency.

[0041] In principle, increasing the electrical power supplied to and consumed by an electrolysis stack, which may be referred to as the ‘applied load’, increases the output flow rate from the stack. However, due to the non-linear relationship between efficiency and output flow rate, an increased applied load does not result in a corresponding linear increase in the rate at which the stack produces hydrogen. Instead, electrolysis stacks tend to operate most efficiently at low power, and become progressively less efficient as the applied load rises.

[0042] In this respect, in accordance with Faraday’s laws of electrolysis, the current density, namely the electrical current flowing through the electrodes, achieved in an electrolysis stack is proportional to the output flow rate of hydrogen. However, the current density is not linearly dependent on the potential difference applied to the electrodes of the stack. Instead, the relationship between the applied voltage and the current density is known as the polarisation curve. In turn, the applied load, which corresponds to the product of the applied voltage and the current density, is also influenced by the polarisation curve, in that a given applied load corresponds to specific values for voltage and current density for a given electrolysis stack, as dictated by the polarisation curve.

[0043] As the relationship between the current density, and therefore the output flow rate, and the applied voltage follows a curve, the efficiency of the electrochemical reactions taking place inside the cells of a stack is typically inversely proportional to the applied voltage. In turn, efficiency falls with increasing flow rate in the manner shown in Figure 1. By way of example, doubling the output flow rate from a stack entails approximately doubling the current density. However, to achieve this the voltage applied to the electrodes may need to increase by more than double due to the shape of the polarisation curve. In turn, the applied load also increases by more than double.

[0044] Figure 1 also shows that the electrolysis stack has a minimum output flow rate of hydrogen, which may correspond to a minimum rate at which the electrochemical reactions that take place in the cells of the stack are sustainable over extended periods. This minimum flow rate, which occurs at a low applied load, also corresponds to a maximum efficiency of the stack, namely the point at which the highest proportion of the input electrical energy is converted to hydrogen.

[0045] Ordinarily, when electrolysis stacks of similar capacity are operated in parallel the available electrical power is divided equally between them. In consequence, all of the stacks operate less efficiently if the available power increases. Recognising this, in embodiments of the invention the electrical power used for powering multiple electrolysis stacks may be divided unevenly between the stacks, so that the stacks operate at different efficiency levels. If the stacks are of similar capacity, dividing the electrical power unevenly entails supplying different levels of power to each stack. If the stacks are of differing capacities, dividing the electrical power unevenly may entail supplying a disproportionate amount of power to one of the stacks with respect to the respective capacities of the stacks. Different power levels for the stacks, whether in relative or absolute terms, may entail power levels that differ by at least 1 %, and potentially by significantly more as shall become clear from the description that follows.

[0046] For example, one stack may be treated as a sacrificial stack by allocating to it a larger share of the input power, so that the sacrificial stack operates at a higher power level and current density, and therefore at lower efficiency, relative to another electrolyser stack, to enable that other electrolyser stack to operate in a more efficient region by allocating it a lesser portion of the input power. In this way, due to the non-linear relationship between efficiency and output flow rate, the overall efficiency of the stacks collectively improves, meaning that the stacks may collectively produce more hydrogen for a given total electrical power input than if that power input were divided equally between the stacks. In this respect, the efficiency of the stack operating at a lower power level increases to an extent that outweighs the decrease in efficiency of the sacrificial stack operating at a higher power level, as a result of the non-linear relationship between efficiency and output flow for the stacks. Alternatively, the same flow rate of hydrogen may be achieved with a reduced power consumption, meaning the input power can reduce. It may also be possible to reduce the number of electrolysis stacks required to produce a target level of hydrogen, thereby reducing the cost of the electrolysis system.

[0047] Operating an electrolyser stack in a low efficiency region may also increase a degradation rate of the stack, whereas conversely operating in a high efficiency region may decrease the degradation rate. The degradation rate may be proportionate to, or otherwise related to, the efficiency of the electrolyser stack, so that the degradation rate varies in a non-linear manner with respect to the output flow rate of the electrolyser stack in a similar manner to efficiency. In some embodiments this relationship is exploited, in that one stack, or sub-group of stacks, may be consistently designated as a sacrificial stack so that it is deliberately degraded at an accelerated rate, to protect other stacks from degradation. Thus, early replacement of the sacrificial stack(s) may be planned to delay replacement of other stacks, thereby reducing the overall rate of replacement for the stacks collectively.

[0048] It follows that embodiments of the invention may involve operating two electrolysis stacks differently, and more specifically at respective power levels that cause the stacks to operate at different efficiency levels. The stacks may be part of the same electrolysis module but operable independently, or they may be in different electrolysis modules. In the latter case, if the electrolysis modules include multiple stacks that cannot be operated independently, the modules may be operated differently to each other.

[0049] Some embodiments of the invention find application in renewable energy power plants that incorporate an electrolysis system having multiple electrolysis stacks alongside a group of generators that supply electrical power to the electrolysis system. Some embodiments find application in power plant arrangements in which an electrolysis system is separate from a power plant, and connected to the power plant by a private transmission line, for example, to receive electrical power generated by generators of the power plant, which generators may be wind turbines for example. In such embodiments, the combination of the power plant and the electrolysis system represents a power plant arrangement. Conversely, it is also possible for an electrolysis system to be coupled directly to a WTG of a wind power plant, and embodiments of the invention are also applicable to such arrangements.

[0050] In this respect, Figure 2 illustrates a typical architecture in which a renewable energy power plant is connected to a main grid or wider power network. In the example shown in Figure 2, the renewable energy power plant is a wind power plant (WPP). As will be understood by the skilled reader, a WPP comprises a plurality of wind turbine generators (WTGs). A WTG is commonly referred to as a ‘wind turbine’. The example shown is representative only and the skilled reader will appreciate that other architectures are possible. The WPP 12 includes electrolysis stacks that perform as shown in Figure 1 , such that the WPP 12 represents a power plant arrangement, as shall be described.

[0051] In other examples, the power plant may include alternative renewable energy sources such as a solar power plant, a bio energy power plant, an ocean / wave / tidal energy plant, or a hybrid power plant having a combination of different types of renewable energy power plants. Thus, the invention relates to renewable energy power plants and renewable energy generators in general, rather than being specific to wind power plants and generators as in the Figures.

[0052] The components of the wind power plant and power network are conventional and as such would be familiar to the skilled reader. It is expected that other known components may be incorporated in addition, or as alternatives, to the components shown and described in Figure 2. Such changes would be within the capabilities of the skilled person.

[0053] More specifically, Figure 2 shows a power system 10 incorporating a WPP 12. In this example, the WPP 12 includes a plurality of WTGs 14 and a control system 21 including a power plant controller (PPG) 22. Each of the WTGs 14 converts wind energy into electrical energy, which is transferred from the WPP 12 to a main power network, or ‘main grid’ 16, as active power and / or current, for distribution.

[0054] Each of the WTGs 14 is associated with a respective local WTG controller 15 in this example. In other implementations, a set of WTGs may share a single, semi-centralised WTG controller, such that there are fewer WTG controllers than WTGs. As will be understood by the skilled person, WTG controllers 15 can be considered to be local computer systems capable of operating a WTG 14 in the manner prescribed herein, and may comprise multiple modules that control individual components of the WTG orjust a single controller. The computer system of the WTG controller 15 may operate according to software downloaded via a communications network or programmed onto it from a computer-readable storage medium.

[0055] The WPP 12 also includes a connecting network 18 for connecting the WPP 12 to the main grid 16. In this example, the WPP 12 and the main grid 16 are connected at a Point of Interconnection (Pol) 20, which is an interface between the WPP 12 and the main grid 16. The Pol 20 may also be referred to as the Point of Common Coupling, which may be abbreviated to ‘PCC’ or ‘PoCC’. The PPC 22 is connected to the main grid 16 at a Point of Measurement (PoM) 24, such as a power meter, and is connected to each of the WTG controllers 15. For example, the PPC 22 may be configured to receive one or more measurement signals from the PoM 24, comprising measurements of the power supply from the WPP 12 to the main grid 16 and / or a frequency level of the main grid 16. The role of the PPC 22 is to act as a command and control interface between the WPP 12 and the grid 16 and, more specifically, between the WPP 12 and a grid operator 26, such as a transmission system operator (TSO) or a distribution system operator (DSO). The PPC 22 also receives data and requests from an energy management system (EMS) 27. The WPP 12 is capable of altering its power or current output in reaction to set points received from the PPC 22.

[0056] The PPC 22 is a suitable computer system for carrying out the controls and commands as described herein and so may incorporate a processing module 28, a connectivity module 30, a memory module 32 and a sensing module 34, amongst others, as shown in Figure 2.

[0057] The connectivity module 30, the memory module 32, and / or the sensing module 34 are configured to provide the processing module 28 with information that is indicative of a frequency level of the main grid 16, as well as power levels, current levels and / or voltage levels of the WTGs 14 and / or the WPP 12. For example, the sensing module 34 may receive such information directly from one or more connected sensors or power meters (e.g. at the PoM 24) and communicate the information to the processing module 28. Alternatively, or additionally, the information may be determined by one or more systems that are connected to the connectivity module 30, such as the WTG controllers 15, and the information may be communicated, in turn, through the connectivity module 30 to the processing module 28. In each case, the determined information may be stored permanently, or temporarily, in the memory module 32, from which it may be recalled, on demand, by the processing module 28.

[0058] During normal operation, the PPC 22 generates and sends dispatch signals to the WTG controllers 15. The dispatched signals contain active and reactive current, and / or power, set points determined by the PPC 22 based on the measurements received from the PoM 24, providing frequency and voltage support to the main grid 16. The WTG controllers 15 control the WTGs 14 according to the set points contained within the dispatch signals.

[0059] For example, the PPC 22 and the local WTG controllers 15 may each be arranged to work in a feedback mode in which they compare a reference value, for example from reference inputs, with a measured value, for example from measurement inputs, and produce a control signal based on the difference between the two input values. A bi-directional control network may be arranged between the PPC 22 and the WTG controllers 15 enabling two-way communication. For example, an uplink direction (i.e. the direction from the central PPC 22 to the local WTG controllers 15) is used to send reference values, e.g., for voltage and / or reactive power, from the PPC 22 to the local WTG controllers 15. A downlink direction may be used by the WTGs 15 to return information about their current operational state, for example regarding the amount of active power currently produced, to the central PPC 22. Such a control network may, for example, be implemented as a bus system, i.e. a CAN bus (ISO 11898) or an Ethernet bus (IEEE 802.3).

[0060] It is noted at this point that the system view of Figure 2 is schematic in form and so does not represent a complete practical system, which may include other components such as power inductors, chokes, filters, isolation switches, power dissipation choppers, breakers, and so on. However, such system components are within the purview of the skilled person and so are not discussed in detail in this disclosure.

[0061] The WPP 12 further includes an electrolysis system 36, which receives an electrical power supply from the connecting network 18 through a supply cable 38. The electrolysis system 36 is separate from the WTGs 14 in this example, although it is also possible for an electrolysis system to be integrated with a wind turbine and embodiments of the invention are also applicable to such arrangements. In addition, although the electrolysis system 36 is part of the WPP 12 in this example, in other examples a power plant arrangement may include an electrolysis system that is physically separate from a power plant, but electrically connected to the power plant to receive electrical power produced by electrical generators of the power plant.

[0062] Although the main focus of this disclosure is to green hydrogen generation, in some examples the electrolysis system 36 may be connected to a grid or a source of non-renewable energy. This is known generally as grey hydrogen generation. In such arrangements, the electrolysis system 36 may receive a portion of its input power from the grid or non-renewable energy source, with the remaining power being provided by one or more renewable energy sources. For example, the electrolysis system 36 may consume power from a grid when WTGs or other renewable energy sources are not producing power.

[0063] The electrical power received from the connecting network 18 is supplied by the WTGs 14 in alternating current (AC) form. The WTGs 14 therefore collectively provide an AC input power source to the electrolysis system 36 by way of the supply cable 38, which defines an electrical power input supplied to the electrolysis system 36. In this example, the electrical power input is supplied directly to the electrolysis system 36 from the WTGs 14, via the connecting network 18.

[0064] In this example, the electrolysis system 36 includes a front-end unit 40 that processes the AC power received from the connecting network 18 into direct current (DC) form that is suitable for components of the electrolysis system 36. Accordingly, the front-end unit 40 includes a transformer to provide a voltage step-up and an AC-DC converter to convert the input power to DC. In other examples, the front-end unit may be separate from the electrolysis system 36, or may even be omitted entirely if the input power is in a suitable form for the components of the electrolysis system.

[0065] The electrolysis system 36 comprises a group of electrolysis stacks 42, each stack 42 comprising a stack of electrolysis cells that operate using a DC power supply. Two individual stacks 42 are shown in Figure 2, namely a first stack 42a and a second stack 42b. The electrolysis system 36 may include more than two stacks 42 in practice. Each stack 42 may form part of a respective electrolysis module. As noted above, the stacks 42 shown in Figure 2 perform according to the relationship shown in Figure 1 when operating, in that they have a non-linear efficiency flow dependency.

[0066] The electrolysis stacks 42 are identical in this example and are independently operable, in that the electrolysis system 36 is configured so that the level of electrical power that is supplied to each stack 42 can be adjusted individually, to allocate the total power received from the connecting network 18 between the stacks 42 as desired. The stacks 42 are also electrically and physically distinct from one another, and can be replaced separately and individually, for example if a stack 42 becomes degraded or otherwise reaches the end of its operating life.

[0067] The electrolysis stacks 42 are each fed with an input water stream 44 by an appropriate water source 46. That water source 46 may supply fresh water, for example from storage tanks or from a pipe. Alternatively, in the case of a system based offshore, a de-saliniser may be used to remove salts from seawater and supply fresh water to the electrolysis stacks 42. Such a de-saliniser is a known system that would be understood by the skilled person and so is not described here.

[0068] The electrolysis system 36 provides a hydrogen output stream into a fluid line 48, through which the hydrogen stream can be conveyed to a user 50 of the generated hydrogen. The user 50 may be representative of a direct supply to a distribution network, for example, or it may be a suitable storage capacity such as a set of tanks. The hydrogen user 50 may also include a suitable compressor / dryer system to compress the hydrogen to a suitable pressure level (e.g., approximately 35 bar, or even 700 bar or higher) before storage and / or transportation.

[0069] Although the electrolysis system 36 produces hydrogen gas in this embodiment, in other embodiments an electrolysis system may produce other fluids, such as methane for example.

[0070] In principle, any suitable type of electrolysis system 36 may be used, the specification of which would be within the understanding of a skilled person. For instance, the electrolysis system 36 may be an alkaline system, a polymer-electrolyte membrane (PEM) system, or a solid- oxide electrolyser (SOEC), by way of example.

[0071] The first and second stacks 42a, 42b may be implemented as discrete units, or modules, that are physically and / or electrically separate from one another, and so may also each be referred to as ‘electrolysers’. Alternatively, the first and second stacks 42a, 42b may be part of the same electrolyser unit or module, whilst still being electrically separate and independently operable and replaceable.

[0072] In addition to the PPC 22 and the EMS 27, the control system 21 is represented as having two further functional blocks in Figure 2, namely a balance of stack (BoS) control system 52 and a front-end system 54. It should be noted, however, that this is not intended to infer any physical or logical restrictions on the actual implementation of the control system 21 , which could have more or fewer functional blocks in practice. The functional blocks may be implemented as separate controllers or may form part of the same controller. As such, the control system 21 may be implemented as a standalone computing device that is configured to communicate via a wired or wireless connection with the systems, sub-systems, sensing units and so on under its control. The control system 21 may also be implemented as distributed control units, for example to provide redundancy. The precise physical and logical implementation of the control system 21 is not central to the invention and so a detailed discussion is outside the scope of this disclosure.

[0073] The functional blocks of the control system 21 are electrically connected to one another and to the electrolysis system 36 by control channels 56, some of which are represented in Figure 2 by dashed lines. The control system 21 issues control signals through these channels 56 to control operation of the components of the WPP 12, for example to control the output power that is delivered to the electrolysis system 36 through the supply cable 38 and to control allocation of that power between the electrolysis stacks 42. The control channels 56 are also configured to return sensing information to the control system 21 that it may need to perform its control objectives.

[0074] The control system 21 is hierarchical, in that the BoS control system 52 and the front-end system 54 reside in a first level of control and the PPC 22 resides in a second level of control, to form a master-slave configuration in which the PPC 22 acts as master. The EMS 27 represents a third layer of control, and acts to generate recommendations, or ‘schedules’, for managing flows of power within the WPP 12 based on forecasts of, for example, power production and electricity pricing, to improve long term performance of the WPP 12. Those schedules are forwarded to the PPC 22, which then executes the recommended actions to the extent possible whilst meeting other objectives. To aid in its decision-making, the EMS 27 may receive measurement data relating to the WPP 12 via the PPC 22.

[0075] The grid operator 26 may be regarded as defining a further level of control that acts in parallel with the EMS 27, to the extent that the grid operator 26 supplies data and objectives to the PPC 22. The data supplied by the grid operator 26 could include measurements of frequency or voltage at the Pol 20, for example, while the objectives may include grid demands and grid services, which could include actions such as forced curtailment of the WPP 12 or activation of ancillary services for grid balancing, depending on the configuration of the WPP 12. The PPC 22 balances schedules received from the EMS 27 against the data and objectives received from the grid operator 26. The PPC 22 therefore acts to control operation of the WPP 12 in accordance with the commands and data received from the grid operator 26 alongside schedules received from the EMS 27.

[0076] More generally, the control system 21 may be configured to control substantially all components of the WPP 12, or alternatively separate controllers may be provided for some components, for example a backup system.

[0077] The front-end system 54 is responsible for operation of the front-end unit 40, and so acts under the control of the PPC 22 to control the processing of the electrical power received from the supply cable 38 into DC form for supplying to the electrolysis stacks 42.

[0078] The proportion of the total power output by the WTGs 14 that should be supplied to the electrolysis system 36 is determined by the EMS 27, with the remaining portion of the generated power being transmitted on to the main grid 16 for example. The PPC 22 then implements this schedule as far as possible within the operational constraints of the WPP 12, whilst also meeting demands from the grid operator 26. The EMS 27 may determine and schedule the electrical power input to be supplied to the electrolysis system 36 on the basis of a target hydrogen output from the electrolysis system 36, or alternatively the EMS 27 may determine the scheduled electrical power input for the electrolysis system 36 based on other factors, in which case the output of the electrolysis system 36 may vary.

[0079] The BoS control system 52 is responsible for distributing the electrical power output from the front-end unit 40 among the electrolysis stacks 42, based on specific operational schemes. As Figure 2 shows, the BoS control system 52 also communicates with the front-end system 54 to receive a control setpoint for the electrolysis stacks 42, which the front-end system 54 determines in accordance with the level of AC power that is received from the connecting network 18.

[0080] In general terms, the BoS control system 52 acts to control the load profile for the entire electrolysis system 36. In this embodiment, in a departure from convention the BoS control system 52 is configured to distribute the supplied power unevenly between the first and second stacks 42a, 42b. Specifically, the second stack 42b is designated as a sacrificial stack and so, when the supplied power is below the full combined capacity of the stacks 42a, 42b, is allocated a higher portion of the supplied power to operate less efficiently than the first stack 42a. The first stack 42a may be operated as a baseload to output its minimum output for optimised efficiency where possible, for example. This enables the WPP 12 to exploit the nonlinear efficiency flow dependency of the stacks 42 that is described above with reference to Figure 1.

[0081] To illustrate the practical effect of the non-linear relationship between efficiency and flow rate, Figure 1 shows a tangent line at a first operating point 57 on the curve, the tangent line representing the rate at which the efficiency reduces at the first operating point 57. This rate of reduction in efficiency is sharpest initially, at low flow rates, and then gradually reduces as the flow rate rises. The curve does not level off entirely, however, and so efficiency continues to fall with increasing flow rate up to a maximum output level of the stack 42.

[0082] Figure 1 also shows a second operating point 58 and a third operating point 59, which are positioned one on each side of the first operating point 57. The second and third operating points 58, 59 may be used in a control scheme for operating the stacks 42, as shall become clear from the description that follows. Figure 3 shows a graph representing another performance index of the electrolysis stacks 42, in this case the degradation rate of the stack 42 as a function of current density under a steady load. In this respect, an increase in the degradation rate represents a shortening of the lifetime of the stack 42, and therefore entails earlier replacement of the stack 42. Figure 3 indicates that the degradation rate may increase generally linearly with the current density. In turn, increasing the current density entails proportionally increasing the output flow rate of hydrogen, and so there is also a corresponding relationship between the degradation rate and the efficiency of the electrolysis stacks 42, in that operating a stack 42 at a low efficiency level also entails increased degradation while operating a stack 42 at a high efficiency level reduces degradation.

[0083] It is noted that the graph shown in Figure 3 is illustrative and the actual degradation performance of electrolysis stacks may vary in practice. In this example, the degradation rate is more strongly influenced by the operational setpoint, namely the current density, than by ramping of power during integration with the WTGs 14 or other energy sources.

[0084] Accordingly, and to summarise, increasing the voltage applied to a stack 42 increases the output flow rate, but at a disproportionate cost to efficiency and also with an associated acceleration in degradation of the stack.

[0085] In this context, embodiments of the invention are predicated on the insight that an overall benefit can be achieved by distributing electrical power unevenly between electrolysis stacks, as the higher efficiency of the stack receiving less power outweighs the lower efficiency of the stack receiving a larger share of the input power due to the shape of the curve shown in Figure 1 , so that the combined efficiency of the stacks increases. Accordingly, when the supplied electrical power input is at a level below the maximum capacity of the stacks, there is an opportunity to increase the combined output of the stacks by allowing one stack to operate at a higher efficiency level. Similarly, a given output flow can be achieved using a smaller electrical input if that input is divided unevenly between the stacks.

[0086] This principle is illustrated in a simplified example shown in Figures 4a and 4b, each of which show approaches for operating the first and second stacks 42 through successive stages of operation, the first stack 42a being shown to the left of the second stack 42b at each stage.

[0087] Figure 4a shows a first approach 60, which corresponds to the conventional approach for operating the stacks 42 in parallel, in which both stacks 42 are operated at the same setpoint, which in this example corresponds to the first operating point shown in Figure 1. Accordingly, the electrical power input is divided evenly and equally between the stacks 42 in the first approach 60.

[0088] Figure 4b shows a second approach 62, which is an approach according to an embodiment of the invention that is implemented by the BoS control system 52 for allocating a supplied electrical power input between the stacks 42 at each stage. In the second approach 62, the electrical power input is divided unevenly between the two stacks 42. Specifically, the first stack 42a is controlled using the second operating point 58 shown in Figure 1 , and so operates at low load and high efficiency, and the second stack 42b is controlled using the third operating point 59 shown in Figure 1 and so operates at high load and low efficiency as a sacrificial stack.

[0089] In the approaches 60, 62 shown in Figures 4a and 4b, the stacks 42 are controlled to produce the same combined output. In this example, the stacks 42 are operated to produce a combined output flow of 43kg / h, although this is purely illustrative.

[0090] As noted above, operating the stacks 42 at power levels corresponding to different efficiency levels, as in the second approach 62 shown in Figure 4b, achieves a higher overall efficiency for the stacks 42 collectively. Accordingly, the energy consumption of the stacks 42 to produce the target output flow rate is lower for the second approach 62 than for the first approach 60. By way of example, in the first approach 60 in which the stacks 42 are driven at equal loads, the energy consumption may be approximately 2.22MWfor illustrative purposes, whereas this reduces to approximately 2.16MW for the second approach. The difference in overall efficiency between the two approaches 60, 62 is in the order of 2% in this particular example. Again, these values are purely illustrative and will vary in practice.

[0091] Figures 4a and 4b also show the impact of the different operating approaches 60, 62 on degradation of the stacks 42.

[0092] In this respect, Figure 4a shows a first stage of operation 64a, shown furthest to the left, in which neither stack 42 has yet degraded and so the full target flow is achieved. This stage may correspond to when the electrolysis system 36 is first installed, for example. The output hydrogen flow is represented here as a full storage tank corresponding to the user 50 of Figure 2. Next, at a second stage of operation 66a after the stacks 42 have been operating for some time, both stacks 42 have degraded equally and so are producing a lower output flow rate from the same input power. In this example, the nominal capacity of each stack has degraded by 20% at this stage, and so each stack 42 outputs the same reduced flow rate. Finally, after a further period has elapsed a third stage of operation 68a is reached, at which point both stacks 42 have completely degraded and no longer output any flow of hydrogen at all, and so each have a nominal capacity of zero. Both stacks 42 therefore require replacement at this stage, involving intervention and downtime for the electrolysis system 36.

[0093] It is noted that replacing either of the stacks 42 at the second stage of operation 66a, when they are degraded by 20%, is generally undesirable, as replacing a single stack 42 at this point will yield only a 12.5% improvement in the output of the electrolysis system 36 in this example. It is also unclear which stack 42 to replace, as they are equally degraded. Also, if one stack 42 is replaced then the other will also need to be replaced relatively soon afterwards, creating a complicated servicing schedule and history for the electrolysis system 36 and potentially leading to the electrolysis system 36 being perpetually degraded to some extent. Similarly, replacing both stacks 42 early is also undesirable as it doubles the cost and work involved in the replacement.

[0094] Turning to the second approach 62 shown in Figure 4b, a corresponding first stage of operation 64b appears the same as for the first approach 60, as both stacks 42 commence operation with zero degradation. However, as noted above the distribution of the power input between the stacks 42 is different, meaning that the overall power consumption is lower to produce the target flow rate in the second approach 62.

[0095] A difference becomes apparent at a second stage of operation 66b in the second approach 62, which corresponds to the same elapsed operation time as the second stage of the first approach 60. In this respect, at this stage the first stack 42a, which is being operated at a relatively low load and high efficiency, exhibits little or no degradation. In contrast, the second stack 42b, which is being operated at the third operating point and thus at high load and low efficiency, has degraded significantly and to a greater extent than in the second stage of the first approach 60. Notably, however, the total output flow rate from the two stacks 42 is higher at this stage of the second approach 62 than at the same stage of the first approach 60.

[0096] Since the second stack 42b has degraded significantly due to its designation as a sacrificial stack, it is replaced shortly after the second stage of operation 66b. Replacing the second stack 42b at this stage will yield a 25% improvement in the system performance in this example. Accordingly, when a third stage of operation 68b is reached, after the same amount of time by which the stacks 42 had degraded completely under the first approach 60, both stacks 42 are performing strongly and exhibiting little degradation under the second approach 62. Accordingly, the first approach 60 results in both stacks 42 having to be replaced at around the same time and a reduced output as soon as degradation begins to take effect, whereas the second approach 62 allows the electrolysis system 36 to produce a higher average output with a reduced need for replacement of components, and with reduced energy consumption.

[0097] To summarise, the second approach 62 shown in Figure 4b deliberately accelerates degradation of one stack 42 of the electrolysis system 36 by operating it at a lower efficiency level, to achieve benefits for the wider system in terms of performance and reduced maintenance. Sacrificing the second stack 42b preserves the lifetime of the first stack 42a and provides an overall benefit. The accelerated degradation of the second stack 42b is predictable and so can be planned for as part of operating the second stack 42b sacrificially. In particular, early replacement of the second stack 42b can be scheduled, in the knowledge that the lifetime of the first stack 42a should be extended as a result. More generally, the power distribution approach exploits the flexible load behaviour of water electrolysers to increase the overall system efficiency, prolong stack lifetime, reduce the amount of maintenance required, and to ensure full capacity gain upon stack replacements.

[0098] Relatedly, when implementing the control approach shown in Figure 4b, the stack 42 that is to be operated as the sacrificial stack 42 may be selected by the BoS control system 52 on the basis of its operational history and / or state. For example, the BoS control system 52 may receive data from the stacks 42 providing real-time indication of the status of each stack 42, for example to indicate the respective degradation state of each stack 42. The BoS control system 52 may then select whichever of the stacks 42 is indicated to be the most degraded as the sacrificial stack 42, to which a higher proportion of the available electrical power will be allocated, thereby preserving the stack 42 that is less degraded and so reducing the number of stacks 42 that may require replacement in the short term.

[0099] More generally, a decision-making process for selecting the sacrificial stack may involve implementing an optimization problem, which may consider factors such as: the number of stacks in the electrolysis system; the electrical connection configuration of the stacks; the nonlinearity in efficiency-flow dependency for each stack; the performance distribution; the operational history of each stack during its active lifetime; and the dependency of the degradation on the current density.

[0100] It is noted that in this example the first and second stacks 42a, 42b are of equal capacity, and so dividing the electrical power input between them unevenly entails supplying different absolute levels of power to each stack 42, so that the first stack 42a receives less power, and so operates at a higher efficiency level, than the second stack 42b. In other examples, electrolysis stacks of an electrolysis system may have differing capacities, in which case dividing a supplied electrical power input unevenly between them may entail dividing the power disproportionately, so that one stack receives a level of power that represents a higher proportion of its capacity than another stack, so that the stacks again operate at different efficiency levels. It follows that dividing an electrical power input unevenly between two stacks of different capacities can potentially mean supplying equal absolute levels of power to those stacks.

[0101] It is also noted that an electrolysis system may have more than two stacks, in which case the stacks may be grouped into clusters each having one or more stacks. One cluster may act as a sacrificial cluster, in that each stack within the cluster is driven at a high load and low efficiency level, while another stack is operated as a baseload that is operated in a high efficiency region and for low degradation.

[0102] The skilled person will appreciate that modifications may be made to the specific embodiments described above without departing from the inventive concept as defined by the claims.

[0103] For example, in another variant an electrolysis system of a power plant arrangement may have electrolysis stacks of varying types in a hybrid configuration, for example a mixture of stacks based on alkaline, PEM and SOEC technologies. This may create further opportunities to improve the combined performance of the stacks by exploiting their different performance characteristics and cost of replacement, for example by operating a stack that is less expensive to replace as a sacrificial stack.

Claims

Claims1. A method of operating a power plant arrangement (12), the power plant arrangement (12) comprising at least one electrical generator (14) and first and second electrolysis stacks (42a, 42b), the method comprising: operating the first electrolysis stack (42a) to consume electrical power at a first power level corresponding to a first efficiency level for the first electrolysis stack (42a); and operating the second electrolysis stack (42b) to consume electrical power at a second power level corresponding to a second efficiency level for the second electrolysis stack (42b), the second efficiency level being lower than the first efficiency level; wherein at least a portion of the electrical power consumed by each of the first and second electrolysis stacks (42a, 42b) is produced by the at least one electrical generator (14).

2. The method of claim 1 , comprising dividing an electrical power input unevenly between the first and second electrolysis stacks (42a, 42b).

3. The method of claim 2, wherein dividing the electrical power input unevenly comprises allocating different levels of power to the first and second electrolysis stacks (42a, 42b).

4. The method of claim 2 or claim 3, wherein dividing the electrical power input unevenly comprises allocating power disproportionately relative to respective capacities of the first and second electrolysis stacks (42a, 42b).

5. The method of any preceding claim, wherein the first power level is lower than the second power level.

6. The method of any preceding claim, comprising operating the second electrolysis stack (42b) for accelerated degradation, to extend an operating life of the first electrolysis stack (42a).

7. The method of any preceding claim, comprising determining the first and second power levels to control degradation of the first electrolysis stack (42a) and / or the second electrolysis stack (42b).

8. The method of any preceding claim, comprising sacrificing performance of the second electrolysis stack (42b) to improve performance of the first electrolysis stack (42a).

9. The method of any preceding claim, wherein the at least one electrical generator (14) comprises one or more wind turbines.

10. The method of any preceding claim, wherein the first and second electrolysis stacks (42a, 42b) are electrically separate and / or physically separate from one another.

11. The method of any preceding claim, wherein the first efficiency level represents a proportion of electrical power supplied to the first electrolysis stack (42a) that is converted to an output fluid, and the second efficiency level represents a proportion of electrical power supplied to the second electrolysis stack (42b) that is converted to an output fluid.

12. The method of any preceding claim, wherein the first electrolysis stack (42a) forms part of a first group of electrolysis stacks of the power plant arrangement (12), wherein each stack of the first group is operated at the first power level corresponding to the first efficiency level.

13. The method of any preceding claim, wherein the second electrolysis stack (42b) forms part of a second group of electrolysis stacks of the power plant arrangement (12), wherein each stack of the second group is operated at the second power level corresponding to the second efficiency level.

14. The method of any preceding claim, wherein: the power plant arrangement (12) is embodied as a power plant that comprises the at least one electrical generator (14), the first electrolysis stack (42a) and the second electrolysis stack (42b); or the power plant arrangement (12) comprises a power plant that includes the at least one electrical generator, and the first electrolysis stack and the second electrolysis stack are separate from the power plant.

15. A control system (21) for a power plant arrangement (12), the power plant arrangement (12) comprising at least one electrical generator (14) and first and second electrolysis stacks (42a, 42b), the control system (21) being configured to: operate the first electrolysis stack (42a) to consume electrical power at a first power level corresponding to a first efficiency level for the first electrolysis stack (42a); and operate the second electrolysis stack (42b) to consume electrical power at a second power level corresponding to a second efficiency level for the second electrolysis stack (42b), the second efficiency level being lower than the first efficiency level; wherein at least a portion of the electrical power consumed by each of the first and second electrolysis stacks (42a, 42b) is produced by the at least one electrical generator (14).

16. A power plant arrangement (12) comprising the control system (21) of claim 15.

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