EP4699201A1

EP4699201A1 - A method for producing green hydrogen by electrolysis in a hybrid power plant

Info

Publication number: EP4699201A1
Application number: EP24715762.1A
Authority: EP
Inventors: Guido Schumacher
Original assignee: Cwp H1 Energy Pte Ltd
Current assignee: Cwp H1 Energy Pte Ltd
Priority date: 2023-04-18
Filing date: 2024-03-27
Publication date: 2026-02-25
Also published as: DE102023109764A1; WO2024217840A1

Abstract

A method for producing green hydrogen by electrolysis in a hybrid power plant (10), which comprises at least: - a wind turbine (11 ) with a rotor (11.1), a drive-train and a generator; multiple photovoltaic modules (12), - an electrolysis unit (15) for producing hydrogen by electrical power generated by the wind turbine (11) and/or the photovoltaic modules (12), an internal electrical power grid interconnecting the generator, the photovoltaic modules (12) and the electrolysis unit (15) within the power plant (10) and - a control unit (16); wherein a) electrical energy is generated by using the photovoltaic modules (12) and/or wind turbines (11 ); b) cloud coverage and/or solar radiation is measured by at least one weather sensor (14) which is located in a windward position remote of the power plant (10) and which is connected to the control unit (16) via a data link; According to a first aspect of the invention the wind turbine (11) is used as kinetic energy storage and according to another aspect of the invention the wind turbine (11) is used as an energy absorber by increasing inertia of the rotor (11.1).

Description

A Method for producing green hydrogen by electrolysis in a hybrid power plant

TECHNICAL FIELD

This invention relates to a method for producing green hydrogen by electrolysis in a hybrid power plant.

BACKGROUND

Renewable electrical power is often generated by a combination of arrays of photovoltaic (PV) modules and one or more wind turbine generators (WTG).

US 2022/0112107 A1 discloses a system for producing hydrogen from electrical power generated by either photovoltaic modules, a wind turbine, or both. Thus, there are at least two different sources of renewable electrical power. Such hybrid systems can be used to operate an electrolysis process, for example, to produce hydrogen.

In principle, the use of photovoltaic panels is preferred in such a hybrid system, since solar systems are easy to maintain, and are subject to little wear and tear. During periods of low or no solar radiation, the loss of solar energy can be compensated by a wind turbine in a hybrid system, if wind conditions are favourable or both wind and solar energy can complement each other.

Special weather events with rapidly changing conditions, such as thunderstorms, can cause a significant drop in solar radiation due to rapidly moving clouds. This can be predicted in the short term, but if the system is coupled with an electrolyzer, the reaction times of the chemical processes powered by energy from the renewable power plant are relatively slow. This can result in a need for additional power from the wind farm or a need to reduce the power from the wind farm when solar radiation increases again.

US 2022/0112107 A1 discloses a system for producing hydrogen from electrical power generated by either photovoltaic modules, a wind turbine, or both. A controller is provided to optimize various parameters of the two different sources of renewable electrical power generation and the electrolysis process. However, it does not specify how to compensate for decreasing solar energy so that the electrolysis process remains stable.

WO 2020/038537 discloses a method of operating a hybrid power plant comprising at least one wind turbine and at least one photovoltaic module, at least one of the photovoltaic modules being at a location within the hybrid power plant that is affected by blade shadows, the method comprising monitoring at least one operating parameter for one or more of the wind turbine generators; monitoring at least one operating parameter for one or more of the photovoltaic modules; and controlling operation of the wind turbine generators in dependence on the monitored operating parameters to control blade shadows cast by the wind turbine generators on the photovoltaic modules and thereby optimize power output of the photovoltaic modules.

US 2008/0121525 A1 discloses a power controller for power generation from renewable energy and consumption of that energy, if any, to produce hydrogen. The controller improves overall system efficiency by controlling power generation over a wider range of conditions and by controlling electrical conversion to the level required by the hydrogen converter much more efficiently than systems consisting of independent controllers. An overall system controller dynamically optimizes the entire system to maximize the available inputs, such as renewable and stored energy, while providing the maximum desired outputs, such as electricity, hydrogen, and revenue, considering the ultimate capacity of the components along with historical, current, and predicted future data.

OBJECT OF THE INVENTION

The object of the invention is to improve a hybrid system for producing hydrogen by electrolysis in a hybrid power plant, wherein electrical energy is generated by photovoltaic modules during daylight hours, and wherein at least one wind turbine is provided as an additional power generating system or as a back-up system.

SUMMARY OF THE INVENTION

If energy generation is possible by both photovoltaic and wind turbines, photovoltaic is assigned a higher priority. This allows the wind turbines to be operated below their rated power and leaves a buffer to use the rotor of the wind turbines as an inertia storage device in the sense of the present invention.

According to a first aspect of the present invention, there is provided a hybrid power plant comprising at least one wind turbine and at least one photovoltaic module, wherein the wind turbine and the photovoltaic module can both be operated by a common control unit, and wherein at least one weather sensor capable of cloud detection is provided at a location remote from both the wind turbine and the photovoltaic module. In addition, at least one wind sensor is provided.

The at least one weather sensor and the photovoltaic modules are arranged to be aligned with a wind direction prevailing in the region wherein the photovoltaic modules are located. Multiple photovoltaic modules are arranged in arrays to increase the yield of electrical solar energy. In the system of the present invention, the wind turbine is provided not only as a backup system for the photovoltaic modules to produce energy during cloudy weather or at night, but also as a kinetic energy storage system. The wind turbine used as a kinetic energy storage system allows to compensate the shading of the photovoltaic modules during short time intervals, i.e., a few seconds to a few minutes. The operation of the system is described below.

It is preferable for the hybrid power plant to operate in a predominantly solar mode. In this mode, the electrical energy is generated primarily by the photovoltaic modules. The wind turbine is operated either in idle mode or in a normal operating mode. The rotor speed in this mode can be less or equal to 100% of the nominal rotor speed, i.e. the rotational speed of the rotor (in rpm) which the wind turbine enables to generate its full electric power.

The sky in windward direction is scanned by the cloud detection sensor at the remote site. When the sensor detects cloud shading, the extend and direction is the cloud field is gauged and an information signal is sent to the controller via a wired or wireless control line. The direction and time of movement of the clouds toward the photovoltaic modules can be calculated by the controller. The controller can calculate if and when the clouds will arrive near or over the site with the photovoltaic module arrays and will at least partially shade them.

The controller also has access to the electrical power output of the wind turbine generator, the rotor speed, and the wind speed at the site. Using this data, the controller also calculates the required rotor speed, which is sufficient to store enough kinetic energy to bridge the period in which the photovoltaic modules are shaded by converting the kinetic energy back into electrical energy in the hybrid system or to lower the ramp rate of the electrolyser(s).

In another application the method is used to reduce the ramp rate only so that electrolysers can adopt better to changes in electrical energy supply.

After calculating the target rotor speed, the time required to accelerate to the desired rotor speed in kinetic energy storage mode is calculated based on the current wind speed, and a start time is set at which the controller begins to accelerate the rotor. All these actions take place before arrival of the cloud filed detected at the remote site.

By reducing the electrical output of the wind turbine generator, the rotor is accelerated to the desired rotor speed which can be greater than 100% of the nominal rotor speed for a short time which is sufficient to mitigate effects of PV shading on the operation of the electrolyzer.

After the acceleration period, the rotor will rotate at a speed that is greater than the typical rotor speed at the equivalent electrical power output of the generator and which might not be proportional to the electrical power output of the wind turbine generator. The additional percentage of rotor speed charges the hybrid power plant system with inertial energy provided by the rotating wind turbine rotor. When the clouds which were detected at the remote sensor site arrive at the site with the photovoltaic modules and begin to shade them, the controller stops accelerating the rotor.

Since the mass inertia of the wind turbine rotor and drive train is high, the spinning rotor contains a large amount of kinetic energy. This energy can be extracted by deceleration due to increased torque on the generator and converter side. The temporary increase in generator torque results in higher electrical power output. The rotor is slowed down to the minimum speed for power production, which is determined by the possible frequency range of the converter system and the overall wind turbine design regarding tolerances towards frequencies. Further deceleration would force the turbine to disconnect from the grid or otherwise lead to challenges or faults in the electrical distribution system.

Ideally, the extraction of kinetic energy takes place while the photovoltaic modules are in the shade of the cloud field. Once the clouds have cleared, the solar energy output of the photovoltaic modules increases again, and the wind turbine can be operated in a normal power generation mode. The system according to the invention is not only advantageous in case of shortterm interruptions of solar radiation since the energy loss due to shading can be compensated for a short time.

It is also advantageous in connection with the operation of an electrolysis plant when the solar radiation decreases rapidly and does not return in the short term, for example in the case of large cloud fields or at sunset in regions near the equator where there are only short twilight periods or when larger cloud fields pass over the PV site. By proactively controlling the wind farm, the expected drop in overall power supply, such as at sunset, and the associated curtailment of the chemical process in the electrolyzer can be delayed and the speed of power decrease can be slowed down by lowered ramp rates, which is advantageous for the process of generating hydrogen in an electrolysis plant.

Thus, the first preferred scenario for using the system and method of the invention is the expected lack of energy. Due to change in external conditions the wind turbine generators are demanded to supply more power than the current output for a limited period. The required period can take from a few seconds to a few minutes. Depending on the wind condition and the actual operational mode, different reaction modes are possible for the wind turbine generator. Furthermore, the situation of a power drop requires the support of the electrical grid with reactive power to stabilize voltage and frequency. This can be provided by modern wind turbines based on available Low or Zero voltage ride through “LVRT” or “ZVRT” -strategies through the provision of reactive power in case of voltage drops in the electrical grid.

A second preferred scenario for using the system and method of the invention is an energy oversupply. Due to a sudden change of the external conditions e.g., fast increase of the solar radiation in the internal electrical grid or the electrolyzer on the consumer side there might be an oversupply of energy in the renewable power plant. This requires support for the electrical grid to compensate potential overvoltage situations. This is manageable by modern wind turbine generators but must be foreseen in the windfarm control unit by using sensors and implementing a local weather forecast for the plant site.

Pursuant to the invention two operation modes of the wind turbine are possible wherein both have in common that the mechanical energy related to the inertia of the rotating rotor and drive train is converted to electrical power provided to the internal electrical power grid or vice versa. Besides they have in common that the conversion mode lasts at least as long that the energy level in the chemical process in the electrolysis unit can be adapted to a lower energy level slowly such that the electrolysis process is not jeopardized.

For this sake the rotor speed can even be increased up to above nominal rotor speed for a short period. The service life of the system - depending on the overall design, especially the dynamic behaviour - may not be materially affected if a rotor overspeed of approximately 120%, preferably 110%, of the nominal rotor speed is maintained for a period of a maximum of about 120 seconds.

In a first operation mode the weather sensor and the control unit detect that the photovoltaic modules will be shaded soon. From the data about cloud coverage and wind speed and wind direction provided by the sensor the control unit calculate the estimated time of arrival of the clouds at the site of the photovoltaic modules and can also calculate the expected loss in electrical energy when clouds will have arrived at the scene.

Preferably, the control unit also weights the expected loss of electrical energy against the demand for electrical energy in the electrolysis process and the time that will be needed for the electrolysis process to be safely adjusted to a lower energy level.

Having calculated the estimated time of arrival (“ETA”) and the amount of required additional electrical energy which will have to be provided by the wind turbine once the photovoltaic modules will be shaded by the detected clouds the rotor speed of the wind turbine is increased well before ETA. When the clouds arrive at the scene and the drop in electrical energy generated by photovoltaic modules is recognized, the mechanical inertia energy of the rotor is re-converted into electrical energy which is provided to the internal electrical power grid.

In the second scenario the conversion of energy is performed vice versa i.e. the inertia of the rotor and drive train is increased first to absorb a surplus in electrical energy provided by the photovoltaic modules and subsequently the rotor is decreased slowly.

The method according to the invention can be used in the two different scenarios, as already explained above, namely on the one hand to bridge or at least to mitigate a power drop in the photovoltaic modules that is to be foreseen, and on the other hand to convert excess energy occurring in the local network in the event of a foreseeable power increase into kinetic energy of the spinning rotor and thus to absorb energy from the local network and to relieve it. The operating state of the wind turbine before the expected event plays an important role in whether and to what extent the above tasks can be fulfilled.

In each scenario at least two operating states should be distinguished, which are differentiated by a minimum wind speed i.e. the wind speed above which the rotor is to be set in motion.

Preferably, even three operating states are distinguished, which are determined by the current wind speed at on site the wind turbine:

1 . The wind speed is below the minimum wind speed which is required to set the rotor in motion i.e. the “cut-in” wind speed of the WTG. Typically, the cut-in wind speed of an WTG is about 3 m/s.

2. The wind speed is above the cut-in wind speed and the rotor is already rotating, but the nominal wind speed and the nominal power for which the wind turbine is designed have not yet been reached. Typically, the range of the wind speed is between 3 m/s and 10 m/s.

3. The wind speed is higher than the nominal wind speed for which the wind turbine is designed, and the nominal power is exceeded. The typical wind-speed for this operating state is in a range between 10 m/s and 20m/s which is below a cut-out wind speed of about 25 m/s above which no operation of the wind turbine is possible. Curtailment is started at about 16-17m/s.

The most important parameters of the wind turbine to be set in the method of the invention are the pitch of the rotor blades and the generator load. The rotor speed is a result of the given wind speed on site and the parameters set.

In each of the following charts the parameters and the resulting effects on the rotor speed are compiled for all three operating states as defined above. The indicated rotor speeds are exemplarily for a rotor with a diameter of 200m.

Base scenario: normal wind turbine operation

The first chart represents a base scenario which is the normal behaviour of a wind turbine which is controlled solely by its proprietary controller, without an application of additional steps pursuant to the invention.

Scenario 1 : energy supply / Phase 1 : increasing rotor speed

The second chart represents a first phase of the first scenario of the invention where the wind turbine is “charged” with kinetic energy by deliberately increasing the rotor speed in advance of an anticipated event at the PV modules.

Scenario 1 : energy supply / Phase 2: extracting electrical energy

The third chart represents a second phase of the first scenario. When the predicted event occurs at the photovoltaic modules, additional electrical energy is “harvested” from the previously “charged” rotor, i.e. inertia of the spinning rotor at higher speed than usual is used to supply an additional amount of electrical energy to the electrolyser. The rotor speed is decreased thereby.

If the wind speed is below the cut-in wind speed the rotor speed can be reduced to zero as the wind turbine cannot be operated normally at this wind speed.

If the wind speed is sufficient for normal operation mode of the wind turbine the rotor speed is reduced to the minimum rotor speed only while kinetic energy is extracted. Thus, the normal operation can be started without the rotor standing still in between.

Scenario 2: energy absorption / Phase 1 : increasing rotor speed

In this scenario a surplus of electrical energy needs to be absorbed from the internal electrical grid for a short period. This is achieved by using electrical power from the grid to increase the inertia of the rotor by supply of energy to the generator and with that changing the generator into a motor or using a separate auxiliary motor preferably at the WTG drive train to accelerate the rotor.

Scenario 2: energy absorption / Phase 2: decreasing rotor speed

In the second phase of the second scenario the rotor speed must be reduced to a normal value for the current wind speed to continue the normal operation of the wind turbine after this phase.

OPTIONAL FEATURES

According to an aspect of the invention, the sensor data is continuously monitored and the time between the beginning and end of photovoltaic shading at the sensor location is recorded. From this data, the speed of cloud movement and the extent of the cloud field can be calculated. This is useful for predicting not only the estimated time of arrival (ETA) of the clouds at the photovoltaic modules site, but also the percentage of the area expected to be covered by the clouds.

To further improve solar radiation prediction, a LIDAR scanner can be used to determine the extent, path, and height of clouds, even if the clouds do not pass directly over the sensor location.

In accordance with another aspect of the invention, the system comprises a plurality of sensors located in different directions from the site in a windward direction so that all major wind directions of the site are covered. By combining data from adjacent sensors, the path in which clouds are expected to move can be more accurately determined and the probability of partial or total shading at the photovoltaic module site can be more accurately predicted. BRIEF DESCRIPTION OF THE DRAWINGS

Various embodiments of the present invention will be described in detail with reference to the drawings, wherein like reference numbers represent like parts throughout the various views. Reference to various embodiments is not intended to limit the scope of the invention. The figures shown herein are not limitations on the various embodiments of the invention and are presented for exemplary illustration of the invention.

One or more embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings in which:

Fig. 1a - 1e schematically illustrates a hybrid power with moving clouds used according to a first mode of the invention;

Fig. 2 schematically illustrates the hybrid power with moving clouds used according to a second mode of the invention;

Fig. 3 is a diagram in which a rated power is plotted as a function of the wind speed;

Fig. 4 is a diagram in which a rated rotor speed is plotted as a function of the wind speed;

Fig. 5 is a diagram in which rated power is plotted as a function of the rated rotor speed;

Fig. 6 is a flow chart of an exemplary process.

DETAILED DESCRIPTION

Figure 1 shows a hybrid power plant 10 in a north-facing side view. In the basic configuration shown, the power plant 10 comprises a wind turbine 11 , two arrays of photovoltaic modules 12, an electrolysis unit 15 for producing hydrogen, a control unit 16, and a weather sensor 14 located at a remote site. The remote weather sensor 14 is connected to the control unit 16 via a data line 13. The photovoltaic modules 12, the wind turbine 11 with a rotor 11.1 and the electrolysis unit 15 have further sensors or control units, each of which is also connected to the control unit 16 via a data line, so that a complete situation picture of the operating state of all components of the hybrid power plant 10 can be obtained in the control unit 16.

In the operating state shown schematically in Figure 1a, the sun 1 shines unobstructed on the solar collectors 12, so that the electricity required to operate the electrolysis unit 15 is primarily generated via these collectors.

In the area in which the hybrid power plant 10 is built, a wind direction W from west to east prevails. Cloud fields 2 therefore move from left to right in the figure 1a. Their occurrence, direction and speed of movement and, depending on the type of weather sensor 14 used, also their extent is detected by the weather sensor 14 and transmitted to the control unit 16.

The rotor 11.1 of the wind turbine 11 is in idle mode or is spinning at low speed. The wind turbine 11 is positioned at such a distance from the photovoltaic modules 12 that, on the one hand, a shadow cast by the rotor blades on the photovoltaic modules 12 is avoided and, on the other hand, is arranged at such a distance from the weather sensor 14 that the wind data recorded there are also valid for the location of the wind turbine 11 .

The weather sensor 14 forms a windward outpost and is located at such a distance from the photovoltaic modules 12 that the cloud fields 2 must travel for such a long time before reaching the photovoltaic modules 12 that there is sufficient reaction time at the wind speeds prevailing at the location. The reaction time is required to increase the rotor speed to a target value calculated in the control unit 16 and thus to temporarily store as much kinetic energy in the rotor 11.1 of the wind turbine 11 as it is required to bridge the calculated duration of the shading of the photovoltaic modules 12. In the situation shown in Figure 1a, the cloud field 2 is still within the detection range of the weather sensor 14.

In the following situation according to Figure 1 b, the cloud field 2 has already moved on with the wind speed W towards the site of the PV modules 12 and has left the detection range of the weather sensor 14. The weather sensor 14 registers that no further clouds are following the cloud field 2 for the time being, so that a measure can now be carried out via the control unit to compensate for the expected temporary shading of the photovoltaic modules 12 caused by the cloud field 2 moving eastwards with a known extent and speed.

The procedure carried out by the control unit to compensate for the expected drop in electrical power comprises several steps 101 to 10x, which are shown in the flowchart according to Figure 6. In the first step 101 , it provides for continuous monitoring of the sky by means of the weather sensor, to detect a cloud field 2. Once a cloud field 2 has been detected, in step 102 the extent, path and speed of the cloud field 2 are calculated. From this, in step 103, the controller 16 again calculates the expected time for the start of the expected drop in electrical power, as well as its duration and magnitude. In step 104, the required countermeasure is determined by calculating a target value for the increased rotor speed of the rotor to charge the wind turbine 11 with sufficient kinetic energy. Considering the given wind speed at the wind turbine 11 and/or the expected wind speed at the time of the start of the shading, the time required for the acceleration of the rotor 11.1 can also be determined, and from this the start time for the speed increase can be determined.

In the next situation, shown in Figure 1c, the cloud field 2 has moved even further towards the photovoltaic modules 12. The rotor of the wind turbine 11 is already rotating at an increased speed.

In the next stage, shown in Figure 1 d, the cloud field 2 is above the photovoltaic modules 12, so that a shadow 3 is formed. The reverse direction of the arrow on the rotor in Figure 1 d, compared to Figure 1c, indicates that the rotor is slowed down at this stage by its electric generator, which provides additional power to compensate for the shading of the photovoltaic modules.

Figure 1e shows the situation after the passage of the cloud field 2. The shadow 3 has left the field of the photovoltaic modules 12. The wind turbine 11 rotates at normal speed again. The weather sensor 14 is still facing the wind to detect further cloud fields.

In the operating state shown schematically in Figure 2, the sun 1 shines on the photovoltaic modules 12 through higher, diffuse cloud layers 4 over a longer period. These cloud layers 4 do not cause strong shading and therefore only slightly reduce the yield of the photovoltaic modules 12. However, the electrical power increases abruptly after passing through the cloud layers 4, which affects the chemical processes in the electrolysis unit 15. To delay a sudden increase in electrical power, the hybrid power plant 10 is now used in such a way that an already running wind turbine 11 is reduced in power or a stationary wind turbine 11 is started.

Both measures are designed to buffer a sharp increase in electrical power after the cloud layers have passed, by converting the increase in electrical power into kinetic energy of the rotor, whose rotor speed is increased for a short time by using the generator of the wind turbine as a motor or an auxiliary motor on the generator shaft.

Then, when the solar radiation has returned to a constant value, the rotor can be slowed down by the wind turbine generator, which slowly feeds electrical energy back into the local grid. The fact that this energy feedback is much slower than the time it takes for the energy generated by the photovoltaic modules 12 to increase after passing through the cloud field means that the chemical processes in the electrolysis unit 15 can be easily adjusted.

For this type of process control, the weather sensor 14 and the control unit are used in the same way as in the first variant of the process, whereby here only the end of the cloud field 4 lying in the wind direction and the wind speed need to be detected. This can then be used to calculate the time of the expected increase in solar radiation and, from this, the lead time required to prepare the wind turbine 11 to receive additional electrical energy from the photovoltaic panels 12.

Figure 3 is a diagram in which the rated power is plotted as a function of the wind speed. A rated power of 100% corresponds to the nominal power for which the wind turbine is designed. No power can be generated with the wind turbine at a wind speed below about 2,5 m/s (5 knots) which is the cut-in wind speed. Line 200 represents the rated power depending on the current wind speed if operational parameters of the wind turbine like the pitch of the rotor etc. are set according to standard routines implemented in the wind turbine’s controller for targeting the aerodynamic optimum.

Figure 4 is a diagram in which the rated rotor speed is plotted as a function of the wind speed. A rated rotor speed of 100% corresponds to the nominal rotor speed for which the wind turbine is designed. Line 210 represents the rated rotor speed that corresponds to the current wind speed on site if operational parameters of the wind turbine like the pitch of the rotor are set according to standard routines implemented in the wind turbine’s controller.

Figure 5 combines the functions plotted in figures 3 and 4 thereby showing the rated power as a function of the rated rotor speed. The rated power is represented by line 220.

All diagrams in figure 3 to 5 show specific operational areas in which the method of the invention can be carried out:

- The dotted areas 204, 214 in figures 3 and 4 refer to a curtailed operation, i.e. less energy is produced by the WTG than could be produced in the given wind conditions.

- Operation in the cross hatched areas 203, 213, 223 will take place in case rotor speed is already at rated speed and the wind conditions allow higher rated power output.

- Each of the areas 201 , 202, 211 , 212, 221 , 222 hatched with horizontal or vertical lines in figures 3 to 5 defines an operational mode which allows extraction of additional energy by deceleration of the turbine rotor. This is only possible for a limited period of time as too much reduction of rotor speed below the cutin speed would lead to a complete turbine stop.

- Each of the areas 201 , 211 , 221 hatched with vertical lines represents a field of operation with pure deceleration from actual rotor speed down to a defined limit e.g., the cut-in speed as mentioned above.

- Each of the areas 202, 212, 222 hatched with horizontal lines defines a range of rotor speed and power which is optimized for short-time power extraction by prior charging of the rotor with kinetic energy. Prior charging of the rotor is achieved by slight reduction of the drive train torque by control of the power conversion system. With reduction of torque the rotor starts accelerating. The faster spinning rotor stores more kinetic energy, which can be extracted in a subsequent next phase. This assumes that the wind conditions remain nearly constant during this event. This is expected to be realistic as the overall time period of this event is short e.g. up to 20 or 30 seconds.

Both operational strategies allow extraction of extra energy, but it must be considered that it is followed by a drop of energy and a ramp-up of power will be required with related impact on the overall balance of energy.

The stored kinetic energy of the rotating drive train can be quantified as:

J = Mass/lnertia of the rotor and drive train including generator 1 = Rotational speed

The period of energy extraction can be controlled and adapted to the actual power demand. The current wind speed defines the operational mode, out of which the energy extraction starts.

In case the turbine is in partial load due to lower wind speeds, the rotational speed is also lower, and the amount of extractable energy is reduced in relation to operation at higher wind speeds. Furthermore, the capabilities of the mechanical and the electrical system can be limiting factors due to:

- the maximum allowable torque of the mechanical drive train;

- the maximum current capacity of the power conversion system and

- the temperature limits of the cooling circuit, which is normally defined by the actual power of the gearbox, the generator, and the power conversion system.

Claims

Patent claims:

1 . A method for producing green hydrogen by electrolysis in a hybrid power plant (10), which comprises at least:

- a wind turbine (11) with a rotor (11.1), a drive-train and a generator;

- multiple photovoltaic modules (12),

- an electrolysis unit (15) for producing hydrogen by electrical power generated by the wind turbine (11) and/or the photovoltaic modules (12),

- an internal electrical power grid interconnecting the generator, the photovoltaic modules (12) and the electrolysis unit (15) within the power plant (10) and

- a control unit (16) wherein electrical energy is generated by using the photovoltaic modules (12) and/or wind turbines (11 ) characterized in that:

- higher priority in producing electrical energy is assigned to the photovoltaic modules (12) in comparison to the wind turbine (11 );

- the rotor speed of the rotor (11 .1 ) of the wind turbine (11 ) is kept at or below the maximum of nominal rotor speed;

- cloud coverage and/or solar radiation is measured by at least one weather sensor (14) which is located in a windward position remote of the power plant (10) with the photovoltaic modules (12) and which is connected to the control unit (16) via a data link;

- depending on the expected weather development at the site of the photovoltaic modules (12) the wind turbine (11 ) is either used either as a kinetic energy storage by accelerating the rotor speed of the wind turbine (11) or as an energy absorber by increasing inertia of the rotor (11.1) and drive train due to an increase of the rotor speed to mitigate effects of variable coverage of the sky above the photovoltaic modules (12) on the electrolysis unit (15).

2. The method according to claim 1 , wherein the wind turbine (11 ) is used as a kinetic energy storage by accelerating the rotor speed of the wind turbine (11 ) if increasing cloud coverage and/or decreasing solar radiation has been detected by the weather sensor (14) in the remote location wherein the rotor (11 .1 ) of the wind turbine (11 ) being coupled to the generator via the drive train is decelerated thereby compensating at least partial loss of electrical energy when at least some photovoltaic modules (12) are shaded by the clouds previously detected by the weather sensor (14).

3. The method according to claim 2, wherein for using the wind turbine (11 ) as a kinetic energy storage:

- for the previously detected clouds an ETA at the photovoltaic modules (12) is calculated,

- the expected loss in electrical energy when the photovoltaic modules (12) will be shaded by the previously detected clouds is calculated and

- the rotor (11 .1 ) is accelerated before the ETA to such rotor speed over minimum rotor speed that the kinetic energy of the rotor (11.1 ) and drive train will be able to compensate at least partially the expected electrical energy loss during the expected shading period of the photovoltaic modules (12).

4. The method according to any of claims 1 to 3, wherein the rotor (11 .1 ) is accelerated by adapting the pitch of the rotor blades if the wind speed at the wind turbine (11 ) is below cut-out wind speed.

5. The method according to any of claims 1 to 4, wherein the rotor (11 .1 ) is accelerated by decoupling the rotor (11 .1 ) from the generator and adapting the pitch of the rotor blades if the wind speed at the wind turbine (11 ) is above cut-in wind speed.

6. The method according to any of claims 1 to 5, wherein the rotor speed is increased up to above nominal maximum rotor speed for a short-time period sufficient to supply such an amount of additional electrical energy that the energy level in the chemical process in the electrolysis unit (15) can be adapted to a lower energy level.

7. The method according to claim 6, wherein the rotor speed is increased up to 120% of nominal maximum rotor speed for up to 120 seconds.

8. The method according to claim 1 , wherein:

- the wind turbine (11) is used as an energy absorber by increasing inertia of the rotor (11.1) and drive train due to an increase of the rotor speed when electrical output of at least some photovoltaic modules (12) increases due to reduced cloud coverage and

- the rotor (11 .1 ) of the wind turbine (11 ) being coupled to the generator via the drive train is decelerated thereby re-converting inertia to electrical energy provided to the electrical power grid.

9. The method of claim 8, wherein for using the wind turbine (11 ) as a energy absorber:

- for the previously detected reduced cloud coverage an ETA at the site of the photovoltaic modules (12) is calculated,

- the expected increase in electrical energy when the photovoltaic modules (12) will be shaded less by the previously detected reduced cloud coverage is calculated.

10. Method of claim 9, wherein before ETA of clouds at the site of the photovoltaic modules (12) the rotor (11.1 ) is decelerated to a rotor speed over minimum rotor speed that kinetic energy of the rotor (11.1) and drive train can compensate the expected energy increase during expected reduced shading period photovoltaic modules (12).

11. Method of any of claims 8 to 10, wherein for using the wind turbine (11 ) as an energy absorber the rotor speed is increased up to above nominal maximum rotor speed for a short-time period sufficient to convert such an amount of electrical energy from the internal electrical power grid to inertia that the energy level in the chemical process in the electrolysis unit (15) can be adapted to a lower energy level.

12. Method as of claim 11 , wherein the rotor speed is increased up to 120% of nominal maximum rotor speed for up to 120 seconds.