The present invention relates to a method for the temporary storage of energy, a wind turbine assembly, and an energy buffer system.

Renewable energy sources, such a wind turbines, solar panels, devices generating energy from tidal effects, devices generating energy from wave energy, etc. are employed increasingly more throughout the world. A property of such renewable energy sources is that the amount of electric energy generated by said renewable energy sources is, in general, non-constant and unpredictable. For example, solar panels generate less energy on a cloudy day than on a sunny day, or even no energy at all. When snow has fallen, solar panels generate no energy or virtually no energy. On a day with little wind, wind turbines generate little energy. On a very windy or stormy day, wind turbines generate much more energy than they do on average. Energy generated from tidal effects is supplied to the electricity grid in relatively short duration peaks (compared to a day’s length), followed by a substantial time of up to several hours in which no energy is generated. Hence, the amount of electric energy generated by renewable energy sources is, generally, both non-constant and unpredictable. At present, energy production by renewably energy sources in Europe accounts for approximately 10% of total energy production.

A peak in the generation of electric energy by renewable energy sources can cause problems to the electricity grid, e.g. as transformers in the grid cannot deal with high peaks in energy deliverance and may, as a default protective setting to prevent damage, shut off. When little energy is generated by renewable energy sources on the other hand, demand for energy may exceed supply, and energy may become very expensive and/or fossil fuel, e.g. coal-burning, power plants are still needed as a back-up, hampering a true‘shift of energy production’.

At present in most regions, fluctuations in the amount of energy generated by renewable energy sources can be handled by the electricity grid. However, when a larger percentage of the total electric energy is to be produced with renewable energy sources, it will become more and more difficult for the electricity grid to cope with the fluctuations in production. Some sources say that the above problems are a major reason why renewable energy can never be the main source of energy supply for the (Western) world, with the present technology.

So reasons exists for the development of effective storage of energy, e.g. resulting from an overproduction of energy (production peak) generated by a renewable energy source. On the one hand, such storage may buffer excess energy when supply of energy exceeds demand for energy, allowing to later make said stored energy available. On the other hand, such storage may produce additional energy when demand for energy exceeds the maximum supply of energy by renewable energy sources (demand peak).

Current energy buffering systems include batteries, fly-wheels, and the conversion of electrical energy into potential energy, e.g. by pumping water from a low location to a high location during excess supply of energy. Batteries and other chemical energy storage solutions are considered to be non-optimal solutions for the long term, for example as these solution (much like oil and gas) rely on rare or at least depletable Earth materials, such as lithium. Also problems are foreseen with regard to recycling of batteries. Fly-wheels are considered relatively inefficient and can only store a relatively small amount of energy. Pumping water to a higher location is considered a sub-optimal solution as it is only economically possible in regions of the world where there are natural height elevations, for Europe e.g. in Switzerland and Norway. When for example considering the problem of how to efficiently store excess electrical energy generated by offshore wind turbines, in the case of converting it to potential energy this excess electrical energy then first needs to be transported to the mountains of Switzerland or the fjords of Norway, which makes the process inefficient. Also, such systems would require a vast extension of the electric grid which is very expensive.

Solutions have been proposed to store energy by compressing air. For example,

DE102014104675 discloses to compress air in the monopile foundation of an offshore wind turbine when excess energy is generated, thereby buffering energy in the monopile of the offshore wind turbine, and to relief the compressed air when there is a demand for electric energy. The compressed air drives a generator, said generator generating electrical energy. This process is conceived to be relatively inefficient.

While it is recognized in the industry that the present solutions are sub-optimal, the quest for a more attractive or at least alternative solution remains active. The present invention aims to provide a method and system for the storage of energy. More specifically, the invention aims to provide a (cost) efficient method and system for the storage of energy.

The invention provides according to a first aspect thereof a method according to claim 1.

As will be explained in more detail herein, e.g. with reference to the figures, according to the first aspect of the invention the carbon dioxide is at least temporarily in a condensation state during each of the first step and of the second step. Preferably the carbon dioxide is in a condensation state throughout the cycle.

When a fluid is in its condensation state, it is at a pressure, volume, and temperature wherein the fluid is partially in its liquid state and partially in its gaseous state. In other words, while the conditions are stable, the fluid comprises both gas and liquid. For carbon dioxide, this occurs for example at a temperature of 21°C, a pressure of about 63 bar, and an appropriate specific volume. In the condensation state, the behaviour of a gas deviates from the‘perfect gas law’. More specifically, for C02 that is in the condensation state, when the volume of the space that contains the C02 is reduced, and when the temperature is kept substantially the same, i.e. when the compression process is isothermal, the pressure of the C02 remains the same, while the portion of the C02 that is in a gaseous state gradually condenses to a liquid state. Note that this is a“special” (but known) phenomenon, deviating from the‘ideal gas law’.

According to the ideal gas law namely, for“normal” conditions, isothermally reducing the volume of a gas results in an increase in pressure. In the condensation state on the contrary, the pressure remains the same while reducing the volume of the body containing the fluid has the effect that the fluid transforms (condenses) from a gas into a liquid. In other words, when a fluid is in its condensation state, reducing the volume of the body containing the fluid squeezes gas into liquid. The greater density of the liquid compensates for the reduction in volume, resulting in a constant pressure of the fluid. When using this principle in the method of the first aspect of the invention, the volume of the C02 can be reduced significantly, without increasing the pressure. On the one hand, this allows to precisely control the process, and requires only a moderate strength pressure vessel (in terms of the pressure it has to withstand). On the other hand, this significantly increases the energy storage capacity of the pressure vessel, as the volume difference resulting from the condensation of the carbon dioxide may now be occupied by (pressurized) liquid. This all results in the inventive method for the storage of energy that allows for a higher efficiency than e.g. pumping air into a pressure vessel, pressurizing the vessel, and regenerating energy from the compressed gas that is released from the vessel.

An advantage of the first aspect of the invention may reside in the pressure vessel not being subjected to (substantial) changes in internal pressure, which may translate into a reduced fatigue effect on the pressure vessel compared to methods wherein the pressure within the vessel must vary significantly in order to achieve substantial energy storage therein. This is in particular true for an embodiment wherein the carbon dioxide is in a condensation state throughout the cycle.

In embodiments of the invention, as will be explained herein, the pressure vessel may be embodied by a foundation element, e.g. a foundation pile, e.g. a monopile, of a wind turbine. In these embodiments it may even be advantageous, e.g. in view of fatigue, to have the, preferably rather constant, pressure within the pressure vessel stressing the wall of the foundation element in view of fluctuating loads on the foundation element by the wind turbine and/or waves at sea.

Another advantage of the first aspect of the invention is that pumping a liquid into the pressure vessel to charge the energy buffer, and generating electrical energy when de charging the energy buffer by discharging the liquid from the pressure vessel via a liquid driven generator, is much more efficient than pumping air into a pressure vessel and recovering energy by driving a generator with compressed air. Firstly, pumping a liquid is about twice as energy efficient compared to pumping a compressed gas. Also recovering energy from a compressed liquid by driving a liquid-driven generator, is up to twice as energy efficient compared to driving a generator with a compressed gas.

Another advantage of the present invention, related to the use of carbon dioxide in a condensation state thereof, is that the total amount of energy that can be stored in the pressure vessel is much higher than when the carbon dioxide is compressed whilst remaining in gaseous state during a charging step of an energy buffer.

A further advantage of the first aspect of the invention relates to the use of carbon dioxide as pressurization fluid. C02 is a gas that is naturally present in the atmosphere, and that may for example be taken out of the atmosphere to be used in the invention. Alternatively, the C02 may be captured from e.g. a coal-fired power station, preventing said C02 from entering the atmosphere. In any case, C02 is readily available, in endless quantities, for a low price. This is a first advantage of using carbon dioxide in the energy buffer. A second advantage of using C02 as pressurization fluid is that, in the unforeseen case of a leakage, it is not perceived as harmful to the environment, given the fact that C02 is present in the atmosphere anyways. Especially given the amount of C02 that is conceivably used in the pressure vessel, any leakage of carbon dioxide is quickly absorbed by the atmosphere.

In the first aspect of the invention the pressure vessel contains pressurized carbon dioxide therein. The carbon dioxide will at least partially be in a gaseous state and at least partially be in a liquid state while the energy buffer is charged and may also be so, preferably is so, in the fully de-charged condition of the energy buffer and in the fully charged condition thereof. Hence, the carbon dioxide preferably is in its condensation state in both the charged condition and the de-charged condition of the energy buffer.

The carbon dioxide may for example be pure carbon dioxide, e.g. having only trace amounts of other gasses in it, such that the pressurized fluid contains at least 98% carbon dioxide. The pressurized C02 may also be less pure, but preferably contains at least 90% or at least 95% carbon dioxide.

The maximum temperature at which carbon dioxide can be in a condensation state is 31.1°C. This is an inherent physical property of C02. As explained the condensation of the C02 is preferably done in isothermal manner at a temperature below said maximum.

In view of control of the temperature of the C02 in the buffer a wide range of measures may be employed.

In embodiments the pressure vessel, preferably lacking thermal insulation, is in thermal communication, e.g. in direct physical contact, with an external body of water, e.g. the external body of water being a natural body of water, like the sea, a river, a lake, a canal, a man made basin, etc. This is for instance appropriate in situations where the temperature of the external body of water allows heat to be removed from the pressure vessel, e.g. during the charging of the buffer.

In a practical embodiment the pressure vessel is preferably at least partially submerged in water, e.g. in sea water, e.g. for at least 30% of its volume, preferably for at least 50% of its volume, such as for at least 70% of its volume, in such a manner that the temperature of the sea water allows heat to be removed from the pressure vessel, e.g. during the charging of the buffer.

It will be appreciated that at sea the surface layer may warm up in summer which may impair heat transfer to the sea water. This may be taken into account when designing an offshore energy buffer, e.g. by designing the pressure vessel to be, at least in part, in direct physical and heat transferring contact with deeper water layers that are often not affected by seasonal changes.

When charging the energy buffer, if sea water is pumped into the pressure vessel, preferably said sea water is obtained from a deeper water layer that is relatively cold. When charging the energy buffer relatively fast, the process may be non-isothermal. More specifically, the temperature of the carbon dioxide may rise. The cold water may absorb some of the heat that is generated when reducing the volume of the carbon dioxide. This allows to charge the energy buffer faster, while ensuring that the process is (semi)- isothermal.

In an embodiment, during the first step, i.e. during charging, pumping liquid, e.g. water into the pressure vessel comprises spraying liquid, e.g. water into the pressure vessel, preferably at a level in the pressure vessel where the carbon dioxide is in a gaseous state. An advantage of spraying the liquid into the pressure vessel is that the liquid sprayed into the pressure vessel absorbs some of the heat generated when reducing the volume of the carbon dioxide, wherein the spraying results in a more efficient heat absorption. To this end one or more spraying devices may be provided in fluid communication with a liquid pump.

When de-charging the energy buffer, sea water from the surface layer may be added into the pressure vessel while discharging the liquid. While de-charging the energy buffer, especially when de-charging the energy buffer at a relatively large de-charging rate, de charging may be non-isothermal and the temperature of the carbon dioxide may drop.

When relatively warm sea water is introduced in the pressure vessel, this warm sea water may add heat to the carbon dioxide, ensuring a (semi)-isothermal process while allowing to de-charge the pressure vessel faster.

It will be appreciated that control of the temperature of the C02 may also, at least in part, be effected via the liquid that is pumped into (and later discharged from) the pressure vessel. E.g. this water may be taken from deeper water layers at sea, that are often not affected by seasonal changes.

For example the pressure vessel is of metal and lacks insulation to enhance transfer of heat to an external body of water.

In an embodiment the pressure vessel is provided with a heat exchanging system configured to cool at least the C02 and possibly also the liquid that is pumped into the pressure vessel, e.g. with an external ducting arrangement through which a coolant, e.g. water, e.g. sea water, flows, e.g. under influence of a coolant pump. For example the pressure vessel is arranged on dry land, e.g. embodied as a foundation pile or mast of a wind turbine, with water being pumped from a nearby natural body of water through the external heat exchanging system

According to the invention the pressure vessel is operated as an energy buffer, that has a first, de-charged, condition wherein the total amount of potential energy stored therein is low, and a second, charged, condition wherein the total amount of potential energy stored therein is high. Hence, the energy buffer may be charged with energy, much like one charges a battery with energy. The energy buffer may for example be charged when production of electrical energy by a renewable energy source, such as a wind turbine, exceeds demand from the grid, such that excess energy produced by said renewable energy source is stored in the energy buffer. The energy buffer may then be de-charged when demand from the grid is higher than the electrical energy produced by the renewable energy source, e.g. when the wind conditions are relatively silent.

In embodiments the method is performed such that bringing the buffer from the first, de- charged state into the second, charged state, by pumping liquid into the pressure vessel so that the liquid level rises from a lowest operational level to a highest operational level in the pressure vessel, is done in a period between 1 and 12 hours, e.g. between 6 and 12 hours. This is for instance practical in embodiments wherein the pressure vessel is embodied by a pile, e.g. monopile, foundation element of an offshore wind turbine or of another offshore renewable energy device. For example the part of the pile or monopile defining the pressure vessel herein is at least partly submerged and not provided with thermal insulation. In embodiments the method is performed such that bringing the buffer from the second, charged state into the first, de- charged state, is done in a time frame between 1 and 12 hours, e.g. between 6 and 12 hours.

It will be appreciated that in a cycle the second step of discharging the liquid from the pressure vessel may follow the first step with a delay wherein the volume of the liquid remains substantially the same. This delay could be brief, e.g. half an hour, but could also be a long time period, e.g. like 6 hours, 12 hours, or even longer.

A series of cycles may involve the pumping and following release of liquid with a liquid volume that varies from step to step depending on factors like the need for storage of energy, energy price, etc.

It is noted that the terms“low” and“high” in relation to the potential energy stored in the buffer are relative to the other condition. Hence, the energy contents of the total amount of potential energy in the energy buffer is low in the de-charged condition, with respect to the charged condition. Vice versa, the total amount of potential energy in the energy buffer are high in the charged condition, with respect to the de-charged condition. In an absolute sense, the total amount of potential energy in the de-charged condition of the pressure vessel may still be substantial. For example, in the de-charged state, the pressure vessel may contain pressurized carbon dioxide and, preferably, a minimum volume of liquid, pressurized to a pressure of up to 73.8 bar.

It is preferred for the method to include variation of the liquid level in the pressure vessel between a preset minimum level wherein there is a non-zero minimum volume of liquid in the vessel and a preset maximum level wherein there is a maximum volume of liquid in the pressure vessel. Of course a charge and decharge cycle may be done between these levels, or between any levels intermediate these preset minimum and maximum. Hence, in embodiments, the method involves variation of the volume of liquid in the pressure vessel between a non-zero minimum volume and a maximum volume such that only liquid is discharged from the vessel during the second step and the carbon dioxide remains in the pressure vessel throughout the cycle.

According to the invention, in a first method step, a liquid is pumped into the pressure vessel. When liquid is pumped into the pressure vessel, energy is thereby added to said energy buffer in the form of potential energy, and the energy buffer is charged. Preferably, the liquid is a relatively cheap substance. For example, the liquid may be water, e.g. taken from a natural body of water. For example, the liquid may be sea water, glycol, a mixture of water and glycol, or alcohol.

In an embodiment wherein the pressure vessel is arranged submerged or partially submerged in a natural body of water, e.g. the sea, a lake, a river, etc., it may be advantageous to use water taken from said natural body of water as the liquid that is pumped into the pressure vessel. So an offshore wind turbine installed at sea according to the invention may take in sea water in order to charge the buffer and discharge said sea water back into the sea upon de-charging the buffer.

A pump will be provided to perform the first method step, the pump e.g. comprising a liquid pump and a pump motor, wherein the pump motor is preferably electrically operated.

Preferably the pump has a pressure rating of at least 50 bars, e.g. of about 75 bars.

For example the pump is a piston pump.

A suitable pump is for example a piston pump that is nowadays employed in desalination plants to pump sea water with a pressure of about 60 bar through a membrane to desalinate the sea water.

It is noted that in the context of the present invention, a liquid is a substance that is in the liquid state, wherein a fluid is a substance that may be in the gaseous state, in the liquid state, or partially in the gaseous state and partially in the liquid state.

It is preferred that the carbon dioxide that is present in the pressure vessel remains in said pressure vessel at all times, so no relief of C02 takes place (e.g. except for a potential emergency relief and/or during inspection and maintenance). It is the liquid that is pumped into the pressure vessel and relieved from the pressure vessel to charge respectively de charge the pressure vessel and to generate power.

In an embodiment a C02 storage tank and/or source may be provided for, e.g. to refill the pressure vessel after a maintenance event and/or to replenish any operational loss of C02 (e.g. C02 that has been dissolved in the liquid and so has been discharged from the pressure vessel). One could also envisage, e.g. in an offshore situation, that C02 is supplied from a ship or lorry to the pressure vessel, the pressure vessel being equipped with a suitable connection.

According to the invention, in the second step, the liquid, and preferably only the liquid, is discharged from the pressure vessel. Thereby, the energy buffer is de-charged and electrical energy may be produced with said pressurized liquid. As explained, preferably, a minimum volume of liquid is retained in the pressure vessel in the fully discharged operational state. For example, the second step may include driving a liquid-driven generator with the liquid that is discharged from the pressure vessel to generate electrical energy. The liquid-driven generator is in use operated by the liquid that has been previously pumped into the pressure vessel and is configured to convert the potential energy that is stored in the energy buffer into electrical energy when said liquid is discharged from the pressure vessel, thereby driving the liquid-driven generator and de-charging the energy buffer. For example, a hydromotor is known as one (relatively cheap) example of a liquid- driven generator that is able to generate electrical energy when it is fed with pressurized liquid. For example, the pressurized liquid interacts with a rotary impeller or with a piston pump to drive the generator.

Preferably, a combination of an electric motor and a liquid pump connected to said electric motor acts as pump in the first step to pump liquid, e.g. water, into the pressure vessel in the first step, and acts as liquid-driven generator in the second step where the discharged water operates the pump and the pump drives the electric motor which then serves as generator.

In embodiments, the liquid pumped into the pressure vessel and discharged from the pressure vessel is water, e.g. sea water. Especially when the pressure vessel is part of an offshore device for generating renewable energy, such as an offshore wind turbine, a tidal current device, a wave generator device, sea water is readily available, for free.

Preferably, when using water from a natural body of water, e.g. the sea, as the liquid, the water is filtered before pumping it in the pressure vessel.

In preferred embodiments, the pressure vessel is at least partially submerged in a natural body of water, e.g. in the sea, a lake, a river, etc., and is directly in contact with said water, wherein the pressure vessel lacks thermal insulation, e.g. the pressure vessel being made of steel. In embodiments, the liquid pumped into the pressure vessel and discharged from the pressure vessel is fresh water. Fresh water is a cheap liquid, that is readily available in large quantities. An advantage of using fresh water compared to sea water is that it contains less contaminants, such that filtering may not be required.

In embodiments, the first step includes pumping liquid, e.g. water, from a storage tank into the pressure vessel and the second step includes discharging said liquid from the pressure vessel into said storage tank. In this embodiment, a closed-loop liquid system may be provided, wherein the same volume of liquid is used repeatedly to charge and de-charge the energy buffer. A closed-loop liquid system advantageously has zero or only a very small impact on the environment of the system. For example, it allows for the use of a mixture of water and glycol as liquid.

The storage tank for the liquid may, for example, be a flexible tank, e.g. accommodated on the bed of the body of water near an offshore wind turbine according to the invention or integrated/accommodated in the foundation of an offshore wind turbine.

In embodiments, the molar quantity of the carbon dioxide in the pressure vessel is substantially the same in the charged condition and in the de-charged condition of the energy buffer, i.e. the carbon dioxide is contained inside the pressure vessel when charging and de-charging the energy buffer. Advantageously, this results in a closed carbon dioxide system, wherein the same carbon dioxide is repeatedly decreased and increased in volume (compressed and decompressed) when charging respectively de-charging the energy buffer. A closed carbon dioxide system advantageously has zero or only a very small impact on the environment of the system. It is noted that some leakage or loss of C02 could occur in practice. Therefore, the wording“closed system” does not imply that there are no leaks; it merely implies that substantially the same C02 is used repeatedly.

When the method is performed such that the pressure in the pressure vessel is always the same, and when the temperature is substantially constant, the carbon dioxide is always in its condensation state.

In embodiments, the pressure of the carbon dioxide is between 29.6 bar (30 atm.) and 72.8 bar (73.8 atm.) or between 30 bar and 73.8 bar. For example, the pressure of the carbon dioxide and the liquid in the charged condition of the pressure vessel may be between 29.6 bar (30 atm.) and 72.8 bar (73.8 atm.) or between 30 bar and 73.8 bar.

The maximum pressure at which a condensation state may be achieved with carbon dioxide is 73.8 bar. This is an inherent physical property of carbon dioxide; above this pressure carbon dioxide is either in a supercritical, a liquid, or a solid state; depending on the specific pressure and temperature. A minimum value of 29.6 bar (30 atm.) or 30 bar is a practical one, e.g. for embodiments wherein the liquid is water. If carbon dioxide is to be in a condensation state at 29.6 bar (30 atm.) or 30 bar, the temperature of the carbon dioxide is below 0°C.

Preferably, the process of charging and de-charging is isothermal, wherein preferably the temperatures of the liquid and the carbon dioxide are similar. Of course, when water is below 0°C, at pressures between ambient and 69.1 bar (70 atm.) or 70 bar, it is frozen and it cannot be pumped or discharged. Sea water must be a bit colder before it freezes;

approximately -4°C.

In an embodiment, the temperature of the carbon dioxide and/or the liquid, at least in the charged state of the pressure vessel, is between -4°C and 31.1°C, when the liquid is sea water.

In another embodiment, the temperature of the carbon dioxide and/or the liquid, at least in the charged state of the pressure vessel, is between 0°C and 31.1°C, when the liquid is fresh water.

In another embodiment, the temperature of the carbon dioxide and/or the liquid, during the cycle, is at most 31.1°C.

Preferably, the temperature of the carbon dioxide and the liquid is substantially the same, to allow an isothermal process when charging and de-charging the energy buffer.

Sea water freezes at approximately -4°C, hence, preferably the temperature of the liquid is above -4°C when the liquid is sea water. Water freezes at 0° C, hence, preferably the temperature of the liquid is above 0°C when the liquid is water. When the liquid is another substance, other minimum boundaries may apply to ensure a practical working. When the temperature of carbon dioxide exceeds 31.1°C, it cannot reach its condensation state. Hence the temperature thereof is maximally 31.1°C.

In embodiments, the method further comprises the steps of monitoring the level of the condensed carbon dioxide in the pressure vessel and/or monitoring the level of the liquid in the pressure vessel. In embodiments a release of carbon dioxide and/or liquid from the pressure vessel is effected when the level thereof exceeds a pre-determined value.

Preferably, in the fully charged condition of the energy buffer, some of the carbon dioxide is still in the gaseous state, such that the pressure in the pressure vessel is below 72.8 bar (73.8 atm.) or 73.8 bar.

When all carbon dioxide would be in the liquid state, and when even more liquid is pumped into the pressure vessel, the pressure inside the pressure vessel would rise very rapidly.

This may cause damage to the buffer system. Hence, to prevent this from happening and/or for other reasons, the level of the condensed carbon dioxide and/or the level of the liquid may be monitored. When too much of the liquid is pumped into the pressure vessel, and/or when too much of the gaseous carbon dioxide has condensed to a liquid state, and when the pump is still active (e.g. because it hampers or cannot be shut off), it may be desired to release some of the carbon dioxide and/or liquid from the vessel.

In embodiments, the method further comprises the step of monitoring the pressure, e.g. of the carbon dioxide and/or the liquid, in the vessel. In embodiments a release of carbon dioxide and/or liquid from the pressure vessel is effected when said pressure exceeds a pre-determined value. Instead of monitoring automatically opening relief valve(s) may also be provided for, e.g. releasing water used as liquid back into the natural body of water from which it was taken.

Preferably, the temperature of the liquid and/or of the carbon dioxide is monitored.

In embodiments the temperature of the pressure vessel wall is monitored.

In embodiments wherein the liquid is taken from a natural body of water, the temperature of said natural body of water is monitored. In preferred embodiments, the pressure vessel is at least partially submerged in a natural body of water, e.g. in the sea, a lake, a river, etc., and is directly in contact with said water, wherein the pressure vessel lacks thermal insulation, e.g. the pressure vessel being made of steel.

Preferably, in the fully charged condition of the energy buffer, some of the carbon dioxide is still in the gaseous state. When all carbon dioxide is in the liquid state, and when more liquid is pumped into the pressure vessel, the pressure inside the pressure vessel rises very rapidly. To monitor the contents of the pressure vessel, it may be undesired that the pressure inside the pressure vessel exceeds 72.8 bar (73.8 atm.) or 73.8 bar, or depending on the temperature, it may be undesired that the pressure inside the pressure vessel exceeds another upper limit, the limit being lower than 72.8 bar (73.8 atm.) or 73.8 bar. While during the condensation of the carbon dioxide the pressure in the pressure vessel remains substantially constant, when all the gaseous carbon dioxide has condensed to the liquid state and still more liquid is added to the vessel, the pressure rises rapidly. In that case, e.g. when the pump cannot be switched off, it may be desired to release some of the carbon dioxide and/or liquid from the vessel.

In embodiments, the power to perform the first step is obtained from a renewable energy source, such as a wind turbine, e.g. an offshore renewable energy source, such as an offshore wind turbine.

In embodiments, the pressure vessel is integrated with the wind turbine, e.g. the pressure vessel is embodied by a foundation element of the wind turbine, and e.g. the pressure vessel is embodied by a foundation element of a wind turbine that is at least partially submerged in a natural body of water.

In an embodiment, the pressure vessel is formed by a monopile forming the foundation of a wind turbine, e.g. an offshore wind turbine, wherein the monopile is at least partially submerged in a natural body of water, e.g. at sea. When embodying a monopile as a pressure vessel, the pressure vessel is obtained almost‘for free’, as the monopile is already very strong to withstand (piling) loads, and applying a constant internal pressure of at most 72.8 bar (73.8 atm.) or 73.8 bar to a monopile construction hardly changes the design thereof, or does not change the design thereof at all. Therefore, when planning an offshore wind turbine assembly, the energy buffer may be obtained almost for free when planning for it in advance. This makes a huge step towards the feasible and economical implementation of offshore wind.

In embodiments, especially when the pressure vessel is able to heat up during the day, e.g. when the pressure vessel is exposed to sunlight, the first step may be performed at night, i.e. between sunset and sunrise, and the second step may be performed after 3 o’clock PM. During the day, the liquid and the carbon dioxide in the pressure vessel may heat up, resulting in an increased amount of energy stored in the buffer. This increased amount of energy is gained for free, and is preferably released before the sun settles again.

In an embodiment, the first step is performed when demand for electrical energy by the electrical grid is lower than the amount of electrical energy generated by a renewable energy source to which the energy buffer is linked or with which it is integrated. In other words, preferably the first step is performed when the renewable energy source

‘overproduces’ electrical energy.

In an embodiment, the second step is performed when demand for electrical energy is higher than the amount of electrical energy generated by the renewable energy source to which the energy buffer is linked or with which it is integrated. In other words, preferably the second step is performed when the renewable energy source‘under produces’ electrical energy.

In embodiments, use is made of a fluid barrier that at least partially separates the C02 from the liquid in the pressure vessel.

According to a second aspect of the invention, an energy buffer according to claim 15 is provided.

A third aspect of the invention relates to a wind turbine assembly according to claim 16.

The third aspect of the invention relates to an wind turbine, e.g. an offshore wind turbine, equipped to perform the method according to the first aspect of the invention. Embodiments discussed in relation to the first aspect of the invention may thus also be advantageous for the third and other aspects thereof. The wind turbine according to the third aspect of the invention may benefit from the same advantages as described in relation to the first aspect of the invention. Besides advantages discussed in relation to the method according to the first aspect of the invention, the third aspect of the invention benefits from additional advantages.

An advantage of the third aspect of the invention relates to embodying a pipe of the floating or seabed-bound foundation as a pressure vessel. In general, the foundation of an offshore wind turbine is very strong and significantly overdesigned for the loads it has to withstand. Hence, embodying the pipe as a pressure vessel requires little to no changes in the design of the pipe, especially when the internal pressure is substantially constant. The foundation is arranged in a harsh environment: sea water, where it is for example subject to rust forming. Also, the dynamic forces on the foundation, due to e.g. the rotation of the blades of the wind turbine, wind gusts, and the tidal forces of waves, are significant. Some types of foundations, e.g. the monopile, may additionally require the foundation pipe to be piled in the seabed. The above plays an important role in the design of the foundation of an offshore wind turbine and typically result in a foundation pipe that is very strong. In general, embodying (a portion of) the foundation pipe as a pressure vessel that is e.g. capable of containing a fluid that is pressurized up to 80 bars, the pressure preferably being constant, is only a minor additional design requirement, that only marginally influences the design of the pipe. This additional design requirement thus increases the price of the foundation pile only a bit or not at all.

Another advantage of the third aspect of the invention relates to the fact that excess energy can be stored locally, at the offshore wind turbine assembly itself. This may be mainly advantageous when considering the layout of a wind farm. A wind farm comprises a number of wind turbines, e.g. a dozen, dozens, up to a hundred or even more. The wind farm comprises one or more transformers, that receive electrical energy from the electrical generator of the wind turbine and that deliver said electrical energy to the electrical grid system. When there are fluctuations in the amount of energy generated by the wind turbines, in particular when there is an excess of energy (overproduction), the transformer has to‘protect’ the grid. That is, the transformer must ensure that the electrical grid is not overloaded with electrical energy. Such transformers, especially when they are installed offshore, can be very costly; costing up to five million euros or more. When a considerable amount of excess energy generated by the wind turbine can be stored in the wind turbine itself, i.e. when the excess energy generated by the wind turbine can be managed without supplying it to the transformer, the transformer may have less stringent design requirements, resulting in a reduction in size and/or a lower number of transformers to equip a wind farm. This may significantly reduce the cost of a wind farm.

In other words, by embodying a foundation pipe of an offshore wind turbine assembly as a pressure vessel, the pressure vessel needed to store energy, i.e. the energy buffer, is obtained at almost no additional cost, and the energy buffer can be added to an offshore wind turbine assembly almost‘for free’, especially when the added costs are compared to the total cost of the offshore wind turbine assembly.

A further advantage of the third aspect of the invention relates to the use of carbon dioxide as pressurization fluid. Carbon dioxide is, inter alia, in a condensation state in the temperature range between approximately -4°C and 31°C, at pressures of between approximately 29.6 bar (30 atm.) and 73 bar (74 atm.) or between approximately 30 bar and 74 bar. This is a physical fact, and an inherent property of carbon dioxide. Sea water, e.g. in the North Sea, at a depth of a few meters and lower, is generally well below 31 °C and well above -4°C. When the pipe, embodied as a pressure vessel, is submerged in sea water and in thermal communication with the sea water, the internal temperature of the pressure vessel may be substantially constant throughout the timespan of a day, but also throughout the timespan of a year. This means that the occurrence of the condensation state of the carbon dioxide is well predictable, and also that the pressure for which the pressure vessel is to be designed is well predictable. By submerging the pressure vessel in sea water, these conditions, especially the temperature, may be obtained without any further special provisions / measures, making the system for the storage of energy cheap, well predictable and controllable, and easy to implement.

In an embodiment, the total storage capacity of the energy buffer may for example be equal to the amount of energy that is generated in 12 minutes of full production capacity.

Typically, it may take much longer to completely fill the energy buffer, as only a portion of the total electrical energy generated by the offshore wind turbine is excess energy and the process is preferably isothermal. The major portion of the produced energy is send to the grid, while only the overproduction is stored in the energy buffer. For example, when the offshore wind turbine is able to generate 5MW of electrical energy at its maximum production capacity, when the storage capacity of the energy buffer is 1 MWh, and when 4 of the 5 MW is sent to by the grid, only 1MW of the 5 MW is excess energy that can be used to fill the energy buffer. Hence, in the present example, up to one hour of excess energy capacity may be stored in the energy buffer (not accounting for any losses). Of course, the above numbers are chosen merely as an example and are not limiting the invention in any way. In embodiments, it may take up to 12 hours to completely charge the energy buffer.

Preferably, the pump is operated by the electrical energy generated by the offshore wind turbine. For example, the pump may receive electrical energy from the electrical generator of the wind turbine, possibly with a transformer in between. Alternatively, the pump may receive electrical energy directly from the rotor of the wind turbine, e.g. when the rotor is coupled to a shaft, the shaft having a branch that drives the electrical generator and a branch that drives the pump. The shaft that drives the pump may then only be in operation when the pump requires electrical energy. The rotor of the wind turbine may drive the pump in alternative ways, e.g. via a chain.

Hence, advantageously, no air/gas pump and no air/gas-driven generator is provided to charge and/or de-charge the pressure vessel.

In embodiments, the wind turbine is an offshore wind turbine, wherein the wind turbine and/or the pressure vessel is e.g. located in the North Sea, wherein, preferably, the pressure vessel is at least partially submerged in a natural body of water, e.g. the sea.

In embodiments, the wind turbine assembly further comprises a fluid barrier that separates the carbon dioxide and the liquid in the pressure vessel. A fluid barrier prevents the mixing or dissolution of the carbon dioxide (pressurization fluid) in the liquid (water), ensuring efficient charging and de-charging of the energy buffer.

Preferably, the fluid barrier can move with respect to walls of the pressure vessel, following e.g. the quantity of liquid inside the pressure vessel. Preferably, the pressure vessel comprises single-curved side walls.

For example, the pressure vessel may comprise two chambers, separated by a fluid barrier, wherein the carbon dioxide or pressurization fluid is contained in a first chamber and the liquid can be pumped in the second chamber, wherein the volume of the first chamber reduces when the liquid is pumped in the second chamber and wherein the volume of the second chamber increases when the liquid is pumped in the second chamber. The fluid barrier can for example be a membrane, a layer of oil or other substance, a piston, a plate, a rolling diaphragm, or any other device that is suitable to prevent or counter the carbon dioxide (pressurization fluid) from mixing and/or dissolving with/in the liquid.

For example, a collector may be provided inside the pressure vessel, wherein the collector collects the condensed carbon dioxide.

In embodiments, the liquid-driven generator and the pump are embodied as a pump- generator, which pump-generator can be configured to operate as a generator and to operate as a pump. In other words, it is not required that the offshore wind turbine assembly or system for the temporary storage of energy comprises both a pump and a liquid-driven generator. These two components may be embodied in the same device, that can be operated as both a pump and a liquid-driven generator; depending on the setting of the device.

In embodiments, the liquid-driven generator and/or the pump are arranged inside the monopile, above an upper end cap of the pressure vessel.

In embodiments, the lower end cap of the monopile, seen in the vertical direction, is formed by the seabed. This is mainly practical when the seabed is‘rocky’.

In embodiments wherein a monopile or other offshore foundation pole extends into the seabed and is embodied as a pressure vessel, only the upper part of the monopile or foundation pole, i.e. the part nearest to the wind turbine or furthest away from the seabed, may be embodied as a pressure vessel, while a lower part of the monopile of foundation pole may extend beyond the lower end cap of said pressure vessel. Preferably, said lower part of the monopile or foundation pile extends into the seabed. At some point in time, e.g. at the end of economic life of the offshore wind turbine assembly or any other offshore construction making use of foundation piles, the offshore construction is to be

decommissioned. When the upper part of the foundation pole is embodied as a pressure vessel, decommissioning may be facilitated when a pressure can also be applied internally of the lower part of the foundation pole. For example, a pressure may be applied at the lower part of the foundation pole by making a hole in the lower part of the foundation pole (below the pressure vessel) attaching a liquid or gas (air) pump to the hole, and applying a pressure inside the lower part of the foundation pole. This may‘pop’ the foundation pole out of the seabed, easing the decommissioning of the foundation. Also for piling it is advantageous when e.g. only the part of the foundation pole that is to extend beyond the seabed is embodied as a pressure vessel. Such a construction induces the lowest forces on the lower end cap of the pressure vessel during piling of the foundation pole.

In embodiments, one or more liquid level sensors are provided that are configured to measure the level of the liquid in the pressure vessel and/or the level of the condensed carbon dioxide in the vessel, as described in the above.

In embodiments, a pressure sensor is provided that is configured to measure the pressure of the carbon dioxide and/or the liquid in the pressure vessel, as described in the above.

In embodiments a transformer is provided, arranged between an electric generator of the offshore wind turbine and the pump, the transformer being configured to transform the voltage generated by the electric generator of the offshore wind turbine into the operating voltage of the pump. Typically, the operating voltage of the pump differs from the voltage generator by the electrical generator. To operate the pump with the electrical energy generated by the renewable energy source, a transformer may be needed.

In embodiments, a mast of the wind turbine is embodied as a (second) pressure vessel that is embodied as a (second) pressure vessel that, in use,

o stores both pressurized carbon dioxide and a pressurized liquid therein, and o is operable as a (second) energy buffer that has a first, de-charged, condition wherein the total amount of potential energy stored therein is low, and a second, charged, condition wherein the total amount of potential energy stored therein is high;

wherein the offshore wind turbine further comprises:

a pump, configured to pump a liquid into the (second) pressure vessel in a first step of a cycle, thereby adding energy to said (second) energy buffer in the form of potential energy and at least temporarily charging said (second) energy buffer, wherein the pump is preferably configured to be operated by electrical energy, e.g. generated by the wind turbine, and

a liquid-driven generator, arranged in fluid communication with the (second) pressure vessel, the liquid-driven generator in use being driven by said liquid discharged from the pressure vessel in a second step of the cycle to convert the potential energy that is stored in the (second) energy buffer into electrical energy I; Wherein, in use, the carbon dioxide in the (second) pressure vessel is at least temporarily in a condensation state during each of the first step and of the second step, preferably throughout the cycle.

Hence, not only the foundation of the wind turbine may be embodied as a pressure vessel, but also the mast of the wind turbine may be embodied as a pressure vessel. In the present embodiment, it is not necessary that an additional pump and liquid-driven generator are provided. It is well conceived that one pump and/or one liquid-driven generator in use is arranged in fluid communication with both the (first) pressure vessel and the (second) pressure vessel in the mast. In embodiments the first pressure vessel, located in the foundation, and the second pressure vessel, located in the mast of the wind turbine, may be arranged in fluid communication with each other.

In embodiments, the wind turbine assembly possibly comprises multiple pups, wherein the total pumping power of the pump or pumps is between 0,1 and 5 MW. Depending on the total production power of the wind turbine, the pump may be used to shave off a shallow peak of excess production, for a relatively longer timespan, i.e. tens of minutes up to several hours and/or to shave of a steep production peak for a shorter time span, for several minutes or shorter. The higher the total pumping power of the pump or pumps, the more advanced and expensive the system will be in general, although the total costs may still be small compared to the cost of an offshore wind turbine assembly.

In embodiments, the foundation of the wind turbine is a monopile, which monopile at least partially embodies the pressure vessel. Preferably, the monopile has a steel tubular wall, wherein an upper end cap and a lower end cap of the pressure vessel are joined at different elevations inside said monopile to the steel tubular wall, such that said steel tubular wall is exposed to pressure in the pressure vessel.

It is conceived that in a (offshore) wind farm, e.g. comprising dozens up to a few hundred or hundreds of wind turbines, each individual wind turbine comprises an energy buffer.

In embodiments, the wind farm comprises at least one wind turbine according to the above, and wherein possibly some of the wind turbines of the park lack an energy buffer. Some of the wind turbines may then have a relatively large total pumping power, to correct for steep peaks in productions, the peaks lasting a few seconds up to several minutes. Other wind turbines may then be equipped with relatively less pumping power, which allows to shave off excess production for a longer time. In cooperation, the total production capacity of the wind park can be controlled, allowing peaks in production of different kinds (i.e. both short and high peaks and longer, shallow, peaks) to be absorbed in the energy buffers of the wind turbines.

In embodiments, the carbon dioxide in the pressure vessel may be used as a fire extinguisher medium, e.g. when one or more of the foundation piles of an electrical transformer of a wind farm is used as an energy buffer and comprise carbon dioxide. If a fire is detected inside the transformer, the pressurized carbon dioxide in the foundation pipe of the transformer may be utilized to extinguish the fire. The same may be done for an offshore wind turbine.

Therefore the present invention also relates to an offshore wind park component, e.g. a wind turbine or an electrical transformer, wherein the component comprises a foundation at least one element, e.g. a pipe, a pile, a monopile, that is embodied as a pressure vessel, which pressure vessel, in use,

- stores both pressurized carbon dioxide and a pressurized liquid therein,

- is, preferably, at least partially submerged in a natural body of water, e.g. the sea, and

- is operable as an energy buffer that has a first, de-charged, condition wherein the total amount of potential energy stored therein is low, and a second, charged, condition wherein the total amount of potential energy stored therein is high. The energy buffer for example being embodied and/or operated as described herein.

For example one or more C02 fire extinguishing lines extend from the pressure vessel up through a mast of wind turbine to the nacelle where, for example, the electrical generator is arranged.

For example one or more C02 fire extinguishing lines extend from the pressure vessel up through a mast of wind turbine to the rotor blades.

For example one or more C02 fire extinguishing lines extend from the pressure vessel up through a mast of wind turbine are automatically controlled, so as to actively release C02 in case of fire, e.g. due to lightning strike, drive train failure, generator failure, etc.

The invention also relates to a method for extinguishing fire as disclosed herein. The present invention further relates to a method for generating electricity and/or buffering energy, wherein use is made of a wind turbine assembly according to the above.

In embodiments, another pressurization fluid may be used instead of carbon dioxide. In principle, any gas can be brought in a condensation state, depending on the specific temperature, pressure, and volume. Hence, many of the advantages that are achieved with the invention according to the first aspect are also achieved when another gas than carbon dioxide is used.

Improved energy storage systems are desired also outside of the (offshore) wind industry, such as in the renewable energy industry, or even outside of that industry. For example, when considering the current electricity grid, when peaks in energy supply are asked from the grid by users of electrical energy, it may be advantageous when energy buffers are locally present, to supply this peak in energy supply. For such purposes, an efficient energy buffer may be required.

In another example, there are industrial companies which need a lot of energy, only some days of the year. Such companies may be charged a relatively high“connection fee” for the possibility to receive a peak capacity from the grid, even though this is needed only a few days of the year. Asking a peak capacity from the grid namely poses problems to the grid and requires an advanced grid infrastructure, for which these companies are charged said “connection fee”. It is not so much the amount of energy they consume for which such companies are charged, but merely the ability to receive peak loads from the grid. For such companies, it may be desired to store energy locally, to slowly charge an energy buffer, and to empty the buffer these few days of the year wherein a peak load is desired, via an internal distribution system. This then significantly reduces their energy bill: the total amount of energy consumed is a bit higher due to inefficiencies in charging and de-charging the energy buffer, but the peak capacity needed from the grid is much lower, which reduces this “connection fee”.

Also in urban environments, where people drive electrical vehicles, peaks in demand are foreseen to cause problems to the grid in the near future. Working people, driving electric cars, all arrive home within a relatively short window of an hour or two hours. All these people charge their vehicle upon arriving home, causing a steep peak in demand for electrical energy. As more people buy electrical cars, this problem becomes bigger. On top of that, when the electricity grid more and more relies upon renewable energy sources for electricity, plugging in such electric cars after sun-set, e.g. in the winter, causes problems as solar panels do not produce any energy when demand is highest, and it may be impossible to meet the peak demand.

Hence, there is a need for efficient energy storage systems for all these kinds of applications.

Accordingly, a further aspect of the invention relates to method for the temporary storage of energy, wherein use is made of a pressure vessel that contains a pressurization fluid, such as carbon dioxide, therein, the pressure vessel being operated as an energy buffer that has:

- a first, de-charged, condition wherein the total amount of potential energy stored in the energy buffer is low, and

- a second, charged, condition wherein the total amount of potential energy stored in the energy buffer is high;

wherein the method comprises a cycle of:

- a first step of pumping a liquid into the pressure vessel, thereby decreasing the volume of the pressurization fluid and charging the energy buffer, followed by

- a second step of discharging the liquid from the pressure vessel, thereby de charging the energy buffer and producing electrical energy with said liquid;

wherein the pressurization fluid is at least temporarily in a condensation state during each of the first step and of the second step, preferably throughout the cycle.

In embodiments, pressure vessel is at least partially submerged in water, e.g. sea water or a flowing water source such as a river or a canal.

According to a further aspect of the invention, a system for the temporary storage of energy from a renewable energy source is provided, comprising:

- a renewable energy source, configured to generate electrical energy;

- a pressure vessel that, in use,

- stores both pressurized carbon dioxide and a pressurized liquid therein,

- is, preferably, at least partially submerged in a natural body of water, e.g. the sea, and - is operable as an energy buffer that has a first, de-charged, condition wherein the total amount of potential energy stored therein is low, and a second, charged, condition wherein the total amount of potential energy stored therein is high,

- a pump, configured to pump a liquid into the pressure vessel in a first step of a cycle, thereby adding energy to said energy buffer in the form of potential energy and charging said energy buffer, wherein the pump is preferably configured to be operated by electrical energy, e.g. generated by the wind turbine, and

- a liquid-driven electric generator, arranged in fluid communication with the pressure vessel, the liquid-driven generator in use being driven by said liquid discharged from the pressure vessel in a second step of the cycle to convert the potential energy that is stored in the energy buffer into electrical energy;

wherein, in use, the carbon dioxide in the pressure vessel is at least temporarily in a condensation state during each of the first step and of the second step, preferably throughout the cycle.

In embodiments, the pressure vessel is at least partially submerged in water, e.g. sea water or a flowing water source such as a river or a canal. For the controllability of the process of charging and de-charging the energy buffer, and for controlling the pressure inside the energy buffer, especially when it is in the charged condition, the temperature of the surroundings of the pressure vessel is preferably relatively constant. Such a constant temperature is for example achieved when the pressure vessel is submerged in water, preferably a flowing water source such as the sea. For example, the pressure vessel may be comprised in a foundation pole of a wind turbine, in a foundation pole of a bridge, in a foundation pole of a dike, or in a foundation pole of an (artificial) water storage lake. For example, the pressure vessel may be connected to one of the above foundations or foundation poles. For example, the pressure vessel may be anchored to a bed of the water source.

Alternatively, the pressure vessel may e.g. be kept underground, e.g. in an (exploited) cave. Also here, the temperature is advantageously substantially constant throughout one day and throughout the year.

In embodiments, the pump is operated by electrical energy that is generated by the renewable energy source or a renewable energy source. Preferably, the pump is only operated when the production of electrical energy by the renewable energy source is higher than demand from the grid. That is, preferably the pump is only operated at times when there is a production peak. That is, the pump, and therefore the energy buffer, may be used for peak-shaving.

In embodiments, the renewable energy source or a renewable energy source is for example a solar energy source, such as a solar (PV) panel, or a wind energy source, such as a wind turbine. In general, all renewable energy sources suffer from production peaks (or dips) at times. Therefore, all renewable energy sources may benefit from an efficient energy storage system.

In embodiments, the renewable energy source or a renewable energy source is an offshore renewable energy source, such as an offshore wind turbine, a tidal operated energy generating device, or a wave operated energy generating device.

It is specifically noted that some embodiments of the present invention have only been described in relation to one or a limited number of aspects of the invention. It is possible to apply the technical advantages of such embodiments also for other aspects of the invention, even when this is not explicitly mentioned.

The same may be better understood with reference to the drawings, in which :

Figure 1A shows a frontal view of a first embodiment of a wind turbine assembly and/or energy buffer system according to the invention;

Figure 1 b shows a side view of the wind turbine assembly and energy buffer system according to figure 1A;

Figure 2A shows a frontal view of a second embodiment of a wind turbine assembly and/or energy buffer system according to the invention;

Figure 2B shows a frontal view of a third embodiment of a wind turbine assembly and/or energy buffer system according to the invention;

Figure 2C shows a frontal view of a fourth embodiment of a wind turbine assembly and/or energy buffer system according to the invention;

Figure 2D shows a frontal view of a fifth embodiment of a wind turbine assembly and/or energy buffer system according to the invention;

Figure 2E shows a frontal view of a sixth embodiment of a wind turbine assembly and/or energy buffer system according to the invention;

Figure 2F shows a frontal view of a seventh embodiment of a wind turbine assembly and/or energy buffer system according to the invention Figure 3A shows a PV-diagram of substances;

Figure 3B shows a P-T diagram for carbon dioxide;

Figure 4A shows an energy buffer according to the invention in a de-charged condition thereof;

Figure 4B shows an energy buffer according to the invention in a condition in between the de-charged condition and the charged condition thereof;

Figure 4C shows an energy buffer according to the invention in a charged condition thereof; Figure 5 shows a more detailed outline of a system according to the invention.

Figures 1A - 2F show embodiments of wind turbines assemblies 1 , here offshore wind turbine assemblies, each comprising at least one pressure vessel 122 that is operable as an energy buffer in the manner as disclosed herein.

The wind turbine assemblies 1 further comprise a wind turbine 11 , here an offshore wind turbine, configured to generate electrical energy from wind energy.

The offshore wind turbine 11 of the offshore wind turbine assembly 1 here is of the horizontal axis type, but may e.g. also be of the vertical axis type, or any other type.

The depicted wind turbine 11 generally comprises at least a set of rotor blades 111 , e.g. three rotor blades, a mast 112, and a hub 113. The rotor blades 111 are configured to rotate with respect to a rotation axis thereof, under the influence of wind. Upon rotation of the rotor blades 111 an electrical generator, e.g. located in the hub 113, is driven, said electrical generator generating electrical energy. Thereby, wind energy is converted into electrical energy. One skilled in the art is familiar with the general lay-out of a wind turbine 11 , and therefore these components of a wind turbine 11 are not elaborated upon in detail.

The offshore wind turbine assemblies 1 furthermore comprises a floating foundation or a seabed-bound foundation 12, configured to stabilise the offshore wind turbine 11 , e.g. with respect to the sea.

In Figures 1A and 1 B, the foundation is a monopile foundation.

In Figure 2A, the foundation is a seabed-bound tripod.

In Figure 2B, the foundation is a floating tripod.

In figure 2C, the foundation is a seabed-bound jacket. In Figure 2D, the foundation is a floating spar, wherein the foundation 12 is secured to the seabed with mooring lines 113.

In Figure 2E, the foundation is a floating semi-sub, wherein the foundation 12 is secured to the seabed with mooring lines 113.

In Figure 2F, the foundation is a floating TLP, wherein the foundation 12 is secured to the seabed with tendons 114.

Many other types of foundations for wind turbines are however known. A person skilled in the art is familiar with the different types of foundations for wind turbines, and therefore these components are not elaborated upon in detail.

Visible for example in figures 1A and 1 B is that a part, e.g. an upper part, of a monopile 121 is embodied as a pressure vessel 122. Another, e.g. lower part of the monopile 121 extends beyond a lower end cap 1222 of said pressure vessel 122. The lower part has been driven into the soil, e.g. using pile driving equipment or installed into a hole made in the soil, etc.

The lower end cap 1222 may be arranged at such an elevation that the end cap 1222 is above the floor of the body of water when the monopile 121 has been installed. In another embodiment, as shown here, the lower end cap 1222 may be arranged at such an elevation that the end cap 1222 is below the floor of the body of water when the monopile 121 has been installed.

In embodiments an excavation or other removal of soil within the monopile below the lower end cap 1222 may be performed, e.g. in view of the lower end cap 1222 ending up below the floor of the body of water, e.g. the sea floor.

Visible in e.g. Figures 2A - 2F is that also another element of the foundation 12 may be embodied as a pressure vessel 122, e.g. in case of a non-monopile foundation.

In embodiments, the wind turbine 11 is an offshore wind turbine, e.g. located in the North Sea.

Preferably, the pressure vessel 122 is at least partially submerged in a natural body of water, e.g. the sea. The foundation 12 comprises at least one element 121 , e.g. a pipe, a pile, a monopile, etc. that is embodied as a pressure vessel 122. For example, the pressure vessel 122 can be made of steel. As is known in the field a monopile is often made of steel.

As is visible in Figure 2C, it is not necessary that the pressure vessel 122 is a structural component of the foundation 12. It may e.g. also be arranged at the centre of the shown jacket.

In use, the pressure vessel 122 stores both pressurized carbon dioxide and a pressurized liquid therein, as will be explained in more detail with respect to Figures 4A - 4C.

In figures 1 A - 2F, the pressure vessel 122 is in use at least partially submerged in the sea. The pressure vessel 122 is operable as an energy buffer that has a first, de-charged, condition wherein the total amount of potential energy stored therein is low, and a second, charged, condition wherein the total amount of potential energy stored therein is high.

As is better visible in Figure 5, the wind turbine further comprises a pump 13, configured to pump a liquid into the pressure vessel 122 in a first step of a cycle, here through fluid line 131. In the shown condition of Figure 5, the energy buffer is relatively un-charged, as relatively a lot of gaseous carbon dioxide is contained in the pressure vessel 122. By pumping liquid into the pressure vessel 122, pressurized liquid L enters the energy buffer, energy is added to the energy buffer in the form of potential energy, and the energy buffer is charged. In the embodiment of Figure 5, the pump 13 is electrically connected to electrical generator 18, allowing the pump 13 to be operated by electrical energy generated by the wind turbine 11. In the embodiment of figure 5, the pump 13 is arranged inside a monopile 121 of the wind turbine 11 , above an upper end cap 1221 of the pressure vessel 122. In the embodiment of figure 5, power to perform the first step of the method is obtained from the wind turbine, which is a renewable energy source, more specifically an offshore renewable energy source.

More specifically, in the embodiment of figure 5, the foundation 12 of the wind turbine 11 is a monopile 121 , which monopile 121 at least partially embodies the pressure vessel 122. Here, the monopile 121 has a steel tubular wall 1223, an upper end cap 1221 and a lower end cap 1222 that are joined at different elevations inside said monopile 121 to the steel tubular wall 1223. Thereby, the steel tubular wall 1223 is exposed to pressure in the pressure vessel 122. Preferably at least one of the upper end cap 1221 and a lower end cap 1222 is made of steel and/or (reinforced) concrete.

Further with reference to Figure 5, a liquid-driven electric generator 14 is visible, arranged in fluid communication with the pressure vessel 122 via fluid line 141 , the liquid-driven generator 14 in use is driven by said liquid discharged from the pressure vessel 122 in a second step of the cycle to convert the potential energy that is stored in the energy buffer into electrical energy. Hence, the second step of the method according to the invention may include driving the liquid-driven generator 14 with the liquid that is discharged from the pressure vessel 122. As will be appreciated the same pump 13 used to pump water into the pressure vessel 122 is now used in the discharge of water from the vessel 122 as well as for driving the generator 14. For example, the pump 13 is a piston pump. For example, the electrical energy generated by the liquid-driven electrical generator may be supplied to the electricity grid via wire 142. In figure 5, the liquid-driven generator 14 is arranged inside the monopile 121 , above an upper end cap 1221 of the pressure vessel 122.

In a non-shown embodiment, the liquid-driven generator 14 and the pump 13 are embodied as a pump-generator, which pump-generator can be configured to operate as a generator and to operate as a pump.

It is noted that the liquid circuit between pump 13, pressure vessel 122, generator 14 and storage tank 23 is merely an example of a liquid circuit that may be employed according to the invention. Many different liquid circuits are conceivable, depending on the specific embodiment of the invention.

According to the invention, the carbon dioxide F, FG, FL in the pressure vessel 122 is at least temporarily in a condensation state during each of the first step and of the second step, preferably throughout the cycle. This phenomenon of condensation will be explained in more detail below, with reference to figures 3A and 3B.

As visible in Figure 5, the wind turbine assembly 1 further comprises a fluid barrier 15 that separates the carbon dioxide F, FG, FL and the liquid L in the pressure vessel 122.

Visible in Figure 5 are level sensors 16A, 16B that are configured to measure the level of the liquid L in the pressure vessel 122 respectively the level of the condensed carbon dioxide FL in the pressure vessel 122. With the level sensors 16A, 16B, it is possible to monitor the level of the condensed carbon dioxide FL in the pressure vessel 122 respectively monitor the level of the liquid L in the pressure vessel 122, and, possibly, releasing carbon dioxide F, FG, FL and/or liquid L from the pressure vessel 122 when the level thereof exceeds a pre-determined value, e.g. through relief valve 19.

Visible in Figure 5 are pressure sensors 17, configured to measure the pressure in the pressure vessel 122. With pressure sensors 17, it is possible to monitor the pressure of the carbon dioxide F, FG, FL and/or the liquid L in the pressure vessel 122 and, possibly release carbon dioxide F, FG, FL and/or liquid L from the pressure vessel 122 when said pressure exceeds a pre-determined value, e.g. through relief valve 19.

The invention further relates to a wind farm having multiple wind turbines, said wind farm comprising at least one wind turbine assembly 1 according to the above. Possibly, some of the wind turbines assemblies in the wind farm lack an energy buffer.

The invention further relates to a method for generating electricity and/or buffering energy, wherein use is made of a wind turbine assembly 1 according to the above.

In other words, with reference to figures 1A, 1 B, 2A, 2B, 2C, 2D, 2E, 2F and 5, an energy buffer system is shown, wherein the energy buffer system comprises:

- a pressure vessel 122 that contains carbon dioxide F, FG, FL therein, the pressure vessel 122 being operable as an energy buffer that has:

- a first, de-charged, condition wherein the total amount of potential energy stored in the energy buffer is low, and

- a second, charged, condition wherein the total amount of potential energy stored in the energy buffer is high,

wherein the system further comprises:

- first means configured to perform said first step of pumping a liquid L into the pressure vessel 122, thereby decreasing the volume of the carbon dioxide F, FG, FL and charging the energy buffer,

- second means configured to perform a second step of discharging the liquid (L) from the pressure vessel 122, thereby de-charging the energy buffer and producing electrical energy with said liquid L,

wherein the system is configured such that the carbon dioxide F, FG, FL is at least temporarily in a condensation state during each of the first step and of the second step, preferably such that the carbon dioxide F, FG, FL is in a condensation state throughout a cycle comprising the first and second step.

Using the wind turbine or the energy buffer system according to the invention, energy can temporarily be stored. Hence, the invention also relates to a method wherein the wind turbine or the energy buffer is used.

More specifically, the invention relates to a method for the temporary storage of energy, wherein use is made of a pressure vessel 21 , 122 that contains carbon dioxide F, FG, FL therein, the pressure vessel 21 , 122 being operated as an energy buffer that has:

- a first, de-charged, condition wherein the total amount of potential energy stored in the energy buffer is low, and

- a second, charged, condition wherein the total amount of potential energy stored in the energy buffer is high;

wherein the method comprises a cycle of:

- a first step of pumping a liquid L into the pressure vessel 21 , 122, thereby decreasing the volume of the carbon dioxide F, FG, FL and charging the energy buffer, followed by

- a second step of discharging the liquid L from the pressure vessel 21 , 122, thereby de-charging the energy buffer and producing electrical energy with said liquid L;

wherein the carbon dioxide F, FG, FL is at least temporarily in a condensation state during each of the first step and of the second step, preferably throughout the cycle.

With reference to Figures 3A and 3B, the occurrence of a fluid in its condensation state may be explained. With reference to figure 3A, a P-V diagram of a (generic) fluid is shown, wherein a liquid-vapor or liquid-gas region is visible. That liquid-vapour or liquid-gas region corresponds to the condensation state of the fluid. In this region, the fluid is partially in the fluid state, and partially in the vapor (gaseous) state, as elaborated upon in the above. When the line indicated T2 is followed, it is visible how, for relatively large volumes near the right of the figure, the fluid is completely gaseous. As the volume in which the fluid is contained is reduced, the pressure rises, until point C in the diagram is reached. At point C, the condensation state of the fluid is achieved, wherein further reduction of the volume does not increase the pressure of the fluid but, instead, condenses some of the gas into a liquid. This process continues until point B, where all the gas is condensed to a fluid. As may be apparent from Figure 3A, when using the phenomenon for the storage of energy, preferably the process is performed isothermally. In the P-T diagram of Figure 3B, the condensation state of C02 is represented by the line GL, indicating the simultaneous liquid and gaseous state of the C02.

As may be apparent from figures 3A and 3B, the temperature of the carbon dioxide is preferably at most 31.1°C during the cycle, as beyond that temperature the condensation state cannot be achieved.

As may be apparent from Figures 3A and 3B, the pressure of the carbon dioxide and the liquid, in the charged condition of the pressure vessel, is preferably between 29.6 bar (30 atm.) and 72.8 bar (73.8 atm.) or between 30 bar and 73.8 bar.

Returning to figure 5, preferably the liquid L is water, e.g. sea water, e.g. is taken from a natural body of water such as the sea. Here, the pressure vessel 122 is partially submerged in said natural body of water and is directly in contact with said water, wherein the pressure vessel 122 lacks thermal insulation e.g. the pressure vessel 122 being made of steel.

Visible in Figure 5 is a storage tank 23. In embodiments, the first step of the invention includes pumping water from the storage tank 23 into the pressure vessel 122 and the second step includes discharging water from the pressure vessel 122 into said storage tank 23.

As is visible in Figure 5, power to perform the first step may be obtained from a wind turbine 11 , here an offshore wind turbine. In the embodiment of Figure 5, the pressure vessel 122 is integrated with the wind turbine. More specifically, the pressure vessel 122 is embodied by a foundation element 12 of the wind turbine 11 , that is, here, e.g. the pressure vessel 122 is partially submerged in a natural body of water.

With reference to Figures 4A - 4C, the charging and de-charging of the energy buffer may be explained.

Shown in Figure 4A is a pressure vessel 122 with an inlet / outlet 1224 through which a liquid may enter the pressure vessel 122. In the condition of figure 4A, some liquid L is present in the pressure vessel 122. The liquid L is separated from the fluid FG by a fluid barrier 15. The fluid FG is completely in the gaseous state in the condition of figure 4A. Preferably, the fluid FG is in a condensation state, but this is not necessary.

Figure 4B shows a condition wherein, compared to figure 4A, more liquid L is entered into the pressure vessel 122. As visible, it is possible to introduce the liquid L into the pressure vessel 122 as the volume of the fluid FG, FL has reduced. More specifically, some of the gaseous fluid FG has condensed to a liquid fluid FL. When condensing, the specific volume of the fluid decreases, reducing the volume of the part of the pressure vessel 122 that contains the fluid FG, FL, while the pressure remains the same. This allows liquid L to be entered into the pressure vessel 122. As, compared to Figure 4A, the pressure vessel 122 of Figure 4B contains more pressurized liquid L, the energy content of the pressure vessel 122 is increased. That is, the energy buffer that is formed by the pressure vessel 122, is charged.

Figure 4C, finally, shows the situation wherein all the fluid FG, FL has condensed to a liquid FL. It is in this situation no longer possible to pump more liquid L into the pressure vessel 122 without significantly increasing the pressure inside. That is, in the condition of Figure 4C, the energy buffer is fully charged.

The process of charging the pressure vessel 122 is explained directly above by considering first figure 4A, then figure 4B, and finally figure 4C. De-charging of the pressure vessel 122 may be explained by considering the figures 4A - 4C in the opposite order.

In figure 4C, the pressure vessel 122 is fully charged. The liquid L therein is pressurized, e.g. at a pressure of between 29.6 bar (30 atm.) and 72.8 bar (73.8 atm.) or between 30 bar and 73.8 bar. When the liquid L is allowed to discharge from the pressure vessel 122, e.g. through outlet 1224, the pressurized liquid L can be used to generate electrical energy, as explained in the above. When liquid L is allowed to discharge from the pressure vessel 122, the volume in the pressure vessel 122 that is available for the fluid F, FL, FG becomes larger, and some of the fluid in the liquid condition FL will evaporate to a gaseous fluid FG. This may continue until all or substantially all liquid L is discharged from the pressure vessel 122, and the energy buffer is fully de-charged in figure 4A.

It is specifically noted that, in the shown embodiment, the fluid F, FG, FL remains inside the pressure vessel 122 while charging and de-charging the energy buffer, while liquid L is pumped into and discharged from the energy buffer to store and regenerate (electrical) energy.

In contrast to what is shown in Figures 4A - 4C, in embodiments the method involves variation of the volume of liquid L in the pressure vessel 21 between a non-zero minimum volume and a maximum volume such that only liquid L is discharged from the pressure vessel 21 during the second step and the carbon dioxide F, FG, FL remains in the pressure vessel 21 throughout the cycle. In embodiments, even in the fully charged condition of the energy buffer, the carbon dioxide F, FG, FL is still partially in the gaseous state. In embodiments, even in the fully de-charged condition of the energy buffer, the carbon dioxide F, FG, FL is still partially in the liquid state.