WIND TURBINE COMPRISING VARIABLE SWEPT AREA AND METHOD OF CONTROLLING A WIND TURBINE

Field of invention

The present invention relates to a wind turbine comprising a variable swept area and to a method of controlling such a wind turbine.

Summary of the Invention

A maximum power output of a wind turbine can be determined by the Betz's law which is derived from the principles of con servation of mass and momentum of the air stream flowing through an idealized "actuator disk" that extracts energy from a wind stream. According to the Betz's law, a turbine cannot capture more than 16/27 (59.3%) of the kinetic energy of the wind. The factor 16/27 is known as Betz's coefficient. In practice, utility-scaled wind turbines can achieve a maxi mum of about 75-80% of the Betz's coefficient. The Betz's co efficient is based on an open-disk actuator. If a diffuser is used to collect and direct an additional wind flow to the turbine, more energy can be extracted, but the limit still applies to the cross-section of the entire structure. Fur thermore, the use of the diffuser to cover a larger swept ar ea turned out to be expensive and complex.

There may be a need for a wind turbine and a method of con trolling such a wind turbine which provide for a higher effi ciency of the wind turbine. This need may be met by the sub ject matters according to the independent claims. The present invention is further developed as set forth in the dependent claims.

According to a first aspect of the invention, a wind turbine comprises a tower, a nacelle mounted at the top of the tower, a rotor mounted rotatable relatively to the nacelle about a rotation axis and comprising at least one blade, wherein the blade, when rotating about the rotation axis, is configured to span a swept area, and a control device which is config ured to control an actuator so as to move the swept area. Moving the swept area can be interpreted that the swept area is moved relatively to a global coordinate system or a global reference point, for example to any reference point at the sea ground.

The swept area is dynamically moved for an improved power output. Sufficiently large translations and rotations of the swept area provide for an increased efficient swept area and can be optimized based on a preview. An immediately advantage is an overall increase in the wind farm power production, which contributes to lower the levelized cost of energy (LCOE).

The wind turbine can be a floating wind turbine that has ad ditional degrees of freedom which may be exploited to cover a larger swept area to increase the power output of the wind turbine and the overall wind turbine park compared with tra ditional stationary wind turbines.

If the swept area is moved fast enough, the wind turbine can remarkably gain more power because the effective swept area is increased. The principle is that the induction zone reduc es the wind speed so that, if a larger swept area is covered, the effective wind speed on the rotor is theoretically in creased. It depends on how quickly the axial induction zone develops relative to the movement speed of the swept area.

In an embodiment, the control device is configured to alter the movement of the swept area based on a measured, estimated or predicted speed of wind to the wind turbine. In an embodi ment, the control device can be configured to set a speed of moving the swept area based on the wind speed, wherein the speed of moving the swept area is preferably increased when the wind speed increases, and the speed of moving the swept area is preferably decreased when the wind speed decreases. The induction zone generally exists a fixed number of rotor diameters upwind (and downwind) of the wind turbine. At high er wind speeds, the wind moves faster through this zone and thereby the induction zone is established faster, thus re quiring a higher movement speed of the swept area.

In an embodiment, the control device is configured to control the actuator so as to reciprocatingly move the swept area. By this measure, a simple control is implemented, while the wind turbine's efficiency can remarkably be increased.

In an embodiment, the swept area is moved by at least one of a translational movement and a rotational movement, for exam ple relatively to the global reference point.

In an embodiment, the wind turbine is an offshore wind tur bine comprising a floating hull onto which the tower is mounted, wherein the hull is connected to a first fixing point at a sea ground via a first mooring line and to a sec ond fixing point at the sea ground via a second mooring line, wherein the actuator comprises means to change at least one of a length of the first mooring line between the first fix ing point and the hull and of a length of the second mooring line between the second fixing point and the hull in a manner that the swept area is moved.

In an embodiment, the wind turbine is an offshore wind tur bine comprising a floating hull onto which the tower is mounted, wherein the actuator comprises a ballast control system which is configured to move a ballast body or a bal last fluid inside the hull in a manner that the swept area is moved.

In an embodiment, the control device is configured to move the swept area, if a at least one of the following conditions is fulfilled: the wind turbine is operated below a rated out- put power of the wind turbine, the wind turbine is within a wake of another wind turbine; if a wind speed is below a ref erence wind speed; if a power, which is consumed by the actu ator to move the rotor, is lower than a first predetermined ratio of an actual output power of the wind turbine; and if the power, which is consumed by the actuator to move the ro tor, is lower than a second predetermined ratio of a power which is gained by moving the swept area. For example, the first predetermined ratio can be 50%, 25% or 10%, and the second predetermined ratio can be 100%, 75% or 50%. The sec ond predetermined ratio provides a kind of hysteresis bearing in mind that an (estimated) power which is gained by moving the swept area 200 may have some fluctuation.

This control strategy considers a reasonable energy balance. The swept area and rotor movements need to be energy effi cient such that the captured extra power by avoiding the wake's impact should exceed the energy used for the movement. This is part of the control strategy to only initiate the movement when an energy gain is possible.

In an embodiment, the control device is configured to decel erate or stop the movement of the swept area, if at least one of the following conditions is fulfilled: if a power, which is consumed by the actuator to move the rotor, exceeds a first threshold value; and if a load parameter, a fatigue pa rameter or an instability parameter, which is caused by the movement of the rotor, exceeds a second threshold value.

This control strategy considers a safe operation. The swept area and rotor movements need to be load efficient such that too much additional load by this movement should be avoided. This is part of the control strategy to only initiate the swept area movement when the load allows the same, e.g. not to drive extreme loads or to cause significant fatigue or in stabilities. This can be realized by monitoring load/acceleration variables online or by scheduling the move ment by designed control strategies. Furthermore, the move- ment should consider park boundaries as well, which may be static or even dynamic with respect to e.g. incoming boats, other turbines and sailing lines.

According to a second aspect of the invention, a wind park control device is configured to control at least two of the above-mentioned wind turbines. The wind park control device is connected to at least one control device of the at least two wind turbines. The wind park control device is configured to perform a coordinated control in a manner that a wake of one of the at least two wind turbines is at least not contin uously directed to the other one of the at least two wind turbines. For example, one of the wind turbines can continu ously be outside a wake of another wind turbine when both wind turbines are synchronously be moved. In another embodi ment, both wind turbines can be moved in opposite directions so that one of the wind turbines can be inside a wake of an other wind turbine only for a short time.

In this approach, the swept area is changed to avoid the im pact of a wake of another wind turbine. Wind turbines placed inside a wind farm interact with each other through their wakes. The wake of a wind turbine is a result of an energy extraction of the incoming flow. The wake causes a reduced wind speed, also known as a wind deficit, and an increased wind speed variation, also known as turbulence. These phenom ena result in a decrease of the overall power production of the wind farm and in an increase of the loads for the down stream wind turbines (downstream wind turbines are turbines behind a front wind turbine that may create the wake). The decrease of power is a direct result of the lower wind speed, while an increase of a mechanical load caused by variations of the mechanical load can be a consequence of the turbulenc es.

Movements trajectories of the swept areas of the wind tur bines of the wind park can be coordinated or optimized to maximize the output power on park level. For example, a control system that has information about park wind direction and wind turbine positions can implement a distributed control strategy (or individual setpoints) to the wind turbines such that they are controlled in order to max imize the output power on park level. This can be done for upwind turbines to coordinate movement with downwind turbines to reduce an impact of wake, e.g. for up- and downwind tur bines to synchronously move or to move in opposite directions when the downwind turbine is in full wake of the upwind tur bine.

According to a third aspect of the invention, a method of controlling a wind turbine is provided. The wind turbine com prises a tower, a nacelle mounted at the top of the tower, and a rotor mounted rotatable relatively to the nacelle about a rotation axis and comprising at least one blade, wherein the blade, when rotating about the rotation axis, is config ured to span a swept area. The method comprises a step of moving the swept area, for example relatively to the global reference point.

It has to be noted that embodiments of the invention have been described with reference to different subject matters.

In particular, some embodiments have been described with ref erence to apparatus type claims whereas other embodiments have been described with reference to method type claims. However, a person skilled in the art will gather from the above and the following description that, unless other noti fied, in addition to any combination of features belonging to one type of subject matter also any combination between fea tures relating to different subject matters, in particular between features of the apparatus type claims and features of the method type claims is considered as to be disclosed with this application. Brief Description of the Drawings

The aspects defined above and further aspects of the present invention are apparent from the examples of embodiment to be described hereinafter and are explained with reference to the examples of embodiment. The invention will be described in more detail hereinafter with reference to examples of embodi ment but to which the invention is not limited.

Fig. 1 shows a wind turbine and the different elements there of;

Fig. 2 shows a translational movement of a swept area;

Fig. 3 shows a rotational movement of a swept area; and Fig. 4 shows a heave movement of a swept area.

Detailed Description

The illustrations in the drawings are schematically. It is noted that in different figures, similar or identical ele ments are provided with the same reference signs.

Fig . 1 shows a wind turbine 100. The wind turbine 100 com prises a nacelle 160 and a tower 130. The nacelle 160 is mounted at the top of the tower 130. The nacelle 160 is mounted rotatable relative to the tower 130 by means of a yaw bearing in order to establish a nacelle yaw. The axis of ro tation of the nacelle 160 relative to the tower 130 is re ferred to as yaw axis (cf. reference sign 104).

The wind turbine 100 also comprises a rotor having, for exam ple, three rotor blades 140. Each blade 140 can usually be rotated about a longitudinal axis of the blade 140 to estab lish a desired blade pitch or pitch angle of the blade 140. The rotor is rotatable mounted relative to the nacelle 160 by means of a bearing. The rotor is mounted rotatable about a rotation axis 107. When the blades 140 are subjected to a wind 111, they are rotated together with the rotor about the rotation axis 107. The blades 140, when rotating about the rotation axis 107, span a swept area 200.

The wind turbine 100 furthermore comprises a generator. The generator is accommodated within the nacelle 160. The genera tor is arranged and prepared for converting rotational energy from the rotor into electrical energy in the shape of an AC power.

The wind turbine 100 is an offshore wind turbine 100 compris ing a floating hull 120 onto which the tower 130 is mounted. The hull 120 floats on a sea level 112 of a sea 114. The hull 120 is connected to a first fixing point 153 at a sea ground 113 via a first mooring line 151 and to a second fixing point 156 at the sea ground 113 via a second mooring line 154. The first mooring line 151 is connected to the hull 120 at a third fixing point 152, and the second mooring line 154 is connected to the hull 120 at a fourth fixing point 155. The installed offshore wind turbine 100 floats in the water 114 and is normally held in position by the mooring lines 151 which either stabilize the system or prevent the same from drifting away.

The wind turbine 100 has six individual degrees of freedom in which the wind turbine 100 may move. Namely three transla tions along a surge axis 103, a sway axis 102, and a heave axis 101 along with three rotations along a floater roll axis 106, a floater pitch axis 105 and a floater yaw axis 104.

The pitch about the floater pitch axis 105 is thus different from the blade pitch because the pitch about the floater pitch axis 105 is the rotation of the floating structure (for example the tower 130 or the entire wind turbine 100) around its point of rotation, while the blade pitch defines the con- trolled pitch angle of the blades 140 about the longitudinal axis thereof. Furthermore, the yaw about the floater yaw axis 104 is different from the nacelle yaw because the yaw about the floater yaw axis 104 is the rotation around the vertical axis of the floating structure (for example the tower 130 or the entire wind turbine 100), while the nacelle yaw is the yaw of the nacelle 160 which can be measured relatively to the tower 130 and actively be controlled by the wind turbine 100.

The wind turbine 100 comprises an actuator (not shown) which is configured to move the tower 130 or the entire wind tur bine 100 and thus the swept area 200 at least in one of the three translations along the surge axis 103, the sway axis 102, and the heave axis 101, and the three rotations about the floater roll axis 106, the floater pitch axis 105 and the floater yaw axis 104. In order to realize the present inven tion, it is not necessary to move the entire wind turbine 100 in order to move the swept area 200. It is sufficient if only the rotor is moved by the actuator so as to move the swept area 200. In the following, the specification sometimes re fers to a movement of the swept area 200 or the rotor even though the entire wind turbine 100 is moved by the actuator.

The wind turbine 100 comprises a control device (not shown) which is configured to control the actuator to move the swept area 200, for example along the wind turbine 100. In other words, the actuator is configured to move the swept area 200 relatively to a global reference point.

In an embodiment, the actuator can comprises means to change at least one of a length of the first mooring line 151 be tween the first fixing point 153 and the third fixing point 152 at the hull 120 and of a length of the second mooring line 154 between the second fixing point 156 and the fourth fixing point 155 at the hull 120 in a manner that the swept area 200 is moved. In another embodiment, the actuator can comprise a ballast control system which is configured to move a ballast body or a ballast fluid inside the hull 120 in a manner that the swept area 200 is moved. However, the present invention can be realized by any other actuator which is capable to move the swept area 200.

Fig . 2 shows a translational movement of a swept area 200.

The swept area 200 is translationally moved along the sway axis 102 in Fig. 1. The control device is preferably config ured to control the actuator to reciprocatingly move the swept area 200 back and forth.

Fig . 3 shows a rotational movement of a swept area 200. The swept area 200 is rotated about the floater roll axis 106 in Fig. 1. The control device is preferably configured to con trol the actuator to reciprocatingly rotate the swept area 200 back and forth.

Fig . 4 shows a heave movement of a swept area 200. The swept area 200 is translationally moved along the heave axis 101 in Fig. 1. The control device is preferably configured to con trol the actuator to reciprocatingly move the swept area 200 back and forth.

The rotor of the individual wind turbine 100 is moved to al ter the swept area 200 so that the rotor captures more wind energy from an "extended" swept area 200 than if the rotor would be at a fixed position.

If the rotor is moved fast enough, the wind turbine 100 can gain wind energy because it covers an enlarged effective swept area 200 being swept by the rotor. The principle is that the induction zone reduces a wind speed, so if a larger swept area 200 is covered, the effective wind speed on the rotor can be increased. It depends on how quickly the axial induction zone develops relative to the movement speed of the swept area. The control device can control this movement of the swept area along the rotor. For example, the control de vice can be configured to set a speed of moving the swept ar ea 200 based on a wind speed, wherein the speed of moving the swept area 200 is preferably increased when the wind speed increases, and the speed of moving the swept area 200 is preferably decreased when the wind speed decreases.

By the above-mentioned control strategies, the wind turbine 100 can be controlled without inter-turbine coordination, that means like a stand-alone turbine.

The control device can also be configured to move the swept area 200, if a at least one of the following conditions is fulfilled: the wind turbine 100 is operated below a rated output power of the wind turbine 100; the wind turbine 100 is within a wake of another wind turbine (wherein the wake can be determined based on the actual positions of the corre sponding wind turbines and a wind flow direction); if a wind speed is below a reference wind speed; if a power, which is consumed by the actuator to move the rotor, is lower than a first predetermined ratio of an actual output power of the wind turbine 100; and if a power, which is consumed by the actuator to move the rotor, is lower than a second predeter mined ratio of a power which is gained by moving the swept area 200. This may be a function of power or wind speed and can be computed in advance or online. For example, the first predetermined ratio can be 50%, 25% or 10%, and the second predetermined ratio can be 100%, 75% or 50%. The second pre determined ratio provides a kind of hysteresis bearing in mind that the (estimated) power which is gained by moving the swept area 200 may have some fluctuation.

The above-mentioned condition, where the power is considered, which is consumed by the actuator to move the rotor, enables an optimized energy balance. The movement of the rotor needs to be energy efficient such that the extra energy captured from a reduction of a wake shall exceed the energy used for the movement of the rotor. This measure can be part of the control strategy to only initiate the movement of the rotor when an energy gain is possible.

The control device can be configured to alter the movement of the swept area 200 based on a measured, estimated or predict ed speed of wind 111 to the wind turbine 100. For example, a LIDAR (light detection and ranging) or a radar could be used to scan the incoming wind field in advance, and this infor mation can be utilized to optimize the strategy to capture the highest amount of wind (power), e.g. exploiting shear, wakes, etc. The control device can be configured to set a speed of moving the swept area 200 based on the measured, es timated or predicted wind speed, wherein the speed of moving the swept area 200 is preferably increased when the wind speed increases, and the speed of moving the swept area 200 is preferably decreased when the wind speed decreases.

The control device can be configured to decelerate or stop the movement of the swept area 200, if at least one of the following conditions is fulfilled: if a power, which is con sumed by the actuator to move the rotor, exceeds a first threshold value; and if a load parameter, a fatigue parameter or an instability parameter, which is caused by the movement of the rotor, exceeds a second threshold value. The consider ation of the load parameter, the fatigue parameter or the in stability parameter provides for a safe operation. The rotor movement needs to be load efficient such that an excessive additional load by this movement is not allowed. This can be part of a control strategy only to initiate the rotor's move ment when the load is allowable. This can be carried out by monitoring load and acceleration variables online, or by scheduling the rotor's movement by a designed control strate gy. Furthermore, the rotor's movement should consider park boundaries as well, which may be static or even dynamic with respect incoming boats, other turbines and sailing lines, for example. A plurality of the wind turbines 100 can be controlled by a wind park control device (not shown). The wind park control device can be connected to each control device of each wind turbine 100. The wind park control device can be configured to perform a coordinated control in a manner that a wake of one wind turbine 100 is at least not continuously directed to another wind turbine 100. For example, one of the wind tur bines 100 can continuously be outside a wake of another wind turbine 100 when both wind turbines 100, 100 are synchronous ly be moved in the same direction. In another embodiment, both wind turbines 100, 100 can be moved in opposite direc tions so that one of the wind turbines 100 may be inside a wake of another wind turbine 100 only for a short time.

The wind turbines 100 placed inside the wind farm interact with each other through their wakes. The wake of the wind turbine 100 is a result of energy extraction of the incoming wind 111. The wake causes a reduced wind speed, also known as a wind deficit, and an increased wind speed variation, also known as turbulence. These phenomena result in a decrease of the overall power production of the wind farm and an increase of the loads for a downstream wind turbine 100. A downstream wind turbine 100 is a wind turbine 100 that is placed behind another wind turbine 100 downstream of the wind 111, wherein the other wind turbine 100 in front row generates the wake. The decrease of power production or output power is a direct result of the wind speed which is decreased due to the wake, while the increase in the wind speed variation caused by the turbulences can cause variations in mechanical loads.

The above-mentioned control strategies use an inter-turbine coordination. For example, trajectories of the rotor move ments of the wind turbines 100 can be optimized on wind park level to maximize the power generation.

The wind park control device that has information about a di rection of the wind 111 in the wind park and about positions of the wind turbines 100 can distribute control strategies (or setpoints) to the individual wind turbines 100 such that they are controlled to maximize the power generation on wind park level. For example, this can be done for upwind wind turbines 100 to coordinate the rotor movement with downwind wind turbines 100 to reduce an impact of the wakes, e.g. for up- and downwind wind turbines 100 to synchronously move or to move in opposite directions when a downwind wind turbine 100 is substantially completely in a wake of an upwind wind turbine 100.

For capturing the wind energy, it is only necessary to per form this rotor movement whenever the wind turbines 100 are operating below a rated power. It may be beneficial to the mechanical load to perform the above-mentioned control strat- egy also above a rated wind speed as well, when the wind tur bines 100 are in a wake.

It should be noted that the term "comprising" does not ex clude other elements or steps and "a" or "an" does not ex- elude a plurality. Also elements described in association with different embodiments may be combined. It should also be noted that reference signs in the claims should not be con strued as limiting the scope of the claims.