WO2020091601A1

WO2020091601A1 - Horizontal axis wind turbine with stabilizing wing

Info

Publication number: WO2020091601A1
Application number: PCT/NL2019/050719
Authority: WO
Inventors: William VAN DER SCHEE
Original assignee: Dutch Ventus Ip B.V.
Priority date: 2018-11-02
Filing date: 2019-11-01
Publication date: 2020-05-07
Also published as: NL2021921B1

Abstract

A horizontal axis wind turbine, comprising a support structure, a generator, a rotatable turbine unit, a turbine unit supporting frame and a stabilizing wing, wherein in use wind acting on the turbine unit generates a moment force with respect to a Y axis onto a yaw bearing mechanism of the turbine unit supporting frame, the stabilizing wing being secured to the turbine unit supporting frame remote from the rotatable turbine unit, wherein in use wind directed onto the front of the rotatable wind turbine also acts on the stabilizing wing and generates a lift force offset from the Z-axis, the lift force counteracting said moment force with respect to said Y-axis on said yaw bearing mechanism.

Description

HORIZONTAL AXIS WIND TURBINE WITH STABILIZING WING

The present invention relates to a horizontal axis wind turbine comprising a support structure, a generator and a rotatable turbine unit. The rotatable turbine unit is of the type that has at least one rotor blade and an outer shroud, wherein a tip of the at least one rotor blade is secured to the outer shroud, and wherein the at least one rotor blade and the outer shroud are configured to rotate about the rotation axis together.

Such a wind turbine is e.g. known from W02015/190916, which discloses a device for converting kinetic energy of wind to electrical energy. The device comprises a wind driven rotor and an electrical generator connected to the rotor. The rotor comprises a tubular ring or shroud with at least one vane or rotor blade mounted on the inner side of the tubular ring and extending radially to the centre thereof, wherein the tubular ring is mounted for rotation about a substantially horizontal axis. When the rotor of such a known device is installed on a support structure with a yaw bearing mechanism, a relatively high moment force about a Y- axis is generated onto the yaw bearing mechanism, also due to the co-rotation of the tubular ring or shroud with the vane(s) or rotor blade(s). Due to these forces, the service life of the yaw bearing mechanism may be relatively short, or a relatively expensive yaw bearing mechanism may be required. In addition, the forces introduced on the support structure may be undesirably large.

It is an object of the present invention to provide an improved horizontal axis wind turbine.

This object is achieved, in a first aspect of the invention, with a horizontal axis wind turbine according to claim 1.

The lift force generated by the stabilizing wing reduces the net moment acting about the Y- axis on the yaw bearing mechanism. Therefore, the design of the yaw bearing mechanism can be simplified and the total cost of the wind turbine can be reduced. Also, the yaw bearing mechanism may function longer without wear or defects as the forces acting on it are reduced.

The stabilizing wing may also reduce the net moment acting on the support structure. This may, for example, allow to install the wind turbine on a roof of an existing building, without making any major modifications or reinforcements to said roof to accommodate the wind turbine.

Preferably, the wind turbine has a single stabilizing wing.

The horizontal axis wind turbine comprises a support structure, that is in a simple

embodiment e.g. a mast which is arranged substantially vertical.

In embodiments, an imaginary Z-axis of a Cartesian coordinate system that has its origin in the yaw bearing mechanism aligns with the mast. When the support structure is a mast, said mast may, for example, be at least 5 meters high, such as between 10 and 20 meters, up to 40 meters or higher.

The support structure may alternatively be any other structure for supporting the wind turbine and the rotatable turbine unit above a ground surface. For example, the support structure may be a tripod structure, e.g. embodied to allow the wind turbine to be installed on a roof of a building.

The height of the support structure may, for example, be at least 0.5m, to separate the rotating turbine unit from the ground surface or the roof.

When the wind turbine is installed on a roof of a building, it may be slightly tilted with its front towards the ground, e.g. at an angle of the horizontal rotation axis of between 2 and 15 degrees relative to the horizon.

On a roof or on top of a building, the direction of the incoming wind may not be horizontal, but may be somewhat inclined due to the presence of the building, as the building forces the wind orientation to be locally deflected with respect to the horizontal orientation. Preferably, the rotational axis is substantially aligned with the averagely expected incoming wind direction, whether this is horizontal, or with the front tilted with respect to the horizon.

The horizontal axis wind turbine comprises a generator that is configured for generating electrical energy, and may principally be any known generator suitable for use in a wind turbine. A rotor of an electrical generator may be directly coupled to the turbine unit, so in absence of any mechanical transmission in between, which simplifies the wind turbine. The horizontal axis wind turbine comprises a rotatable turbine unit supported on one or more bearings by the turbine unit supporting frame such that the turbine unit is rotatable about a rotation axis to drive the generator. The at least one bearing allows the turbine unit to rotate with respect to the turbine unit support frame about the rotation axis parallel to the X-axis.

The bearing or bearings may be any known bearing suitable for use in turbine units of horizontal axis wind turbines. The imaginary X-axis of the above- introduced Cartesian coordinate system coincides with the horizontal axis of the turbine unit.

One or more bearings supporting a rotor within a stator housing of the electrical generator may also act as bearing or bearings of the wind turbine unit, e.g. at the rear of the unit. This may allow to dispense with an additional bearing at the rear for the unit.

The X-axis is arranged substantially horizontally, e.g. when the wind turbine is installed on top of a mast. Alternatively, the X-axis may be slightly tilted with respect to the horizontal, e.g. at an angle of the horizontal rotation axis of between 2 and 15 degrees relative to the horizon, e.g. the front of the unit facing slightly downward, e.g. when the wind turbine is installed on top of a roof and the front of the unit is slightly tilted towards the ground.

The rotatable turbine unit comprises at least one rotor blade configured to rotate about the rotation axis. For example, the rotor blade may be a helical blade, having a revolution between a root and a tip thereof. The revolution of a blade may span e.g. 30 degrees or more about the rotation axis. Alternatively, the rotor blades may only extend in a radial direction with respect to the rotation axis.

The rotatable turbine unit further comprises an outer shroud, secured to a tip of the at least one rotor blade and configured to rotate about said rotation axis together with said at least one rotor blade. Hence, the outer shroud and the tip of the at least one rotor blade are fixed to each other, e.g. welded, bolted, glued, cured, or formed as one part, etc., and in use rotate in conjunction with each other. When the outer shroud and the tip of the rotor blade are secured to each other, this has two main technical effects. Firstly, tip vortices are reduced as effectively there is no rotor blade tip. Secondly, noise from the wind turbine is low compared to noise generated by other wind turbines, e.g.“normal” shrouded wind turbines wherein the shroud is stationary with respect to the rotation axis of the rotor blades, as noise typically results from said tip vortices. These two technical effects allow the wind turbine to be installed in an urban environment, on top of a building, in an industrial environment, or on a street crossing, without complains about noise from the population of the urban environment, people within the building, etc. The horizontal axis wind turbine further comprising a turbine unit supporting frame including a forward turbine supporting arm in front of the rotatable turbine unit and a rearward turbine supporting arm behind the rotatable turbine unit. The forward turbine supporting arm and the rearward turbine supporting arm together support the turbine unit on said one or more bearings. For example, a bearing may be provided at the forward turbine supporting arm and another bearing at the rearward supporting arm. In an embodiment, a supporting arm, e.g. the rear supporting arm, may support the electrical generator, wherein the one or more bearings of the rotor of the generator also act as bearing(s) for the turbine unit.

The forward turbine supporting arm and/or the rearward turbine supporting arm may, for example, each be embodied as a single arm body that extends substantially vertically from a central frame member of the supporting frame. Preferably, each arm body is streamlined to avoid undue interference with the airflow.

The forward turbine supporting arm and/or the rearward turbine supporting arm may, for example, each be substantially parallel to the Z-axis.

The turbine unit supporting frame further includes a yaw bearing mechanism mounting the turbine unit supporting frame with respect to the support structure, such that the turbine unit supporting frame is rotatable about a Z-axis, e.g. to keep the front of the turbine unit facing into the wind. For example, said rotation about the Z-axis may allow the turbine unit supporting frame to optimally align the turbine unit in the incoming wind, to maximize the electrical energy generated by the horizontal axis wind turbine. This may be a free yaw arrangement and/or a motor driven yaw arrangement when desired, e.g. on the basis of a wind direction sensor.

As preferred, the turbine unit supporting frame is rigid, thereby only having a single degree of freedom of rotating about said Z-axis due to said yaw bearing mechanism.

As preferred, the support structure is rigid, e.g. a mast or tripod, etc.

In use, wind acting on the turbine unit generates a moment force with respect to a Y-axis onto said yaw bearing mechanism. The Y-axis is typically arranged in the horizontal plane, and completes the X-Y-Z Cartesian coordinate system. Especially for a horizontal axis wind turbine comprising an outer shroud that is configured to rotate together with the at least one rotor blade, said moment force onto the yaw bearing mechanism, about a Y axis thereof, may be significant. A yaw bearing mechanism is typically an expensive component of a wind turbine, that wears relatively fast. This moment force about the Y-axis may be one of the main factors of wear of the yaw bearing mechanism.

The forces acting on the yaw bearing mechanism may also act on the support structure, potentially requiring modifications, such as structural reinforcements, to the underground of the support structure. When the wind turbine is installed on top of an existing building, such modifications may be very expensive and undesired.

The horizontal axis wind turbine further comprises a stabilizing wing, secured to the turbine unit supporting frame remote from the rotatable turbine unit. Hence, while the rotatable turbine unit in use rotates with respect to the rotation axis, the stabilizing wing is substantially stationary with respect to the rotation axis. In use, wind directed onto the front of the rotatable wind turbine also acts on the stabilizing wing and generates a lift force offset from the Z-axis, the lift force counteracting said moment force with respect to said Y-axis on said yaw bearing mechanism.

So, of all the wind in the stream field of the wind turbine, a part of the wind goes through the turbine unit. Another portion of the wind flows around the outer shroud, so along the exterior thereof. Some of the wind that flows around the outer shroud flows over the stabilizing wing. Typically, wind entering and exiting the turbine unit does not flow over the stabilizing wing. However, while it may not be the same wind that flows through the turbine unit and over the stabilizing wing, said separate wind flows do have the same general characteristics in terms of e.g. direction, speed and density. When wind entering the inlet of the turbine unit has e.g. a higher speed, the same holds for the wind flowing over the stabilizing wing. When the wind entering the inlet of the turbine unit e.g. has a lower speed or changes direction, the same holds for the wind flowing over the stabilizing wing. Hence, the wind flowing through the turbine unit is proportional in wind characteristics (direction, speed, density) to wind flowing over the stabilizing wing. Hence, when the (positive) moment about the Y axis generated by the force of wind onto the turbine unit is larger, also the (negative) moment about the Y axis generated by the force of wind onto the stabilizing wing is larger, and vice versa.

The counteracting moment force generated by the stabilizing wing, as a percentage of the moment force generated by wind acting on the turbine unit, may be substantially constant, as said wind streams are practically roughly proportional to each other.

The moment on the yaw bearing mechanism resulting from the lift force generated by the stabilizing wing counteracts the moment on the yaw bearing moment induced by wind on the turbine unit. Hence, when the moment about the Y-axis generated by the wind on the turbine unit has a positive sign, the moment about the Y-axis generated by the lift force of the stabilizing wing has a negative sign, and vice versa.

To increase the moment generated on the yaw bearing mechanism by the lift force, the stabilizing wing is offset, i.e. positioned at a distance, with respect to the Z-axis. For a same lift force, the larger the offset distance in X-direction, the larger the moment. Hence, when the offset is relatively large, the wing can be relatively small to generate a certain counteracting moment on the yaw bearing mechanism.

The presence of the stabilizing wing in practice will reduce the forces on the yaw bearing mechanism and the support structure. Therefore, a less complex and less expensive yaw bearing mechanism may be used, less ground surface reinforcements may be needed, and/or a longer lifespan of the yaw bearing mechanism and/or support structure may be ensured.

In an embodiment, the stabilizing wing is arranged at a level below the turbine unit, e.g. near an outlet side thereof. Preferably, the stabilizing wing receives an airflow that is relatively laminar. Preferably, the turbine unit receives an airflow that is relatively laminar. Preferably, the airflow received by the turbine unit and the airflow received by the stabilizing wing are separated from each other, at least until the airflow has moved past the outlet of the wind turbine and the trailing edge of the stabilizing wing. When the stabilizing wing is arranged below the turbine unit it may e.g. by secured to the turbine unit supporting frame, such that the wind turbine does not need additional components to mount the stabilizing wing on.

Preferably, the distance between the yaw bearing mechanism and the stabilizing wing (measured along the X-axis) is as large as possible. When the yaw bearing mechanism is arranged e.g. relatively close to the inlet of the of the turbine unit, it is advantageous to arrange the stabilizing wing near an outlet side of the turbine unit. When the mast is e.g. arranged relatively close to the outlet side of the turbine unit, it may be advantageous to arrange the stabilizing wing near an inlet side thereof. For example, the stabilizing wing may be arranged near an inlet side of the turbine unit.

Alternatively, the stabilizing wing may be arranged above the turbine unit, e.g. near the inlet side thereof or near the outlet side thereof. This design, compared to an arrangement below the turbine unit, is expected to require a larger and/or more complex support frame and may therefore be less attractive. Preferably, the yaw bearing mechanism, seen in a direction along the X-axis, is arranged near the centre of gravity of the rotatable turbine unit.

In an embodiment, the stabilizing wing in use is mounted at an angle of attack of between -5 and 15 degrees with respect to the X-axis, e.g. at an angle of attack of between 2 and 10 degrees. Although a wing typically generates lift also when it is mounted at an angle of attack outside of these limits, the amount of generated lift increases when a wing is mounted at a moderate angle of attack - depending of course on the airfoil shape. Mounting the stabilizing wing at a pre-determined angle of attack hence may increase the lift force generated by said stabilizing wing, and further reduces the moment about the Y-axis acting on the yaw bearing mechanism.

The angle of attack at which the stabilizing wing is mounted may be adjustable, such that the angle of attack can e.g. be adapted to the specific location at which the wind turbine unit is installed, and/or adjusted during the life of the wind turbine.

In an embodiment the stabilizing wing, at wing tips thereof, comprises winglets. The winglets may be embodied as vertical wind vanes that assist in orienting of the turbine unit to the actual wind direction and/or stabilizing the orientation of the turbine unit to the actual wind direction. As such the provision of these winglets on the stabilizing wing may further be beneficial for the yaw bearing mechanism, e.g. as it may reduce wear and tear of the yaw bearing mechanism. Also the stabilization provided by the winglets may increase the efficiency of the wind turbine. As the winglets are arranged at the wing tips of the stabilizing wing, they are offset laterally in Y-direction relative to the X-axis. Therefore, they may be relatively small while being efficient in said stabilizing of the orientation of the turbine unit.

In an embodiment, the winglets are of the sharklet type, being activated both by wind flowing over the top side of the stabilizing wing and by wind flowing over the lower side of the stabilizing wing. Other types of winglets are however also conceivable.

In an embodiment, the length of the stabilizing wing, in a direction from wing tip to wing tip, is non-uniform. For example, the length of the stabilizing wing may increase in a direction from the leading edge to the trailing edge, e.g. widening towards the trailing edge. This widening of the wing may be proportional to the increase in diameter of the outer shroud of the turbine unit towards the outlet thereof. In an embodiment, the counteracting lift force generated by the stabilizing wing induces a counteracting moment on the yaw bearing mechanism that has a magnitude of at least 10%, such as at least 15%, preferably approximately 20% or more of the moment force generated by the wind acting on the turbine unit. This may e.g. be achieved with a stabilizing wing that has an area of between 10% and 33% of the frontal area of the rotatable turbine unit.

In an embodiment, the yaw bearing mechanism is a free yaw bearing mechanism, i.e. of the type that allows a self-orienting of the turbine unit with respect to incoming wind. Alternatively, the yaw bearing mechanism is a steered mechanism, comprising a steering device and a sensor, wherein the sensor is configured to determine the wind direction and wherein the steering device is configured to steer the turbine unit into the optimal direction.

In an embodiment, the yaw bearing mechanism of the wind turbine is arranged between the forward turbine supporting arm and the rearward turbine supporting arm, seen in a direction along the X-axis. That is, preferably the yaw bearing mechanism of the wind turbine is arranged between the turbine unit inlet and the turbine unit outlet. With such an arrangement, forces on the yaw bearing mechanism (e.g. a moment about the Y-axis) are typically smaller than when the yaw bearing mechanism is e.g. arranged in front of the turbine unit inlet or behind the turbine unit outlet. To increase the offset between the yaw bearing mechanism and the stabilizing wing, the yaw bearing mechanism may e.g. be arranged closer towards the forward turbine supporting arm than towards the rearward turbine supporting arm, the stabilizing wing then being arranged near the rearward turbine supporting arm (or vice versa).

In an embodiment, the forward turbine supporting arm and the rearward turbine supporting arm of the turbine unit supporting frame are interconnected by a central frame member, that is preferably formed as a wind vane to assist the orienting of the turbine unit to the actual wind direction. The central frame member may be positioned below the turbine unit.

In an embodiment, the turbine unit supporting frame comprises a central shaft mounted between the forward turbine supporting arm and the rearward turbine supporting arm, the central shaft being rotatable together with the outer shroud and the rotor blades, thereby e.g. driving the electrical generator. The central shaft may be arranged coaxially with respect to the turbine unit.

In an embodiment, a length of the outer shroud is 0.5 - 1.2 times the maximum outer diameter of the outer shroud. In an embodiment, the turbine unit further comprises an inner cone, e.g. arranged coaxial with respect to the outer shroud and arranged at an inner side of said outer shroud, wherein a root side of the at least one rotor blade is secured to the inner cone, and wherein the inner cone is configured to rotate with respect to the rotation axis together with the outer shroud and the at least one rotor blade. Hence, the inner cone and the root of the at least one rotor blade may be rigidly fixed to each other, e.g. welded, cured, or formed as one part, and in use rotate in conjunction with each other. When the rotor blade is secured to both an outer shroud and an inner cone, the turbine unit is very stiff and strong, resulting in a robust turbine unit.

In an embodiment, the turbine unit has an inlet and an outlet, the diameter of the turbine outlet exceeding the diameter of the turbine unit inlet. In an embodiment, the turbine unit has an inlet and an outlet, the diameter of the turbine inlet exceeding the diameter of the turbine unit outlet.

In an embodiment a cross section of the outer shroud, in a direction parallel to the X axis, has an airfoil shape. Typically, as wind goes through the turbine unit it has a net deceleration, an outlet speed of the wind being lower than an inlet speed, potential energy of the wind being used to drive the electrical energy. Also, the wind may be directed outwardly when exiting the turbine unit. When the outer shroud has an airfoil profile, the speed and direction of wind flowing along the outer side of the outer shroud may also be influenced. Preferably, when wind exiting the turbine unit mixes with wind that has bypassed the turbine unit, said wind flows are similar in terms of direction, speed and density, such that little energy is lost when the flows are mixed and the efficiency of the wind turbine is increased.

In an embodiment, the turbine unit supporting frame further comprises an inlet shroud, arranged in front of the rotary turbine unit. The inlet shroud is, preferably, generally in line with the outer shroud. The inlet shroud may e.g. have a converging shape, so reducing downwind in diameter for the airflow, to increase the density of the wind before it enters the turbine unit.

Preferably, the inlet shroud is a rigid and stationary part of the turbine unit supporting frame.

In an embodiment, the inlet shroud has a maximum diameter, e.g. at a front end thereof, that exceeds the diameter of the inlet portion of the outer shroud of the turbine unit. For example, the diameter of the inlet shroud may be a few centimetres larger than the diameter of the inlet portion of the rotatable outer shroud. There may, in practical embodiments, be a spacing between the (stationary) inlet shroud and the rotatable outer shroud, of e.g. one to a few centimetres, e.g. less than 5 centimetres.

In an embodiment, the inlet shroud comprises inwardly protruding vortex inducing stator blades, the vortex inducing stator blades preferably being mounted stationary with respect to the inlet shroud. These blades cause the airflow to have a vortex, or increase any vortex effect of the incoming airflow. In an alternative structural embodiment however, the angle of attack of the stator blades is adjustable, e.g. manually or by a motorized drive, e.g. remotely controlled, by rotating the stator blades about an axis defined by the longitudinal direction (perpendicular to the chord line) of the stator blade.

In an embodiment, the forward turbine supporting arm is formed as a stator blade of the inlet shroud. This stator is then stationary. Other stator blades of the inlet shroud may either be stationary or adjustable, as described.

It will be appreciated that technical features described herein in relation to embodiments may be combined in various combinations, e.g. in order to obtain the effects associated with said embodiments.

The first aspect of the invention also relates to a method for generating electrical energy wherein use is made of a horizontal axis wind turbine as described herein. In the method, wind directed onto the front of the rotatable turbine unit also acts on the stabilizing wing and said interaction with the stabilizing wing generates a lift force at a location that is offset from the Z- axis said lift force counteracting said moment force with respect to the Y-axis on the yaw bearing mechanism.

A second aspect of the invention relates to a horizontal axis wind turbine, comprising a support structure, a generator, at least one rotor blade, e.g. said at least one rotor blade being part of a rotatable turbine unit, a supporting frame and a stabilizing wing, wherein the generator is configured for generating electrical energy;

the at least one rotor blade is supported on one or more bearings by the supporting frame such that the at least one rotor blade is rotatable about a rotation axis parallel to an X axis to drive the generator,

the supporting frame including:

- a forward supporting arm in front of the at least one rotor blade and a rearward turbine arm behind the at least one rotor blade, supporting the at least one rotor blade on said one or more bearings, and - a yaw bearing mechanism for mounting the supporting frame to the support structure such that the supporting frame is rotatable about a Z axis;

wherein the stabilizing wing is secured to the supporting frame remote from the at least one rotor blade, wherein in use wind directed onto the front of the at least one rotor blade also acts on the stabilizing wing,

and wherein the stabilizing wing, at wing tips thereof, comprises winglets.

The winglets may be embodied as vertical wind vanes that assist in orienting of the turbine to the actual wind direction and/or stabilizing the orientation of the turbine to the actual wind direction. As such the provision of these winglets on the stabilizing wing may be beneficial for the yaw bearing mechanism, e.g. as it may reduce wear and tear of the yaw bearing mechanism. Also the stabilization provided by the winglets may increase efficiency of the wind turbine. As the winglets are arranged at the wing tips of the stabilizing wing, they are offset laterally in Y-direction relative to the X-axis. Therefore, they may be relatively small while being efficient in said stabilizing of the orientation of the turbine unit.

In an embodiment - in use - wind acting on the at least one rotor blade generates a moment force with respect to a Y axis onto the yaw bearing mechanism, and the stabilizing wing is embodied such that interaction of the incoming wind with the stabilizing wing generates a lift force at a location that is offset from the Z axis, said lift force counteracting said moment force with respect to said Y axis on said yaw bearing mechanism.

It will be appreciated that the horizontal axis wind turbine of the second aspect of the invention may comprise one or more features as discussed herein with reference to the first aspect of the invention. For example the wind turbine may include the rotatable turbine unit.

The second aspect of the invention also relates to method for generating electrical energy wherein use is made of a horizontal axis wind turbine as described herein, wherein wind directed onto the front of the at least one rotor blade, e.g. part of a rotatable turbine unit as described herein, also acts on the stabilizing wing and said interaction with the stabilizing wing having winglets causes a stabilization of the orientation of the turbine to the actual wind direction. The present invention also relates to a building on which a horizontal axis wind turbine as described herein is installed, e.g. wherein the wind turbine is installed slightly tilted towards the ground, e.g. at an angle of the horizontal rotation axis of between 2 and 15 degrees relative to the horizon.

The invention is further elucidated in the below, with reference to the attached drawings. In these figures:

Figure 1 schematically shows a side view of a horizontal axis wind turbine;

Figure 2 schematically shows a rear view of a part of the horizontal axis wind turbine of Figure 1 ;

Figure 2A schematically shows a cross sectional view along line 2A of the turbine unit of Figure 2;

Figure 3 schematically shows an isometric view from the rear of a part of the horizontal axis wind turbine of Figure 1 ;

Figure 4 is similar to Figure 3, wherein the turbine unit is not shown;

Figure 5 schematically shows a top view of the part of the horizontal axis wind turbine of Figure 5;

Figure 5A shows a detail of Figure 5;

Figure 6 schematically shows a rear view of the part of the horizontal axis wind turbine of Figure 4;

Figure 6A shows a detail of Figure 6;

Figure 7 schematically show some of the forces acting on the yaw bearing mechanism of the wind turbine of Figure 1.

With reference to Figure 1 , a horizontal axis wind turbine is shown. Visible are a support structure, here mast 11 , a rotatable turbine unit 13, a turbine unit supporting frame 14 and a stabilizing wing 15. The turbine unit supporting frame comprises a forward turbine supporting arm 141 , in front the turbine unit 13, a rearward turbine supporting arm 142, behind the turbine unit 13, and a yaw bearing mechanism 143.

Defined in the centre of the yaw bearing mechanism 143 is an imaginary Cartesian XYZ axis system. The Z-axis Z of the axis system here coincides with the vertical direction and the orientation of the mast. The X-axis X here is defined parallel to the rotation axis R of the turbine unit 13, and is arranged horizontally. The Y-axis Y completes the axis system and is also arranged horizontally; transverse to the X-axis X. Hence, the horizontal plane is defined by the X-axis X and the Y-axis Y. Compared to the diameter of the turbine unit 13, the length LS of the outer shroud 136 is relatively long. For example, the length LS of the outer shroud 136 may be 0.5 - 1.5 times the maximum diameter S2 of the outer shroud 136.

The stabilizing wing 15 is here mounted on the supporting frame at a location below the turbine unit 13.

In embodiments, an outlet of the outer shroud 136 and the trailing edge of the wing 15 may approximately align.

The wing 15 here starts, seen in X direction, in front of the outlet of the turbine unit 13 and behind the inlet of the turbine unit 13, i.e. between the inlet and the outlet of the turbine unit 13.

The wing 15 may extend downwind to behind the outlet of the outer shroud 136.

Here, the wing 15 is mounted at an pre-determined angle of attack a of between -5 and 15 degrees relative to the rotation axis R of the turbine unit 13.

The yaw bearing mechanism 143 mounts the turbine unit supporting frame 14, here on top of the mast 11 , in such a way that the turbine unit supporting frame 14 is rotatable about the Z- axis Z. For example, the yaw bearing mechanism 143 is of the type that allows a self- orienting of the turbine unit with respect to incoming wind, also called a free yaw bearing mechanism.

Well visible in Figure 1 is that the yaw bearing 143 mounting the frame 14 on the mast 11 of the wind turbine 13 is arranged between the forward turbine supporting arm 141 and the rearward turbine supporting arm 142. In fact, as preferred, the yaw bearing 143 is positioned closer towards the forward arm 141 than towards the rearward arm 142. That is, a distance d1 between the forward turbine supporting arm 141 and the mast 11 is smaller than a distance d2 between the rearward turbine supporting arm 142 and the mast 11.

The turbine unit 13 is configured to rotate about substantially horizontal rotation axis R to drive an electrical generator 12. The unit comprises an outer shroud 136 rotating along with the one or more rotor blades. A rear view of inter alia the turbine unit 13 is shown in Figure 2, while a cross sectional view of the turbine unit 13 along line 2A in Figure 2 is visible in Figure 2A. An isometric rear view of Figure 2 is shown in Figure 3. Visible in Figures 2 and 3 is not only outer shroud 136, but also rotor blades 133 and inner cone 139.

The inner cone 139 widens, that is increases in diameter, towards the rear or outlet of the turbine unit.

As preferred, generator 12 is accommodate in an open ended hollow rear portion of the inner cone, preferably the generator 12 having a rotor aligned with the rotation axis R and directly coupled to the inner cone 139 and thus to the turbine unit 13.

As preferred one or more bearings within the generator 12, said bearings supporting the rotor of the generator, also support the rotor unit 13, preferably in absence of any further bearing between the rotor unit 13 and the frame 14 at the rear.

The rotor blades 133 are mounted to the inner cone 139 with a root side 133A thereof (visible in Figure 2A) and to the outer shroud 136 with a tip side 133B thereof (visible in Figure 2A).

Upon operation of the wind turbine, outer shroud 136, rotor blades 133 and inner cone 139 together rotate about rotation axis R, driving the electrical generator 12. The wind turbine is powered by wind entering the turbine unit 13. The direction of the incoming wind is indicated by arrows in Figure 2A. Upon driving the electrical generator 12, electrical energy may be generated by the wind turbine.

In Figure 2, a total of five rotor blades 133 is shown. This number is purely indicative; any number of rotor blades 133 may be included in the turbine unit 13.

Visible in Figure 4 are turbine unit supporting frame 14 and stabilizing wing 15. The wing 15 is mounted onto a central frame member 147 of the turbine unit supporting frame 14. The central frame member 147 connects forward supporting arm 141 and rearward supporting arm 142 with each other, besides supporting wing 15.

As visible in Figure 4, the central frame member 147 has a relatively large dimension in a direction parallel with the Z-axis; the central frame member 147 thereby acting as (horizontal) wind vane. Also the rearward supporting arm 142 is embodied as a wind vane, here a vertical wind vane, by having a relatively large dimension in a direction parallel to the X-axis. This shaping of the central frame member 147 and the rearward turbine supporting arm 142 as wind vanes assist in orienting the turbine unit 13 in the actual wind direction.

Further visible in Figure 4 is a schematically illustrated central shaft 144, arranged between bearings 131 , 132. The turbine unit 13 is in use supported on these bearings 131 , 132, such that the turbine unit is rotatable with respect to the rotation axis R. The central shaft 144 may in use rotate together with the turbine unit 13, to drive generator 12.

Also visible in Figure 4 is an inlet shroud 145, arranged in front of the turbine unit and generally in line therewith (see e.g. Figure 3). Here the inlet shroud 145 comprises a stator blade 146 directed inwardly, generally towards the axis R.

In an embodiment, the forward turbine supporting arm 141 is embodied as a stator blade 146 of the inlet shroud 145. In an embodiment, for example, the blade 146 can pivot about an axis parallel to the longitudinal axis thereof, here radial to the axis R, e.g. to form a vortex inducing stator blade 146 of the inlet shroud. It is conceivable that the inlet shroud 145 comprises multiple such vortex inducing stators 146.

Also well visible in Figure 4 are winglets 151 , arranged at wing tips 15A of the stabilizing wing 15. The wing 15 has a wing portion on either side, seen in Y-direction, of the frame 14, here of central frame member 147. At the end of each wing portion, a winglet 151 is present.

The wingtips 151 here function mainly as wind vanes that orient and/or stabilize the turbine unit optimally with respect to the incoming wind.

As is well visible in figures 5 and 6, the wing 15 widens in a direction from the leading edge towards the trailing edge. Preferably, the width of the wing 15, measured from wing tip to wing tip, at any cross section, equals at least the diameter of the outer shroud 136. As the outer shroud 136 here increases in diameter towards the outlet side thereof, also the wing 15 increases in width towards the trailing edge thereof. This may ensure a more uniform flow field over the wing 15, leading to less vibrations and a longer lifespan.

The technical effect of the creating a countermoment by means of the stabilizing wing 15 is now explained with reference to Figure 7.

In use of the wind turbine, wind acts on the turbine unit 13. This wind generates electrical energy by rotating the unit of rotor blade(s) and the outer shroud, thereby driving the electrical generator. However, the wind also generates a moment force MY1 about the Y-axis in the yaw bearing member 143, as the turbine unit is offset from the yaw bearing member 143 in the direction of the Z-axis. This moment force MY1 equals the average wind pressure Pwind times the maximum surface area (function of the maximum diameter S2) of the turbine unit 13 times the distance hf along the Z axis between the centre line R of the turbine unit 13 and the yaw bearing member 143.

When wind acts on the turbine unit 13, wind of similar characteristics also acts on the stabilizing wing 15, the wing 15 generating a lift force L. The lift force L is substantially perpendicular with respect to the force Pwind of the wind acting on the turbine unit 13. The lift force L also generates a moment force MY2 about the Y-axis in the yaw bearing member 143, as the wing 15 is offset of the yaw bearing member 143 in the direction of the X-axis.

The lift force L, and the moment MY2 resulting from it, counteracts at least in part the moment force MY1 generated by the force of the wind Pwind acting on the turbine unit 13.

As the two moments MY1 , MY2 are directed to counteract each other, the net moment about the Y-axis in the yaw bearing member 143 is reduced compared to a design wherein the wing 15 would be absent. As explained herein this may lead to the yaw bearing member 143 having a longer lifespan, being of simpler design, etc.

Preferably the wing 15 is configured to generate a counter moment MY2 that is at least 10%, e.g. at least 25%, of the moment MY1 about the Y-axis in the bearing member.

For example, the area of the stabilizing wing 15 is between 10% and 33% of the frontal area of the outer shroud.

Preferably, the wind turbine is optimised for wind speeds between 1m/s and 10m/s, preferably for wind speeds between 4m/s and 8m/s, allowing to use the wind turbine at relatively low heights at both inland and near-shore locations.

Claims

1. A horizontal axis wind turbine (1), comprising a support structure (11), a generator (12), a rotatable turbine unit (13), a turbine unit supporting frame (14) and a stabilizing wing (15), the generator (12) being configured for generating electrical energy;

the rotatable turbine unit (13) being supported on one or more bearings (131 , 132) by the turbine unit supporting frame (14) such that the turbine unit (13) is rotatable about a rotation axis (R) parallel to an X axis (X) to drive the generator (12), the rotatable turbine unit (13) comprising:

- at least one rotor blade (133) configured to rotate about said rotation axis (R), and

- an outer shroud (136), secured to a tip (133A) of the at least one rotor blade (133) and configured to rotate about said rotation axis (R), together with said at least one rotor blade (133);

the turbine unit supporting frame (14) including:

- a forward turbine supporting arm (141) in front of the rotatable turbine unit (13) and a rearward turbine supporting arm (142) behind the rotatable turbine unit (13), supporting the turbine unit (13) on said one or more bearings (131 , 132), and

- a yaw bearing mechanism (143) for mounting the turbine unit supporting frame (14) to the support structure (11) such that the turbine unit supporting frame (14) is rotatable about a Z- axis (Z);

wherein in use wind acting on the turbine unit (13) generates a moment force (MY1) with respect to a Y-axis (Y) onto said yaw bearing mechanism (143),

the stabilizing wing (15) being secured to the turbine unit supporting frame (14) remote from the rotatable turbine unit (13), wherein in use wind directed onto the front of the rotatable turbine unit (13) also acts on the stabilizing wing (15) and said interaction with the stabilizing wing (15) generates a lift force (L) at a location that is offset from the Z-axis (Z), said lift force (L) counteracting said moment force (MY1) with respect to said Y-axis (Y) on said yaw bearing mechanism (143).

2. Horizontal axis wind turbine according to claim 1 , wherein the stabilizing wing (15) is arranged below the turbine unit (13), e.g. in proximity of an outlet side (138) thereof.

3. Horizontal axis wind turbine according to one or more of the preceding claims, wherein the stabilizing wing (15) is mounted at an angle of attack (a) of between -5 and 15 degrees with respect to the X-axis (X), preferably at an angle of attack (a) of between 2 and 10 degrees.

4. Horizontal axis wind turbine according to one or more of the preceding claims, wherein the stabilizing wing (15), at wing tips (15A) thereof, comprises winglets (151).

5. Horizontal axis wind turbine according to one or more of the preceding claims, wherein the lift force (L) generated by the stabilizing wing (15) induces a counteracting moment (MY2) on the jaw bearing mechanism (143) that has a magnitude of at least 10% of the moment force (MY1) generated by the wind acting on the turbine unit (13).

6. Horizontal axis wind turbine according to one or more of the preceding claims, wherein the yaw bearing mechanism (143) is a free yaw bearing mechanism, i.e. of the type that allows a self-orienting of the turbine unit (13) with respect to incoming wind.

7. Horizontal axis wind turbine according to one or more of the preceding claims, wherein, seen in a direction along the X-axis (X), a connection between the support structure (11) and the yaw bearing mechanism (143) of the wind turbine (1) is arranged between the forward turbine supporting arm (141) and the rearward turbine supporting arm (142), wherein preferably a distance (d1) between the forward turbine supporting arm (141) and the yaw bearing mechanism (143) is smaller than a distance (d2) between the rearward turbine supporting arm (142) and the yaw bearing mechanism (143).

8. Horizontal axis wind turbine according to one or more of the preceding claims, wherein the rearward turbine supporting arm (142) is embodied as a vertical wind vane assisting the orienting of the turbine unit (13) to the actual wind direction

9. Horizontal axis wind turbine according to one or more of the preceding claims, wherein a length (LS) of the outer shroud (136) is 0.5 - 1.5 times the maximum diameter (S2) of the outer shroud (136).

10. Horizontal axis wind turbine according to one or more of the preceding claims, wherein the turbine unit (13) further comprises an inner cone (139), e.g. arranged coaxial with respect to the outer shroud (136) and arranged at an inner side thereof, wherein a root side (133B) of the at least one rotor blade (133) is secured to the inner cone (139), and wherein the inner cone (139) is configured to rotate with respect to the rotation axis (R) together with the outer shroud (136) and the at least one rotor blade (133).

11. Horizontal axis wind turbine according to one or more of the preceding claims, wherein the turbine unit supporting frame (14) further comprises an inlet shroud (145), arranged in front of the rotary turbine unit (13) and generally in line with the outer shroud (136).

12. Horizontal axis wind turbine according to one or more of the preceding claims, wherein the inlet shroud (145) has a maximum diameter that exceeds the diameter of the inlet portion of the outer shroud (136) of the turbine unit (13) or vice versa.

13. Horizontal axis wind turbine according to one or more of the preceding claims, wherein the turbine unit supporting frame (14) further comprises an inlet shroud (145), arranged in front of the rotary turbine unit (13), and wherein the inlet shroud (145) comprises vortex inducing stator blades (146), the vortex inducing stator blades (146) e.g. being stationary with respect to the inlet shroud (145).

14. Horizontal axis wind turbine according to any of claims 11 - 13, wherein the forward turbine supporting arm (141) is formed as a vortex inducing stator blade (146) of the inlet shroud (145).

15. A method for generating electrical energy wherein use is made of a horizontal axis wind turbine according to one or more of the preceding claims, and wherein wind directed onto the front of the rotatable turbine unit (13) also acts on the stabilizing wing (15) and said interaction with the stabilizing wing (15) generates a lift force (L) at a location that is offset from the Z-axis (Z), said lift force (L) counteracting said moment force (MY1) with respect to said Y-axis (Y) on said yaw bearing mechanism (143).