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Title:
A FLOATING VERTICAL AXIS WIND TURBINE WITH PERIPHERAL WATER TURBINE ASSEMBLIES AND A METHOD OF OPERATING SUCH
Document Type and Number:
WIPO Patent Application WO/2019/190387
Kind Code:
A1
Abstract:
The present invention relates to a floating vertical axis wind turbine, VAWT, with a central column which at its lower end is connected to a mooring system. The VAWT comprises at least one rigid peripheral power generating unit comprising a blade, a peripheral buoyancy element and a water turbine assembly. The peripheral buoyancy element extends from the lower end of the blade and connects the water turbine assembly to the blade, and wherein the peripheral buoyancy element at least partly supports the rigid peripheral power generating unit by buoyancy. A bearing assembly is rotatably attached to the central column and at least one first strut connects the rigid peripheral power generating unit with the bearing assembly.

Inventors:
RAHM, Magnus (Lönnviksvägen 7-, Uppsala, 756 51, SE)
Application Number:
SE2019/050278
Publication Date:
October 03, 2019
Filing Date:
March 27, 2019
Export Citation:
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Assignee:
MAGNUS RAHM ENERGY CONSULTING AB (Lönnviksvägen 7-, Uppsala, 756 51, SE)
International Classes:
F03D3/00; F03D9/00; F03B13/10; F03D13/25
Domestic Patent References:
WO2015116016A12015-08-06
WO2015086033A12015-06-18
WO2015140415A12015-09-24
WO2003016714A12003-02-27
Foreign References:
US20140339832A12014-11-20
DE10064472A12002-06-20
CN106121923A2016-11-16
CN107829869A2018-03-23
GB2351124A2000-12-20
Attorney, Agent or Firm:
BRANN AB (Box 3690-, Stockholm, 111 51, SE)
Download PDF:
Claims:
Claims

1. A floating vertical axis wind turbine (1) comprising a central column (3)

connected to a support structure (10, 11) and at least one vertical blade (5a) arranged at the perimeter of the floating vertical axis wind turbine (1), the blade (5a) making the floating vertical axis wind turbine (1) rotate by the force of the wind, the floating vertical axis wind turbine (1) characterized by

-at least one rigid peripheral power generating unit (5b) comprising the at least one blade (5a), at least one peripheral buoyancy element (13) and at least one water turbine assembly (8), wherein the blade (5a), the peripheral buoyancy element (13) and the water turbine assembly (8) are mechanically

interconnected, the water turbine assembly (8) is adapted to be below a water surface, and wherein the peripheral buoyancy element (13) at least partly supports the rigid peripheral power generating unit (5b) including the blade (5a) by buoyancy;

- a bearing assembly (7) rotatably attached to the central column (3);

-at least one first strut (4a) connecting the rigid peripheral power generating unit (5b) with the bearing assembly (7); and

- at least one hydrofoil assembly (9) provided in connection to the rigid peripheral power generating unit (5b).

2. The floating vertical axis wind turbine (1) according to claim 1, wherein the hydrofoil assembly (9) comprises adjusting means adapted for adjusting the upward or downward force exerted by the hydrofoil assembly (9) during operation.

3. The floating vertical axis wind turbine (1) according to claim 2, wherein the hydrofoil assembly (9) comprises at least one hydrofoil wing which angle of attack is adjustable by means of a mechanical rod connected between a surface- sensing floating body and the hydrofoil assembly (9).

4. The floating vertical axis wind turbine (1) according to claim 2, wherein the hydrofoil assembly (9) comprises at least one hydrofoil wing, an electronic depth sensor and an actuator, wherein the angle of attack of the hydrofoil wing is adjustable by the electric or hydraulic actuator and the electronic depth sensor is in communication with the electric or hydraulic actuator. 5. The floating vertical axis wind turbine (1) according to any of claims 1 to 4, wherein the first strut (4a) is provided with at least one hinge joint (6) so that the rigid peripheral power generating unit (5b) is connected with the bearing assembly (7) via the at least one hinge joint (6) in so that the rigid peripheral power generating unit (5b) and the central column (3) may have varying relative inclination.

6. The floating vertical axis wind turbine (1) according to claim 5, wherein a first hinge joint (6) is provided between the bearing assembly (7) of the central column (3) and the first strut (4a) and a second hinge joint (6) is provided between the rigid peripheral power generating unit (5b) and the first strut (4a).

7. The floating vertical axis wind turbine (1) according to any of claims 1 to 5, further comprising a lower strut (4a) provided below the first strut (4a), the lower strut (4b) supporting the rigid peripheral power generating unit (5b).

8. The floating vertical axis wind turbine (1) according to claim 7, wherein the lower strut (4b) is arranged to be at least partly below the sea surface during operation. 9. The floating vertical axis wind turbine (1) according to claim 8, wherein the hydrofoil assembly (9) is provided on the lower strut (4b).

10. The floating vertical axis wind turbine (1) according to any of the preceding claims, further comprising a plurality of rigid peripheral power generating units (5b) wherein the lower part of each blade (5a) of each rigid peripheral power generating unit (5b) is connected to the blade (5a) of another rigid peripheral power generating unit (5b) by a connecting member (14).

11. The floating vertical axis wind turbine (1) according to any of the preceding claims, wherein the water turbine assembly is an axial flow upstream turbine, an axial flow downstream turbine or a cross flow turbine.

12. The floating vertical axis wind turbine (1) according to any of the preceding claims, further comprising an electrical cable arranged inside at least one first (4a) or lower strut (4b) for transmitting power from the water turbine assembly (8) to the central column (3).

13. The floating vertical axis wind turbine (1) according to any of the preceding claims, wherein the rigid peripheral power generating unit (5b) is connected to the central column only by one first strut (4a).

14. The floating vertical axis wind turbine (1) according to any of the preceding claims, wherein the central column (3) is rotating along with the first strut (4a) and rigid peripheral power generating unit (y) and the interior of the central column (3) is geo-stationary.

15. The floating vertical axis wind turbine (1) according to any of the preceding claims, wherein the central column (3) is a buoyancy column and thereby the floating vertical axis wind turbine (1) is completely floating.

16. The floating vertical axis wind turbine (1) according to claim 15, wherein the support structure is a mooring system (10).

17. A method of operating a floating vertical axis wind turbine (1) comprising a central column (3) connected to a support structure (10, 11) and at least one vertical blade (5a) arranged at the perimeter of the floating vertical axis wind turbine (1), the blade (5a) making the floating vertical axis wind turbine (1) rotate by the force of the wind, the floating vertical axis wind turbine (1), wherein at least one rigid peripheral power generating unit (5b) comprising the at least one blade (5a), at least one peripheral buoyancy element (13) and at least one water turbine assembly (8), the water turbine assembly (8) adapted to be below a water surface and the peripheral buoyancy element (13), wherein the peripheral buoyancy element (13) at least partly supports the rigid peripheral power generating unit (5b) including the blade (5a) by buoyancy, a bearing assembly (7) rotatably attached to the central column (3), at least one first strut (4a) connecting the rigid peripheral power generating unit (5b) with the bearing assembly (7), and at least one hydrofoil assembly (9) provided in connection to the rigid peripheral power generating unit (5b), and the water turbine assembly (8) is coupled to an electrical generator,

the method comprises

matching the axial thrust force on the individual water turbine assembly (8), with the tangential wind force component on the individual blade (5a) of the rigid peripheral power generating unit (5b) so that the water turbine thrust force equals the tangential component of aerodynamic force on the blade (5a) in all angular positions of the floating vertical axis wind turbine's (1) rotation, wherein the axial thrust force is controlled by the breaking torque of the generator.

18. The method according to claim 17, further comprising the step of controlling the hydrofoil assembly (9) to counteract the tipping moment exerted by the normal force component of the aerodynamic wind force on the individual blade (5a) by increasing or decreasing the lift force of the hydrofoil assembly (9).

Description:
A FLOATING VERTICAL AXIS WIND TURBINE WITH PERIPHERAL WATER

TURBINE ASSEMBLIES AND A METHOD OF OPERATING SUCH

Technical Field

The present invention relates to the area of offshore wind power. More specifically, the invention relates to a floating vertical axis wind turbine which comprises one or more rigid peripheral power generating unit comprising a blade, a peripheral buoyancy element and a water turbine assembly and a method of operating such

Background

Offshore wind power globally has very large potential to contribute to the transition from a fossil-based society to one where only renewable energy is used. The potential for offshore wind power only in Europe has been estimated to 4000 GW which is more than the global electricity use today. A challenge is that 80% of these 4000 GW is located at ocean depths of more than 60 m where bottom fixed foundations are not economically viable. Also in places like Japan, the US west coast, large parts of the Mediterranean, many volcanic islands in the Atlantic and the Pacific oceans, the ocean depth increases rapidly outside the coast. An alternative for these markets are floating wind turbines which can be anchored at large depths.

Today’s floating wind turbines are mainly based on four different foundation types all of which offer buoyancy and stability: barge, spar (buoy), semisubmersible and tension leg platforms (TLP). The stability requirement is high since the foundations are intended to carry conventional Horisontal Axis Wind Turbines (HAWTs) which can only withstand small inclination angles. Conventional wind turbines have a rotor with three blades that is mounted on a relatively heavy nacelle which in turn is placed at the top of a steel tower. The steel tower is, from the base to the top, designed as a tapered steel pipe which is welded and bolted in sections. The sections are generally about 20 m long for ease of handling. The wind acts on the entire rotor disc resulting in a tipping moment and subsequently a high tower base moment. This is not a problem per se. However, as the wind pressure on the rotor disc increases, the wind turbine will experience an inclination angle. The tower top weight will then add to the tipping moment on the foundation and to the mechanical tower base moment. The static inclination angle has to be limited in order for the rotor blades to stay clear of the tower during its rotation. During dynamic motions, especially in the pitch degree of freedom which is rotation around an axis perpendicular to the wind direction and parallel with the sea surface, the inertial loads may be significant especially in terms of blade root moments and tower base moments. Large accelerations result in large inertial loads which requires more material to reduce stress levels. More material increases the weight which in turn requires more material in the foundation to increase the buoyancy to carry the increased weight. A negative weight spiral is attained. The moment from the wind turbine’ s tower base is for semisubmersible typically resisted by buoyancy columns via stiff construction elements. To secure that this design, that has large distributed masses, is not excited by energy from environmental loads, the load-carrying construction elements (beam sections) between the masses (RNA and buoyancy columns respectively) must be designed stiff enough which requires a lot of material. This is the case also for other known designs of wind turbines.

A general disadvantage of HAWTs is that they need to be pointed into the prevailing wind direction. Most Vertical Axis Wind Turbines (VAWTs) converts wind energy from any direction equally efficient and is thus less sensitive than HAWTs to a rapidly changing wind direction. A problem with today’s floating wind turbine technology is how to exchange major components such as main bearing, gearbox and generator. The nacelle can be mounted on 80-90 m or more above sea level and a gearbox for a 5 MW wind turbine can weigh more than 60 tonnes. Today, there are only very few crane vessels in the world that can manage the combination of this lifting height and weight. The operation is also made significantly more complex by the fact that it is a lift from one floating vessel to another, not between a fixed reference point (as on a bottom-mounted offshore wind turbine) and a floating vessel.

A problem with today’s floating turbine foundations is the firm stability requirement. This is due to that HAWTs must operate within a narrow window of inclination in the pitch (nodding back and forward) degree of freedom. Spar-buoy type foundations solve the high stability requirement by providing a very deep draft configuration (up to 70-80 m) so that the center of buoyancy is located sufficiently above the center of gravity.

This makes it impossible to tow the floating turbine in the upright position from keyside/dockside to site since most harbours are much too shallow. The wind turbines thus have to be mounted on the spar buoy foundation at sea at deeper water using very large and expensive crane vessels in complex marine operations.

Semisubmersible floating foundations solve the high stability criteria through buoyancy columns which have sufficiently large water plane area that is located at a sufficiently large distance from the centre of the wind turbine tower. This requires that the construction elements separating them are sufficiently stiff and sturdy in order not to buckle or fail due to high stress. This requires a lot of material which causes challenges in handling and logistics and will often require dry dock fabrication which is limiting and drives high fabrication cost.

Tension-leg floating foundations are believed to be more material-efficient than other foundations, but they are by design inherently unstable without the tension-legs. They are therefore unstable during tow to the installation site and before the mooring tethers are attached and tensioned. The installation sequence is therefore more complex than for spar buoys and semisubmersible platforms.

A problem with VAWTs that are arranged with the blades rotating around a central turbine shaft to which an electrical generator is mounted is that if the generator experiences a failure, e.g. a short circuit, the central shaft and the struts that secure the blades to the central shaft will be subjected to a very high torque transient. The design will have to withstand this high stress transient. The shaft also has to be dimensioned for high torsional stiffness to attain a suitable natural period which often is chosen to be shorter than occurring environmental loads to avoid harmful structural excitation. This requires additional material which increases cost and makes handling during fabrication and installation more onerous. The same measures have to be taken to handle an ESD event, Emergency Shut Down event.

A problem is that the mooring system of a floating VAWT has to provide yawing stiffness i.e. it has to counteract the torque generated by the turbine if the electrical generator is placed on a geo-stationary (and moored) part in relation to the rotating turbine shaft. If the mooring lines are attached at or near the perimeter of the floating foundation, if the foundation for example is of the semisubmersible type, the torque resistance will inherently be high due to the large moment arm. But a semisubmersible platform requires a lot of construction material. If the turbine torque is resisted by a geo-stationary (and moored) central shaft, e.g. via a turret assembly, the mooring spread will have to be large due to the relatively small diameter of the turret chain table to which the mooring lines are attached.

WO2015116016 discloses a floating wind turbine solution with one stationary part and one rotating part. The stationary part is the lower part and consists of a circular monorail resting on a number of vertically oriented semisubmersible columns arranged around the perimeter of the monorail. The monorail is fixed to the floating

semisubmersible columns. The rotating, upper, part of the floating wind turbine solution is a vertical axis rotor which consists of a central shaft, spokes or struts connecting the central shaft to annular rings arranged at the perimeter of the rotor. The annular rings can be stacked in the vertical direction by interconnecting braces to increase the wind- absorbing area of the rotor. Sails are arranged between the annular rings at the perimeter to convert energy from the wind into rotational motion and kinetic energy in the rotor. The rotation of the rotor is resisted by water turbines mounted either on radially arranged arms or struts from the lower part of the central shaft of the rotor or by vertical consoles mounted on the lower part of the rotor rim or spokes alternatively struts or carrying arms between the lowest annular ring and the central shaft. An electrical cable is arranged to exit the central shaft through a rotating connection. The rotor rotates on the circular monorail by wheels with integrated electromagnetic breaks that can be turned on or off. One disadvantage of the prior art is that the floating monorail is of very large diameter and needs to be structurally very stiff to be able to carry the rotor with the sails resting on wheels running on the floating monorail. Either the monorail has to be very stiff and will therefore be heavy and expensive or, the number of vertical semisubmersible columns will have to be increased to reduce the free-span of the monorail resting on the semisubmersible columns to provide a smooth and planar running surface for the wheels of the rotating turbine rotor. The wheels will require a lot of maintenance due to being subjected to salt water spray from the ocean.

A further disadvantage of the floating wind turbine according to WO2015116016 is the large weight of the rotating part being the rim and the central shaft (hub) which is interconnected by spokes (struts). The driving moment from the windforce-ab sorbing sails is transferred to the central shaft via the spokes and further from the central shaft to the consoles on the lower part of the shaft, the consoles carrying water turbines for power production. The moment is thus transferred from the perimeter sails via stiff construction members to the central shaft and the spokes and its attachments will have to be dimensioned against transient fault conditions such as loss of grid load or generator short circuit. In both these cases, the resisting force of the water turbines will suddenly be released followed by torsional mode oscillation in the structure. A yet further disadvantage is that the electrical cable is connected to the central shaft via a rotating connection. The floating foundation composed of semisubmersible floating columns and a monorail is anchored by mooring lines attached at the perimeter. The friction in the rotating connection thus has to be resisted by torsional stiffness of the electrical cable.

Summary of the invention

An object of the invention is to provide an improved vertical axis wind turbine that overcomes the drawbacks associated with prior art floating wind turbines. In particular to provide a structure that is relatively lightweight and easy to transport and still capable of handling the high and varying forces and the harsh environment associated with offshore wind power. This is achieved by the device as defined in claim 1 and by the method as defined by claim 17

The floating vertical axis wind turbine, VAWT, according to the invention has a central column which at its lower end may be connected to a mooring system which is anchored with at least one anchor. The VAWT comprises at least one rigid peripheral power generating unit comprising a blade at least one peripheral buoyancy element and at least one water turbine assembly which is adapted to be below a water surface. The peripheral buoyancy element extends from the lower end of the blade and may also connect the water turbine assembly to the blade. The peripheral buoyancy element does at least partly supports the rigid peripheral power generating unit by buoyancy. A bearing assembly is rotatably attached to the central column and at least one first strut connects the rigid peripheral power generating unit with the bearing assembly. The rigid peripheral power generating unit of the VAWT is provided with at least one hydrofoil assembly. By introducing a hydrofoil assembly the rigid peripheral power generating unit may be supported, i.e. balancing downward forces being gravitation or forces from the wind during operation, by a static force provided by the buoyancy of the peripheral buoyancy element and a dynamic force of the hydrofoil assembly.

The hydrofoil assembly may comprise adjusting means adapted for adjusting the upward or downward force exerted by the hydrofoil assembly during operation. One way of adjusting the upward or downward force comprises adjusting the angle of attack of at least one hydrofoil wing by means of a mechanical rod connected between a surface-sensing floating body and the hydrofoil assembly. Alternatively the hydrofoil assembly comprises at least one hydrofoil wing, an electronic depth sensor and an electric or hydraulic actuator, wherein the angle of attack of the hydrofoil wing is adjustable by the electric or hydraulic actuator and the electronic depth sensor is in communication with the electric or hydraulic actuator. According to one aspect of the invention the VAWT, the first strut is provided with at least one hinge joint so that the rigid peripheral power generating unit is connected with the bearing assembly via the at least one hinge joint in so that the rigid peripheral power generating unit and the central column may have varying relative inclination. An alternative hinge arrangement comprises a first hinge joint provided between the bearing assembly of the central column and the first strut and a second hinge joint is provided between the rigid peripheral power generating unit and the first strut.

According to one aspect of the invention the VAWT comprises a lower strut provided below the first strut, the lower strut supporting the rigid peripheral power generating unit. The lower strut may be arranged to be at least partly below the sea surface during operation. A hydrofoil assembly may be provided on the lower strut.

According to one aspect of the invention the VAWT comprises a plurality of rigid peripheral power generating units and at least one connecting member and wherein the lower part of each blade is connected to the blade of another rigid peripheral power generating unit.

The method of operating a floating vertical axis wind turbine comprises matching the axial thrust force on the individual water turbine assembly, with the tangential wind force component on the individual blade of the rigid peripheral power generating unit so that the water turbine thrust force equals the tangential component of aerodynamic force on the blade in all angular positions of the floating vertical axis wind turbine's rotation. The axial thrust force is controlled by the breaking torque of a generator coupled to the water turbine.

According to one aspect the method further comprises the step of controlling the hydrofoil assembly to counteract the tipping moment exerted by the normal force component of the aerodynamic wind force on the individual blade by increasing or decreasing the lift force of the hydrofoil assembly.

One advantage afforded by the present invention, is that the blades are supported by buoyancy at the perimeter and can mounted to a central hub by hinge joints allowing a change of inclination between the blade and the central column. The structure will thus not have to be dimensioned for bending moments imposed by change in draft of structural elements located on the perimeter compared to structural elements located in the center of rotation of the rotor.

A further advantage is that the largest component is the rigid peripheral power generating unit which will be a very light structure in comparison with the rim or the monorail of the prior art and thus will be easy to move around in a fabrication yard or to deploy from keyside by commonly available mobile cranes. The weight of the strut and blade sections will be much less than for example the weight of the nacelle in conventional horizontal axis wind turbines of a comparable size which has a weight of typically between 350 and 450 tonnes. In the present invention, there is no transfer of wind force or moment from the perimeter to a central shaft. Thereby the risk of torsional oscillation will not require stiffness-dimensioning of the struts and the attachments of the struts to the rotating part of the central column or to the blades.

Yet a further advantage of the present invention is that the central semisubmersible column is anchored to the seabed by mooring lines attached to the geo-stationary part of the central column. The electrical cable is fixed to the geo-stationary part of the central column and a standard commercially available electrical swivel is arranged inside the column at the location of the interface between the rotating hub, to which the struts are attached, and the geo-stationary structural parts of the central column. In this way, the electrical swivel can also be arranged in an electrical room inside the column where it is protected from the weather and also being located above mean sea level.

Brief Description of the drawings

A more complete understanding of the above mentioned and other features and advantages of the present invention will be apparent from the following detailed description of preferred embodiments in conjunction with the appended drawings, wherein: Figures 1 a-f are schematic illustrations of the floating vertical axis wind turbine a according to the invention, in a slightly elevated side view (a) and a top view (b) of one embodiment and a side view (c) of another embodiment;

Figures 2 a-b are schematic illustrations of the details of the floating vertical axis wind turbine according to an embodiment of the invention;

Figure 3 is a schematic illustration of the floating vertical axis wind turbine according to the invention during operation;

Figures 4 a-j are schematic illustrations of the different embodiments of the strut and blade arrangement of the according to embodiments of the invention; and Figures 5 a-b are schematic illustrations of the different embodiments of the invention.

Detailed description

The floating vertical axis wind turbine 1, VAWT, according to the invention is schematically illustrated in figures 1 a-c, wherein la is a slightly elevated side view, lb is a view from the top and lc is an alternative embodiment. The invention is a floating VAWT 1. A 5 MW version of such a VAWT may be typically 110 m to 130 m in diameter and having a total height above mean sea level of about 120 m to 150 m, although the invention is not limited to these, or any other size ranges given herein. According to one embodiment of the invention, schematically illustrated in figures la-b, a floating central column 3 is, when the VAWT is installed and during use, moored to the seabed 12 with a support structure comprising a mooring system 10 connected to one or several anchors 11. For the 5 MW version the central column 3 typically has a diameter of 6 m to 8 m. Different types of mooring systems are known in the art. At least one vertical blade 5a is arranged at the perimeter of the turbine. The blade 5a is part of a rigid peripheral power generating unit 5b also comprising at least one peripheral buoyancy element 13 and at least one water turbine assembly 8. least one but alternatively two or more struts 4a, 4b are arranged to connect each rigid peripheral power generating unit 5b to the central column 3. The strut may be connected to the central buoyancy column by a hinge joint 6 allowing both the central column 3 and rigid peripheral power generating unit 5b to change their inclinations. The upper part of the central column 3 carrying the hinges 6 is allowed to rotate by a bearing assembly 7 forming an integral part of the central column. This could typically be a combination of a two-bearing assembly such that on a turret bearing assembly used on Floating

Production Storage and Offloading vessels, FPSOs, allowing the FPSO to weathervane. Such bearing assemblies are custom-designed and are manufactured on a regular basis for the oil and gas industry. An interior part or a lower part of the VAWT’s central column may thus be geo-stationary and connected to a mooring system.

The blade 5a the peripheral buoyancy element 13 and the water turbine assembly 8 are mechanically interconnected to form the rigid peripheral power generating unit 5b, although how the interconnect between the components may vary. The blade 5a of the VAWT 1 may extend downwards by the peripheral buoyancy element 13 so that it penetrates the water surface and float in the body of water and thereby provides buoyancy to the blade 5a and other parts. The aerodynamically active part of the blade is located primarily above the lower 4b of the two supporting struts if two struts are used. Below the lower strut 4b, the blade 5a is primarily optimized for structural strength to resist wave loads. The section intermittently being wetted by waves is apart from structural strength optimized for low hydrodynamic drag. The peripheral buoyancy element 13 is in figure la depicted as direct extension downwards from the blade 5a. The peripheral buoyancy element 13 may also be positioned a distance in radial direction from the blade 5a, for example on the lower strut 4b. A further alternative is that the peripheral buoyancy element 13 is connected both to the blade 5b and the lower strut 4b. The skilled person would have the knowledge to balance the design to give sufficient buoyancy and rigidity, among other design criteria. Schematically illustrated in figure 2a is an embodiment wherein the peripheral buoyancy element 13 comprises one part extending essentially downwards l3b from the blade 5a and one part forming a stay l3c extending inwards in a radial direction to join with the lower strut 4b, in this case the lower strut 4b is rigidly joint to the blade 5a. Alternatively no stay l3c is utilized, for example if the lower strut 4b is connected to the blade 5a with a hinge joint.

At or near the lower end of each blade a water turbine assembly 8 is arranged and is connected to e.g. an electrical generator 8b for production of electricity. The blade 5a, the peripheral buoyancy element 13, and the water turbine 8 forms a rigid peripheral power generating unit 5b. The water turbine assembly 8 may be connected to the peripheral buoyancy element 13, which would serve a double function as the buoyancy element and the connector between the blade 5a and the water turbine assembly 8. Alternatively the water turbine assembly 8 is connected to the blade 5a and/or the lower strut 4b with a separate support (not shown).

Depicted is a VAWT with three rigid peripheral power generating units 5b, this is advantageous for certain aspects, which will be further discussed below. However, the VAWT according to the invention may have one, two or more rigid peripheral power generating units 5b. The modular design gives an ability to adapt to power production need, economical factors and environmental factors such as tidal currents and waves.

A suitable water turbine assembly 8 is schematically illustrated in figure 2b and may be similar to a tidal energy high flow speed free-flow turbine and is commercially available from companies such as Schottel Hydro or Tocardo. The turbine may also be ducted (schrouded) to alter the axial flow pattern, in the figure 2 exemplified with duct 8c An electrical generator 8d can be directly connected to the water turbine assembly 8.

Typically the water turbine assembly 8 also comprises supports 8d. The arrangement solves the problem of dimensioning the struts and shaft of a VAWT for ESD events or generator failure due to that the driving aerodynamic force on the wind turbine blade is resisted directly at the lower end of each blade. There is thus no transfer of the tangential driving force on the wind turbine blade via the struts to a central shaft of the VAWT. This saves material. The rigid peripheral power generating unit 5b does for the same reason neither need to satisfy the same firm global structural stiffness

dimensioning criteria as a VAWT that transfers the driving force to a generator on a central shaft. The problem of structural excitation in torsional mode is thus heavily reduced. This also saves material. The water turbine assembly 8 will also run at high rotational speed which makes it possible to use a high-speed electrical generator which is much cheaper than a low-speed and high-torque generator mounted on a central shaft.

A hydrofoil assembly 9 may be provided on the lower end of peripheral buoyancy element 13 as schematically depicted in figures 2a-b and integrated with, or in connection with, the water turbine assembly 8. Alternatively the hydrofoil assembly is provided on the strut 4a or the lower strut 4b, if more than one strut is utilized, for example a distance from the perimeter of the VAWT 1. A further alternative is to provide the hydrofoil assembly 9 on the peripheral buoyancy element 13 above or below the water turbine assembly 8. The hydrofoil assembly 9 may have two wings as illustrated, but may also be realized as a single plane or even a more complex structure. The person skilled in hydromechanics will be able to select an appropriate design.

According to one embodiment of the invention the hydrofoil assembly 9 comprises adjusting means adapted for adjusting the upward or downward force exerted by the hydrofoil assembly 9 when in operation. The adjusting means may be mechanical, for example the hydrofoil assembly 9 comprising at least one hydrofoil wing which angle of attack is adjustable by means of a mechanical rod connected between a surface- sensing floating body and the hydrofoil assembly 9. Alternatively the hydrofoil assembly 9 an electronic depth sensor and an electric or hydraulic actuator, wherein the angle of attack of the hydrofoil wing is adjustable by means of an actuator for example an electric, hydraulic or pneumatic actuator. The angle of attack is based on signals communicated from the electronic depth sensor is in communication with the actuator. Further ways of adjusting the upward and downward force from the hydrofoil assembly 9, includes, but is not limited to, altering the size of the wings/plane, altering the profile of the wings/plane and folding/unfolding a further wing/plane.

According to embodiments of the invention the central column 3 is adapted to be mounted on a fixed support structure lOb which reaches to the seabed which is schematically illustrated in Figure lc. Figures ld-f schematically illustrate embodiments of the invention comprising a fixed support structure in the form of a monopile foundation lOc a jacket foundation lOd and a gravity base foundation lOe, respectively. Also other types of support structures may be used, for example a tripod foundation (not shown). Depending of the nature of the seabed, for example if solid rock seabed is not available, the fixed support structure may be complemented with a gravity base foundation as illustrated in Figure lf. The term floating vertical axis wind turbine, VAWT, does for the purpose of this application, include also these embodiments, wherein the central column may be supported by a fixed support structure and still major parts, the rigid peripheral power generating units, are supported by buoyancy. For the purpose of manufacturing and transportation, the central column may still be made as a floating member, or possible to equip with floatation devices, allowing the VAWT to be towed to its final position in the same manner as the embodiments described above.

Figure 3 illustrates the VAWT during operation. The hinge joint 6 in the connection between the rigid peripheral power generating unit 5b and the central column 3 allows each rigid peripheral power generating unit 5b to rotate around the hinge centre axis of rotation. This degree of freedom allows adjusting at which draft the water turbine assembly 8 operates. The draft is controlled by the adjustment of the hydrofoil wing.

The invention separately solves resisting the tipping moment and resisting the useful driving aerodynamic tangential force on the blade. The useful tangential aerodynamic force is resisted by the water turbine assembly 8 rotor disc at or near the lower end of the water-penetrating part of the blade. The rotation of the water turbine is resisted by the torque e.g. of an electrical generator. The normal force on the blade, which is the main contribution to the tipping moment on the turbine, is resisted by buoyancy when the turbine is not operating. When the turbine is in operation, the normal force is handled by the counterforce of an adjustable hydrofoil assembly at or near the water turbine assembly at the lower end of each blade.

When the turbine is in operation, the hydrofoil contributes with lifting force on the rigid peripheral power generating unit 5b if adjusted so as to have a positive angle of attack. Assuming a counter-clockwise revolution of the turbine, this is useful when the blade is in the downwind part of its revolution around the central column 3. In this area, the normal force on the blade is directed radially outward and will thus cause a tipping moment on the rigid peripheral power generating unit 5b around the hinge centre of rotation according to the right hand rule with the thumb pointing in the tangential direction of revolution. The hydrofoil contributes with a downward force on the rigid peripheral power generating unit 5b if adjusted to have a negative angle of attack. This is utilized when the blade rotates in the upwind part of its path around the central column 3. In this area, the normal force on the blade is directed radially inward. This causes a reversed and negative moment around the hinge centre of rotation according to the right hand rule convention. The hydrofoil has an integrated mechanism for adjusting the hydrofoil angle of attack that can be active or passive. A passive system can be arranged e.g. by a surface-sensing floating body connected by a stiff wing profiled strut fixed at an angle to the hydrofoil. When the rigid peripheral power generating unit 5b and thus the hydrofoil is further submerged under the action of the aerodynamic loads, the floating surface-sensing body will mechanically and directly adjust the angle of attack accordingly until the set design depth of water turbine operation is achieved. An active system can e.g. be based on the use of an electronic depth-sensor measuring the distance from the water turbine assembly to the seabed or to the water surface. The signal is used e.g. by electric motors to adjust the hydrofoil angle of attack until the desired depth of operation of the water turbine is achieved. In case of utilizing an active system, this constitutes a method of operating the VAWT according to the invention.

Various embodiments of the invention with regards to the strut(s) 4a, 4b and how the struts 4a, 4b are joined to the central column 3 and the blade 5a of the rigid peripheral power generating unit 5b are schematically illustrated in figures 4 a-j.

Figures 4 a-b schematically illustrate a VAWT with one, two or more struts connecting central column 3, to each blade. The central column 3 can rotate, the geo-stationary part of it being an interior part. Figure 4 c schematically illustrates that the central column 3, can be divided in an upper part that is rotating and a lower part that is geo-stationary and where the strut(s) are mounted on the upper rotating part of the central column 3.

Figure 4 d schematically illustrates that strut(s) can be mounted to the blade with only one hinge joint and arranging a second hydrofoil to control the inclination of the blade or be rigidly attached to the blade, as illustrated in figure 4 e. The strut(s) can be mounted to the central column 3 by hinge joint(s) 6, which can be placed at different positions on central column 3, as schematically illustrated in figure 4 e-g or be rigidly attached to the central column 3. If two struts are used, they can be attached to the central column 3 in the same place, i.e. to one hinge joint. Alternatively, as schematically illustrated in figure 4 h the struts are placed at different places on the central column 3, e.g. that an upper strut is mounted on the upper part of the central column 3, and the lower strut is mounted on the lower part of the central column 3. This configuration requires hinge joints for both the upper and lower strut 4a, 4b at both the central column 3 and at the rigid peripheral power generating unit 5b, and a parallel arrangement of the upper and lower strut 4a, 4b. Figure 4 schematically illustrates the VAWT 1 rotating around a mainly vertical axis of rotation wherein the central column can tilt during operation if strut(s) are attached to it by hinge joints 6.

Figures 4c, e, f, g and i depict the upper strut 4a and lower strut 4b as being joined at the hinge joint 6. Alternatively the two struts may be joined slightly before the hinge joint 6. Such minor design variations are apparent for the skilled person. In a similar manner, as depicted in for example figure 4c by the dashed strut, rigid joints may be provided with support struts or other means to increase the strength of the construction.

The blade(s) do not need to be straight even though this is preferred from a cost of fabrication perspective. From an operational efficiency perspective, the blades are preferably curvedto attain a rotor with a radius that increases with height above mean sea level. This is schematically illustrated in figure 4 j. Such a VAWT turbine rotor design enables equal and optimized local TSR, tip speed ratio, in relation to a power law profile. According to one embodiment, schematically illustrated in figure 5 a (perspective) and 5 b (top view), the blades of the VAWT are rigidly attached to the struts and the struts are rigidly attached to the central column. Further, the blades are interconnected by at least one connecting member such as a beam, a steel wire, line or a rod to the adjacent blade. A cable for transmission of electrical power from the generator can be arranged inside the water-penetrating part of the blade and through the lower or upper of the blade- supporting struts. An electrical swivel, also referred to as a marine slip ring, is arranged in the central column to transfer power from the outer, rotating, side to the geo- stationary, inner, side of the bearing assembly. Such electrical swivels are available on the market for example from Moog Focal which offer medium voltage marine renewable swivels up to a voltage of 36 kV. SBM Offshore is another large provider of such electrical swivels. Such swivels are today in use around the world in the oil and gas industry for example in turrets on FPSO vessels. A turret is a large bearing and swivel arrangement. Such swivels are used for transferring for example power, sensor signals and fluids between a rotating and a stationary side. In on-shore applications slip rings are common on e.g. gantry cranes in synchronous electrical generators for turbo or hydropower applications.

An electrical step-up transformer can be arranged inside the column, either between the electrical swivel and the connection to the export cable if a low-voltage swivel is used or between the generators and electrical swivel if a medium-voltage swivel is used.

A dynamic electrical export cable is arranged inside the column, is made to exit at the lower end of the column and is led in a suitable manner to the seabed where is can transcend to or be joined to a static export cable, whichever is the most practical and economical. Such dynamic cables are available on the market and are provided e.g. by companies such as JDR, Prysmian or NKT Cables.

A suitable mooring system is arranged to provide station-keeping for the floating VAWT. The mooring system is chosen according to what is most suitable for the depth, bathymetry and soil conditions at the site of installation. Since the useful aerodynamic force is directly used for production of electricity at the lower end of each blade, the general problem of floating VAWTs that the mooring system need to resist the turbine torque is solved. The mooring system of this invention only needs to provide station keeping.

The invention solves the stability problem requirement through that the central column has positive buoyancy and supports its own weight and the weight of the rigid peripheral power generating unit 5b resting its inner end on the hinge on the rotating upper part of the column. The rigid peripheral power generating unit 5b is self- supporting by its own own buoyancy. The floating VAWT can thereby be floated out directly from a fabrication yard on shore and through shallow harbours as opposed to spar buoy type foundations which have very large draft. The hinge arrangement and the integral buoyancy support of the surface-penetrating blade sections solve the problem of global stiffness dimensioning without having to use large, heavy and material-intensive beam sections connecting the buoyancy columns of e.g. a semisubmersible type floating foundation.

The invention also solves the problem of large component replacement. Since each blade is self- supported by buoyancy at the perimeter and the bearing assembly is resting on the central column, a crane vessel can, thanks to the arrangement with a hinge at the central attachment, lift the blade section out of the water to exchange or perform service or maintenance on the water turbine, hydrofoil assembly or the generator. The blade can be lifted and the lower end can then conveniently be set down on e.g. a floating barge for safe and easy access while performing maintenance at sea. The central column can be fabricated e.g. in mild steel as a conventional

semisubmersible buoyancy column. The struts, blades, water surface-penetrating hydrodynamical struts as well as the hydrofoil wings and the generator enclosure can be fabricated in composite plastic, e.g. in glass-fibre reinforced plastic with reinforcements in carbon fibre or e.g. aramid (Kevlar). The water-penetrating peripheral buoyancy element, hydrofoil wing, as well as the water turbine assembly are due to their submersion in water not as weight sensitive as the struts and blades so they can be fabricated e.g. in mild steel and corrosion protected by paint. Submersed parts could also be fabricated in stainless steel or thermoplastic material which would be beneficial from a corrosion protection perspective. The method according to the invention of operating a VAWT comprises actively controlling the individual water turbine assemblies and optionally individual hydrofoil assemblies.

The method of control of the individual water turbine assemblies is based on matching the axial thrust force on the individual water turbine assembly at the lower end of the rigid peripheral power generating unit with the tangential wind force component on the individual VAWT rotor blade so that in all angular positions of the VAWT’s rotation. A control signal to indicate this matching can e.g. be the measured forward inclination (in the tangential direction) of the VAWT rotor blade compared to the vertical. The higher the VAWT thrust force and tangential aerodynamic force component, the higher the inclination of the VAWT rotor blade.

The axial thrust force on the water turbine assembly is controlled typically via an electrical generator coupled to the shaft of the water turbine. As the applied breaking torque of the generator is increased e.g. by means of an active (controlled) rectifier, if there is enough available power in the water flow surrounding the water turbine, the mechanical power on the water turbine’s shaft will increase equivalently to the axial thrust force times the speed of travel of the water turbine assembly. Assuming the VAWT rotates in the counterclockwise direction and the free stream wind direction is 0 degrees, we here refer to the wind direction as the upwind sector of the VAWT’ s rotation. In this sector, the individual VAWT rotor blade generates the highest aerodynamic tangential driving force. The water turbine assembly’s axial thrust is gradually increased to reach its maximum approximately when the VAWT rotor blade moves perpendicularly to the free stream wind velocity, i.e. is at 0 degree azimuthal angle. When the VAWT blade approaches the position at which it moves in the free stream wind direction (90 degree), the relative wind flow over the wind turbine blade does not generate any tangential driving force. In this sector, the water turbine assembly’s thrust is therefore gradually adjusted to reach zero. As the VAWT rotor blade enters the downwind sector, the axial thrust force on the water turbine assembly is gradually increased to match the increasing tangential wind force component on the VAWT rotor blade to reach a local maxima when the VAWT rotor blade moves perpendicularly to the free stream wind direction (180 degrees). As the individual VAWT rotor blade approaches the position at which is moves in a pure headwind (270 degrees), the blade only exerts drag and the water turbine assembly’s axial thrust force is adjusted accordingly to reach zero. Due to the inertia originating from the kinetic energy of each individual rigid peripheral power generating unit as it rotates around the central column, the water turbine assembly’s axial thrust may have to be adjusted to a non-zero value in the headwind and tailwind positions (90 and 270 degrees

respectively) despite the fact that the aerodynamic tangential driving force component is zero in these positions.

Another aspect of the method of controlling the floating VAWT is associated with the performance of the hydrofoil assembly which for example can be located at the lower end of the peripheral buoyancy element. In conjunction with a positive tangential wind force component on the VAWT rotor blade (for example in the upwind sector, 0 degrees, or the downwind sector, 180 degrees, of its rotation around the central column) the hydrofoil assembly is pitched to counteract the tipping moment exerted by the normal force component of the aerodynamic wind force on the VAWT rotor blade. In the upwind sector, this normal force component is directed radially inwards. The hydrofoil assembly will then be adjusted so as to decrease its lift force (its force in the positive vertical direction) e.g. by decreasing the angle of attack. The angle of attack can even be adjusted to reach negative lift force if needed. In the downwind sector, the normal force component is directed radially outwards. The hydrofoil assembly will then be adjusted so as to increase its lift force in the positive vertical direction e.g. by increasing the angle of attack.

The method of controlling the uplift force on the hydrofoil assembly is also a means of achieving storage of potential energy during the upwind and downwind sectors of an individual peripheral power generating unit’s rotation. The tangential force on the VAWT rotor blade can be extracted in two ways. The first way, which is perhaps the most intuitive way, is by using the water turbine assembly to exert an axial thrust force. This directly counteracts the driving aerodynamic tangential force component. The second way is to elevate the whole peripheral power generating using the hydrofoil assembly’s vertical lift force component to increase the height above mean sea level of the centre of mass of the peripheral power generating unit. This increase in potential energy of the rigid peripheral power generating unit can be extracted and converted back to kinetic energy (speed) in the headwind sector following the downwind sector of the peripheral power generating unit’s rotation, i.e. when the aerodynamic tangential force is zero or very low. The same principle applies to the upwind sector followed by the tailwind sector of the rotation. The axial thrust force exerted by the water turbine assembly can, using the second means of control based on a varying elevation of the rigid peripheral power generating unit above the mean sea level, be more even seen over one revolution of the VAWT than if the first means of control is used.

Terminology and acronyms

Column Cylindrically or rectangularly shaped vertical construction element

providing buoyancy

Draft Alt. Draught , submerged distance of structure below sea level

ESD Emergency Shut-Down, a term defined in the IEC standard for design of wind turbines.

FAWT Floating Axis Wind Turbine

FPSO Floating Production Storage and Offloading vessel (is ship-shaped and weathervaning).

HAWT Horisontal Axis Wind Turbine Nacelle The housing with machinery (main shaft, gearbox, generator) at top of a

HAWT tower

RNA Rotor Nacelle Assembly

Rotor The blades and the central hub of a HAWT

Semi sub Semisubmersible, a class of partly submerged floating foundations

utilizing distributed buoyancy columns with connection members between them

Spar buoy A cylinder-shaped, elongated deep-draft floating foundation with small diameter

Strut Blade-supporting construction element for a Vertical Axis Wind Turbine Tether Typically a tensioned mooring line

TLP Tension-Leg Platform, a class of floating foundations stabilized through a set of vertical tensioned mooring lines to e.g. a gravity foundation type foundation on the seabed

TSR Tip Speed Ratio, ratio of blade tangential speed divided by the free wind speed.

Turret Rotating bearing assembly with rotating transfer of electrical power and/or fluids

VAWT Vertical Axis Wind Turbine

Elements in figures:

1) Floating Vertical Axis Wind Turbine

2) Sea level

3) Central column

4) Struts

a. Strut

b. Lower strut

5) Peripheral parts

a. Blade

b. Rigid peripheral power generating unit

6) Hinge joint

7) Bearing assembly

8) Water turbine assembly

a. Water turbine

b. Electrical generator

c. Ducting

d. Supports

9) Hydrofoil assembly

10) Mooring lines

11) Anchor

12) Seabed ) Peripheral buoyancy element) Connecting member