Partial pitch wind turbine with floating foundation

Abstract

The present invention relates a wind turbine comprising a wind turbine tower with a nacelle provided on the top to which a rotor hub with one or more wind turbine blades is rotatably mounted so that they form a rotor plane. A floating foundation having a upper section is mounted to the bottom of the wind turbine tower, wherein the foundation has a buoyant body configured to be installed at an offshore position having a water depth of about 40 m or more. The wind turbine blade comprises an inner blade section coupled to an outer blade section by a pitch junction in which a pitch mechanism is coupled to a pitch control system configured to regulate the pitch of the outer blade section relative to the inner blade section at wind speeds above a first wind speed. This allows the pitching to be used to counteract the tilting of the wind turbine caused by the different thrusts acting on the structure. This allows for a more linear control of the bending moment induced in the structure, since the blade sections provides a more constant thrust acting on the rotor hub which in turn allows the large negative damping loads and stresses introduced in the wind turbine to be eliminated.

Claims

1. A wind turbine comprising: a wind turbine tower having a top and a bottom; a nacelle provided on top of the wind turbine tower; a rotor hub rotatably mounted to the nacelle; one or more wind turbine blades mounted to the rotor hub, wherein the wind turbine blades form a rotor plane, wherein the rotor plane is facing upwind relative to a wind direction; a floating foundation having a upper section configured to be mounted to the bottom of the wind turbine tower, wherein the foundation has a buoyant body and is configured to be installed at an offshore position having a predetermined water depth, wherein each of the one or more wind turbine blades comprises at least an inner blade section with a first aerodynamic profile and an outer blade section with a second aerodynamic profile, wherein the inner blade section has a stall-controlled aerodynamic profile and the outer blade section has a pitch-controlled aerodynamic profile, wherein the inner blade section is mounted to the rotor hub and the outer blade section is coupled to the inner blade section by at least one pitch junction, the wind turbine having a pitch control system which is configured to pitch the outer blade section relative to the inner blade section at wind speeds above a first wind speed, and wherein the wind speed acting on the inner blade sections defines a first thrust value, the wind speed acting on the outer blade sections defines a second thrust value, wherein the wind speed acting on the inner and outer blade sections defines a resultant thrust value acting on the rotor hub, and wherein the pitch control system is adapted and configured to use the first thrust value as a reference thrust value as a parameter, and wherein the pitch control system is adapted to control the pitching of the outer blade sections and the pitch control system is adapted to regulate the second thrust value relative to the first thrust value and to control the pitching of the outer blade sections so that the resultant thrust value is maintained at a substantially constant value.

2. The wind turbine according to claim 1, wherein the inner blade section extends from a root end of the wind turbine blade to a position of the pitch junction and the outer blade section extends from a tip end of the wind turbine blade to the position of the pitch junction, and wherein the pitch junction is positioned at a relative length between 0.18 and 0.88 relative to the root end.

3. The wind turbine according to claim 1, wherein the aerodynamic profile of the inner blade section defines a first surface area which defines both a suction side and a pressure side of that inner blade section, where the aerodynamic profile of the outer blade section defines a second surface area which defines both a suction side and a pressure side of that outer blade section, and wherein a ratio between the first surface area relative to the second surface area is between 0.45 and 1.65.

4. The wind turbine according to claim 1, wherein the pitch control system is configured to regulate a pitch angle of the outer blade section according to a predetermined power output.

5. The wind turbine according to claim 1, wherein the pitch control system is electrically coupled to at least one measuring unit which is configured to measure the tilting of the wind turbine tower relative to its vertical position, and wherein the pitch control system is configured to regulate the pitch of the outer blade sections based on the measured tilt.

6. The wind turbine according to claim 1, wherein the first wind speed has a mean value of 10 to 14 m/s or more.

7. A method of controlling a wind turbine comprising providing one or more wind turbine blades mounted to a rotor hub, which rotor hub is rotatably mounted to a nacelle provided on top of a wind turbine tower, which wind turbine tower is mounted to a floating foundation having a buoyant body, wherein the wind turbine blades have inner blade sections and outer sections, wherein the outer blade sections are relatively unpitched relative to the inner blade sections at wind speeds up to W.sub.1 and wherein the outer blade sections increasingly are pitched relative to the inner sections at mean wind speeds above a wind speed W.sub.1 to a higher wind speed W.sub.2, the method further comprising regulating with a pitch control system the pitch of the outer blade sections of the wind turbine blades relative to the inner blade sections of the wind turbine blades, defining with the wind speed acting on the inner blade sections a first thrust value, and defining with the wind speed acting on the outer blade sections a second thrust value, which is used as a reference thrust value and a reference parameter for pitching the outer blade sections, defining with the wind speed acting on the inner and outer blade sections a resultant thrust value acting on the rotor hub, and wherein the pitch control system is adapted to regulate the second thrust value relative to the first thrust value by pitching the outer blade sections relative to the inner blade sections so that the resultant thrust value is maintained at a substantially constant value.

8. The method according to claim 7, wherein the pitching is further regulated relative to a predetermined power output.

9. The method according to claim 7, wherein at least one measuring unit arranged on the structure measures the tilting of the wind turbine tower, and wherein the pitching of the outer blade sections are further regulated based on the measured tilt.

10. The wind turbine according to claim 1, wherein the pitch junction is positioned at a relative length between 0.27 and 0.77 relative to the root end.

11. The wind turbine according to claim 1, wherein the ratio between a surface area of the outer blade parts relative to a surface area of the inner blade parts is between 0.5 and 1.

Description

DESCRIPTION OF THE DRAWING

(1) The invention is described by example only and with reference to the drawings, wherein:

(2) FIG. 1 shows an exemplary embodiment of a wind turbine installed on a floating foundation;

(3) FIG. 2 shows the wind turbine shown in FIG. 1 with the outer blade section pitched relative to the inner blade section; and

(4) FIG. 3 shows a graph of the thrust acting on the rotor hub relative to the wind speed for a wind turbine according to the invention compared to a traditional pitch-regulated wind turbine.

(5) In the following text, the figures will be described one by one and the different parts and positions seen in the figures will be numbered with the same numbers in the different figures. Not all parts and positions indicated in a specific figure will necessarily be discussed together with that figure.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

(6) The term “tilting” is defined as the rotational movement of the wind turbine relative to a rotation point in at least one predetermined direction. The tilting movement may refer to the rolling of the wind turbine around the x-axis (in parallel with the wind direction). The tilting movement may alternatively or additionally refer to the yawing of the wind turbine around the z-axis (in parallel with the longitudinal direction of the wind turbine tower). The tilting movement may alternatively or additionally refer to the pitching of the wind turbine around the y-axis (perpendicular to the wind direction and the longitudinal direction of the wind turbine tower).

(7) FIG. 1 shows an exemplary embodiment of a wind turbine 1 mounted on a floating foundation 2. The wind turbine 1 may comprise a wind turbine tower 3 having a bottom facing the foundation 2 and a top facing away from the foundation. One or more tower sections (not shown) mounted on top each other may form the wind turbine tower 3. A nacelle 4 may be provided on the top of wind turbine tower 3. A rotor hub 5 may be coupled to the nacelle 4, e.g. via a rotor shaft (not shown) which at one end is mounted to the rotor hub 5 and at the other end is rotatably mounted to a generator (not shown) arranged inside the nacelle 4. One or more wind turbine blades 6 may be mounted to the rotor hub 5 wherein the wind turbine blades 6 form a rotor plane. At least two or three wind turbine blades 6 are preferably mounted to the rotor hub 5, as shown in FIG. 1.

(8) At least one yawing mechanism comprising at least a yawing bearing (not shown) may be arranged at the top of the wind turbine tower 3. The nacelle 4 may be coupled to the yawing mechanism which may be configured to actively or passively yaw at least the nacelle 4 relative to the wind turbine tower 3 so that the rotor plane is moved into the wind. This allows the rotor plane to be positioned substantially perpendicularly to the main wind direction.

(9) The floating foundation 2 may comprise a buoyant body 7 configured to be partly or fully submerged below the water surface 8. The body 7 may comprise at least one buoyant chamber 9 in the form of a ballast chamber configured to be at least partly filled with a ballast material. The ballast material may be a liquid in the form of water, e.g. seawater, or a solid in the form of rocks, sand/gravel, concrete, metal or another suitable material. The body 7 may be shaped as an elongated and/or cylindrical body where the buoyant chamber 9 may be positioned in the lower section of the body 7. The upper section of the body 7 may be hollow and may be filled with a gaseous material, such as air, helium or another suitable gaseous material. The body 7 may form a closed body 7 wherein the gaseous material and/or the ballast material is contained inside the body 7. The body 7 may have a first end facing the water level 8 and a second end facing the seabed 10. An upper section 11 may be arranged at the first end and may be configured to be mounted to the bottom of the wind turbine tower 3. The body 7 may be made of iron, steel or another suitable material.

(10) Means for adjusting the amount of ballast and/or gaseous material, e.g. a pump system, may be arranged on the floating foundation 2. This allows the buoyant body 7 of the floating foundation 2 to be positioned at a predetermined depth below the water level 8, e.g. so that the upper section 11 is partly submerged below the water level 8, as shown in FIG. 1. Alternatively, the upper section 11 may be positioned above the water level 8 for service or maintenance purposes or positioned below the water level 8 for added stability of the wind turbine 1, e.g. during extreme loads in form of storms, typhoons, or the like.

(11) The floating foundation 2 may be configured to be installed at an offshore position having a water depth of about 40 m or more. The floating foundation 2 may be positioned about 5 m or more below the water level 8 measured from the centre of gravity or middle of the body 7. The floating foundation 2 may also be positioned so that the first end of the body 7 is positioned about 5 m or less from the water level 8. One or more anchoring means 12 in the form of two, three or more anchoring cables or other suitable anchoring mechanisms or systems may be configured to hold the floating foundation 2 and thus the wind turbine 1 in its position. The anchoring cables may be coupled to the floating foundation 2 and one or more anchoring supports (not shown) placed or secured to the seabed 10.

(12) The wind turbine 1 may be configured as a partial pitch wind turbine, as shown in FIG. 1, where at least one of the wind turbine blades 6 comprises at least two blade sections 13, 14. The wind turbine blade 6 may comprise an inner blade section 13 coupled to an outer blade section 14 by a pitch junction 15. The inner blade section 13 may comprise a blade root 16 which faces the rotor hub 5 and is configured to be mounted to the rotor hub 5 by fastening means, such as bolts/nuts or the like. The outer blade section 14 may comprise a tip end 17 which faces away from the rotor hub 5.

(13) The pitch junction 15 may be configured to pitch the outer blade section 14 relative to the inner blade section 13. A pitch mechanism (not shown) in the form of a mechanic, hydraulic or electrical unit may be arranged at the pitch junction 15. A pitch control system (not shown) may be coupled to the pitch mechanism and may be configured to regulate the pitch of the outer blade section 14. This allows the outer blade section 14 to be pitched into or out of the wind. FIG. 1 shows the blade sections 13, 14 in an unpitched position where the cross-sectional profiles of the two blade sections 13, 14 at the pitch junction 15 are aligned with each other. The pitch mechanism may be configured to pitch the outer blade section 14 between 0 and 90° relative to the inner blade section 13.

(14) The wind turbine blade 6 can be made of fibre reinforced plastics or composites forming a laminate, wherein the fibres can be made of glass, carbon, or organic fibres. The laminate can be infused by using a resin, e.g. epoxy, supplied by an external system, e.g. a vacuum infusion or injection system.

(15) FIG. 2 shows the wind turbine 1 with the outer blade section 14 pitched relative to the inner blade section 13. The pitch control system may be configured to regulate the pitch of the outer blade section 14 relative to the inner blade section 13 according to a selected power output profile or a constant thrust value. The two blade sections 13, 14 provides a better control of the bending moment induced in the structure which in turn reduces the tilting movement experienced by the wind turbine 1 and/or the floating foundation 2. The pitch control system may be activated at wind speeds above a predetermined mean wind speed of 10 to 14 m/s, e.g. 12 m/s or more. This allows for a more linear control of the bending moment induced in the structure due to the size and weight of the nacelle 4 and the wind turbine blades 6. The outer blade section 14 may be pitched so that the wind turbine 1 tilts no more than ±2° relative to its vertical position.

(16) The inner blade section 13 may have a stall-controlled aerodynamic profile defined by a predetermined set of specifications providing a first thrust value. The outer blade section 14 may have a pitch-controlled aerodynamic profile defined by a predetermined set of specifications providing a second thrust value. The first and second thrust values of the two blade sections 13, 14 may define a resultant thrust value acting on the rotor hub 5. The pitch control system may be configured to use the first thrust value as a reference parameter when regulating the second thrust value by pitching the outer blade section 14.

(17) The inner blade section 13 may extend from the root end 16 to a position of the pitch junction 15 and define an inner swept area. The outer blade section 14 may extend from the tip end 17 to the position of the pitch junction 15 and may define an outer swept area. The inner and outer swept areas define a total area swept by the wind turbine blades 6. The relative length of the two blade sections 13, 14 may define the ratio between the outer swept area relative to the inner swept area.

(18) The wind turbine blade 6 may have a length of 35 m or more from root end 16 to the tip end 17, corresponding to a relative length of 1. The pitch junction 15 may be positioned at a relative length between 0.20 and 0.80±10% relative to the root end 17. The inner blade section 13 may have a relative length between 0.20 and 0.80±10%. The outer blade section 14 may have a relative length between 0.20 and 0.80±10%.

(19) The aerodynamic profile of the inner blade section 13 may form the suction and pressure sides of the blade section which define a first surface area. The aerodynamic profile of the outer blade section 14 may form the suction and pressure sides of the blade section which define a second surface area. The two surface areas may define the lift and drag forces of the two blade sections 13, 14. The thickness distribution and/or the chord distribution of each blade section 13, 14 may define the ratio between the first surface area relative to the second surface area, which can be between 0.5 and 1.5±10%.

(20) At least one measuring unit (not shown) in the form of an accelerometer, a GPS unit, an angular sensor or another suitable measuring unit may be electrically coupled to the pitch control system. The measuring unit may be configured to measure the tilting of the wind turbine 1 relative to its vertical position. The pitch control system may be configured to regulate the pitch of the outer blade section 14 based on the measured tilt. Alternatively or additionally at least one other measuring unit in the form of an anemometer, a meteorological mast, a tachometer or another suitable measuring unit may be electrically coupled to the pitch control system. The measuring unit may be configured to measure the wind speed and/or the rotor speed of the drive train in wind turbine 1. The pitch control system may be configured to regulate the pitch of the outer blade section 14 based on the tilt of the wind turbine 1 and/or the wind speed hitting the rotor plane so that the wind turbine 1 compensates for not only for the thrust induced by the wind speed but also the thrusts induced by the marine currents and waves.

(21) FIG. 3 shows a graph of the thrust acting on the rotor hub 5 relative to the wind speed for the wind turbine 1 according to the invention compared to a traditional pitch-regulated wind turbine.

(22) At wind speeds below the first wind speed W.sub.1, the outer blade section 14 may be positioned in its unpitched position, e.g. pitch angle of zero. The thrust 18, 19 acting on the rotor hub 5 may increase as the wind speed hitting the rotor plane and the wind turbine 1 increases. As the wind speed increases above the first wind speed W.sub.1 the pitch control system activates the pitch mechanism which then regulates the pitch angle of the outer blade section 14. The first wind speed W.sub.1 may define the wind speed at which the wind turbine 1 reaches its rated power output. If the wind speed increases above a second wind speed W.sub.2 the wind turbine activates its extreme load procedure and uncouples the wind turbine 1 from the power grid. The second wind speed W.sub.2 may define the cut-off wind speed of the wind turbine 1. The first wind speed W.sub.1 may be between 10 to 14 m/s, e.g. 12 m/s or more. The second wind speed may be 25 m/s or more.

(23) The pitch control system regulates of the pitch of the outer blade section 14 relative to the inner blade section 13 so that the wind turbine blade 6 maintains a substantially constant resultant trust value. The first curve 18 defines the thrust value expired by a traditional pitch-regulated wind turbine when pitching the entire wind turbine blade. The second curve 19 defines the resultant thrust value experienced by the wind turbine 1 according to the invention. A first point 20 may define a position where the wind turbine 1 is in a tilted position while a second point 21 may define a position where the wind turbine 1 is in an untilted position, as illustrated in FIG. 3. A third point 22 may define an intermediate position where the wind turbine 1 is pitching into the wind so that the thrust value 18, 19 moves/changes from the second point 21 towards the first point 20.

(24) As illustrated in FIG. 3, the pitching of the outer blade section 14 relative to the inner blade section 13 provides a more linear control of the bending moment induced in the structure due to the size and weight of the nacelle 4 and the wind turbine blades 6. The pitch control system may use the first thrust value of the inner blade section 13 as a reference parameter when regulating the pitch of the thrust value of the outer blade section 14. This allows the pitch control system to maintain the resultant thrust value acting on the rotor hub 5 at a substantially constant value which reduces the tilting movement of the wind turbine 1 and thus reduces the bending moment induced in the structure. As illustrated in FIG. 3, the regulation 18 of the pitch in the traditional pitch-regulated wind turbine causes the thrust value to change significantly which in turn produce large negative dampening loads and stresses in the wind turbine. These loads and stresses may be at least significantly reduced by the use of the partial pitch wind turbine 1.

(25) The pitch control system may regulate the pitch of the outer blade section 14 based on the tilt of the wind turbine 1 and/or the wind speed hitting the rotor plane which is measured by one or more measuring units arranged on the structure, e.g. the wind turbine 1.

(26) The resultant thrust value may be regulated to within ±10% of the constant value. The constant value may be determined at the rated wind speed, e.g. W.sub.1, of the wind turbine 1 and may be between 80 and 120% of the resultant thrust value at that wind speed. The pitching may be performed by the pitch control system so that the wind turbine 1 tilts no more than ±2° relative to its vertical position.