Abstract
A method of reducing the load of a rotor blade of a wind turbine during installation of the wind turbine, whereby the rotor blade includes an aerodynamic device such as a vortex generator or a noise reducer is provided. The method includes the steps of attaching a cover on the rotor blade for covering at least a part of the aerodynamic device before lifting the rotor blade to the top of the tower of the wind turbine, and detaching the cover subsequently. An arrangement including a rotor blade of a wind turbine and such a cover, is also provided.
Claims
1. A method of reducing a load of a rotor blade of a wind turbine during installation of the wind turbine, wherein the rotor blade comprises an aerodynamic device arranged at a surface of the rotor blade, of the method comprising: attaching a cover) on the rotor blade such that at least a part of the aerodynamic device is covered by the cover; lifting the rotor blade to a top of a tower of the wind turbine ; and detaching the cover from the rotor blade.
2. The method according to claim 1further comprising mounting the rotor blade to a hub of the wind turbine after lifting the rotor blade to the top of the tower of the wind turbine.
3. The method according to claim 1, wherein attaching the cover to the rotor blade is carried out before transporting the rotor blade to an installation site of the wind turbine.
4. The method according to claim 1, wherein the rotor blade is stored at a temporary storage siteafter attaching the cover to the rotor blade and before lifting the rotor blade to the top of the tower.
5. The method according to claim 1, wherein detaching the cover from the rotor blade comprises pulling the cover away by means of a string which is attached to the cover.
6. The method according to claim 1, wherein detaching the cover from the rotor blade comprises rotating the rotor with a rotor speed being high enough such that the cover flies off due to the centrifugal force acting on the cover during rotation of the rotor blade.
7. An arrangement comprising: a rotor blade of a wind turbine and a cover, wherein the rotor blade comprises an aerodynamic device arranged at a surface of the rotor blade, the cover covering at least a part of the aerodynamic device such that airflow which flows across the aerodynamic device is deflected at the cover, wherein the cover is configured and arranged such that a maximum lift coefficient of the arrangement is reduced compared to a maximum lift coefficient of the rotor blade during installation of the wind turbine, wherein the covens prepared to be detached from the rotor blade after installation of the wind turbine.
8. The arrangement according to claim 7, wherein the aerodynamic device is a vortex generator.
9. The arrangement according to claim 7, wherein the aerodynamic device is a noise reducer which is arranged at a trailing edge section of the rotor blade.
10. The arrangement according to claim 7, wherein the aerodynamic device is a flap for modifying the lift coefficient of the rotor blade.
11. The arrangement according to claim 7, wherein at least a part of the cover comprises a convexly shaped outer surface.
12. The arrangement according to claim 7, wherein a cross-section of the cover has a substantially elliptical shape in a mounted state to the rotor blade.
13. The arrangement according to claim 7, wherein a cross-section of the cover is shaped as a ramp away from the surface of the rotor blade with a substantially constant slope in an upstream section of the cover.
14. The arrangement according to claim 7, wherein the cover is made of a water soluble material.
15. The arrangement according to claim 7, wherein the cover is made of a foam material, and the aerodynamic device is submerged into a foam material of the cover.
16. The method of claim 1, wherein the aerodynamic device is a vortex generator, a noice reducer, or a flap.
Description
BRIEF DESCRIPTION
[0033] Some of the embodiments will be described in detail, with reference to the following figures, wherein like designations denote like members, wherein
[0034] FIG. 1 shows a wind turbine;
[0035] FIG. 2 shows a rotor blade of a wind turbine in a top view;
[0036] FIG. 3 shows an airfoil of a rotor blade with a vortex generator;
[0037] FIG. 4 shows the same airfoil as in FIG. 3, covered by a cover for deflecting the air-flow flowing across the vortex generator;
[0038] FIG. 5 graphically shows the impact of the cover on the lift coefficient of the airfoil;
[0039] FIG. 6 shows an embodiment of covering a vortex generator mounted to the surface of a rotor blade;
[0040] FIG. 7 shows an embodiment of covering a vortex generator mounted to the surface of a rotor blade;
[0041] FIG. 8 shows an embodiment of covering a vortex generator mounted to the surface of a rotor blade;
[0042] FIG. 9 shows a rotor blade with a noise reducer which is arranged at the trailing edge section of the rotor blade;
[0043] FIG. 10 shows a first embodiment of covering the noise reducer of FIG. 9;
[0044] FIG. 11 shows a second embodiment of covering the noise reducer of FIG. 9; and
[0045] FIG. 12 graphically shows the impact of the different covers on the lift coefficient of the rotor blade.
[0046] The illustration in the drawings is schematic. Note that similar reference signs refer to similar or identical parts of the invention in different drawings.
DETAILED DESCRIPTION
[0047] FIG. 1 shows a wind turbine 10. The wind turbine 10 comprises a tower 11 by which the wind turbine 10 is installed on the ground or at the sea. The tower 11 comprises a bottom 111 and a top 112. At the top 112 of the tower 11 a nacelle 12 is mounted rotatably. The nacelle 12 can be rotated about a substantially vertical axis, the so called yaw axis. The nacelle accommodates a generator for transforming rotational energy being provided by the rotor of the wind turbine into electrical energy. The wind turbine 10 furthermore comprises a rotor wherein the rotor axis 14 is slightly inclined from a horizontal orientation. One part of the rotor is the hub 13 of the wind turbine 10. The hub 13 is rotatably connected with regard to the nacelle 12. A plurality of rotor blades 20 are mounted to the hub 13. Each rotor blade 20 comprises a pitch axis 15 around which the rotor blade 20 can be rotated or at least pivoted with regard to the hub 13. During rotation of the rotor blades, centrifugal forces are generated and are directed into a direction 16 away from the hub 13 and along the pitch axis 15.
[0048] FIG. 2 shows a top view on a rotor blade 20. The rotor blade 20 comprises a root section 21 with a root 211 and a tip section 22 with a tip 221. Furthermore, the rotor blade 20 comprises a trailing edge section 23 with a trailing edge 231 and a leading edge section 24 with a leading edge 241. Furthermore, a virtual line connecting the root section 21 with a tip section 22 is referred to as the span 25 of the rotor blade 20. A set of further virtual lines are referred to as the chords 26 of the rotor blade 20. The span 25 and the chords 26 define the spanwise direction 251 and the chordwise directions 261 of the rotor blade 20, respectively. The length of the chord 26 is referred to as the chord length. The shoulder 27 of the rotor blade 20 is defined to be the position wherein the chord length is maximum.
[0049] FIG. 3 shows an airfoil of a rotor blade 20, i.e. it shows a cross-sectional view perpendicular to the span of the rotor blade. The leading edge 241 and the trailing edge 231 can be seen. The straight line which is connecting the trailing edge 231 and the leading edge 241 is referred to as the chord 26. The surface of the rotor blade, which is equivalent to the contour line of the airfoil, can be divided into a suction side 281 and a pressure side 282. In the example of FIG. 3, a vortex generator 51, which is an example of an aerodynamic device, is mounted and attached to the suction side 281 of the rotor blade 20. The technical effect of the vortex generator 51 is the generation of a vortex in the airflow 44, the airflow flowing from the leading edge section to the trailing edge section of the rotor blade. These vortices downstream of the vortex generator 51 have the effect of reenergizing the boundary layer downstream of the vortex generator 51. This delays stall of the airflow 44 and increases the lift coefficient of the airfoil. As lift and load are closely correlated, by covering the vortex generator 51 by a cover 30, as illustrated in FIG. 4, the vortex generator 51 is not effective anymore. In other words, a vortex generator covered by a cover deflects the airflow 44 differently compared to an uncovered vortex generator. This is due to the fact that the vortex generators 51 which are typically mounted in pairs are not active once that they are completely or almost completely covered. Therefore, lift and also load on the rotor blade is reduced.
[0050] This is valid for the case that the airflow 44 is flowing from the leading edge section to the trailing edge section, but is also valid if the airflow 44 comes from another direction and flows across the aerodynamic device differently. In any case, covering a lift generating device such as a vortex generator may have a beneficial effect regarding load reduction of the arrangement comprising the rotor blade and the cover.
[0051] FIG. 4 shows a cover 30 which more or less imitates or follows the shape of the aerodynamic device to be covered. In other words, in the example of FIG. 4, the cover has a ramp wherein a substantially constant slope can be discerned in the direction of the airflow 44 until the top of the vortex generator 51. Subsequently and suddenly, the contour line of the cover decreases towards the surface of the rotor blade.
[0052] However, the cover 30 may also have different shapes such as, for instance, shown in FIGS. 6 and 7. In FIG. 6, the cover 30 has a substantially elliptical shape, thus covering generously the vortex generator 51. As another example, FIG. 7 again shows a ramp-like cover 30 with an upstream section of the cover, now followed by an almost constant slope and an irregular downstream section 32 of the cover 30.
[0053] Note that, in principle, any aerodynamic device may be covered by the cover according to embodiments of the present invention. FIG. 8, for example, shows a vortex generator 51 which is mounted to the pressure side instead of the suction side of a rotor blade. Again, similar to FIG. 6, the cover has an elliptical shape in the cross-sectional view of FIG. 8.
[0054] Back to the general concept of embodiments of the present invention, FIG. 5 shows the impact of a cover on the lift coefficient of the airfoil. In FIG. 5, a lift curve 431 of a rotor blade with an uncovered vortex generator is compared with the lift curve 432 of a rotor blade with a covered vortex generator. The rotor blade with the covered vortex generator is also referred to as a “first arrangement” in the following. The first arrangement refers to an arrangement as illustrated in FIG. 4. The lift curve 431 refers to a rotor blade such as illustrated in FIG. 3. It can be seen that for small and medium size angles of attack 41, the lift coefficient 42 is almost the same. However, for higher angles of attack 41, the lift coefficient of the first arrangement 432 is considerably reduced compared to the lift coefficient of the rotor blade with the uncovered vortex generator. As the maximum lift coefficient is closely related to the maximum load on the rotor blade and the wind turbine, reduction of this maximum lift coefficient is beneficially in terms of maximum loads which are supported by the rotor blade.
[0055] FIG. 9 shows a noise reducer 52 which is arranged at the trailing edge section 23 of a rotor blade. Note that the furthest downstream point of the airfoil represents the trailing edge 231 of the airfoil. The noise reducer 52 is attached to the pressure side 282 of the rotor blade. It may comprise just a standard flap or a serrated flap which is also referred to as trailing edge serrations or DinoTail. The lift curve of the rotor blade with the noise reducer is shown as the curve 433 in FIG. 12. The airflow 44 which flows along the pressure side 282 of the rotor blade is shown in FIG. 9.
[0056] FIG. 10 and FIG. 11 show two embodiments of a cover 30 covering a noise reducer 52. In the first option (FIG. 10), the cover 30 only covers the pressure side part of the noise reducer 52, while in the second option (FIG. 11), the cover 30 consists of two parts, one part covering the suction side of the noise reducer 52 and one part covering the pressure side of the noise reducer 52.
[0057] The effect on the lift coefficient of these two arrangements, namely the first arrangement referring to a cover in FIG. 10 and the second arrangement referring to the cover in FIG. 11 are illustrated in FIG. 12. Both lift curves 434 and 435 are considerably shifted and reduced compared to the lift curve with the rotor blade and the uncovered airflow 44. This is due to the fact that also the airflow 44 is considerably deflected, in other words deviated, compared to the uncovered scenario as illustrated in FIG. 9. Therefore, it can be seen that by providing a cover 30 covering at least a part of an aerodynamic device, a reduction of the maximum lift coefficient can be achieved. Consequently, a reduction of the load on the rotor blade and the wind turbine as a whole can be achieved. This in turn ultimately allows installation of the wind turbine, in particular lifting of the rotor blades to the top of tower, at even higher wind speeds compared to rotor blades with uncovered aerodynamic devices.
