Abstract
A wind turbine blade (6) having a span-wise direction between an inner tip region (6a) and an outer tip region (6b), and a chord-wise direction (AA) perpendicular to the span-wise direction is disclosed. The wind turbine blade (6) comprises a hinge (7), an outer blade part (8) and an inner blade part (9). The hinge (7) is arranged to connect the wind turbine blade (6) to a blade carrying structure (5) of a wind turbine (1). The hinge (7) is arranged at a distance from the inner tip region (6a) and at a distance from the outer tip region (6b). The outer blade part (8) is arranged between the hinge (7) and the outer tip region (6b) and the inner blade part (9) is arranged between the hinge (7) and the inner tip region (6a). The inner blade part (9) comprises at least two inner blade portions (20) each having a profile and wherein the inner blade portions (20) are arranged such that the profiles are spaced from each other in the chord-wise direction (AA).
Claims
1. A wind turbine blade having a span-wise direction between an inner tip region and an outer tip region, a chord-wise direction perpendicular to the span-wise direction and a thickness direction, the wind turbine blade comprising: a hinge arranged to connect the wind turbine blade to a blade carrying structure of a wind turbine, the hinge being arranged at a distance from the inner tip region and at a distance from the outer tip region, an outer blade part arranged between the hinge and the outer tip region, and an inner blade part arranged between the hinge and the inner tip region, wherein the inner blade part comprises at least two inner blade portions each having a profile, the inner blade portions being arranged such that the profiles are spaced from each other in the chord-wise and/or thickness direction.
2. The wind turbine blade according to claim 1, wherein the profiles of the inner blade part have a lift generating profile.
3. The A wind turbine blade according to claim 1, wherein the inner blade part and the outer blade part are two separate parts being joined to each other.
4. The wind turbine blade according to claim 3, further comprising a hinge part interconnecting the inner blade part and the outer blade part.
5. The wind turbine blade according to claim 1, wherein the inner blade part and the outer blade part form one piece.
6. The wind turbine blade according to claim 1, the wind turbine blade being configured to have a biasing mechanism attached thereto, the biasing mechanism being arranged to apply a biasing force to the wind turbine blade which biases the wind turbine blade towards a position defining a minimum pivot angle.
7. The wind turbine blade according to claim 1, wherein the inner blade portions are joined at the inner tip region.
8. The wind turbine blade according to claim 1, wherein at least one of the inner blade portions is provided with a balancing mass.
9. The wind turbine blade according to claim 1, wherein at least one of the inner blade portions is provided with a winglet.
10. The wind turbine blade according to claim 1, wherein the profiles of the inner blade portions are different from each other.
11. The wind turbine blade according to claim 1, wherein the inner blade portions have different length.
12. The wind turbine blade according to claim 1, wherein the profiles of the inner blade portions are identical.
13. The wind turbine comprising a tower, a nacelle mounted on the tower via a yaw system, a hub mounted rotatably on the nacelle, the hub comprising a blade carrying structure, and a wind turbine blade according to claim 1, the wind turbine blade being connected to the blade carrying structure via a hinge at a hinge position of the wind turbine blade, the wind turbine blade thereby being arranged to perform pivot movements relative to the blade carrying structure between a minimum pivot angle and a maximum pivot angle.
14. The wind turbine according to claim 13, wherein the blade carrying structure comprises an arm, the wind turbine blade being mounted on the arm, and wherein the arm is configured to pass between at least two of the inner blade portions of the wind turbine blade being mounted thereon during pivoting movements of the wind turbine blade.
15. The wind turbine according to claim 13, further comprising a biasing mechanism arranged to apply a biasing force to the wind turbine blade which biases the wind turbine blade towards a position defining a minimum pivot angle.
16. The wind turbine according to claim 13, wherein the inner blade portions are arranged at a distance from the blade carrying structure, and wherein the distance changes as the wind turbine blade performs pivot movements.
Description
BRIEF DESCRIPTION OF THE DRAWINGS
[0059] The invention will now be described in further detail with reference to the accompanying drawings in which
[0060] FIG. 1 is a side view of a wind turbine according to an embodiment of the invention,
[0061] FIGS. 2a-2i show cross-sectional views of a blade carrying structure arm and inner blade part of a wind turbine blade according to nine different embodiments of the invention,
[0062] FIGS. 3a-3c show a part of a wind turbine blade according to three different embodiments of the invention,
[0063] FIG. 4 shows an exploded view of a part of a blade carrying structure for a wind turbine according to an embodiment of the invention,
[0064] FIGS. 5a-5c show cross-sectional views of a blade carrying structure arm and inner blade part of a wind turbine blade according to an embodiment of the invention with the wind turbine blade at three different pivot angles, and
[0065] FIG. 6 is a graph showing a lift coefficient as a function of angle of attack in the three situations shown in FIGS. 5a-5c.
DETAILED DESCRIPTION OF THE DRAWINGS
[0066] FIG. 1 is a side view of a wind turbine 1 according to an embodiment of the invention. The wind turbine 1 comprises a tower 2 and a nacelle 3 mounted on the tower 2. A hub 4 is mounted rotatably on the nacelle 3, the hub 4 comprising a blade carrying structure 5 with three arms (two of which are visible). A wind turbine blade 6 is connected to each of the arms of the blade carrying structure 5 via a hinge 7. Thus, the wind turbine blades 6 rotate along with the hub 4, relative to the nacelle 3, and the wind turbine blades 6 can perform pivoting movements relative to the blade carrying structure 5, via the hinges 7.
[0067] Each wind turbine blade 6 has a span-wise direction between an inner tip region 6a and an outer tip region 6b. The hinge 7 is arranged at a distance from the inner tip region 6a as well as at a distance from the outer tip region 6b. An outer blade part 8 is thereby arranged between the hinge 7 and the outer tip region 6b. Similarly, an inner blade part 9 is arranged between the hinge 7 and the inner tip region 6a. The inner blade part 9 comprises a number of inner blade portions 20, three of which are shown. The inner blade portions 20 are spaced from each other in a thickness direction, i.e. in a direction perpendicular to a chord defined by the wind turbine blade 6, and indicated by AA line.
[0068] The wind turbine blade 6 with inner blade portions 20 enables various designs of the blade 6. Various cross-sections of the inner blade portions 20 can define more optimal aerodynamic profiles which contribute to extracting more energy from the wind. At low wind speeds, the wind turbine blade 6, and in particular the inner blade part 9, is closest to the blade carrying structure 5 creating a high lift acting on close proximity to the blade carrying structure 5. The high lift is created as the flow is guided by the inner blade portions 20. Thus, the wind turbine blade 6 can be designed in a manner which optimizes the aerodynamic performance of the wind turbine blade 6 along the entire length of the wind turbine blade 6.
[0069] By having spacing between the inner blade portions 20 the total mass of the inner blade part 9 is significantly reduced as the inner blade part 9 is partitioned and a large portion of material which would normally form part of the blade 6 is omitted. Having lighter wind turbine blades would enable development of a new generation high efficiency wind turbines with lighter rotors. The lower mass of the wind turbine blade 6 decreases the loads on the wind turbine blade itself, as well as on other parts of the wind turbine 1, in particular the hub 4, the drivetrain and the tower 2. This allows these parts of the wind turbine 1 to be designed for handling lower loads, and this will result in lower mass and lower manufacturing costs for these parts, and thereby in lower total manufacturing costs for the wind turbine 1.
[0070] In FIG. 1 the wind turbine blades 6 are in a position defining a minimum pivot angle. In this position the inner blade part 9 is arranged adjacent to the blade carrying structure arm 5 defining a minimum distance between the inner blade part 9 and blade carrying structure 5 while maximizing the lift force as the flow is guided by the inner blade portions 20 and the blade carrying structure 5 adjacent to the inner blade portion 9. However, the wind turbine blade 6 can perform pivot movements, thereby increasing the pivot angle as well as increasing a distance between the inner blade part 9 and the blade carrying structure arm 5. In this scenario, the lift is not influenced by the guided flow as the inner blade part and the blade carrying structure are far apart.
[0071] FIGS. 2a-2i show cross sectional views of a blade carrying structure arm 5 and inner blade part 9 of a wind turbine blade according to nine different embodiments of the invention.
[0072] FIG. 2a shows the blade carrying structure arm 5 having a circular cross-section and three inner blade portions 20 spaced from each other in a chord-wise direction. In the embodiment illustrated in FIG. 2a, the inner blade portions 20 are in the form of identical airfoils. The inner blade portions 20 are positioned in such a manner relative to the rotational axis of the hinge 7 that, during pivoting movements, they all move away from the blade carrying structure arm 5 in the same manner. At low wind speeds the inner blade portions 20 are in a position defining a minimum distance between the inner blade part 9 and the blade carrying structure arm 5. This is the position illustrated in FIG. 2a. When the inner blade portions 9a-9c are close to the blade carrying structure arm 5 the lift coefficient provided by the interaction between the blade carrying structure arm 5 and the inner blade portions 20 is increased, as described above. When the wind turbine blade 6 pivots the inner blade portions 20 move away from the blade carrying structure 5 along an upwards direction in the Figure, substantially synchronously.
[0073] In another embodiment shown in FIG. 2b the inner blade part 9 comprises four identical airfoils representing the inner blade portions 20. The inner blade portions of FIG. 2b are covering about a quarter of the blade carrying structure arm 5. All the inner blade portions 20 have the same orientation such that all the airfoils experience a positive angle of attack. The embodiment of FIG. 2b is beneficial as it provides for well guided flow of the wind. When the inner blade portions 20 are closest to the blade carrying structure arm 5, the lift coefficient will be increased, partly due to the proximity of the inner blade portions 20 and the blade carrying structure arm 5. The inner blade portions 20 of FIG. 2b are positioned in a manner which is different from the embodiment of FIG. 2a. During pivoting movements of the wind turbine blade, the distances between the inner blade portions 20 and the blade carrying structure arm 5 increase, and the lift coefficient will change in a different manner compared to embodiment of FIG. 2a.
[0074] FIG. 2c shows yet another embodiment of the inner blade part 9 having nine inner blade portions 20 occupying approximately half of the surface of the blade carrying structure 5 and providing even better guided flow compared to the embodiment of FIG. 2b, because the inner blade portions 20 and the blade carrying structure 5 define a large overlap.
[0075] In the embodiments shown in FIG. 2a-2c the distance between two neighbouring inner blade portions 20 is approximately the same.
[0076] FIG. 2d shows the inner blade part 9 having only two inner blade portions 20 widely separated in the chord-wise direction allowing for pivot angles greater than 90° as the blade carrying structure arm 5 can pass between the inner blade portions 20 during pivoting movements of the wind turbine blade 6.
[0077] The inner blade portions 20 may have different cross-sections and may not be placed equidistantly from each other. Such embodiments are shown in FIG. 2e-2f. FIG. 2e shows inner blade portions 20a-20c in the form of airfoils with a narrow chord. The chords of the inner blade portions 20a-20c differ from each other, i.e., the inner blade portion 20b has the largest chord length, while the two inner blade portions 20c has the shortest chord length. The chord lengths of the inner blade portions 20d-20f shown in FIG. 2f are relatively large and they have three different cross-sections. Thus, inner blade portion 20d has a cross-section in the form of a rectangle, inner blade portion 20e has a cross-section in the form of a circle, and inner blade portion 20f has an inner cross-section in the form of a droplet.
[0078] FIG. 2g shows a cross-sectional view along a cut AA indicated in FIG. 1. It shows the blade carrying structure arm 5 having a circular cross-section and three inner blade portions 20 spaced in a thickness direction. In the embodiment illustrated in FIG. 2a, the inner blade portions 20 are in the form of identical airfoils each of them having a different distance to the blade carrying structure arm 5.
[0079] FIGS. 2h and 2i show the blade carrying structure 5 with three inner blade portions 20 spaced both in the chord-wise direction and the thickness direction. The inner blade portions 20 have a different distance to blade carrying structure arm 5. In these two embodiments, the inner blade portions 20 have different cross sections and they are not placed equidistantly from each other and from the blade carrying structure 5.
[0080] All the embodiments of FIGS. 2a-2i provide an inner blade part 9 with a reduced mass compared to prior art hinged wind turbine blades as it is formed from multiple inner blade portions 20, and a large amount of material which would normally form part of the blade is therefore omitted. The lower mass of the wind turbine blade decreases the loads on the wind turbine blade itself, as well as on other parts of the wind turbine, in particular the hub, the drivetrain and the tower. This allows these parts of the wind turbine to be designed for handling lower loads, and this will result in lower mass and lower manufacturing costs for these parts, and thereby in lower total manufacturing costs for the wind turbine. A specific design of the inner blade part 9 depends on wind conditions at the site of a wind turbine and different requirements set for the power generation of the wind turbine.
[0081] FIG. 3a is an exploded view of a part of a wind turbine blade 6 according to an embodiment of the invention. According to this embodiment, the wind turbine blade 6 is formed from an outer blade part 8 and an inner blade part 9 formed separately. The outer blade part 8 has an airfoil profile. The inner blade part 9 comprises three inner blade portions 20, spaced apart in a chord-wise direction. The inner blade portion 20g has a different length than the two other inner blade potions. The outer blade part 8 and the inner blade part 9 can be joined to each other via a hinge part 10 interconnecting the inner blade part 9 and the outer blade part 8, thereby assembling these three parts into the wind turbine blade 6. The hinge part 10 can be designed to meet requirements at the hinge, e.g. with respect to strength and material thickness, without having to consider other requirements which may be relevant for other parts of the wind turbine blade 6, e.g. with respect to weight, aerodynamic properties, flexibility, etc.
[0082] The hinge part 10 is provided with protrusions 11 which enable connection with a mating part formed on a blade carrying structure in order to form the hinge ensuring the connection of the wind turbine blade 6 with the blade carrying structure. The hinge part 10 further comprises separate mounting interfaces 12 allowing each of the inner blade portions 20 to be mounted on the hinge part 10 and thereby interconnecting it with the outer blade part 8. These separate mounting interfaces 12 are in the form of separate slots for each inner blade portion 20. One of the advantages of this embodiment is that the outer blade part 8 and the inner blade portions 20 are all manufactured separately. This is drastically simpler than manufacturing the wind turbine blade 6 in one piece as it typically requires larger moulds for moulding the blade. Furthermore, when the blade portions 8, 20 are manufactured as separate pieces, their transportation is easier and they can be assembled at a site of the wind turbine, i.e., there is no need for transporting the wind turbine blade 6 in one piece, which may require special means of transportation due to large size and large weight of the wind turbine blade. Providing the inner blade part 9 and the outer blade part 8 as two separate parts allows for assembling the wind turbine blade 6 at the site.
[0083] Also shown in FIG. 3a is a flow fence 15. The flow fence is provided in the vicinity of the hinge and is provided to prevent spanwise flow of air along the blade, in other words to reduce any flow disturbances away from the hinge. Flow fences can be provided on the suction and/or pressure sides of the outer blade part; and flow fences can be provided on the suction and/or pressure sides of the inner blade portions 20.
[0084] FIG. 3b is an exploded view of a part of a wind turbine blade 6 according to yet another embodiment of the invention. The wind turbine blade 6 is formed of the outer blade part 8 and the inner blade part 9 formed separately. The inner blade part 9 comprises two inner blade portions 20. The outer blade part 8 and the inner blade part 9 are joined to each other via a hinge part 10 interconnecting the inner blade part 9 and the outer blade part 8. The hinge part 10 forms protrusions 11 which enable a connection with the mating part 13. The hinge part 10 further comprises separate mounting interfaces 12 allowing each of the inner blade portions 20 to be mounted on the hinge part 10 and thereby interconnecting it with the outer blade part 8. Each of the inner blade portions 20 is provided with a separate winglet 14. The winglets 14 may have precisely selected mass thereby acting as balancing mass which moves a centre of mass of the wind turbine blade 6 at rest in a direction towards the inner tip portion 6a.
[0085] Each of the winglets 14 may have a wire 16 attached thereto. The wires are connected to a biasing mechanism (not shown), via a pulley. The biasing mechanism is configured to apply a biasing force to the inner blade part 9 biasing the wind turbine blade 6 towards a position defining a minimum pivot angle. The winglets 14 form a suitable position for attaching the wires 16 to the inner blade portions 20, because they are structurally strong and therefore able to withstand the forces involved when the biasing mechanism pulls the wires 16. In other examples, the wires may be connected directly to the inner blade part 9 even if a winglet is present or not.
[0086] FIG. 3c shows a part of a wind turbine blade 6 according to yet another embodiment of the invention. This embodiment is similar to one described above with reference to FIG. 3b and therefore will not be described in detail here. The only difference compared to the wind turbine blade 6 of FIG. 3b is that the inner blade portions 20 are joined to each other by a single winglet 14. As in the above described embodiment, the winglet 14 may also have a specially selected mass and may therefore act as a balancing mass defining the centre of mass of the wind turbine blade 6. The biasing mechanism (not shown) is attached to the winglet 14 via a single wire 16.
[0087] FIG. 4 shows an exploded view of a part of the blade carrying structure 5 for a wind turbine according to an embodiment of the invention. The portion of the hinge can be attached to the blade carrying structure 5 and comprises mating parts 13 which are configured to receive the protrusions (not shown) of the hinge part.
[0088] FIGS. 5a-5c show cross-sectional views of a blade carrying structure arm and inner blade part of a wind turbine blade with the wind turbine blade at three different pivot angles.
[0089] FIG. 5a illustrates the blade carrying structure arm 5 and the inner blade portions 20 arranged adjacent to each other, i.e. at a minimum pivot angle, and defining a minimum distance between each other. This distance changes as the wind turbine blade 6 performs pivot movements. Typically, the wind turbine blade 6 is in the position defining the minimum pivot angle and thereby a minimum distance between the inner blade part 9 and the blade carrying structure, as shown in FIG. 5a, at low wind speeds.
[0090] When the inner blade part 9 is close to the blade carrying structure arm 5 the lift coefficient C.sub.L provided by the interaction between the blade carrying structure 5 and the inner blade portions 20 is increased. An increased lift coefficient C.sub.L results in improved aerodynamic properties of the wind turbine blade 6 by the inner blade part 9 having multiple inner blade portions 20.
[0091] As the wind turbine blade pivots towards larger pivot angles the distance between the inner blade portions 20 and the blade carrying structure 5 increases as illustrated in FIGS. 5b and 5c. In FIG. 5b the pivot angle is larger than the pivot angle illustrated in FIG. 5a, and in FIG. 5c the pivot angle has increased even further. In this case, the inner blade portions 20 do not influence the aerodynamic properties to the same extent, since they are arranged further away from the blade carrying structure arm 5. The lift coefficient C.sub.L provided by the interaction between the blade carrying structure 5 and the inner blade portions 20 is therefore decreased compared to the situation illustrated in FIG. 5a. This typically happens at higher wind speeds, but the pivot angle could also be increased for other reasons.
[0092] FIG. 6 is a graph showing a lift coefficient C.sub.L as a function of angle of attack α relative to the blade carrying structure 5 in the three situations shown in FIGS. 5a-5c. The lift coefficient C.sub.L represents a combination of the lift acting on the blade carrying structure 5 and the inner blade portions 20. Curve a corresponds to the scenario illustrated in FIG. 5a when the inner blade portions 20 are arranged adjacent to the blade carrying structure arm 5. In this scenario, the lift coefficient C.sub.L is strongly dependent on the angle of attack α. Thereby, even a small change in angle of attack α will result in a large change in lift coefficient C.sub.L. Accordingly, the aerodynamic properties of the combined blade carrying structure arm 5 and inner blade portions 20 is, in this case, highly sensitive to changes in angle of attack α. It can further be seen that the lift coefficient C.sub.L can be relatively high for certain angles of attack α in this case.
[0093] Curves b and c correspond to situations illustrated in FIGS. 5b and 5c, respectively. It can be seen from curve b that the lift coefficient C.sub.L still depends significantly on the angle of attack α. However, the variations in lift coefficient C.sub.L are not as pronounced as it is the case in the situation illustrated in curve a. Furthermore, the maximum obtainable lift coefficient C.sub.L is also smaller in this case. Accordingly, the impact on the aerodynamic properties is less pronounced in this case, but it is still significant.
[0094] It can be seen from curve c that the lift coefficient C.sub.L in this case remains almost constant as the angle of attack α varies, and it is relatively small. Accordingly, the impact on the aerodynamic properties is more or less insignificant, because the inner blade portions 20 have been moved so far away from the blade carrying structure arm 5 that there is in reality no interaction there between.
