Abstract
A wind turbine (1) comprising a tower (2), a nacelle (3) mounted on the tower (2) via a yaw system, a hub (4) mounted rotatably on the nacelle (3), the hub (4) comprising a blade carrying structure (5), and one or more wind turbine blades (6) connected to the blade carrying structure (5) via a hinge (7) is disclosed. Each wind turbine blade (6) is thereby arranged to perform pivot movements relative to the blade carrying structure (5) between a minimum pivot angle and a maximum pivot angle. The blade carrying structure (5) is provided with one or more elements (8) configured to improve aerodynamic properties of a surface of the blade carrying structure (5) by increasing a lift and/or decreasing a drag of the blade carrying structure. The increase in lift and/or decrease in drag varies as a function of angle of attack (AOA) between the blade carrying structure (5) and the incoming wind.
Claims
1. A 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 one or more wind turbine blades connected to the blade carrying structure via a hinge, each 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, wherein each wind turbine blade does not rotate about a longitudinal axis of the respective blade, wherein the blade carrying structure is provided with one or more elements configured to improve aerodynamic properties of a surface of the blade carrying structure by increasing a lift and/or decreasing a drag of the blade carrying structure, and wherein the increase in lift and/or decrease in drag varies as a function of angle of attack (AOA) between the blade carrying structure and the incoming wind.
2. The wind turbine according to claim 1, wherein the increase in lift varies as a function of angle of attack (AOA) in such a manner that the lift decreases as the angle of attack (AOA) increases.
3. The wind turbine according to claim 1, wherein the decrease in drag varies as a function of angle of attack (AOA) in such a manner that the drag decreases as the angle of attack (AOA) increases.
4. The wind turbine according to claim 1, wherein the blade carrying structure comprises one or more arms, each wind turbine blade being mounted on one of the arms, and wherein one or more of the elements are arranged on the arms.
5. The wind turbine according to claim 4, wherein one or more of the elements are distributed on the arms along a radial direction of the arms and/or along a circumference of the arms.
6. The wind turbine according to claim 1, wherein each of the wind turbine blades defines an aerodynamic profile between an inner tip and an outer tip, and wherein the hinge is arranged on the wind turbine blade at a non-zero distance from the inner tip and at a non-zero distance from the outer tip.
7. The wind turbine according to claim 1 wherein a part of the wind turbine blade is arranged adjacent to a part of the blade carrying structure when the wind turbine blade is in a position defining minimum pivot angle, thereby forming an overlapping region between the wind turbine blade and the blade carrying structure, and wherein the elements are arranged on the blade carrying structure outside of the overlapping region.
8. The wind turbine according to claim 1 wherein at least one of the elements is a vortex generating element.
9. The wind turbine according to claim 1, wherein at least one of the elements is configured to guide a flow along the surface of the blade carrying structure.
10. The wind turbine according to claim 1, wherein at least one of the elements is a spoiler.
11. The wind turbine according to claim 1, wherein at least one of the elements is glued onto the surface of the blade carrying structure.
12. The wind turbine according to claim 1, wherein at least one element is an actively controlled device.
13. The wind turbine according to claim 1, wherein at least one element is a passive device.
Description
BRIEF DESCRIPTION OF THE DRAWINGS
(1) The invention will now be described in further detail with reference to the accompanying drawings in which
(2) FIG. 1 is a side view of a wind turbine according to an embodiment of the invention,
(3) FIG. 2 is a side view of a wind turbine according to another embodiment of the invention,
(4) FIG. 3 shows a part of a wind turbine according to a first embodiment of the invention with a blade carrying structure arm provided with elements,
(5) FIG. 4 shows a blade carrying structure arm of a wind turbine according to a second embodiment of the invention,
(6) FIGS. 5a and 5b show a blade carrying structure arm of a wind turbine according to a third embodiment of the invention,
(7) FIG. 6 is a graph showing lift coefficient as a function of angle of attack of the blade carrying structure arm shown in FIGS. 5a and 5b,
(8) FIG. 7 shows a blade carrying structure arm and an inner blade part of a wind turbine according to a fourth embodiment of the invention,
(9) FIGS. 8a and 8b show a blade carrying structure arm of a wind turbine according to a fifth embodiment of the invention,
(10) FIGS. 9a and 9b show a blade carrying structure arm of a wind turbine according to a sixth embodiment of the invention,
(11) FIGS. 10a-10c show a blade carrying structure arm of a wind turbine according to a seventh embodiment of the invention,
(12) FIG. 11 shows a blade carrying structure arm of a wind turbine according to an eighth embodiment of the invention, and
(13) FIG. 12 is a graph showing lift coefficient and drag coefficient as a function of angle of attack of a blade carrying structure of a wind turbine according to an embodiment of the invention.
DETAILED DESCRIPTION OF THE DRAWINGS
(14) 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. The blade carrying structure 5 comprises 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. The wind turbine blades 6 perform pivot movements relative to the blade carrying structure 5 between a minimum pivot angle and a maximum pivot angle. FIG. 1 illustrates the wind turbine blades 6 defining a minimum pivot angle with the blade carrying structure 5, and thereby maximum rotor diameter.
(15) Each of the wind turbine blades 6 defines an aerodynamic profile between an inner tip 6a and an outer tip 6b, and the hinge 7 is arranged on the wind turbine blade 6 at a non-zero distance from the inner tip 6a and at a non-zero distance from the outer tip 6b. Thereby the wind turbine blade 6 is hinged to the blade carrying structure 5 at a position which is not at an end (6a or 6b) of the wind turbine blade 6. When the wind turbine blade 6 forms a minimum pivot angle with the blade carrying structure arm 5, an overlapping region 10 between the wind turbine blade 6 and the blade carrying structure 5 is formed.
(16) The blade carrying structure 5 is provided with elements 8 configured to improve aerodynamic properties of a surface of the blade carrying structure 5. The elements 8 are distributed on the arms 5 along a radial direction of the arms 5 and along the entire length of the arm 5. Additionally, the elements 8 may also be distributed along a circumferential direction of the arm 5.
(17) The blade carrying structure 5 occupies part of the swept area of the rotor which may be utilized if its aerodynamic properties can be improved. Placing the elements 8 at carefully chosen places on the blade carrying structure 5 results in lowering drag forces and/or increasing lift forces imposed on the blade carrying structure 5, especially at low wind speeds. By lowering the drag forces and/or increasing the lift forces of the blade carrying structure 5, the blade carrying structure 5 with the elements 8 thereon contributes to the energy conversion of the wind turbine 1. Thereby the part of the swept area, which is occupied by the blade carrying structure 5, is also utilized for energy production, i.e. it is ‘activated’, and thereby the total swept area is utilized to a greater extent.
(18) FIG. 2 is a side view of a wind turbine 1 according to another embodiment of the invention. FIG. 2 is similar to FIG. 1 and it will therefore not be described in details here.
(19) In the embodiment of FIG. 2, the elements 8 are arranged on the blade carrying structure 5 outside of the overlapping region 10. Namely, a part of the blade carrying structure 5 in combination with an inner blade part 9 which defines the overlapping region 10, contribute to the conversion efficiency of the wind turbine 1 as the portions of the blade carrying structure 5 and adjacent part of the wind turbine blade 6 can work together improving the lift forces as the flow is guided by the inner blade part 9 and the blade carrying structure 5 adjacent to the inner blade part 9. Therefore, placing the elements 8 in the overlapping region 10 is unnecessary. The elements 8 arranged on the blade carrying structure 5 outside of the overlapping region 10 are contributing to the aerodynamic properties of the blade carrying structure 5. This is in particular important during start-up of the wind turbine 1 as it will allow the wind turbine 1 to accelerate more quickly and start power production immediately upon start-up.
(20) On the other hand, when the wind turbine blades 6 are arranged at a position defining a larger pivot angle, which may be the case at higher wind speeds, the wind turbine blades 6 are positioned at a larger distance from the blade carrying structure 5. In this scenario, the part of the swept area which corresponds to the overlapping region 10 of the blade carrying structure 5 does not contribute to the lift coefficient, because no elements 8 are arranged in this region 10, and because the aerodynamic profiles of the wind turbine blades 6 are also not arranged in this area 10. This is desirable because high lift may, in this case, be undesirable and may have detrimental effects on the wind turbine at high wind speeds as the rotor already rotates with high rotational speeds and further increase is not needed. Accordingly, positioning the elements 8 outside the overlapping region 10 provides a structure in which the aerodynamic properties of the blade carrying structure 5 are improved at low wind speeds, but not at high wind speeds.
(21) FIG. 3 shows a part of a wind turbine 1 according to a first embodiment of the invention. The wind turbine 1 comprises a blade carrying structure 5 with three conical-cylindrical arms providing required support to the wind turbine blades 6, each arm being provided with elements 8 in the form of vortex generators 8, which increase wind circulation for given angles of attack, thus generating lift at low wind speeds. Only the vortex generators 8 of one of the arms can be seen. The vortex generators 8 are placed at a portion of the blade carrying structure arm 5 closest to the hub 4, and well away from an overlapping region 10, thereby improving aerodynamic properties of the widest part of the arm 5 which is otherwise not utilized as a swept area of the wind turbine 1. The vortex generators 8 may be designed to control turbulent separated flow along the blade carrying structure 5 and especially at low wind speeds, thereby improving the aerodynamic properties of the blade carrying structure 5.
(22) FIG. 4 shows a blade carrying structure arm 5 of a wind turbine according to a second embodiment of the invention. The blade carrying structure arm 5 is provided with an element 8 in the form of a gurney flap 8. The gurney flap 8 can be designed and positioned on the blade carrying structure arm 5 in such a manner that it increases air circulation for given angles of attack, thereby generating lift at low wind speeds. The gurney flap 8 has a height-diameter ratio defined in percentages (%) as a ratio between a height h of the gurney flap 8 and a diameter D of the arm 5. The height-diameter ratio may range from 1% and up to 15% and it depends on an exact position of the gurney flap along the blade carrying structure 5. The gurney flap 8 can also be designed to act as a vortex generating element.
(23) FIGS. 5a and 5b show a blade carrying structure arm 5 of a wind turbine according to a third embodiment of the invention. The blade carrying structure arm 5 is provided with elements 8 in the form of five gurney flaps distributed along and circumferentially around the arm 5 such that each gurney flap 8 experiences different angles of attack, and thereby individually contributes to the lift coefficient. FIG. 5a shows a cross sectional view of the arm 5 showing how the gurney flaps 8 are distributed around the circumference of the arm 5 with different attachment angles. The gurney flaps 8 differ among each other in their height-diameter ratio (h/D) and their length, i.e., R5 has 6% h/D, R10 has 5% h/D, R20 has 4% h/D, R30 has 3% h/D, and R40 has 2% h/D.
(24) FIG. 5b shows a side view of the arm 5 where difference in length between different gurney flaps 8 is show. FIG. 5b also shows that R5 is placed closest to the hub, e.g. 5 m from the hub, while R40 is placed furthest from the hub, e.g., at 40 m distance from the hub. Each gurney flap R5-R40 experiences different angles of attack as they are placed at different positions around the arm 5, thereby experiencing the incoming wind differently and thereby individually contributing to the lift coefficient. Lift coefficient dependency as a function of angle of attack for each gurney flap R5-R40 is shown in FIG. 6.
(25) FIG. 6 is a graph showing lift coefficient C.sub.L as a function of angle of attack AOA of the blade carrying structure shown in FIG. 5. The graph shows five individual curves for each of the gurney flaps R5-R40. Each gurney flap R5-R40 will contribute differently to the lift coefficient C.sub.L of the blade carrying structure depending on an angle of attack AOA to which it is exposed. The angle of attack AOA which the gurney flaps R5-R40 experience changes as rotational speed of the rotor changes. The rotational speed of the rotor changes when the wind speed changes. Therefore, the angle of attack which the gurney flaps R5-R40 experience changes with the wind speed. The gurney flaps R5-R40 are designed and positioned in such a manner that they drastically increase the lift coefficient C.sub.L at low wind speeds while at high wind speeds their influence on the lift coefficient C.sub.L is minor. This is explained in Table 1. For example, at low wind speeds, 2-22 m/s, gurney flap R5 is positioned on the blade carrying structure so as to experience wind with an angle of attack within the range 45°-56°, thereby maximally increasing the lift coefficient of the blade carrying structure, as can be seen on R5 curve of the graph of FIG. 6. As the wind speed increases, the gurney flap R5 will experience higher angles of attack and therefore lower lift coefficient C.sub.L of the blade carrying structure. From Table 1 it can be seen that, at low wind speeds, all the gurney flaps R5-R40 experience angles of attack which result in maximum lift coefficient, while it is opposite for high wind speeds.
(26) TABLE-US-00001 TABLE 1 AOA at AOA at AOA at AOA at 2-22 m/s 24 m/s 42 m/s 50 m/s R5 45°-56° ~80° ~85° ~87° R10 25°-36° ~68° ~76° ~80° R20 17°-25° ~58° ~67° ~72° R30 10°-15° ~42° ~56° ~63° R40 8°-13° ~38° ~52° ~57°
(27) FIG. 7 shows a cross sectional view of a blade carrying structure arm 5 and an inner blade part 9 of a wind turbine according to a fourth embodiment of the invention. The blade carrying structure arm 5 comprises elements 8a and 8b which, together with the blade carrying structure 5, form an airfoil. The element 8a is in the form of a leading edge portion of the airfoil and the element 8b is in the form of a trailing edge portion of the airfoil. By mounting the leading edge portion 8a and the trailing edge portion 8b on the blade carrying structure 5 with a circular cross-section, as shown in FIG. 7, the lift of the blade carrying structure is increased while decreasing the drag at the same time. The airfoil portions 8a, 8b may be bolted or glued to the arm 5.
(28) The airfoil elements 8a, 8b are attached to the arm 5 in an overlapping region between the arm 5 and the inner blade part 9 of the wind turbine blade. In FIG. 7 the inner blade part 9 is shown in a position defining minimum pivot angle, thereby arranging the inner blade part 9 close to the airfoil elements 8a, 8b. This allows the inner blade part 9 and the airfoil elements 8a, 8b to cooperate in order to increase lift of the blade carrying structure, in particular at low wind speeds, and thereby small angles of attack. This improves the performance of the inner blade part 9 an prevents stall within a larger range of angles of attack.
(29) In FIG. 7 the direction of the incoming wind, v.sub.wind, as well as the direction of the relative movement between the air, on the one hand, and the inner blade part 9 and the arm 5, on the other hand, due to rotation of the hub, v.sub.Rot, are also shown. The resulting direction of the incoming wind, relative to the inner blade part 9 and the arm 5, defining the angle of attack, is a vector summation of these two.
(30) FIGS. 8a and 8b show a blade carrying structure arm 5 of a wind turbine according to a fifth embodiment of the invention. The blade carrying structure arm 5 is provided with an element 8 being formed by two bended profiles 8c and 8d positioned against each other forming a shell-like structure. This element is hereinafter referred to as a ‘sea shell’. The bended profiles 8c and 8d are bonded to each other and also to the blade carrying structure 5. These two bended profiles 8c and 8d can be optimized for increased lift at specific angles of attack intervals. Alternatively or additionally, the sea shell 8 can be designed to act as a vortex generating element thereby controlling separated turbulent flow along the blade carrying structure 5. The sea shell 8 may be formed in plastic or another material. Bonding the plastic profiles 8c and 8d to the surface of the blade carrying structure 5 makes the sea shells 8 an extremely robust solution which may last for a long period of time. FIG. 8a shows a cross sectional view of the blade carrying structure 5 with the sea shell 8, and FIG. 8b shows a perspective view of the blade carrying structure 5 with the sea shell 8.
(31) FIGS. 9a and 9b show a blade carrying structure arm 5 of a wind turbine according to a sixth embodiment of the present invention. The blade carrying structure arm 5 is provided with an element 8 in the form of an undulating pattern mimicking whale-like curves. This type of element 8 is hereinafter referred to as ‘whale curves’. FIG. 9a shows the whale curves 8 which are added to the blade carrying structure 5 by, e.g., bonding. They have a shape which is similar to one which whales have to help them swim in the water. The whale curves 8 guide airflow around the blade carrying structure 5, thereby generating lift which acts on the portion of the blade carrying structure 5 where the whale curves 8 are arranged. FIG. 9b shows a cross-sectional view of the blade carrying structure 5 with the whale curves 8 arranged thereon.
(32) FIGS. 10a-10c show a blade carrying structure arm 5 of a wind turbine according to a seventh embodiment of the present invention. According to this embodiment, the blade carrying structure arm 5 is provided with an element 8 which mimics the skin of sharks, which is created by nature for reducing drag in water flow, resulting in the efficient movement of sharks through the water. This type of element 8 is hereinafter referred to as a ‘shark skin’. The shark skin element 8 is configured to reduce drag coefficient of the blade carrying structure 5. FIG. 10a shows a mat of shark skin element 8 placed onto the surface of the blade carrying structure 5 thereby reducing drag forces imposed to the blade carrying structure 5. The shark skin 8 may be glued to the blade carrying structure 5, enabling manufacturing the blade carrying structure 5 in a simple manner without the shark skin 8 (or any other elements 8), thereby simplifying the manufacturing process significantly.
(33) FIG. 10b shows a mat of shark skin element 8 which can be arranged onto the blade carrying structure 5 along direction AA.
(34) FIG. 10c shows a cross-sectional view of the shark skin 8. The shark skin 8 comprises tiny scales 12 also known as dermal denticles which are shaped like small riblets and aligned in the direction of wind flow as indicated by a directional symbol 13.
(35) FIG. 11 shows a blade carrying structure arm 5 of a wind turbine according to an eighth embodiment of the invention. According to this embodiment the, the blade carrying structure arm 5 is provided with a vortex generator 8e and a spoiler 8f. The vortex generator 8e is arranged on a part of the arm 5 which may be regarded as near a ‘leading edge’ and on a ‘suction side’. The spoiler 8f is arranged on a part of the blade carrying structure arm 5 which may be regarded as near a ‘trailing edge’ and on a ‘pressure side’. The spoiler 8f extends from the surface of the blade carrying structure arm 5 along a direction which is neither perpendicular to, nor parallel to the surface. It can be seen from the insert that the spoiler 8f may extend along a linear direction, or it may extend along a curved direction.
(36) When subjected to air flow, the vortex generator 8e creates vortices which tend to retain the air flow towards the surface of the blade carrying structure arm 5. When subjected to air flow, the spoiler 8f guides the air in a direction defined by the direction in which the spoiler 8f extends from the blade carrying structure arm 5.
(37) In FIG. 11, the direction of the incoming wind, v.sub.wind, as well as the direction of the relative movement between the air the arm 5, due to rotation of the hub, v.sub.Rot, are also shown. The resulting direction of the incoming wind, v.sub.Res, relative to the arm 5, derived from these two and defining the angle of attack (AOA), is also shown.
(38) At low wind speeds, the angle of attack is small, and the resulting wind direction, v.sub.Res, is close to v.sub.Rot. Thereby the incoming wind reaches the vortex generator 8e as well as the spoiler 8f, and the two will cooperate in increasing the lift of the arm 5.
(39) At high wind speeds, the angle of attack is large, and the resulting wind direction, v.sub.Res, is close to v.sub.Wind. Thereby the incoming wind reaches the spoiler 8f, but not the vortex generator 8e. This results in reduced lift as well as reduced drag, as compared to the angles of attack where the incoming wind also reaches the vortex generator 8e. In particular, at very high wind speeds, where the wind turbine is shut down in order to protect the wind turbine, the drag is lower as compared to the characteristics at low angles of attack, thereby providing improved protection of the wind turbine.
(40) FIG. 12 is a graph showing lift coefficient 14 and drag coefficient 15 as a function of angle of attack of a blade carrying structure of a wind turbine according to an embodiment of the invention. The blade carrying structure could, e.g., be the blade carrying structure illustrated in FIG. 11.
(41) It can be seen that the lift coefficient 14 is significantly higher at small angles of attack than at large angles of attack, and that a maximum lift coefficient is defined within the angle of attack interval designated ‘Max lift range’. It can further be seen that the lift coefficient 14 decreases drastically as the angle of attack approached 90°. Thus, at low wind speeds, where it is desirable to extract as much energy as possible from the wind, resulting in small angles of attack, the lift coefficient 14 is large, thereby improving the ability of the blade carrying structure to extract energy from the wind significantly. On the other hand, at high wind speeds, where sufficient energy may already be extracted from the wind by the wind turbine blades, resulting in large angles of attack, the lift coefficient 14 is small, thereby providing only a small increase in the energy production contribution from the blade carrying structure.
(42) It can further be seen that the drag coefficient 15 is also higher at small angles of attack than at large angles of attack. The drag coefficient 15 is substantially constant within the angle of attack interval designated ‘Max lift range’. Within the angle of attack interval designated ‘Min drag range’, the drag coefficient 15 decreases towards a minimum drag coefficient at an angle of attack of approximately 90°. Thus, at low wind speeds, resulting in small angles of attack, a drag is introduced. However, this is more than outbalanced by the increased lift described above. At high wind speeds, resulting in large angles of attack, the drag is low. This is particularly advantageous at very high wind speeds, where the wind turbine is shut down in order to protect the wind turbine. In this case the low drag provides additional protection to the wind turbine, in particular to the blade carrying structure.
