ROTOR BLADE ACTIVE FLAP

Abstract

The invention describes a wind turbine rotor blade active flap (1) comprising a primary body (1P) adapted for mounting to the trailing edge (20.sub.TE) of a wind turbine rotor blade (20); a flap turning means (10, 11, 12) adapted to turn the active flap (1) between a neutral position (R.sub.0), in which the active flap (1) directs airflow (A.sub.20S, A.sub.20P) towards the suction side (P.sub.20S) of the rotor blade (20), and a working position (R-R.sub.max), in which the active flap (1) directs airflow (A.sub.20S, A.sub.20P) towards the pressure side (P.sub.20P) of the rotor blade (20); and a secondary body (1S) mounted to the primary body (1P) and configured to hold the active flap (1) in its neutral position (R.sub.0). The invention further describes a wind turbine (2) comprising a number of rotor blades (20) mounted to a hub; and an active flap (1) according to the invention, mounted to the trailing edge (20.sub.TE) of each rotor blade (20).

Claims

1. An active flap (1) comprising a primary body (1P) adapted for mounting to the trailing edge (20.sub.TE) of a wind turbine rotor blade (20); a flap turning means (10, 11, 12) adapted to turn the active flap (1) between a neutral position (R.sub.0), in which the active flap (1) directs airflow (A.sub.20S, A.sub.20P) towards the suction side (P.sub.20P) of the rotor blade (20), and a working position (R-R.sub.max), in which the active flap (1) directs airflow (A.sub.20S, A.sub.20P) towards the pressure side (P.sub.20P) of the rotor blade (20); and a secondary body (1S) mounted to the primary body (1P) and configured to hold the active flap (1) in its neutral position (R.sub.0).

2. An active flap according to claim 1, wherein the secondary body (1S) is shaped to achieve a positive hinge moment (1.sub.HM) about a hinge region (1H) of the active flap (1).

3. An active flap according to claim 1, wherein a suction surface of the secondary body (1S) has a convex shape.

4. An active flap according to claim 1, wherein the secondary body (1S) is an essentially rectangular band (14).

5. An active flap according to claim 1, wherein the secondary body (1S) comprises an auxiliary airfoil (16).

6. An active flap according to claim 4, wherein the secondary body (1S) comprises an auxiliary airfoil (16) mounted to the pressure surface of an essentially rectangular band (14).

7. An active flap according to claim 4, wherein the auxiliary airfoil (16) is an active device realized to alter the direction of airflow over its surfaces.

8. An active flap according to claim 1, wherein the material stiffness of the primary body (1P) is chosen according to a desired range between a neutral position (R.sub.0) and a maximum working position (R.sub.max).

9. An active flap according to claim 1, wherein the flap turning means comprises an elastic body (11) configured to maintain the secondary body in a working position (R.sub.max).

10. An active flap according to claim 1, wherein the flap turning means comprises an inflatable chamber (10, 12) arranged in an interior cavity (100) of the primary body (1P).

11. An active flap according to claim 1, wherein an inflatable chamber (12) arranged in the interior cavity (100) of the primary body (1P) comprises an orifice (120) with an area chosen on the basis of a desired spring damping coefficient.

12. A wind turbine (2) comprising at least a number of rotor blades (20) mounted to a hub; and an active flap (1) according to claim 1 mounted to the trailing edge (20.sub.TE) of each rotor blade (20).

13. A wind turbine according to claim 12, comprising a compressor assembly (22) adapted to provide pressurized air to the flap turning means (10) of the active flap (1) of a rotor blade (20).

14. A wind turbine according to claim 13, comprising a controller (24) configured to issue control commands to the compressor assembly (22) on the basis of a desired active flap position (R.sub.0, R, R.sub.max).

15. A method of operating a wind turbine (2) according to claim 12, comprising a step of actuating the flap turning means (10) of the active flap (1) of a rotor blade (20) to turn that active flap (1) from a working position (R, R.sub.max) to its neutral position (R.sub.0) in order to reduce the lift force (F20.sub.A_lift) on that rotor blade (20); and comprising a step of actuating the flap turning means (10, 11, 12) of the active flap (1) of a rotor blade (20) to turn that active flap (1) to a working position (R, R.sub.max) in order to increase the lift force (F20.sub.A_lift) on that rotor blade (20).

Description

[0027] Other objects and features of the present invention will become apparent from the following detailed descriptions considered in conjunction with the accompanying drawings. It is to be understood, however, that the drawings are designed solely for the purposes of illustration and not as a definition of the limits of the invention.

[0028] FIG. 1 shows a wind turbine with rotor blades equipped with active flaps;

[0029] FIG. 2 is a cross-section through a rotor blade airfoil equipped with an embodiment of the inventive active flap in a first position;

[0030] FIG. 3 is a cross-section through the rotor blade airfoil of FIG. 2, with the active flap in a second position;

[0031] FIG. 4 shows a wind turbine rotor blade equipped with a prior art active flap;

[0032] FIGS. 5-7 show further embodiments of the inventive active flap;

[0033] FIG. 8 and FIG. 9 illustrate the effect of adjusting the flap end angle of the inventive active flap;

[0034] FIG. 10 shows a further embodiment of the inventive active flap.

[0035] In the diagrams, like numbers refer to like objects throughout. Objects in the diagrams are not necessarily drawn to scale.

[0036] FIG. 1 shows a wind turbine 2, with rotor blades 20 mounted to a hub. Each rotor blade 20 is equipped with one or more active flaps 1, 4 along a portion of its airfoil region. During operation of the wind turbine 2, the rotor blades 20 can be pitched to increase or decrease the amount of energy extracted from the wind.

[0037] In exemplary embodiments of the invention, a compressor assembly 22 can control each active flap 1, 4 independently of the other active flaps, so that each active flap 1 can be moved to a desired position between a neutral position and a maximum working position. This is done by adjusting the amount of pressurized air in an inflatable hose arranged in the interior of the active flap as will be explained below. A wind turbine controller 24 can issue appropriate commands to the compressor assembly 22, which then opens and closes appropriate valves to inflate or deflate the hoses.

[0038] FIG. 2 is a cross-section through an airfoil 20A of a wind turbine rotor blade 20 equipped with an embodiment of the inventive active flap 1. The diagram indicates a plane P that includes the chord plane of the rotor blade 20. A rotor blade airfoil gradually transitions from a wide and thick shape at the inboard region towards a narrow and flat shape at the tip end of the rotor blade, and the chord plane comprises the chord line of each possible airfoil cross-section, including the chord line 20C of the airfoil cross-section shown here.

[0039] Since a wind turbine rotor blade can be pre-bent in the flapwise direction, the skilled person will appreciate that the chord plane is not necessarily flat. The plane P divides the space through which the rotor blade 20 moves into a suction side P.sub.20S (which includes the suction surface 20S of the airfoil 20) and a pressure side P.sub.20P (which includes the pressure surface 20P of the airfoil).

[0040] The active flap 1 has been turned to a working position R as shown here, i.e. to a position within the possible range of motion bounded by a neutral position R.sub.0 and a maximum working position R.sub.max.

[0041] The active flap 1 comprises a primary body 1P and a secondary body 1S. The primary body 1P is constructed to be able to turn about a hinge 1H indicated by the dotted region. To this end, an inflatable hose 10 is arranged in an interior cavity 100 of the primary body 1P. When the hose 10 as shown here is inflated (for example by a suitable quantity of pressurized air from a compressor arrangement), the shape of the primary body 1P is altered and its upper surface is deflected downwards, turning the active flap 1 to the desired working position R. This shape alteration is achieved by suitable design of the primary body 1P, for example by incorporating an elastic element 12 as shown here that extends when the hose 10 is inflated and which returns to its original shape when the hose 10 is deflated. Here, flap turning is achieved by the inflatable hose 10 and the elastic element 12.

[0042] The diagram illustrates the manner in which the airflow A.sub.20S, A.sub.20P (passing over both surfaces 20S, 20P of the airfoil 20A) is guided by the active flap 1 more towards the suction side P20S (and away from the pressure side P20P), thereby achieving a large aerodynamic force F.sub.20A in the direction shown. As the skilled person is aware, the aerodynamic force F20A on an airfoil 20A has a lift component F.sub.20A_lift and a drag component F.sub.20A_drag.

[0043] Usually, it is desirable for a wind turbine to generate as much power as possible, andin addition to suitable pitch commandsthe controller will issue active flap commands to move one or more active flaps 1 to working positions that help achieve a desired aerodynamic force for each rotor blade. For example, the active flap 1 of FIG. 2 can be moved to a maximum working position R.sub.max in which the active flap 1 makes its maximum contribution to the lift component F.sub.20A_lift.

[0044] In other situations, it may be necessary to minimize the aerodynamic force F.sub.20A on the rotor blade 20, and the controller issues control commands to return the active flap 1 to its neutral position R.sub.0 as shown in FIG. 3. The hose 10 is deflated, and the suction surface of the active flap 1 moves upwards again. The diagram shows how the shape of the secondary body 1S guides and combines the suction-side airflow A.sub.20S (the airflow that passed over the suction surface of the airfoil 20A) and the pressure-side airflow A.sub.20P (the airflow that passed over the pressure surface of the airfoil 20A) to deflect upward at the trailing edge, in the direction of the suction side P.sub.20S. The effect of this airflow guidance is to generate a positive hinge moment 1.sub.HM acting on the hinge region 1H, effectively maintaining the active flap 1 in its neutral position R.sub.0.

[0045] FIG. 3 also shows a mounting plane 30 at the interface between the primary body 1P and the secondary body 1S of the active flap 1. The curved shape of the secondary body 1S can be defined by the relative orientation of the mounting plane 30 and a plane 31 through the outer edge of the secondary body.

[0046] FIG. 4 shows a similar situation in the case of a wind turbine rotor blade 20 equipped with a prior art active flap 4 of a similar design, i.e. a flexible add-on with an inflatable hose in an interior cavity. Even though the active flap 4 has been put in its neutral position R.sub.0, the force of the airflow A.sub.20S over the rotor blade suction surface 20S can exert a negative hinge moment 4.sub.HM on the active flap 4 which acts to deflect it from its neutral position and into a working position R.sub.x. This may result in an increase in lift force F20.sub.A_lift on the rotor blade 20. Besides the possibly unwanted increase in lift from this deflection, a later actuation of the active flap 4 by the wind turbine controller (for example in order to increase the power output of the wind turbine) may fail to achieve a desired blade loading, since the wind turbine controller issues control commands on the assumption that the active flap 4 is in its neutral position, and is unaware that the active flap 4 is unintentionally already in a working position R.sub.x. As a result, the resultant position of the active flap 4 may be non-optimal, since it will be closer than intended to the maximum working position R.sub.max. Effectively, the range of the prior art active flap 4 is reduced by the passive deflection R.sub.x, owing to its inability to maintain its neutral position.

[0047] FIG. 5 shows a perspective view of an embodiment of the inventive active flap 1 in place on a rotor blade 20. Viewed from above, the secondary body 1S has an elongate shape and exhibits a slight convex form along its length. The cross-section through the airfoil 20A and active flap 1 clearly shows this curved shape of the secondary body 1S. The diagram also indicates the effect of the active flap 1 on the incoming airflow A.sub.20S, A.sub.20P when the flap 1 is in its neutral position R.sub.0 as shown here.

[0048] FIG. 6 shows a perspective view of a further embodiment of the inventive active flap 1 in place on a rotor blade 20. Viewed from below, the secondary body 1S comprises two parts, namely an elongate part 14 with serrations 140 along its trailing edge, and a number of small auxiliary airfoils 16.

[0049] Each auxiliary airfoil 16 is attached to the elongate part 14 by means of streamlined struts 160. FIG. 7 illustrates the effect of this embodiment on the incoming airflow A.sub.20S, A.sub.20P when the flap 1 is in its neutral position R.sub.0 as shown here.

[0050] A negative hinge moment may be deliberately designed into the active flap in order to avoid excessive positive hinge moment when the active flap is in its neutral position. A negative hinge moment can convey a greater control authority to the active flap and result in a faster response time when the active flap is returned to its neutral position.

[0051] FIG. 8 and FIG. 9 illustrate the effect of adjusting the flap end angle of the inventive active flap. FIG. 8 shows an exemplary embodiment of the inventive active flap 1 in place on the trailing edge of a wind turbine rotor blade, corresponding to an embodiment as described in FIG. 3 above (only the outer portion of the primary body 1P is shown for clarity, an inflatable chamber or similar is not shown but may be assumed). The diagram indicates a reference plane 90 relative to which the flap end angle is defined. The reference plane 90 may correspond to the mounting plane 30 described in FIG. 3 above. The solid outline of the secondary body 1S indicates a flap end angle do associated with a baseline hinge moment, and the dotted outlines for other positions of the secondary body 1S indicate flap end angles .sub.1-.sub.4 associated with other baseline hinge moments as explained in FIG. 9, which shows graphs of hinge moment (Nm) against flap deflection (degrees) for embodiments of the inventive active flap with different flap end angles. A baseline hinge moment curve 10B illustrates the relationship between hinge moment and flap deflection for a certain flap end angle do as indicated in FIG. 8. This flap end angle do may be associated without any pronounced hinge moment during operation of the rotor blade. Several exemplary hinge moment curves 101-104 illustrate the effect of altering the flap end angle. The two hinge moment curves 101, 102 above the baseline hinge moment curve 10B exhibit positive offset, i.e. the hinge moment is increased or made more positive by choosing a large flap end angle .sub.1, .sub.2. The two hinge moment curves 103, 104 below the baseline hinge moment curve 90 exhibit negative offset, i.e. the hinge moment is decreased or made more negative by choosing a reduced flap end angle .sub.3, .sub.4.

[0052] Of course, the inventive active flap can combine the effects described above. For example, at one end of its mounting length, the active flap can be shaped to exhibit a small amount of negative hinge moment. At the other end of its mounting length, the active flap can be shaped to exhibit only positive hinge moment. The shape and contours of the active flap shape can segue smoothly over its mounting length so that the transition from negative hinge moment to positive hinge moment (i.e. with a flap end angle do) occurs at a desired spanwise position of the rotor blade trailing edge.

[0053] FIG. 10 shows a further embodiment of the inventive active flap 1. Here, the flap turning means is realised as an elastic body 11in this case a springwhich acts to place the secondary body 1S in its working position. In this embodiment, the flap turning means is passive in the sense that it is not actuated by a wind turbine controller. Here, the spring 11 maintains the secondary body 1S in its maximum working position R.sub.max, as long as the spring is not compressed. The diagram shows an air bladder 12 with an outlet or orifice 120 whose diameter or cross-sectional area is chosen to achieve a desired damping of the spring 11. The bladder 12 may be connected to a reservoir 13 or tank in the rotor blade interior as shown here. At higher wind speeds, the force exerted on the secondary body 1S causes the spring 11 to compress. This forces air out of the bladder 12 (and, in this case, into the reservoir). The active flap 1 is compressed further as wind speed increases, and ultimately reaches its neutral position when the bladder 12 is completely emptied. The bladder 12 will refill again (drawing air from the tank 13) as wind speed decreases and the force on the secondary body decreases, allowing the spring 11 to extend again.

[0054] Although the present invention has been disclosed in the form of preferred embodiments and variations thereon, it will be understood that numerous additional modifications and variations could be made thereto without departing from the scope of the invention.

[0055] For the sake of clarity, it is to be understood that the use of a or an throughout this application does not exclude a plurality, and comprising does not exclude other steps or elements.