Wind turbine blade

Abstract

The present wind turbine blade comprises an airfoil structure comprising an airfoil shape, an internal support structure arranged spanwise along the length of the blade within the airfoil structure, and an elastic connection joining a portion of an inner surface of the airfoil structure with a portion of the internal support structure. The airfoil structure can be passively pitched relative to the internal support structure according to aerodynamic pressure distribution at different blade locations.

Claims

1. A wind turbine blade comprising: an airfoil structure comprising an airfoil shape; an internal support structure arranged spanwise along a length of the blade and within the airfoil structure; and an elastic hinge connection joining a portion of an inner surface of the airfoil structure with a portion of the internal support structure, wherein the elastic hinge connection is adapted to cause the airfoil structure to passively pitch relative to the internal support structure around the position of the elastic hinge connection according to an aerodynamic pressure distribution at different locations of the blade section; and wherein the elastic hinge connection comprises a single hinged connection between the airfoil structure and the internal support structure, the single hinged connection comprising a first connecting portion that is fixed to or is part of an inner surface of a pressure side of the airfoil structure, and a second connecting portion that is fixed to or part of a corresponding lower portion of an outer surface of the internal support structure, said single first and single second connecting portions being hinged to each other such that they are capable to pivot with respect to each other.

2. The wind turbine of claim 1, wherein the single hinged connection comprises a ball joint.

3. The wind turbine blade of claim 1, wherein the single hinged connection is placed at a location within a cross-section of the airfoil structure such that the aerodynamic moment of the internal support structure is zero.

4. The wind turbine blade of claim 1, wherein at least one of a plurality of biasing mechanisms is arranged between the airfoil structure and the internal support structure.

5. The wind turbine blade of claim 4, wherein each of the plurality of biasing mechanisms is comprised of a flexible foam.

6. The wind turbine blade of claim 4, wherein a plurality of the biasing mechanisms are provided along the length of the blade in different blade sections, wherein the plurality of biasing mechanisms have different properties in the different blade sections.

7. The wind turbine blade of claim 1, wherein the internal support structure is a beam.

8. The wind turbine blade of claim 1, wherein the internal support structure is a spar-box.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Particular examples of the present wind turbine blade will be described in the following by way of non-limiting examples, with reference to the appended drawings, in which:

(2) FIG. 1 illustrates a typical power curve of a wind turbine;

(3) FIGS. 2a-2c illustrate aerodynamics of wind turbine blades and aerodynamic profiles in general;

(4) FIG. 3a illustrates schematically the curves of drag coefficient and coefficient of loads in the chordal plane in static and dynamic conditions;

(5) FIG. 3b shows the decomposition into loads in the chordal plane and perpendicular to the chordal plane, and lift and drag polar curve; and

(6) FIGS. 4a-4c show cross-sectional views of examples of the present wind turbine blade.

DETAILED DESCRIPTION OF EMBODIMENTS

(7) The wind turbine blade 100 shown in FIGS. 4a-4c comprises an airfoil structure 110 with an airfoil shape. In the present design, the airfoil structure 110 does not need to be a closed structure as long as an inner space 115 is enclosed. Inner space 115 is suitable for receiving an internal support structure 120 therein. The internal support structure 120 is arranged spanwise along the length of the blade 100. The elastic connection is arranged at a location within the airfoil structure 110 such that the aerodynamic moment is at least close to zero for low wind conditions.

(8) In the examples shown, the internal support structure 120 is a longitudinal hollow beam, referred to as a spar-box, having a substantially rectangular cross-section. Other shapes in cross-section, such as circular or polygonal, may be used according to the requirements for the internal support structure, and in general it may have a closed or open cross section. Furthermore, the support structure 120 could be solid instead of hollow if required. In any case, one end of the support structure 120 is fixed to a blade root or a to blade root extender (not shown) of the wind turbine.

(9) As shown in FIGS. 4a and 4c, the internal support structure 120 is hinged to the airfoil structure 110 through an elastic hinge connection 130.

(10) In the example shown in FIG. 4a, the elastic connection 130 comprises a first connecting portion 140 that is fixed to or is part of a lower portion of the inner surface of the airfoil structure 110 (i.e. the inner surface of the pressure side of the airfoil), and a second connecting portion 150 that is fixed to or is part of a corresponding lower portion of the outer surface of the internal support structure 120. The first and second connecting portions 140, 150 of the elastic connection 130 are hinged to each other such that the first portion 140 can pivot with respect to the second portion 150. In this way, the internal support structure 120 and the airfoil structure 110 can be passively pitched relative to each other according to aerodynamic pressure distribution present at different locations of the blade 100. The hinge in this case may be a ball joint.

(11) In the example shown in FIG. 4b, the first connecting portion 140 of the elastic connection 130 is fixed to or is part of the inner surface of the airfoil structure 110 that substantially corresponds the blade leading edge, while the second connecting portion 150 is fixed to or is part of a forward portion of the outer surface of the internal support structure 120. The first and second connecting portions 140, 150 are hinged to each other such that the first portion 140 can pivot with respect to the second portion 150. In this way, the internal support structure 120 and the airfoil structure 110 can be passively pitched relative to each other according to aerodynamic pressure distribution present at different locations of the blade 100. Also in this case, the hinge may be a ball joint.

(12) The way in which stall is avoided by the blade according to this aspect may be further explained with reference to FIGS. 3a and 3b. FIG. 3a illustrates schematically the curves of C.sub.d and C.sub.c in static conditions (in interrupted lines) and in dynamic conditions (in continuous lines). In dynamic conditions, the drag increases quite rapidly before stall and drops off. The loads in the chordal plane, represented by coefficient C.sub.c show a peak in the negative direction before stall occurs. This peak is significantly more pronounced under dynamic conditions than under static conditions. This negative peak means that the blade experiences a forward load.

(13) Another way of looking at the behaviour of blade sections is by looking at the lift and drag polar curve (represented on the left hand side of FIG. 3b). Again, the static behaviour is represented by the interrupted line, whereas the dynamic behaviour (unsteady aerodynamics) is represented by the continuous line. On the right hand side of FIG. 3b, the decomposition into loads in the chordal plane C and perpendicular to the chordal plane N is used rather than lift L and drag D. The negative peak in the loads in the chordal plane (negative C.sub.c) mentioned earlier can be seen again quite clearly in this figure.

(14) Examples of blades according to the present disclosure are based on using the pronounced negative peak of loads in the chordal plane before stall occurs. The aerodynamic profile (skin of the blade) is thus pulled forwards with respect to the internal support structure before stall occurs. The elastic connection 130 between the internal support structure 120 and the airfoil structure 110 can ensure that this forward pull is translated into a local pitch movement of the blade section. The local angle of attack may thus be reduced and stall may be avoided or the negative effects of stall may at least be reduced. This is carried out without any active control of the blade 100.

(15) Within the inner space 115 defined inside the internal support structure 120 a suitable biasing mechanism 160 is provided. The biasing mechanism 160 acts between the airfoil structure 110 and the internal support structure 120. In the example shown in FIGS. 4a and 4c, the biasing mechanism 160 comprises a flexible foam surrounding the internal support structure 120. This flexible foam is fixed to the inner side of the airfoil structure 110 or it may be fitted between walls attached to of being part of the inner side of the airfoil structure 110.

(16) In the example shown in FIG. 4b, the biasing mechanism 160 comprises a number of springs arranged between the upper and lower sides of the internal support structure 120 and the upper and lower internal sides of the airfoil structure 110, respectively.

(17) In the example shown in FIG. 4c the biasing mechanism 160 comprises a flexible foam surrounding the internal support structure 120 and at least one elastic support element, such as a plate or spring, arranged between the lower external side of the internal support structure 120 and the lower internal side of the airfoil structure 110. In this particular example, the foam-spring connection between the airfoil structure 110 and the internal support structure 120 itself acts as an elastic hinge connection. Both in this case and in the example shown in FIG. 4a, the flexible foam is partially surrounding the internal support structure 120, although the foam could also be arranged completely surrounding the internal support structure 120. In any case, the flexible foam is arranged such that the airfoil structure 110 can be passively pitched relative to the internal support structure 120 according to the aerodynamic pressure distribution present at different locations of the blade 100.

(18) The blade 100 can be divided into sections along its length. A number of biasing mechanisms 160 having the same or different properties may be provided in said blade sections such that each can be resiliently twisted degrees locally by the action of the wind at an angle, for example, of the order of 1-2.

(19) Although only a number of particular embodiments and examples of the wind turbine blade have been disclosed herein, it will be understood by those skilled in the art that other alternative embodiments and/or uses and obvious modifications and equivalents thereof are possible. Furthermore, the present disclosure covers all possible combinations of the particular embodiments described. Thus, the scope of the present disclosure should not be limited by particular embodiments, but should be determined only by a fair reading of the claims that follow.