Abstract
A root assembly of a wind turbine blade for a wind turbine is provided. A wind turbine blade including the root assembly and a wind turbine including the wind turbine blade are also provided.
Claims
1. A root assembly of a wind turbine blade for a wind turbine, wherein (a) a root portion of the root assembly comprises a root segment, the root segment having a first centerline located in the center of a thickness of the root segment measured along a radial direction of the root segment and extending along a circumferential direction of the root segment, (b) a root attachment face of the root portion is attached to a bearing or a hub flange of the root assembly by multiple bolts or root inserts, the multiple bolts or root inserts being arranged with their centers along a second centerline extending along the circumferential direction of the root segment, wherein, the second centerline is offset from the first centerline.
2. The root assembly according to claim 1, wherein, the second centerline is offset from the first centerline in a direction towards an inside of the root portion.
3. The root assembly according to claim 1, wherein, the second centerline is offset from the first centerline by less than 15% of the thickness of the root segment.
4. The root assembly according to claim 1, wherein, the second centerline is offset from the first centerline by 0.5% to 5% of the thickness of the root segment.
5. The root assembly; according to claim 1, wherein, the first centerline extends through the multiple bolts or root inserts.
6. The root assembly according to claim 1, wherein, at least half of the multiple bolts or root inserts of the root assembly are arranged with their centers along the second centerline.
7. The root assembly according to claim 1, wherein, each of the multiple bolts is connected to one of multiple bushings, fixedly arranged within the root segment such that the multiple bolts are arranged adjacent to each other along a circumference of the root portion, and the bushings are arranged adjacent to each other along the circumference of the root portion, and adjacent bushings are offset from one another in a way such that adjacent bushings are provided at an axial distance from one another, the axial distance (d.sub.A) being measured in an axial direction from the root attachment face to the bushings and between centers of the adjacent bushings.
8. The root assembly according to claim 7, wherein, a quotient between the axial distance (d.sub.A) and a bushing diameter (d.sub.11) of the bushings is 2.5 or greater.
9. The root assembly according to claim 7, wherein, a quotient between the axial distance (d.sub.A) and a bushing diameter (d.sub.B) of the bushings is in the range of 2.5 to 5.
10. The root assembly according to claim 7, wherein, a quotient between the axial distance (d.sub.A) and a bushing diameter (d.sub.B) of the bushings is within the range of 2.7 to 4.8.
11. The root assembly according to claim 7, wherein, a quotient between the axial distance (d.sub.A) and a bushing diameter (d.sub.B) of the bushings is within the range of 3 to 4.5.
12. The root assembly according to claim 7, wherein, the multiple bolts have a first length or a second length, wherein the second length is greater than the first length, and wherein the bolts of the multiple bolts having the first length and the bolts of the multiple bolts having the second length are alternatingly connected to the adjacent offset bushings.
13. The root assembly according to claim 1, wherein, the multiple bolts are secured against the bearing or the hub flange by nuts.
14. A wind turbine blade comprising the root assembly according to claim 1.
15. The wind turbine comprising at least one wind turbine blade according to claim 14.
Description
BRIEF DESCRIPTION
[0025] Some of the embodiments will be described in detail, with references to the following Figures, wherein like designations denote like members, wherein:
[0026] FIG. 1 shows a side perspective view on a wind turbine;
[0027] FIG. 2 shows a side perspective view on a part of a root portion of a wind turbine blade according to a first embodiment:
[0028] FIG. 3 shows a side perspective view on a T-bolt:
[0029] FIG. 4 shows a cross-section view on a part of a root assembly of a wind turbine blade according to a second embodiment:
[0030] FIG. 5 shows a perspective view on a part of a root segment of the root assembly of FIG. 4:
[0031] FIG. 6 shows a side sectional view on the wind turbine blade having the root assembly of FIG. 4;
[0032] FIG. 7 shows a perspective view on a part of a T-bolt according to a design as generally used in the conventional art:
[0033] FIG. 8 shows a perspective view on a part of a T-bolt according to a design desirably used for the root assembly of FIG. 4;
[0034] FIG. 9 shows a strain profile diagram for the wind turbine blade of FIG. 6;
[0035] FIG. 10 shows a strain profile diagram for different combinations of radial distances between staggered rows of bolts and axial distances between the bushings:
[0036] FIG. 11 shows a strain profile diagram depending on the axial distance of bushings and the lateral distance of bushings for a given bushing diameter:
[0037] FIG. 12 shows a top view on a strain contours representation of the part of the root segment of FIG. 5 having as design parameter a first quotient between the axial distance of bushings and the bushing diameter; and
[0038] FIG. 13 shows a top view on a strain contours representation of the part of the root segment of FIG. 5 having as design parameter a second quotient between the axial distance of bushings and the bushing diameter.
DETAILED DESCRIPTION
[0039] FIG. 1 shows a wind turbine 1 according to an embodiment of the invention. The wind turbine 1 comprises a rotor 2 having three wind turbine blades 5.1, 5.2, 5.3 connected to a hub 3. However, the number of wind turbine blades 5 may be at least one wind turbine blade 5, two wind turbine blades 5 or more than three wind turbine blades 5 and chosen as required for a certain setup of a wind turbine 1.
[0040] The hub 3 is connected to a generator (not shown) arranged inside a nacelle 4. During operation of the wind turbine 1, the wind turbine blades 5 are driven by wind to rotate, and the wind's kinetic energy is converted into electrical energy by the generator in the nacelle 4.
[0041] The nacelle 4 is arranged at the upper end of a tower 8 of the wind turbine 1. The tower 8 is erected on a foundation 9 such as a monopile or tripile. The foundation 9 is connected to and/or driven into the ground or seabed.
[0042] Each of the wind turbine blades 5.1, 5.2, 5.3 has a root portion 6.1, 6.2. These root portions 6.1, 6.2 are connected to the hub 3 by bearings 7.1, 7.2 or hub flanges 7.1, 7.2. In this particular view, the root portion 6 and bearing 7 or hub flange 7 of the wind turbine blade 5.3 is covered by the hub 3.
[0043] FIG. 2 shows a side perspective view on a part of a root portion 6 of a wind turbine blade 5 according to a first embodiment. Multiple bolted connection means in the form of bushings 11 are arranged within cavities 64 located in the root portion 6 along the circumference of it. Bolts 10 are attached to the bushings 11. The bolts 10 may be attached to a hub flange 7 or bearing 7 as shown in FIG. 1. As an alternative to the bolts 10 and bushings 11, root inserts (not shown) may be used as connection means. Such root inserts may be bonded within the laminate material of the root portion 6. As an alternative to the arrangement of the bushings 11 in a single row along the root portion 6 as shown in FIG. 2, a second embodiment shown in FIG. 4 realizes a staggered configuration of the bolts 10 and bushings 11 and will be explained in more detail with reference to FIG. 4.
[0044] FIG. 3 shows a side perspective view on a bolt 10 with a bushing 11, generally referred to as a T-bolt when assembled together, and a nut 12. The bushing 11 has a cylindrical shape so as to positively fit into the cavities 64. It may be placed in corresponding cavities 64 within the root portion 6, as can be seen in FIG. 2. When the bolt 10 is secured by the bushing 11 within the root portion 6, and the hub flange 7 or bearing 7 is attached thereto, the root portion 6 may be secured to the hub flange 7 or bearing 7 by the nut 12.
[0045] FIG. 4 shows a cross-section view on a part of a root assembly 20 of a wind turbine blade according to a second embodiment. In FIG. 4, the root portion 6 is only shown with one root segment 61. However, depending on the design of the wind turbine blade 5, the root portion 6 may comprise two or more root segments 61. For example, in a butterfly design of a wind turbine blade 5, the root portion 6 typically consists of two root segments 61. In an integral design of a wind turbine blade 5, the root portion 6 may consist of multiple root segments 61 joined together at respective root segment interfaces. All root segments 61 of the root portion 6 of the wind turbine blade 5 may be designed as explained below with reference to FIG. 4 and the further figures of the drawings.
[0046] The root segment 61 shown in FIG. 4 comprises multiple staggered bushings 11.1 . . . 11.8 such that respectively adjacent bushings 11.1 . . . 11.8 are alternately located at two different distances d.sub.s1 and d.sub.s2 from a root attachment surface 63, at which the root portion 6 with its root segment 61 is attached to the bearing 7 or hub flange 7. The distances d.sub.s1 and d.sub.s2 of the bushings 11.1 . . . 11.8 are measured from the centers of the bushings 11 to the root attachment face 63 in the Z direction indicated in the coordinate system with coordinates X, Y, Z depicted in FIG. 4. The Z direction corresponds to a longitudinal or axial direction of the root portion 6 or wind turbine blade 5. The Y direction corresponds to a radial direction or thickness direction of the root portion 6, along which its thickness t may be measured (see FIG. 5). And the X direction corresponds to a circumferential direction along which the circumference of the root portion 6 or wind turbine blade 5 may be measured.
[0047] By the two different distances d.sub.s1 and d.sub.s2 of the bushings 11.1 . . . 11.8 from the root attachment face 63, the multiple bolts 10.1 . . . 10.8 are staggered, such that the bolts 10.1 . . . 10.8 alternatingly have a first length L. 1 and a second length L.2, the first length L. 1 being smaller than the second length L.2. Each of the bolts 10.1 . . . 10.8 is secured within the root segment 61 by a nut 12, thereby securing the bearing 7 or hub flange 7 to the root segments 61 and root portion 6 and securely fastening the wind turbine blade 5 having the root portion 6 to the hub 2 of the wind turbine 1.
[0048] As seen in FIG. 4, the adjacent bushings 11.1 . . . 11.8 alternatingly extend along a first staggered row S. 1 and a second staggered row S.2. The staggered rows S.1, S.2 extend through centers of the bushings 11.1 . . . 11.8 while running perpendicular to the Z direction or, in other words, running in the X direction. An axial distance d.sub.A between the staggered rows S. 1, S.2 or the centers of each one of the adjacent bushings 11.1 . . . 11.8 may be measured in the Z direction. The axial distance d.sub.A indicates the distance or spacing between two adjacent bushings 11.1 . . . 11.8 having the different distances d.sub.s1 and d.sub.s2 from the root attachment surface 63. Note that the axial distance d.sub.A is not the shortest distance between two adjacent ones of the bushings 11.1 . . . 11.8 but is measured in the Z direction or, in other words, perpendicular to the root attachment face 63 and from the center of one bushing 11 to the center of the other bushing 11. FIG. 4 further indicates a diameter d.sub.11 of the bushings 11.1 . . . 11.8 at exemplary bushing 11.1. All bushings 11.1 . . . 11.8 have the same diameter d.sub.11.
[0049] FIG. 5 shows a part of the root portion 61 in a perspective view not yet having the bolts 10 and bushings 11 inserted therein so as to connect to the hub flange 7 or bearing 7. Indicated are once again the staggered rows S.1, S.2 and the axial distance d.sub.A measured between the two rows S. 1, S.2 or, in other words, centers of adjacent bushings 11.1 . . . 11.5. The adjacent bushings 11.1 . . . 11.5 are not shown in FIG. 5: instead, their positions in respective cavities 64.1 . . . 64.5 configured for receiving the bushings 11.1 . . . 11.5 are shown. The centers of the cavities 64.1 . . . 64.5 correspond to the centers of the bushings 11.1 . . . 11.5.
[0050] Besides the axial distance d.sub.A, a further design parameter may be measured in the form of a lateral distance or spacing di between adjacent bushings 11.1 . . . 11.5 or cavities 64.1 . . . 64.5, which is shown in FIG. 5. The lateral distance di is measured between the centers of adjacent bushings 11.1 . . . 11.5 in the X direction, i.e., parallel to the root attachment face 63.
[0051] At the root attachment face 63, the respective cavities 62.1 . . . 62.5 configured for receiving the bolts 10 may be seen. Also, the root segment 61 may be seen with its entire above-mentioned thickness t. At half of the thickness t of the root segment 61 or, in other words, at the center or middle of the root segment 61 along its extension in the Y direction, a first centerline C.sub.62 may be drawn running along the circumferential direction X. This first centerline C.sub.62 separates the surface of the root attachment face 63 into two equally sized surfaces.
[0052] A second centerline C.sub.10 may be drawn extending in the circumferential direction X and in parallel to the first centerline C.sub.62. This second centerline C.sub.10 connects the centers of the adjacent bolts 10 or centers of the cavities 62.1 . . . 62.5 configured for receiving the bolts 10. In other words, the bolts 10 or cavities 62.1 . . . 62.5 are (radially) offset with their centers from the (thickness) center or middle of the root segment 61. This radial offset is defined by a radial distance d.sub.R measurable between the two centerlines C.sub.10, C.sub.62 in the radial direction Y.
[0053] Generally, a laminate of the wind turbine blades 5 must include the above-described cavities 62, 64 for the T-bolts formed by the bolts 10 and bushings 11. These cavities 62, 64 create stress concentrations which are magnified the closer together the cavities 62, 64 are spaced together. A root capacity of the root portion 6 defined by the maximum load that the root portion can carry is related to the number of T-bolts that are placed in the root portion 6 and is thus linked to the placement of the T-bolts around the root circumference, and stronger root portions 6 often need larger diameters to fit more bolts 10. Larger root diameters require larger hubs 2, and the production of a larger hub 2 is very expensive. Therefore, there is considerable interest in increasing root capacity without increasing the diameter of the root portion 6.
[0054] To increase the root capacity without also requiring an increase of the diameter of the root portion 6, the design of the root assembly 20 shown in FIG. 4 is improved by optimal choice of the above-identified design parameters, in particular axial distance d.sub.A, bushing diameter d.sub.11 and/or radial distance d.sub.R. This is explained in the following in more detail.
[0055] FIG. 6 shows a side sectional view on a wind turbine blade 5 having the root assembly 20 of FIG. 5. Past the bushings 11.1, 11.2 towards a tip of the wind turbine blade 5, a thickness of the wind turbine blade 5 is reduced through a tapered geometry to transition from the root portion 6 to a shell 51 of the wind turbine blade 5. This tapering offsets a plane of the first centerline C.sub.62 towards the outer surface of the blade 5, and subsequently, the point at which the root portion 6 is loaded differs between the hub 2 and tip end of the wind turbine blade 5. This may be seen from FIG. 6, where the applied load N is closer to the outer surface of the wind turbine blade 5 than the bolt 10. The result of this is that the outer surface of the wind turbine blade 5 carries more load and therefore experiences more strain. This strain bias through the laminate of the wind turbine blade 5 is disadvantageous because the laminate material is not loaded evenly.
[0056] The radial offset between the two centerlines C.sub.10, C.sub.62 shifts the cavities 62 and thereby the bolts 10 away from the outer blade surface and towards the inside or center of the root portion 6. By this shift, the strain profile through the thickness of the wind turbine blade 5 can be equalized. This equalization also comes with a reduction in strain that can be used to increase the root capacity.
[0057] FIG. 7 shows a perspective view on a part of a T-bolt according to a design as commonly used in the art. This T-bolt is designed symmetrically. This means that the bolt 10 is attached to the bushing 11 at a center or middle of the bushing 11. In this symmetric design of the T-bolt, a radial offset is not foreseen.
[0058] FIG. 8 shows a perspective view on a part of a T-bolt according to a design desirably used for the root assembly 20 of FIG. 4 with the radially offset cavities 62. The T-bolt has a non-symmetric design, meaning that the attachment point of the bolt 10 at the bushing 11 is offset from a center or middle of the bushing 11. This is indicated by the radially offset centerlines C.sub.10, C.sub.62 showing the respective positioning of the bolt 10 and the bushing 11 in the root segment 61.
[0059] It has been found that for a radial offset of 0?d.sub.R/t?0.08, a very good strain reduction may be achieved. This means that the position of the centers of the cavities 62 or bolts 10 is shifted towards the inner surface of the wind turbine blade 5 by a distance less than or equal to 8% of the thickness t of the (laminate) material of the root portion 6 of the wind turbine blade 5. In other words, the second centerline C.sub.10 is offset from the first centerline C.sub.62 by an amount equal to or less than 8% of the thickness t of the root segment 61.
[0060] FIG. 9 shows a diagram of the result of a simulation comparing normalized strain profiles ?z adjacent to the bushing through the thickness t (measured in the Y direction) of the laminate of the wind turbine blade 5 from the inside (surface) of the wind turbine blade 5 (Y/t=0) to the outside (surface) of the wind turbine blade 5 (Y/t=1). Three cases are illustrated in this diagram for the radial distance d.sub.R, namely d.sub.R=0 mm, d.sub.R=0.75 mm and d.sub.R=2 mm. The result of the last two is an equalization of the strain profile ?z through the laminate and a reduction in overall strain ?z in the root portion 6. A radial distance of d.sub.R=0.75 mm delivers the best result. In lab-scale, the radial offset has proven to be most beneficial for d.sub.R=0.1 to 0.3 mm. However, the exact radial distance d.sub.R that is most beneficial depends on the axial distance d.sub.A and the lateral distance d.sub.L.
[0061] This is explained in more detail with reference to FIG. 10 showing a diagram of maximum overall strains ?z for different combinations of radial distances d.sub.R and axial distances d.sub.A. In other words, FIG. 10 shows different design parameters of the root portion 6 and the overall strains ?z resulting therefrom.
[0062] When evaluating exactly which radial distance d.sub.R to use as design parameter, the reduction of the strain in the laminate of the wind turbine blade 5 must be considered. This is why the design parameters radial distance d.sub.R and axial distance d.sub.A desirably should be considered and chosen in combination. This is best illustrated in FIG. 10. Different combinations of radial distances d.sub.R and axial distances d.sub.A can be used in combination to achieve similarly low strain levels. The proposed radial offset of the T-bolts can either reduce the maximum strain in the laminate or achieve equivalent strain values at a lower axial distance or spacing d.sub.A Both behaviors are achieved by equalizing the strain profile (as seen in FIG. 9), but different radial distances d.sub.R may be used to achieve them.
[0063] However, when a larger axial distance d.sub.A is not possible, it is possible to achieve a further strain reduction at a set axial distance d.sub.A or an equivalent strain performance at a lower axial distance d.sub.A by radially offsetting the two centerlines C.sub.10, C.sub.62 from one another.
[0064] FIG. 11 shows a diagram of numerical strain results ?z depending on the axial distance d.sub.A and the lateral distance d.sub.L for a given bushing diameter d.sub.11. A quotient range of 2.7 to 4.3 between the axial distance d.sub.A and the bushing diameter d.sub.11 of the bushings 11 is indicated in the diagram of FIG. 11. With this quotient range, the distance between the two staggered rows S.1, S.2 of bushings 11 are increased to a region of optimal strain reduction as may be taken from the strain profile ?z corresponding thereto.
[0065] The main advantage of the increased axial spacing between the adjacent bushings 11 is lower strain levels around the bolts 10. This creates two key advantages: Firstly, the laminate of the root portion 6 can carry a higher load, and thus support longer wind turbine blades 5, and secondly, the bushings 11 can be spaced closer together (meaning laterally closing together or, in other words, reducing the lateral distance d.sub.L), allowing for more bolts 10 around the circumference of the root portion 6, which also allows for stronger and/or longer wind turbine blades 5 by way of lower loads per bolt 10.
[0066] FIG. 12 illustrates a top view on a strain contours representation of the part of the root segment 61 of FIG. 5 with a quotient of 2.3 between the axial distance d.sub.A and the bushing diameter d.sub.11.
[0067] FIG. 13 on the other hand illustrates a top view on a strain contours representation of the part of the root segment 61 of FIG. 5 with a quotient of 4.3 between the axial distance d.sub.A and the bushing diameter d.sub.11. The greater axial spacing of the bushings 11 significantly reduces the strain magnitude in the root segment 61 of FIG. 13 over the one of the root segment 61 of FIG. 12.
[0068] Although the present invention has been disclosed in the form of 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.
[0069] 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.
