ROOT ASSEMBLY OF A WIND TURBINE BLADE FOR A WIND TURBINE, WIND TURBINE BLADE AND WIND TURBINE

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.

FIELD OF TECHNOLOGY

The following relates to a root assembly of a wind turbine blade for a wind turbine, a wind turbine blade and a wind turbine.

BACKGROUND

Wind turbine blades are typically attached to the hub via one of two connection methods: T-bolts or root inserts. By these connection methods, large loads from the wind turbine blades to the hub must be transferred. The industry trend towards longer turbine blades increases these loads so that stronger roots are needed.

Root assemblies referred to herein are understood as an assembly comprising a root portion of a wind turbine blade connected to a bearing or a hub flange of the hub of the wind turbine. As explained above, typically, the bearing or hub flange is connected to the root portion of the wind turbine blade by multiple T-bolts, that is bolts secured within bushings (seeFIG.2), or by root inserts introduced into the laminate material of the root portion.

To resist the very high loads in large wind turbines having long wind turbine blades, a large number of bolts and bushings or inserts need to be provided in the root assembly. However, the space for the bolts and bushings or inserts on a common circumference of the root portion is limited. Therefore, it is known to provide the root assembly with a staggered configuration of the multiple bolts to increase the so-called root capacity, which indicates the number of T-bolts or root inserts in a root assembly.

However, the root portions, in particular when having a staggered configuration of bolts, still experience large strain and are therefore susceptible to failure.

SUMMARY

An aspect relates to an improved root assembly, wind turbine blade and wind turbine not having the previously described disadvantage, in particular having a long service life.

Further details of embodiments of the invention unfold from the other claims as well as the description and the drawings. Thereby, the features and details described in connection with the root assembly of embodiments of the invention apply in connection with the wind turbine blade of embodiments of the invention and with the wind turbine of embodiments of the invention, so that regarding the disclosure of the individual aspects of embodiments of the invention, it is or can be referred to one another.

According to a first aspect of embodiments of the invention, the aspect is solved by a root assembly of a wind turbine blade for a wind turbine. 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. 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. The second centerline is offset from the first centerline.

Embodiments of the invention are based on the finding that by offsetting the second centerline from the first centerline, whereby the bolts or root inserts are shifted away from the thickness center of the root segment either towards the inside or the outside of the wind turbine blade, the strain profile through the thickness of the wind turbine blade can be equalized. This equalization also comes with a reduction in strain that can be used to increase the root capacity related to the number of bolts or root inserts placed in the root portion. In this regard, the centerlines represent geometric lines that can be drawn in the root assembly to determine the offset. In particular, a radial distance may be measured between both centerlines, indicating the amount or size of the offset. The centerlines must not actually be drawn or be visible in the root assembly but merely be imaginary or drawable by the instruction given herein, meaning that the first centerline is drawn in the center of the thickness of the root segment measured along the radial direction of the root segment and extending along the circumferential direction of the root segment, and the second centerline is drawn through the centers of the bolts or root inserts or cavities, in which they are inserted, along the circumferential direction of the root segment.

Depending on the design of the wind turbine blade, the root portion may comprise one or more root segments. For example, in a butterfly design of a wind turbine blade, the root portion typically consists of two root segments. In an integral design of a wind turbine blade, the root portion may consist of multiple root segments joined together at respective root segment interfaces. The root segments may in particular have a round shape, i.e., be rounded and moreover in particular have a partially circular or elliptical shape. In other words, the root segments may form an arc or have an arc shape. The root segments may have equal or different arc lengths among them. Thereby, a cylindrically shaped root portion of the wind turbine blade may be provided. The cross section of the root portion may have a circular or an elliptical shape. The root segments may be reinforcement blocks (such as pre-cured laminate blocks) that are placed on or within a shell laminate of the wind turbine blade during manufacture of the shell, or they may simply be reinforced areas of the shell constructed by additional layers placed and cured together with the rest of the shell. The root segments may be manufactured from a fiber composite material, in particular a fiber composite lay-up. Accordingly, the material of the root segment may also be referred to as a laminate. The fiber composite material may have glass fibers and/or carbon fibers, for example.

In particular, the second centerline may be offset from the first centerline in a direction towards an inside of the root portion. This is desirable because past the bushings and towards the blade tip of the wind turbine blade, the thickness of the root portion is typically reduced through a tapered geometry to transition from the root region to the shell of the wind turbine blade. This tapering offsets the first centerline towards the outer surface of the wind turbine blade, and subsequently, the point at which the root portion is loaded differs between the hub and tip ends. The result of this is that the outer surface of the wind turbine blade carries more load and thus more strain. This strain bias through the laminate is disadvantageous because the material is not loaded evenly. By offsetting the second centerline towards the inside of the root portion or, in other words, towards the center (radially) of the root portion or wind turbine blade, the strain bias can be compensated for so as to reduce the strain on the outer surface and provide the above-mentioned equalization of strain in the wind turbine blade.

Further, the second centerline may be offset from the first centerline by less than 15%, in particular less than 10%, of the thickness of the root segment. It has been found that a radial offset larger than that is detrimental to equalizing the strain profile of the wind turbine blade.

Moreover, the second centerline may be offset from the first centerline by 0.5% to 5%, in particular 1% to 3%, of the thickness of the root segment. The equalization of the strain profile through the laminate of the wind turbine blade and the reduction in overall strain in the root portion has been found to be most desirably in this area.

Also, it may be provided that the first centerline extends through the multiple bolts or root inserts. In this case, the first centerline does not extend through the centers of the bolts or root inserts but offset from their centers and still close enough to the second centerline such that the first centerline extends through the bolts or root inserts. In this case, the radial distance between the two centerlines cannot be greater than half of the diameter of the bolts, root inserts or cavities, in which they are inserted.

It may be provided that at least half or more than half, at least two-thirds or more than two-thirds or all of the multiple bolts or root inserts of the root assembly are arranged with their centers along the second centerline. Accordingly, there may only be one row of bolts or root inserts on the root attachment face along which the bolts or root inserts are introduced into the root segment. The provision of, for example, two or more rows of bolts or root inserts at the root attachment face would introduce further strain between the bolts and can thereby be avoided.

Each of the multiple bolts may be 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 that the bushings are arranged adjacent to each other along the circumference of the root portion. Adjacent bushings may be offset from one another in a way such that adjacent bushings are provided at an axial distance from one another, the axial distance being measured in an axial direction from the root attachment face towards the bushings (or, in other words, the tip of the blade) and between centers of the adjacent bushings. The combination of bolt and bushing is commonly referred to as a T-bolt. In particular, the bolts may have threads on an outer circumference thereof. By these outer threads, they may be interlocked with inner threads of the bushings. By the axial spacing of adjacent bushings, a so-called staggered configuration of t-bolts may be achieved. The staggered configuration of t-bolts is characterized by adjacent bushings being spaced apart from one another in the axial direction or, in other words, being alternatingly located at different distances from the root attachment surface. Accordingly, adjacent bolts alternatingly have a different length to realize the staggered configuration. The staggered configuration allows an increase of root capacity in the axial direction without increasing the spacing in the circumferential direction of the root segment.

However, the staggered configuration still has some drawbacks. In particular, a root segment having a quotient between the axial distance and a bushing diameter of the bushings of 2.3 or less results in a high strain profile that is disadvantageous with respect to the service life of the wind turbine blade.

Therefore, a quotient between the axial distance and a bushing diameter of the bushings should be 2.5 or greater. A lateral spacing in a circumferential direction of the root segments between bolts is typically set by external parameters: however, the axial distance or, in other words, axial spacing between the centers of the bolts or cavities, in which the bolts are fitted, can be adjusted. By changing the axial distance in dependence to the bushing diameter as described herein, the strain in the laminate of the root portion can be reduced. Thereby, the distance between the two staggered rows of bolts may be optimized to a region of optimal strain reduction. Accordingly, a root bolt pattern using alternating near and far bolts may be implemented. Or, in other words, a root bolt pattern having inner bolts located in a first staggered row and outer bolts in a second staggered row, with an axial distance between the rows being according to a quotient between the axial distance and bushing diameter of the bushings of at least 2.5, may be implemented. For calculating the quotient, it is sufficient to calculate the quotient based on the axial distance of merely one pair of adjacent bushings with respect to one bushing diameter of one bushing of this pair of adjacent bushings. In an embodiment, most or all of the bushings have the same diameter. In an embodiment, the axial distance between adjacent bushings is the same for most or all pairs of adjacent bushings.

In an embodiment, the quotient between the axial distance and the bushing diameter of the bushings may be in the range of 2.5 to 5. In an embodiment, the quotient between the axial distance and the bushing diameter of the bushings may be within the range of 2.7 to 4.8. In an embodiment, the quotient between the axial distance and the bushing diameter of the bushings may be within the range of 3 to 4.5 or 3.5 to 4.5. This quotient range has been found to be optimal for the laminate of the root segment to become able to carry a higher load, and for the root portion having the root segment or root segments thereby becoming able to support a longer wind turbine blade, and for spacing the bushings closer together, allowing for more bolts around the circumference of the root portion, which also allows for stronger and/or longer wind turbine blades.

The multiple bolts may 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. The multiple bolts may be secured against the bearing or the hub flange by nuts. This is a particularly simple and easy way of securing the bearing or hub flange to the root portion.

According to a second aspect of embodiments of the invention, the aspect is solved by a wind turbine blade comprising the root assembly according to the first aspect of embodiments of the invention.

According to a third aspect of embodiments of the invention, the aspect is solved by a wind turbine comprising at least one wind turbine blade according to the second aspect of embodiments of the invention.

The wind turbine may be a direct drive wind turbine or a geared wind turbine, for example. Further, the at least one wind turbine blade may be mounted on an outer ring of a pitch bearing of the wind turbine or on an inner ring of the pitch bearing.

DETAILED DESCRIPTION

FIG.1shows a wind turbine1according to an embodiment of the invention. The wind turbine1comprises a rotor2having three wind turbine blades5.1,5.2,5.3connected to a hub3. However, the number of wind turbine blades5may be at least one wind turbine blade5, two wind turbine blades5or more than three wind turbine blades5and chosen as required for a certain setup of a wind turbine1.

The hub3is connected to a generator (not shown) arranged inside a nacelle4. During operation of the wind turbine1, the wind turbine blades5are driven by wind to rotate, and the wind's kinetic energy is converted into electrical energy by the generator in the nacelle4.

The nacelle4is arranged at the upper end of a tower8of the wind turbine1. The tower8is erected on a foundation9such as a monopile or tripile. The foundation9is connected to and/or driven into the ground or seabed.

Each of the wind turbine blades5.1,5.2,5.3has a root portion6.1,6.2. These root portions6.1,6.2are connected to the hub3by bearings7.1,7.2or hub flanges7.1,7.2. In this particular view, the root portion6and bearing7or hub flange7of the wind turbine blade5.3is covered by the hub3.

FIG.2shows a side perspective view on a part of a root portion6of a wind turbine blade5according to a first embodiment. Multiple bolted connection means in the form of bushings11are arranged within cavities64located in the root portion6along the circumference of it. Bolts10are attached to the bushings11. The bolts10may be attached to a hub flange7or bearing7as shown inFIG.1. As an alternative to the bolts10and bushings11, root inserts (not shown) may be used as connection means. Such root inserts may be bonded within the laminate material of the root portion6. As an alternative to the arrangement of the bushings11in a single row along the root portion6as shown inFIG.2, a second embodiment shown inFIG.4realizes a staggered configuration of the bolts10and bushings11and will be explained in more detail with reference toFIG.4.

FIG.3shows a side perspective view on a bolt10with a bushing11, generally referred to as a T-bolt when assembled together, and a nut12. The bushing11has a cylindrical shape so as to positively fit into the cavities64. It may be placed in corresponding cavities64within the root portion6, as can be seen inFIG.2. When the bolt10is secured by the bushing11within the root portion6, and the hub flange7or bearing7is attached thereto, the root portion6may be secured to the hub flange7or bearing7by the nut12.

FIG.4shows a cross-section view on a part of a root assembly20of a wind turbine blade according to a second embodiment. InFIG.4, the root portion6is only shown with one root segment61. However, depending on the design of the wind turbine blade5, the root portion6may comprise two or more root segments61. For example, in a butterfly design of a wind turbine blade5, the root portion6typically consists of two root segments61. In an integral design of a wind turbine blade5, the root portion6may consist of multiple root segments61joined together at respective root segment interfaces. All root segments61of the root portion6of the wind turbine blade5may be designed as explained below with reference toFIG.4and the further figures of the drawings.

The root segment61shown inFIG.4comprises multiple staggered bushings11.1. . .11.8such that respectively adjacent bushings11.1. . .11.8are alternately located at two different distances ds1and ds2from a root attachment surface63, at which the root portion6with its root segment61is attached to the bearing7or hub flange7. The distances ds1and ds2of the bushings11.1. . .11.8are measured from the centers of the bushings11to the root attachment face63in the Z direction indicated in the coordinate system with coordinates X, Y, Z depicted inFIG.4. The Z direction corresponds to a longitudinal or axial direction of the root portion6or wind turbine blade5. The Y direction corresponds to a radial direction or thickness direction of the root portion6, along which its thickness t may be measured (seeFIG.5). And the X direction corresponds to a circumferential direction along which the circumference of the root portion6or wind turbine blade5may be measured.

By the two different distances ds1and ds2of the bushings11.1. . .11.8from the root attachment face63, the multiple bolts10.1. . .10.8are staggered, such that the bolts10.1. . .10.8alternatingly have a first length L.1and a second length L.2, the first length L.1being smaller than the second length L.2. Each of the bolts10.1. . .10.8is secured within the root segment61by a nut12, thereby securing the bearing7or hub flange7to the root segments61and root portion6and securely fastening the wind turbine blade5having the root portion6to the hub2of the wind turbine1.

As seen inFIG.4, the adjacent bushings11.1. . .11.8alternatingly extend along a first staggered row S.1and a second staggered row S.2. The staggered rows S.1, S.2extend through centers of the bushings11.1. . .11.8while running perpendicular to the Z direction or, in other words, running in the X direction. An axial distance dAbetween the staggered rows S.1, S.2or the centers of each one of the adjacent bushings11.1. . .11.8may be measured in the Z direction. The axial distance dAindicates the distance or spacing between two adjacent bushings11.1. . .11.8having the different distances ds1and ds2from the root attachment surface63. Note that the axial distance dAis not the shortest distance between two adjacent ones of the bushings11.1. . .11.8but is measured in the Z direction or, in other words, perpendicular to the root attachment face63and from the center of one bushing11to the center of the other bushing11.FIG.4further indicates a diameter d11of the bushings11.1. . .11.8at exemplary bushing11.1. All bushings11.1. . .11.8have the same diameter d11.

FIG.5shows a part of the root portion61in a perspective view not yet having the bolts10and bushings11inserted therein so as to connect to the hub flange7or bearing7. Indicated are once again the staggered rows S.1, S.2and the axial distance dAmeasured between the two rows S.1, S.2or, in other words, centers of adjacent bushings11.1. . .11.5. The adjacent bushings11.1. . .11.5are not shown inFIG.5: instead, their positions in respective cavities64.1. . .64.5configured for receiving the bushings11.1. . .11.5are shown. The centers of the cavities64.1. . .64.5correspond to the centers of the bushings11.1. . .11.5.

Besides the axial distance dA, a further design parameter may be measured in the form of a lateral distance or spacing di between adjacent bushings11.1. . .11.5or cavities64.1. . .64.5, which is shown inFIG.5. The lateral distance di is measured between the centers of adjacent bushings11.1. . .11.5in the X direction, i.e., parallel to the root attachment face63.

At the root attachment face63, the respective cavities62.1. . .62.5configured for receiving the bolts10may be seen. Also, the root segment61may be seen with its entire above-mentioned thickness t. At half of the thickness t of the root segment61or, in other words, at the center or middle of the root segment61along its extension in the Y direction, a first centerline C62may be drawn running along the circumferential direction X. This first centerline C62separates the surface of the root attachment face63into two equally sized surfaces.

A second centerline C10may be drawn extending in the circumferential direction X and in parallel to the first centerline C62. This second centerline C10connects the centers of the adjacent bolts10or centers of the cavities62.1. . .62.5configured for receiving the bolts10. In other words, the bolts10or cavities62.1. . .62.5are (radially) offset with their centers from the (thickness) center or middle of the root segment61. This radial offset is defined by a radial distance dRmeasurable between the two centerlines C10, C62in the radial direction Y.

Generally, a laminate of the wind turbine blades5must include the above-described cavities62,64for the T-bolts formed by the bolts10and bushings11. These cavities62,64create stress concentrations which are magnified the closer together the cavities62,64are spaced together. A root capacity of the root portion6defined by the maximum load that the root portion can carry is related to the number of T-bolts that are placed in the root portion6and is thus linked to the placement of the T-bolts around the root circumference, and stronger root portions6often need larger diameters to fit more bolts10. Larger root diameters require larger hubs2, and the production of a larger hub2is very expensive. Therefore, there is considerable interest in increasing root capacity without increasing the diameter of the root portion6.

To increase the root capacity without also requiring an increase of the diameter of the root portion6, the design of the root assembly20shown inFIG.4is improved by optimal choice of the above-identified design parameters, in particular axial distance dA, bushing diameter d11and/or radial distance dR. This is explained in the following in more detail.

FIG.6shows a side sectional view on a wind turbine blade5having the root assembly20ofFIG.5. Past the bushings11.1,11.2towards a tip of the wind turbine blade5, a thickness of the wind turbine blade5is reduced through a tapered geometry to transition from the root portion6to a shell51of the wind turbine blade5. This tapering offsets a plane of the first centerline C62towards the outer surface of the blade5, and subsequently, the point at which the root portion6is loaded differs between the hub2and tip end of the wind turbine blade5. This may be seen fromFIG.6, where the applied load N is closer to the outer surface of the wind turbine blade5than the bolt10. The result of this is that the outer surface of the wind turbine blade5carries more load and therefore experiences more strain. This strain bias through the laminate of the wind turbine blade5is disadvantageous because the laminate material is not loaded evenly.

The radial offset between the two centerlines C10, C62shifts the cavities62and thereby the bolts10away from the outer blade surface and towards the inside or center of the root portion6. By this shift, the strain profile through the thickness of the wind turbine blade5can be equalized. This equalization also comes with a reduction in strain that can be used to increase the root capacity.

FIG.7shows 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 bolt10is attached to the bushing11at a center or middle of the bushing11. In this symmetric design of the T-bolt, a radial offset is not foreseen.

FIG.8shows a perspective view on a part of a T-bolt according to a design desirably used for the root assembly20ofFIG.4with the radially offset cavities62. The T-bolt has a non-symmetric design, meaning that the attachment point of the bolt10at the bushing11is offset from a center or middle of the bushing11. This is indicated by the radially offset centerlines C10, C62showing the respective positioning of the bolt10and the bushing11in the root segment61.

It has been found that for a radial offset of 0≤dR/t≤0.08, a very good strain reduction may be achieved. This means that the position of the centers of the cavities62or bolts10is shifted towards the inner surface of the wind turbine blade5by a distance less than or equal to 8% of the thickness t of the (laminate) material of the root portion6of the wind turbine blade5. In other words, the second centerline C10is offset from the first centerline C62by an amount equal to or less than 8% of the thickness t of the root segment61.

FIG.9shows a diagram of the result of a simulation comparing normalized strain profiles εz adjacent to the bushing through the thickness t (measured in the Y direction) of the laminate of the wind turbine blade5from the inside (surface) of the wind turbine blade5(Y/t=0) to the outside (surface) of the wind turbine blade5(Y/t=1). Three cases are illustrated in this diagram for the radial distance dR, namely dR=0 mm, dR=0.75 mm and dR=2 mm. The result of the last two is an equalization of the strain profile εz through the laminate and a reduction in overall strain εz in the root portion6. A radial distance of dR=0.75 mm delivers the best result. In lab-scale, the radial offset has proven to be most beneficial for dR=0.1 to 0.3 mm. However, the exact radial distance dRthat is most beneficial depends on the axial distance dAand the lateral distance dL.

This is explained in more detail with reference toFIG.10showing a diagram of maximum overall strains εz for different combinations of radial distances dRand axial distances dA. In other words,FIG.10shows different design parameters of the root portion6and the overall strains εz resulting therefrom.

When evaluating exactly which radial distance dRto use as design parameter, the reduction of the strain in the laminate of the wind turbine blade5must be considered. This is why the design parameters radial distance dRand axial distance dAdesirably should be considered and chosen in combination. This is best illustrated inFIG.10. Different combinations of radial distances dRand axial distances dAcan 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 dABoth behaviors are achieved by equalizing the strain profile (as seen inFIG.9), but different radial distances dRmay be used to achieve them.

However, when a larger axial distance dAis not possible, it is possible to achieve a further strain reduction at a set axial distance dAor an equivalent strain performance at a lower axial distance dAby radially offsetting the two centerlines C10, C62from one another.

FIG.11shows a diagram of numerical strain results εz depending on the axial distance dAand the lateral distance dLfor a given bushing diameter d11. A quotient range of 2.7 to 4.3 between the axial distance dAand the bushing diameter d11of the bushings11is indicated in the diagram ofFIG.11. With this quotient range, the distance between the two staggered rows S.1, S.2of bushings11are increased to a region of optimal strain reduction as may be taken from the strain profile εz corresponding thereto.

The main advantage of the increased axial spacing between the adjacent bushings11is lower strain levels around the bolts10. This creates two key advantages: Firstly, the laminate of the root portion6can carry a higher load, and thus support longer wind turbine blades5, and secondly, the bushings11can be spaced closer together (meaning laterally closing together or, in other words, reducing the lateral distance dL), allowing for more bolts10around the circumference of the root portion6, which also allows for stronger and/or longer wind turbine blades5by way of lower loads per bolt10.

FIG.12illustrates a top view on a strain contours representation of the part of the root segment61ofFIG.5with a quotient of 2.3 between the axial distance dAand the bushing diameter d11.

FIG.13on the other hand illustrates a top view on a strain contours representation of the part of the root segment61ofFIG.5with a quotient of 4.3 between the axial distance dAand the bushing diameter d11. The greater axial spacing of the bushings11significantly reduces the strain magnitude in the root segment61ofFIG.13over the one of the root segment61ofFIG.12.