A wind turbine rotor blade root extension

Abstract

A wind turbine rotor blade root extension having an inner end configured to be mounted to a wind turbine rotor hub and an outer end configured to be mounted to a wind turbine rotor blade main part, wherein the root extension comprises a plurality of end-to-end steel assemblies each having a first thread at the inner end and a second thread at the outer end, characterized in that the steel assemblies are fixedly embedded in a fiber-reinforced composite material.

Description

[0001] A wind turbine rotor blade root extension

[0002] A wind turbine rotor blade root extension is a longitudinal segment of a wind turbine rotor blade configured to be connected between a wind turbine rotor hub and a main part of a wind turbine rotor blade.

[0003] US 7,393, 184 B 1 discloses a method for making a wind turbine rotor blade by stacking a plurality of modular segments having formed conduits and tensioning cables guided through the conduits.

[0004] EP 2 108 819 A2 discloses a wind turbine rotor blade extension with a hollow, cylindrical concrete body mounted between a wind turbine rotor blade root and a wind turbine rotor blade hub. The concrete body is preloaded by bolts guided through plastic tubes used as space-holders when casting the concrete body.

[0005] US 8,337,161 B2 discloses a wind turbine rotor blade extension having a hollow, corrugated metal cylinder with flanges at both ends. The blade extension is mounted between a wind turbine rotor blade hub and a root end of a wind turbine rotor blade by means of long, pre-loaded bolts guided through both flanges. The bolts have one end screwed into the root end of the wind turbine rotor blade and another end locked behind a blade bearing with a nut.

[0006] EP 2 363 601 A2 discloses a wind turbine rotor hub having a connecting body made of a composite material for each rotor blade. One end of the connecting body is fastened to a drive shaft, the other end is connected to a wind turbine rotor blade.

[0007] EP 2 758 657 Bl discloses a wind turbine rotor blade having longitudinal sections connected to one another. Between the sections, box-shaped structures are arranged comprising two rib-shaped bearing plates and a number of tubes therebetween. The bearing plates are clamped together by clamping rods passed through the tubes. EP 2 735 732 A2 discloses a coupling element of a wind turbine rotor blade arranged between a wind turbine rotor blade hub and a wind turbine rotor blade section. It comprises carbon fiber spar caps arranged in a longitudinal direction.

[0008] EP 2 905 464 Al discloses a root end device for a wind turbine rotor blade. The root end device is arranged between a wind turbine rotor blade hub and a blade section of a wind turbine rotor blade. Tension elements are guided through the root end device from end to end to transfer a tension force between the hub and the blade section.

[0009] WO 2024 / 132069 Al discloses a root extender positioned between a root end of the wind turbine blade and a mounting structure of a wind turbine rotor hub. The root extender comprises a plurality of pipes and a plurality of elongate tension elements connecting the root end of the blade to the mounting structure. The pipes are held in compression between the root end of the blade and the mounting structure. The pipes are made of steel and are arranged in a body of a composite material.

[0010] Departing therefrom, it is an object of the invention to provide a wind turbine rotor blade root extension that has superior mechanical characteristics and is easy to manufacture, as well as a method of manufacturing such a wind turbine rotor blade root extension.

[0011] This object is solved by the wind turbine rotor blade root extension of claim 1 and by the method of claim 22. Aspects of the invention are given in the dependent claims.

[0012] The wind turbine rotor blade root extension has an inner end configured to be mounted to a wind turbine rotor hub and an outer end configured to be mounted to a wind turbine rotor blade main part, wherein the root extension comprises a plurality of end- to-end steel assemblies each having a first thread at the inner end and a second thread at the outer end, wherein the steel assemblies are fixedly embedded in a fiber- reinforced composite material.

[0013] The root extension is adapted to be used in a wind turbine having a wind turbine rotor, in particular a wind turbine rotor with a horizontal axis. The wind turbine rotor has a wind turbine rotor hub and one or more wind turbine rotor blades. Each of these rotor blades comprises a wind turbine rotor blade main part and the root extension mounted with its outer end to the wind turbine rotor blade main part. Therefore, the root extension forms an integral part of the wind turbine rotor blade. An outer surface of the root extension forms a part of the aerodynamic surface of the wind turbine rotor blade. The inner end of the root extension is mounted to the wind turbine rotor hub. Typically, the wind turbine rotor will have three wind turbine rotor blades each having a wind turbine rotor blade main part and a root extension mounted between the wind turbine rotor blade main part and the wind turbine rotor hub.

[0014] The root extension may have a length, measured along a longitudinal direction between the inner end and the outer end, in the range of for example 2 m to 20 m, preferably in the range of 4 m to 10 m. The root extension is a hollow, tubular structure. The cross-section may be circular, in particular at the inner end. The crosssection may vary along the longitudinal direction of the root extension, so that the root extension may have, for example, a conical shape. The cross-section may also be constant, so that the root extension has a cylindrical shape. At the outer end, the crosssection of the root extension may also be circular, but it may also be for example elliptical.

[0015] The root extension needs to transfer substantial forces from the wind turbine rotor blade main part to the wind turbine rotor hub. These include weight as well as aerodynamic forces, with static and peak components altogether causing substantial extreme and fatigue loads on the root extension.

[0016] ,.. / 4 In accordance with the invention, these loads will be carried by the combination of the steel assemblies and the surrounding fiber-reinforced composite material. For example, the steel assemblies may carry about 30 % to 40 % of the loads and the fiber- reinforced composite material (in particular when the fiber-reinforced composite material comprises unidirectional fibers) may carry about 60 % to 70 % of the loads. With these ranges, an optimal ratio between stiffness and weight can be achieved. The steel assemblies can be aligned with a longitudinal direction of the root extension. The steel assemblies can be arranged side by side, wherein neighboring steel assemblies typically will not get in direct contact with one another because some of the fiber- reinforced composite material is arranged therebetween. The root extension may have any number of the end-to-end steel assemblies, for example in the range of 100 to 200. The end-to-end steel assemblies may be the only elements of the root extension directly connected to the wind turbine rotor blade main part and the wind turbine rotor hub.

[0017] The steel assemblies are end-to-end, that means they extend from the inner end to the outer end of the root extension. They have a first thread at the inner end for connection to the wind turbine rotor hub and a second thread at the outer end for connection to the main wind turbine rotor blade main part. Each steel assembly therefore forms a direct load path between the wind turbine rotor blade main part and the wind turbine rotor hub. This load path may accept tensile forces, but may also accept compression forces because it is fixedly embedded in the fiber-reinforced composite material, so that the position and a straight alignment of the steel assemblies will be maintained under compression loads. The combination of the steel assemblies and the fiber- reinforced composite material is also well suited to accept torsional loads. Overall, the root extension has superior strength at lower weight and lower costs as compared to conventional root extenders. The connections between the inner end of the root extension and the wind turbine rotor hub will be made via the first threads. To this end, the steel assemblies may have a front face near the first thread facing the wind turbine rotor hub which is tensioned against a counter face at the wind turbine rotor hub by a fastening member cooperating with the first thread, for example a nut or a bolt. In this way, each steel assembly is in direct and secure connection to the wind turbine rotor hub. The front face of the steel assembly may protrude from the surrounding fiber-reinforced composite material, so that the fiber-reinforced composite material may not get damaged when connecting the steel assembly via the first thread to the wind turbine rotor hub. A similar design can be applied at the outer end of the root extension. Here, the steel assemblies near the second threads may each have a front face facing towards the wind turbine rotor blade main part and tensioned against a counter face at the wind turbine rotor blade main part. This front face may also protrude from the surrounding fiber-reinforced composite material. The counter face at the wind turbine rotor blade main part may be provided for example by a connection part such as a steel plate or flange connected to or integrated into the wind turbine rotor blade main part or placed between the outer end of the root extension and the wind turbine rotor blade main part.

[0018] The steel assemblies may be fixedly embedded in the fiber-reinforced composite material along their entire length, or along at least a major part thereof, such as along more than 70%, more than 80% or more than 90%. The first and second threads and possibly any contact surfaces for connecting the steel assemblies to the wind turbine rotor blade main part and to the wind turbine rotor hub, respectively (such as the front faces described above), will typically not be covered by the fiber-reinforced composite material. Shoud a steel assembly consist of more than one part, the feature that the steel assembly is fixedly embedded in the fiber-reinforced composite material means that at least those parts of the steel assembly that comprise the first and second threads cannot be moved with reference to the fiber-reinforced composite mateiral.

[0019] ...16 The fiber-reinforced composite material comprises reinforcing fibers and a polymer. In addition, other materials such as a core material or pultruded profiles may form an integral part of the fiber-reinforced composite material. The fiber-reinforced composite material may also have a sandwich construction comprising an inner skin and an outer skin and a core material arranged there between.

[0020] In an aspect, the plurality of steel assemblies accounts for 25 % to 50 % of a total weight of the root extension. An even more preferred range is from 35 % to 45 %. The remainder of the total weight is contributed by the fiber-reinforced composite material, whereas a minor part of the total weight may also be contributed by other elements, some of which will be mentioned below. Forming 25 % to 50 % of the total weight of the root extension by the steel assemblies leads to a good compromise between strength, weight and costs.

[0021] In an aspect, the first thread is an inner thread and / or the second thread is an inner thread. The respective connection to the wind turbine rotor hub and / or the wind turbine rotor blade main part can then be made via bolts comprising an outer thread, comparable to a conventional connection of a wind turbine rotor blade root to a wind turbine rotor hub by means of embedded steel bushings having inner threads. The front faces mentioned above, which will be brought in contact with counter faces at the wind turbine rotor blade main part (or a connection part described above) and the wind turbine rotor hub, respectively, will typically be formed by ring-shaped surfaces surrounding an opening, in particular a bore, in which the inner thread is formed.

[0022] In general, the first threads and / or the second threads may also be outer threads. In this case, the steel assemblies will have a bolt-like section at the inner and / or outer end which can, for example, be locked with a nut behind a flange of the wind turbine rotor blade main part (or of a connection part described above) and / or the wind turbine rotor hub. When using an outer thread, the front faces explained above may also be ,.. / 7 ring-shaped surfaces surrounding a lower diameter section provided with the respective thread.

[0023] In an aspect, at least one of the steel assemblies has a first diameter at the inner end and / or at the outer end and a second diameter in an intermediate section, wherein the second diameter is smaller than the first diameter. Along the length of the steel assemblies, the (outer) diameter may change in a continuous or stepwise manner. In particular, the steel assemblies may comprise a conical segment at the inner end and / or at the outer end. A reduced diameter in an intermediate section helps obtaining high strength at low weight and / or costs.

[0024] In an aspect, at least one of the steel assemblies is wrapped in a layer of reinforcing fibers. In particular, the layer of reinforcing fibers may be an unidirectional layer (UD- layer) comprising or consisting of reinforcing fibers aligned with the longitudinal direction of the respective steel assembly. Wrapping a steel assembly in a layer of reinforcing fibers leads to a strong bond between the steel assembly and the surrounding composite material. It also adds to the strength of the composite material.

[0025] In an aspect, the layer extends from the inner end to the outer end. A layer extending over the entire length also adds to a strong bond between the steel assembly and the surrounding composite material, as well as to the strength of the fiber-reinforced material.

[0026] In an aspect, the layer has been cut to shape before the at least one steel assembly is wrapped in the layer such that the steel assembly in combination with the layer wrapped thereabout has a constant outer diameter. The shape of the layer may include a trapezoidal section wrapped about the steel assembly beginning at one of the parallel edges of the trapezoidal section. The diagonal arrangement of the remaining edges of the trapezoidal section then, when wrapped about the steel assembly, compensate for

[0027] ,.. / 8 changes in the outer diameter of the steel assembly, in particular when the same comprises an intermediate section with smaller diameter and conical sections extending therefrom at both ends. The constant outer diameter of the wrapped steel assembly makes it easy to integrate the wrapped steel assemblies in the root extension.

[0028] In an aspect, protruded profiles forming a tubular structure are arranged about the intermediate section. The pultruded profiles are comprised of aligned reinforcing fibers embedded in a cured polymer. A tubular structure of pultruded profiles adds to the strength of the root extension and also leads to a particularly strong fixation of the steel assembly within the fiber-reinforced composite material.

[0029] In an aspect the tubular structure has an outer diameter corresponding to the first diameter. In this way, the varying diameter of the steel assembly is compensated by the tubular structure. An inner diameter of the tubular structure may correspond to the second diameter. It is also possible to combine a tubular structure formed from pultruded profiles with a layer of reinforcing fibers wrapped about the tubular structure. The tubular structure may consist of more than one part. It can be divided in a longitudinal direction and / or in a direction rectangular to the longitudinal direction.

[0030] In an aspect, the steel assemblies each include a tubular section extending over at least 50% of a total length of the steel assembly. The tubular section is hollow. The tubular section may also extend over at least 80% of the total length of the steel assembly or over the entire length of the steel assembly. A tubular section helps obtaining high strength at low weight and costs, in particular when the interior of the tubular section remains a free space in the finished root extension which is not filled with a polymer.

[0031] In an aspect, the steel assemblies each comprise at least two longitudinal segments connected to one another by a threaded connection and / or by a welded connection ...19 and / or by a glued connection. Each of the segments is made of steel. Combining several longitudinal segments to an end-to-end steel assembly may be easier than forming one integral structure from end-to-end, in particular if an inner and / or outer diameter of the steel assemblies changes along the length. For example, an intermediate segment can be formed as a simple tube with constant inner and outer diameters. However, it is critical to obtain a strong connection between the at least two longitudinal segments. This is possible by a threaded connection or by a welded connection, in particular when using friction welding. A glued connection is also possible, in particular when the longitudinal segments glued together have an interface extending over a length larger than the diameter of the segments at the interface position.

[0032] In an aspect, the steel assemblies each comprise at least three longitudinal segments including a first segment having the first thread, a second segment having the second thread, and at least one intermediate segment. As mentioned before, this makes it easy to provide the steel assemblies with the desired geometry varying along the length of the steel assemblies.

[0033] In an aspect, the first segment is a steel bushing and / or the second segment is a steel bushing and / or the at least one intermediate segment is a steel tube. The steel tube may have a constant inner diameter and a constant outer diameter over its entire length. The steel bushings may have a constant outer diameter as well or may be provided with a conical shape forming a transition to the intermediate segment.

[0034] In an aspect, the first segment and / or the second segment consists of steel having a first steel grade and the at least one intermediate segment consists of steel having a second steel grade different from the first steel grade. Each steel grade is characterized by certain material properties that may be selected according to the different requirements of the different parts. For example, the first steel grade may be selected

[0035] ... / IO such that strong threads can be formed, whereas the second steel grade may be selected such that the intermediate segment has optimal stiffness.

[0036] In an aspect, an outer diameter of the first segment and an outer diameter of the second segment are greater than an outer diameter of the at least one intermediate segment.

[0037] In an aspect, the at least one intermediate segment is wrapped in the layer of reinforcing fibers. As mentioned above, the shape (and also the thickness) of the layer can be selected to form a common outer diameter along the entire length of the steel assemblies. If desired, the first and second segments may be wrapped in a layer of reinforcing fibers as well.

[0038] In an aspect, pultruded profiles forming a tubular structure are arranged about the intermediate segment.

[0039] In an aspect, the tubular structure has an outer diameter corresponding to an outer diameter of the first segment and / or to an outer diameter of the second segment. An inner diameter of the tubular structure may correspond to an outer diameter of the intermediate segment.

[0040] In an aspect, an inner end of the tubular structure is arranged within a conical recess of the first segment and / or an outer end of the tubular structure is arranged within a conical recess of the second segment. The inner end of the tubular structure refers to the end of the tubular structure that is directed towards the inner end of the root extension; the outer end of the tubular structure refers to the end of the tubular structure that is directed towards the outer end of the root extension. When assembling the steel assemblies, for example using a screw connection, the pultruded profiles can be arranged in the desired position and will then be held within the recess of the first segment and / or within the recess of the second segment, thereby maintaining the desired arrangement of the pultruded profiles when handling the steel assemblies during manufacture.

[0041] In an aspect, the root extension comprises two, three, four or more sections manufactured separately, wherein each of the sections extends from the inner end to the outer end and includes at least one of the steel assemblies. These sections can be connected to each other for example by gluing and / or by adding additional layers of a laminate, such as inner and outer skin layers. Each of the sections extends over a certain angular segment of the entire root extension. Dividing the root extension in several sections manufactured separately has advantages in relation to controlling of the production process as well as handling and storage of the sections.

[0042] In an aspect, the root extension comprises a first ring-shaped stiffening rib arranged at an inner side of the root extension near the inner end and / or a second ring-shaped stiffening rib arranged at an inner side of the root extension near the outer end and / or a stiffening web arranged along a longitudinal direction inside the root extension. The stiffening rib(s) and / or the stiffening web are firmly attached to an inner surface of the root extension, for example by an adhesive. When a stiffening rib and a stiffening web are used, these may also be connected to one another. The stiffening ribs and the stiffening web can be made of metal or of a fiber-reinforced composite material, for example. Each of the mentioned stiffening elements provides additional stiffness to the root extension.

[0043] The method of claim 22 is used for manufacturing a wind turbine rotor blade root extension having an inner end configured to be mounted to a wind turbine rotor hub and an outer end configured to be mounted to a wind turbine rotor blade main part, and comprises the following steps: • providing a plurality of end-to-end steel assemblies each having a first thread at an inner end and a second thread at an outer end, and

[0044] • fixedly embedding the steel assemblies in a fiber-reinforced composite material.

[0045] With regard to the features and advantages of the method, reference is made to the above explanations of the root extension, which apply correspondingly. In particular, a connection part as described above can be used for mounting the outer end to the wind turbine rotor blade main part. The elements and steps of the method can be implemented to include any one of the features explained above with reference to the root extension. For example, the method may include a step of wrapping a steel assembly in a layer of fiber-reinforced material, as has been explained above.

[0046] In an aspect, the step of fixedly embedding the steel assemblies in a fiber-reinforced composite material is carried out by placing at least one of the steel assemblies and at least some reinforcing fibers in a mold, filling a liquid plastics material into the mold, and curing of the liquid plastics material. The liquid plastics material may be filled into the mold for example by injection or by vacuum infusion. However, using preimpregnated fiber material (pre-pregs) is also possible, as is adding of the liquid plastics material manually in a hand lay-up process. The steel assemblies may be heated, for example by electrical resistance or induction heating, and / or may be provided with an activated surface prior to placing them in the mold to ensure a strong bonding with the plastics material.

[0047] In an aspect, two, three, four or more sections are manufactured separately, each of the sections extending from the inner end to the outer end and including at least one of the steel assemblies. After having been manufactured separately, the sections can be connected to each other by an adhesive to form the complete root extension. In the following, the invention is explained in greater detail based on drawings. The drawings show:

[0048] Fig. 1 a wind turbine rotor in a schematic view,

[0049] Fig. 2 a root extension connected to other elements in a perspective, partly exploded view,

[0050] Fig. 3 a root extension in a perspective view,

[0051] Fig. 4 four sections of a root extension in a perspective view,

[0052] Fig. 5 a part of a root extension in a longitudinal section,

[0053] Fig. 6 an enlarged section of Fig. 5,

[0054] Fig. 7 a cross-section through the elements shown in Fig. 6 along the line A,

[0055] Fig. 8 an illustration of wrapping a steel assembly in a layer of reinforcing material,

[0056] Fig. 9 a part of a root extension in a longitudinal section,

[0057] Fig. 10 an illustration showing four steps of manufacturing a root extension,

[0058] Fig. 11 a root extension in a perspective view cut along a longitudinal midplane.

[0059] The wind turbine rotor 10 shown in Fig. 1 comprises a wind turbine rotor hub 12 and three wind turbine rotor blades 14 attached thereto. Each wind turbine rotor blade 14 comprises a wind turbine rotor blade main part 16 and a root extension 18. The root ,.. / 14 extension 18 has an inner end 20 mounted to the wind turbine rotor hub 12 and an outer end 22 mounted via a connection part (not shown) to the wind turbine rotor blade main part 16. The wind turbine rotor blade main part 16 has a root end 24 which is mounted via the connection part to the outer end 22 of the root extension 18, and a blade tip 26. The wind turbine rotor blade main part 16 in cross-section has an aerodynamic profile with a maximum chord 28. The maximum chord 28 is larger than a diameter 30 of the root extension 18.

[0060] Fig. 2 illustrates how the root extension 16 is mounted between the wind turbine rotor hub 12 and the wind turbine rotor blade main part 16. Only the inner bearing ring 30 of a pitch bearing of the wind turbine rotor hub 12 is shown. It is connected to the inner end 20 of the root extension 18 by a plurality of threaded bolts 32 guided through openings in the bearing ring 30 and screwed into correspondingly arranged first threads 34 (see Fig. 5) of the root extension 18. A ring-shaped connection part 36 is attached to the outer end 22 of the root extension 18 also via a plurality of threaded bolts 32 which are guided through corresponding holes in the connection part 36 arranged near an outer circumference of the connection part 36. These threaded bolts 32 screwed into the second threads 38 of the steel assemblies 40 (see Fig. 5). At an inner circumference, the connection part 36 comprises a plurality of second holes which are used for fastening the connection part 36 to the root end 24 of the wind turbine rotor blade main part 16 by a further set of threaded bolts 32.

[0061] Fig. 3 shows a root extension 18 having a tubular, cylindrical cross section along its entire length, from the inner end 20 to the outer end 22. First front faces 42 of a plurality of steel assemblies 40 (see Fig. 5) can be seen at the root extensions’s outer end 22, which is facing the viewer. Each steel assembly 40 is fixedly embedded in a fiber-reinforced composite material 44. Each steel assembly 40 extends from the inner end 20 to the outer end 22 and has first and second threads 34, 38 (see Fig. 5) at the inner end 20 and the outer end 22, respectively. Fig. 4 shows four sections 46 of a root extension 18 which each extend over an angular area of 90°. When connected to each other, the four sections 46 will form a tubular root extension 18 similar to the one shown in Fig. 3. Each section 46 comprises a plurality of end-to-end steel assemblies 40 (see Fig. 5).

[0062] Fig. 5 shows a longitudinal section through one of the sections 46 of Fig. 4 or through a part of the root extension 18 of Fig. 3. A midsection has been cut out, as indicated by the two curved lines, so that the construction can be seen more clearly. The longitudinal section comprises an end-to-end steel assembly 40 having a first thread 34 at the inner end 20 of the root extension 18 and a second thread 38 at the outer end 22 of the root extension 18. The steel assemblies 40 are hollow and tubular along their entire length. The first and second threads 34, 38 are spaced from the ultimate inner and outer ends 20, 22 of the root extension 18 by a certain distance allowing to prestress a threaded bolt 32 (not shown) inserted into the respective first or second thread 34, 38.

[0063] A first front face 42 of the steel assembly 40 is arranged at the inner end 20, near the first thread 34, and, when mounting the root extension 18 to a wind turbine rotor hub 12, can be tensioned against a counter face (not shown) of the wind turbine rotor hub 12 by inserting a threaded bolt 32 into the first thread 34. At the outer end 22, the steel assembly 40 comprises a second front face 48 facing towards the wind turbine rotor blade main part 16. This second front face 48 is arranged near the second thread 38 and can be tensioned against a counter face of the connection part 36 (not shown) by a threaded bolt 32 inserted into the second thread 38.

[0064] As can be seen more clearly in the enlarged view of Fig. 6, the first front face 42 protrudes from the surrounding fiber-reinforced composite material 44 a few millimetres, so that the fiber-reinforced composite material 44 will not be damaged when the required tension is applied. The fiber-reinforced composite 44 material ,.. / 16 includes several layers of unidirectional fibers 50 arranged along the longitudinal direction of the steel assembly 40. In addition, the fiber-reinforced composite material 44 comprises inner and outer skin layers 52 of reinforcing fibers arranged in a biaxial fabric.

[0065] In the cross-sectional view of Fig. 7, which corresponds to the plane A-A indicated in Fig. 6, one recognizes a steel assembly 40 with a first thread 34, the layers of unidirectional fibers 50 and the skin layers 52. In addition, one can see four segments of core material 54 surrounding the circular arrangement of the steel assembly and the surrounding layers of unidirectional fibers 50. These segments of core material 54 are also arranged between the inner and outer skin layers 52.

[0066] Again, with reference to Fig. 6, one can see that the steel assembly 40 at the inner end 20 has a first diameter which is larger than a second diameter in an intermediate section 56. In-between, the steel assembly 40 has a conical outer surface 58. The inner diameter of the steel assemblies 40 between the first front face 42 and the first thread 34 is somewhat larger than the inner diameter of the first thread 34. The inner diameter of the steel assemblies 40 between the second front face 48 and the second thread 38 is somewhat larger than the inner diameter of the second thread 38. The inner diameter of the first and second threads 34, 38 can differ from each other depending on the size of the threaded bolts. Over the entire section of the steel assembly 40 between the first thread 34 and the second thread 38, the inner diameter of the steel assembly 40 is constant and corresponds to the inner diameter of the first and second threads 34, 38.

[0067] Fig. 8 illustrates how the steel assemblies 40 can be provided with the surrounding layer of unidirectional fibers 50. To this end, Fig. 8 to the top shows only the steel assembly 40 with the conical / larger end sections and the thinner intermediate section 56. In the middle of Fig. 8, a precut layer of unidirectional fibers 50 having a specific shape is shown. The direction of the fibers extends from left to right, that is parallel to the long edges 60 of the shape. The shape has a rectangular portion which in Fig. 8 forms a lower part of the entire shape, and a trapezoidal portion forming the upper part in Fig. 8. When wrapping the steel assembly 40 into the layer, one will start with the trapezoidal section, from the upper long edge 60, so that the increasing length of the layer will compensate for the varying diameter in the conical sections 56 of the steel assembly 40. When the entire trapezoidal section is wrapped about the steel assembly 40, the remaining, rectangular portion is also wrapped around the steel assembly 40 to form one or more additional layers of fiber material extending from end-to-end with a constant thickness.

[0068] To the bottom of Fig. 8, the resulting combination of the steel assembly 40 and the surrounding fiber material 50 is shown. It has a constant diameter over the entire length. When all of the steel assemblies 40 have been wrapped in the reinforcing fibers in this manner, they can be placed in a manufacturing mold, in particular in combination with the elements of core material 54 and fibers of the skin layers 52 explained with reference to Fig. 7. The mold is then filled with a liquid plastics material which is cured in order to form the fiber-reinforced composite material 44 with the steel assemblies 40 fixedly embedded therein.

[0069] Fig. 9 shows another root extension 18 comprising an end-to-end steel assembly 40 assembled from three longitudinal segments connected to one another by a threaded connection. These include a first longitudinal segment 62 having the first thread 34, a second longitudinal segment 64 having the second thread 38 and a tubular, intermediate segment 66. The threaded connection between these segments is formed by means of steel connectors 68 comprising first and second outer threads. These are inserted in-between the intermediate segment 66 and the first longitudinal segment 62 or the second longitudinal segment 64, respectively. Similar to the steel assembly 40 shown in Figs. 5 to 7, the outer diameters at the inner end 20 and at the outer end 22 are larger and a section with a conical outer surface 58 forms a transition to the smaller diameter of the intermediate segment 66. The conical sections are formed on the first longitudinal segment 62 and on the second longitudinal segment 64 in this example, while the intermediate segment 66 has a constant cross-section.

[0070] This steel assembly 40 of Fig. 9 is also wrapped in a layer of unidirectional reinforcing fibers 50 which, in contrast to what has been explained with reference to Fig. 8, extend only along a part of the entire length, namely from where the conical sections begin. In addition to these reinforcing fibers 50 wrapped about the steel assembly 40, the root extension 18 of Fig. 9 also has inner and outer skin layers 52, so that the entire steel assemblies 40 are fixedly embedded in the fiber-reinforced composite material 44. For illustrative purposes only, Fig. 9 also shows threaded bolts 32 screwed into the first and second threads 34, 38, respectively, and a nut 70 placed on each of the free ends of these threaded bolts 32.

[0071] Fig. 10 shows four steps of manufacturing a root extension 18 labelled 1 to 4. Step 1 shows a first longitudinal segment 62 comprising a first thread 34 and a first front face 42. These are arranged similar as has been explained above with reference to Figs. 5 and 6. At the end away from the first front face 42, the first longitudinal segment 62 has a conical recess 72. The first thread 34 is an inner thread formed along a relatively long longitudinal section of the first longitudinal segment 62, up to where the conical recess 72 begins. The length of the first thread 34 is dimensioned such that not only a threaded bolt 32 for making connection to the wind turbine rotor hub 12 can be inserted, but also an outer thread from an intermediate segment 66 of the steel assembly 40 which is screwed into the first thread 34 from the end with the conical recess 72, as illustrated in step 1 by the arrow. In step 2, a plurality of pultruded profiles 74 are arranged about the intermediate segment 66 of the steel assembly 40 such that they form a tubular structure. An outer diameter of the tubular structure corresponds to the outer diameter of the first longitudinal segment 62. The pultruded profiles 74 at both ends have a decreasing thickness, such that the tubular structure has a conical outer surface which fits into the conical recess 72 in the first longitudinal segment 62.

[0072] In step 3, a second longitudinal segment 64 comprising the second thread 38 is screwed onto the outer thread of the intermediate section 66. The second longitudinal segment 64 also has a conical recess 72 which, after having screwed the second thread 38 onto the outer thread of the intermediate section 66, covers the ends of the pultruded profiles 74 to maintain these in the desired arrangement.

[0073] Step 4 shows how the arrangement obtained after step 3 has been wrapped in several layers of unidirectional fibers 50. Because the arrangement resulting from Fig. 3 has a constant outer diameter, it is particularly simple to wrap the fiber layers thereabout.

[0074] Fig. 11 shows a root extension 18 cut along a longitudinal direction. The root extension 18 has a plurality of end-to-end steel assemblies 40 fixedly embedded in a fiber-reinforced composite material 44 as has been explained above. Also shown is a ring-shaped connection part 36 at the outer end 22 and a root end 24 of a wind turbine rotor blade main part 16. The root end 24 includes a plurality of steel bushings screw- connected to the connection part 36 as has been described above.

[0075] At the inner end 20, a plurality of threaded bolts 32 are shown, which are screwed into the first threads 34 provided in the end-to-end steel assemblies 40. In addition, Fig. 11 shows a first ring-shaped stiffening rib 76 arranged at an inner side of the root extension 18 near its inner end 20, and a second ring-shaped stiffening rib 78 arranged at the inner side near the outer end 22. Both stiffening ribs 76, 78 are manufactured from a fiber-reinforced composite material, for example in a sandwich construction.

[0076] Also shown is a stiffening web 80 arranged along the longitudinal direction of the root extension 18. It extends over the entire inner diameter. Lengthwise, the stiffening web 80 extends from the first ring-shaped stiffening rib 76 to the second ring-shaped stiffening rib 78, and is adhered to these two stiffening ribs 76, 78. List of reference numerals

[0077] 10 wind turbine rotor

[0078] 12 wind turbine rotor hub

[0079] 14 wind turbine rotor blade

[0080] 16 wind turbine rotor blade main part

[0081] 18 root extension

[0082] 20 inner end

[0083] 22 outer end

[0084] 24 root end

[0085] 26 blade tip

[0086] 28 maximum chord

[0087] 30 bearing ring

[0088] 32 threaded bolt

[0089] 34 first thread

[0090] 36 connection part

[0091] 38 second thread

[0092] 40 steel assembly

[0093] 42 first front face

[0094] 44 fiber-reinforced composite material

[0095] 46 section

[0096] 48 second front face

[0097] 50 unidirectional fibers

[0098] 52 skin layer

[0099] 54 core material

[0100] 56 intermediate section

[0101] 58 conical outer surface

[0102] 60 long edge

[0103] ...122 first longitudinal segment second longitudinal segment intermediate segment steel connector nut conical recess pultruded profile first stiffening rib second stiffening rib stiffening web

Claims

- 23 -Claims1. A wind turbine rotor blade root extension (18) having an inner end (20) configured to be mounted to a wind turbine rotor hub (12) and an outer end (22) configured to be mounted to a wind turbine rotor blade main part (16), wherein the root extension (18) comprises a plurality of end-to-end steel assemblies (40) each having a first thread (34) at the inner end (20) and a second thread (38) at the outer end (22), characterized in that the steel assemblies (40) are fixedly embedded in a fiber-reinforced composite material (44).

2. The wind turbine rotor blade root extension (18) of claim 1 , wherein the plurality of steel assemblies (40) accounts for 25 % to 50 % of a total weight of the root extension (18).

3. The wind turbine rotor blade root extension (18) of claim 1 or 2, wherein the first thread (34) is an inner thread and / or the second thread (38) is an inner thread.

4. The wind turbine rotor blade root extension (18) of any of the claims 1 to 3, wherein at least one of the steel assemblies (40) has a first diameter at the inner end (20) and / or at the outer end (22) and a second diameter in an intermediate section, wherein the second diameter is smaller than the first diameter.

5. The wind turbine rotor blade root extension (18) of any of the claims 1 to 4, wherein at least one of the steel assemblies (40) is wrapped in a layer of reinforcing fibers.

6. The wind turbine rotor blade root extension (18) of claim 5, wherein the layer extends from the inner end (20) to outer end (22).

7. The wind turbine rotor blade root extension (18) of claim 5 or 6, wherein the layer has been cut to shape before the at least one steel assembly (40) is wrapped in the layer such that the steel assembly (40) in combination with the layer wrapped thereabout has a constant outer diameter.

8. The wind turbine rotor blade root extension (18) of claim 4, wherein pultruded profiles (74) forming a tubular structure are arranged about the intermediate section.

9. The wind turbine rotor blade root extension (18) of claim 8, wherein the tubular structure has an outer diameter corresponding to the first diameter.

10. The wind turbine rotor blade root extension (18) of any of the claims 1 to 9, wherein the steel assemblies (40) each include a tubular section extending over at least 50 % of a total length of the steel assembly.

11. The wind turbine rotor blade root extension (18) of any of the claims 1 to 10, wherein the steel assemblies (40) each comprise at least two longitudinal segments connected to one another by a threaded connection and / or by a welded connection and / or by a glued connection.

12. The wind turbine rotor blade root extension (18) of any of the claims 1 to 11, wherein the steel assemblies (40) each comprise at least three longitudinal segments including a first longitudinal segment (62) having the first thread (34), a second longitudinal segment (64) having the second thread (38), and at least one intermediate segment (66).

13. The wind turbine rotor blade root extension (18) of claim 12, wherein the first longitudinal segment (62) is a steel bushing and / or the second longitudinalsegment (64) is a steel bushing and / or the at least one intermediate segment (66) is a steel tube.

14. The wind turbine rotor blade root extension (18) of claim 12 or 13, wherein the first longitudinal segment (62) and / or the second longitudinal segment (64) consist of steel having a first steel grade and the at least one intermediate segment (66) consist of steel having a second steel grade different from the first steel grade.

15. The wind turbine rotor blade root extension (18) of any of the claims 12 to 14, wherein an outer diameter of the first longitudinal segment (62) and an outer diameter of the second longitudinal segment (64) are greater than an outer diameter of the at least one intermediate segment (66).

16. The wind turbine rotor blade root extension (18) of any of the claims 12 to 15, wherein the at least one intermediate segment (66) is wrapped in a layer of reinforcing fibers.

17. The wind turbine rotor blade root extension (18) of any of the claims 12 to 16, wherein pultruded profiles (74) forming a tubular structure are arranged about the intermediate segment (66).

18. The wind turbine rotor blade root extension (18) of claim 17, wherein the tubular structure has an outer diameter corresponding to an outer diameter of the first longitudinal segment (62) and / or to an outer diameter of the second longitudinal segment (64).

19. The wind turbine rotor blade root extension (18) of claim 17 or 18, wherein an inner end (20) of the tubular structure is arranged within a conical recess (72) of- 26 - the first longitudinal segment (62) and / or an outer end (22) of the tubular structure is arranged in a conical recess (72) of the second longitudinal segment (64).

20. The wind turbine rotor blade root extension (18) of any of the claims 1 to 19, wherein the root extension (18) comprises two, three, four or more sections (46) manufactured separately, wherein each of the sections (46) extends from the inner end (20) to the outer end (22) and includes at least one of the steel assemblies (40).

21. The wind turbine rotor blade root extension (18) of any of the claims 1 to 20, wherein the root extension (18) comprises a first ring-shaped stiffening rib (76) arranged at an inner side of the root extension (18) near the inner end (20) and / or a second ring-shaped stiffening rib (78) arranged at an inner side of the root extension (18) near the outer end (22) and / or a stiffening web (80) arranged along a longitudinal direction of the root extension (18).

22. A method of manufacturing a wind turbine rotor blade root extension (18) having an inner end (20) configured to be mounted to a wind turbine rotor hub (12) and an outer end (22) configured to be mounted to a wind turbine rotor blade main part (16), the method comprising the following steps:• providing a plurality of end-to-end steel assemblies (40) each having a first thread (34) at an inner end (20) and a second thread (38) at an outer end, and• fixedly embedding the steel assemblies (40) in a fiber-reinforced composite material (44).

23. The method of claim 22, wherein the step of fixedly embedding the steel assemblies (40) in a fiber-reinforced composite material (44) is carried out by placing at least one of the steel assemblies (40) and at least some reinforcing- 27 - fibers in a mold, filling a liquid plastics material into the mold, and curing of the liquid plastics material.

24. The method of claim 22 or 23, wherein two, three, four or more sections (46) are manufactured separately, each of the sections extending from the inner end (20) to the outer end (22) and including at least one of the steel assemblies (40).

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