Patent Publication Number: US-2022228553-A1

Title: Method for manufacturing a wind turbine blade and wind turbine blade

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to PCT Application No. PCT/EP2020/063605, having a filing date of May 15, 2020, which claims priority to EP Application No. 19179026.0, having a filing date of Jun. 7, 2019, the entire contents both of which are hereby incorporated by reference. 
    
    
     FIELD OF TECHNOLOGY 
     The following relates to a method for manufacturing a wind turbine blade and to a wind turbine blade. 
     BACKGROUND 
     One way to produce more power using a wind turbine under given wind conditions is to increase the size of the blades. However, the manufacture of wind turbine blades and their transportation to wind harvesting sites is becoming increasingly difficult for increasing blade sizes. Therefore, wind turbine blades are, for example, manufactured section-wise and joined on-site. It is desirable to keep the number of processing steps necessary for joining the blade sections as small as possible. Furthermore, the joints must be configured sufficiently strong to withstand the forces acting on the blade during operation of the wind turbine. Blade sections are connected to each other either by bonding (using an adhesive or a laminate), bolting or by a combination of both. 
     WO 2016 198 075 A1 discloses a modular wind turbine blade comprising first and second blade modules having spar caps embedded within their outer shells. The spar caps taper in thickness such that they are tapered from the blade outer surface to the blade inner surface as the thickness decreases resulting in inwardly tapered recesses being defined in the outer shells. The tapered recesses in the first and second blade modules are aligned when the modules are placed end-to-end to form a continuous double-tapered channel. A connecting member having a double-tapered structure is bonded in the channel to connect the modules together. An adhesive is used to bond the connecting member in the double-tapered channel. This bonding process has to be performed from the blade outside due to the inwardly tapered recesses. 
     To avoid problems with glue joints such as cracks in the glue line, EP 1 310 351 A1 proposes a method for joining modular wind turbine blades of composite materials mainly in one piece by vacuum infusion avoiding any glue joints. 
     SUMMARY 
     An aspect relates to an improved method for manufacturing a wind turbine blade and an improved wind turbine blade. 
     Accordingly, a method for manufacturing a wind turbine blade is proposed. The method comprises the steps of:
         arranging a joining portion comprising a fiber lay-up inside adjacent blade sections,   covering the joining portion and the adjacent blade sections at least partially with a vacuum bag, and   applying vacuum to a space covered by the vacuum bag, infusing at least the fiber lay-up with a resin and curing the resin to obtain a cured joining portion joining the blade sections inside.       

     Thus, a light-weight and at the same time strong blade section joint is provided. In particular, the strength of this laminate joint formed by vacuum infusion is comparable to the strength of the pristine laminate. Compared to a connection using an adhesive, the laminate joint formed by vacuum infusion provides a lighter and stronger blade section joint, in particular, a better weight-to-strength performance. This is because in the case of an adhesive, the weight of the adhesive is added in the bond line and the interlaminar shear strength drops as the bond line thickness increases. Further, the laminate joint formed by vacuum infusion avoids the problem of glue joints of having a different material in the glue than in the rest of the blade. 
     Furthermore, the proposed method for manufacturing a wind turbine blade allows to perform the joining process of the blade sections from inside the blade cavity. This facilitates the manufacturing process as there are joining processes, such as a web connection, which can only be performed from inside the blade cavity anyway. A wind turbine blade usually comprises a (shear) web in its interior cavity. Connecting the web of one blade section to the web of another blade section can only be done from inside the blade cavity. Furthermore, as wind turbine blades are increasing in size, the sizes of the cross-sections of the blade sections are also increasing in size. In particular, the inner cavities of the blade sections are large enough to allow workers to perform work, such as joining processes, from inside the blade. 
     In addition, with the proposed joining method using the joining module arranged inside the blade sections and the vacuum infusion process, the joining of the two blade sections can be realized with a reduced number of wet processes and curing steps. For example, only one wet process, i.e., resin infusion, and only one curing cycle might be necessary. 
     The wind turbine blade is part of a rotor of a wind turbine. The wind turbine is an apparatus to convert the wind&#39;s kinetic energy into electrical energy. The wind turbine comprises, for example, the rotor having one or more of the blades connected each to a hub, a nacelle including a generator, and a tower holding, at its top end, the nacelle. The tower of the wind turbine may be connected via a transition piece to a foundation of the wind turbine, such as a monopile in the seabed. 
     The wind turbine blade comprises two or more blade sections dividing the blade in a lengthwise direction. In particular, the adjacent blade sections are arranged adjacent to each other in a lengthwise direction of the blade. A first one of the adjacent blade sections is, for example, an inboard blade section. The inboard blade section comprises, for example, a root section connected to the hub of the rotor. A second one of the adjacent blade sections is, for example, an outboard blade section. The outboard blade section comprises, for example, a blade tip. In addition to the described first and second blade sections, the wind turbine blade may also comprise one or more further blade sections. The one or more further blade sections may be joined with each other and/or with the described first and/or second blade sections by the same joining process by which the first blade section is joined with the second blade section. 
     The wind turbine blade, e.g., the root section, is, for example, fixedly connected to the hub. The wind turbine blade is, for example, directly bolted to the hub. 
     Alternatively, the wind turbine blade, e.g., the root section, is rotatably connected to the hub. For example, the wind turbine blade is connected to a pitch bearing of the wind turbine, and the pitch bearing is connected to the hub. The pitch bearing is configured to adjust the angle of attack of the blade according to the wind speed to control the rotational speed of the blade. 
     Apart from the (cylindrical) root section connected with the hub, the wind turbine blade is formed aerodynamically. The wind turbine blade, i.e., each of its blade sections, comprises, for example, a pressure side (upwind side) and a suction side (downwind side). The pressure side and the suction side are connected with each other at a leading edge and a trailing edge. The pressure and suctions sides and the leading and trailing edges define an interior cavity of the wind turbine blade. 
     Two adjacent blade sections might be arranged adjacent to each other such that the blade sections abut each other. Alternatively, there might be a small gap between the adjacent blade sections which may be filled by the (cured) joining portion. The gap may be minimal. 
     Arranging the joining portion inside the adjacent blade sections includes, in particular, overlapping the fiber lay-up of the joining portion with the blade sections in a joining area of the blade sections. 
     The resin is infused and cured from inside the wind turbine blade cavity. Before infusing and curing the resin, the vacuum bag is sealed and a vacuum is generated inside the sealed vacuum bag. The resin is, for example, cured by applying heat. 
     The fiber lay-up embedded in the infused and cured resin is forming the cured joining portion. The cured joining portion is, in particular, a fiber-reinforced resin laminate. 
     The phrase “a cured joining portion joining the blade sections inside” is to say that the cured joining portion joins or connects the blade sections on their inside surfaces, i.e., surfaces of the blade sections that face an open or closed cavity within each blade section. 
     According to an embodiment, each of the adjacent blade sections comprises an outwardly tapered portion such that the outwardly tapered portions form a common recess. Further, the joining portion is arranged inside the adjacent blade sections by arranging the joining portion in the common recess. 
     By having the outwardly tapered portions of the adjacent blade sections, the outer surfaces of the blade sections can be finished prior to the joining process. In particular, there is no need for additional filling, coating and/or painting of the outer surface of the blade after the joining process. 
     Each of the adjacent blade sections comprises at an end portion thereof an outwardly tapered portion. In particular, each of the blade sections is tapered outwardly towards a joining interface with the other blade section. Further, each of the blade sections is tapered outwardly such that each of the blade sections forms an inner recess. The inner recesses of the blade sections form a common recess when the blade sections are arranged adjacent to each other. The common inner recess is filled by the fiber lay-up of the joining portion. 
     The joining portion is arranged inside the common recess by, firstly, arranging the joining portion in the inner recess of a first one of the blade sections. Then, a second one of the blade sections is arranged adjacent to the first one of the blade sections such that the inner recess of the second one of the blade sections receives the joining portion. 
     A thickness of a blade shell, beam and/or web decreases towards the interface with the other blade section to form the outwardly tapered portion. The outwardly tapered portion may, for example, be configured such that the thickness of its blade shell, beam and/or web decreases linearly towards the interface with the other blade section. The outwardly tapered portion may, for example, be configured such that the thickness of its blade shell, beam and/or web decreases stepwise or curved towards the interface with the other blade section. 
     In embodiments, each blade section may comprise at a certain end portion thereof two or more outwardly tapered portions. In particular, each of the two or more outwardly tapered portions of a first blade section corresponds to an outwardly tapered portion of a second blade section. Further, each of the two or more outwardly tapered portions of the first and second blade sections form a common recess. Further, in this case, the joining portion is arranged inside the adjacent blade sections by arranging the joining portion in the two or more common recesses. 
     According to a further embodiment, the joining portion is provided on a mandrel. Further, the joining portion is arranged inside the adjacent blade sections by arranging the mandrel with the joining portion inside the adjacent blade sections. 
     Using a mandrel allows to pre-pack the fiber lay-up on the mandrel. For example, the fiber-lay-up may be pre-packed on the mandrel off-site and transported to the assembling site. The assembling site is, for example, a harbour site in the case of an offshore wind turbine, or an erection site in the case of an onshore wind turbine. Further, the fiber lay-up may be pre-packed on the mandrel in parallel to the manufacturing of the blade sections. Thus, the manufacturing time for the blade can be reduced. 
     Further, a mandrel allows a better forming of the fiber lay-up infused with resin during curing by using the mandrel as a mould. 
     The mandrel may comprise two or more mandrel portions. 
     Before packing the fiber lay-up on the mandrel, a vacuum bag may be arranged on the mandrel. The fiber lay-up may then be arranged on the vacuum bag. 
     In embodiments, the method might comprise the step of removing the vacuum bag and/or the mandrel, after infusing and curing the resin. The mandrel and/or the vacuum bag is, for example, removed through the blade root section. 
     According to a further embodiment, the fiber lay-up comprises a fiber lay-up forming, once cured, one or more beam joints joining one or more beams of a first one of the adjacent blade sections with one or more corresponding beams of a second one of the adjacent blade sections. 
     Having the joining portion comprising a fiber lay-up forming one or more beam joints allows to provide a lightweight and strong blade section joint including one or more beam joints in a more efficient manufacturing process. 
     According to a further embodiment, the one or more beams comprise a pressure-side beam, a suction-side beam, a leading-edge beam or reinforcement and/or a trailing edge beam or reinforcement. 
     A pressure-side beam is, in particular, a beam on the pressure side of the wind turbine blade. A suction-side beam is, in particular, a beam on the suction side of the wind turbine blade. A leading-edge beam or reinforcement is, in particular, a beam and/or reinforcement on the leading edge of the wind turbine blade. A trailing edge beam or reinforcement is, in particular, a beam and/or reinforcement on the trailing edge of the wind turbine blade. 
     Having the joining portion comprising a fiber lay-up for a pressure-side beam, a suction-side beam, a leading-edge beam and/or a trailing edge beam allows to provide a light-weight and strong blade section joint including a pressure-side beam joint, a suction-side beam joint, a leading-edge beam joint and/or a trailing edge beam joint. In particular, the joining portion may include the fiber lay-up forming several or all of these beams. Thus, several or all of these beams may be joined in a single process step by infusing and curing the resin. 
     According to a further embodiment, the fiber lay-up comprises a fiber lay-up forming, once cured, a shell joint joining a shell of a first one of the adjacent blade sections with a shell of a second one of the adjacent blade sections. 
     Having the joining portion comprising a fiber lay-up for the shell allows to provide a light-weight and strong blade section joint including a shell joint in a more efficient manufacturing process. In particular, the shell and several or all of the beams may be joined in a single process step by infusing and curing the resin. 
     According to a further embodiment, the fiber lay-up comprises a fiber lay-up forming, once cured, a web joint joining a web of a first one of the adjacent blade sections with a web of a second one of the adjacent blade sections. 
     The (shear) web connects, in particular, the blade shells of the pressure side and the suction side in the interior cavity of the blade. The web provides shear strength to the blade. 
     Having the joining portion comprising the fiber lay-up for the web allows to provide a light-weight and strong blade section joint including a web joint. In particular, the web and the shell and/or several or all of the beams may be joined in a single process step by infusing and curing the resin. 
     According to a further embodiment, the joining portion is arranged inside the adjacent blade sections comprising a first one of the blade sections and a second one of the blade sections by:
         inserting the joining portion partially into the first one of the blade sections such that a portion of the joining portion protrudes from the first one of the blade sections, and   arranging the second one of the blade sections adjacent to the first one of the blade sections such that the second one of the blade sections receives the portion of the joining portion protruding from the first one of the blade sections.       

     Inserting the joining portion partially into the first one of the blade sections and arranging the second one of the blade sections on the protruding portion of the joining portion allows to more easily arrange the joining portion. Further, it allows to better position the second one of the blade sections adjacent to the first one of the blade sections. 
     According to a further embodiment, the joining portion is arranged inside the adjacent blade sections by a lifting tool. 
     The lifting tool is, in particular, a crane or a lifting vehicle. Having the lifting tool allows to better arrange and position the joining portion. 
     According to a further embodiment, the fiber lay-up includes glass fibers, carbon fibers, aramid fibers and/or natural fibers. 
     According to a further embodiment, the fiber lay-up comprises fibers in dry condition. 
     Fibers in dry condition are, in particular, fibers without a resin. Fibers in dry condition are more flexible compared to fibers with resin such as fibers casted in resin or pre-impregnated fibers (prepreg). Thus, using fibers in dry condition for the joining portion and infusing and curing the resin only after arranging the joining portion inside the blade sections allows to better match the shape of the joining portion to the shape of the adjacent blade sections. Hence, the requirements of manufacturing adjacent blade sections with geometries matching the joining portion is reduced. 
     According to a further embodiment, the fiber lay-up includes a core material such as wood, balsa, PET foam and/or PVC foam. 
     When the fiber lay-up including the core material is infused and cured with the resin, a fiber-reinforced resin laminate with a core structure made from the core material is obtained. For example, a sandwich-structured fiber-reinforced resin laminate may be obtained in which a layer of the core material is arranged between layers of fiber-reinforced resin. 
     Having the core material allows to reduce the weight of the final fiber-reinforced resin laminate while maintaining a sufficient rigidity and/or strength of the blade. 
     According to a further embodiment, the resin includes thermosets, thermoplastics, epoxy, polyurethane, vinyl ester and/or polyester. 
     According to a further embodiment, the method comprises, after infusing and curing the resin, the step of applying a sealing band covering a separation line between the adjacent blade sections from the outside. 
     Applying the sealing band allows to cover a small gap between the adjacent blade sections. 
     According to a further aspect, a wind turbine blade is provided. The wind turbine blade comprises adjacent blade sections and a joining portion arranged inside the adjacent blade sections. Further, the joining portion comprises a fiber lay-up embedded in cured resin and joins the adjacent blade sections inside. 
     The embodiments and features described with reference to the method of the present invention apply mutatis mutandis to the wind turbine blade of embodiments of the present invention. 
     Further possible implementations or alternative solutions of embodiments of the invention also encompass combinations—that are not explicitly mentioned herein—of features described above or below with regard to the embodiments. The person skilled in the art may also add individual or isolated aspects and features to the most basic form of embodiments of the invention. 
    
    
     
       BRIEF DESCRIPTION 
       Some of the embodiments will be described in detail, with reference to the following figures, wherein like designations denote like members, wherein: 
         FIG. 1  shows a wind turbine according to an embodiment; 
         FIG. 2  shows a perspective view of a joining portion on a mandrel according to an embodiment, the joining portion being configured for joining two blade sections of a blade of the wind turbine of  FIG. 1 ; 
         FIG. 3  shows the joining portion of  FIG. 2  without the mandrel; 
         FIG. 4  shows a perspective view of the joining portion of  FIG. 2  during inserting it into a first blade section; 
         FIG. 5  shows a perspective view of a second blade section during arranging it adjacent to the first blade section of  FIG. 4 ; 
         FIG. 6  shows a cross-section view of the first and second blade sections of  FIG. 5  arranged adjacent to each other, the cross-section being taken along line B of  FIG. 5 ; 
         FIG. 7  shows a fiber lay-up for a beam joint of the joining portion of  FIG. 2  in cross-section taken along line B of  FIG. 5 ; 
         FIG. 8  shows a view similar as  FIG. 7  but with the joining portion inserted into the first blade section; 
         FIG. 9  shows a view similar as  FIG. 8  but with the second blade section arranged adjacent the first blade section; 
         FIG. 10  shows a view similar as  FIG. 9  but with a vacuum bag attached and sealed; 
         FIG. 11  shows a fiber lay-up for a shell joint of the joining portion of  FIG. 2  in cross-section taken along line C of  FIG. 5 ; 
         FIG. 12  shows a view similar as  FIG. 11  but with the joining portion inserted into the first blade section; 
         FIG. 13  shows a view similar as  FIG. 12  but with the second blade section arranged adjacent the first blade section; 
         FIG. 14  shows a view similar as  FIG. 13  but with a vacuum bag attached and sealed; 
         FIG. 15  shows a fiber lay-up for a web joint of the joining portion of  FIG. 2  and the first blade section in cross-section taken along line D in  FIG. 2 ; 
         FIG. 16  shows a view similar as  FIG. 15  but with the joining portion inserted into the first blade section; 
         FIG. 17  shows an outside view of the first and second blade sections joined with each other by the joining portion; and 
         FIG. 18  shows a flowchart illustrating a method for manufacturing the wind turbine blade of the wind turbine of  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  shows a wind turbine  1  according to an embodiment. The wind turbine  1  comprises a rotor  2  having one or more blades  3  connected to a hub  4 . The hub  4  is connected to a generator (not shown) arranged inside a nacelle  5 . During operation of the wind turbine  1 , the blades  3  are driven by wind to rotate and the wind&#39;s kinetic energy is converted into electrical energy by the generator in the nacelle  5 . The nacelle  5  is arranged at the upper end of a tower  6  of the wind turbine  1 . The tower  6  is erected on a foundation  7  such as a monopile or tripile. The foundation  7  is connected to and/or driven into the ground or seabed. 
     In the following an improved method for manufacturing a wind turbine blade  3  comprising two or more blade sections  20 ,  24  is described with respect to  FIGS. 2 to 18 . 
     In step S 1  of the method, a joining portion  8  is provided on a mandrel  9 , as shown in  FIG. 2 . The mandrel  9  comprises a first mandrel portion  10  and a second mandrel portion  11 . The joining portion  8  comprises a fiber lay-up  12  for a pressure-side beam joint and a fiber lay-up  13  for a suction-side beam joint. Further, the joining portion  8  comprises a fiber lay-up  14  for a leading-edge beam joint and a fiber lay-up  15  for a trailing edge beam joint. Furthermore, the joining portion  8  comprises a fiber lay-up  16  for a web joint. The fiber lay-up  16  for the web joint is arranged between the first and second mandrel portions  10  and  11 . The joining portion  8  comprises in addition a fiber lay-up  17  for a shell joint. 
     In the described example, the fiber lay-ups  12 ,  13 ,  14 ,  15  and  17  for the beams and the shell comprise fibers in dry condition, i.e., without resin. Further, in the described example, the fiber lay-up  16  for the web joint comprises both fibers in dry condition as well as pre-casted fibers. 
       FIG. 3  shows the joining portion  8  of  FIG. 2  without the mandrel  9  for illustration purposes. 
     For pre-packing the joining portion  8  as shown in  FIG. 2 , firstly a mould  18  is provided. On the mould  18 , the fiber lay-up  13  for the suction-side beam joint and a part of the fiber lay-up  17  for the shell joint are provided. Then, vacuum bags  19  are provided on the fiber lay-ups  13 ,  17 . In the next step, the mandrel  9 , i.e., the mandrel portions  10  and  11 , is arranged. The vacuum bags  19  are wrapped around the mandrel portions  10  and  11  at the leading edge and trailing edge side. Also, the fiber lay-up  16  for the web joint is arranged. Next, the fiber lay-up  12  for the pressure-side beam joint, the fiber lay-up  14  for the leading-edge beam joint, the fiber lay-up  15  for the trailing edge beam joint, and the remaining of the fiber lay-up  17  for the shell joint are provided. Finally, the vacuum bags  19  are arranged such that all fiber lay-ups  12 ,  13 ,  14 ,  15 ,  16  and  17  are covered on one side thereof by a vacuum bag  19 . When using a mandrel  9  comprising more than one mandrel portion  10 ,  11 , more than one vacuum bag  19  may be required. 
     In step S 2  of the method, the joining portion  8  comprising the fiber lay-ups  12 ,  13 ,  14 ,  15 ,  16  and  17  for the beam, shell and web joints is inserted together with the mandrel  9  partially into a first blade section  20 . 
       FIG. 4  illustrates step S 2  of arranging the joining portion  8  in the first blade section  20  of the wind turbine  1 . The first blade section  20  has, in particular, been manufactured using fiber-reinforced resin. The first blade section  20  has, for example, been manufactured simultaneously to the step S 1  of pre-packing the joining portion  8 . In this example, the first blade section  20  is an inboard section of the blade  3 . However, the joining portion  8  could also be inserted into an outboard section of the blade  3 . 
     In  FIG. 4 , the first blade section  20  is fixed in position by alignment jigs  21 . The joining portion  8  is inserted into the first blade section  20  by a lifting tool  22 . The lifting tool  22  in this example is a lifting truck. The joining portion  8  is, in particular, inserted partially into the first blade section  20  such that a portion  23  of the joining portion  8  is protruding from the first blade section  20 , as shown in  FIG. 5 . 
     Inserting the joining portion  8  into the first blade section  20  includes matching the fiber lay-ups  12 ,  13 ,  14 ,  15 ,  16 ,  17  of the joining portion  8  with corresponding layers of the first blade section  20 . 
     In order to more easily insert the joining portion  8  into the first blade section  20 , the cross-section size of the joining portion  8  may be reduced temporarily. This could be done by temporarily bending one or more of the fiber lay-ups  12 ,  13 ,  14 ,  15  for the beams. Another or an additional option would be inward folding of the hollow mandrel  9  in areas were no fiber lay-ups  12 ,  13 ,  14 ,  15  for the beams are present. After inserting the joining portion  8  into the first blade section  20  the original size of the cross section of the joining portion  8  may be re-established. 
     In step S 3  of the method, a second blade section  24  of the blade  3  is arranged adjacent to the first blade section  20 . In particular, the second blade section  24  is arranged adjacent to the first blade section  20  such that it accommodates the portion  23  of the joining portion  8  protruding from the first blade section  20 , as shown in  FIG. 5 . In particular, the second blade section  24  is shoved onto the portion  23  of j oining portion  8  until the second blade section  24  abuts the first blade section  20 . The second blade section  24  is arranged adjacent to the first blade section  20  by an alignment jig  26 . The alignment jig  26  is, for example, mounted such that the adjacent blade sections  20 ,  24  can be moved, e.g., on rails  27 . Reference sign A denotes a longitudinal axis of the blade  3 . 
     Arranging the second blade section  24  adjacent to the first blade section  20  includes matching the fiber lay-ups  12 ,  13 ,  14 ,  15 ,  16 ,  17  of the joining portion  8  with corresponding layers of the second blade section  24 . 
     The second blade section  24  in this example is an outboard blade section. Further, in this example, the second blade section  24  comprises a blade tip  25 . However, the second blade section  24  could also be an inboard blade section. 
       FIG. 6  shows a cross-section view of a part of the blade  3  taken along line B in  FIG. 5 . Shown in  FIG. 6  are the first and second blade sections  20  and  24  arranged adjacent to each other as well as the joining portion  8  arranged inside the first and second blade sections  20  and  24 . The cross-section is taken through a pressure-side beam and a suction-side beam of the blade  3 . In the upper part of  FIG. 6  are shown the pressure-side beam  28  of the first blade section  20 , the pressure-side beam joint  12  of the joining portion  8 , and the pressure-side beam  29  of the second blade section  24  in cross-section. In the lower part of  FIG. 6  are shown the suction-side beam  30  of the first blade section  20 , the suction-side beam joint  13  of the joining portion  8 , and the suction-side beam  31  of the second blade section  24 . 
     Each of the pressure-side beams  28  and  29  and of the suction-side beams  30  and  31  comprises an outwardly tapered portion  32 ,  33 ,  34 ,  35 . The outwardly tapered portions  32  and  33  of the pressure-side beams  28  and  29  of the first and second blade sections  20 ,  24  form a common recess  36 . The fiber lay-up  12  for the pressure-side beam joint of the joining portion  8  is arranged in the common recess  36 . Likewise, the outwardly tapered portions  34  and  35  of the suction-side beams  30  and  31  of the first and second blade sections  20 ,  24  form a common recess  37 . The fiber lay-up  13  for the suction-side beam joint of the joining portion  8  is arranged in the common recess  37 . 
     In step S 4  of the method, the joining portion  8  and the adjacent first and second blade sections  20 ,  24  are at least partially covered with a vacuum bag  19 ,  38 . In this example, one or more first vacuum bags  19  have already been provided in step  51  during prepacking the mandrel  9 . In step S 4 , a second vacuum bag  38  is provided at the outside surfaces  39 ,  40  of the first and second blade sections  20 ,  24 . In step S 4 , the vacuum bags  19 ,  38  are sealed around the inner surfaces  41 ,  42  and the outer surfaces  39 ,  40  of the first and second blade sections  20 ,  24 .  FIG. 6  shows the sealing of the vacuum bags  19 ,  38  covering the fiber lay-up  12  for the pressure-side beam joint and the fiber lay-up  13  for the suction-side beam joint. The sealing is schematically and exemplarily indicated by sealing points  42  in  FIG. 6 . The sealing of the vacuum bags  19 ,  38  might be done by applying a sealing tape or any other suitable method. Although not shown in 
       FIG. 6 , the vacuum bags  19  and  38  are also sealed around the fiber lay-ups  14 ,  15 ,  16 ,  17  for the leading and trailing edge beam joints, the web joint and the shell joint. 
     In step S 5  of the method, a vacuum is generated within a cavity  54  defined by the sealed vacuum bags  19 ,  38 . Then, a resin  43  is infused into the cavity  54  defined by the sealed vacuum bags  19 ,  38 .  FIG. 6  shows the resin  43  starting to fill the cavity  54  defined by the sealed vacuum bags  19 ,  38  and to embed the fiber lay-up  12  for the pressure-side beam joint. The infused resin  43  is fully embedding the fiber lay-ups  12 ,  13 ,  14 ,  15 ,  16 ,  17  of the joining portion  8 . The resin  43  is then cured to obtain a cured joining portion. In  FIG. 6  the reference sign  44  indicates the cured joining portion which is formed when the fiber lay-ups  12 ,  13  for the pressure-side and suction-side beam joints are fully embedded in the infused and cured resin  43 . Although not shown in  FIG. 6 , the cured joining portion  44  also comprises the fiber lay-ups  14 ,  15 ,  16 ,  17  when embedded in the infused and cured resin  43 . The vacuum infusion process may be a Vacuum Assisted Resin Transfer Moulding (VARTM) process. For further details of the generation of the vacuum, the infusion and curing of the resin  43 , it is referred to EP 1 310 351 A1. 
       FIGS. 7 to 10  show a further and more detailed illustration of the steps S 1  to S 5  of the method.  FIGS. 7 to 10  show the joining of the pressure-side beam of the blade  3  by the joining portion  8  including the fiber lay-up  12  for the pressure-side beam joint. However, the beam joint of  FIGS. 7 to 10  could be any of the pressure-side beam joint  12 , suction-side beam joint  13 , leading edge beam joint  14  or trailing edge beam joint  15 . 
       FIG. 7  shows the step S 1  of prepacking the vacuum bag  19  and the joining portion  8  on the mandrel  9  (the mandrel  9  is only shown in  FIG. 2 ). Firstly, the vacuum bag  19  is arranged on the mandrel  9 , then the fiber lay-up  17  for an inner shell joint, the fiber lay-up  12  for the pressure-side beam joint, and the fiber lay-up  17  for an outer shell joint are arranged on the vacuum bag  19 . 
       FIG. 8  shows step S 2  of partially inserting the prepacked joining portion  8  into the first blade section  20 . The first blade section  20  comprises layers for the pressure-side beam  28  as well as layers  45  for the inner and outer shell. The joining portion  8  is inserted into the first blade section  20  such that the fiber lay-ups  12 ,  17  of the joining portion  8  match the corresponding layers  45 ,  28  of the first blade section  20 . 
       FIG. 9  shows step S 3  of shoving the second blade section  24  onto the joining portion  8  such that the second blade section  24  abuts the first blade section  20  and the common recess  36  is formed. 
       FIG. 10  shows step S 4  of applying the second vacuum bag  38  and sealing the vacuum bags  19 ,  38 . In the next step S 5 , a vacuum is generated inside the cavity defined by the sealed vacuum bags  19 ,  38  in order to infuse and cure the resin  43 . 
       FIGS. 11 to 14  show a further illustration of steps S 1  to S 5  of the method displaying the joining of the shell in areas without a beam.  FIGS. 11 to 14  each show a cross-section view taken along line C of  FIG. 5 . 
       FIG. 11  shows step S 1  of prepacking the vacuum bag  19  and the joining portion  8  on the mandrel  9  (the mandrel  9  is only shown in  FIG. 2 ). Firstly, the vacuum bag  19  is arranged on the mandrel  9  and the fiber lay-up  17  for an inner shell joint is arranged on the vacuum bag  19 . Then, a core material  46  such as balsa is arranged on the fiber lay-up  17  for the inner shell joint. Finally, the fiber lay-up  17  for an outer shell joint is arranged on the core material  46 . 
       FIG. 12  shows step S 2  of partially inserting the prepacked joining portion  8  from  FIG. 11  into the first blade section  20 . The first blade section  20  comprises corresponding layers  47  for the shell and a corresponding core material  48 . The joining portion  8  is inserted into the first blade section  20  such that the fiber lay-ups  17  and the core material  46  of the joining portion  8  match the corresponding layers  47  and corresponding core material  48  of the first blade section  20 . 
       FIG. 13  shows step S 3  of shoving the second blade section  24  onto the joining portion  8  such that the second blade section  24  abuts the first blade section  20  and a common recess  49  is formed. 
       FIG. 14  shows step S 4  of applying the second vacuum bag  38  and sealing the vacuum bags  19 ,  38 . In the next step S 5 , a vacuum is generated inside the cavity defined by the sealed vacuum bags  19 ,  38  in order to infuse and cure the resin  43 . 
       FIGS. 15 and 16  illustrate joining a web of the blade  3  by the fiber lay-up  16  for the web joint.  FIGS. 15 to 16  each show a cross-section view taken along line D of  FIG. 2 . In the example of  FIGS. 15 and 16 , the fiber lay-up  16  for the web joint comprises a dry fiber lay-up portion  49  and a pre-casted fiber lay-up portion  50 . The dry fiber lay-up portion  49  is comprised of dry fibers without resin. The pre-casted fiber lay-up portion  50  is, in particular, comprised of fibers embedded in cured resin. 
     On the left part of  FIG. 15 , the prepacking of the joining portion  8  in step Si is illustrated for the case of the web joint  16 . Firstly, the vacuum bag  19  is arranged on the mandrel  9  (only shown in  FIG. 2 ), in particular on the first and second portions  10 ,  11  of the mandrel  9 . Then, the fiber lay-up  16  for the web joint is arranged between the vacuum bags  19 . In step  2 , the joining portion  8  is inserted in the first blade section  20  ( FIG. 4 ). Thereby, the dry portion  49  of the fiber lay-up  16  for the web joint is accommodating a corresponding web element  51  of the first blade section  20 . 
     In step S 3 , the second blade section  24  (not shown in  FIGS. 15 and 16 ) is arranged adjacent to the first blade section  20 . Thereby, the fiber lay-up  16  for the web joint of the joining portion  8  is joined with a corresponding web element (not shown) of the second blade section  24  in similar way as the illustrated joining of the fiber lay-up  16  of the joining portion  8  with the web element  51  of the first blade section  20 . 
     In step S 4  ( FIG. 16 ), the vacuum bag  19  is sealed (sealing points  42  in  FIG. 16 ). In step S 5 , vacuum is applied to a cavity defined by the sealed vacuum bag  19 . Further, resin  43  is inserted into the cavity and cured. 
     As shown in  FIG. 17 , in step S 6  of the method, a sealing band  52 , such as an adhesive tape, is applied to the outside surfaces  39 ,  40  of the first and second blade sections  20 ,  24 . The sealing band  52  may cover any remaining gap at the boundary line between the first and second blade sections  20 ,  24 . 
     In step S 7 , the mandrel  9  and the vacuum bags  19 ,  38  are removed from the blade  3 , e.g., through a root section of the blade  3 . 
     With the described method the whole joining process of joining two blade sections  20 ,  24  can be performed in one single process from inside the cavity of the blade  3 . Furthermore, apart from eventually applying the sealing band  52 , there is no finishing treatment of the outer surfaces  39 ,  40  of the blade  3  necessary. 
     Although the present invention has been disclosed in the form of preferred embodiments and variations thereon, it will be understood that numerous additional modifications and variations could be made thereto without departing from the scope of the invention. 
     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.