Patent Publication Number: US-2015086369-A1

Title: Wind turbine rotor

Description:
BACKGROUND 
     The present subject matter relates to a wind turbine rotor and a wind turbine comprising such a wind turbine rotor 
     In a commonly known wind turbine, a plurality of blades is mounted to a hub which is connected to a generator system for generating electricity based on the rotational power caused by wind energy exerted on the blade. In the commonly known wind turbine installations, the hub is rotatably mounted with the axis being aligned substantially horizontally which installation is referred to as horizontal wind turbine. In such commonly known horizontal wind turbines, wind turbine rotors are employed in which a blade pitch angle can be adjusted so that a predetermined rotation speed is obtained in accordance with specific wind conditions. 
     In recent years, the length of wind turbine blades has increased in order to provide wind turbine installations with an output of as much as 1 MW(el) or more in a single turbine. Consequently, specific restrictions relating to the transportation of elements of such wind turbine installations are introduced. However, large wind turbine installations provide an increased output, an enhanced efficiency and, besides others, several economic improvements. 
     Conventional rotor systems employ rotatable supports for the blades in order to adjust the pitch of the blades to current wind conditions. As the complete blade is designed as rotatable element, this support is located close to the hub. For such a support, bearings are used which are adapted to transmit the bending torque of the blade to the hub. In commonly known arrangements, the bearing is formed as compact assembly for mounting this assembly in the designated space which is limited in such known arrangements. 
     SUMMARY 
     The present subject matter relates to a wind turbine rotor which provides the advantages of large-scale wind turbines while the restrictions regarding the transportation of the elements of such a wind turbine rotor are reduced. 
     Further, the present subject matter relates to an improved arrangement for adjusting the pitch of a blade mounted to the wind turbine rotor which is easily mountable and which provides an enhanced safety in particular in situations with high wind speeds. 
     According to the basic concept of the present subject matter, a wind turbine rotor comprises a hub and at least one blade which is connected to the hub. According to the subject matter, the blade comprises a first blade section and a second blade section. The first blade section is mounted to the hub stationary with respect to the hub, wherein the second blade section is supported by the first blade section rotatably adjustable about a longitudinal axis of the blade. The second blade section is supported by the first blade section by at least two bearings which are spaced with respect to the longitudinal axis of the blade. 
     The blades of the wind turbine rotor which are designed for a high specific rated output power comprise a predetermined length in the radial direction of the wind turbine rotor when mounted. Consequently, reducing the total length of such a blade is not reasonable as the rated output power of such a wind turbine installation is reduced at the same time. 
     That is, the total length of the blade of the wind turbine rotor according to the subject matter is determined by sum of the length of the first blade section which is mounted to the hub and the length of the second blade section which is supported by the first blade section. Consequently, the second blade section is shorter than the total length of the entire blade such that the second blade section can be easily transported on the road or railway. The support of the second blade section by the first blade section using at least two bearings which are spaced with respect to the longitudinal axis of the blade enhances the stability of the rotatable support while simplifying the design of the bearings and related elements, such as bearing mounts. 
     According to an embodiment of the present subject matter, the first blade section extends substantially in the radial direction of the wind turbine rotor and the second blade section forms an extension at the radial end of the first blade section and substantially extends in the radial direction of the wind turbine rotor. Based on such an arrangement, a blade with a total length can be provided which meets the predetermined conditions of the wind turbine installation design. 
     According to yet another embodiment of the present subject matter, the first blade section forms a cover and/or mount for a support structure rotatably supporting the second blade section. A rotatable support for the second blade section enables a pitch control in order to adapt the operational conditions of the wind turbine rotor to the current wind conditions which are subject to fluctuations. According to the concept of the present subject matter, the first blade section is not rotatable, while the second blade section which is rotatably supported by the first blade section can be rotated for adjusting a pitch of at least a part of the entire blade. 
     According to yet another embodiment of the present subject matter, the support structure comprises a shaft mounted to a radial inner end of the second blade section and said at least two bearings rotatably supporting the shaft. According to the concept of the present subject matter, the shaft supports the second blade section which is mounted in said at least two bearings. According to the present subject matter, said at least two bearings provides a support of the shaft at locations which are spaced in the radial direction of said wind turbine rotor, in particular, in the axial direction of said shaft. The shaft can be manufactured in a simple way and can be held by said at least two bearings in order to provide a sufficient stability providing the capability of transferring a moment to the hub to be converted into the rotation of the hub. 
     According to yet another embodiment of the present subject matter, one of said at least two bearings is arranged at or close to the radially outer end of the first blade section and the other of said at least two bearings is arranged at or close to the radially inner end of the first blade section. 
     According to yet another embodiment of the present subject matter, each of said at least two bearings is mounted in a bearing mount. Specific bearing mounts can be adapted to the predetermined shape or construction of the bearings which are employed for supporting the shaft. 
     According to yet another embodiment of the present subject matter, each of the bearing mounts is formed as plate member or at least partly as plate member with its surface forming the main surface being arranged substantially perpendicular with respect to the radial direction of the wind turbine rotor, wherein each of the bearing mounts is provided with at least one manhole for maintenance and/or installation works. Due to the fact that the support structure includes elements which are subject to maintenance, the arrangement employing plate members as bearing mounts and providing at least one manhole in each plate member enables required maintenance work to be performed in the area of the first blade section, in particular, inside the first blade section, which includes multiple mechanical arrangements. Further, the installation of the wind turbine rotor is simplified by providing such manholes for transporting the elements of the rotor inside the first blade section through the manholes. 
     According to yet another embodiment of the present subject matter, the first blade section is formed as hollow member which is fixedly connected to the hub and encloses the support structure inside said hollow member such that said support structure is arranged outside the hub. Such an arrangement enables a simplified construction of the hub due to the fact that the forces acting from the second blade section are applied to locations which are positioned further outside the hub. That is, the forces are applied to the first blade section which includes the support structure for the second blade section. Stresses created by such an arrangement can be decreased in relation to a structure in which the support structure is provided inside or close to the hub. 
     According to yet another embodiment of the present subject matter, an actuating device is provided at least partly in the first blade section which is configured to rotate the second blade section about the longitudinal axis of the blade. Such an actuating device enables the pitch control of the second blade section and the specific arrangement in the first blade section simplifies the construction and the maintenance work. 
     According to yet another embodiment of the present subject matter, the actuating device is formed as at least one electric motor which is capable of rotating the shaft. An electric motor is easily controllable and can be designed with a small dimension which is important due to the limited space inside the rotor structure. 
     According to yet another embodiment of the present subject matter, the at least one motor is provided with a drive gear which is engaged or engageable with a gear section provided on the shaft. A geared transmission is simple and accurate for controlling the pitch of the blade. Further, the maintenance of such a system is simple and economic. 
     According to yet another embodiment of the present subject matter, the at least one motor is directly coupled to the shaft thus forming a direct drive of the shaft, in particular, without any speed reduction between the motor and the shaft. Such an arrangement is even more simplified as a transmission structure is not required which enhances the service life of the entire system. Further, the at least one motor can be mounted at the axial end of the shaft or even inside the shaft. 
     According to yet another embodiment of the present subject matter, the at least one electric motor is formed as synchronous motor which is controllable by a frequency converter. Such a system further enhances the controllability of the pitch of the second blade section. This system can be further enhanced by using a positional sensor for providing a signal indicating the rotational position of the shaft or of the second blade section. However, such a sensor is not essential for the subject matter. 
     According to yet another embodiment of the present subject matter, the second blade section is rotatably adjustable to an operational pitch position between a maximum pitch position and a minimum pitch position. Consequently, the second blade section can be operated to the desired rotational position in order to meet the requirements taking into account current wind conditions besides others. 
     According to yet another embodiment of the present subject matter, the outer surface of the first blade section is formed with an aerodynamic shape arranged for applying a torque to the hub when a wind force or wind load is exerted to the first blade section from the axial direction the wind turbine rotor, and the outer surface of the second blade section is formed with an aerodynamic shape arranged for applying a torque to the hub via the first blade section when a wind force or wind load is exerted to the second blade section approximately in the axial direction of the wind turbine rotor and the second blade section is rotatably adjusted to the operational pitch position. 
     According to this yet another embodiment, the surface of the first blade section contributes to the generation of a torque in the hub based on the specific aerodynamic shape, in particular, when the axis of the rotor is aligned with the wind direction. In addition, the outer surface of the second blade section also contributes to the generation of the torque due to the specific aerodynamic shape. Other than the first blade section, the pitch of the second blade section can be adjusted. As consequence, the effect of generating a torque which is applied to the hub can be adjusted by adjusting the rotational position of the second blade section, whereas the pitch of the first blade section is constant at all times. 
     A specific effect of this arrangement is remarkable in conditions with high wind speeds or in cases where the wind turbine rotor is to be inhibited from rotating. When the pitch of the second blade section is maximized, the pitch of the first blade section is still the same, i.e. the operational pitch. As consequence, irrespective of the direction from which the wind flows towards the turbine in relation to the axis of the rotor, the torque applied to the hub is reduced or minimized due to the fact that the torques applied to the hub from the first blade section and the second blade section, respectively, will never reach the maximum or operational torque at the same time. That is, at least one of the torque of the first blade section and of the torque of the second blade section is less than the maximum possible torque in case that the second blade section is rotated to the maximum pitch position. 
     According to yet another embodiment of the present subject matter, the outer surface or appearance of the blade which is formed by the outer surface of the first blade section and the outer surface of the second blade section forms a continuous surface or appearance at least in a predetermined pitch position of the second blade section. Preferably, this predetermined pitch position is the operational pitch position with the optimum torque output taking into account the rated output at the rated wind speed. 
     That is, the design of the outer surface of the entire blade formed by the first blade section and the second blade section is such that a continuous outer surface is formed in an operational pitch position of the second section which is the position under normal conditions of operation of the wind turbine installation. That is, the outer appearance of the entire blade and the aerodynamics thereof are optimized to the operational conditions and do not differ remarkably from outer appearances commonly known from aerodynamically optimized blades which are manufactured as a single part. 
     According to yet another embodiment of the present subject matter, the torque applied to the hub by the blade is minimized by adjusting the pitch position of the second blade section to the maximum pitch position. As stated above, the pitch position of the first blade section is constant at all times whereas the pitch position of the second blade section can be adjusted. In case that the pitch position of the second blade section is adjusted to the maximum pitch position, the pitch position of the first blade section remains at the operational pitch position. Consequently, the system with such a wind turbine rotor can perform a safety or fail safe operation in which the wind turbine rotor can be stopped even under conditions with high wind speeds. 
     According to yet another embodiment of the present subject matter, at least one support base is provided at the outer circumferential surface of the hub for supporting the first blade section, wherein said support base comprises a mounting surface which faces at least partly to the radial outward direction and which is tangential to at least a part of the outer periphery of said hub or parallel to a tangent of the outer periphery of said hub. The support base at the outer circumference of the hub can be adapted to the mounting properties of the first blade section such that the mounting work of the first blade section can be simplified and the strength of such an arrangement is improved. 
     According to yet another embodiment of the present subject matter, the mounting surface of the support base is arranged with an outer contour which is adapted to an outer contour of the first blade section at the transition between said support base and the first blade section. The adaptation of the contours of the first blade section and of the support base improves the mounting and in particular enables an optimized coupling or attachment such as an attachment based on welding or the like. However, any other couplings or attachments are possible and not restricted to welding and even a detachable mounting structure is possible. 
     According to yet another embodiment of the present subject matter, the length of the first blade section in the radial direction of said wind turbine rotor is a fraction of the length of the second blade section in the radial direction of said wind turbine rotor, preferably 5-50%, more preferably 5-25% and most preferably 10-25% of the length of said second blade section. The above ranges provide the effect of reducing the transport efforts and, at the same time, the optimized operation of the wind turbine rotor according to the subject matter. The above ranges represent preferable ranges, but other arrangements are within the scope of the present subject matter as long as the effects are achieved, that is, as long as the improved mounting of the rotatable second blade section is possible and the above discussed fail safe operation is remarkable. In particular, the above proportion is selected for optimizing the balance between pitch control and structural rigidity of the entire wind turbine rotor. 
     Optionally, the support base protrudes from the outer circumferential surface of the hub. As the first blade section is formed as hollow member to be mounted to the hub as stationary member in relation to the hub the support base enhances the transfer of forces and moments, i.e. bending moments, to the hub such that the entire arrangement can be reduced in mass. The first blade section can be fixedly mounted to the support base or can be detachably mountable to the support base in order to reduce the dimensions of the hub with the first blade section for transportation. In any case, an optimization is desired between transportation issue and reduction of mass of the first blade section and the hub. 
     According to yet another embodiment of the present subject matter, a wind turbine is provided which comprises a housing, a generator, which is accommodated in said housing and a wind turbine rotor according to the above presented basic concept of the subject matter, which is optionally further developed by one or more of the above discussed embodiments, wherein the hub of the wind turbine rotor is drivingly connected to the generator. The wind turbine comprising the above mentioned concept of the present subject matter exhibits the same advantageous effects as indicated above. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows an embodiment of the wind turbine rotor in a side view in a partially cut away condition as schematic drawing. 
         FIG. 2  shows the wind turbine rotor of  FIG. 1  in a three dimensional schematic view. 
         FIG. 3  shows the wind turbine rotor of  FIG. 1  in a further three dimensional schematic view. 
         FIG. 4  shows a schematic view of the wind turbine rotor of  FIG. 1  in a in a condition where there second blade sections are positioned at the maximum pitch position. 
     
    
    
     In the following, embodiments of the present subject matter are explained based on the drawings. It is noted that the drawings show a specific embodiment as explained below and further alternative modifications as specified in the description are at least in part not illustrated. 
     DESCRIPTION OF THE EMBODIMENTS 
       FIG. 1  shows the wind turbine rotor of the present subject matter and details thereof. In order to understand the advantages and function of this wind turbine rotor, the general function thereof is explained based on  FIG. 4  showing the wind turbine rotor according to an embodiment in a mounted condition. The wind turbine rotor comprises a hub  2  which is rotatably supported on a nacelle  1 . The hub  2  is rotatable such that a rotating axis of the hub  2  is substantially horizontally aligned. The rotational axis can be slightly inclined such that the direction of the extension of the rotational axis opposite to the wind direction is slightly tilted upwards. Such a deviation shall be considered as being covered by the meaning of the horizontal alignment of the axis. Further, the front side of the hub  2  faces against the wind such that the wind stream is directed towards the hub  2 . The hub  2  is formed with a specific aerodynamic shape for reducing disturbances by the wind stream passing by the surface of the hub  2 . The outer surface of the hub  2  and the outer surface of the nacelle  1  are formed substantially as continuous surface in order to avoid further disturbances at the transition between the hub  2  and the nacelle  1 . 
     The nacelle  1  shown in  FIG. 4  is provided with a top section  5  which includes equipment such as cooling system and position lights. However, the top section  5  as shown in  FIG. 4  is not essential for the present embodiment and rather forms an advantageous development thereof as discussed below. 
     Furthermore, the nacelle  1  is supported on a tower which is not shown in the drawings. The nacelle  1  is rotatably supported such that the horizontally directed axis of the hub  2  can be positioned in an optimum relation to the wind direction. 
       FIG. 1  shows the wind turbine rotor in a side view when mounted to the nacelle  1 . The hub  2  comprises a support base  12  which is formed in the outer surface of the hub  2 . The support base  12  according to the present embodiment forms at least partially a plane for supporting at least a part of a blade. 
     Referring back to  FIG. 4 , the blade  20  comprises a first blade section  3  and a second blade section  4 . Consequently, the first blade section  3  and the second blade section  4  form the complete blade  20  in the present embodiment. As shown in the drawings, the wind turbine rotor according to the present embodiment comprises three blades  20  which are mounted to the hub  2  at equal angular distances. However, it is possible to provide the rotor with a different number of blades  20 . Consequently, a single blade, two blades or more than three blades are possible. Thus, the number of three blades is not essential for the present subject matter. In case that only one blade  20  is mounted to the hub  2 , a counter balance must be designed and arranged at the hub  2  in order to enable a proper operation of the wind turbine rotor. 
     As shown in  FIG. 4 , the first blade section  3  is arranged at the outer circumference of the hub  2 . In the present embodiment, the first blade section  3  is mounted to the support base  12  which is shown in  FIGS. 1-3 . The support base  12  is, as already discussed, formed in the outer circumference of the hub  2  and comprises a mounting surface  14  which is basically a plane. The outer contour of the support base  12  is adapted to the outer contour to the first blade section  3 . In the present embodiment, the first blade section  3  is a hollow member which is arranged at the support base  12  and welded to the mounting surface  14  which faces in the radial direction of the hub  2 . However, any other coupling arrangement can be selected as long as the first blade section  3  is fixedly mounted to the hub  2 . Even a detachable arrangement is possible. 
     The first blade section  3  basically extends in the radial direction of the hub  2 . As shown in  FIG. 4 , the second blade section  4  forms an extension of the first blade section  3  in the radial direction. 
     According to the basic concept of the present subject matter, the first blade section  3  is not rotatably mounted to the hub  2  and as such stationary mounted with respect to the hub  2 . The second blade section  4  is rotatably mounted such that the rotational position of the second blade section  4  with respect to the longitudinal direction of the blade  20  can be changed. The torque generated by specific longitudinal portions of the blade depends on the distance of the respective portion from the rotational axis of the hub. That is, the radially outer portions of the blade contribute to a higher extent to the torque generation than the radially inner portions. Therefore, the incorrect adjustment of the pitch of the first blade section has less influence than any incorrect influence of the pitch of the second blade section. Therefore, it is important to provide the second blade section with rotatable properties and the disadvantages of the non-adjustable property of the first blade section are still acceptable. 
     In the following, the mechanism for supporting the second blade section  4  and for adjusting the rotational position thereof is discussed. In the illustration of  FIG. 1 , the hollow member forming the first blade section  3  is eliminated such that the support structure for the second blade section  4  is visible. 
     The support structure comprises a shaft  8  which is supported by bearings  7   a ,  7   b . Each of the bearings  7   a ,  7   b  is supported by one of bearing mounts  6   a ,  6   b.    
     Specifically, the bearings  7   a ,  7   b  are formed as roller bearings with an inner ring and an outer ring. The bearings  7   a ,  7   b  are arranged with the inner ring at the outer circumference of the shaft  8 . The outer ring of each of the bearings  7   a ,  7   b  is supported on inner circumferential surfaces formed in the bearing mounts  6   a ,  6   b . The bearings  7   a ,  7   b  are preferably mounted in a manner that the axial displacement thereof is inhibited. 
     The bearing mounts  6   a ,  6   b  according to the present embodiment are formed as plate members or at least partly plate shaped members with the support holes for supporting the outer rings of the bearings  7   a ,  7   b . The bearing mounts  6   a ,  6   b  which are formed as plate members are arranged such that the main surfaces thereof are approximately perpendicular to the axial direction of the shaft  8 . 
     The bearing mounts  6   a ,  6   b  are formed with an outer contour which is adapted to the inner contour of the first blade section  3 . The first blade section  3  is provided as the above discussed hollow member which is arranged on the outer peripheral surfaces of the bearing mounts  6   a ,  6   b . Consequently, the bearing mounts  6   a ,  6   b  are supported inside the first blade section  3  and the first blade section  3  is supported on the support base  12  which is part of hub  2 . 
     According to the basic concept of the present subject matter, the first blade section  3  includes the support structure for the second blade section  4 . The support structure is based on the shaft  8  which is rotatably supported by the bearings  7   a ,  7   b . The bearing  7   a ,  7   b  are positioned in a spaced relationship with respect to the axial direction of the shaft  8 . Consequently, the shaft  8  can be rotated while forces acting on the shaft  8 , such as bending moments, can be supported by the bearings  7   a ,  7   b  and the bearing mounts  6   a ,  6   b , which forces are transferred via the first blade section  3  to the support base  12  and eventually to the hub  2 . 
     The second blade section  4  is mounted at the radial outer end of the shaft  8 . This arrangement can be achieved by screwing with the use of bolts in the shaft  8  and the second blade section  4  using flanges or other equivalent means, such as forming connections in composite structures of the second blade section  4 . Consequently, the outer end of the shaft  8  provides a rotatable support for the second blade section  4 . That is, based on the rotation of the shaft  8 , the second blade section  4  can be adjusted with respect to its rotational position about the longitudinal axis of the blade  20  or an axis which is approximately aligned with the longitudinal direction of the blade  20 . 
     As shown in  FIG. 1 , at the radial inner end of the shaft  8 , a geared portion  11  is provided which is fixedly mounted to the shaft  8 . Further, an actuating device  10  is provided for actuating the shaft  8  in the rotational direction. According to the present embodiment which is shown in  FIGS. 1-4 , the actuating device  10  is formed with three electric motors  10   a ,  10   b ,  10   c , wherein each of the electric motors  10   a ,  10   b ,  10   c  comprises a drive gear which is in engagement with the geared portion  11 . Consequently, the electric motors  10   a ,  10   b ,  10   c  can rotate the shaft  8  with respect to the hub  2  which forms, at the same time, the support for the electric motors  10   a ,  10   b ,  10   c.    
     Referring back to  FIG. 4 , the first blade section  3  is formed with an aerodynamic shape which is adapted for generating a torque based on the wind force which is applied thereto from the axial direction of the hub  2 . The second blade section  4  is formed with an aerodynamic shape having a similar effect, that is, an effect for applying a torque to the hub  2  based on the wind force which is applied to the second blade section  4  from the axial direction of the hub  2 . The torque which is applied to the hub  2  can be adjusted by twisting the second blade section  4 , that is, the pitch thereof. The present embodiment provides a wind turbine rotor in which the second blade section  4  can be adjusted in the rotational position in order to optimize the output of the wind turbine. The rotation of the second blade section  4  is performed based on the actuating device  10  in cooperation with the specific support structure which is arranged in the first blade section  3 . 
     In a specific condition, the design rotational speed of the wind turbine rotor is achieved by a specific operation pitch position of the second blade section  4 . In the specific operational pitch position of the second blade section  4 , the outer appearance, that is, the outer surface of the complete blade  20  which is formed by the first blade section  3  and the second blade section  4  is continuous in order to optimize the aerodynamic properties of the complete wind turbine rotor. In order to adapt the aerodynamic action of the blade  20 , the second blade section  4  can be adjusted in order to optimize the output torque taking into account the design rotational speed of the wind turbine besides other conditions. 
     Based on the design of the first blade section  3  and the second blade section  4  which form at least in the operational condition of the wind turbine rotor a substantially continuous surface, disturbances at the transition between the first blade section  3  and the second blade section  4  can be reduced to a minimum. 
     In cases where the wind turbine rotor shall be inhibited from rotating, it is required to reduce the torque applied from the blades  20  to the hub  2  to a minimum. In such cases, the pitch of the second blade section  4  can be maximized such that the rotational position of the second blade section  4  is approximately at 90° with respect to the operational pitch position or at least a pitch position which is remarkably larger that the operational pitch position. 
     In this case, the torque applied from the blade  20  is reduced due to the fact that irrespective of the direction of the wind stream towards the turbine, either the torque of the first blade section  3  or the torque of the second blade section  4  is reduced, such that the overall torque generated by the blade  20  is very low such that the wind turbine rotor can be stopped. Such an action is very important for cases with high wind speeds in order to increase the safety of fail safe operation of the system. 
     As shown in  FIG. 2 , the bearing mounts  6   a ,  6   b  comprise manholes  13   a ,  13   b  which are used for maintenance and/or installation work. In the support structure of the present embodiment, several elements are provided which are subject to maintenance. Such a maintenance can be simplified by the arrangement of a hollow member forming the first blade section  3  and the bearing mounts  6   a ,  6   b  comprising such manholes  13   a ,  13   b . Equivalent manholes can be provided in the support base  12  such that access is provided from the inner space of the hub  2  towards the support structure inside the first blade section  3 . 
     In the above discussed embodiment, three motors  10   a ,  10   b ,  10   c  are provided. As alternative embodiment, a single motor  10  can be provided which can be formed as direct drive actuating device. As specific advantageous arrangement, the electric motor  10  can be mounted coaxially with respect to the shaft  8 . As further advantage, such a direct drive electric motor  10  can be mounted inside the shaft  8  which can be formed as hollow member. 
     In the above embodiment, an actuating device is shown which is formed as three motors  10   a ,  10   b ,  10   c  which are in engagement with a geared section  11  mounted on the shaft  8 . However, the present subject matter can be realized by any other actuating mechanism, such as a single motor  10 , as explained above, or even other actuating mechanisms based on hydraulic, pneumatic or other mechanic or electric systems. 
     In the above embodiment, two bearings  7   a ,  7   b  are shown. However, it is possible to use more than two bearings for rotatably supporting the second blade section  4  with respect to the first blade section  3 . That is, the number and the type of the bearings  7   a ,  7   b  are not essential for the subject matter as long as at least two bearings  7   a ,  7   b  are provided which are spaced in the radial direction of the blade  20  which arrangement contributes to the specific solution of the present subject matter. The spaced arrangement of the bearings in the context of the present subject matter is not limited to a specific distance between the at least two bearings. However, it is advantageous to provide the at least two bearings with a spaced arrangement which is adapted to the design of the first blade section  3 . Due to the fact that the inner space in the first blade section  3  is employed for covering the support structure comprising the at least two bearings, the distance between the at least two bearing can be adapted to the available space depending on the design of the first blade section  3 . It is clear that the maximum effect of the spaced arrangement of the at least two bearings  7   a ,  7   b  is achieved if each of the two bearings is positioned at extreme radial ends inside the first blade section  3 . However, this option is not restrictive and the distance between the bearings  7   a ,  7   b  can be less than maximum possible distance as long as the effect of enhancing the support properties of the support structure is achieved. 
     In the above discussed embodiment, three blades  20  are shown. However, it is clear that the number of blades  20  is not restricted to three blades. Rather, two blades or more than three blades can be arranged with the same advantageous effect of the present subject matter. Even a single blade  20  can be employed wherein care must be taken that a counterbalance weight is provided. The first blade section  3  is shown as separate member which is mounted to the support base  12  of the hub  2 . However, it is possible to provide the hub  2  with the first blade section  3  as single piece e.g. based on casting or the like. In this case, the dimension of the hub  2  including the first blade section  3  is increased. However, the reduction of the length of the second blade section  4  is still preferable in order to reduce the restrictions regarding transportation. 
     The length of the first blade section  3  in the radial direction of the wind turbine rotor is a fraction of the length of the second blade section  4  in the radial direction of the wind turbine rotor, as stated above. In the present embodiment, the advantageous effects of this arrangement remarkable in case that the length of the first blade section  3  is 5-50% of the length of the second blade section  4  in the radial direction of the wind turbine rotor. That is, even a very short first blade section  3  provides an improved support structure and reduces the length of the second blade section  4  to a specific extent. On the other hand, the length of the first blade section  3  which is very large and ranges up to 50% of the length of the second blade section  4  provides an optimized operation, an enhanced safety while the transportation restrictions are dramatically reduced. 
     The blade  20  according to the present embodiment comprises the first blade section  3  and the second blade section  4 . Consequently, the blade  20  in this embodiment is formed by two sections. However, it is possible that the blade  20  comprises the first blade section  3  which is stationary with respect to the hub  2  and the second blade section  4  which is rotatable with respect to the hub  2  in the longitudinal direction of the blade  20  and, in addition one or more further blade sections which are stationary or rotatable. It is only essential for the subject matter that the blade  20  comprises at least the first blade section  3  and the second blade section  4  with the above discussed arrangements and effect without excluding the provision of further sections between the first blade section  3  and the second blade section  4  or at other positions of the blade  20 . 
     The subject matter is not restricted to the shown and explained embodiment as long as a wind turbine rotor is provided which includes a first blade section  3  which is fixedly mounted to the hub  2  and a second blade section  4  which is arranged radially outside with respect to the first blade section  3  and rotatably supported on the first blade section  3  or at least on the hub  2  with respect to the longitudinal axis of the blade  20  which comprises the first blade section  3  and the second blade section  4  and in which the second blade section  4  is supported by the first blade section  3  by at least two bearings  7   a ,  7   b  which are spaced with respect to the longitudinal axis of the blade  20 .