Abstract:
A method of manufacturing wind turbine blades of variable length with connection elements with the rotor hub. The method includes providing and using enlarged manufacturing molds having a common zone of a predetermined length and, at least, an adaptive zone arranged with the length needed for manufacturing the blades with a desired length, particularly the length required for optimizing the annual energy production (AEP) of a predetermined wind turbine model in a predetermined site.

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to a manufacturing method of wind turbine blades of variable length. 
     BACKGROUND 
     Wind turbines include a rotor that supports a number of blades extending radially therefrom for capturing the kinetic energy of the wind and causing a rotational motion of a driving train coupled to an electric generator for producing electrical power. 
     The amount of energy produced by wind turbines is dependant on the rotor blade sweeping surface that receives the action from the wind and consequently increasing the length of the blades leads normally to an increase of the power output of the wind turbine. 
     The energy produced by a wind turbine depends, among other factors, on the diameter of the rotor and on the characteristics of the wind at the site where the wind turbine is installed. 
     To optimize the energy produced at the wind turbine site, the usual procedure in selecting the most appropriate wind turbine is the following:
         Identifying the wind characteristics at the site (maximum speed, average speed, turbulence . . . ).   Determining the site class (I, II or III) according to international standards (IEC-61400-1).   Selecting the suitable wind turbine model for the site. The wind turbine manufacturers usually offer wind turbines having specific blades for each class. In low wind sites they have larger blades than in high wind sites, so that the loads they induce on the wind turbine are similar.       

     This process leads to a very wide classification of sites (only three levels), so that if a site is midway between the limits of two classes an oversized wind turbine can be selected. In this case, the choice of the optimum diameter of the rotor for the intermediate subclass, would entail an optimization of the annual energy production (AEP). 
     The adaptation of a wind turbine to the site constrains including the selection of a blade length adapted to the features of the site it is discussed in US 2009/0169390. It is proposed the provision a kit of parts comprising several modules for assembling a modular rotor blade, wherein the several modules comprise at least one root-type module and at least one tip-type module and at least one further module of the root-type or the tip-type, wherein the at least one further module has a different shape compared to the other module of the same type. Finally a method for adapting a rotor of a wind turbine to a site constraint is provided. 
     Therefore the teaching of US 2009/0169390 is limited to the provision of a number of modules of wind turbine blades, manufactured according to standard procedures, for selecting a suitable set for assembling a blade adapted to a site constraint. 
     The problem of this approach is that it does not provide blades with the optimum length for each particular site. 
     This invention is addressed to the solution of this problem. 
     SUMMARY OF THE INVENTION 
     An object of this invention is to provide blades for wind turbines of predetermined features (i.e. specific wind turbine models) with the optimum length for a particular site. 
     Another object of this invention is to provide a manufacturing method of wind turbine blades of variable length. 
     These and other objects are met by a method of manufacturing wind turbine blades of variable length with connection means with the rotor hub comprising steps of providing and using enlarged manufacturing moulds having a common zone of a predetermined length and, at least, an adaptive zone arranged with the length needed for manufacturing the blades with a desired length. 
     In embodiments of the invention said desired length is the length required for optimizing the annual energy production (AEP) of a predetermined wind turbine model in a predetermined site. Hereby it is achieved a manufacturing method allowing the manufacturing of wind turbine blades with the optimum length for a given site improving therefore the current situation where there is only an offer of a limited set of blades of different length for a given wind turbine model. 
     In embodiments of the invention said enlarged manufacturing moulds are provided and used for the whole blades or, in blades divided in at least an inboard module and an outboard module, for the inboard module; said adaptive zone is located at the side corresponding to the root of the blade. Hereby it is achieved a manufacturing method of wind turbine blades of variable length where said variable length is concentrated in the root zone where there are small variations in the blade geometry. 
     In embodiments of the invention for blades divided in an inboard module and an outboard module, said enlarged manufacturing moulds are provided and used for the inboard module and/or the outboard module; said adaptive(s) zone(s) is/are located at the side in contact with the other module. Hereby there are achieved manufacturing methods of wind turbine blades of variable length where said variable length is concentrated on the distal end of the inboard module and/or on the frontal end of the outboard module where an increase of the length may have a significant effect in the AEP. 
     In embodiments of the invention for blades divided in an inboard module, an outboard module and an intermediate module between them, said enlarged manufacturing moulds are provided and used for the intermediate module; said adaptive(s) zone(s) is/are located at any side in contact with the inboard or outboard modules or at both sides. Hereby there are achieved manufacturing methods of wind turbine blades of variable length where said variable length is concentrated on an intermediate module. 
     In embodiments of the invention, each adaptive zone in a central section of the blade is configured with the same transversal section than the contiguous transversal section of the common zone in all its length. Hereby it is achieved a manufacturing method of wind turbine blades of variable length that allows an adaptation of the length of the blade to the features of its foreseen site at a low cost. 
     In embodiments of the invention, each adaptive zone in a central section of the blade is configured with a variable transversal section corresponding to a blade optimal shape in said adaptive zone (using for example flexible moulds). Hereby it is achieved a manufacturing method of wind turbine blades of variable length that allows an full adaptation of the length of the blade to the features of its foreseen site for optimizing the AEP. 
     In embodiments of the invention the length of each adaptive zone in each of said enlarged manufacturing moulds is comprised between the 1%-15% of the length of the common zone. On the other hand the length of the intermediate module is comprised between the 10%-30% of the length of the blade. Theses proportion avoid eventual detrimental effects due to the use of the above-mentioned enlarged manufacturing moulds. 
     Other features and advantages of the present invention will be understood from the following detailed description in relation with the enclosed drawings. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1 a    shows in schematic perspective views the main components of the inboard module of a wind turbine blade. 
         FIG. 1 b    shows in schematic perspective views the main components of the outboard module of a wind turbine blade. 
         FIG. 2  shows schematically the enlarged manufacturing moulds with an adaptive zone in the root that are used for manufacturing an inboard module with a variable length according to the present invention and 
         FIG. 3  shows schematically the enlarged manufacturing moulds of this zone. 
         FIG. 4  shows schematically the enlarged manufacturing moulds with an adaptive zone at the end that are used for manufacturing an inboard module with a variable length according to the present invention. 
         FIG. 5  shows schematically the enlarged manufacturing moulds with an adaptive zone at the beginning that are used for manufacturing an outboard module with a variable length according to the present invention. 
         FIG. 6  shows a wind turbine blade divided in an inboard module and an outboard module with an optimum length for a given site manufactured using enlarged manufacturing moulds with three adaptive zones. 
         FIG. 7  shows a wind turbine blade divided in an inboard module, an intermediate module and an outboard module. 
         FIG. 8  shows schematically enlarged manufacturing moulds with two adaptive zones that are used for manufacturing an intermediate module with a variable length according to the present invention. 
         FIG. 9  shows schematically an example of an enlarged manufacturing mould that can be used in the manufacturing methods illustrated in  FIGS. 4, 5 and 8 . 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     This invention refers to wind turbine blades adaptable in length comprising wind turbine blades manufactured as single parts and to wind turbine blades manufactured by modules, particularly an inboard module and an outboard module. To solve the transportation problems posed particularly by lengthy blades, the division of the blade in two or more longitudinal sections provided with joining means is a well-known solution in the art. 
     As illustrated in  FIGS. 1 a  and 1 b    the inboard module  23  of a blade of a length L 1  is formed by an spar  25  (that may be divided in several panels) and upper and lower shells  27 ,  29 , and the outboard module  33  of a blade of a length L 2  is formed by an spar  35  and upper and lower shells  37 ,  39 . Many other configurations of the blade are possible. 
     In any case all the basic components of said inboard and outboard modules  23 ,  33  are manufactured using suitable manufacturing moulds (preferable female moulds) for each single component of the module having, respectively, the same length as the length L 1  or L 2  of the corresponding module. Then all the components are bonded. During the manufacturing procedure the inboard and outboard modules are provided with joining means between them and with the rotor hub such as those disclosed for example in WO 2005/100781, WO 2006/103307, WO 2007/051879 in the name of the applicant. 
     In a first embodiment, the manufacturing method according to the present invention for manufacturing an inboard module  23  of a length L 1  (or a whole blade manufactured as a single part), with joining means  18 ,  20  with, respectively, the rotor hub and the outboard module  33 , is done, as illustrated in  FIG. 2 , using enlarged manufacturing moulds  41  (comprising all the moulds needed for manufacturing all its components) having a common zone  13  of a length L 0   1  and an adaptive zone  15  in the root of a length AL 0   1  comprised preferably between the 1-15% of L 0   1 . 
     If the inboard module  23  (or the whole blade) to be manufactured shall have for example a length of L 1 =L 0   1 +0.05*L 0   1 , the enlarged moulds  41  shall be arranged for said length during the manufacturing process placing in the position  17 ′ determined by said length L 0 +0.05*L 0   1  the template of the joining means  18  (preferably metallic inserts) with the rotor hub. 
     As illustrated in  FIG. 3 , said adaptive zone  15  is a zone without significant variations in its transversal section between the extreme positions  17 ,  17 ″ of said joining means  18 . 
     In a second embodiment, the manufacturing method according to the present invention for manufacturing an inboard module  23  of a length L 1 , with joining means  18 ,  20  with, respectively, the rotor hub and the outboard module  33 , is done, as illustrated in  FIG. 4 , using enlarged manufacturing moulds  43  having a common zone  13  of a length L 0   1  and an adaptive zone  15  at the end of a length AL 0   1  comprised preferably between the 1-15% of L 0   1 . 
     If the inboard module  23  to be manufactured shall have for example a length L 1 =L 0   1 +0.10*L 0   1  the enlarged manufacturing moulds  43  shall be arranged with said length during its manufacturing. Therefore suitable configured additional manufacturing moulds for said adaptive zone  15  shall be provided whether with the same transversal section than the final transversal section of the common zone  13  or with a transversal section of variable shape. 
     In a third embodiment, the manufacturing process according to the present invention for manufacturing an outboard module  33  of a length L 2  with joining means  30  with the inboard module  23 , is done, as illustrated in  FIG. 5 , using enlarged manufacturing moulds  45  having a common zone  13  of a length L 0   2  and an adaptive zone  15  at the beginning of a length AL 0   2  comprised preferably between the 1-15% of L 0   2 . 
     If the outboard module  33  to be manufactured shall have for example a length L 2 =L 0   2 +0.07*L 0   2  the enlarged manufacturing moulds  45  shall be arranged with said length during its manufacturing. Therefore suitable configured additional manufacturing moulds for said adaptive zone  15  shall be provided whether with the same transversal section than the initial transversal section of the common zone  13  or with a transversal section of variable shape. 
     The above mentioned embodiments can be combined, so that the length of the adaptive zones  15  can reach a length, as illustrated in  FIG. 6 , comprised between the 1-30% of L 0   1  in the inboard module  23  plus a length comprised between the 1-15% of L 0   2  in the outboard module  33 . 
     In a fourth embodiment, the manufacturing process according to the present invention is arranged, as illustrated in  FIGS. 7 and 8 , for manufacturing an extra intermediate module  29 , with joining means  32 ,  34  with, respectively, the inboard module  23  and the outboard module  33 , using enlarged manufacturing moulds  47  having a common zone  13  of a length L 0   3  and an adaptive zone  15  at the beginning and/or at the end of a length AL 0   3  comprised preferably between the 1-15% of L 0   3 , the maximum length of the intermediate module  29  being comprised between the 10-30% of the total length of the blade. 
     If the intermediate module  29  to be manufactured shall have for example a length L 3  the enlarged moulds  47  shall be arranged with said length during its manufacturing. Therefore suitable configured additional manufacturing moulds for said adaptive zones  15  shall be provided whether with the same transversal section than the initial transversal section of the common zone  13  or with a transversal section of variable shape. 
     The configuration of the transversal section of said adaptive zones  15  at the end of the inboard module  23 , at the beginning of the outboard module  33  or at the beginning or end of the intermediate module  29  with a constant shape facilitates the manufacturing of blades of variable length but the final shape of the blade is not an aerodynamic optimal shape.  FIG. 9  shows schematically an example of a mould  50  for a shell in said adaptive zones  15  with a movable template  51 . 
     The configuration of the transversal section of said adaptive zones  15  at the end of the inboard module  23  or at the beginning of the outboard module  13  or at the beginning or end of the intermediate module  29  with a variable shape so that the final shape of the blade is as close as possible to an aerodynamic optimal shape requires suitable moulds that can be provided as ad-hoc rigid moulds or as adapted flexible moulds. 
     The installation of blades of different length in a same type of wind turbine in sites of different characteristics for optimizing the AEP may produce effects in several wind turbine features such as the blade tip noise, the deflection of the blade and the eigenfrequencies of the blade that shall be taken into account during the tuning of the wind turbine control system. 
     In any case, the above-mentioned defined margins for the adaptive zones  15  have been set up for minimizing any detrimental effect. 
     Although the present invention has been fully described in connection with preferred embodiments, it is evident that modifications may be introduced within the scope thereof, not considering this as limited by these embodiments, but by the contents of the following claims.