Abstract:
A rotor blade extension for mounting over a tip of a wind turbine rotor blade, including an airfoil extension portion) configured to extend a length of the rotor blade, a leading edge portion configured to cover a profile of the leading edge of the rotor blade, and a trailing edge portion configured to cover a profile of the trailing edge of the rotor blade, is provided. A wind turbine including a number of rotor blades and a rotor blade extension mounted to each rotor blade is also provided.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority to EP Application No. 14199679.3 having a filing date of Dec. 22, 2014, the entire contents of which are hereby incorporated by reference. 
       FIELD OF TECHNOLOGY 
       [0002]    The following describes a rotor blade extension and a wind turbine. 
       BACKGROUND 
       [0003]    Most of the wind energy that acts to turn the rotor of a wind turbine generator is harvested in the outermost part of wind turbine blades, and present-day wind turbines are designed to have increasingly large rotor diameters, whereby the rotor diameter is the diameter of a circle described by the rotor blade tips as they rotate. 
         [0004]    The outermost end of the blade, also referred to as the tip region, is subject to several strict design criteria, such as leading edge radius, profile contour, twist orientation, trailing edge thickness, etc. When a blade fails to meet these design criteria, either through direct manufacturing deviations or through deterioration of the leading and/or trailing edges during the lifetime of the wind turbine, the result will be a decrease in annual energy production (AEP) and an increase in noise levels for the turbine. 
         [0005]    The cost of wind energy produced by a wind turbine, i.e. the cost of manufacturing, erecting, commissioning, and maintaining the wind turbine, is inversely proportional to the AEP of the wind turbine. Therefore, it is desirable to increase the AEP of an existing rotor. One way of increasing the AEP of a wind turbine is to increase the rotor swept area, i.e. increasing the diameter of the rotor circle described by the blade tips. Extension of the rotor swept area by means of a blade extension can be done by either a root extension, in which an insert is placed between the hub and the root end of the blade; or by a tip extension placed outboard of the tip section of the blade. A root extension is generally a large and heavy section, since it must bear the complete loading of the blade. Furthermore, a root extension is aerodynamically inefficient. 
         [0006]    A further drawback of a root extension is that the airfoil part of the blade is effectively shifted “outward”, with the result that the aerodynamic properties of the altered blade can significantly deviate from the originally intended design, since the blade regions will operate in altered aerodynamic inflow conditions. In contrast, an extension mounted onto the blade tip can be designed to be aerodynamically efficient. For already installed wind turbines, tip extensions can be mounted onto each blade in a retro-fit step, and serve to extend the airfoil portion of the blade tip. A drawback of known tip extensions is that the increased tip speed increases the risk of deterioration of the edges in the blade tip region. 
         [0007]    The quality and shape of the blades in their outermost regions has a significant influence on the power output and AEP of the wind turbine. It follows that the overall performance of a wind turbine is highly sensitive to any deviations from a designed blade shape in the tip region of the blade. In known approaches, protection against deterioration of an edge such as the leading edge can be accomplished by applying a coating or a thin sheet of polymer tape. Corrective measures are effectively limited to being carried out for a new installation, and seldom performed on the blades of an existing rotor. In another approach, the correction of leading-edge or trailing-edge defects of a new rotor blade can to some extent be accomplished by making local repairs to the blade surface during manufacturing. Again, such repairs are economically feasible for deviations that are detected prior to installing the blades, and the scope of this type of repair can be limited by blade design constraints. 
       SUMMARY 
       [0008]    An aspect relates to a rotor blade extension for mounting over a tip of a rotor blade of a wind turbine, comprising an airfoil extension portion realized to extend the blade length; a leading edge portion realized to cover a portion of the profile of the leading edge of the rotor blade; and a trailing edge portion realized to cover a portion of the profile of the trailing edge of the rotor blade. 
         [0009]    An advantage of the rotor blade extension according to embodiments of the invention is that it allows for a correction of deviations in the blade shape. The correction of such deviations can be performed prior to installation of new rotor blades on a wind turbine, or in a retro-fit step of renovating damaged rotor blades of an existing wind turbine. The rotor blade extension according to embodiments of the invention also allows a remodeling of parts of the outer blade surface, for example, to introduce an entirely new shape at the blade tip region. 
         [0010]    A further advantage of the rotor blade extension according to embodiments of the invention is that the risk of leading edge erosion can be reduced, since the edges at the vulnerable outer end of the blade are effectively covered by the leading edge portion of the rotor blade extension which are designed to withstand higher levels of erosion. 
         [0011]    According to embodiments of the invention, the wind turbine comprises a number of rotor blades, and a rotor blade extension according to embodiments of the invention mounted to each rotor blade. 
         [0012]    An advantage of the wind turbine according to embodiments of the invention is the favorable increase in the AEP that can be achieved on account of the effective increase in rotor swept area. Furthermore, the aerodynamic behavior of the blades of the wind turbine according to embodiments of the invention is actively improved in a region further outward, to an extent greater than previously possible. 
         [0013]    Particularly advantageous embodiments and features of embodiments of the invention are given by the dependent claims, as revealed in the following description. Features of different claim categories may be combined as appropriate to give further embodiments not described herein. 
         [0014]    The airfoil portion of a rotor blade is defined by a number of parameters such as its chord length, i.e. the distance between the leading edge and trailing edge. In the following, the terms “airfoil extension portion” and “chord extension portion” may be used interchangeably. 
         [0015]    The blade length is the distance from the root end of the blade at its attachment to the hub and to the rest of the wind turbine to the distal tip end. In the following, the term “blade length” is used to denote the length of the original blade without the tip extension described herein. 
         [0016]    Preferably, the rotor blade extension is realized in one piece, i.e. the airfoil extension portion, the leading edge portion and the trailing edge portion are manufactured in one piece in a seamless manner. The airfoil extension portion covers the blade tip, while the edge portions extend inward in the direction of the blade root or hub. The blade extension may therefore have the appearance of a “hood” with a cut-out between the lateral edge portions. 
         [0017]    In a particular embodiment of the invention, the airfoil portion extends outboard, i.e. effectively increasing the rotor diameter, by a distance of at least 0.5% of the blade length, more preferably at least 2% of the blade length. Note that most preferably, the airfoil extension portion extends outboard of the tip of the rotor blade by a distance between 2% of the blade length and 5% of the blade length. 
         [0018]    As explained above, the quality of the leading and trailing edges of the blade is very important, since these have a direct influence on the annual energy production of the wind turbine. Therefore, in a preferred embodiment of the invention, the leading edge and/or trailing edge portion extends inboard by a distance of at least 1% of the blade length, more preferably at least 5% of the blade length, most preferably at least 10% of the blade length. The outer surfaces of an edge portion effectively assumes the function of the edge which it replaces, i.e. the outer surface of the leading edge portion assumes the function of the leading edge for the extent covered by the leading edge portion. 
         [0019]    In another embodiment of the invention, the trailing edge portion not only covers the trailing edge of the rotor blade, but also significantly extends the chord length of the arrangement comprising the original rotor blade and the rotor blade extension in chordwise direction. More particularly, the chord length of the arrangement is increased towards the trailing edge of the rotor blade, compared to the rotor blade alone, without the rotor blade extension. 
         [0020]    Likewise, in another embodiment of the invention, the leading edge portion not only covers the leading edge of the rotor blade, but also significantly extends the chord length of the arrangement comprising the original rotor blade and the rotor blade extension in chordwise direction. More particularly, the chord length of the arrangement is increased towards the leading edge of the rotor blade, compared to the rotor blade alone, without the rotor blade extension. 
         [0021]    Both embodiments may also be combined such that the arrangement is extended both at the trailing edge and at the leading edge compared to the rotor blade alone, without the rotor blade extension. 
         [0022]    A problem with known tip extensions is the increased blade loading, since the additional aerodynamic loading is transferred to the rest of the blade. Another critical issue is the increase in tip deflection compared to the original blade configuration. In many cases, and in particular for modern long flexible slender blades, an increase in loading, and in particular an increase in tip deflection, must be avoided on account of the permissible load envelope and the required tower clearance of the rotor. 
         [0023]    Therefore, in a particularly preferred embodiment of the invention, the rotor blade extension comprises a degree of “pre-bend” such that the airfoil extension portion is tilted outward from a longitudinal axis of the rotor blade. Here, the term “tilted outward” is to be understood such that the airfoil extension portion is bent to face “into the wind”, i.e. away from the tower. The pre-bent airfoil extension portion allows a favorable increase in rotor swept area, while maintaining an essentially constant, (or only negligibly increased) tip deflection. An advantage of the outwardly pre-bent airfoil extension portion of the rotor blade extension is that it becomes possible to upgrade existing wind turbine for which a tip deflection constraint applies. 
         [0024]    In a preferred embodiment of the wind turbine according to embodiments of the invention, a rotor blade exhibits an “outwardly bent” shape when stationary, i.e. it bends away from the tower when stationary, and assumes a straighter shape when rotating. With a rotor blade extension according to embodiments of the invention, for which the airfoil extension portion has an outwardly pre-bent form, the rotor swept area of such a pre-bent blade can be increased even further, since the rotor blade extension is designed in such a way as to further increase the distance between the extended blade tip and the tower. 
         [0025]    In a further preferred embodiment of the rotor blade extension, a longitudinal axis of the airfoil extension portion is skewed relative to the longitudinal axis of the rotor blade. When such a rotor blade extension is mounted onto a rotor blade, the blade exhibits “sweep”. The sweep introduced by the blade extension serves to further increase the bend-twist coupling, with the effect that blade loading is passively reduced. The positive effect is even more pronounced when the blade tip extension is mounted onto a “swept-back” blade, i.e. a blade for which the outer tip portion is already angled towards the trailing edge. 
         [0026]    The aerodynamic noise generated by a wind turbine rotor blade is directly related to the tip speed of the blade. Therefore, in a preferred embodiment of the invention, the rotor blade extension comprises a number of serrated or saw-toothed elements arranged on the trailing edge portion. Such elements serve to favorably reduce or even eliminate the additional aerodynamic noise impact that would otherwise be generated by the extended tip region. The rotor blade extension according to embodiments of the invention therefore makes it possible to achieve a noise- neutral tip extension, or with a marginal noise penalty, permitting a direct decrease in the cost of energy (by increasing the rotor swept area) while at the same time not increasing, or increasing marginally the overall noise level. 
         [0027]    Vortex generators can be used to avoid separation of the boundary layer in the outboard sections of the blade, and to increase the robustness of the turbine. Therefore, in a particularly preferred embodiment of the invention, the rotor blade extension comprises a number of vortex generators arranged on a surface of the rotor blade extension. Preferably the vortex generators are applied on the suction surface of the blade extension, for example along the leading edge portion. With such an arrangement of vortex generators, aerodynamic noise resulting from the rotor blade extension can be counteracted or even eliminated. 
         [0028]    In a further preferred embodiment of the invention, the rotor blade extension comprises a number of active devices realized to alter a surface shape of the rotor blade extension. For example, an active device can comprise a hydraulic, pneumatic, or piezoelectric device, or any other type of active actuator used to operate e.g. a flap at the trailing edge and can act to alleviate the loading of the turbine under different loading conditions. 
         [0029]    In a further preferred embodiment of the invention, the rotor blade extension comprises least one flow control device realized to react in response to a passive impulse or signal. For example, a flow control device such as a flexible flap can be activated according to the relative wind speed of the rotor blade. 
         [0030]    When the rotor blades rotate, the flow meets the leading edge first, which is therefore more exposed to environmental influences in comparison to the trailing edge. For this reason, the leading edge can tend to wear faster than the trailing edge. In a preferred embodiment of the invention, therefore, the leading edge portion extends further inboard than the trailing edge portion. 
         [0031]    Lightning strikes to rotor blades are quite common, and a rotor blade is therefore generally equipped with some means of lightning protection, for example a lightning conductor arranged along the length of the blade and a receptor at the tip end. A lightning strike will generally hit the blade at its outer end. Therefore, in a preferred embodiment of the invention, the rotor blade extension comprises a lightning protection arrangement. For example, a receptor and conductor can be arranged such that an electrical connection is made to an existing lightning protection arrangement of the blade to which the extension is mounted. 
         [0032]    A rotor blade is generally not hermetically sealed, and water can enter the blade in various ways. To prevent a build-up of water in the blade and to avoid the resulting damage during freezing conditions, a rotor blade generally comprises a drainage arrangement with small openings at the blade tip to allow moisture to leak out of the blade. In a preferred embodiment of the invention, the rotor blade extension also comprises a drainage arrangement, for example one or more holes at an outer end, so that water escaping from the original blade can also escape from the rotor blade extension. 
         [0033]    Preferably, the rotor blade extension is realized to fit flush over the blade tip. This can be achieved by applying a suitable manufacturing technique to make the rotor blade extension. For example, the rotor blade extension is preferably made using a casting technique, since this allows a greater degree of accuracy than the lamination procedures generally used to construct the original rotor blade. 
         [0034]    In a further preferred embodiment of the invention, the rotor blade extension can be made to have a signal color, so that the extensions (and therefore the rotor swept area of the wind turbine) are made visible, for example for aviation purposes or bird collision mitigation purposes. The signal color can be applied as a coating and/or can be incorporated in the material of the rotor blade extension. 
     
    
     
       BRIEF DESCRIPTION 
         [0035]    Some of the embodiments will be described in detail, with reference to the following figures, wherein like designations denote like members, wherein: 
           [0036]      FIG. 1  shows an extended rotor blade comprising a first embodiment of a rotor blade extension in place over a rotor blade tip; 
           [0037]      FIG. 2  shows schematically a cross-section of the first embodiment of the extended rotor blade of  FIG. 1 ; 
           [0038]      FIG. 3  shows a second embodiment of a rotor blade extension; 
           [0039]      FIG. 4  shows a cross-section of the second embodiment of the rotor blade extension of  FIG. 3 ; 
           [0040]      FIG. 5  shows a third embodiment of a rotor blade extension; 
           [0041]      FIG. 6  shows a fourth embodiment of a rotor blade extension; 
           [0042]      FIG. 7  shows an embodiment of a wind turbine, wherein rotor blades are not rotating; and 
           [0043]      FIG. 8  shows an embodiment of a wind turbine, wherein the wind turbine is operational. 
       
    
    
       [0044]    In the diagrams, like numbers refer to like objects throughout. Objects in the diagrams are not necessarily drawn to scale. 
       DETAILED DESCRIPTION 
       [0045]      FIG. 1  shows the outboard part of an extended rotor blade  4  of a wind turbine  3 . The extended rotor blade  4  comprises a first embodiment of a rotor blade extension  1  according to embodiments of the invention, which is in place over the tip  20  of the rotor blade  2 . The rotor blade  2  comprises a trailing edge TE and a leading edge LE. The rotor blade  2  is tapered, i.e. pointed, and comprises a tip  20 . Reference sign  20  refers to the tip of the “original” rotor blade  2 , i.e. the rotor blade  2  without any rotor blade extension. Once the rotor blade extension  1  is attached to the rotor blade  2 , the extended rotor blade  4  is obtained. Reference sign  21  refers to the tip of the extended rotor blade  4 , thus, it is also referred to as the “extended tip”  21 . 
         [0046]    The rotor blade extension  1  is shown to comprise an airfoil extension portion  1 _AF that effectively extends the length of the rotor blade  2 ; a leading edge portion  1 _LE that is used to cover or extend a portion of the leading edge LE of the rotor blade  2 ; and a trailing edge portion  1 _TE that is used to correct a profile of the trailing edge TE of the rotor blade  2 . Conventional tip extensions generally only comprise an airfoil extending portion that is used to cover or extend a portion of the trailing edge TE of the rotor blade  2 . In the blade extension  1  according to embodiments of the invention, not only is the overall blade length favorably increased by the airfoil portion  1 _AF, but the leading edge and trailing edges are also corrected and/or optimized by the portions  1 _LE,  1 _TE. 
         [0047]      FIG. 2  shows a cross-section of the extended rotor blade  4  at the line A-A of  FIG. 1 . The cross-sectional illustration shows how the leading edge portion  1 _LE covers or extends the original leading edge LE of the rotor blade  2 , and how the trailing edge portion  1 _TE covers or extends the original trailing edge TE of the rotor blade  2 . Deviations during manufacturing and/or sub-optimal design features can be covered, i.e. corrected by the correction portions  1 _LE,  1 _TE, as well as new designs can be elaborated with the use of these portions  1 _LE and  1 _TE. 
         [0048]      FIG. 3  shows a second embodiment of a rotor blade extension  1  according to embodiments of the invention. Here, the rotor blade extension  1  exhibits a significant degree of pre-bend, i.e. the airfoil portion  1 _AF is bent outwards so that, when mounted onto a rotor blade, the airfoil portion  1 _AF will face “into the wind”. 
         [0049]    The extent of the pre-bend is shown in cross-section in  FIG. 4 , which shows a longitudinal axis X 2  of the rotor blade  2  and a longitudinal axis X 1  extending through the airfoil portion  1 _AF of the rotor blade extension  1 . 
         [0050]    A first approach to characterize and quantify the extent of the pre-bend of the rotor blade extension  1  is an angle α subtended by these axes X 2 , X 1 . 
         [0051]    A second approach to characterize and quantify the extent of the pre-bend of the rotor blade extension  1  is the distance  22  between the extended tip  21  and the longitudinal axis X 2 . This distance  22  relates to the amount by which the distance between the tip of the rotor blade and the tower of the wind turbine is increased. 
         [0052]    For a rotor blade  2  with a blade length of fifty meters, a distance  22  between the extended tip  21  and the longitudinal axis X 2  of at least 0.5 meters is advantageous. Preferably, the distance  22  between the extended tip  21  and the longitudinal axis X 2  is at least one meter. 
         [0053]    Likewise, for a rotor blade  2  with a blade length of one hundred meters, a distance  22  between the extended tip  21  and the longitudinal axis X 2  of at least one meter is advantageous. Preferably, the distance  22  between the extended tip  21  and the longitudinal axis X 2  is at least two meters. 
         [0054]      FIG. 5  shows a third embodiment of a rotor blade extension  1  according to embodiments of the invention. In this example, the portions  1 _LE,  1 _TE have essentially the same length. The rotor blade  2  comprises a swept-back tip, i.e. the tip of the blade is angled more towards the trailing edge TE than towards the leading edge LE. In this embodiment, the airfoil portion  1 _AF is also “swept back”, so that the sweep of the original blade design is repeated or even increased by the sweep of the rotor blade extension  1 . This embodiment can be combined with the embodiment of  FIG. 4 , for example, to give a rotor blade extension  1  that exhibits sweep as well as pre-bend. 
         [0055]    The rotor blade extension  1  of  FIG. 5  also comprises a drainage hole  6  for enabling liquid, such as water, to exit the inside of the extended rotor blade  4 . Furthermore, the rotor blade extension  1  comprises an extension lightning receptor  7  for attracting lightning strikes, which typically strike a rotor blade in the proximity of the tip. The extension lightning receptor  7  is added to the lightning receptor  8  of the original rotor blade  2 , or it substitutes the lightning receptor  8  of the original rotor blade  2 . In both cases, the extension lightning receptor  7  and the lightning receptor  8  of the original rotor blade  2  are electrically connected by a connection element  8 . This enables to connect the extension lightning receptor  7  to an existing lightning conductor system of the rotor blade  2  and the wind turbine  3  as a whole. 
         [0056]      FIG. 6  shows a fourth embodiment of a rotor blade extension  1  according to embodiments of the invention. Here, vortex generator elements  11  have been applied to the leading edge portions  1 _LE on the suction side. Note that, additionally, vortex generator elements  11  may also be applied further outboard on the rotor blade extensionl, i.e. on the airfoil extension portion  1 _AF. In the fourth embodiment of a rotor blade extension  1 , also serrated elements  10  have been attached to the trailing edge portion  1 _TE. Alternatively, the serrated elements  10  may also be extended all the way to the distal end of the rotor blade extension  1 . The effect of these elements  10  is to reduce or eliminate the noise that would otherwise be generated on account of the increase in rotor swept area resulting from the blade extension. In the diagram, the lengths of the edge portions  1 _LE,  1 _TE are shown to be approximately equal, but their lengths may also be different. 
         [0057]      FIG. 7  and  FIG. 8  show an embodiment of a wind turbine  3  according to embodiments of the invention. 
         [0058]    In  FIG. 7 , the wind turbine  3  is at stand-still, so that the rotor blades  2  are not rotating. The diagram illustrates the position of a rotor blade  2  relative to the tower  30  of the wind turbine  3 , and indicates the distance d between the “original” tip  20  of the rotor blade  2  and the tower  30 . The diagram also indicates the increased distance d 1  between the extended tip  21  of the extended rotor blade  4  and the tower  30 . 
         [0059]    In  FIG. 8 , the wind turbine  3  is operational, and the rotor blades  2  are rotating. The diagram shows that the distance d load  between the extended tip  21  and the tower  30  is still favorably sufficient, i.e. the rotor blade extension  1  does not significantly alter the distance between tip of the extended rotor blade  4  and the tower  30  during operation of the wind turbine  3 . 
         [0060]    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. 
         [0061]    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.