Patent Publication Number: US-2022235735-A1

Title: Rotor blade and wind turbine

Description:
BACKGROUND 
     Technical Field 
     The invention relates to a rotor blade of a wind turbine, an associated wind turbine and an associated method. 
     Description of the Related Art 
     Attachments to rotor blades of wind turbines, for the purpose of increasing aerodynamic efficiency, are known. Known inter alia are eddy generators, also known as vortex generators, which are often attached in the region near the hub, preferably on the suction side of the rotor blade, and which ensure mixing of the boundary layer flow, as a result of which the flow remains close against the surface of the rotor blade over a longer distance before a flow separation occurs. 
     The delayed flow separation makes it possible to realize greater angles of attack and correspondingly greater maximum lift coefficients on the profile section. 
     In addition to the eddy generators, other aerodynamic attachments are also known. For example, aerodynamic flaps, or also so-called spoilers, also known as Gurney flaps in the technical literature, are used. 
     The Gurney flaps in this case are preferably attached in the region near the hub of the rotor blade, on the pressure side in the region of the trailing edge, i.e., on cylindrical or elliptical profiles in the region located just before the flow separation. 
       FIG. 2  shows, in schematic form, an example of a profile section  200  through a rotor blade of a wind turbine. A suction side  202  is located opposite a pressure side  204  and connects a leading edge  206  to a trailing edge  208 . A profile chord that has a rotor blade depth C extends between the leading edge  206  and a center of the trailing edge  208 . In this example, the profile is one that has a thick trailing edge  208 , having a thickness T. The thickness T is defined perpendicular to the rotor blade depth C. Inclined thick trailing edges  208  are also conceivable, there being a projection to the perpendicular to the rotor blade depth C, for comparability. Both a Gurney flap  220  and a splitter plate  230  are attached to, or in the region of, the trailing edge  208 . The Gurney flap  220  is realized, for example, in the form of a metal plate angle and in this example is perpendicular to the rotor blade depth C. The height H of the Gurney flap  220  denotes the extent of the Gurney flap  220  perpendicular to the rotor blade depth C from the trailing edge  208 . A length L of the splitter plate  230  denotes a dimension of the splitter plate  230  parallel to the rotor blade depth C beyond the trailing edge  208 . 
       FIG. 2  therefore shows a schematic rotor blade profile that has a flat or blunt trailing edge  208 , also referred to as a flatback profile. 
     A Gurney flap in principle enables the curvature of a profile to be significantly increased at the trailing edge, which can result in a significant increase in lift. However, this also has the disadvantage of increasing the aerodynamic moment c m  and, most importantly, also the drag coefficient c d . Gurney flaps are therefore useful where a high lift is required but a low drag coefficient is not absolutely necessary. In wind turbines, this is mainly the case in the region of the rotor blade root. Profiles having a flat trailing edge are also used in this region. 
     The splitter plate  230 , shown for example in  FIG. 2 , is attached approximately perpendicular to the trailing edge  208 , i.e., in particular in the chord direction, to reduce drag. This component is called a splitter plate because it divides the wake region of the trailing edge  208  into two parts. The use of the splitter plate  230  prevents the formation of a high-energy periodic vortex street behind profiles that have a flat trailing edge  208 . 
     The process of unsteady vortex shedding in the so-called Kármán vortex street extracts energy from the flow around the profile, and causes a high flow resistance. Often, acting at the profile trailing edge there is a pressure lower than ambient pressure, which causes the resulting force on the trailing edge of the profile to be contrary to the direction of movement of the profile. By deliberately interrupting the vortex interaction at the flat trailing edge  208 , the aim is to replace the unsteady vortices with two or more stationary vortices that extract significantly less energy from the flow around the profile. With a suitable design, this measure can significantly reduce the resistance of flatback profiles. A disadvantage of this solution is that slight losses in lift have to be accepted due to the dishing effect of the splitter plate  230 . 
     In summary, the following can be stated with regard to the use of attachments for passive flow control on flatback profiles. The Gurney flaps, which are already frequently used on wind turbines, increase the lift coefficient, but also increase the drag coefficient on the profile section, thereby reducing the lift-to-drag ratio in total. The splitter plates, on the other hand, which hitherto are mainly known from research, reduce the pressure resistance at the flat trailing edge and thus the drag coefficient with, however, reduced lift coefficients, thereby weakening in total the desired increase in the lift-to-drag ratio. 
     Each attachment, applied in isolation, cannot optimally increase the turbine yield, due to the described properties. Only an increase in lift with a simultaneous reduction in drag would be optimally advantageous with respect to increasing the performance of the rotor blade. 
     The German Patent and Trade Mark Office has searched the following prior art in the priority application for the present application: DE 10 2011 012 965 A1, DE 10 2018 103 678 A1, DE 20 2016 101 461 U1, US 2010/0 047 070 A1, US 2019/0 024 631 A1, U.S. Pat. No. 5,265,830 A, EP 2 806 156 A1, WO 2016/055 076 A1. 
     BRIEF SUMMARY 
     Provided is a rotor blade having improved properties. In particular, provided is a rotor blade combining a Gurney Flap and a splitter plate. 
     Provided is a rotor blade of a wind turbine, having a rotor blade length, a rotor blade depth extending over the rotor blade length, a rotor blade thickness extending over the rotor blade length, and a thickness of a trailing edge of the rotor blade extending over the rotor blade length. In a region of the rotor blade length, the rotor blade simultaneously has a splitter plate that has a predetermined length and a Gurney flap that has a predetermined height. A ratio of the height of the Gurney flap to the length of the splitter plate at a particular position in the direction of the rotor blade length is selected in such a manner that a threshold value that decreases with a decreasing relative profile thickness, which is defined as a ratio of the rotor blade thickness to the rotor blade depth, is not reached. 
     The relative profile thickness of a rotor blade profile defines a threshold value that, in turn, influences a ratio of the length of the splitter plate and the height of the Gurney flap. The higher the relative profile thickness, the higher is the threshold value of the ratio of the height of the Gurney flap and the length of the splitter plate. In other words, the higher the relative profile thickness, the higher is the Gurney flap in relation to the length of the splitter plate. 
     The inventors of the present invention have specifically discovered that, in particular, excessively high Gurney flaps have a negative effect on the performance of the rotor blade, but this effect can be compensated by longer splitter plates, higher relative profile thicknesses, etc. It is thus a particular perception that the threshold value, which is compared with the ratio of the height of the Gurney flap and the length of the splitter plate, is dependent on the relative profile thickness and decreases with the same. 
     Preferably, the height of the Gurney flap is in any case less than the length of the splitter plate. 
     Preferably, a quantity ϵ is determined as a ratio of a de-dimensioned height of the Gurney flap and a de-dimensioned length of the splitter plate, wherein the height of the Gurney flap and the length of the splitter plate of the particular position in the direction of the rotor blade length are selected in such a manner that a threshold value that decreases with a decreasing relative profile thickness, which is defined as a ratio of the rotor blade thickness to the rotor blade depth, is not reached. 
     The quantity ϵ affords a substantially profile-independent evaluation, since both the height of the Gurney flap and the length of the splitter plate are de-dimensionalized, and thus non-dependent on the specific design of the rotor blade. The quantity ϵ therefore affords comparability and a generally valid evaluation in order to determine the length of the splitter plate, or the height of the Gurney flap. 
     At the particular position in the direction of the rotor blade length means that a particular profile section of the rotor blade is used for evaluation. Both the splitter plate and the Gurney flap have a certain extent in the longitudinal direction. The evaluation of the ratio is effected at a particular position, i.e., with respect to a particular profile section. 
     It is particularly preferred that the ratio be maintained over a predetermined length in the direction of the rotor blade length. In particular, the ratio evaluated is achieved over the entire region in which the splitter plate and the Gurney flap extend together in the longitudinal direction of the rotor blade. 
     Preferably, the de-dimensioned height of the Gurney flap is a ratio of the height of the Gurney flap and the rotor blade depth, and/or the de-dimensioned length of the splitter plate is a ratio of the length of the splitter plate and the thickness of the trailing edge. 
     Preferably, the Gurney flap and the splitter plate are dimensioned in such a manner that the quantity ϵ over the region of the Gurney flap and the splitter plate in the longitudinal direction of the rotor blade is in a range of between 0 and a function of the relative thickness ϵ limit =ƒ(relTh(r)): 
       0≤ϵ(relTh( r ))≤ϵ limit (relTh( r )).
 
     Preferably, the function of the relative thickness ϵ limit  is defined as follows: 
     
       
         
           
             
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     Preferably, the length of the splitter plate is in a range of from 25% to 150% of the thickness of the trailing edge at the particular position in the longitudinal direction of the rotor blade. 
     Preferably, the height of the Gurney flap is at most 30%, in particular at most 10%, and especially preferably at most 3% of the rotor blade depth at the particular position in the longitudinal direction of the rotor blade. The more closely the profile at the particular position in the longitudinal direction of the rotor blade approaches a circular cylinder, i.e., the greater the relative thickness becomes, the greater preferably is the maximally fitted Gurney flap, relative to the rotor blade depth. 
     Preferably, therefore, the maximum height of the Gurney flap is a function of the relative thickness. The lower the relative thickness, the lower preferably is also the height of the Gurney flap, particularly preferably relative to the rotor blade depth. 
     Preferably, the rotor blade thickness in the region of the Gurney flap and the splitter plate is at least 16% of the rotor blade depth. 
     Provided is a wind turbine having at least one rotor blade according to an embodiment of the invention. 
     Provided is a wind farm having a plurality of wind turbines according to an embodiment of the invention. 
     Both the wind turbine and the wind farm may be combined with any preferred designs of the rotor blade to achieve the same advantages. 
     Provided is a method for designing a rotor blade of a wind turbine, having a rotor blade length, a rotor blade depth extending over the rotor blade length, a rotor blade thickness extending over the rotor blade length, and a thickness of a trailing edge of the rotor blade extending over the rotor blade length. In a region of the rotor blade length, the rotor blade simultaneously has a splitter plate that has a predetermined length and a Gurney flap that has a predetermined height. A ratio of the height of the Gurney flap to the length of the splitter plate at a particular position in the direction of the rotor blade length is determined in such a manner that a threshold value that decreases with a relative profile thickness, which is defined as a ratio of rotor blade thickness to the rotor blade depth, is not reached. 
     The method may be combined with the preferred designs of the rotor blade, the same advantages being achieved analogously. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       Further advantages and preferred embodiments are described below with reference to the accompanying drawings. There are shown: 
         FIG. 1  shows an example of a wind turbine, in schematic form, 
         FIG. 2  shows an example of a profile section of a rotor blade, in schematic form, 
         FIG. 3  shows various diagrams of aerodynamic index quantities, as examples, in schematic form, 
         FIG. 4  shows an example of a characteristic of particular variables over a relative thickness of the rotor blade, in schematic form, and 
         FIG. 5  shows an example of a characteristic of a quantity over the relative thickness of the rotor blade, in schematic form. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  shows a schematic representation of a wind turbine according to the invention. The wind turbine  100  comprises a tower  102 , and a nacelle  104  on the tower  102 . Provided on the nacelle  104  there is an aerodynamic rotor  106  that has three rotor blades  108 , each having a rotor blade length R, and a spinner  110 . When the wind turbine is in operation, the aerodynamic rotor  106  is caused to rotate by the wind, and thus also rotates an electrodynamic rotor of a generator that is directly or indirectly coupled to the aerodynamic rotor  106 . The electrical generator is arranged in the nacelle  104  and generates electrical energy. The pitch angles of the rotor blades  108  can be varied by pitch motors at the rotor blade roots of the respective rotor blades  108 . 
       FIG. 2  shows, as already described, a profile section  200  of the rotor blade  108 . Due to the flat, or blunt, trailing edge  208 , the profile shown by way of example is regularly located in the region of the rotor blade root, i.e., in a region of the rotor blade  108  close to the hub. 
     The profile section  200  shows in particular the combination of two aerodynamic attachments, namely the Gurney flap  220  and the splitter plate  230 . The two attachments are now ingeniously combined and dimensioned in such a manner that an improved rotor blade is obtained as a result. 
       FIG. 3  shows, in schematic form, examples of characteristics of important index quantities of a rotor blade for a bare rotor blade, for one having only a Gurney flap  220 , only a splitter plate  230 , and a combination of a splitter plate  230  and a Gurney flap  220 , which achieves the optimization described herein. A bare rotor blade means, in particular, a rotor blade without aerodynamic attachments. 
     In a first diagram  310 , the profile of a lift coefficient Cl is plotted on the vertical against the profile angle of attack AoA on the horizontal. In a diagram  320 , a drag coefficient Cd is also plotted against the profile angle of attack AoA. A diagram  330  plots the lift-to-drag ratio L/D against the profile angle of attack AoA, and a diagram  340  plots the moment coefficient Cm against the profile angle of attack AoA. 
     In each of the diagrams  310 ,  320 ,  330 ,  340  the characteristic of the bare rotor blade is shown as a graph  302 , the characteristic with only the Gurney flap  220  is shown as a graph  304 , the characteristic of the rotor blade with only the splitter plate  230  is shown as a graph  306 , and the characteristic the rotor blade with the combination of splitter plate  230  and Gurney flap  220  optimized is represented as a graph  308 . 
     It can be seen that the combination of Gurney flap  220  and splitter plate  230  produces a greatest possible increase in performance. In this case, the drag reducing effect of the splitter plate  230  and, at the same time, the lift increasing effect of the Gurney flap  220  are obtained, because the attachments, especially their relation to the rotor blade profile, are optimized. Thus, the lift-to-drag ratio L/D, in particular of flatback airfoils, can be significantly increased, which in turn has a positive effect on the efficiency of the wind turbine. 
     Preferably, the quantity ϵ is therefore defined for dimensioning the combined application of Gurney flap  220  and splitter plate  230 . 
     
       
         
           
             ɛ 
             = 
             
               
                 H 
                 ⁢ 
                 T 
               
               
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                 ⁢ 
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     Therein:
         H: height of the Gurney flap  220     L: length of the splitter plate  230     T: thickness of the trailing edge  208  of the flatback profile   C: blade depth, i.e., length of the chord of the rotor blade  108 .       

     Shown in schematic form in  FIG. 4 , as an example, is a diagram  400 , in which characteristics  410  of a de-dimensioned splitter plate length L/T and characteristics  420  of a de-dimensioned Gurney flap height H/C are represented as a function of a relative thickness, on the horizontal axis, for different variants of suitable combinations of a Gurney flap  220  and splitter plate  230 . 
     The scale of the de-dimensioned splitter plate length L/T shown on the left in the diagram  400  thus relates to the characteristics  410 , while the scale of the de-dimensioned Gurney flap height H/C shown on the right side relates to the characteristics  420 . Related variants are represented in the characteristics  410 ,  420  by the same symbols, i.e., by rectangles or lozenges. It can be seen that in an optimized region the relative Gurney flap heights H/C decrease with decreasing relative thickness and remain constant from a certain range onwards, in the example above from a relative thickness of 0.45 at most. In contrast, it has been found that the optimized de-dimensioned splitter plate length L/T remains constant, or even increases, with decreasing relative thickness. 
     If the characteristics  410  and  420  are correlated to each other and the above-mentioned quantity ϵ is used for this purpose, the range to be protected can be found, which can be seen in schematic form, as an example, in the diagram  500  shown in  FIG. 5 . 
     In  FIG. 5  characteristics  510  for the dimensionless quantity ϵ are plotted on the vertical for different relative thicknesses on the horizontal. It can be seen that the characteristics of all preferred variants of combinations of relative thickness, length L of the splitter plate  230  and height H of the Gurney flap  220  lie within an advantageous quantity range  520 . The quantity range  520  is determined by an upper bound that decreases with decreasing relative thickness, in particular decreases linearly, and by a lower bound, which in this example is 0. 
     
       
         
           
             
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     The relative thickness, “relTh”, is a function of a usually relative position r in the longitudinal direction of the rotor blade R, where here a position of 0 corresponds to the rotor blade root or, in an alternative definition, to the axis of rotation, and a position of 1 corresponds to the rotor blade tip. 
     Against this background, the quantity range  520  may be described by the formula 
       0≤ϵ(relTh( r ))≤ϵ limit (relTh( r ))
 
     As long as the quantity ϵ lies in this range, with the upper bound decreasing as the relative profile thickness decreases, i.e., usually with increasing relative radius r, the result is a particularly attractive rotor blade that combines the advantages of the splitter plate  230  and the Gurney flap  220 . Thus, when both components are used simultaneously in the longitudinal direction of the rotor blade, the in each case both drag-reducing effect of the splitter plate  230  and the lift-increasing effect of the Gurney flap  220  are obtained. 
     A field of application is, in particular, rotor blades  108  for and on wind turbines  100  having rotor blade profiles  200  that have a flat trailing edge  208 , so-called flatback profiles. Furthermore, the invention is also applicable to rotor blades  108  having profiles that have a pointed trailing edge, to the trailing edge of which a Gurney flap has been attached for the purpose of increasing lift.