Abstract:
A blade for a wind turbine rotor with a hub is described. The object of the invention is to provide a wind turbine blade of the modern, aerodynamic type with improved regulating properties to enhance the adjustment of the wind turbine to various wind conditions, and where no mechanical parts are needed near the tip area. At least one slot or a number of holes arranged in at least one longitudinally extending zone, thereby allowing an interior cavity of the blade to communicate with the exterior. The amount of air emitted from the interior cavity to the exterior is also regulated to alter the aerodynamic properties of the blade.

Description:
This is a National Phase Application filed under 35 U.S.C. 371 as a national stage of PCT/DK2008/000004, filed 4 Jan. 2008, an application claiming priority benefit from Denmark Application No. PA200700013, filed 5 Jan. 2007, the entire content of each of which is hereby incorporated by reference in its entirety. 
     TECHNICAL FIELD 
     The present invention relates to a blade for a wind turbine rotor with a hub, the blade extending in a longitudinal direction along a longitudinal axis and having a root area closest to the hub and a tip region furthest away from the hub, the blade along at least a portion of the longitudinal direction of the blade having an envelope, which defines an airfoil profile having, in normal operation, a pressure side and a suction side, and further having a leading edge and a trailing edge, which edges define an airfoil chord line extending between them, the blade including adjustable lift-regulating means extending in the longitudinal direction of the blade, and activating means by means of which the lift-regulating means can be adjusted and thus alter the aerodynamic properties of the blade, the lift-regulating means being adapted and arranged so that by activation of the activating means, the lift can be reduced in a zone extending in the longitudinal direction of the blade from a first position in proximity to the blade tip to a second position between the first position and the root area and this second position being variable in the longitudinal direction of the blade. The invention further relates to a wind turbine rotor including such blades, to a wind turbine and to a method of controlling such a wind turbine. 
     BACKGROUND 
     Modern wind mills, also called wind turbines or wind engines, are used to produce electricity. They are often very large structures with blades of up to and in excess of 60 meters and made from fibre-reinforced polymer structures, such as shell elements. These wind turbines are provided with control devices which may prevent an overloading of the wind turbine and the blades at wind gusts and high wind speeds. Such control devices can also be used to slowing the turbine down and bringing it to a complete halt, if the wind speed becomes too high. In addition to these devices the turbine may comprise a braking device arranged in communication with the main shaft of the wind turbine. 
     The control devices may be formed of pitch-controlled blades mounted such on the hub that they are able to turn about the longitudinal axis. The blades may thus be continuously adjusted to provide the lift rendering the desired power. In so-called stall-controlled wind turbines the blades are fixedly mounted on the hub and thus unable to turn about their longitudinal axis. The stall properties of the blades are used to reduce the aerodynamic lift and thus the power output. 
     U.S. Pat. No. 6,361,275 (Wobben) discloses a wind turbine, wherein the pitch angles of each wind turbine blade can be adjusted independently such that corrections for varying wind speeds in the swept area of the wind turbine blades can be made. The wind speed often increases with the distance to the surface of the ground and it may thus be advantageous to adjust the lift power of the blades such that the load on the blade is substantially uniform during its entire rotation. However, this solution may also be used to compensate for other types of variations in the wind speed in the rotor plane. 
     The wind turbine according to U.S. Pat. No. 6,361,275 is, however, encumbered by the drawback that the adjustment cannot be made particularly fast, the conventional pitch hydraulics of the blade being used for this purpose. The hydraulics is not adapted for fast and almost instantaneous movements, since a wind turbine blade, whose dead load may exceed 10 tons, has a comparatively large inertia. 
     As mentioned above, particularly at large wind turbines, e.g. offshore wind turbines, the wind speed may vary greatly locally in the area swept by the rotor. The rotor may have a diameter of more than 120 meters, for which reason the wind speed may vary greatly due to local wind gusts and turbulence both in height and width. 
     U.S. Pat. No. 4,692,095 discloses a wind turbine blade according to the preamble to claim  1 . 
     WO 2004/099608 also discloses a wind turbine blade according to the preamble of claim  1 , wherein the size of the zone with reduced lift can be varied gradually by use of active flaps. 
     DISCLOSURE OF THE INVENTION 
     According to a first aspect, the object of the invention is to provide a wind turbine blade of the modern, aerodynamic type with improved regulating properties to enhance the adjustment of the wind turbine to various wind conditions, and where no mechanical parts are needed near the tip area. 
     In accordance with the invention, this object is obtained by the lift-regulating means being formed of at least one slot or a number of holes arranged in at least one longitudinally extending zone, thereby allowing an interior cavity of the blade to communicate with the exterior, and the lift-regulating means being adjustable by means of one or more activating means to regulate the amount of air emitted from the interior cavity to the exterior to alter the aerodynamic properties of the blade. 
     Thereby, a simple solution is provided for lowering or removing the lift of the longitudinally extending zone, at the same time lowering the load or bending moment at the hub. No mechanical parts are needed at the tip region of the blade in order to lower the lift of the tip region. Therefore, the blade according to the invention is in practice maintenance-free. 
     The wind turbine may operate at high wind speeds as the lift from the blade tip and inwards can be reduced as the wind speed increases. As a result, the turbine is operational at wind speeds above the so-called “cut-out wind speed”, at which the turbine otherwise would have been stopped due to an excessive load. These options make it possible to provide a wind turbine with longer blades than usual, said blades being relieved of the load as the wind speed increases, whereby the operating range of the wind turbine as regards wind speed is increased. A considerable advantage is that the bending moment to which the blade root is subjected can be reduced. The forces acting on the outermost portion of the blade adjacent the blade tip have a comparatively heavy impact on the bending moment at the blade root, since the moment is given by distance times force, for which reason it is advantageous to reduce the lift in the outermost portion of the blade. 
     By lift-regulating means are meant means that in any way can lower the lift of the blade. Therefore, these means can also be stall-regulating means. 
     According to a preferred embodiment, the blade comprises a number of adjacent longitudinally extending zones, each comprising at least one slot or a number of holes. Thereby, it is possible to reduce the lift gradually along the longitudinal extent of the blade by reducing the lift of the individual zones, thereby also gradually lowering the load at the hub. 
     According to another preferred embodiment, the longitudinally extending zone is provided at the aerodynamic pressure side of the blade. This provides the most efficient way of reducing the lift. 
     According to yet another preferred embodiment, the at least one slot or group of holes are provided near the trailing edge of the blade. Thereby, the function of the slots or holes corresponds to the function of an active flap arranged at the trailing edge of the blade. 
     Alternatively or in addition thereto, the slots or holes can be provided at the leading edge of the blade, thereby being adapted to altering the stall properties of the blade. Such means promote the stall tendency of the blade, i.e. the formation of separated air flows on the aerodynamic suction side of the blade, whereby the lift is reduced. 
     According to one embodiment of the blade, the slots or group of holes each are angled between 0 and 90 degrees compared to the tangent line of the envelope of the blade, or between 5 and 60 degrees or between 10 to 45 degrees. These angles provide the best performance for reducing the lift. The lift reduction also depends on the pressure or velocity of the emitted air. In general it will be preferred to have as low an emitting angle as possible in order to keep the aerodynamic resistance or drag as low as possible. However, in this case the emission pressure should be relatively high in order to achieve the desired lift reduction. 
     According to another embodiment of the blade, the longitudinally extending zones comprise a number of slots or holes, which have different angles compared to the tangent line of the envelope of the blade. For instance, every second hole can be angled in a first angle, whereas the rest are angled in a second angle. If the blade is adapted to only emit air through the holes having the first angle or the holes having the second angle or both sets of holes or not emitting air at all, the lift-regulating means can take four different settings, thereby functioning similar to an active flap. 
     Alternatively, the slots can all be angled in the same angle, but the emission of air can be controlled separately. For instance, every second hole can be controlled by first activation means, whereas the rest are controlled by second activation means. Thereby, the lift can be lowered gradually in each zone. 
     According to a preferred embodiment, the lift-regulating means are adjustable by use of pressurised air. Thereby, a simple method for rapidly reducing the lift of the blade tip and thus also the load at the hub is obtained. The activating means can in this case for instance be an air compressor. 
     According to another preferred embodiment, the activating means are one or more valve means. Thereby, an alternative method for rapidly changing the load at the hub is provided. 
     According to yet another preferred embodiment, the blade comprises an air intake communicating with the interior of the blade. Preferably, the intake is provided at the root end of the blade. Thereby, air drawn into the interior cavity of the blade near the root area is therefore directed in the longitudinal direction of the blade inside the interior cavity due to the centrifugal force from the rotor movement, and the air is emitted via the at least one slot or group of holes. Thereby, the lift of the blade can be altered passively using the centrifugal force of the blade only. 
     According to a preferred embodiment of the blade, the interior cavity of the blade comprises a number of individual cavities, each communicating with the exterior via separate slots or groups of holes, which are provided in separate longitudinal sections of the longitudinal direction of the blade. Each of these cavities can be provided with a separate valve means. Thereby, the lift of the individual longitudinal sections can be altered separately by opening the corresponding valve member. The individual cavities can be formed by separating walls. Alternatively, the cavities can be formed by tubes or similar, for instance connected to an air compressor. 
     According to a particular embodiment of the blade, the valve members are adapted to open automatically, when the rotational velocity of the rotor exceeds a predetermined value. The centrifugal force may for instance be sufficient to open the valves, when the centrifugal force exceeds a certain value. 
     Alternatively, the blade comprises load sensors arranged in or on the blade and measuring wind loads in form of for instance wind pressure or strain. These sensors may suitably be used to adjust the lift-regulating means. 
     According to an advantageous embodiment, the blade comprises a control system with a control unit connected with the activating means and the load sensors, so that the control unit may activate the activating means and thus adjust the amount of air emitted via the at least one slot or groups of holes in accordance with the measurements mad by the load sensors. Such a wind turbine blade may thus be “automatically controlled” in that it per se adapts its lift according to the load and thus requires no external control. 
     According to a particularly advantageous embodiment of the blade, the control system is adapted so that it can be linked with other corresponding blades on the same wind turbine to allow activation of the activating means on the basis of the load measurements from one or more of the other blades. As a result, for instance when the turbine is hit by a wind gust, the blade may more easily attain optimum lift properties based on the load data received from another blade before it is subjected to the loads to which the other blade just has been subjected. In a typical situation the blades are subjected to wind speeds, which are higher when they are in the upper portion of the rotor plane during their rotation than when they are in the lower portion of the rotor plane. A blade may thus receive data from an upwardly facing blade and make adaptation before it per se reaches its upward position during rotation. At yaw errors, i.e. when the nacelle is not adjusted in the optimum manner in relation to the wind direction, the blades are also subjected to loads, which vary according to their position in the rotational plane. Such varying loads may also be compensated by means of the blade according to the invention. 
     A second aspect of the invention relates to a wind turbine rotor having a number of such wind turbine blades, preferably two or three, said wind turbine rotor including a central control system with a control unit, e.g. built-in in the rotor hub, and connected with the load sensors and activating means of each blade to allow the control unit to adjust the amount of air emitted via the at least one slot or groups of holes of one or more of the blades based on the load measurements from each blade and/or the setting of lift-regulating means. 
     The blade is preferably fabricated as a shell element, the holes or slots thus being formed in the shell wall. 
     The invention also relates to a wind turbine comprising the above wind turbine blades or the above rotor. Preferably, the wind turbine has a near horizontal rotor shaft. 
     The method is particularly suitable for controlling a wind turbine having pitch-controlled blades and wherein the rotational speed of the blades is kept substantially constant. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention is explained in greater detail below with reference to the drawings illustrating embodiments of the invention and in which 
         FIG. 1  shows a wind turbine, 
         FIG. 2  is a schematic view of a first embodiment of a blade according to the invention, 
         FIG. 3  is a cross section along the line I-I in  FIG. 2 , 
         FIG. 4  is a schematic view of a second embodiment of a blade according to the invention, 
         FIG. 5  is a schematic view of a third embodiment of a blade according to the invention, 
         FIG. 6  is a schematic view of a wind turbine rotor with a control system, 
         FIG. 7  is a side view of a wind turbine blade, 
         FIG. 8  is a graph showing the lift of a wind turbine blade profile as a function of the distance to the blade root, and 
         FIG. 9  is a graph as the one shown in  FIG. 8 , the lift here being reduced in a zone along an outer portion of the blade. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  illustrates a conventional modern wind turbine with a tower  4 , a nacelle  6  and a rotor including a hub  8  and three blades  10  extending from the hub  8 . The blades  10  may be provided with lift-regulating means, which by activation of the activating means at high wind speeds can gradually reduce the lift of the blade in a zone extending from the blade tip  14  to a position between the blade tip and the blade root  16 . As a result, at high wind speeds the blade  10  is relieved in a zone extending from the blade tip  14  and inwardly towards the root  16  such that a wind turbine  2  with a given blade  10  can remain in operation at higher wind speed than usual. The invention also renders it possible to provide a wind turbine  2  with longer blades  10 ′ than usually, said blades merely being relieved by the lift-regulating means at increasing wind speeds and thus at increasing loads such that an overloading of the turbine and the blade is prevented. 
       FIG. 2  shows a schematic view of a first embodiment of a wind turbine blade according to the invention. The blade  10  comprises three elongated slots  12 ,  13 ,  15 . A first slot  12  is placed near the tip  14  of the blade. The first slot is connected to a first internal cavity  22  or first air reservoir, which thereby can communicate with the exterior of the blade. A first valve means  32  is arranged within the first cavity  22 . A second slot  13  is placed adjacent the first slot  12 . The second slot  13  is connected to a second internal cavity  23  or second air reservoir, which thereby can communicate with the exterior of the blade. A second valve means  33  is arranged within the second cavity  23 . A third slot  15  is placed adjacent the third slot  15 . The third slot  15  is connected to a third internal cavity  25  or third air reservoir, which thereby can communicate with the exterior of the blade. A third valve means  35  is arranged within the third cavity  25 . The first  22  and the second  23  cavity are separated by a first separating wall  27 , and the second  23  and the third  25  cavity are separated by a second separating wall  28 . 
     The blade  10  comprises an intake in the root area  16 . The intake is preferably arranged so that air is drawn in through the hub, but the intake can also be arranged as for instance a slot in the root area  16 . 
       FIG. 3  shows a cross-section through the blade along the line I-I in  FIG. 2 . The blade  10  comprises a leading edge  18  and a trailing edge  17 , a pressure side  20 , and a suction side  19 . As seen, the slots  12  are arranged at the pressure side  20  near the trailing edge  17  of the blade. The slot  12  is angled with the angle □ compared to the tangential line of the envelope of the blade. By emitting air through the slot  12 , the aerodynamic properties of the blade are changed and the lift can be significantly reduced. Therefore, the slots functions as lift-regulating means. The slot can also be arranged so that it emits air exactly at or in the close vicinity of the trailing edge  17 . In this case, the slot should preferably be angled in an upward angle compared to the chord line extending between the trailing edge  17  and the leading edge  18 . 
     By opening the first valve means  32 , air is drawn in via the hub (or the intake slot near the root  16 ) and is moved in the longitudinal direction of the blade  10  due to the rotational force from the rotor movement. Due to the rotational force, the air pressure in the internal cavity will also increase in the longitudinal direction of the blade. As a consequence an overpressure builds up, and the air is emitted through the first slot  12 , thereby reducing the lift in the longitudinally extending region in which the first slot  12  is arranged. Similarly, the lift in a second longitudinally extending region can be reduced by opening the second valve means  33 , and the lift can be reduced a third longitudinally extending region by opening the third valve means  35 . Thereby, the lift can be reduced gradually along the longitudinal direction of the blade. 
     The blade may additionally comprise holes or slots near the leading edge  18  of the blade  10 . Such leading edge holes can function as stall-generating means. 
       FIG. 4  shows a second embodiment of a blade according to the invention. The blade  10 ″ of the second embodiment corresponds to the first embodiment shown in  FIG. 2  with the exception that the cavities are replaced by air tubes, so that a first tub  22 ″ is connected to a first slot  12 ″, a second tube  23 ″ is connected to a second slot  23 ″, and a third tube  25 ″ is connected to a third tube  25 ″. The tubes may optionally be connected to the slots via air reservoirs formed near the slots. 
     It is advantageous that the lift of the different longitudinally extending regions can be reduced relatively fast, especially if the lift has to be adjusted one or more times for each rotation of the wind turbine blade. Therefore, the tubes can in the other end be connected to an air compressor. Thereby, the emission of air can quickly be turned on or off. The passive solution using valve means and the rotational force for emission of air is also sufficient to achieve this effect. However, using an air compressor has the additional advantage, that the velocity of emitted air can be regulated, thereby the lift reduction can be controlled even further. 
       FIG. 5  shows a third embodiment of a blade  10 ′″ according to the invention. Instead of slots, the lift-regulating means are formed by groups of holes  42 ,  43 ,  45 ,  52 ,  53 ,  55 , so that a first longitudinally extending zone comprises a group of first holes  42  and a group of second holes  52 , a second longitudinally extending zone comprises a group of first holes  43  and a group of second holes  53 , and a third longitudinally extending zone comprises a group of first holes  45  and a group of second holes  55 . 
     The first groups of holes and second groups of holes can be connected to separate air reservoirs or tubes. Thereby, it is possible to reduce the lift to a first setting by emitting air through the first group of holes, reduce the lift to a second setting by emitting air through the second group of holes, and to a third setting by emitting air through both group of holes. Thus, it is possible to reduce the lift gradually in the different longitudinally extending zones. 
     However, the groups of holes can also be arranged in single rows. In this case, it can also be possible to reduce the lift gradually in the different zones by letting for instance every second hole be connected to separate air reservoirs or tubes. 
       FIG. 6  is a schematic view of a rotor with a hub  8  and three wind turbine blades according to the invention extending from the hub  8 . Each blade is provided with a control system including an electronic control unit  3  connected to the activating means of the lift-regulating means  12 ,  13 ,  15 , i.e. for instance controlling valve means or an air compressor. The control unit  3  is further connected to load sensors  5  measuring the load on the wind turbine blade. The load sensors  5  may for instance be strain gauges mounted on the inner face of the blade shell or a bracing interconnecting the inner faces of the blade shell and thus measures the strain caused by the wind load. Optionally pressure gauges measuring the wind pressure on the blade may be used as load sensors. 
     As evident in  FIG. 6 , the control units  3  of each blade may be interconnected such that data about the load on a first blade or the lift setting thereof is transmitted to the other blades. As a result the lift regulating means  12 ,  13 ,  15  thereof can be adjusted to the wind action to which they are subjected, when they adopt the position of the first blade during rotation. 
     The control systems of the blades may be interconnected in such a manner that a single control unit  3  controls all of the lift-regulating means of the blades  10 . This control unit  3  may be placed in one of the blades, in the hub, in the nacelle, in the turbine tower  4  or outside of the turbine. 
       FIG. 7  is a side view of a wind turbine blade.  FIG. 8  shows a graph diagrammatically illustrating the lift L of the blade as a function of the distance X from the blade root. The lift of the blade thus increases gradually along the blade and decreases to 0 at the blade tip. 
       FIG. 9  illustrates a situation, in which the lift-regulating means  12 ,  13 ,  15  are activated along an outer zone of the blade. In this zone the lift has been reduced to nearly 0. The length of this zone is adjustable depending on the wind load, the abrupt transition can be moved inwards towards the blade root—to the left in FIG.  9 —or outwards towards the blade tip—to the right in  FIG. 9 . The lift-regulating means and the activating means may be formed and/or adjustable such that the lift is gradually reduced in the direction towards the blade tip. By using a number of stepwise adjustable settings, the graph in the activated zone may be shaped as a flight of steps. 
     In addition to wind speed registrations and strain measurements, power output data from the generator in the nacelle may also be used to adjust the lift-regulating means. 
     Wind predictions may also be performed using the solution described in WO 98/42980, in which the wind speed at a certain distance in front of the wind turbine is measured by means of laser equipment. This method is advantageously combined with the flaps described above, the blades thus being “ready” when the predetermined wind situation occurs in the rotor plane. 
     The method of adjusting the aerodynamic properties of the blades described above makes it easier—particularly with pitch-controlled wind turbines—to maintain a substantially constant rotational speed even if the wind turbine is affected by wind gusts, e.g. due to turbulence, and other variations in the wind speed. Furthermore, the load on the blades is significantly more uniform in that variations in the speed profile of the wind can be part of the regulating parameters such that corrections for wind variations in the rotor plane can be made. It is also possible to make corrections for the wind shade exerted by the wind turbine tower on the rotor. 
     The invention may be used in connection with stall-controlled wind turbines with a constant speed of revolution and in connection with wind turbines with a varying speed of revolution. The invention is, however, particularly advantageous when used to operate pitch-controlled wind turbines with a substantially constant rate. The cost of the expensive electric components required for converting the alternating current of varying frequency generated by wind turbines with variable rotational speed to an alternating current of constant frequency is thus saved. 
     LIST OF REFERENCE NUMERALS 
     
         
           2  wind turbine 
           3  control unit 
           4  tower 
           5  load sensor 
           6  nacelle 
           8  hub 
           10  blade 
           12  first slot 
           13  second slot 
           14  blade tip 
           15  third slot 
           16  blade root 
           17  trailing edge 
           18  leading edge 
           19  suction side 
           20  pressure side 
           22  first cavity/first air reservoir 
           23  second cavity/second air reservoir 
           25  third cavity/second air reservoir 
           22 ′ first tube 
           23 ′ second tube 
           25 ′ third tube 
           27  first separating wall 
           28  second separating wall 
           32  first valve means 
           33  second valve means 
           35  third valve means 
           42  first holes of first longitudinally extending zone 
           43  first holes of second longitudinally extending zone 
           45  first holes of third longitudinally extending zone 
           52  second holes of first longitudinally extending zone 
           53  second holes of second longitudinally extending zone 
           55  second holes of third longitudinally extending zone