Abstract:
The invention relates to a wind turbine having rotating means including at least two wind turbine blades which are turnable around the longitudinal axis of said blades, and a wind turbine hub connecting said at least two wind turbine blades with at least one shaft. Further, the wind turbine includes hydraulic means for actuating said at least two wind turbine blades, said hydraulic means being incorporated in said rotating means. The invention also relates to a hydraulic system, an air bleed system and a method of controlling at least two wind turbine blades.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a wind turbine, a hydraulic system, an air bleed system and a method of controlling at least two wind turbine blades. 
     BACKGROUND OF THE INVENTION 
     In modern wind turbines, one of two systems is used to control the power generated by the turbine generators, especially in situations of excess winds. Both control systems involve a reduction in the lifting force transferred from the wind to the rotor blades of the wind turbine. 
     By the first method, the pitch system, control is obtained by pitching the blades around the longitudinal axis. 
     By the second method, the stall system, control is obtained by using a blade profile designed to stall when situations of excess winds occur. In large wind turbines, the stall system may be combined with pitchable blades in order to establish a so-called active stall system. 
       FIG. 1  shows a prior art pitch system in which the necessary force for pitching the blades is supplied by a hydraulic system. The main part of the hydraulic system is positioned in the nacelle of the wind turbine and provides hydraulic pressure to a number of actuators through a hollow low-speed shaft. The actuators are positioned in the wind turbine hub which connects the blades with the shaft. The actuators transform the hydraulic pressure into mechanical force used when pitching the blades. 
     With the prior art control systems comprising hydraulics, a number of problems have come into view. In particular, the arrival of large, megawatt wind turbines has exposed problems with the hydraulic systems and the transmission of hydraulic pressure from stationary to rotating components of the hydraulic systems in the wind turbines. Also, the necessary hallow shafts are not suitable or useable in connection with large wind turbines and are generally too expensive in production. The distance between the different hydraulic components may result in significant pressure capacity loss and a knocking noise affecting the lifetime of the hydraulic components. 
     SUMMARY OF THE INVENTION 
     The invention establishes a wind turbine without the above-mentioned disadvantages. Especially, the invention establishes a hydraulic system that may be used in all wind turbines regardless of size. 
     In accordance with the invention, a wind turbine comprising rotating means including at least two wind turbine blades ( 4 ) which are turnable around the longitudinal axis of said blades, and a wind turbine hub connecting said at least two wind turbine blades ( 4 ) with at least one shaft ( 7 ,  16 ), hydraulic means ( 17 ) for actuating said at least two wind turbine blades ( 4 ), said hydraulic means ( 17 ) being incorporated in said rotating means. 
     By including all of the hydraulic means in the rotating means, the distance between the hydraulic components and thus the pressure capacity loss may be kept low. 
     Further, the development in the wind turbine industry has led to more and more compact gears. When creating compact gears, the use of hollow shafts is not possible as the diameter of these shafts is too large and if the shafts of the compact gears are hollowed out, the risk of collapse increases significantly. 
     Even further, the costs of hollowing out the shafts and the the accompanying hydraulic components may be avoided. 
     At the same time, space may be saved in the nacelle which allows for a reduction in size while using “free space” in connection with the hub. 
     The term “rotating means” is to be understood as the means of a wind turbine which rotates in relation to the nacelle of the wind turbine such as the rotor, the hub and the shafts. 
     The term “actuating” is to be understood as force used to move a wind turbine blade into a new position or to maintain the wind turbine blade in its position when other forces are trying to move it. 
     In an aspect of the invention, said hydraulic means ( 17 ) may include at least one hydraulic tank ( 20 ), at least one hydraulic pump ( 21 ,  25 ), at least two hydraulic actuators ( 22 ) actuating said at least two wind turbine blades ( 4 ), at least one air bleed system ( 43 ), at least one hydraulic filter ( 26 ), and heating means ( 18 ) and/or cooling means ( 27 ). 
     The actuators may hereby be provided with hydraulic pressure from the pump over a limited distance as the pump (and tank) and the actuators may be positioned in close proximity of each other. 
     In a further aspect of the invention, said hydraulic means ( 17 ) is positioned in relation to a rotation center line (cl) of said rotating means. 
     Hereby, it is possible to add hydraulic means to the rotating means without creating symmetric balance problems in the rotating means of the wind turbine. Preferably, the hydraulic means is positioned in a compact manner around the center line e.g. by positioning the hydraulic tank central in the center line with the other hydraulic components positioned as symmetrically as possible on the sides of the tank. 
     In an even further aspect of the invention, said hydraulic means ( 17 ) is attached to the wind turbine hub ( 14 ). 
     Hereby, it is possible to create a very rigid construction with a high degree of dependability. 
     In accordance with the invention, a hydraulic system for controlling the rotating means includes at least two wind turbine blades, and that said system comprises a hydraulic tank ( 20 ), at least one hydraulic pump ( 21 ,  25 ), at least two hydraulic actuators ( 22 ) actuating said at least two wind turbine blades ( 4 ) around the longitudinal axis of said blades with said at least one hydraulic pump ( 21 ,  25 ) pumping hydraulic oil from said hydraulic tank to said least two hydraulic actuators, and said hydraulic system being positioned and rotating around a rotation center line (cl). 
     In an aspect of the invention, said system further comprises at least one air bleed system ( 43 ). 
     In accordance with the invention, an air bleed system ( 43 ) for hydraulic systems incorporated in rotating means, said air bleed system comprising an air bleed tube ( 19 ) positioned at the rotation center line (cl) of said hydraulic systems and pivotally connected to said hydraulic systems, an air bleed pipe ( 34 ,  41 ) positioned in said hydraulic system and with an air intake away from said center line (cl), a weight block with a weight-plumbing bar positioned outside said hydraulic system, said air bleed pipe and weight block each being pivotally connected to said air bleed tube, and said air bleed pipe being connected to said weight block in a fixed connection. 
     In an aspect of the invention, said air bleed pipe ( 41 ) is connected to said air bleed tube ( 38 ) through an elbow connection ( 40 ) and said pivotal connection comprises at least one bearing ( 39 ). 
     In a further aspect of the invention, said weight block ( 29 ) weighs more than said air bleed pipe ( 41 ). 
     In an even further aspect of the invention, said system comprises at least two and preferably three capacitive sensors ( 31 ) positioned symmetrically around said center line (cl). 
    
    
     
       THE FIGURES 
       The invention will now be described in detail with reference to-the drawings, in which 
         FIG. 1  shows a prior art hydraulic system for a wind turbine, 
         FIG. 2  shows a hydraulic system for a wind turbine according to the invention, 
         FIG. 3  shows a flow chart of the functionality of a preferred embodiment according to the invention, 
         FIG. 4  shows an exploded view of a hydraulic air bleed system according to the invention, 
         FIG. 5  shows a first view of a hydraulic tank according to the invention, 
         FIG. 6  shows a second view of the hydraulic tank, 
         FIG. 7  shows a preferred embodiment of a hydraulic system according to the invention, and 
         FIG. 8  shows a third view of the hydraulic tank. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  shows a prior art pitch system in a wind turbine. The wind turbine includes a nacelle  2  positioned on top of a wind turbine tower and a yaw mechanism  5  in order to erect the wind turbine rotor blades into the wind. The rotor blades are incorporated in the rotating means of the wind turbine in which a wind turbine hub  14  and a pitch mechanism  15  in a nose cone are also included together with low  10  and high-speed shafts  7  on opposite sides of a gear  9 . The high-speed shaft further comprises a mechanical brake and is connected to the electric generator  6 . 
     The necessary force for pitching the blades  4  is supplied by a hydraulic system. The main part of the hydraulic system, such as pumps and a tank  8 , is positioned in the nacelle  2  of the wind turbine  1  and supplies hydraulic pressure force to a number of actuators  14  through the low-speed shaft  10  which is hollow as indicated on the figure. 
     The hydraulic pressure is supplied through the ends of the low-speed shaft in order to supply hydraulic fluid from stationary hydraulic components to the rotating components, e.g. from the pump to the actuators. The low-speed shaft is suspended between a first and a second bearing  11 ,  12  and is connected to the wind turbine blades  4  through a wind turbine hub  14  and a pitch mechanism  15 . 
     The actuators  13  are positioned in the wind turbine hub or in close proximity of the hub and are connected to the pitch mechanism  15  of each wind turbine blade. The actuators transform the hydraulic pressure into mechanical force used when pitching the blades through the pitch mechanism  15 . 
     Normally, the pitch system may be used to automatically adjust the rotor blades in order to adapt to the wind. 
     The pitch system may also be used to turn the blades into a position in which the blades are perpendicularly angled in relation to the wind. In this position, the lifting force on the blades disappears and the rotating means of the wind turbine will slow down and finally stop rotating e.g: in situations of excess winds. 
       FIG. 2  shows a hydraulic system for a wind turbine according to the invention. The entire hydraulic system  17  is positioned next to the hub  14  on a rotating center line cl in the nose cone  3  and rotates with the rest of the rotating means. The hub is connected to the hydraulic system  17  and the gear  9  through a very short low-speed shaft  16 . The length of the shaft means that it is suspended in bearings in front of the gear  9 . The remaining components on the figure correspond to the components of  FIG. 1 . 
       FIG. 3  shows a flow chart of the functionality of a preferred embodiment according to the invention. 
     The chart involves a number of hydraulic components all forming part of the hydraulic system  17 . In a normal 3-rotor blade wind turbine, the components are as follows: 
     A hydraulic tank  20  which contains hydraulic oil for the system. The oil level in the hydraulic tank is normally around ⅔ of the full tank and the rest is filled with air. 
     A hydraulic high-pressure pump  21  which pumps the hydraulic oil from the tank to the hydraulic actuators  22  at high pressure. The hydraulic high-pressure pump  21  further supplies the back-up accumulators  24  with the necessary pressure during normal functionality. 
     Each of the three hydraulic actuators  22  is mechanically connected to a pitch mechanism  15  of a wind turbine blade  4 , and may turn the blade around the longitudinal axis of the blade. 
     A low-pressure pump  25  which receives the hydraulic oil from the actuators and the tank and pumps it through hydraulic filters  26  in order to clean out different particles and dirt from the oil. Further, the oil is pumped through cooling means  27  such as an oil cooler with electric fans before it is returned to the hydraulic tank. 
     The air bleed tube  19  in the hydraulic tank ensures that the relation between air and oil is kept stable by letting air in or out of the tank if necessary. 
     Three back-up accumulators  24  comprise pressured gas in order to turn the blades to a halt position, e.g. if the hydraulic or the electric system fails and the hydraulic pressure disappears. 
     In additions to the hydraulic components, the hydraulic system comprises further elements such as heating means  18  including a number of electric heating elements inside the hydraulic tank ensuring that the oil has a certain preferable temperature. 
     The pumps and some other hydraulic components are supplied with power from an electric power supply  23  which may be positioned away from the hydraulic system and the rotating means. 
       FIG. 4  shows an exploded view of a preferred embodiment of the hydraulic air bleed system according to the invention. 
     The hydraulic air bleed system  43  is partly positioned inside the hydraulic tank in which the sensor and bearing bracket  33  defines the line between parts inside and outside the hydraulic tank. 
     The part of the system positioned inside the tank comprises an air bleed pipe  41  shaped as an inverted J. The air bleed pipe  41  is connected to the air bleed tube  38  through the bracket  33  by an elbow connection pipe  40  and a bearing  39 . The bearing allows the air bleed tube  38  and the pipes  40 ,  41  to rotate in relation to the bracket  33 . 
     At the other end of the air bleed tube  38 , a weight-plumbing bar  36  is connected through a rectangular hole  37 . The hole ensures that the bar follows the air bleed tube when it turns as a kind of shaft in relation to the bracket which is possible by the bearing  39  and a tightening ring  42  being positioned on opposite sides of the bracket. The weight-plumbing bar  36  is provided with a weight block  29  at its low end. 
     The air bleed tube  38  extends into an air filter  30 . 
       FIG. 5  shows a first view of a hydraulic tank according to the invention and illustrates the functionality of the air bleed system. 
     The hydraulic tank is symmetrically positioned around a rotating center line cl. 
     The hydraulic tank  20  comprises a number of sections filled with hydraulic oil  28  and air  35 . The air floats above the oil irrespective of the fact that the tank rotates around the rotating center line cl as indicated by the rotation arrow ra. The separation of oil and air is indicated by the oil surface  32  and, as shown, by the air bleed pipe rising above the oil surface  32 . 
     During the turning of the tank, the weight block  29  and the air bleed pipe  34  will always be vertical with the weight block  29  facing downwards. As the weight block and the pipe are attached to the air bleed tube in a fixed connection and the weight block is heavier than the pipe, gravity will ensure that the air bleed pipe  34  is always vertical in the upwards direction and that the weight block is vertical in the downwards direction. 
     If the oil pressure in the tank rises, the air pressure will rise and the air bleed system will ensure that the pressure is removed through the tube. Further, the system will ensure that air is sucked in from the outside if the air pressure inside the tank drops significantly. The pressure inside the tank and the oil level may change when oil is pumped in or out of the tank during control of the rotor blades. 
     The bracket  33  also comprises a number of capacitive sensors  31  which are positioned symmetrically away from the center line cl, e.g. three sensors positioned at given distance from the line and 120 degrees apart. During rotation, the sensors are always in different positions in relation to the oil surface  32  and may as such provide an indication of the oil level in the tank by e.g. one of the three sensors being free of oil, which may indicate that maintenance should soon be carried out, of two of three sensors being free indicating an immediate stop of the wind turbine as the oil level is much too low. 
       FIG. 6  shows a second view of the hydraulic tank in which rotation is clearly indicated by the rotation arrow ra. Further, the relation between the air bleed pipe  41 , the weight-plumbing bar  36  and the weight block  29  and their respective positions are indicated. 
     The weight-plumbing bar  36  and weight block  29  and the air bleed pipe  41  are fixedly connected to the air bleed tube as mentioned above. The air bleed tube, and thus the weight-plumbing bar  36  and the air bleed pipe  41 , are pivotally connected to the tank and the rest of the hydraulic system through the bearing  39 . 
       FIG. 7  shows a preferred embodiment of a hydraulic system according to the invention (in an exploded view). 
     The figure shows the hydraulic tank  20  as the center component in the hydraulic system in which other hydraulic components are positioned as symmetrically as possible. The hydraulic components comprise the back-up accumulators  24 , the high-pressure pump  21 , the low-pressure pump  25 , the hydraulic filter  26 , the cooling means  27  and the air bleed system  43 . 
     The figure also indicates the necessary hydraulic hoses connecting the different components, including the hose connection  44  connecting the high-pressure pump with one of the actuators  22 . 
       FIG. 8  shows a further embodiment of the hydraulic tank  20 . The tank has inlet and outlet pipes  45 ,  46  which extend from the outside hose connections  47 ,  48  into the tank and they are preferably provided with openings  49 ,  50  at the center of the tank or close to the center. The center of the tank is represented by the rotating center line cl. 
     The figure further illustrates the electric heating means  51  ensuring a preferred temperature in the hydraulic oil. 
     The number or inlet and outlet pipes and the position at which they extend from the surface of the tank may be changed. In an embodiment of the invention, the inlet and outlet hoses may be combined into one hose which acts as inlet and outlet pipes. 
     LIST 
     
         
         
           
               1 . Wind turbine 
               2 . Nacelle 
               3 . Nose cone 
               4 . Wind turbine blades 
               5 . Wind turbine tower and yaw mechanism 
               6 . Generator 
               7 . Mechanical brake and high speed shaft 
               8 . Hydraulic system incl. hydraulic pump etc. 
               9 . Gear 
               10 . Hollow low-speed shaft 
               11 . First bearing for low-speed shaft 
               12 . Second bearing for low-speed shaft 
               13 . Hydraulic actuators 
               14 . Hub 
               15 . Pitch mechanism 
               16 . Low-speed shaft 
               17 . Hydraulic system 
               18 . Electric heating means 
               19 . Air bleed tube 
               20 . Hydraulic tank 
               21 . Hydraulic high-pressure (hp) pump 
               22 . Hydraulic actuators 
               23 . Electric power supply 
               24 . Back-up accumulators 
               25 . Hydraulic low-pressure (lp) pump 
               26 . Hydraulic filters 
               27 . Cooling means 
               28 . Hydraulic oil 
               29 . Weight block 
               30 . Air filter 
               31 . Capacitor sensors 
               32 . Hydraulic oil surface 
               33 . Sensor and bearing bracket 
               34 . Air bleed pipe 
               35 . Air 
               36 . Weight-plumbing bar 
               37 . Rectangular hole 
               38 . Air bleed tube and shaft 
               39 . Bearing 
               40 . Elbow connection pipe 
               41 . Air bleed pipe 
               42 . Tightening ring 
               43 . Air bleed system 
               44 . Hydraulic connection hose 
               45 . Outlet pipe 
               46 . Inlet pipe 
               47 . Outside hose connection—outlet 
               48 . Outside hose connection—inlet 
               49 . Opening of outlet pipe inside the tank 
               50 . Opening of inlet pipe inside the tank 
               51 . Electric heating means 
             cl. Center line 
             ra. Rotation arrow