Abstract:
A method for moving a double-bladed rotor of a wind power plant into a parking position characterized by the orientation of the rotor blades in a horizontal plane and braking the rotor.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application represents a National Stage application of PCT/DE2009/001727 entitled “Wind Power Plant” filed Dec. 3, 2009, pending. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to a method for moving a double-bladed rotor of a wind power plant into a parking position. 
     2. Discussion of the Prior Art 
     Transferring wind power plants from an operating position into a parking position is always necessary when climatic conditions, in particular high wind loads, occur, that may damage the wind power plants on account of the immense forces that partly act only briefly on the wind power plants. Thus wind power plants are vulnerable in particular in regions with a high occurrence rate of extreme winds, in particular seasonally occurring winds and whirlwinds such as hurricanes or typhoons that also exhibit a high degree of turbulence. 
     In particular three-bladed rotors are affected especially by cross flows due to the geometric arrangement of the blades, that could lead to extremely high loads and finally to the destruction of the plant. To dimension the wind power plant components such that they can withstand these forces seems impossible for these cases. 
     So that wind power plants can be installed even in these regions and their potential to extract energy can be used, several mechanisms have already been developed for protecting the wind power plants from damage by the storms that have been mentioned. 
     EP 0 709 571 A2 thus introduces a wind power plant with a double-bladed rotor that has rotor blades with rotor blade sections that can be rotated freely relative to each other, so that on the one hand the entire rotor can be oriented parallel to the wind direction or in the case of a frontal flow at least the load acting on the rotor can be reduced. 
     A disadvantage of this design is however the very complex blade structure that is complicated and is likewise susceptible for high wind loads. 
     In contrast, DE 101 41 098 A1 totally dispenses with braking and locking the rotor. There the rotor is to be stabilized without braking and without locking in a rotating position by preventing the rotor from leaving the parking position on account of turbulences by adjusting the rotor blades into a position that counteracts the rotational movement. The control is to effect an “idling” of the rotor in this way without leaving the parking position. 
     However, it seems doubtful that the control introduced in DE 101 41 098 A1 without further details can react with such a speed to turbulences that the wind power plant having this type of control can avoid damage by high wind loads. Rather it has to be expected that the components of the wind power plant that move while high wind loads occur generate further variable moments that would have to be taken into account when designing and dimensioning the components. The construction of such wind power plants can however hardly be calculated because the parameters cannot be predetermined. 
     SUMMARY OF THE INVENTION 
     The object of the invention is therefore to reduce the loads occurring in a wind power plant with a braked and locked rotor such that they can sufficiently withstand also turbulent flows. 
     In this context, it also has to be taken into account that the electricity network is switched off during the conditions of extreme weather that have been previously mentioned and that the wind power plants depend on passive mechanisms that have to make do without an additional energy supply. 
     The basic idea of the invention is to bring the double-bladed rotor of a wind power plant by braking into a parking position in which the rotor blades are oriented horizontally. The rotor is preferably locked positively by means of locking means, the locking means being designed particularly preferably as hydraulically driven locking bolts that engage into locking recesses. Particularly preferred, the blades are also oriented relative to each other in terms of their pitch angle to the longitudinal axis that considering the inclination of the rotor axis and the aerodynamic properties of the blades on the rotor axis a minimal torque and if possible minimal loads that act on the blade roots occur. Averaged over time, preferably only the dead-weight moments are to be present at the blade flanges as load average. 
     Most particularly preferred, the blades are to be oriented to each other relative to the longitudinal axis in terms of their pitch angle such that blade uplift forces are generated that (averaged over time) compensate the dead-weight force of the blades. These uplift forces are preferably also to counter-act the pitching moment that is generated by the inclination of the rotor axis. 
     In particular a control system is provided that controls the braking force of the rotor brake by means of a pressure-regulating valve such that the rotational speed of the rotor is braked as a result of the angle and/or position detection of the rotor in such a way that the rotor comes to a standstill in the horizontal parking position. Then a checking device is being used that checks the precision of the position that has been assumed. In a position in which the locking means cannot engage in their recesses with which they co-operate the control system again releases the rotor, after which the rotor can again rotate and a new braking and checking procedure is being triggered. If however the position that has been assumed coincides with the predetermined tolerances, preferably locking bolts are inserted into the locking recesses that fix the rotor in its horizontal position by means of a positive engagement. 
     According to the invention, the rotor of the double-bladed-rotor wind power plant is moved into a horizontal parking position by a regulated target-braking, braked and locked preferably by inserting hydraulically operated bolts into recesses provided on the rotor for this purpose. Then the rotor blades are brought into a position that is a function of the inclination of the rotor axis of the wind power plant and the aerodynamic properties of the blade for an inflow from below and from above and in which they preferably rest permanently. It is also the case that the blade angles of the two blades differ on account of their usually asymmetric construction relative to the chord of the blade. 
     If the wind power plant is designed as a downwind machine and braked in the horizontal position, and the yaw brake(s) released and the yaw motors decoupled, i.e. the output toothing of the yaw motor no longer engages the toothing ring gear arranged on the tower, to avoid excess revolutions, independent tracking of the rotor can take place when the wind direction changes. This ensures that the blades do not have a cross flow and only small uplift forces can develop. The advantage of the inventive plant is that moving parts to regulate the parking position and thus plants required during the parking position to generate auxiliary energy can be dispensed with. Compared to conventional plants, the wind power plant according to the invention is at the same time subject to less wear and also has a higher degree of stability and safety in comparison to those. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention is therefore explained by way of a specific example using a particularly preferred exemplary embodiment shown in the drawings. In the Figures: 
         FIG. 1  shows a sectional drawing of an aerodynamic profile of a rotor blade of a wind power plant; 
         FIG. 2  shows a diagram that illustrates the uplift coefficients (CO against the angle of attack of a profile; 
         FIG. 3  shows a diagrammatic representation, cut in different levels of the blade, of a rotor blade; 
         FIG. 4  shows a second diagram to illustrate the torque acting on the rotor axis of a rotor blade as a function of the blade pitch angle; 
         FIG. 5   a  shows a perspective view of a wind power plant according to the invention with the rotor braked and locked in the parking position, to illustrate the geometric conditions and axes; 
         FIG. 5   b  shows a frontal view of the wind power plant of  FIG. 5   a;    
         FIG. 6   a  shows an elevation of a wind power plant having a double-bladed rotor; 
         FIG. 6   b  shows a profile section of the one blade of the wind power plant at approximately half the blade length; 
         FIG. 6   c  shows a profile section of the other blade of the wind power plant at approximately half the blade length; 
         FIG. 7   a  shows the position of the one blade of the wind power plant from  FIG. 6   b  when using the inventive method; 
         FIG. 7   b  shows the position of the other blade of the wind power plant from  FIG. 6  when using the inventive method; 
         FIG. 8   a  shows a diagram that shows over time the loading of the rotor axis with the torque M ZR  for a double-bladed rotor of a conventional wind power plant; 
         FIG. 8   b  shows a diagram that illustrates over time the loading of the rotor axis of a double-bladed rotor with the torque M ZR  when using the inventive method. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  shows a diagrammatic sectional view of an aerodynamic profile  10 . The rotor-blade profile  10  is of asymmetric design relative to its chord—it can be seen clearly that the top side  12  of the profile  10  is shaped differently than its bottom side  14 . Here, the angle α designates the angle between the inflow and the chord, the chord being the straight connecting line between the leading edge and the trailing edge of the profile. 
       FIG. 2  shows that in the case of asymmetrically shaped profiles a different inflow angle α leads to different profile uplift coefficients C L . In the example shown, an angle α of −4° for example leads to an uplift force that is lower by more than one half than an inflow angle α of +4°. 
       FIG. 3  shows diagrammatically the different profiles that form along the axis of a rotor blade, the profiles at the blade tip having a thinner design than at the blade root and the profiles being warped relative to each other. 
       FIG. 4  shows a second diagram to illustrate the torque M ZR  acting on the rotor axis as a function of the blade pitch angle β, that is to say the angle between the optimum production position and the rotor plane (that is defined as 0°) in the case of a constant inflow. 
       FIGS. 5   a  and  5   b  show a wind power plant according to the invention with the rotor braked and locked in the inventive parking position. The wind power plant  100  is equipped with a double-bladed rotor  20 . It can be seen clearly that the rotor  20  has been brought into a horizontal position. 
     To better understand the description, the wind power plant  100  is drawn in a coordinate system with x, y and z axes. The R point designates the intersection of y and z axes, and the T point designates the intersection of the x and z axes. 
       FIG. 5   b  shows the wind power plant from  FIG. 5   a  in a front view, the blade position, as described below relative to  FIG. 7 , of the left rotor blade  30  and the right rotor blade  40  differing from each other. 
     The diagrammatic elevation shown in  FIG. 6  of the wind power plant  100  from  FIG. 5  clarifies the blade-angle position of the left rotor blade  30  and the right rotor blade  40  for conventionally operated plants. Here, z designates the rotor axis, the angle γ the inclination of the rotor axis relative to the horizontal, and p the blade angle relative to the rotor plane a that stands vertically on the rotor axis. 
       FIG. 6   b  shows a section through the left blade  30  for a pitch angle of β=90°.  FIG. 6   c  shows a section through the right blade  40  for a pitch angle of likewise β=90°. It can be recognized here that differing uplift coefficients will result for an inflow from the left side in the drawing plane, which again leads to a torque loading of the shaft. 
       FIG. 7  now shows the adjustment of the two rotor blades when using the inventive method. The two blades  30  and  40  are positioned relative to each other such that the rotor axis has a minimum torque and the rotor-blade flanges have a minimum loading by the rotor blades. Therefore the blade pitch angle for the blade  30  shown in  FIG. 7   a  thus for example amounts to between 92° and 95°, at the same time the blade pitch angle for the other blade  40  from  FIG. 7   b  amounting to between 88° and 85°. 
     The differing inflow angles resulting from the different blade pitch angles thus lead to an adaptation of the uplift coefficients and thus to a reduction in the rotor torque. 
       FIG. 8  finally shows clearly the load distribution on the rotor axis that has been improved by the inventive method, using charts that represent the loading of the rotor axis (the torque M ZR ) of a double-bladed rotor against the time. 
       FIG. 8   a  shows that over time peak loads can occur in a conventionally operated double-bladed rotor with β=90° for both blades that can lead to the wind power plant being damaged. 
     In contrast, it can be seen in  FIG. 8   b  that both the average loading and the peak loads can be reduced by the inventive control of the blade angles of both rotor blades (in the example shown, β=95° for the one blade and β=85° for the other blade). This leads to the components of the plant being protected against damage.