Abstract:
The invention relates to a wind energy installation ( 10 ) having a rotor ( 18 ) which can be driven by wind and has at least one rotor blade ( 22 ), having a generator for conversion of the mechanical energy of the rotor ( 18 ) to electrical energy, and having a tower ( 14 ) on which the rotor ( 18 ) is arranged, in which the rotor blade ( 22 ) has one or more additional masses ( 36, 40 ) and/or active and/or passive oscillation dampers ( 24 ), which are designed in such a manner that movements of the rotor blade ( 22 ), in particular oscillations, which are initiated by external influences and are directed towards the tower or away from it are prevented and/or damped. The invention also relates to a method for operation of a wind energy installation, preferably of an off-shore wind energy installation, in which one or more components of the wind energy installation, preferably the tower ( 14 ), have opposing vibration applied to them in order to reduce/prevent sound waves which result from component vibration and disturb animals and/or people, which opposing vibration counteracts vibration (which produces sound) of the component ( 14 ), and reduces or prevents this component vibration.

Description:
STATEMENT OF RELATED APPLICATIONS  
       [0001]     This application claims convention priority on German Patent Application No. 10 2005 053 145.8, having a filing date of 4 Nov. 2005, and German Patent Application No. 10 2006 022 266.0, having a filing date of 11 May 2006, both of which are incorporated herein by this reference.  
       BACKGROUND OF THE INVENTION  
       [0002]     1. Technical Field  
         [0003]     The present invention relates to a wind energy installation having a rotor which can be driven by wind and has at least one rotor blade, having a generator for conversion of the mechanical energy of the rotor to electrical energy, and having a tower on which the rotor is arranged. The present invention also relates to a method for operation of a wind energy installation such as this.  
         [0004]     2. Related Art  
         [0005]     Various components in wind energy installations such as these, in particular the rotor blades as well as the tower, have a tendency to start to oscillate, in particular as a result of external influences. Both the rotor blades and the tower are excited, for example, by impulses which are caused whenever each rotor blade passes the tower. The forces which are exerted by the wind on the respective rotor blade are less while passing the tower than in the other phases of its rotational movement. This is because the tower, which is arranged behind the rotor blade with respect to the wind direction, represents an obstruction for the wind.  
         [0006]     Because the forces which act on the rotor blade while it is passing the tower are less than in the other phases of rotational movement, the rotor blade and the tower receive a forwards movement impulse, that is to say in the direction of the side of the wind energy installation facing the wind. The certain amount of elasticity which all of the components originally have leads to the rotor blade and the tower swinging backwards again from this forwards-deflected position initiated by the movement impulse, as soon as the rotor blade has ceased to pass the tower.  
         [0007]     These processes take place whenever each rotor blade passes the tower. Even with the normal dimensions of the individual components, in particular of the rotor blades, nowadays, oscillations such as these have a negative effect on the life of the components. This problem is becoming more serious because of the trend to design wind energy installations with ever larger dimensions.  
         [0008]     A further reason for inadvertent movements, in particular oscillations, of individual components during operation of wind energy installations is, for example, changes in the wind incidence angle of the individual rotor blades, as well as wind turbulence.  
         [0009]     The component oscillations which are based on the external influences described above become more critical, of course, the greater the extent to which the excitation frequency of the movement impulses acting on them matches the natural frequency of the components being used. In the worst case, this can lead to catastrophic resonance (as is known in systems which can oscillate), resulting in destruction of the components.  
         [0010]     Furthermore, internal and external influences can excite various components in the wind energy installation to oscillate in a particular manner, whose frequencies are in the range which is audible by people and/or animals. These oscillations, which are referred to in the following text as vibration, lead to acoustic signals, that is to say noise which is disturbing or even dangerous to people and/or animals. Component vibration such as this is frequently caused by natural oscillations of the generator or of other components in the wind energy installation. Component vibration can be extremely problematic, particularly in the case of off-shore wind energy installations. Vibration of the tower, in particular of that tower section which is located in the water, can lead to noise which disturbs or even drives away animals in the sea, for example all types of fish.  
       BRIEF SUMMARY OF THE INVENTION  
       [0011]     One object of the present invention is to specify a wind energy installation in which the abovementioned damaging movements/oscillations, in particular vibration, of the rotor blade and/or of other components, in particular of the tower, of the wind energy installation are prevented, reduced and/or damped. A further object of the present invention is to specify a method for operation of the wind energy installation such as this.  
         [0012]     This object is achieved by a wind energy installation having a rotor which can be driven by wind and has at least one rotor blade, having a generator for conversion of the mechanical energy of the rotor to electrical energy, and having a tower on which the rotor is arranged, characterized in that the rotor blade has one or more additional masses and/or active and/or passive oscillation dampers, which are designed in such a manner that movements of the rotor blade, in particular oscillations, which are initiated by external influences and are directed towards the tower and/or away from it are prevented and/or damped.  
         [0013]     The rotor blade accordingly and advantageously has additional masses and/or active and/or passive oscillation dampers, which are designed in such a manner that movements or movement components of the rotor blade, in particular oscillations, which are initiated by external influences and are directed towards the tower and/or away from it are prevented and/or damped or reduced. Thus, according to the invention, the aim is to restrict or prevent those movement components of the movements of the rotor blade which run out of the rotor plane or rotor blade blade plane which runs at right angles to the rotor shaft.  
         [0014]     The rotor blade mass or masses which is or are additional to the normal rotor blade design lead, with suitable positioning and design, to the overall system comprising the rotor blade and the additional mass or masses having a different natural oscillation frequency to that of the rotor blade. In this case, the additional mass or masses is or are preferably arranged and/or designed, in order to vary the natural oscillation frequency of the rotor blade, in such a manner that the resultant natural oscillation frequency of the system comprising the additional mass and the rotor blade is outside the excitation frequency value range to be expected for the installation in the given conditions. The theoretically feasible excitation frequencies, that is to say those frequencies at which, in principle, the movements or movement components of the rotor blade which are directed towards or away from the tower can be initiated, can be determined for respective theoretically predictable wind speeds, for the given dimensions of the installation, for every wind energy installation. If, as a result of suitable adaptation of the additional masses of the rotor blade, the natural oscillation frequencies are well outside the value range of the excitation frequencies to be expected, then this, in particular, effectively prevents so-called catastrophic resonances.  
         [0015]     The additional masses are preferably mounted in the interior of the rotor blades. Alternatively, of course, it is also possible to arrange them partially or completely outside the rotor blades. In order not to interfere with the aerodynamics, the additional masses should preferably be arranged on the side of the rotor blade facing away from the wind. The masses may, of course, have any shape identified by a person skilled in the art in this field. Depending on the arrangement and weight of the masses, the natural oscillation behaviour of the rotor blade, preferably of all of the rotor blades in the wind energy installation, can be varied as desired.  
         [0016]     As an alternative to the masses which vary the natural oscillation behaviour, the active and/or passive oscillation dampers are provided according to the invention, and can likewise be part of the rotor blades. These oscillation dampers make it possible to initiate movement impulses for the rotor blade, which counteract those movements of the correspondingly equipped rotor blade which are directed towards or away from the tower, and at least partially, and preferably completely, compensate for these movements. The above statements relating to the arrangement of the additional masses apply to the arrangement and design of the active and/or passive oscillation dampers.  
         [0017]     The movement impulses which compensate for the movements which are directed towards or away from the tower can preferably be initiated as soon as the movement which is directed towards or away from the tower starts, or at a time immediately before or after this. If, by way of example, a movement which is directed away from the tower is initiated by a rotor blade passing the tower, a movement impulse which directs the rotor blade towards the tower can be initiated whenever the rotor blade passes the tower, counteracting that impulse which is initiated externally by passing the tower. In short, a movement impulse which runs in the opposite direction is used to counteract the movement impulse of the rotor blade, which is caused by an external influence and moves it away from the tower.  
         [0018]     The object of the present invention is also achieved by a wind energy installation having a rotor which can be driven by wind and has at least one rotor blade, having a generator for conversion of the mechanical energy of the rotor to electrical energy, and having a tower on which the rotor is arranged, characterized in that the wind energy installation has an active oscillation damper for production of movement impulses which counteract any movement of the tower of the wind energy installation which is initiated by external influences.  
         [0019]     According to this embodiment, the wind energy installation has one or more active oscillation dampers for production of movement impulses, which specifically counteract any movement of the tower of the wind energy installation which is initiated by external influences. In this case, the tower and/or the pod of the wind energy installation preferably has the active oscillation damper. This active oscillation damper can be arranged in the interior of the tower, and/or in the interior of the pod. Alternatively, of course, it is feasible for it also to be arranged at least partially externally on the components.  
         [0020]     In one preferred embodiment, the active oscillation damper is designed in such a manner that the movement impulse which can be initiated by it counteracts a tower movement which is produced as each rotor blade passes the tower. In this case, the active oscillation damper may have at least two masses which contrarotate about, in particular, a common rotation axis, with each mass being unbalanced about the rotation axis. In this case, the rotating masses can be arranged, and the rotation frequencies of the rotating masses can be matched to one another, in such a manner that an opposing impulse, in the opposite direction to the movement which is initiated by external influences, can be initiated.  
         [0021]     The frequency at which the masses rotate about the preferably vertical rotation axis is in this case matched to the number of rotor blades. The majority of conventional wind energy installations nowadays have three rotor blades. Rotor blades accordingly pass the tower three times during one complete revolution of the rotor. These three passes by the tower can be compensated for, for example, by the frequency of the masses with which they rotate about the rotation axis corresponding to three times the frequency of the rotor blades, since each complete rotation of the masses results in one and only one opposing impulse.  
         [0022]     It is particularly preferable for the wind energy installation to have sensors by means of which it is possible to detect movements of the rotor blade which are directed away from or towards the tower, and/or movements of the tower. The at least partially compensating opposing pulses from the active oscillation dampers can then be initiated as a function of movements detected in this way.  
         [0023]     Furthermore, a sensor can be provided which in each case detects the instantaneous speed of revolution of the rotor blade or blades during operation, and/or a defined null-point position of the rotor blades, so that it is possible to predict each rotor blade passing the tower. The sensor can accordingly determine the instantaneous speed of revolution of the rotor blade, in which case the time at which the rotor blade subsequently passes the tower can be determined in advance from the speed of revolution by means of a suitable computer unit, and in which case impulses (in the opposite direction to the expected movements of the rotor blade) from the active oscillation dampers can be initiated at a time of passing the tower that is calculated by the computation unit.  
         [0024]     The present invention also relates to a method for operation of a wind energy installation, characterized in that, after detection of any movement, which is initiated by external influences, of components of the wind energy installation, in particular rotor blade and/or tower movements, and/or at times at which such movements are expected, movement impulses which counteract the movements are initiated by means of one or more active oscillation dampers.  
         [0025]     The object of the present invention is also achieved by a method for operation of a wind energy installation, preferably of an off-shore wind energy installation, in particular according to one or more of the preceding claims, with the wind energy installation having a rotor with at least one rotor blade, a generator for conversion of the mechanical energy of the rotor to electrical energy, as well as a tower on which the rotor is arranged, characterized in that one or more components of the wind energy installation, preferably the tower, have opposing vibration applied to them in order to reduce/prevent sound waves which result from component vibration and disturb animals and/or people, which opposing vibration counteracts vibration (which produces sound) of the component, and reduces or prevents this component vibration.  
         [0026]     This embodiment specifies a method for operation of a wind energy installation, preferably of an off-shore wind energy installation, in which one or more components of the wind energy installation, preferably the tower, have opposing vibration applied to them in order to reduce/prevent sound waves which result from component vibration and disturb animals and/or people, which opposing vibration is in the opposite direction to the component vibration which produces the sound, and reduces or prevents this component vibration. One or more components of the wind energy installation accordingly also have corresponding opposing movements, specifically opposing vibration, applied to them in the oscillation range which is audible by animals and/or people.  
         [0027]     If the wind energy installation is in the form of an off-shore wind energy installation, at least that tower section which is covered with water preferably has opposing vibration applied to it, so that vibration of this tower section which produces sound and disturbs marine animals is reduced or prevented. Component vibration may be extremely problematic, particularly in the case of off-shore wind energy installations such as these. This is because vibration of the tower, in particular of that tower section which is in the water, can lead to noise which disturbs or even drives away marine animals, such as all types of fish.  
         [0028]     In one particularly advantageous embodiment, one or more sensors detects or detect component vibration which causes sound waves. The opposing vibration is preferably applied as a function of the component vibration detected in this way, in a particular as a function of the frequency and the amplitude of the detected component vibration.  
         [0029]     The component, such as the tower, can have opposing vibration applied to it, whose amplitude and/or frequency at least approximately match/matches the amplitude of the detected component vibration, and are/is preferably identical to it.  
         [0030]     One or more components of the wind energy installation, preferably the tower, may have a vibration generator via which the component can have opposing vibration applied to it in order to reduce/prevent sound waves which result from component vibration and are disturbing to animals and/or people, which opposing vibration is in the opposite direction to the component vibration which produces the sound, and reduces or prevents this component vibration.  
         [0031]     A further wind energy installation in order to achieve the object according to the invention has the features of a method characterized in that the frequency or frequencies of the opposing vibration originates or originate from the frequency range which is audible by animals and/or people.  
         [0032]     According to this embodiment, the wind energy installation, in particular one or more components of the wind energy installation and preferably the tower, has or have a vibration generator, via which the component can have opposing vibration applied to it in order to reduce/prevent sound waves which result from component vibration and are disturbing to animals and/or people, which opposing vibration is in the opposite direction to the vibration of the component which produces the sound, and produces or prevents this component vibration.  
         [0033]     In a further embodiment of the invention, the wind energy installation has a vibration generator with masses or mass bodies which can be moved controllably, are directly or indirectly connected to the component and can be moved controllably relative to the component in order to produce the vibration.  
         [0034]     The vibration generator may have a plurality of mass bodies in the interior of the tower, which mass bodies can be moved controllably relative to the tower and can be moved on a plane which runs at least approximately horizontally.  
         [0035]     Each movable mass body is advantageously guided along a straight line which runs at least approximately from the tower centre to the tower edge area.  
         [0036]     In one particular embodiment, the movable mass bodies are distributed approximately in the form of a star over the tower cross section.  
         [0037]     In order to detect component vibration, one or more sensors are arranged on the component, in particular on the tower, in order to detect this vibration.  
         [0038]     Those components of the present wind energy installation, in particular the active oscillation damper and/or the vibration generator and/or the sensors for detection of oscillations and/or vibration, which can be controlled and/or regulated, can be controlled and/or regulated via one or more suitable control and/or regulation devices. Furthermore, the methods which have been described in the context of this application can be implemented by means of suitable control and/or regulation devices.  
         [0039]     Finally, in a further embodiment, the wind energy installation may have a standby power supply set. In this case, the standby power supply set is used to supply one or more, and preferably all, of the electrical power components in the present wind energy installation with electrical power and energy in the event of a power supply system failure, in particular one or more control and/or regulation devices and/or sensors, for example the sensors for detection of oscillations and/or vibration, and/or the vibration generator and/or the active oscillation damper. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0040]     Further features of the present invention are specified in the attached dependent claims, in the following description of specific exemplary embodiments of the present invention, and in the attached drawings, in which:  
         [0041]      FIG. 1  shows a side view of a wind energy installation according to the invention, with an active oscillation damper.  
         [0042]      FIG. 2  shows a side view of a rotor of a wind energy installation with an additional mass.  
         [0043]      FIG. 3  shows a side view of a further embodiment of a wind energy installation according to the invention, specifically an off-shore wind energy installation with a vibration generator arranged in the tower interior.  
         [0044]      FIG. 4  shows a horizontal cross section through the tower of the wind energy installation shown in  FIG. 3 . 
     
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0045]      FIG. 1  illustrates a wind energy installation  10  which has a pod  16 , which is arranged at the top of the tower, at the upper end of a vertical tower  14  which is arranged on a horizontal foundation  12 . As those skilled in the art in this field will know, a wide range of embodiments are feasible for the detailed design of a tower for a wind energy installation. The invention is not, of course, restricted to the truncated-conical form of the tower  14  described in the drawing.  
         [0046]     A rotor  18  is arranged at an end of the pod  16  facing the wind and has a hub  20 . Three rotor blades  22  are connected to the hub  20 , with the rotor blade roots of the rotor blades  22  being inserted into corresponding openings in the hub  20 , and being connected to the hub  20  in a known manner.  
         [0047]     The rotor  18  rotates about an axis which is inclined slightly upwards with respect to the horizontal. As soon as wind strikes the rotor blades  22 , the rotor  18  is caused to rotate together with the rotor blades  22  about the rotor axis. The movement of the rotor axis is converted to electrical power by a generator which is arranged within the pod. The rotor blades  22  cover a circular area during rotation. The positions of the rotor blades  22  with respect to the wind can be varied individually, that is to say the incidence angles of the rotor blades  22  with respect to the wind can be adjusted, by means of an adjustment device which is not illustrated but is known to those skilled in the art in this field.  
         [0048]     The basic design of the wind energy installation  10  with an at least approximately horizontal rotor axis is known from the prior art, and it will therefore not be described in detail.  
         [0049]     The reference symbol  1  in  FIG. 1  in each case denotes a detail of the wind energy installation  10 , which is illustrated enlarged in the right-hand upper section of  FIG. 1 . This shows an active oscillation damper  24 , as is arranged in the interior of the tower  14  and of the pod  16 . The active oscillation damper  24  has two coaxial shafts  26 ,  28 , which are arranged vertically one above the other. The upper shaft  26  is caused to rotate by a motor  30 . The lower shaft  28  is caused to rotate by a motor  32 . One end of a rigid arm  34  is connected to the upper shaft  26  such that they rotate together. A mass or a weight  36  is arranged at the opposite end of the rigid arm  34 .  
         [0050]     A rigid arm  38  with a corresponding weight or mass  40  at the end is arranged in a mirror-image form with respect to the rigid arm  34  and the weight  36  on the lower shaft  28 .  
         [0051]     The upper shaft  26  and the lower shaft  28  contrarotate, so that the weights  36  and  40  are each arranged once on one and the same vertical plane per complete revolution, that is to say they point in the same direction. In this position, the impulses of the weights  36 ,  40  are added so that, overall, a corresponding movement impulse is exerted on the tower  14 , which is connected to the active oscillation damper, or on the pod  16 , which is connected to the oscillation damper  24 .  
         [0052]     The rotation frequency of the individual weights  36 ,  40  is matched to the frequency at which the rotor blades  22  rotate. As can be seen in the drawing, a rotor blade  22  which is pointing downwards is passing the tower, that is to say the rotor blade is passing the tower  14  at the instant shown in the drawing. In a front view of the wind energy installation  10 , which is not illustrated, this rotor blade  22  will at least partially cover the upper area of the tower  14 . During this coverage phase, the forces which act on this rotor blade  22  are less than during the rest of its rotation phase. A forward impulse which is represented by the arrows, that is to say to the left in the drawing, is exerted in a corresponding manner on the rotor blade  22 , and thus on the pod  16  and the tower  14 .  
         [0053]     In order to compensate for this movement impulse, the rotation frequencies of the weights  36 ,  40  of the active oscillation damper  24  are matched such that they are located on the common vertical plane when the rotor blade  22  completes its pass by the tower. In a corresponding manner, while passing by the tower, opposing impulses are initiated by the active oscillation dampers  24 , as illustrated by the arrows pointing to the right in the drawing. This provides compensation for the movements.  
         [0054]     Since the compensation must be carried out for each rotor blade whenever it passes the tower, the frequency at which the weights  36 ,  40  rotate is three times as high as the rotation frequency of the rotor blades  22 .  
         [0055]     As those skilled in the art will be aware, widely differing types of other active oscillation dampers are feasible within the scope of the invention, and can be arranged in or on the tower  14 , the pod  16  or the rotor  18 , or the rotor blades  22 . The same applies to passive oscillation dampers, which can be positioned in or on the rotor blades  22 .  
         [0056]     By way of example,  FIG. 2  shows two different positions at which additional masses or weights  42  can be arranged in the rotor blade  22 . These masses  42  result in a change to the natural oscillation behaviour of the rotor blade  22 , that is to say the natural frequency, with respect to the natural frequency that is predetermined by the given physical shape. This change in the natural frequency or in the natural oscillation behaviour is used to ensure that the resultant natural oscillation frequency and the resultant natural oscillation behaviour of the system comprising the additional mass  42  and the rotor blade  22  is away from the excitation frequencies to be expected for the installation in the predetermined conditions. This prevents the rotor blades  22  from starting to oscillate in a damaging manner.  
         [0057]      FIG. 3  illustrates an off-shore wind energy installation  10 . Components having the same function are provided with the same reference symbols as in the case of the wind energy installation shown in  FIG. 1 .  
         [0058]     A pod  16  is arranged at the top of the tower, at the upper end of a vertical tower  14 . The tower  14  is anchored in the sea bed in a manner that will not be described in any more detail, and water  13  flows around it. As in the case of the wind energy installation  10  in  FIG. 1 , a rotor  18 , which has a hub  20 , is arranged at the end of the pod  16  facing the wind. Three rotor blades  22  are connected to the hub  20 , with the rotor blade roots of the rotor blades  22  being inserted in corresponding openings in the hub  20 , and being connected to it in a manner known per se. The rotor  18  rotates about an axis which is inclined slightly upwards with respect to the horizontal. As soon as wind strikes the rotor blades  22 , the rotor  18  together with the rotor blades  22  is caused to rotate about the rotor axis. The movement of the rotor axis is converted to electrical power by a generator which is arranged within the pod  16 . The position of the rotor blades  22  with respect to the wind can be varied individually, that is to say the incidence angle of the rotors blades  22  with respect to the wind can be adjusted, by means of an adjustment device.  
         [0059]     A horizontally running helicopter landing platform  44  is arranged at the top of the pod  16 . A further helicopter landing platform  46  is arranged in the same manner in the lower area of the tower  14 . The platforms  44 ,  46  are attached to the wind energy installation  10  via suitable connecting and supporting structures. A helicopter, which is not illustrated, can land both on the platform  44  and on the platform  46 . Once the helicopter has landed on one of the platforms  44  or  46 , any required maintenance or repair measures can be carried out on the lower and/or upper parts of the wind energy installation  10 .  
         [0060]     The basic design of the off-shore wind energy installation  10  with an at least approximately horizontal rotor axis is known from the prior art, so that this will not be described in detail.  
         [0061]     A vibration generator  48 , which is illustrated only schematically in  FIG. 3 , is arranged in the interior of the tower  14 . The vibration generator  48  makes it possible to compensate for oscillations, specifically vibration of the tower, at a frequency which is within the range that is audible for marine animals. Without the compensation according to the invention, vibration such as this can lead to the production and propagation of sound waves which are damaging to marine animals.  
         [0062]     Structure-borne sound sensors and/or vibration sensors, which are not illustrated, in this case detect that vibration of the tower or of other components of the wind energy installation  10  which is within the frequency range that is audible for marine animals, such as dolphins  50 . This vibration, and/or the parameters which characterize this vibration, is or are measured, such as the frequency and the amplitude of the vibration. The measured vibration may, of course, comprise vibration at different frequencies being superimposed. The individual frequencies can in this case be filtered out by means of suitable analysis methods, such as Fourier analysis.  
         [0063]     Opposing vibration is applied to the tower by means of the vibration generator  48 , as a function of the measured vibration. The relative parameters for the opposing vibration are in this case selected and controlled such that the opposing vibration counteracts the component vibration producing the sound and cancels it out, or at least reduces it.  
         [0064]      FIG. 4  shows a horizontal cross section through the tower  14 . One particular embodiment of a vibration generator  48  arranged in the interior of the tower can be seen well.  
         [0065]     The vibration generator  48  has mass bodies  52  which are distributed in the form of a star over the tower cross section. Each mass body  52  is guided in each case along an approximately radially running guide, specifically a guide rod  54 , such that it can be moved controllably along the horizontal plane, to be precise in the form of a carriage which can be moved along the guide rod  54 .  
         [0066]     In this case, each of the guide rods  54  is connected at one end at the tower centre to a central holding ring  56 , specifically by being attached to it. In a corresponding manner, the ends of the guide rods  54  are arranged distributed around the holding ring  56  in the circumferential direction. At the respective other end, each guide rod  54  is connected to an outer connecting ring  58 , and is attached to it.  
         [0067]     The central holding ring  56  has a considerably smaller diameter than the connecting ring  58 . Both the outer connecting ring  58  and the central holding ring  56  are arranged concentrically with respect to the tower wall  60 , which has a circular cross section. In this case, the connecting ring  58  runs at a small radial distance from the tower wall  60 , that is to say its diameter is only slightly smaller than the diameter of the tower wall  60 . The connecting ring  58  is firmly connected to the tower wall, in particular by means of screws, via connecting webs  62 . The central holding ring  56  may be supported in the ground, in particular the tower foundation, for example by means of suitable supporting structures.  
         [0068]     Two guide rods  54 , and thus two mass bodies  52  in each case, are each arranged in the same radial direction. These guide rods  54 , which are located opposite with respect to their arrangement on the holding ring  56 , accordingly include an angle α of 180° with one another. All of the guide rods  54  include identical angles with respectively adjacent guide rods, with the sum of all the individual angles being 360°.  
         [0069]     The individual mass bodies  52  can be moved along the respective guide rods  54  in the direction of the tower center, that is to say as far as the holding ring  56 , and in the opposite direction as far as the connecting ring  58 , by means of drive means, which are not illustrated explicitly, for example electric motors as well as corresponding transmission means. When these linear movements of the mass bodies  52  are carried out with positive or negative acceleration, that is to say the mass bodies  52  are accelerated or braked, forces are transmitted to the tower  14 . Vibration can be applied to the tower  14  by an appropriate time sequence of suitable forward and backward movements of the mass bodies  52 , that is to say with the acceleration and braking processes being controlled in a suitable manner.  
         [0070]     During an accelerated movement of one of the mass bodies  52  towards the holding ring  56 , or in the case of a braking process during a movement towards the connecting ring  58 , forces are transmitted to the tower wall  60 , which are directed radially outward, that is to say away from the tower center. During the opposite acceleration and braking processes, forces are likewise transmitted to the tower wall  60 , but in the direction towards the tower center.  
         [0071]     In the simplest case, the braking processes take place directly adjacent to the holding ring  56  or the connecting ring  58 , by the mass bodies  52  being stopped there by coming into contact with appropriate stops  64 . Dampers, for example oil-pressure dampers, can be provided on the holding ring  56 , and brake a movement of the mass bodies towards the holding ring  56  or the tower center.  
         [0072]     However, alternatively or additionally, it is also feasible to provide controllable braking means, which can also be used to brake the mass bodies  52  during the course of their movements along the guide rods  54 .  
         [0073]     All of the mass bodies  52  are controlled independently of one another by an appropriate control device, in particular being accelerated and/or braked at adjustable times, for time periods which are likewise adjustable. The control device processes the signals which originate from the sensors, and controls the mass bodies  52  as a function of these signals. In addition to other parameters, it is possible to adjust the magnitudes of the acceleration and/or braking characteristic values as well as the frequency at which the respective acceleration and braking processes are carried out.  
         [0074]     Superimposed vibration movements at an adjustable frequency and with an adjustable deflection, and which counteract the component-dependent measured vibration, can be applied to the tower by suitable superimposition and control of the individual acceleration and braking movements of the mass bodies  52 . It is thus even possible to generate complex vibration movements.  
         [0075]     As those skilled in the art will know, there are a wide range of options for the physical form of a vibration generator according to the invention. The invention is accordingly not restricted to the exemplary embodiments mentioned above.  
       LIST OF REFERENCE SYMBOLS  
       [0000]    
       
           10  Wind energy installation  
           12  Foundation  
           13  Water  
           14  Tower  
           16  Pod  
           18  Rotor  
           20  Hub  
           22  Rotor blade  
           24  Active oscillation damper  
           26  Uppershaft  
           28  Lowershaft  
           30  Motor  
           32  Motor  
           34  Rigid arm  
           36  Weight  
           38  Rigid arm  
           40  Weight  
           42  Mass  
           44  Landing platform  
           46  Landing platform  
           48  Vibration generator  
           50  Dolphins  
           52  Mass body  
           54  Guide rod  
           56  Holding ring  
           58  Connecting ring  
           60  Tower wall  
           62  Connecting web  
           64  Stop