Abstract:
The invention relates to a method of operating a wind turbine ( 1 ) to guard against oscillations of the wind turbine blades ( 5 ) when they are at a standstill or idling at low speeds, and to a wind turbine blade anti-oscillation apparatus ( 10 ). The apparatus comprises a releasably attachable blade cover ( 10 ) that provides a non-aerodynamic surface for a region of the blade. This has been found to prevent air flow adhering to the blade and periodically detaching in a phenomenon known as vortex shedding, and therefore prevents oscillations of the blade becoming problematic. The blade cover can comprise a sleeve ( 10 ) of a net-like material, that can be positioned on the blade either before installation or in the field by service engineers using guide lines 16 and 17.

Description:
[0001]    This invention relates to an anti-oscillation apparatus and to technique for securing wind turbine blades against oscillations. 
         [0002]    A typical horizontal axis wind turbine is illustrated in  FIG. 1  to which reference should now be made.  FIG. 1  illustrates a wind turbine  1 , comprising a wind turbine tower  2  on which a wind turbine nacelle  3  is mounted. A wind turbine rotor  4  comprising at least one wind turbine blade  5  is mounted on a rotor hub  6 . 
         [0003]    The hub  6  is connected to the nacelle  3  through a shaft (not shown) extending from the nacelle front. The nacelle  3  can be turned, using a yaw drive positioned at the top of the tower  1 , to change the direction in which the rotor blade hub  6  and the blades  5  are facing. The blades are aerodynamically profiled so that they experience a ‘lift’ or pressure from the wind as the wind flows past the surface of the blade. The angle or pitch at which the leading surface of the blade aerodynamic profile meets the incident wind can be altered using a pitch drive, that turns the blades  5  with respect to the hub  6 . 
         [0004]    The wind turbine illustrated in  FIG. 1  may be a small model intended for domestic or light utility usage, or may be a large model, such as those that are suitable for use in large scale electricity generation on a wind farm for example. A typical commercial wind turbine, for example one that is designed to generate say 3 MW of power, can stand approximately 100 meters high and have wind turbine blades with a length of around 40 m or more. The size of the wind turbine blade, and in particular the area that is swept out by the blades as they turn in the wind is linked to the amount of energy the turbine can extract from the wind. In commercial energy generation wind turbines are therefore large so that they provide the greatest generation capacity. 
         [0005]    In normal generation, the yaw drive turns the nacelle  3  so that the blades  5  of the wind turbine are pointed into the wind. The pitch of the blades can then be adjusted so that they force they experience from the wind is maintained within safe operating parameters, while generating as much energy from the incident wind as possible. 
         [0006]    When the wind turbine is not generating power, such as before connection to the grid or during times of maintenance, the blades of the turbine are to as great an extent as possible turned out the wind by means of the yaw drive and pitch drives so that the blades do not experience much force from the incident wind. In this case, the wind turbine blades may be brought to a stand still and locked. 
         [0007]    When the blades of the wind turbine are locked and at a stand still, they are prone to edge wise blade oscillations (edge wise is the direction from the leading edge to the trailing edge of the blade) resulting from the air flow across or around the blade, when the wind hits the blade from the side. If the wind turbine blades can be feathered with respect to the wind, then the wind flows around the blades smoothly, but without power to continuously adjust the blade pitch, the wind direction will inevitably change and result in a situation where the flow around the blade is not smooth. The worst case is when the incident wind meets the blade perpendicular to one of its flat surfaces and has to flow around the leading and trailing edge of the blade. If the oscillations are of a large enough magnitude, they can result in physical damage to the structure of the wind blade that is costly and time consuming to repair. 
         [0008]    The problem is caused by the largely unstable flow of air around the rotor blade, particularly the leading edge, when the blade is pitched out of the wind. The air flows around the curved surface of the blade, but cannot stay attached to the blade surface in laminar flow because the blade is not pitched correctly to support lift. As a result, a laminar air flow may briefly occur but is then lost, resulting in the formation of a vortex in the flow on the leeward side of the blade. This situation is often referred to as a vortex shedding flow, and it has the characteristic that the separations in the air tend to shift from one side of the blade to the other in a rhythmic air flow. These oscillations are typically experienced first at the tip of the blade, where the reduced diameter and lighter frame offer less resistance to the rhythmic shifting force provided by the air. If the rhythmic oscillation provided by the air flow corresponds to the resonance frequency of the blade itself, the blade can eventually suffer a catastrophic failure. 
         [0009]    Many wind turbines therefore comprise mechanical or hydraulic devices that dampen oscillations induced in the wind turbine blade before they can develop a magnitude that is sufficient to damage the blade. Although, such devices do solve the problem, they can be expensive and difficult to install. 
         [0010]    We have therefore appreciated that there is a need for an apparatus and method for addressing such oscillations in the blade. 
       SUMMARY OF THE INVENTION 
       [0011]    In a first aspect, the invention provides a method of operating a wind turbine to inhibit oscillations induced by the air flow across the blades when the wind turbine is in a non-operational mode, the method comprising: releasably locking the wind turbine blades in place; securing a releasably attachable temporary blade cover to the wind turbine blade so that the blade cover covers a region of the blade surface and provides a non-aerodynamic outer surface for inducing turbulence in the air flow across the blade. Air flowing across the surface of the blade cannot therefore adhere sufficiently well to the blade for laminar flow to be established, even briefly, and as a result the likelihood of rhythmic vortex shedding occurring, and the magnitude of any vortex shedding oscillation that does occur, have been found to be significantly reduced. 
         [0012]    In one embodiment, the blade cover is a sleeve, and the method comprises: attaching a first guide line to an attachment point at one end of the sleeve; 
         [0013]    pulling the sleeve onto the wind turbine blade; tying the guide line to the wind turbine to secure the sleeve in place. This allows the sleeve to be attached either at the point of manufacture, so that when the blade is installed on the wind turbine the blade cover remains in place until the turbine is ready for use, or alternately allows attachment in the field. 
         [0014]    In one embodiment, the sleeve is formed from netting as this has been found to provide a non-aerodynamic surface to the blade once in place, and has the advantages of being relatively light and easy to fabricate. Alternatively, the blade cover may be a sleeve having protrusions or indentations in its surface to create the non-aerodynamic outer surface. 
         [0015]    In one embodiment, the method comprises: lowering one end of the first guide line from the nacelle or the hub of the wind turbine to a service engineer at the ground. The service engineer at the ground can then attach the guide line to the sleeve. 
         [0016]    In one embodiment, the method comprises at the ground, attaching a second guide line to the sleeve, so that pulling on the ends of the first and second guide lines pulls the length of the sleeve taut. The service engineer at the ground can attach the second ground line to provide a positioning line as the sleeve is hoisted upwards towards the blade. 
         [0017]    In a further embodiment the blade cover comprises an adhesive surface section for attachment directly to the surface of the blade, and the method comprises attaching the adhesive surface section to the blade surface. This allows the surface section to easily be attached to the blade before transport or installation. 
         [0018]    In one embodiment, the blade cover is formed from a net-like material. The net material preferably has an open mesh having a mesh spacing in at least one direction in the range 25 mm to 100 mm. The blade cover may also have protrusions or indentations in its surface to create the non-aerodynamic outer surface, preferably having a depth in the range of 5 mm to 10 mm. 
         [0019]    A wind turbine blade anti-oscillation apparatus is also provided. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0020]    The invention will now be described in more detail, and by way of example, with reference to the drawings, in which: 
           [0021]      FIG. 1  is a schematic illustration of a wind turbine; 
           [0022]      FIG. 2  is a schematic illustration of a wind turbine according to the invention, including an anti-oscillation apparatus fitted to the wind turbine blades; 
           [0023]      FIG. 3  is a side elevation view of a first example of an anti-oscillation sleeve for a wind turbine blade; 
           [0024]      FIG. 4  is a top elevation view of a first example of an anti-oscillation sleeve for a wind turbine blade shown in a closed position; 
           [0025]      FIG. 5  is a top elevation view of a first example of an anti-oscillation sleeve for a wind turbine blade shown in an open position; 
           [0026]      FIG. 6  is a schematic illustration of a technique for attaching the anti-oscillation apparatus according to a first example to the blade of a wind turbine; and 
           [0027]      FIG. 7  is a schematic illustration of a second example of an anti-oscillation device for a wind turbine blade. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0028]    A first example of the invention will now be described in more detail with reference to  FIGS. 2 and 3  of the drawings. 
         [0029]    The example anti-oscillation apparatus comprises a sleeve  10  that is fitted over the tip of a wind turbine blade  5  when the blade is in a stationary position. Attachment of the sleeve  10  can be carried out in situ when the wind turbine blades of an operational wind turbine have been locked in position for maintenance or repair. Alternatively, the sleeve  10  can be attached to the blade  5  in the factory, prior to installation of the blade  5  on the wind turbine tower  2 , and before the wind turbine is connected to the grid to output electricity. In both cases, therefore, the wind turbine can be thought of as being in a non-operational mode. Once the sleeve is in place, the blade can be unlocked and can idle in a feathered position, if desired. 
         [0030]    The construction of an example anti-oscillation sleeve  10  is shown in more detail in  FIG. 3 . The sleeve  10  comprises a substantially net-like or otherwise coarse material  11  having at least one open end  12  for manoeuvring onto the tip of the wind turbine blade  5 . In the example shown in  FIG. 3 , the sleeve  10  has the shape of a sock or pocket and the end  13  of the sleeve  10  opposite the open end  12  is therefore closed. In other examples the end  13  may also be open, in which case the sleeve  10  is closer in shape to a tubular wind sock having two open ends. Although, the sleeve  10  can be thought of as substantially tubular, in that its length dimension is larger than its width, it will be appreciated the sleeve  10  tapers towards its end  13  to fit more snugly around the smaller diameter of wind turbine blade tip. Further, it may also have a flattened cross-section to follow the flattened cross-section of the blade away from the root. 
         [0031]    The open end  12  of the sleeve  10  comprises a resilient reinforcement  14  that both provides structural strength to the open end  12  of the sleeve and in a default position keeps the end  12  of the sleeve sprung open for ease of positioning the sleeve  10  on the blade tip  5 . This is shown more clearly in  FIG. 5 . The resilient reinforcement  14  can be comprised of a metal, plastic or other suitable material as desired. 
         [0032]    As seen more clearly in  FIGS. 4 and 5 , the resilient reinforcement  14  has a loop or eye  15  at its centre that provides an attachment point for one  16  of two guide ropes or lines  16  and  17  by which the sleeve  10  can be hoisted into position. The resilient reinforcement  14  is attached to the sleeve  10  so that the loop  15  is conveniently disposed at one end of the sleeve  10 . The other guide rope  17  is secured to a second loop or eye  18  attached at the other end  13  of the sleeve  10  directly below and on the same edge of the sleeve. Preferably, this edge of the sleeve comprises a reinforcing cord  19 , such as a rope or cable, connected between the two loops  15  and  18 . The loop  18  can in fact be attached directly to the reinforcing cord  19 . The reinforcing cord  19  prevents damage to the sleeve  10  when the force on the guide ropes  16  and  17  becomes large, such as when the sleeve  10  is being pulled into position. 
         [0033]    In one embodiment, the sleeve  10  can be made from a single piece of material, with the reinforcing cord  19  being located in its centre, and the edges of the material being folded over and connected together, by any suitable means, such as stitching or sewing. In this case, the opposite edge of the sleeve  10  to the reinforcing cord may comprise a stitch line  20 . 
         [0034]    The material  11  making up the sleeve  10  may be any material that can be formed into the sock or pocket shape mentioned above, but that will not be too coarse to damage the surface of the wind turbine blade as the sleeve  10  is attached. Fibre materials that are soft and flexible are therefore advantageous, such as but not limited to organic fibres like hemp, sisal, jute, and cotton; synthetic or artificial fibres such as polyamide, polypropylene, polyethylene or any suitable thermoplastic fibrous material; and monofilament materials, such as polyethylene or rubber. In the examples described here, the weave or mesh size of the netting is in the range 10×10 mm to 100×100 mm in mask. Depending on the application, it could also have a finer or a more open mesh. 
         [0035]    The sleeve  10  can be secured over the end of the wind turbine blade during the manufacturing process and attached using ropes or pulleys (not shown) to the root of the blade where the blade is attached to the hub. Alternatively, a key advantage with the sleeve  10 , described above, is that it can be secured over the end of the wind turbine blades by service engineers, when the turbine is put into a non-operational mode and the blades are locked down for repair or maintenance. 
         [0036]    The process of securing the anti-oscillation sleeve  10  over the blades of a wind turbine is shown in  FIG. 6  to which reference should now be made. First, the blades  5  are rotated into a position where one of the blade points towards the ground, and the wind turbine is stopped. For safety, the wind turbine blades may be locked in this position, at least temporarily, while the service engineers are at work. 
         [0037]    A service engineer in the hub of the rotor blade lets down the pull guide rope  16  from a position in the nacelle  3  or the hub  6  of the wind turbine  1 , while securing the other end to the wind turbine body. A service engineer at the ground secures the pull guide rope  16  to the loop  15 , and threads the ground line  17  guide rope through the loop  18 . The service engineer in the hub  6  or nacelle  3  then applies a force to the guide rope  16 , while the service engineer on the ground holds both ends of line  17 . The service engineer in the hub  6  or the nacelle  3  can then pull on the pull guide rope  16  to pull the open end of the sleeve  10  towards the tip of the wind turbine blade  5 . As shown in  FIG. 5 , the resilient reinforcement  14  keeps the end of the sleeve  10  open allowing it, with some care and attention, to be threaded over the tip of the blade  5 . The ground engineer, having a better view point of the blade tip, which for obvious reasons is preferably directed towards the ground for this procedure, is crucial in guiding the open end  13  of the sleeve  10  into place. Once the sleeve  10  has been threaded over the blade  5 , the service engineer in the nacelle  3  or hub  6  can pull further on the pull guide rope  16  sliding the resilient reinforcement  14  upwards and along the length of the blade until it can slide no further. The increasing diameter of the blade  5  in the direction of the root will mean that it will eventually fill out the sleeve  10  preventing further movement, or otherwise the tip of the blade  5  will meet the closed end  13  of the sleeve  10 . In this situation the diameter of the blade  5  will have taken up the slack in the sleeve  10  material, and resilient reinforcement, so that the sleeve will adopt the shape shown in  FIG. 4 , pulled tight across the chord direction of the blade. 
         [0038]    Once the sleeve  10  is in place, the service engineer in the nacelle  3  or hub  6  secures the end of the pull guide rope  16  not attached to the loop  15  to a suitable location on the wind turbine structure (a dedicated tie-off point can be provided for this purpose if required, though one is not strictly necessary). The hub  6  or blade root is preferred for the tie-off point however, as it allows the tie-off points to rotate with the wind turbine blades. The service engineer at the ground then lets go of one end of the ground line  17  and pulls the rope from the lower loop  18 . The sleeve  10  is then in place, and can be detached from the wind turbine simply by releasing the attachment of the pull guide rope  16  to the wind turbine structure. To attach the sleeve  10  to the other blades  5  of the wind turbine, the blades are manoeuvred to face towards the ground, and the process described above is repeated. Once a sleeve is attached to each blade, the wind turbine blades  5  can be locked in place. 
         [0039]    The sleeve  10 , once in place on wind turbine blade  5 , then prevents vortex shedding induced oscillation of the wind turbine blade, by deliberately causing turbulent air flow at the blade surface and preventing the air flow from adhering to the blade. It is only when the air can adhere to the blade that the phenomenon of rhythmic vortex shedding occurs, as the air sticks, detaches, sticks and detaches in turn. We have therefore appreciated that a sleeve  10  or other similar structure attached to the blade to provide a rough, non-aerodynamic surface will cause turbulence and prevent the air from sticking to the blade, therefore breaking the cycle, and preventing or significantly reducing the magnitude of any vortex shedding that occurs. 
         [0040]    For this reason, the material is preferably a net-like material, as this has been found to be effective in causing turbulence at the blade surface and in reducing vortex shedding, by efficiently covering the leading edge of the blade. The open mesh or weave of the net ensures an irregular surface air boundary between the air and the blade, and is advantageously used as the sleeve  10  as it is easy to produce, and therefore not costly. The mesh or weave of the net can for example leave open spaces in the netting having dimensions of around 25 mm to 100 mm, in at least one dimension or square, with a preferable dimension of 50 mm. If the net is too open of course the disruption to the laminar flow of air around the blade will not be reduced significantly for the net to have the desired effect. Additionally, it has been found desirable if the diameter of the cord from which the net is fashioned be in the range 1 mm to 5 mm, with a typical value in use being 2 mm to 3 mm. The net cording can have greater diameter, but then the weight of the net needs to be carefully assessed. 
         [0041]    The net sleeve can also suppress oscillations caused by negative aerodynamic damping at airflow directions causing transition between lift and stall. The net gives a poor lift performance, which reduces the force variation between the lift and stall conditions, and thus reduces the amount of energy available for oscillation. 
         [0042]    It is not however necessary to use a net-like material however as the anti-oscillation sleeve  10 , and it will be appreciated from the description that any material could be used that has a coarse external surface, either due to the weave of the material or due to the presence of protrusions, indentations specifically engineered into its surface. The temporary surface section could for example be engineered to resemble plastic or foam packaging sheets or filler material. A height 5 to 10 mm for example in the depth of the protrusions or indentations of any surface shape has been found more than sufficient to cause a severe disruption to the laminar flow. 
         [0043]    A second example of an anti-oscillation apparatus will now be described with reference to  FIG. 7 . In this example, instead of sleeve  10 , temporary surface sections  25  are attached to the blade surface by means of adhesive or mechanical fastenings. As with the first example, the surface of the temporary surface section  25  comprises protrusions or indentations in its surface that cause turbulent air flow. The temporary surface section  25  can be manufactured from similar material to that from which the sleeve  10  is made, and attached via a backing sheet and suitable adhesive directly to the blade surface. The second example of  FIG. 7  is harder to implement than the first when the temporary surface sections  25  are to be applied to wind turbine blades in the field, but the second example is relatively easy to install on the blades  5  in the factory before the blades are attached to the wind turbine and wind turbine begins operation. The temporary surface section can be peeled off from the blade by providing a suitable cable attached to the section that can be grasped by a service engineer. 
         [0044]    As with the example above, it is preferable that the temporary surface section  25  covers at least the leading edge of the blade as it is the leading edge where the phenomenon of vortex separation typically occurs. 
         [0045]    It will be appreciated that the temporary anti-oscillation surface provided by the sleeve  10  or the surface section  25  may cover only the region of the blade from the tip to an intermediate portion of the blade, or may cover the entire length of the blade, or solely even cover just an intermediate region away from tip of the blade  5 . As noted above, however, it is usually preferred to cover the tip of the blade  5  as this is the region most susceptible to oscillation. 
         [0046]    It will be appreciated that as the length of a wind turbine blade can be of the order of 40 m in length, the length of the blade cover or temporary surface section can be anywhere in the range of a few meters to a few tens of meters.