Patent Application
20110223021
The invention relates to wind turbines and in particular to wind turbine rotor blades.
A wind turbine's ability to generate power is dependent on the air density around the wind turbine. If no other factors are changed, the relationship between air density and power generation is proportional. A drop in air density will lead to a drop in power generation.
Air density is a function of temperature and pressure. Air density therefore changes on a seasonal basis and even an hourly basis in accordance with local weather patterns. Moreover, air density can vary significantly between wind turbine sites, depending, for example, on the altitude at which the sites are located.
For example, at a height of 800 meters above sea level, the air density is approximately 10 percent less than at sea level. Wind turbines are generally designed for operation at sea level. If such a wind turbine is operated at 800 meters, the annual energy production (AEP) will be reduced, depending on wind conditions, by up to approximately 3 to 10 percent. The lower figure is that for a very high wind site. It will be appreciated that high altitude sites often experience higher winds than sites at sea level. Higher winds increase energy production, but the net AEP drop is still significant. The result is a loss of revenue for the wind turbine operator and a reluctance to erect wind turbines at higher altitudes.
We have appreciated that it would be desirable to mitigate the effects of operation in differing air densities. For the above example, the potential savings, in terms of increased energy production, are considerable. For seasonal changes, the potential savings, albeit not as large, can still be significant, especially for sites subject to extreme seasonal variations.
One way of mitigating the effects of air density variation is to adjust the tip speed of the rotor blade. If the density drops by 10 percent, the tip speed would need to be increased by 3 percent to maintain the same energy output. For a fixed or semi-fixed generator this would require a different gear box, which is impractical. However, a full power converter would allow for such adjustments, but requires a complex control system. Complex designs are unattractive to wind turbine designers as they are often expensive and unreliable. Moreover, wind turbines are often erected in hostile environments where maintenance is difficult and sometimes impossible.
Another alternative is to produce a different blade design for use at higher altitudes. However, the cost of tooling and producing different blade designs is so high that this alternative is undesirable.
Therefore, what is needed is a wind turbine blade and associated methods which enable the effects of operating in differing air densities to be mitigated in a simple, robust manner.
To address these and other deficiencies, in one embodiment of the invention, a method of adjusting a wind turbine rotor blade having a blade surface includes operating a mechanism for acting over a region of the blade surface. The region includes a portion of the trailing edge of the blade and the mechanism is configured for modifying the camber of the blade at the region of the blade surface. Additionally, the trailing edge of the blade is continuous in the vicinity of the region and the operating step described above is performed prior to the blade being mounted on a wind turbine.
Preferably, operating the mechanism comprises actuating the mechanism to modify the camber of the blade at the region of the blade surface, whereby the blade when mounted on a wind turbine has a modified camber at the region of the blade surface.
Preferably, the camber of the blade is modified by the mechanism to account for air density conditions at the site at which the blade is to be mounted on the wind turbine.
According to an embodiment of the invention, there is further provided a wind turbine rotor blade comprising an upper blade surface, a lower blade surface, a leading edge, and a trailing edge. The wind turbine rotor blade further comprises a mechanism for acting over a region of one of the upper or lower blade surfaces and configured to modify the camber of the blade at the region of the one of the upper or lower surfaces of the blade. The region of the one of the upper or lower blade surfaces at which the mechanism acts includes a portion of the trailing edge of the blade. Moreover, the trailing edge of the blade is continuous in the vicinity of the region.
Modifying the camber of the blade enables the maximum lift that can be produced on the blade to be changed. Thus, embodiments of the invention have the advantage that blades can be optimised for operation in differing air densities. If a site has lower density, the maximum lift, and thus energy capture, can be increased. In addition, if a site has higher density the maximum lift can be decreased, so that the rotor design is optimal. Advantageously, decreasing lift will also decrease loading on the blades, thus reducing fatigue and the likelihood of wind turbine breakdown.
Thus, advantageously, embodiments of the invention enable blades to be produced to the same design regardless of intended location, which is cost effective, but also enable the blades to be optimised for particular local air density conditions in a simple manner.
Preferably, a plurality of the mechanisms are provided along the length of the blade, each for modifying the camber of the blade at a different region of the blade surface. This enables the camber of a blade to be modified over a larger portion of the blade and thus increase the possible change in lift.
In one embodiment, the mechanism may be actuated mechanically. Alternatively, the mechanism may be actuated hydraulically. The mechanism may be operated remotely, for example, from the hub of the wind turbine. This is particularly desirable where modifications to the camber may be required after erection, which may occur, for example, if the blade is to be optimised for seasonal variations. It is not cost effective or practical to remove the blades and is difficult to gain access to the blades in situ.
In a preferred embodiment, the blade comprises an upper surface and a lower surface, at least one of the surfaces comprising two overlapping skins, whereby the mechanism is arranged to move the skins in relation to one another thereby acting over a region of the blade surface, the region including a portion of the trailing edge of the blade, and modifying the camber of the blade at the region of the blade surface.
In an alternative embodiment, the mechanism comprises a lever arranged to act on a blade surface, the mechanism thereby acting over the region of the blade surface, the region including a portion of the trailing edge of the blade, and modifying the camber of the blade at the region of the blade surface.
Preferably, an opening is provided in the surface of the blade proximate the mechanism to provide access thereto. For example, the opening allows an operator to access the mechanism and mechanically actuate the mechanism.
Embodiments according the invention also provide a wind turbine having a rotor comprising at least one wind turbine rotor blade. The blade includes a blade surface and a mechanism for acting over a region of the blade surface, the region including a portion of the trailing edge of the blade, the mechanism configured for modifying the camber of the blade at the region of the blade surface, wherein the trailing edge of the blade is continuous in the vicinity of the region.
Preferably, the rotor comprises a hub, the camber modifying mechanism being actuatable from the hub.
Preferred embodiments of the invention will now be described by way of example only and with reference to the accompanying figures in which:
As is well known in the art, the camber of a blade refers to the asymmetry between the upper and lower surfaces 10, 20 of the blade. In
One or more mechanisms 60 may be provided along the length of the blade. Preferably, the mechanisms 60 are spaced substantially equidistantly along the blade. Each mechanism 60 may adjust the camber of a local region of the blade by an individual amount or by the same amount for each mechanism 60.
The blades of commercial scale wind turbines are typically made of resiliently deformable material such as GRP (Glass Reinforced Plastics). This may be glass reinforced epoxy or polyester. The skin may be continuous over the blade surface or the blade may be modular and formed from several sections.
In the following examples, the mechanism 60 is arranged to modify the camber of the blade by modifying the position of the trailing edge 30 of the blade. Preferably, the mechanism is provided inside the blade.
In the embodiment of
In this example, the actuation device 80 is located on the upper surface 10 of the blade and is coupled to an end of the lever. A fulcrum 100 about which the lever 90 pivots is located on the lower surface 20 of the blade. This location of the actuation device 80 is convenient, particularly, as will be discussed with reference to
The actuation device 80 is provided at the end of the lever 90 and the fulcrum 100 is positioned towards the centre of the lever. However, it will be appreciated that other arrangements are possible, and the position of the fulcrum, for example, may be changed depending on the forces required.
When the actuation device applies a pushing force to the lever 90, the end to which the force is applied is pushed down towards the lower surface 20. The lever 90 pivots about the fulcrum 100 and the opposite end is raised. In this case, as the opposite end is located in the trailing edge 30 of the blade, the trailing edge 30 of the blade is also raised. This causes the curvature of the upper and lower surfaces 10, 20 of the blade to be modified, as indicated by the dotted line. Thus, the mechanism 60 acts over a region of the blade. That is, although not fully shown, over a portion of the trailing edge 30 and the area of the blade surface whose curvature is thereby modified. Thus, the mechanism 60 modifies the camber of the blade at the region of the blade surface.
In order for the blade surface to be modified as described, the blade skin must be sufficiently deformable. The forces required will depend on the stiffness of the blade. The stiffness of the blade skin may need to be reduced from that of a conventional design.
In this embodiment, the mechanism 60 comprises an actuation device 130 and an elongate member 140 connected at its ends, via respective fulcrums 150, 160, to the first and second skins, respectively. The mechanism thus bridges the overlap of the skins 110, 120. In this case the actuation device 130 is located such that the elongate member 140 extends from both sides of the device 130. However, this need not be the case.
The mechanism 60 is configured such that the skins forming one of the blade surfaces can be moved in relation to one another. The overlap of the sheets can therefore be increased or decreased.
When the actuation device 130 acts to lengthen the distance between the ends of the elongate member 140, the skins are moved away from one another and the overlap decreases. This forces the trailing edge 30 of the blade up. It will be appreciated that the fulcrums 150, 160 provide pivot points which allow rotational movement. Similar to the embodiment of
The third embodiment illustrated in
In this case, the actuation device 170 is connected to the member 180 such that the member 180 extends on either side of the device 170. The actuation device 170 is provided towards the end of the member 180 reacted at the intersection between the spar 70 and the upper surface 10. Other locations are possible, however.
When the actuation device 170 applies a force so as to push the member 180 in the direction of the trailing edge 30 of the blade, the upper and lower surfaces of the blade curve upwards. Thus, the mechanism 60 acts over a region of the blade, the curvature of the upper 10 and lower 20 surfaces of the region being modified as indicated by the dotted line 175. It will be noted that the change in curvature of the lower surface of the blade is more prominent in this example. In any event, the mechanism 60 modifies the camber of the blade at the region of the blade surface.
Similar to the embodiment of
The mechanisms 60 may be actuated mechanically, for example, using a jack screw. Alternatively, the mechanisms 60 may be actuated hydraulically, in which case the mechanisms 60 may be actuated remotely, for example from the hub of the wind turbine. Other means of actuation are possible and will occur to a person skilled in the art. For example, the mechanism could be actuated electrically.
Advantageously, the mechanisms 60 described above can be used to mitigate the effects of operation in air densities other than sea level densities, for which blades were designed. Examples of how the mechanisms 60 can be used to modify the camber of the blade have been discussed. In the examples, the camber of the blade is increased through actuation of mechanisms 60. Applying the reverse operation would decrease the camber again. It will also be appreciated that by applying appropriate forces, the camber of the blade could be modified to a greater or lesser degree.
Therefore, a single blade design can be used at multiple sites and have a modified camber at each site, so that it has the optimum aerodynamic profile for the local conditions.
For example, if the air density increases by 10 percent, the lift coefficient would need to decrease by about 5 percent for the rotor to work at optimal energy production. The amount of maximum lift change is a function of geometry and existing camber in the base line airfoil geometry. In order to achieve a 10 percent change, the camber change required is in the order of 0.5 to 1 percent, depending on the airfoil shape.
Preferably, each blade of the rotor will have one or more of the camber adjusting mechanisms 60 described above arranged along the blade. Each mechanism 60 may be used to adjust the camber by the same amount, or each mechanism 60 may be used to adjust the camber by differing amounts. The mechanisms 60 used need not be the same and may be a combination of the embodiments described above.
It is presently envisaged that approximately five mechanisms 60 will be provided. However, the number of mechanisms 60 preferred will depend on the stiffness of the skin along the blade and the length of the blade. If the skin is very compliant then the region of the blade modified by each mechanism 60 will be relatively small and more mechanisms 60 would be required to modify the camber along the same length of the blade. The skin may be re-enforced along the length of the blade such that the region of the blade modified by each mechanism 60 is increased. For example, a beam may be attached to the skin in the spanwise direction. In that case, a mechanism 60 may be provided at each end of the beam.
As is well understood in the art, the blade has a degree of twist such that the angle of attack of the leading edge 40 of the blade is steeper at the root 45 of the blade than at the tip 35. If the mechanisms 60 each adjust the camber of the blade by differing amounts, the twist of the blade will change. Thus, embodiments of the invention enable the twist distribution along the blade to be modified. Different twist distributions may be advantageous in different air densities.
One of the key parameters of wind turbine design is the selection of the optimum tip speed ratio, that is, the ratio of the blade's tip speed to the oncoming wind speed. The optimum tip speed ratio is linked to the twist of the blade and if the twist can be varied, the optimum tip speed will also be varied. According to an aspect of the invention, when the camber of the blade is changed by moving the trailing edge, this has the effect of changing the twist of the blade. Therefore, an optimum tip speed ratio can be selected.
For example, the utility line frequency in Europe is 50 Hz and in the United States the frequency is 60 Hz. It is therefore necessary to have different rotor rotational speeds on turbines in Europe or the United States, and consequently, the tip speed ratios will be different. In accordance with aspects of the invention, the same turbine can be used, and the blade can be modified by changing the camber to ensure optimal performance on both the turbine in Europe or if it is used in the United States with a different rotor rotational speed.
A modification of the airfoil geometry, as described above, may result in an effective change in blade twist where applied, thereby potentially impacting the characteristics for optimal power production, i.e., conditions for peak power coefficient. This result can be used to the designer's advantage in cases where a blade design for a specific application may be improved by such modifications.
Advantageously, embodiments according to the present invention enable blades to be produced to the same design, and then optimised, for example, for sites having different air densities or in view of seasonal air density variations.
The mechanisms 60 according to embodiments of the present invention enable the camber of the blade to be modified without creating a discontinuity in the trailing edge 30 of the blade, such as is created by operation of a flap or aileron. In these embodiments, the trailing edge 30 of the blade is continuous in the vicinity of the region of the blade surface at which the mechanism 60 modifies the camber of the blade. By continuous, we mean that there is no step change in the trailing edge 30 when the trailing edge 30 is viewed from the root or tip 45, 35 of the blade. It will be noted that a mechanism for operating a flap only acts at the hinge line about which the flap is rotated, not over the region of the blade, as in embodiments of the present invention. When a flap is operated, the camber of the blade is modified by a constant amount along the length of the blade comprising the flap, whereas, in embodiments of the present invention, the amount by which the camber of the blade is modified may vary across the region in the spanwise direction. The mechanisms 60, as disclosed herein, are not flaps or ailerons, but operate in a different manner to modify the camber.
In this regard, flaps are useful for rapidly adjusting the lift on a blade so as to prevent loads, caused by sudden gusts of wind, for example, from damaging the blades or to effect short term adjustments. However, they are disadvantageous, as turbulence is generated at the intersection of the blade and the flap, which results in noise. In addition, a complex controller, generally implemented with software, is required. Whilst these disadvantages may be overlooked when continual adjustments are required, these disadvantages may make flaps undesirable for long term operation such as is addressed by the embodiments of the present invention described herein.
With reference to
While the invention has been illustrated by a description of various embodiments, and while these embodiments have been described in considerable detail, it is not the intention of the applicants to restrict or in any way limit the scope of the appended claims to such detail. Additional advantages and modifications will readily appear to those skilled in the art. The invention in its broader aspects is therefore not limited to the specific details, representative methods, and illustrative examples shown and described. Accordingly, departures may be made from such details without departing from the spirit or scope of the general inventive concept.
This application claims the benefit of U.S. Provisional Application Ser. No. 61/312,390, filed Mar. 10, 2010 and is incorporated by reference herein in its entirety.
| Number | Date | Country | |
|---|---|---|---|
| 61312390 | Mar 2010 | US |
