Patent Application
20140093382
This application claims priority of European Patent Office application No. 12186534.9. All of the applications are incorporated by reference herein in their entirety.
The invention describes a wind turbine rotor blade, a wind turbine, and an airflow correction arrangement.
In a conventional wind turbine, a number of blades, usually three, is mounted onto a hub, which can be directed into the wind. Each blade is usually connected to the hub by means of a pitch system so that the pitch angle of the blade or “angle of attack” can be adjusted as necessary, for example to allow the blade to extract more energy from the wind or to pitch the blades towards feather during high wind conditions. During operation, the blades convert kinetic energy from the wind into rotational energy of a rotor in order to drive a generator. Wind turbines with high power ratings in the order of several Megawatts are being developed. For such large wind turbines, it is important to design the blades so that as much energy as possible can be extracted from the wind. This could be achieved by extending the blade length to increase the available surface area. However, the maximum length of a blade is typically constrained by the blade tip speed, to avoid undesirable noise levels. Another problem associated with long blades is that the thinner outer ends may bend and collide with the wind turbine tower. For this reason, the hub (and nacelle) may be tilted upward from the horizontal by a few degrees to minimize the risk of collision.
In one approach to the problem of how to maximize the blade efficiency, the blade's chord length may be increased towards the root, so that the widest point of the blade is close to the hub. However, such a blade design results in higher loads on the tower, higher manufacturing costs, and difficulties in transporting the blade.
In another approach that focuses on the blade collision aspect, a longer blade is made stiffer by using carbon fibre instead of only the usual glass fibre. However, such blades are more expensive than conventional glass fibre blades and may considerably increase the total cost of a wind turbine.
In another approach, the use of a spoiler in combination with a more slender and thereby “thicker” blade design is considered close to the root portion, i.e. the blade has a relatively high thickness coefficient of about 0.5 in a transition region between the round root portion and the flatter airfoil portion. The thickness coefficient at a section of an airfoil is defined as the ratio of the longest perpendicular to the chord length of that section. In this blade design for a circular root portion with a diameter of about 2 m, the chord lengths in the transition region are kept relatively short in order to reduce tensile loads on the blade in its lower regions. The spoiler acts to improve the performance of the blade in the relatively thick transition region. However, the high thickness coefficient in the transition region can result in the airflow detaching from the pressure side of the blade as the blade rotates. As a result, the lift coefficient of the blade is reduced, the effectiveness of the spoiler is diminished, and the efficiency of the blade is also reduced.
It is therefore an object of the invention to provide an improved blade design that overcomes the problems mentioned above.
According to the invention, a wind turbine rotor blade, comprising a root portion and an airfoil portion, comprises a thickened zone in which the blade has a thickness coefficient of at least 0.45, which thickened zone extends outward from an inner hub end of the blade into the airfoil portion of the blade; and an airflow correction arrangement arranged on a pressure side of the blade over at least a portion of the thickened zone, which airflow correction arrangement comprises a spoiler arranged along a trailing edge of the blade and realized to increase blade lift; and a vortex generator arranged between a leading edge and the trailing edge and realized to maintain an attached airflow between the vortex generator and the spoiler, i.e. to ensure that the airflow remains attached over the surface of the pressure side.
An advantage of the wind turbine blade according to the invention is that the aerodynamic properties of the thick blade portion are considerably improved, so that a thick blade design can be implemented over a much longer portion of the blade, even over the entire length or span of the blade, in contrast to prior art blades in which any such thick blade portion is at most restricted to a relatively short shoulder or transition region between root portion and airfoil portion. The spoiler and vortex generator act together to improve the aerodynamic properties of the blade. The vortex generator or “turbulator” acts to cause local turbulence and to mix or stir the energized air, encouraging it to remain attached over the boundary layer along the surface of the pressure side of the blade in the direction of the spoiler. This airflow stabilizes and remains attached to the surface as it travels along the pressure side towards the spoiler in the aft or trailing edge region of the blade. The spoiler can therefore fulfill its purpose in improving the aerodynamic properties of this thicker rotor blade. The combination of vortex generator and spoiler leads to an improved performance, particularly for angles of attack between 5° and 25°, which is a typical range for blade cross sections on large rotors with low rotational speed. Furthermore, because the thickened zone of the blade is associated with less overall drag (skin friction and bare drag) and a higher lift coefficient compared to a “flatter” blade (which must be wider at the shoulder region in order to extract energy from the wind), the blade according to the invention can be realized to have a relatively narrow form or shape in its lower regions adjacent to the root portion. Also, by using the favorable combination of vortex generator and spoiler, the spoiler itself can be made relatively small, since the vortex generator ensures that the airflow remains attached so that even a relatively small spoiler always “sees” the airflow. A “small spoiler” is to be understood as a spoiler with a relatively low height. These favorable aspects allow for a more straightforward manufacturing process with simpler mould shapes and with a simpler spoiler design for lower wind load; a more robust blade design with a less fragile trailing edge; and a more economical transport of the blade to the wind turbine assembly site.
A further advantage of the wind turbine blade according to the invention is that the presence of a thickened zone further outboard on the blade favorably increases the blade stiffness and thereby helps to prevent blade tower collisions. The thickened zone in combination with the airflow correction arrangement ensures that a favorable pressure-side airflow is maintained in spite of the relatively thick blade in the outboard region.
According to the invention, a wind turbine has a number of rotor blades, wherein at least one rotor blade comprises a rotor blade according to the invention. Preferably, such a wind turbine has three essentially identical rotor blades according to the invention.
An advantage of such a wind turbine is that any extended thickened zone of a rotor blade according to the invention minimizes the loss in lift coefficient and the increase in drag that would otherwise be expected for a blade with an increased relative thickness. The wind turbine according to the invention is therefore more effective in extracting energy from the wind. The increased stiffness of the “thick” blade is also beneficial in such a wind turbine, since it is not necessary to incorporate significant amounts of expensive carbon fibre to avoid tower collisions.
According to the invention, an airflow correction arrangement for correcting the airflow over a wind turbine rotor blade comprises a spoiler and a vortex generator realized for attaching to the pressure side of the blade in a region of the blade having a thickness coefficient of at least 0.45; wherein the vortex generator is dimensioned to maintain an attached airflow in the direction of the spoiler.
An advantage of the airflow correction arrangement according to the invention is that the combined action of the vortex generator or “turbulator” can significantly improve the lift coefficient of the blade over any thicker region of the blade. Such an airflow correction arrangement can be incorporated in the blade design from the outset, or can be added to an existing blade in a retrofitting procedure.
Particularly advantageous embodiments and features of the invention are given by the dependent claims, as revealed in the following description. Features of different claim categories may be combined as appropriate to give further embodiments not described herein.
In the following, it may be assumed that the rotor blade is for use in a wind turbine with a horizontal axis shaft that is housed in a nacelle mounted on top of a tower. A blade may be regarded as commencing at the hub and extending outward from the hub. Also, in the following, the terms “vortex generator” and “turbulator” may be used interchangeably. The term “thickened zone”, when used in the following, is to be understood as defined above to mean the portion of the wind turbine blade that has a thickness coefficient of at least 0.45. The terms “thick” or “thickened” relate only to the definition of thickness coefficient describing a blade section geometry, i.e. to the ratio of transverse to chord, and are not necessarily related to any increase in mass or weight of the blade.
As indicated above, the thickness coefficient in prior art blades has generally been kept quite low over the airfoil portion. The thickness coefficient (r) over a typical “flat” airfoil can be about 0.25. Prior art designs generally focus on how to shape the relatively flat airfoil section in order to maximize lift. For such flat airfoils, the airflow over the pressure side will not detach, so that this aspect has not been considered over outboard sections of prior art blades. Thicker blade portions have been regarded as a necessity in the unavoidable shoulder or transition regions, in which the circular root portion segues into the flatter airfoil portion, because in prior art realisations any such thick blade section is associated with a breaking away of the airflow as it passes over the pressure side. For this reason, rior art blade designs generally focus on limiting the unavoidable transition or shoulder portions to as short a zone as possible.
The blade according to the invention was developed as a result of considering a different approach that embraces the possibilities of a high thickness coefficient instead of regarding this as a constraint that needs to be minimized.
Therefore, in a particularly preferred embodiment of the invention, the rotor blade comprises a transition portion in the thickened zone between the root portion and the airfoil portion, which transition portion extends up to at least 30%, more preferably at least 50%, even up to or exceeding 70% of the blade span. Here and in the following, the term “transition portion” refers to a portion of the blade over which the thickness coefficient reduces from a high value (for example from a value of 1.0 in the circular root portion) towards a lower value in the airfoil portion of the blade. The blade according to the invention therefore clearly has a much longer transition portion than a conventional blade (which strives to keep the unavoidable shoulder or transition region as short as possible). The airflow correction arrangement ensures that the airflow remains attached over the pressure side of the thicker blade, even relatively far outboard along the blade span where the angle of attack is low on account of the increased rotational velocity of the blade.
The thickened zone can extend quite some distance outward along the blades length beyond the transition portion, and may even extend over the entire length of the blade. Preferably, the thickened zone extends over at least 30% of the blade span, more preferably at least 50% of the blade span, and may even extend up to 70% or more of the blade span. In a further preferred embodiment of the rotor blade according to the invention, therefore, the thickness coefficient comprises at least 0.45, more preferably at least 0.6, in the airfoil portion of the blade. This contrasts strongly with conventional blade design, in which the thickness coefficient is kept low over the airfoil portion, seldom exceeding values of 0.25-0.3. In a rotor blade according to the invention, a thick blade design can be used for much of the blade's length, and the transition portion can extend over much or even the entire “thickened zone”.
In one preferred embodiment of the rotor blade according to the invention, the vortex generator is arranged to extend along the length of the thickened zone, in particular when the thickened zone does not extend all the way to the blade tip. For example, the thickened zone may extend outward from the hub over two thirds of the length of the blade, and the blade airfoil may be designed to have the usual low thickness coefficient over the remainder of the blade.
A tendency of the airflow to break away from the pressure side at a certain distance along the blade may depend on the thickness coefficient at that point along the rotor blade. For example, as the distance along the blade increases, the thickness coefficient may be gradually reduced, for example from a value of 1.0 in the root section, through values of about 0.7 in a transition region, to values of about 0.45 further outboard along the blade. The tendency of the airflow to break away from the pressure side at a point relatively far out along the blade may be lower than at a point closer to the hub.
Therefore, in a particularly preferred embodiment of the invention, the vortex generator is arranged to extend along a length of the thickened zone over which the thickness coefficient exceeds 0.45.
The incoming airflow over the upwind portion of the pressure side is laminar and may be regarded as travelling essentially perpendicularly to the longitudinal axis of the rotor blade. To generate turbulence, therefore, the vortex generator is preferably realized to deflect the incoming airflow in such a way that its path of travel is altered, but not in such a way as to deflect the airflow away from the blade. To this end, therefore, in a preferred embodiment of the invention the vortex generator comprises an open arrangement of outwardly projecting vortex generator elements or “fins”. These elements are preferably arranged in a linear fashion along the blade. The open arrangement allows the incoming air to pass between the vortex generator elements. Preferably, each vortex generator element “disturbs” the air to some extent, deflecting the air from the path it would otherwise take, causing a wind load on the vortex generator element, which in turn results in the creation or “shedding” of vortices that “mix” the air. For example, the vortex generator can comprise an open arrangement of vortex generator elements that are essentially parallel to the incoming airflow, but which have shaped “tail ends” that deflect the airflow, mixing or stirring the air and making it turbulent. Alternatively, in a further preferred embodiment of the invention, the vortex generator elements are arranged at alternating angles, for example in a zigzag pattern or a staggered pattern. To allow the incoming airflow to enter the vortex generator, adjacent vortex generator elements are preferably separated by a suitable distance. The distance between vortex generator elements may be a multiple of their height. In a particularly preferred embodiment of the invention, a vortex generator element comprises an essentially triangular shape and the vortex generator element is arranged on the blade such that an apex of the vortex generator element is directed essentially towards the leading edge of the blade. In this way, the airflow becomes turbulent only as it exits the vortex generator. Vortex generator elements can be made of any suitable material, for example acrylonitrile butadiene styrene (ABS), or a blend of suitable materials chosen for their durability.
The extent of turbulence can depend on the dimensions of the vortex generator. Preferably, the vortex generator is dimensioned to induce only as much turbulence as is needed to keep the flow attached over the aft portion of the blade's pressure side. Therefore, in a further preferred embodiment of the invention, the height of a vortex generator element comprises at most 2.0%, more preferably at most 1%, most preferably at most 0.125% of the corresponding chord length of the blade. The length of a vortex generator element may be derived from its height, for example, 4 to 5 times its height. In one exemplary embodiment of the airflow correction arrangement according to the invention, the height of a vortex generator element might be 1.5 cm for a chord of about 3 m, it may be about 7 cm in length, and the distance between the ends of adjacent vortex generator elements might be in the order of 6 cm.
In a particularly preferred embodiment of the invention, the spoiler is realized such that the pressure side of the blade comprises a concave surface between the vortex generator and the trailing edge of the blade. In this way, the spoiler acts to increase lift. Preferably, the spoiler comprises an essentially flat outer surface at the trailing edge of the blade. This is to be understood to mean a blunt trailing edge, for example the trailing edge can have a significant depth and can be essentially perpendicular to the chord. Such a spoiler design, having a concave inner surface on the pressure side and a flat trailing edge, means that the blade according to the invention can be narrower than a conventional blade, with a similar or even better performance as regards lift. Furthermore, by implementing a vortex generator to effectively “assist” the spoiler in its function, the spoiler can be made smaller, i.e. with a lower depth or height. This makes for a blade that is easier and more economical to manufacture and to transport.
The airflow correction arrangement according to the invention can be realized on a rotor blade as this is being manufactured. For example, the spoiler can be moulded as part of the blade itself, and the vortex generators can be embedded in the outer surface of blade's pressure side after the moulding procedure and before the blade is mounted to the hub. Of course, existing blades can be retro-fitted with such an airflow correction arrangement. The performance of any blade that has a thickened zone over a portion of its length may be improved by the addition of such an airflow correction arrangement along that thickened zone. To this end, in an airflow correction arrangement according to the invention, the vortex generator preferably comprises a plurality of vortex generator elements mounted onto a carrier strip, which carrier strip is realized for attaching to the rotor blade surface. For example, such a carrier can be flexible to some extent allowing the turbulator to follow a curvature of the blade or to maintain a certain distance to the leading edge or trailing edge. In a particularly favorable realization, the vortex generator is realized as a plurality of vortex generator elements mounted onto an adhesive strip. Similarly, a spoiler can also be realized for mounting to the trailing edge of an already existing blade, or for covering and replacing an already existing spoiler.
For a blade that has a suitable spoiler already in place along such a thickened zone, the addition of a vortex generator can be sufficient to considerably improve the performance of the blade.
Other objects and features of the present invention will become apparent from the following detailed descriptions considered in conjunction with the accompanying drawings. It is to be understood, however, that the drawings are designed solely for the purposes of illustration and not as a definition of the limits of the invention.
In the diagrams, like numbers refer to like objects throughout. Objects in the diagrams are not necessarily drawn to scale.
where V is the wind speed upstream of the rotor, and v1 is the wind speed in the rotor plane. The axial interference factor has a theoretical maximum of ⅓.
As the diagram shows, the axial interference factor for the conventional blade displays a marked dip in the lower blade regions. In those regions, the performance of the blade is rather poor. The inventive blade has considerably higher values of axial interference factor over its entire length, even in the critical lower blade regions.
While specific embodiments have been described in detail, those with ordinary skill in the art will appreciate that various modifications and alternative to those details could be developed in light of the overall teachings of the disclosure. For example, elements described in association with different embodiments may be combined. Accordingly, the particular arrangements disclosed are meant to be illustrative only and should not be construed as limiting the scope of the claims or disclosure, which are to be given the full breadth of the appended claims, and any and all equivalents thereof. It should be noted that the term “comprising” does not exclude other elements or steps and the use of articles “a” or “an” does not exclude a plurality.
| Number | Date | Country | Kind |
|---|---|---|---|
| 12186534.9 | Sep 2012 | EP | regional |
