Source: https://patents.google.com/patent/WO2010133591A1/en
Timestamp: 2018-04-26 19:12:17
Document Index: 124114127

Matched Legal Cases: ['art.\n3', 'art 34', 'art. 4', 'art 36', 'art 236', 'art 241', 'art 243', 'art 241', 'art 243', 'arts 241', 'art 44', 'art 42', 'art 44', 'art 44', 'art 42', 'art 42', 'art 42', 'art 42', 'art 436', 'art 441', 'art 443', 'art 441', 'art 443', 'arts 441', 'arts 441']

WO2010133591A1 - A method of operating a wind turbine - Google Patents
WO2010133591A1
WO2010133591A1 PCT/EP2010/056814 EP2010056814W WO2010133591A1 WO 2010133591 A1 WO2010133591 A1 WO 2010133591A1 EP 2010056814 W EP2010056814 W EP 2010056814W WO 2010133591 A1 WO2010133591 A1 WO 2010133591A1
PCT/EP2010/056814
Title: A method of operating a wind turbine
The present invention relates to a method of operating a wind turbine including a rotor comprising a wind turbine blade and having a substantially horizontal rotor shaft, the rotor comprising a hub, from which the blade extends substantially in a radial direction when mounted to the hub, the blade comprising: a profiled contour comprising a pressure side and a suction side as well as a leading edge and a trailing edge with a chord extending between the leading edge and the trailing edge, the profiled contour generating a lift when being impacted by an incident airflow, the profiled contour in the radial direction being divided into a root region with a substantially circular or elliptical profile closest to the hub, an airfoil region with a lift generating profile furthest away from the hub, and preferably a transition region between the root region and the airfoil region, the transition region having a profile gradually changing in the radial direction from the circular or elliptical profile of the root region to the lift generating profile of the airfoil re- gion, wherein the airfoil region comprises a first base part having a leading edge and a trailing edge with a chord extending between the leading edge and the trailing edge, the airfoil region further being divided into at least a first longitudinal segment and a second longitudinal segment, the first longitudinal segment extending along at least 20% of a longitudinal extent of the airfoil region.
WO 01/14740 discloses ways of modifying wind turbine blade profiles in order to pre- vent stall problems. EP 2 031 242 discloses a blade element for mounting on a wind turbine blade in order to change the profile from an airfoil shape with a pointed trailing edge to an airfoil profile with a truncated trailing edge.
According to an aspect of the invention, this is obtained by the first base part having an inherent non-ideal aerodynamic design so that a substantial longitudinal part of the base part without flow altering devices at a design point deviates from a target axial induction factor, wherein the method comprises the steps of: a) adjusting a pitch of the blade and a rotational speed of the rotor so as to meet the target axial induction factor of the second longitudinal segment, and b) providing and arranging flow altering de- vices to the first longitudinal segment so as to meet the target axial induction factor of the first longitudinal segment.
Using a base part with an inherent non-ideal aerodynamic design, such as a non- optimum twist or chordal length distribution, for a large part of the airfoil region makes it possible to achieve a modular blade design, in which the base part with the non- optimum design can be used for several different blade types and blade lengths. Thus, it is possible to reuse the base part of an existing blade further outboard on a larger/longer blade, or alternatively reuse the base part of an existing blade further inboard on a smaller/shorter blade. All in all, it is possible to make a blade design in such a way that the blade design of the airfoil region is put together from pre-designed sections and that blades of different lengths can be composed partly from sections already existing from previous blades.
Further, a blade segment with an inherent sub-optimum twist or chordal length distribu- tion has a number of advantages with respect to manufacturing of the base part, since the shape of the base part can be kept much simpler than the shape of conventional, modern wind turbine blades with a length of more than 40 meters. For instance double curvatured blade profiles may be avoided. This also makes the production of mould parts for manufacture of the blades simpler. All in all, the time for initial start-up for de- veloping the design of a new blade type to the product launch of the new blade type may be lowered significantly, and the overall production costs may also be lowered. However, putting such a constraint on the design parameters of the blade segment means that the blade segment deviates from the optimum design with respect to aerodynamics, in which the twist and chordal length for an ideal blade segment has a non- linear dependency on the radial position of the blade section. Thus, such a constrained blade segment will inherently be non-ideal with respect to aerodynamics and in particular in relation to the axial induction factor. This deviation is compensated for by using flow altering devices in order to adjust the design lift and inflow properties to the appropriate near-optimum axial induction as a function of the blade radius. However, the tar- get axial induction factor may deviate from the aerodynamic optimum axial induction of 1/3 due to structure and loading considerations.
The target axial induction may be seen as an average over the entire longitudinal ex- tent of the first longitudinal segment and the second longitudinal segment, or it may be seen as an individual target for a plurality of smaller radial segments within the first longitudinal segment. Yet again, it may be seen as an individual target for each cross- section of the first longitudinal segment and the second segment of the blade.
According to a first embodiment, the second longitudinal segment extends along at least 20% of the airfoil region. Thus, the blade comprises at least one longitudinal segment extending along a substantial part of the airfoil region of the blade. According to a first embodiment, the airfoil region includes a blade tip region of the blade. According to a second embodiment, the blade further comprises a blade tip region abutting the airfoil region. Thus, the blade tip region may be seen as either part of the airfoil region or as a separate part. Typically, the tip region covers the outer 5-10% of the longitudinal extent of the airfoil region.
According to an advantageous embodiment, the first base part has an axial induction factor, which without flow altering devices deviates at least 5% from a target axial induction factor at a design point, and the first longitudinal segment is provided with a number of first flow altering devices arranged so as to adjust the aerodynamic properties of the first longitudinal segment to substantially meet the target axial induction fac- tor at the design point.
According to yet another advantageous embodiment, the induction factor of the first base part without flow altering means deviates from the target axial induction factor along substantially the entire longitudinal extent of the first longitudinal segment. According to one embodiment, the target axial induction factor is substantially equal to the aerodynamic optimum target axial induction factor. Thereby, it is possible to substantially maximise the energy extracted from the wind and thus maximise the power production of a wind turbine utilising such blades.
The loading of the first base part may without flow altering devices at the design point deviate at least 5%, or 10%, or 20% or 30% from the target loading. In other words, the loading of the first longitudinal segment is shifted on average over the entire longitudinal extent by at least 5% or 10% by applying the flow altering devices to the first longitudinal segment of the blade. Advantageously, the loading of the first longitudinal segment with flow altering devices deviates no more than 2% from the target loading at the design point. Advantageously, the deviation is no more than 1 % from the target loading at the design point.
However, according to a particularly advantageous embodiment, the airfoil region of the blade is substantially straight. In other words, the orientation of the chord plane is substantially the same in the entire radial direction of the blade. Accordingly, each ra- dial section can be provided with flow altering devices in order to optimise the lift of the substantially straight blade. This provides a large number of possibilities for the design of blades, since the blades can be designed without twist and still be optimised for the local radial velocity of the blade during normal use, i.e. at the design point. This means that the blade can be manufactured from individual sectional blade parts, e.g. as indi- vidual blade parts, which are mutually connected afterwards, or by use of sectional mould parts as for instance shown in DE 198 33 869. Alternatively, a given blade can be fitted with a hub extender without changing the direction of the chord for a given radial position of the blade. This also makes it possible to design the blade without an ideal double curvatured pressure side, i.e. without the need of having both a convex and a concave surface profile on the pressure side of the blade. In this situation, the flow altering devices may be utilised to compensate for a non-ideal profile. Thus, the mould assemblies can be manufactured with a much simpler shape. Also, such a blade may make it possible to manufacture the blade via simpler fabrication methods, such as extrusion or the like.
According to a second embodiment, the first longitudinal segment has an inherent twist angle so that the first base part without flow altering devices at the rotor design point comprises a first segment, in which the inflow angle is lower than the optimum inflow angle, and a second segment, in which the inflow angle is higher than the optimum inflow angle. In this situation, it may be necessary to employ different types of flow altering devices in order to accommodate for the non-ideal aerodynamic structure of the first base part. Such a blade may occur, if the inherent twist of the first base part is linearly dependent on the radial distance from the hub and where the inherent twist "crosses" the ideal twist, which has a non-linear dependency on the radial position. Since the ideal twist has an inverse proportional dependency on the radial distance from the hub, a blade having a first base part with an inherent linear twist dependency may comprise - seen from the hub towards the blade tip - a first segment having an inherent twist being lower than the ideal twist, a juxtaposed second segment having an inherent twist being higher than the ideal twist, and a juxtaposed third segment having an inherent twist being lower than the ideal twist. Advantageously, the root mean square difference over the longitudinal extent of the first longitudinal section between the average inflow angle and the optimum inflow of attack at the design point is more than 1 degree, or more than 2 degrees, or more than 2.5 degrees for the first longitudinal segment without flow altering devices. Thus, the root mean square difference is calculated as an absolute spatial deviation in the longitudinal direction of the blade. This deviation is further observed over a given time interval, e.g. one full cycle for a wind turbine rotor. Advantageously, the root mean square difference over the longitudinal extent of the first longitudinal section between the aver- age inflow angle and the optimum inflow angle at the design point is less than 1 degree, or less than 0.5 degrees for the first longitudinal segment with the flow altering means.
According to another advantageous embodiment, the first base part is pre-bent, and the airfoil region comprises longitudinal segments comprising base parts with no pre- bending. Thus, the pre-bend may be located in one or two segments of the blade only, for instance the outboard part of the airfoil region and/or in the root region. According to an advantageous embodiment, the first base part is a pultruded or extruded profile. Such base parts are feasible to manufacture due to the linearly varying inner dimensions and simplify the manufacturing process significantly.
Such a base part will in itself have inherent non-optimum aerodynamic properties for a conventional wind turbine blade having a profile, which is twisted in the radial direction of the blade. However, the use of profile with such properties makes it possible to simplify other properties of the blade, such as the twist or the chordal shape of the blade. For example, it is made possible to provide a longitudinal segment having no or a linear twist and/or having a linearly varying chord length in the radial direction of the blade. However, putting such constraints on the design of the base part of the blade will inherently entail that the segment deviates substantially from the near-optimum target axial induction of that segment. In order to compensate for such deviations, it is neces- sary to change the overall inflow properties and the lift coefficient of the segment. However, since the novel profile has a relationship between lift coefficient and angle of attack, which differs significantly from conventional blade profiles, this may be sufficient to even out the deviations or at least change the axial induction towards the target axial induction so that the flow altering devices only have to change the axial induction slightly. Thus, the blade comprises at least one longitudinal segment extending along a substantial part of the airfoil region of the blade. According to a first embodiment, the airfoil region includes a blade tip region of the blade. According to a second embodiment, the blade further comprises a blade tip region abutting the airfoil region. Thus, the blade tip region may be seen as either part of the airfoil region or as a separate part. Typically, the tip region covers the outer 5-10% of the longitudinal extent of the airfoil region.
Overall, the shape of the base part can be kept much simpler than the shape of con- ventional, modern wind turbine blades with a length of more than 40 meters. For instance double curvatured blade profiles may be avoided. This also makes the production of mould parts for manufacture of the blades simpler. All in all, the time for initial start-up for developing the design of a new blade type to the product launch of the new blade type may be lowered significantly, and the overall production costs may also be lowered.
Thus, according to an advantageous embodiment, the first base part has an inherent non-ideal twist and/or chordal length, and wherein the cross-sectional profile is adapted to compensate for the non-ideal twist and/or chordal length by shifting an axial induc- tion towards a target axial induction. However, putting such a constraint on the design parameters of the blade segment means that the blade segment deviates from the op- timum design with respect to aerodynamics. Thus, such a blade segment will inherently be non-ideal with respect to aerodynamics and in particular in relation to an optimum lift coefficient for the segment. This deviation is compensated for by using flow altering devices in order to adjust the design lift to the appropriate near-optimum axial induction as a function of the blade radius. The adjustment of the loading to another blade radius implies the need for use of flow altering devices.
All in all, a first base part comprising: a linear chord, a linear thickness, and a twist, which varies linearly or is constant in the radial direction of the blade, has a number of advantages when designing a modular assembled blade and in respect to manufacturing such blades. Preferably, the length of the wind turbine blade is at least 40 meters, or at least 50 meters, or at least 60 meters. The blades may even be at least 70 meters, or at least 80 meters. Blades having a length of at least 90 meters or at least 100 meters are also possible.
According to one advantageous embodiment, the number of first flow altering devices comprises a multi element section having an airfoil profile with a chord extending be- tween a leading edge and a trailing edge. This multi element section may be formed as an airfoil having a chord length in the interval of 5% to 30% of a local chord length of the first base part. Alternatively, the afore-mentioned profile element has a maximum inner cross-sectional dimension, which corresponds to 5% to 30% of the chord length of the first base part.
The flow altering devices may also comprise a surface mounted element, which alters an overall envelope of the first longitudinal segment of the blade. Advantageously, the surface mounted element is arranged in proximity of the leading edge and/or the trail- ing edge of the first base part. The flow altering devices may also comprise boundary layer control means, such as holes or a slot for ventilation, vortex generators and a Gurney flap. Preferably, the boundary layer control means are used in combination with the multi element sections or the surface mounted elements. Multi element sections or surface mounted elements are typically necessary for achieving the large shift in the axial induction factor, i.e. for rough adjustment to the target. However, the boundary layer control means may be utilised in order to fine adjust the axial induction factor to the target.
According to yet another aspect, the invention provides a wind turbine comprising a rotor including a number of blades, preferably two or three, according to any of the afore- mentioned embodiments.
Advantageously, the wind turbine comprises a substantially horizontal axis rotor shaft. Preferably, the wind turbine is operated in an upwind configuration, e.g. according to the "Danish concept". The invention is explained in detail below with reference to an embodiment shown in the drawings, in which
Figs. 14a-c and Figs. 15a-c illustrate compensatory measures for correcting non- optimum chordal length, Fig. 16 shows the operating point for an actual blade section of a wind turbine blade compared with the airfoil section design point.
Fig. 23c shows graphs of the relationship between the drag coefficient and the lift coefficient and the relationship between the angle of attack and the lift coefficient for an outer part of a blade according to the invention, and Fig. 23d shows graphs of the relationship between the drag coefficient and the lift coefficient and the relationship between the angle of attack and the lift coefficient for an inner part of a blade according to the invention.
Fig. 38 shows a comparison between the twist of a transformable blade and that of an existing blade, Fig. 39 shows graphs of the inflow angle for different blades and wind speeds,
Fig. 1 illustrates a conventional modern upwind wind turbine according to the so-called "Danish concept" with a tower 4, a nacelle 6 and a rotor with a substantially horizontal rotor shaft. The rotor includes a hub 8 and three blades 10 extending radially from the hub 8, each having a blade root 16 nearest the hub and a blade tip 14 furthest from the hub 8. The rotor has a radius denoted R. Fig. 2 shows a schematic view of a first embodiment of a wind turbine blade 10 according to the invention. The wind turbine blade 10 has the shape of a conventional wind turbine blade and comprises a root region 30 closest to the hub, a profiled or an airfoil region 34 furthest away from the hub and a transition region 32 between the root re- gion 30 and the airfoil region 34. The blade 10 comprises a leading edge 18 facing the direction of rotation of the blade 10, when the blade is mounted on the hub, and a trailing edge 20 facing the opposite direction of the leading edge 18.
1. The blade root region 30 next to the hub, which is predominantly circular. 2. The transition region 32 between the blade root region and the remaining blade part.
3. An airfoil shaped part 34', which is the main part of the blade. Typically the airfoil shaped part extends from the area of the blade with maximum chord and towards the blade tip part. 4. A blade tip part 36 - usually less than the outer 10% of the blade. The blade root region 30 is the interface from the blade to the blade bearing and the hub, and therefore this region has to end in a circular flange. The design is therefore mainly structural. The blade chord and thickness in this region correspond to the root flange diameter and the twist cannot be defined in this region. Due to the poor aerody- namic characteristics of a circular section, the resulting normal force component will be significantly too small to balance the rotor flow, the induction will be too small and the inflow angle will be too high leading to a poor local power coefficient.
2 Transformable blades The present invention provides a departure from the traditional process of designing modern wind turbine blades. The invention provides a new design process, in which the production is optimised in relation to effective methods of manufacturing a base part or main blade part of a wind turbine blade, and where the base part of the blade is retrofitted with flow guiding devices in order to obtain the proper aerodynamic specifications, i.e. to obtain the target axial induction factor and loading for each radial section. Thus, the base part of the blade may deviate substantially from the target design point and the optimum aerodynamic design.
In the first embodiment, the airfoil shaped part of the airfoil region is replaced by a single longitudinal section comprising a base part and flow altering devices. However, the airfoil shaped part may be divided into additional longitudinal sections as shown in Figs. 10 and 1 1. Fig. 10 shows a second embodiment of a wind turbine blade according to the invention. Similar to the conventional method of designing a wind turbine blade, the blade is divided into a root region 230, a transition region 232, and an airfoil region 234. The air- foil region comprises a blade tip part 236, a first longitudinal section 240, and a second longitudinal section 242. The first longitudinal section 240 of the blade is divided into a first base part 241 and a number of first flow altering devices 246. The second longitudinal section 242 of the blade is divided into a second base part 243 and a number of second flow altering devices 248. The first base part 241 and the second base part 243 have profiles, which have a simplified structure with respect to for instance modularity of blade parts and/or manufacturing of the base parts 241 , 243, and which at the rotor design point in itself deviates substantially from the target axial induction factor and/or the target loading. Therefore, the longitudinal sections are provided with the flow altering devices, which are here depicted as a first slat 246 and a first flap 248, however; the flow altering devices are not restricted to such flow altering devices only. The first longitudinal section 240 and the second longitudinal section 242 both extend along at least 20% of the longitudinal length of the airfoil region 234.
Further, the blade may be provided with flow altering devices arranged at the transition region 332 and possibly the root region 330 of the blade, here depicted as a slat 333. It will be apparent from the later description that the embodiments shown in Figs. 9-1 1 also may be provided with surface mounted elements, vortex generators and the like.
2.1 Local sub-optimum twist and/or chord length When designing transformable blades having simplified base parts, two profile characteristics of the base parts will typically differ locally from the optimum target condition, viz. the local blade twist and thereby the inflow angle at the rotor design point, and the local chord length.
Fig. 12 shows a first situation illustrating the above mentioned deviation. As explained in section 1.4, the local section design target point is defined from the velocity vector triangle, where the resulting velocity vιnfiOw is composed by the axial flow speed, Vwιncι(1- atarget), and the tangential flow speed, r-ω (1 +a'), see also Fig. 12a. This condition is only met at the rotor design point, when the inflow angle is Φi. At the rotor design point, the local section has an operational lift coefficient C| and an operational drag coefficient cd. The resultant aerodynamic forces may as previously explained be divided into a tangential force T, which is oriented in the rotational plane of the rotor and a loading or thrust, which is the normal force Ntarget oriented normally to the rotor plane 64'.
However, the local blade profile for the base part has an actual twist Q2, which is lower than the target twist G1. Consequently, the inflow angle is shifted to an altered angle O2, which is lower than O1. Furthermore, the angle of attack is changed to an altered angle of attack α2, which is larger than oci. Consequently, the two shown vector triangles are as shown in Fig. 12b shifted and the blade section obtains an inflow condition having an altered resultant velocity vector vιnflow2, at which an actual axial induction factor a2 becomes larger than the target axial induction factor atarget- Further, the lift coefficient is shifted to an altered lift coefficient Q2, and an altered drag coefficient cd2. Consequently, the normal load is shifted to an actual normal load N2, which is larger than the target normal load Ntarget- Consequently flow altering devices are needed in order compensate for the altered inflow conditions and normal load. In order to obtain the target axial induction factor atarget, the inflow angle must be shifted back to Φi as shown in Fig. 12c. Consequently, the compensated angle of attack must equal α3 = O1 - θ2. At this angle of attack, the flow altering devices (here depicted as a flap) must alter the drag coefficient and lift coefficient to altered values cd3 and Q3, at which the resultant normal load becomes equal to the target normal load Ntarget- Thus, the flow altering devices are used to reduce the axial induction factor from a2 to atarget, and reduce the load from N2 to Ntarget-
Fig. 13 shows a similar situation, but where the local blade profile for the base part having a given local chord length c has an actual twist Q2, which is higher than the target twist θi (as shown in Fig. 13b). Consequently, the inflow angle is shifted to an altered angle O2, which is larger than O1. Furthermore, the angle of attack is changed to an altered angle of attack α2, which is smaller than Ot1. Consequently, the two shown vector triangles are as shown in Fig. 12b shifted and the blade section obtains an inflow condition having an altered resultant velocity vector vιnfiOw2, at which an actual axial induction factor a2 becomes smaller than the target axial induction factor atarget. Further, the lift coefficient is shifted to an altered lift coefficient cl2, and an altered drag coefficient cd2. Consequently, the normal load is shifted to an actual normal load N2, which is smaller than the target normal load Ntarget- Consequently flow altering devices are needed in order to compensate for the altered inflow conditions and normal load.
In order to obtain the target axial induction factor atarget, the inflow angle must be shifted back to O1 as shown in Figs. 13a and 13c. Consequently, the compensated angle of attack must equal α3 = O1 - θ2. At this angle of attack, the flow altering devices (here depicted as a flap) must alter the drag coefficient and lift coefficient to altered values cd3 and Ci3, at which the resultant normal load becomes equal to the target normal load Ntarget- Thus, the flow altering devices are used to increase the axial induction factor from a2 to atarget> and increase the load from N2 to Ntarget-
Fig. 14 yet again shows a similar situation. For the given profile of the section having a given pitch angle G1 and angle of attack Ot1, the target conditions for achieving the target axial induction factor atarget and the target normal load Ntargetfor the local blade profile at the rotor design point are only met, when the chord length is equal to a target chord C1 (as shown in Fig. 14a). However, the local blade profile for the base part has an actual chord length C2, which is lower than the target chord C1. Consequently, the inflow angle is shifted to an altered angle O2, which is higher than O1. Furthermore, the angle of attack is changed to an al- tered angle of attack α2, which is larger than Ot1. Consequently, the two shown vector triangles are as shown in Fig. 14b shifted and the blade section obtains an inflow condition having an altered resultant velocity vector vιnfiOw2, at which an actual axial induction factor a2 becomes smaller than the target axial induction factor atarget- Further, the lift coefficient is shifted to an altered lift coefficient Q2, and an altered drag coefficient cd2. Consequently, the normal load is shifted to an actual normal load N2, which is smaller than the target normal load Ntarget- Consequently flow altering devices are needed in order compensate for the altered inflow conditions and normal load.
In order to obtain the target axial induction factor atarget> the inflow angle must be shifted back to O1 as shown in Fig. 14c. Consequently, the compensated angle of attack must equal α3 = O1 - θ2. At this angle of attack, the flow altering devices (here depicted as a flap) must alter the drag coefficient and lift coefficient to altered values cd3 and C|3, at which the resultant normal load becomes equal to the target normal load Ntarget- Thus, the flow altering devices are used to increase the axial induction factor from a2 to atarget, and increase the load from N2 to Ntarget-
Fig. 15 shows yet again a similar situation, but where the local blade profile for the base part having a given twist angle θ2 has an actual chord C2, which is larger than the target chord C1 (as shown in Fig. 15b). Consequently, the inflow angle is shifted to an altered angle O2, which is lower than O1. Furthermore, the angle of attack is changed to an altered angle of attack α2, which is lower than Ot1. Consequently, the two shown vector triangles are as shown in Fig. 15b shifted and the blade section obtains an inflow condition having an altered resultant velocity vector vιnflow2, at which an actual axial induction factor a2 becomes larger than the target axial induction factor atarget. Further, the lift coefficient is shifted to an altered lift coefficient Q2, and an altered drag coefficient cd2. Consequently, the normal load is shifted to an actual normal load N2, which is larger than the target normal load Ntarget- Consequently flow altering devices are needed in order to compensate for the altered inflow conditions and normal load. In order to obtain the target axial induction factor atarget, the inflow angle must be shifted back to O1 as shown in Figs. 15a and 15c. Consequently, the compensated angle of attack must equal 0,3 = Φ1 - Θ2. At this angle of attack, the flow altering devices (here depicted as a flap) must alter the drag coefficient and lift coefficient to altered values cd3 and ca, at which the resultant normal load becomes equal to the target normal load Ntarget- Thus, the flow altering devices are used to decrease the axial induction factor from a2 to atarget, and decrease the load from N2 to Ntarget-
2.2 Flow altering devices and aerodynamic effect In this subsection various flow altering devices, which can be used to compensate for the off-target twist and chord, are described together with their aerodynamic effect. In general multi-element devices, such as flaps and/or slats, as shown in Fig. 19, or surface mounted elements as shown in Fig. 18 are needed in order to compensate for the substantial deviation from target twist and chord of the base part of the longitudinal sections of the blade. However, it may be necessary to use additional flow altering devices, such as high lift devices, e.g. vortex generators and/or Gurney flaps, in order to obtain the correct lift and drag coefficients at the given angle of attack.
Fig. 17 shows a first example of flow altering devices 80 for compensating for off-target design parameters of the base part of the respective longitudinal section of the blade. In this embodiment, the flow altering means consists of a number of ventilation holes 80 for blowing or suction between an interior of the blade and an exterior of the blade. The ventilation holes are advantageously applied to the suction side of the blade as shown in Figs. 17a and 17b. The ventilation holes 80 can be utilised to create a belt of attached flow. Air vented from the ventilation holes 80 may used to energise and re- energise the boundary layer in order to maintain the flow attached to the exterior surface of the blade as shown in Fig 17b. Alternatively, the ventilation holes may be used for suction as shown in Fig. 17a, whereby the low momentum flow in the boundary layer is removed and the remaining flow thereby reenergised and drawn towards the surface of the blade. Alternatively, the ventilation holes may be used to generate a pulsating flow, e.g. as a synthetic jet. Despite not generating a flow, this transfers momentum to the flow and thereby reenergises the boundary layer and alters flow separation. Examples of such embodiments are shown in Figs. 17c and 17d, in which the ventilation holes are provided with membranes. The membranes may be provided near the exterior surface of the blade as shown in Fig. 17d or near the interior surface of the blade as shown in Fig. 17c. The full drawn line in Fig. 17e shows the relationship between the lift coefficient and the inflow angle (or alternatively the angle of attack) for the basic airfoil without suction or blowing. By using suction as shown in Fig. 17a or tangential blowing, i.e. venting air substantially tangentially to the surface of the blade, the boundary layer is energised and reenergised. Likewise a pulsating jet as shown in Figs. 17c and 17d will energise the boundary layer. Consequently, the lift coefficient becomes larger. At the same time, the maximum lift coefficient is found at a slightly higher inflow angle. Thus, the graph is shifted up and slightly to the right as shown with the dashed line in Fig. 17e. Alterna- tively, it is possible to apply blowing in an off-tangential angle, e.g. at an angle of more than 45 degrees and possibly substantially normally to the blade surface. In this case, the boundary layer becomes detached from the surface of the blade. Consequently, the lift coefficient becomes lower. At the same time, the maximum lift coefficient is found at a slightly lower inflow angle. Thus, the graph is shifted down and slightly to the left as shown with the dotted line in Fig. 17e.
The full drawn line in Fig. 18c shows the relationship between the lift coefficient and the inflow angle (or alternatively the angle of attack) for the basic airfoil without the use of surface mounted element. By utilising the leading edge element 182 and the second trailing edge element 181 on the pressure side of the blade, the effective camber of the airfoil is increased and the operating lift coefficient at the rotor design point is increased. The maximum lift coefficient is also increased. By utilising the first trailing edge element 180 on the suction side of the blade, the camber of the airfoil is reduced and the operating lift coefficient at the rotor design point as well as the maximum lift coefficient is decreased. Fig. 19 shows the effect of using multi-element airfoils, such as slats or flaps, as flow guiding devices. The depicted graph shows the relationship between the lift coefficient and the inflow angle (or alternatively the angle of attack) for the basic airfoil without the use of multi-element airfoils. By utilising a trailing edge flap oriented towards the pressure side of the blade, the graph is shifted towards lower angles of attack. By utilising a trailing edge flap oriented towards the suction side of the blade, the graph is shifted towards higher angles of attack. By using a slat near the suction side of the blade, the lift coefficient is increased, and further the maximum lift coefficient is found at a slightly higher angle of attack. By using a slat near the suction side of the blade and a flap oriented towards the pressure side of the blade, the lift coefficient is increased, and further the maximum lift coefficient is found at a lower angle of attack. By using a slat near the suction side of the blade and a flap oriented towards the suction side of the blade, the lift coefficient is increased, and further the maximum lift coefficient is found at a higher angle of attack.
Fig. 20 shows another example of flow altering devices 280 for compensating for off- target design parameters of the base part of the blade. In this embodiment, the flow altering means comprises a device attached to the pressure side at the trailing edge, in this case a Gurney flap 280 as shown in Fig. 20a. Other attachments with similar flow altering means are a triangular wedge or a rip forming an angle of more than 90 degrees with the surface of the profile. The full drawn line in Fig. 20b shows the relationship between the lift coefficient and the inflow angle (or alternatively the angle of attack) for the basic airfoil without the use of a surface mounted element. By utilising the Gurney flap, the operating lift coefficient at the rotor design point is increased as well as the maximum lift coefficient, which is also increased as shown as the dashed line in Fig. 20. Fig. 21 shows yet another example of flow altering devices 380 for compensating for off-target design parameters of the base part of the blade. In this embodiment, the flow altering means comprises a number of vortex generators as shown in Fig. 21 a. The vortex generators 380 are here depicted as being of the vane type, but may be any other type of vortex generators. The vortex generators 380 generate coherent turbulent structures, i.e. vortices propagating at the surface of the blade towards the trailing edge of the blade. The vortex generators efficiently change the optimum angle of attack for the radial section and alter the lift of the blade section by reenergising the boundary layer and delaying separation.
Fig. 21 b shows an advantageous embodiment having an arrangement of pairs of vane vortex generators, which has shown to be particularly suited for delaying separation of airflow. The arrangement consists of at least a first pair of vane vortex generators com- prising a first vane and a second vane, and a second pair of vane vortex generators comprising a first vane and a second vane. The vanes are designed as triangular shaped planar elements protruding from the surface of the blade and are arranged so that the height of the vanes increases towards the trailing edge of the blade. The vanes have a maximum height h, which lies in an interval of between 0.5% and 1 % of the chord length at the vane pair arrangement. The vanes are arranged in an angle b of between 15 and 25 degrees to the transverse direction of the blade. Typically the angle b is approximately 20 degrees. The vanes of a vane pair are arranged so that the end points, i.e. the point nearest the trailing edge of the blade, are spaced with a spacing s in an interval of 2.5 to 3.5 times the maximum height, typically approximately three times the maximum height (s = 3h). The vanes have a length I corresponding to between 1.5 and 2.5 times the maximum height h of the vanes, typically approximately two times the maximum height (I = 2h). The vane pairs are arranged with a radial or longitudinal spacing z corresponding to between 4 and 6 times the maximum height h of the vanes, typically approximately five times the maximum height (z = 5h). The full drawn line in Fig. 21c shows the relationship between the lift coefficient and the inflow angle (or alternatively the angle of attack) for the basic airfoil without the use of vortex generators. By utilising the vortex generators 380, the maximum lift coefficient is shifted towards a higher angle of attack. The dotted line shows the corresponding relationship, when vortex generators are positioned in a forward position, i.e. towards the leading edge of the blade, whereas the dashed line shows the corresponding relationship, when vortex generators are positioned in a backward position, i.e. towards the trailing edge of the blade. It is readily seen that the use of vortex generators can be used to change the design inflow angle as well as the maximum lift.
Fig. 22 shows yet another example of flow altering devices 480 for compensating for off-target design parameters of the base part of the blade. In this embodiment, the flow altering means comprises a spoiler element, which protrudes from the pressure side of the blade as shown in Fig. 22a. The spoiler element is usually used at the transition region of the blade and possibly at an inboard part of the airfoil region of the blade. The full drawn line in Fig. 22b shows the relationship between the lift coefficient and the in- flow angle (or alternatively the angle of attack) for the basic airfoil without the use of surface mounted element. It is seen that the lift coefficient is very low for the transition region. By utilising a spoiler element, the maximum lift coefficient is increased significantly. By utilising a spoiler element positioned at a forward position of the blade, i.e. towards the leading edge of the blade or towards the position of maximum thickness, the operating lift coefficient at the rotor design point is increased as well as the maximum lift coefficient as shown with dashed line in Fig. 22b. By utilising a spoiler element positioned at a backward position of the blade, i.e. towards the trailing edge of the blade or towards the position of maximum thickness, the operating lift coefficient at the rotor design point as well as the maximum lift coefficient is shifted towards a higher value as well as towards a higher angle of attack as shown with dotted line in Fig. 22b.
Therefore, from a design and manufacturing point of view it would be advantageous to obtain a base part of the blade having a simplified twist, such as a linearly dependent twist or a reduced twist compared to an optimum twist. Such simplified twist makes it feasible to achieve a modular blade design, in which the base part with the non- optimum twist can be used for several different blade types and blade lengths. Thus, it is possible to reuse the base part of an existing blade further outboard on a lar- ger/longer blade, or alternatively reuse the base part of an existing blade further inboard on a smaller/shorter blade. All in all, it is possible to make a blade design in such a way that the blade design of the airfoil region is put together from pre-designed sections and that blades of different lengths can be composed partly from sections already existing from previous blades.
Each of the radial sections 38 has an individual average angle of attack for a given design point and the base part of the blade has a sectional airfoil shape, which without flow altering devices has a sectional optimum angle of attack. Flow altering devices, e.g. as shown in Figs. 17-22, may be used to shift the optimum angle of attack of the sectional airfoil shape towards the average angle of attack for the radial section. Fig. 23a shows graphs of an average angle of attack 78 for the blade as a function of the radial distance r from the hub of the rotor. Fig. 23a also shows a graph of an optimum angle of attack 76 of the blade without flow altering devices as a function of the radial distance from the hub. It can be seen that the average angle of attack 78 is higher than the optimum angle of attack 76, which clearly indicates that the blade does not have an optimum blade twist. Therefore, the blade can be provided with flow altering devices as for instance shown in Figs. 17-22 in order to shift the optimum angle of attack with a shift angle Δα, thereby shifting the optimum angle of attack towards the average angle of attack for a given radial distance rfrom the hub.
Figs. 23c and 23d illustrate the effect of providing the blade with flow altering devices for the outer part 44 and the inner part 42 of the blade, respectively. Fig. 23c shows graphs of the relationship between the drag coefficient and the lift coefficient as well the relationship between the angle of attack and the lift coefficient. The graphs are examples of design parameters for the blade at a given radial distance from the hub fal- ling within the outer part 44 of the blade. The design point is depicted with a dot and is chosen based on an optimum relationship between the lift coefficient and the drag coefficient, e.g. by maximising the lift-to-drag ratio. By providing the blade with flow altering devices, the graph showing the relationship between the lift coefficient and the angle of attack is shifted towards higher angles, thereby compensating for the "missing" twist of the outer part 44 of the blade.
Fig. 23d shows similar graphs for the inner part 42 of the blade. It can be seen that the use of flow altering devices on the inner part 42 of the blade has two effects, viz. that the relationship between the lift coefficient and the angle of attack is shifted towards a higher angle of attack and towards a larger lift coefficient. Thus, the flow altering devices not only compensate for the "missing" twist of the inner part 42 of the blade, but also compensate for the non-optimum profile with respect to generating lift of the inner part 42 of the blade, which typically would comprise the root area 30 and the transition area 32 of the blade. However, in relation to modularity of blade parts, it is advantageous that the twist is linearly dependent on the local blade radius. Figs. 24a-d illustrate the relationship between twist, θ, and local radius for various embodiments having a linearly dependent twist. The dashed lines illustrate the optimum twist angle in order to obtain the rotor de- sign target point for a blade without flow altering devices, and the full drawn lines illustrate the relationship between twist angle and local blade radius for a base part having sub-optimum twist and provided with flow altering devices. As shown in Fig. 24a, the twist angle may be lower than the optimum twist angle along the entire longitudinal extend of the longitudinal blade section. Fig. 24b illustrates a second embodiment, in which the twist angle of the base part is equal to the optimum twist angle for a single cross-section only, and where the remainder of the longitudinal blade section has a twist angle, which is lower than the optimum twist angle. However, in principle, the longitudinal blade section may have one part, in which the twist angle is lower than the optimum twist angle, a second part, in which the twist angle is higher than the optimum twist angle, and a third part, in which the twist angle is higher than the optimum twist angle. This is illustrated in Fig. 24c. Yet again, it is highly advantageous, if the base part is designed without any twist, which is illustrated in Fig. 24d.
Figs. 24e-g illustrate the relationship between chord length, c, and local radius for various embodiments having a linearly dependent chord length. The dashed lines illustrate the optimum chord in order to obtain the rotor design target point for a blade without flow altering devices, and the full drawn lines illustrate the relationship between chord length and local blade radius for a base part having a linear chord length distribution and provided with flow altering devices. As shown in Fig. 24a, the chord length may be lower than the chord length along the entire longitudinal extend of the longitudinal blade section. Fig. 24b illustrates a second embodiment, in which the chord length of the base part is equal to the optimum chord length for a single cross-section only, and where the remainder of the longitudinal blade section has a chord length, which is lower than the optimum chord length. Fig. 24c illustrates an advantageous embodiment having a first part, in which the chord length is lower than the optimum chord length, a second part, in which the chord length is higher than the optimum chord length, and a third part, in which the chord length is higher than the optimum chord length. The linear chord length distribution may for instance be chosen as a median line to the optimum chord length distribution.
3.4 Base part with linearly dependent pre-bend Yet again, it may also be advantageous - especially with respect to modularity - to design the base part of the particular longitudinal section with a linear pre-bend, Δy, as illustrated in Fig. 26.
This compensation may be exploited even further by providing at least a part of the longitudinal section with a profile 450 as shown in Fig. 32 having a "negative camber", i.e. a blade where the camber 462 is located closer to the pressure side 452 of the blade than the suction side 454 of the blade (or equivalently, where the camber 462 is located below the chord 460). A blade section has a negative lift coefficient for an incident airflow at an angle of attack of 0 degrees, i.e. the graph illustrating the relationship between lift coefficient and angle of attack is shifted even further towards larger inflow angles. This in turn means that a particular lift coefficient may be obtained at an even higher angle of attack. The use of profiles having a negative camber may be advantageous especially for blades having a very low twist or no twist, since such blades have blade sections, in which the operational angle of attack at the rotor design point is very high. This is particularly relevant for the inboard part of the airfoil region and the transition region. However, the camber 462 need not locally be located nearer the pressure side of the blade than the suction side of the blade along the entire chord of the profile as shown in Fig. 32. As shown in Fig. 33, it is also possible to provide a blade section with profile 550 having a negative camber 562, in which a part of the camber is closer to the suction side 554 of the blade than the pressure side of the blade 552 (or above the chord 560) as long as the camber 562 on average is closer to the pressure side 552 than the suction side of the blade 554. 4 Modularity and reuse of blade sections
Fig. 34 illustrates this principle. A wind turbine blade 410 is divided into a root region 430, a transition region 432, and an airfoil region 434. The airfoil region 434 comprises a blade tip part 436, a first longitudinal section 440, and a second longitudinal section 442. The first longitudinal section 440 of the blade is divided into a first base part 441 and a number of first flow altering devices 446. The second longitudinal section 442 of the blade is divided into a second base part 443 and a number of second flow altering devices 448. The first base part 441 and the second base part 443 have profiles, which have a simplified structure with respect to for instance modularity of blade parts and/or manufacturing of the base parts 441 , 443, and which at the rotor design point in itself deviate significantly from the target axial induction factor and/or the target loading. The base parts 441 are here depicted as having linearly dependent chord lengths, but advantageously, the sections also have a linearly dependent thickness and linearly de- pendent twist or no twist. Therefore, the longitudinal sections are provided with the flow altering devices, which are here depicted as a first slat 446 and a first flap 448, however; the flow altering devices are not restricted to such flow altering devices only. The first longitudinal section 440 and the second longitudinal section 442 both extend along at least 20% of the longitudinal length of the airfoil region 434. The first longitudinal section or base part is located in a first radial distance r?.
6 Examples The following section describes a study of the transformable blades concept via examples. As previously mentioned the transformable blade comprises a base part and an adjustable part. The adjustable part comprises aero-devices or flow altering means, which are fitted to the base part in order to adjust and meet the aerodynamic design target of the blade sections. By adjusting only the flow altering means, the tuning of section aerodynamics allows partial de-coupling between the structural and aerodynamic design. The base part can be designed to have optimal structural properties and not necessarily optimal aerodynamics. Afterward, the flow altering devices will be designed to fill the aerodynamic gaps from non-optimum to near-optimum target condi- tions. Flow altering devices, among others, include flaps, slats, vortex generators and spoilers as described in Sec. 2.2.
6.1 Blade of 40.3 m with DU -91 -w2-250 airfoil and no twist
By changing the outer part of the airfoil region with the DU-91-W2-250 airfoil profile, the power production of a wind turbine using such blades at wind speeds of 8 m/s and 10 m/s without the use of flow altering devices is reduced by 3% compared to a wind turbine using the ideal 40.3 meter blades. Furthermore, the change of profile leads to a deviation from an optimum power coefficient at a wind speed of 8 m/s in a region ranging from 10 meters to 26 meters from the tip (not shown in the graphs). Figs. 40 and 41 show that this deviation is caused by an overloading in this region of the blade. Hence, this part of the transformable blade should be provided with flow altering devices capa- ble of lowering the lift, thereby enhancing the mechanical power output of a wind turbine using such blades.
Fig. 42 shows a first graph 710 showing the relative thickness of the transformable blade compared to a second graph 700 showing the relative thickness of the existing LM40.3p blade as a function of the radial distance rt from the tip. It is seen that the rela- tive thickness of the transformable blade is larger than the relative thickness of the LM40.3p blade. The bending stiffness of a wind turbine blade comprising a shell body is proportional to the cube of the distance between the neutral axis of the blade and the shell body. This means that the shell body of a relative thick profile may be thinner than a relative thinner profile and still obtain the same strength and stiffness. The shell body is typically made as a laminate structure comprising a matrix material reinforced with fibres, such as glass fibres and/or carbon fibres. In the present example the redesigned part of the transformable blade is 14.8% lighter than the corresponding part of the LM40.3p blade, and the overall weight reduction is 7.7%. Thus, it is seen that also the material cost of a transformable blade may be reduced compared to existing blades. A similar study was made of a transformable blade having a relative thickness of 30% for the outer 32 meters. The weight of the redesigned part is reduced with 21.4% compared to the corresponding part of an LM40.3p blade, and the overall weight reduction was 12.3%. Nonetheless, the mechanical power yield of a wind turbine using the trans- formable blade can be maximized from the use of the flow altering devices.
- a profiled contour comprising a pressure side and a suction side as well as a leading edge and a trailing edge with a chord extending between the leading edge and the trailing edge, the profiled contour generating a lift when being impacted by an inci- dent airflow,
- the profiled contour in the radial direction being divided into a root region with a substantially circular or elliptical profile closest to the hub, an airfoil region with a lift generating profile furthest away from the hub, and preferably a transition region between the root region and the airfoil region, the transition region having a profile gradu- ally changing in the radial direction from the circular or elliptical profile of the root region to the lift generating profile of the airfoil region, wherein
- the airfoil region comprises a first base part having a leading edge and a trailing edge with a chord extending between the leading edge and the trailing edge, the airfoil region further being divided into at least a first longitudinal segment and a second lon- gitudinal segment, the first longitudinal segment extending along at least 20% of a longitudinal extent of the airfoil region characterised in that the first base part has an inherent non-ideal aerodynamic design so that a substantial longitudinal part of the base part without flow altering devices at a design point deviates from a target axial induction factor, wherein the method comprises the step of: a) adjusting a pitch of the blade and a rotational speed of the rotor so as to meet the target axial induction factor of the second longitudinal segment, and wherein the first longitudinal segment is provided with flow altering devices so as to meet the target axial induction factor of the first longitudinal segment.
2. A method according to claim 1 , wherein the second longitudinal segment extends along at least 20% of the airfoil region.
3. A method according to claim 1 or 2, wherein the second longitudinal segment comprises a tip region of the blade.
4. A method according to any of the preceding claims, wherein the first longitudinal segment is provided at an inboard position of the airfoil region.
5. A method according to any of the preceding claims, wherein the target axial induc- tion factor of the first longitudinal segment and/or the second longitudinal is close to the aerodynamic optimum target axial induction factor.
6. A method according to any of the preceding claims, wherein the target axial induction factor of the first longitudinal segment and/or the second longitudinal segment lie in the interval between 0.25 and 0.4, or between 0.28 and 0.38, or between 0.3 and 0.36.
7. A method according to any of the preceding claims, wherein the induction factor of the first base part of the first longitudinal segment without flow altering devices at the design point deviates at least 5%, or 10%, or 20%, or 30% from the target axial induc- tion factor.
8. A method according to any of the preceding claims, wherein the first base part of the first longitudinal segment without flow altering devices at the design point further deviates from a target loading, and wherein the first flow altering devices are further ar- ranged so as to adjust the aerodynamic properties of the first longitudinal segment to substantially meet the target loading at the design point.
10. A method according to any of the preceding claims, wherein the flow altering devices comprises devices chosen from the group of:
1 1. A method according to claim 10, wherein the flow altering devices in addition comprises boundary layer control means, such as holes or a slot for ventilation, vortex generators and a Gurney flap.
PCT/EP2010/056814 2009-05-18 2010-05-18 A method of operating a wind turbine WO2010133591A1 (en)
WO2010133591A1 true true WO2010133591A1 (en) 2010-11-25
US20150192106A1 (en) * 2012-02-20 2015-07-09 Alstom Renewable Technologies Wind turbine blade and method of controlling the lift of such a blade
US9803619B2 (en) * 2012-02-20 2017-10-31 Alstom Renewable Technologies Wind turbine blade and method of controlling the lift of such a blade
EP2253838A1 (en) 2010-11-24 application
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