Tip structure for a wind turbine blade

Abstract

Tip structure for a wind turbine blade A wind turbine blade (110) is provided with a bifurcated tip structure (10) comprising a pair of oppositely directed winglets (30, 40). In one aspect, each winglet (30, 40) has an endplate (230, 240) at its distal end. Optionally, each endplate (230, 240) can be made from metal to form a lightning receptor. In another aspect, each winglet (30, 40) is made from metal to form a lightning receptor. In another aspect, each winglet (30, 40) includes a lightning receptor (230, 240) at its distal end, and both lightning receptors (230, 240) are connected to the lightning conductor (113) of the blade (110) via a serrated metal blade tip connector (150) that forms a trailing edge serration of the blade (110). Optionally, both lightning receptors (230, 240) can be connected to the blade tip connector (150) via a serrated metal lightning receptor connector (151) that forms a trailing edge serration of the winglets (30, 40).

Description

[0001] Tip structure for a wind turbine blade

[0002] This invention relates to blade tip structures defining winglets or endplates for use on horizontal axis wind turbine blades.

[0003] It is known to provide wind turbine blades with tip structures defining a winglet or a pair of winglets or endplate, so as to improve aerodynamic efficiency (the ratio of lift to drag, or L / D) by reducing induction, the tendency for the freestream airflow to move radially outwardly around the rotor plane along the length of the blade.

[0004] It is also known to configure the tip of a wind turbine blade to form a receptor for lightning strikes.

[0005] By way of example, CN106762389 A discloses a pointed blade tip forming a lightning receptor.

[0006] WO2019214280 Al discloses a serrated blade tip for use with an ionization discharge unit to form a lightning receptor.

[0007] Although a tip structure can increase lift, it will also add drag, which limits its contribution to aerodynamic efficiency. It will also add mass at the tip of the blade, especially if made from metal to form a lightning receptor, which disadvantageously reduces mass efficiency of the rotor.

[0008] A conventional blade tip without an added winglet or endplate may have a lower mass moment of inertia, which may improve mass efficiency; however, the conventional tip tends to generate an intense vortex as the blade rotates, which in turn generates a somewhat expanding wake downstream of the rotor. The wake persists for a great distance and impinges on downstream wind turbines in a wind farm, reducing their annual energy production.

[0009] It is a general object of the present invention to provide a tip structure for a horizontal axis wind turbine blade that addresses one or more of these problems. In accordance with the present invention there is provided a blade tip for a blade of a wind turbine rotor, as defined in the independent claims.

[0010] The rotor rotates in use about a rotor axis XI, and in use, a mid-portion of the blade extends radially outwardly with respect to the rotor axis, from a blade root to the blade tip.

[0011] The mid-portion defines an aerofoil having a pressure side, a suction side, a leading edge and a trailing edge. The aerofoil is configured, in use, to generate lift in an airflow having an airflow direction from the leading edge to the trailing edge.

[0012] The blade tip includes a pressure side winglet and a suction side winglet joined at a winglet root. The pressure side winglet extends in use along a length axis, from the pressure side of the aerofoil and away from the suction side winglet. The suction side winglet extends in use along a length axis, from the suction side of the aerofoil and away from the pressure side winglet.

[0013] Each winglet has a leading edge, a trailing edge, an inward side joining the respective side of the aerofoil from which the winglet extends, and an outward side opposite the inward side. The inward and outward sides extend in use in the airflow direction from the leading edge to the trailing edge of the respective winglet.

[0014] In accordance with a first aspect of the invention, the blade tip further includes a pressure side endplate mounted at a distal end of the pressure side winglet, and a suction side endplate mounted at a distal end of the suction side winglet.

[0015] Each endplate has at least one of an inward portion and an outward portion. The inward portion extends away from the length axis of the respective winglet, from the inward side of the respective winglet, to an inward free edge of the endplate. The outward portion extends away from the length axis of the respective winglet, from the outward side of the respective winglet, to an outward free edge of the endplate.

[0016] Preferably, each endplate has both of said inward and outward portions extending away from the length axis of the respective winglet and respectively, from the inward and outward sides of the respective winglet to respective, inward and outward free edges of the endplate. Optionally, each endplate may be made from metal and connected, in use, to a lightning conductor of the blade, to form a lightning receptor.

[0017] In accordance with a second aspect of the invention, each winglet is made from metal and connected, in use, to a lightning conductor of the blade, to form a lightning receptor.

[0018] In accordance with a third aspect of the invention, each winglet includes a lightning receptor at its distal end, and a serrated blade tip connector extends from the winglet root to form a serrated trailing edge of the mid-portion of the blade. The lightning receptors are connected electrically to the lightning conductor of the blade via the blade tip connector.

[0019] Further features and advantages of the invention will be appreciated from the various illustrative embodiments that will now be described, purely by way of example and with reference to the accompanying drawings, in which:

[0020] Fig. 1 shows part of the rotor of a horizontal axis wind turbine, including one blade extending horizontally from the hub, as seen in plan view looking down on the rotor.

[0021] Fig. 2 shows a section through the mid-portion of the blade in the chordal plane P123 of Fig. 1.

[0022] Fig. 3 is a schematic view of a simple, bifurcated tip structure including pressure side and suction side winglets, looking towards the leading edge of the mid-portion of the blade in the rotor plane.

[0023] Fig. 4 shows the bifurcated tip structure as shown in Fig. 3, but adapted in accordance with a first embodiment of the invention, wherein endplates are added to the winglets, each endplate having both inward and outward portions extending to respective, inward and outward free edges of the endplate. The geometry is shown as projected onto the second reference plane P2 (discussed below) which corresponds to the plane of the drawing.

[0024] Fig. 4A corresponds to Fig. 4, showing an alternative arrangement in which each of the endplates has only one of the inward and outward portions. Fig. 5 is an enlarged view of the plain (conventional) tip of the blade of Fig. 1, looking towards the leading edge of the blade.

[0025] Figs. 6 - 8 are more detailed views of a bifurcated tip structure without endplates, respectively:

[0026] - looking obliquely towards the tip from the suction side of the blade (Fig. 6);

[0027] - looking in the direction of the rotor axis XI towards the suction side of the blade (Fig. 7); and

[0028] - looking towards the leading edge of the mid-portion of the blade in the rotor plane (Fig. 8).

[0029] Figs. 9 - 11 show the bifurcated tip structure as shown in Figs. 6 - 8, respectively, but adapted with endplates in accordance with another embodiment of the invention.

[0030] Fig. IDA is an enlarged view of part of Fig. 10.

[0031] Figs. 12 - 14 show the vortex pattern generated at the tip of the blade, respectively:

[0032] - by the plain tip of Fig. 5 (Fig. 12);

[0033] - by the bifurcated tip structure of Figs. 6 - 8 (Fig. 13); and

[0034] - by the bifurcated tip structure with endplates of Figs. 9 - 11 (Fig. 14).

[0035] Figs. 15A - 15C present CFD data comparing the wake generated by a full size commercial wind turbine rotor fitted with different tip structures, wherein:

[0036] - Fig. 15A is based on the bifurcated tip structure of Figs. 6 - 8, without endplates;

[0037] - Fig. 15B is based on the bifurcated tip structure of Figs. 9 - 11, with endplates; and

[0038] - Fig. 15C is a scale showing the wake velocity V as represented in Figs. 15A and 15B.

[0039] Figs. 16A - 16C present the same data set as Figs. 15A - 15C, respectively, but at a greater level of detail, wherein:

[0040] - Fig. 16A is based on the bifurcated tip structure of Figs. 6 - 8, without endplates;

[0041] - Fig. 16B is based on the bifurcated tip structure of Figs. 9 - 11, with endplates; and

[0042] - Fig. 16C is a scale showing the wake velocity V as represented in Figs. 16A and 16B. Fig. 17 shows the axial velocity of the wake integrated over the rotor plane, for the same data set as Figs. 15A and 15B and for the plain tip of Fig. 5.

[0043] Fig. 18 presents wind tunnel data comparing a blade with the plain tip of Fig. 5 with the novel bifurcated tip structure with endplates as shown in Figs. 9 - 11.

[0044] Fig. 19 shows a further bifurcated blade tip configured as a lightning receptor in accordance with another embodiment of the invention in its second aspect.

[0045] Fig. 20 shows how the bifurcated blade tip may be retrofitted to a blade in service by screwing a threaded connector into an existing lightning receptor block at the tip.

[0046] Figs. 21 - 23 show a further bifurcated tip structure with feathered endplates.

[0047] Figs. 24 and 25 show another bifurcated tip stucture, generally as illustrated in Fig. 4, but with the endplates adapted as lightning receptors and connected to the lighting conductor of the blade via serrated metal trailing edge fittings.

[0048] Reference numerals and characters that appear in more than one of the figures indicate the same or corresponding elements in each of them.

[0049] Referring to Figs. 1 and 2, a horizontal axis wind turbine rotor 100 is arranged to rotate in use about a rotor axis XI. The rotor has at least one blade, typically three blades, of which only one blade 110 can be seen in the drawing. The blade includes a lightning conductor 113, which may be located inside or on the outside of the blade, for conducting lightning to ground.

[0050] A mid-portion 111 of the blade 110 extends radially outwardly with respect to the rotor axis XI, from a blade root 112 to a blade tip 10", which as shown in Fig. 1 and Fig. 5 is a conventional, plain blade tip without any winglet or endplate structure.

[0051] Referring also to Fig. 2, the mid-portion 111 of the blade defines an aerofoil 120 having a pressure side 130, a suction side 140, a leading edge 121 and a trailing edge 122. The aerofoil 120 defines a chord 123, which is a straight line extending between the leading edge 121 and the trailing edge 122 of the aerofoil 120 in any plane P123, P123' perpendicular to the length axis Xlll of the mid-portion 111 of the blade 110.

[0052] The aerofoil 120 is configured, in use, to generate lift in an airflow having an airflow direction Da from the leading edge 121 to the trailing edge 122. The airflow direction Da will change somewhat with the angle of attack a of the aerofoil, which in turn depends on the pitch of the aerofoil (which maybe adjustable at the hub) and the tip speed ratio (which is to say, the speed of the tip relative to the free airstream, i.e the prevailing wind direction), but can be defined more simply as the chordal direction, which is to say, the direction of the chord 123 from the leading edge 121 to the trailing edge 122 in the plane P123, P123' of the chord 123, referred to herein as the chordal plane.

[0053] Schematic Fig. 3 and more detailed Figs. 6 - 8 show a bifurcated blade tip structure 10' including a pressure side winglet 30 and a suction side winglet 40 joined at a winglet root 11. The blade tip structure 10' may be fitted to the mid-portion 111 of the blade to replace the conventional blade tip 10", either as an original part of the blade or as a retrofit assembly.

[0054] The pressure side winglet 30 extends in use along a length axis X30, from the pressure side 130 of the aerofoil 120 and away from the suction side winglet 40. The suction side winglet 40 extends in use along a length axis X40, from the suction side 140 of the aerofoil 120 and away from the pressure side winglet 30.

[0055] Each winglet 30, 40 has a leading edge 31, 41, a trailing edge 32, 42, an inward side 33, 44 joining the respective side of the aerofoil from which the winglet extends, and an outward side 35, 45 opposite the inward side 33, 44. The inward and outward sides 33, 35, 44, 45 extending in use in the airflow direction Da from the leading edge 31, 41 to the trailing edge 32, 42 of the respective winglet 30, 40.

[0056] The winglets may be formed to define aerofoils, further contributing to lift or enhancing their sealing effect with respect to induced radial flow. The above mentioned features of the winglets 30, 40 of Fig. 3 and Figs. 6 - 8 can also be seen (although not all referenced in the figure) in the generally triangular winglets 30, 40 of the fourth blade tip 10"' of Fig. 19, although they are of quite different shape, as further discussed below. with both inward and outward free

[0057] The same features as seen in Fig. 3 and Figs. 6 - 8 can also be seen in the embodiments of schematic Fig. 4 and more detailed Figs. 9 - 11, wherein the blade tip 10 further includes a pressure side endplate 230 mounted at a distal end of the pressure side winglet 30, and a suction side endplate 240 mounted at a distal end of the suction side winglet 40.

[0058] Optionally, the pressure side winglet 230 may be relatively larger than the suction side winglet 240, as illustrated.

[0059] Each endplate 230, 240 is referred to as an endplate because of its inward and outward portions 233, 235, 244, 245 which extend away from the length axis X30, X40 of the respective winglet 30, 40, and respectively, from the inward and outward sides 33, 35, 44, 45 of the respective winglet 30, 40, to respective, inward and outward free edges 233', 235', 244', 245' of the endplate 230, 240, as shown in Fig. 4 and Figs. 9 - 11.

[0060] By free edges is meant edges that are formed by two, generally oppositely directed outer surfaces that project into the airflow so that the free edge is surrounded on three sides by the airflow.

[0061] A free edge may be formed as an edge of a plate having parallel opposite surfaces, so that the edge is formed in the thickness dimension of the plate. Alternatively, a free edge may be formed between two non-parallel (but still generally oppositely facing) surfaces, such as the opposite major surfaces of an aerofoil.

[0062] As used herein, an edge may be relatively thin, but is not necessarily sharp (although it may be sharp). On the contrary, an edge may be generally square or rounded, defining a surface that extends in the thickness dimension that separates the two major surfaces separated by the edge.

[0063] Optionally, each endplate 230, 240 may be made from metal and connected, in use, to a lightning conductor 113 of the blade, to form a lightning receptor.

[0064] By way of example, each endplate may be formed from a metal plate with constant thickness, so that the free edges define the boundary of the plate and extend in its thickness dimension. Alternatively, the thickness of each endplate may vary across its length and breadth, for example, so that it forms an aerofoil, further contributing to lift or enhancing its sealing effect with respect to induced radial flow.

[0065] Respective free edges 232', 233', 235'; 242', 244', 245' of each endplate 230, 240 may define at least one point 233", 235"; 244", 245", which may further enhance the performance of the endplate when configured as a lightning receptor.

[0066] As illustrated, the inward and outward free edges 233', 235', 244', 245' of each endplate (230, 240) may diverge in the airflow direction (Da) from a leading end (231, 241) to a trailing end (232, 242) of the endplate (230, 240). Each endplate 230, 240 may defines two points 233", 235"; 244", 245" spaced apart at the trailing end 232, 242 of the endplate. The trailing end 232, 242 may define at least one trailing free edge 232', 242' of the endplate 230, 240. Each of the inward and outward free edges 233', 235', 244', 245' of the endplate 230, 240 may form a respective one of the two points 233", 235"; 244", 245" at a respective point of intersection with the at least one trailing free edge 232', 242' of the endplate 230, 240.

[0067] In use, an imaginary rotor plane (P3) perpendicular to the rotor axis (XI) may be defined to pass through or proximate the winglet root 11, and the pressure side winglet 30 and the suction side winglet 40 may extend, respectively, upstream and downstream of the rotor plane (P3) with respect to a direction of the freestream airflow (DA) through the rotor plane (P3).

[0068] Endplates with only one, inward or outward free edge (half endplates) In less preferred embodiments, instead of having both inward and outward portions extending to inward and outward free edges as shown in Fig. 4 and Figs. 9 - 11, each endplate may have only one respective, inward or outward portion extending to a respective, inward or outward free edge.

[0069] Such arrangements might be termed half endplates, compared with the full endplates shown in Fig. 4 and Figs. 9 - 11.

[0070] Fig. 4A shows one example of half endplates (which is the only illustrated example.)

[0071] In such arrangements, each half endplate blends into its respective winglet in a region that has a smaller internal radius than the adjacent surfaces of the endplate and the winglet, thus forming a clearly defined angle of less than 180° between the adjacent surfaces of the endplate and the winglet.

[0072] In the example of Fig. 4A, the pressure side endplate 230 has only an inward portion 233 extending to an inward free edge 233', and the suction side endplate 240 has only an outward portion 245 extending to an outward free edge 245'.

[0073] Half endplates are still advantageous although they will have less power than full endplates. Where half endplates are used, both endplates may have only the inward portion or only the outward portion, or (as shown in Fig. 4A) either one of them can have only the inward portion, and the other one can have only the outward portion.

[0074] Generally in this specification, the term "endplates" means full endplates rather than half endplates.

[0075] However, the various features and advantages described herein with reference to the full endplates can also be seen or applied, mutatis mutandis, in the half endplates. The full endplates will work better than half endplates and so are preferred.

[0076] Endplate cant angles Referring also to Fig. 4, a first reference plane Pl may be defined to contain both the length axis Xlll and the chord 123 of the mid-portion 111 of the blade 110 proximate the winglet root 11.

[0077] For ease of illustration, the lines Pl' are drawn parallel with the first reference plane Pl in the second reference plane P2 which is the plane of the drawing.

[0078] A second reference plane P2 (Fig. 1; the plane of the drawing of Figs. 3 and 4) perpendicular to the first reference plane Pl is defined to contain the length axis Xlll of the mid-portion 111 of the blade 110 proximate the winglet root 11.

[0079] When projected onto the second reference plane P2, each of the inward and outward portions 233, 235, 244, 245 of each endplate 230, 240 extends along a respective endplate portion axis X233, X235, X244, X245 to define a respective cant angle A233, A235, A244, A245 with respect to the first reference plane Pl.

[0080] Each cant angle A233, A235, A244, A245 may lie within a range of 10° either side of the first reference plane Pl.

[0081] Wake decay

[0082] Where multiple wind turbines are spaced apart to form a wind farm, the annual energy production of downstream turbines will be reduced by wake losses due to the wake from upstream turbines impinging on the downstream turbines.

[0083] The novel bifurcated tip structure including twin winglets terminating in endplates is found to enhance mixing of the freestream flow into the wake, compressing the boundary of the wake and promoting earlier decay of the wake, and so reducing wake losses in downstream turbines.

[0084] Figs. 15A and 15B present data from a CFD (computational fluid dynamics) model showing the wake pattern generated by a blade tip mounted on one of the three rotor blades of an otherwise conventional, full scale commercial horizontal axis wind turbine. Fig. 15A shows the wake generated by the tip structure of Figs. 6 - 8 having winglets without endplates, and Fig. 15B shows the wake generated by the tip structure of Figs. 9 - 11 having the same winglets with endplates. Both tip structures are modelled in the same wind conditions, with the winglets being of equal size and proportionate to the blade.

[0085] The figures represent the wake velocity in a vertical plane containing the rotor axis XI, showing the upper half of the wake generated downstream of the rotor. The blade carrying the tip structure extends vertically upwardly at the left hand end of the plot from the upper half of the rotor hub, which can just be seen at the bottom of the plot. The distance downstream from the rotor is indicated by the scale from 0m to 400m.

[0086] Wake velocity V is represented on a scale from 0 m / s to 9.5 m / s by the shade of the plot, as shown in the scale at Fig. 15C.

[0087] The dark shade at the height of the rotor hub (so at the bottom of each of Figs. 15A and 15B) corresponds to the left hand end of the scale of Fig. 15C, indicating low velocity. The shade of the free stream flow above the wake profile (so at the top of each of Figs. 15A and 15B) corresponds to the right hand end of the scale of Fig. 15C, indicating high velocity. The light shade of the wake profile at its boundary with the free stream flow corresponds to the middle of the scale of Fig. 15C, indicating medium velocity.

[0088] To help in comparing the two wake patterns, a horizontal line LI indicates the maximum height of the tip structure, which is too small to show at the scale of the drawing.

[0089] It can be seen that the lower velocity region towards the bottom of the figure is similar in both Figs. 15A and 15B.

[0090] However, the boundary of the wake profile (where it meets the free stream flow) in Fig. 15B, near the rotor, is significantly radially smaller than that of the wake profile of Fig. 15A. In Fig. 15A, the wake widens radially just downstream of the rotor, around the 30m position; whereas in Fig. 15B the wake widens very little downstream of the rotor.

[0091] Further, it can be seen that the wake profile in Fig. 15B decays more rapidly than that of Fig.

[0092] 15A - which is to say, the radial extent of the boundary of the wake profile, with respect to the rotor axis XI, declines more rapidly with distance from the rotor in Fig. 15B compared with Fig.

[0093] 15A.

[0094] The data of Figs. 15A and 15B was derived mathematically from a more detailed data set, which is shown in Figs. 16A and 16B, whereby Figs. 15A and 15B present a simplified view of the data.

[0095] Fig. 16A is a more detailed view of the data set of Fig. 15A, and Fig. 16B is a more detailed view of the data set of Fig. 15B.

[0096] Figs. 16A and 16B are presented in a similar way to Figs. 15A and 15B, with wake velocity V represented on a scale from 0 m / s to 9.5 m / s by the shade of the plot, as shown in the scale at Fig. 16C.

[0097] Fig. 16B shows how high energy air from the freestream flow is injected into the wake downstream of the rotor at a position indicated by the arrow A, about 200m downstream of the rotor; whereas in Fig. 16A, a similar effect is observed at a greater distance of about 300m, also indicated by an arrow A. The earlier injection of high energy air in Fig. 16B illustrates how the wake decays more rapidly downstream of the rotor.

[0098] To assist in identifying the values shown in Figs. 16A and B on the scale of Fig. 16C, the background value in each figure is at the right hand end of the scale, so just below 9.5m / s. The lightest regions have a value of just above 4.75m / s on the scale. The dark region immediately downstream of the rotor is at the left hand end of the scale, so just above 0 m / s. The long, central part of the downstream flow, terminating shortly before the tip of each arrow A, has a value of about 3.0m / s on the scale. The high energy air at the tip of each arrow A has a value of about 8.5m / s on the scale.

[0099] Fig. 17 shows the axial velocity of the wake calculated in the model as shown in Figs. 15A and 15B, integrated over the rotor plane to give a single value Vint that varies with distance D downstream of the rotor. A corresponding data set is shown for the same blade with a plain tip without winglets or endplates, as shown in Fig. 5. In Fig. 17:

[0100] Data set A represents the wake generated by the blade with the tip structure of Fig. X having winglets without endplates, corresponding to Figs. 15A and 16A.

[0101] Data set B represents the wake generated by the blade with the tip structure of Fig. Y having the same winglets with endplates, corresponding to Figs. 15B and 16B.

[0102] Data set C represents the wake generated by the blade with the plain tip of Fig. 5.

[0103] It can be seen that the integrated axial velocity Vint downstream of the rotor is higher for data set B than for data set A, which in turn is higher than data set C.

[0104] Since power is proportional to velocity cubed, the difference between data sets A and B represents a significant increase in power yield, as much as +10% instantaneous increase, or even as much as +20% or more.

[0105] It can be seen that the added endplates have the effect of binding the airflow over the winglets, significantly augmenting the so-called jetting effect of the winglets in redirecting the flow radially inwardly.

[0106] Vortex formation and trajectory

[0107] Referring to Figs. 12 - 14, the advantageous effect of the novel bifurcated tip structure with twin winglets terminating in endplates can be better understood by considering the vortex generating effect of the free edges of the endplates.

[0108] The plain blade tip of Fig. 5 generates a single large vortex which contributes significant induced drag and holds its position axially behind the swept boundary of the rotor, resulting in a radially voluminous wake, as shown in Fig. 12.

[0109] By adding a bifurcated tip structure including both pressure side and suction side winglets (Figs. 6 - 8) this single large vortex is replaced by a pair of counter-rotating vortices 70 as illustrated in Fig. 13. By adding an endplate to each winglet as shown in the embodiment of Figs. 9 - 11, each of these counter-rotating vortices 70 is broken down into a further pair of smaller, counterrotating vortices 70 as shown in Fig. 14, with a further reduction in induced drag.

[0110] Compared with larger, more intense vortices, the smaller vortices are more rapidly deflected radially inwardly downstream of the rotor. As the family of vortices blend together and decay they draw in clean (high energy) air from the freestream flow outside the wake boundary. The higher energy air mixes with the wake, causing the wake to decay more rapidly as it flows away from the rotor, and so increasing the velocity and energy of the air flowing to the downstream turbines.

[0111] As further discussed below, this effect of the endplates is achieved at surprisingly low cost in aerodynamic and mass efficiency, so that the novel tip structure including winglets terminating in endplates is significantly more efficient than a comparable tip structure without the endplates.

[0112] Unsteady loads

[0113] A wind turbine rotor is sensitive to unsteady loads, which can damage the drivetrain from the hub to the generator. By fitting winglets to the blade tip, unsteady loads can be reduced.

[0114] Experimental data indicates that by adding endplates to the winglets, the formation of multiple counter-rotating vortices instead of larger, single vortices can provide a further reduction in unsteady loads of around 2% - 3% of the reduction achieved by the winglets without endplates.

[0115] Fig. 18 shows real time data derived from wind tunnel testing comparing a blade having a plain tip (Fig. 5), represented by trace C (the lighter of the two lines), with a blade having the novel bifurcated tip structure with endplates (Figs. 9 - 11), represented by trace B (the darker of the two lines). The ratio of transient lift Lt to mean lift Lm ( Lt / Lm ) is plotted over time t, providing an indication of the severity of unsteady loading.

[0116] Unsteady loads are produced by periodic separations or stalls. The tip vortex itself is unsteady and forms, breaks down and reforms, exerting periodic loading on the tip. By breaking down the vorticity formed at the blade tip, shear layer mixing occurs at a much smaller scale and so is inherently more stable.

[0117] The reduction in the variation (unsteadiness) of the loads provided by the novel tip structure is apparent from the reduced variability of trace B compared with trace C.

[0118] The rotating blades impede airflow through the rotor plane and so generate a radially outward flow, lengthwise along the blade, as the free airstream diverts around the rotor - the so-called induction effect. The winglets have a sealing effect, restraining this radially outward component of the airflow and force the air to flow around the blade in the chordal direction Da, resulting in increased lift L.

[0119] The endplates are found to augment the sealing effect of the winglets, so that lift is increased on all lifting surfaces (including the mid-portion of the blade and the winglets) with a relatively modest penalty in added drag.

[0120] Compared with the plain blade tip (Fig. 5), the tip structure reduces induced drag but adds parasitic drag, and so contributes a modest overall increase in drag. It is found that the endplates reduce induced drag from the winglets while contributing significant additional lift when compared with the tip structure without endplates. The improvement in lift is greater than the increase in overall drag, resulting in a net increase in aerodynamic efficiency L / D, the ratio of lift to drag of the blade.

[0121] Mass efficiency Wind turbine rotors are particularly sensitive to added mass at the tip of the blades. It is found that by adding endplates to the winglets of Figs. 6 - 8, mass efficiency (P / l, the ratio of average shaft output power P to mass moment of inertia I of the rotor) is improved.

[0122] By way of example, modelling shows that the bifurcated tip structure without endplates shown in Figs. 6 - 8 (when fitted to a commercial wind turbine blade as shown in the modelled examples) can be adapted by reducing the axial length of the pressure side winglet by 400mm, and terminating the pressure side winglet with an endplate. The combined effect of adapting the bifurcated tip structure in this way is to reduce the total mass of the tip structure by an estimated 1.2kg while delivering approximately 0.2% additional torque to the blade.

[0123] In order to obtain the added torque without an endplate, the mass moment of the blade would have to increase, either by increasing the length of the mid-portion of the blade or by increasing the length of the tip structure.

[0124] Feathered endplates

[0125] Figs. 21 - 23 show a further bifurcated tip stucture with endplates, corresponding generally to that of Figs. 9 - 11, but differing from that embodiment in that each of the inward and outward portions 233, 235, 244, 245 of each endplate 230, 240 is divided into two or more portions or feathers 236, 246 extending from the respective winglet 30, 40, adjacent ones of the portions 236, 246 being separated by a gap 237, 247. The portions or feathers generate multiple vortices which blend together downstream of the rotor and further enhance the breakdown of the wake.

[0126] As discussed above, the endplates 230, 240 of the novel tip structure may form lightning receptors.

[0127] Fig. 19 shows a yet further blade tip 10"' having a pair of winglets 30, 40 jointed at a winglet root 11, wherein the winglets are configured to form lightning receptors. The winglets may be generally triangular, as shown, with the bases of the two triangles joined to form the winglet root 11.

[0128] The winglets of the fourth blade tip 10"' have features common to the bifurcated blade tip structure 10' of schematic Fig. 3 and more detailed Figs. 6 - 8, as mentioned above, and so those features will not be further discussed. The inward sides 33, 44 cannot be seen in the figure, but the leading and trailing edges 31, 41 and the outward sides 35, 45 are visible.

[0129] Each winglet 30, 40 is made from metal and connected, in use, to a lightning conductor 113 of the blade, to form a lightning receptor.

[0130] As illustrated, each winglet 30, 40 may have a free edge 31, 32, 41, 42 defining at least one point 30", 40".

[0131] Optionally, the leading and trailing edges 31, 32, 41, 42 of each winglet 30, 40 converge to define a point 30", 40".

[0132] Optionally, the leading and trailing edges 31, 32, 41, 42 of each winglet 30, 40 are straight or smoothly curved.

[0133] Optionally, the blade tip includes a metal plate 153 extending away from the winglet root, the metal plate 153 being configured to connect the blade tip both electrically to the lightning conductor 113 of the blade, and mechanically to the mid-portion of the blade.

[0134] A wind turbine rotor blade may include a blade tip as disclosed above, wherein the metal plate 153 is mounted on a surface of the blade and connected to the lightning conductor 113 of the blade by a threaded fastener 154.

[0135] Fig. 20 shows how the bifurcated blade tip may be retrofitted to a blade in service by screwing a threaded connector 154 into an existing LPS block (i.e. lighting protection system block or lightning receptor block) 114 connected to the lightning conductor 113 at the original blade tip. The LPS block 114 is typically present in any large blade, being a substantial block of metal that resides near the tip of the blade to accept the pin receptor discharges from the conventional lightning receptor pins of the blade.

[0136] In each of its embodimets, the novel blade tip may be retrofitted or formed as an original part of the blade. It may be connected internally or externally of the main body of the blade. It may or may not have rubber or insulating pads or spacers for fitting purposes and better insulation of unwanted paths into the blade.

[0137] To fit the blade tip electrically to the LPS block 114, firstly the existing LPS block 114 may be bored out, e.g. with a drill. The LPS block 114 is often corroded, so re-boring the block provides a satisfactory electrical contact area for the current path. The threaded connector is then inserted into the new bore formed in the LPS block 114.

[0138] When configured as a lightning receptor, the novel blade tip provides an aerodynamic benefit while also defining a bifurcated structure providing two spaced lightning receptors 30, 40 at the extremity of the blade, to more effectively attract lightning strikes away from the midportion of the blade.

[0139] Since the lightning receptor must be made from metal, it adds mass to the blade tip and so adversely affects mass efficiency.

[0140] The added aerodynamic benefit of the bifurcated tip structure provides additional torque which offsets the reduction in mass efficiency.

[0141] Advantageously, each receptor may be configured to form a large point 30", 40" which tends to ionize the surrounding airflow, further attracting the strike to the point 30", 40". Further advantageously, when compared with serrated arrangements as known in the art, a large, single point 30", 40" is more resistant to erosion and so may retain its shape and so remain effective for longer if hit by multiple strikes. The same advantage is obtained by providing large points (e.g. trailing edge points 233", 235"; 244", 245") on the endplates of the earlier described tip structures when adapted as lightning receptors.

[0142] Serrated blade tip connector and serrated endplate connector

[0143] As illustrated in the example of Figs. 24 and 25, the endplates 230, 240 of the bifurcated tip stucture discussed above with reference to Fig. 4 and Figs. 9 - 11 may be made from metal to form lightning receptors and connected to the lighting conductor 113 of the blade via one or more serrated metal trailing edge fittings 150, 151.

[0144] The trailing edge fittings may include a serrated blade tip connector 150 defining serrations 150' and extending from the winglet root (11) to form, in use, a serrated portion (122') of the trailing edge (122) of the mid-portion (111) of the blade (110). The blade tip connector 150 is made from metal and connected in electrically conductive relation to both of the endplates 230, 240 and to the lightning conductor 113 of the blade 110, so that the endplates 230, 240 are connected electrically to the lightning conductor 113 of the blade 110 via the blade tip connector 150.

[0145] As illustrated, the endplates 230, 240 may be connected electrically to the blade tip connector 150 via a serrated endplate connector 151.

[0146] The endplate connector 151 defines serrations 151' and forms a serrated trailing edge 32', 42' of each of the winglets (30, 40). The endplate connector 151 is connected in electrically conductive relation to both of the endplates 230, 240 and, at the winglet root 11, also to the blade tip connector 150.

[0147] The serrated connectors can provide both an electrical and a mechanical connection, while the serrated trailing edges help to reduce turbulence and hence noise at the trailing edge of the blade and the tip structure. The metal trailing edges can provide the strength required in the trailing edge serrations with relatively small material thickness. In this way the enhanced lightning protection of the tip structure is obtained with minimal net added drag, and moreover, with minimal added weight since the metal parts need only extend along the trailing edges.

[0148] As illustrated (and whether or not the serrrated endplate connector is provided), the blade tip connector 150 may be connected electrically to the lightning conductor 113 of the blade 110 via at least one chordwise connector 152.

[0149] One, or (as illustrated) a pair of chordwise connectors 152 can be arranged to extend, each in the airflow direction Da at a respective one of the pressure and suction sides 130, 140 of the mid-portion 111 of the blade 110. These connectors are referred to herein as chordwise connectors because they extend chordwise, which is to say, generally across the blade surface between the leading and trailing edges, so in the direction of the chord of the aerofoil or the airflow direction Da.

[0150] The or each chordwise connector 152 is connected in electrically conductive relation to the blade tip connector 150 and to the lightning conductor 113 of the blade 110.

[0151] The chordwise orientation allows the chordwise connector 152 to react mechanical forces in tension as well as conducting lighting at the surface of the blade. The or each chordwise connector 152 can be fixed to the existing LPS block 114, e.g. using a threaded fastener 154 as shown in Fig. 20.

[0152] Each chordwise connector 152 can be a relatively thin strap or strip of metal, for example, less than about 3mm thick.

[0153] In use, the serrated blade tip connector 150 and (if provided) the serrated endplate connector 151 and / or the chordwise connector 152 conduct any lighting strike from the metal endplates 230, 240 externally along the blade to the lightning conductor 113 of the blade, so that the nonconductive structure of the blade is not damaged by heat from internal conductors near the tip.

[0154] In less preferred embodiments, where a serrated endplate connector 151 is not provided, the metal endplates may be connected electrically to the serrated blade tip connector 150 in any other way, such as via an internal conductor running through the tip structure, or by making the entire winglet structure from metal (which however is less preferred due to the added weight.)

[0155] In further alternative embodiments, instead of forming the metal lightning receptors 230, 240 as endplates, they could be arranged at distal ends of the winglets (as exemplified by endplates 230, 240, e.g. in Figs. 24 and 25, which are located at the distal ends of the winglets) with any other desired shape. For example, the lightning receptors could merely form tip (distal end) portions of the winglets 30, 40. In this case the serrated blade tip connector 150 may be arranged as described above with reference to Figs. 24 and 25, so that it extends from the winglet root 11 to form a serrated portion 122' of the trailing edge 122 of the mid-portion 111 of the blade 110. As in the last described embodiment, the blade tip connector 150 is made from metal and connected in electrically conductive relation to both of the lightning receptors 230, 240 and to the lightning conductor 113 of the blade 110, so that the lightning receptors 230, 240 are connected electrically to the lightning conductor 113 of the blade 110 via the blade tip connector 150.

[0156] Optionally, a lightning receptor connector 151 may be provided, similar to the endplate connector 151 of the last described embodiment. The lightning receptor connector 151 defines serrations 151' and forms a serrated trailing edge 32', 42' of each of the winglets 30, 40, and is connected in electrically conductive relation to both of the lightning receptors 230, 240 and to the blade tip connector 150 at the winglet root 11, so that the lightning receptors 230, 240 are connected electrically to the blade tip connector 150 via the lightning receptor connector 151. This arrangement too can be seen in Figs. 24 and 25.

[0157] Optionally, (and whether or not the serrrated lightning receptor connector 151 is provided), the blade tip connector 150 may be connected electrically to the lightning conductor 113 of the blade 110 via at least one chordwise connector 152.

[0158] As shown in Figs. 24 and 25, and as described with reference to the last described embodimet, at least one, or (as illustrated) a pair of chordwise connectors 152 can be arranged to extend, each in the airflow direction Da at a respective one of the pressure and suction sides 130, 140 of the mid-portion 111 of the blade 110. The at least one chordwise connector 152 is connected in electrically conductive relation to the blade tip connector 150 and to the lightning conductor (113 of the blade 110, so that the blade tip connector 150 is connected electrically to the lightning conductor 113 of the blade 110 via the at least one chordwise connector 152.

[0159] Summary

[0160] In summary, in embodiments, a wind turbine blade (110) is provided with a bifurcated tip structure (10) comprising a pair of oppositely directed winglets (30, 40). In one aspect, each winglet (30, 40) has an endplate (230, 240) at its distal end. Optionally, each endplate (230, 240) can be made from metal to form a lightning receptor. In another aspect, each winglet (30, 40) is made from metal to form a lightning receptor. In another aspect, each winglet (30, 40) includes a lightning receptor (230, 240) at its distal end, and both lightning receptors (230, 240) are connected to the lightning conductor (113) of the blade (110) via a serrated metal blade tip connector (150) that forms a trailing edge serration of the blade (110). Optionally, both lightning receptors (230, 240) can be connected to the blade tip connector (150) via a serrated metal lightning receptor connector (151) that forms a trailing edge serration of the winglets (30, 40).

[0161] As mentioned above, all of the features discussed herein with reference to full endplates (including inter alia: cant angles, feathered endplates, serrated trailing edge fittings, and other lightning protection arrangements) can be applied similarly, mutatis mutandis, to half endplates.

[0162] Many further adaptations are possible within the scope of the claims.

[0163] In the claims, reference numerals and characters are provided in parentheses, purely for ease of reference, and should not be construed as limiting features.

Claims

CLAIMS1. A blade tip (10) for a blade (110) of a wind turbine rotor (100), the rotor rotating in use about a rotor axis (XI), wherein in use, a mid-portion (111) of the blade (110) extends radially outwardly with respect to the rotor axis (XI), from a blade root (112) to the blade tip (10); the mid-portion (111) defining an aerofoil (120) having a pressure side (130), a suction side (140), a leading edge (121) and a trailing edge (122), the aerofoil (120) being configured, in use, to generate lift in an airflow having an airflow direction (Da) from the leading edge (121) to the trailing edge (122); the blade tip (10) including a pressure side winglet (30) and a suction side winglet (40) joined at a winglet root (11); the pressure side winglet (30) extending in use along a length axis (X30), from the pressure side (130) of the aerofoil (120) and away from the suction side winglet (40); the suction side winglet (40) extending in use along a length axis (X40), from the suction side (140) of the aerofoil (120) and away from the pressure side winglet (30); each winglet (30, 40) having a leading edge (31, 41), a trailing edge (32, 42), an inward side (33, 44) joining the respective side (130, 140) of the aerofoil from which the winglet extends, and an outward side (35, 45) opposite the inward side (33, 44), the inward and outward sides (33, 35, 44, 45) extending in use in the airflow direction (Da) from the leading edge (31, 41) to the trailing edge (32, 42) of the respective winglet (30, 40); wherein the blade tip (10) further includes: a pressure side endplate (230) mounted at a distal end of the pressure side winglet (30), and a suction side endplate (240) mounted at a distal end of the suction side winglet (40); each endplate (230, 240) having at least one of: an inward portion (233, 244) extending away from the length axis (X30, X40) of the respective winglet (30, 40), from the inward side (33, 44) of the respective winglet (30, 40), to an inward free edge (233', 244') of the endplate (230, 240); and an outward portion (235, 245) extending away from the length axis (X30, X40) of the respective winglet (30, 40), from the outward side (35, 45) of the respective winglet (30, 40), to an outward free edge (235', 245') of the endplate (230, 240).

2. A blade tip according to claim 1, wherein each endplate (230, 240) has both of said inward and outward portions (233, 235, 244, 245) extending away from the length axis (X30, X40) of the respective winglet (30, 40), and respectively, from the inward and outward sides (33, 35, 44, 45) of the respective winglet (30, 40), to respective, inward and outward free edges (233', 235', 244', 245') of the endplate (230, 240).

3. A blade tip according to claim 1 or claim 2, wherein each endplate (230, 240) is made from metal and connected, in use, to a lightning conductor (113) of the blade, to form a lightning receptor.

4. A blade tip accoding to claim 3, wherein respective free edges (232', 233', 235'; 242', 244', 245') of each endplate (230, 240) define at least one point (233", 235"; 244", 245").

5. A blade tip according to claim 2, wherein: each endplate (230, 240) is made from metal and connected, in use, to a lightning conductor (113) of the blade, to form a lightning receptor; and the inward and outward free edges 233', 235', 244', 245' of each endplate (230, 240) diverge in the airflow direction (Da) from a leading end (231, 241) to a trailing end (232, 242) of the endplate (230, 240); and each endplate (230, 240) defines two points (233", 235"; 244", 245") spaced apart at the trailing end (232, 242) of the endplate; and the trailing end (232, 242) defines at least one trailing free edge (232', 242') of the endplate (230, 240); and each of the inward and outward free edges (233', 235', 244', 245') of the endplate (230, 240) forms a respective one of the two points (233", 235"; 244", 245") at a respective point of intersection with the at least one trailing free edge (232', 242') of the endplate (230, 240).

6. A blade tip according to claim 3, 4 or 5, further including: a blade tip connector (150) defining serrations (150') and extending from the winglet root (11) to form, in use, a serrated portion (122') of the trailing edge (122) of the mid-portionthe blade tip connector (150) being made from metal and connected in electrically conductive relation to both of the endplates (230, 240) and to the lightning conductor (113) of the blade (110), so that the endplates (230, 240) are connected electrically to the lightning conductor (113) of the blade (110) via the blade tip connector (150).

7. A blade tip according to claim 6, further including an endplate connector (151); the endplate connector (151) defining serrations (151') and forming a serrated trailing edge (32', 42') of each of the winglets (30, 40); the endplate connector (151) being connected in electrically conductive relation to both of the endplates (230, 240) and to the blade tip connector (150) at the winglet root (11), so that the endplates (230, 240) are connected electrically to the blade tip connector (150) via the endplate connector (151).

8. A blade tip according to claim 6 or claim 7, further including at least one chordwise connector (152) extending in the airflow direction (Da) at a respective one of the pressure and suction sides (130, 140) of the mid-portion (111) of the blade (110); the at least one chordwise connector (152) being connected in electrically conductive relation to the blade tip connector (150) and to the lightning conductor (113) of the blade (110), so that the blade tip connector (150) is connected electrically to the lightning conductor (113) of the blade (110) via the at least one chordwise connector (152).

9. A blade tip accoding to claim 1 or claim 2, wherein, in use, an imaginary rotor plane (P3) perpendicular to the rotor axis (XI) passes through or proximate the winglet root; and the pressure side winglet and the suction side winglet extend, respectively, upstream and downstream of the rotor plane (P3) with respect to a direction of airflow (DA) through the rotor plane (P3).

10. A blade tip according to claim 1 or claim 2, wherein the aerofoil (120) defines a chord (123), the chord (123) being a straight line extending between the leading edge (121) and the trailing edge (122) of the aerofoil (120) in a plane (P123') perpendicular to a length axis (Xlll) of the mid-portion (111) of the blade (110); and a first reference plane (Pl) contains both the length axis (Xlll) and the chord (123) of the mid-portion (111) of the blade (110) proximate the winglet root (11); anda second reference plane (P2) perpendicular to the first reference plane (Pl) contains the length axis (Xlll) of the mid-portion (111) of the blade (110) proximate the winglet root (11); wherein, when projected onto the second reference plane (P2): for each of the endplates (230, 240), the or each of said at least one of an inward portion (233, 244) and outward portion (235, 245) of the respective endplate (230, 240) extends along a respective endplate portion axis (X233, X235, X244, X245) to define a respective cant angle (A233, A235, A244, A245) with respect to the first reference plane (Pl); and each cant angle (A233, A235, A244, A245) lies within a range of 10° either side of the first reference plane (Pl).

11. A blade tip accoding to claim 1 or claim 2, wherein, for each of the endplates (230, 240), the or each of said at least one of an inward portion (233, 244) and outward portion (235, 245) of the respective endplate (230, 240) is divided into two or more portions (236, 246) extending from the respective winglet (30, 40), adjacent ones of the portions (236, 246) being separated by a gap (237, 247).

12. A blade tip (10) for a blade (110) of a wind turbine rotor (100), the rotor rotating in use about a rotor axis (XI), wherein in use, a mid-portion (111) of the blade (110) extends radially outwardly with respect to the rotor axis (XI), from a blade root (112) to the blade tip (10); the mid-portion (111) defining an aerofoil (120) having a pressure side (130), a suction side (140), a leading edge (121) and a trailing edge (122), the aerofoil (120) being configured, in use, to generate lift in an airflow having an airflow direction (Da) from the leading edge (121) to the trailing edge 122; the blade tip (10) including a pressure side winglet (30) and a suction side winglet (40) joined at a winglet root (11); the pressure side winglet (30) extending in use along a length axis (X30), from the pressure side (130) of the aerofoil (120) and away from the suction side winglet (40); the suction side winglet (40) extending in use along a length axis (X40), from the suction side (140) of the aerofoil (120) and away from the pressure side winglet (30);each winglet (30, 40) having a leading edge (31, 41), a trailing edge (32, 42), an inward side (33, 44) joining the respective side (130, 140) of the aerofoil from which the winglet extends, and an outward side (35, 45) opposite the inward side (33, 44), the inward and outward sides (33, 35, 44, 45) extending in use in the airflow direction (Da) from the leading edge (31, 41) to the trailing edge (32, 42) of the respective winglet (30, 40); wherein each winglet (30, 40) is made from metal and connected, in use, to a lightning conductor (113) of the blade, to form a lightning receptor.

13. A blade tip according to claim 12, wherein each winglet (30, 40) has a free edge (31, 32; 41, 42) defining at least one point (30", 40").

14. A blade tip according to claim 12, wherein the leading and trailing edges (31, 32; 41, 42) of each winglet (30, 40) converge to define a point (30", 40").

15. A blade tip according to claim 14, wherein the leading and trailing edges (31, 32; 41, 42) of each winglet (30, 40) are straight or smoothly curved.

16. A blade tip according to claim 12, wherein the blade tip includes a metal plate (153) extending away from the winglet root, the metal plate (153) being configured to connect the blade tip both electrically to the lightning conductor of the blade, and mechanically to the midportion of the blade.

17. A wind turbine rotor blade including a blade tip according to claim 16, wherein the metal plate (153) is mounted on a surface of the blade and connected to the lightning conductor of the blade by a threaded fastener (154).

18. A blade tip (10) for a blade (110) of a wind turbine rotor (100), the rotor rotating in use about a rotor axis (XI), wherein in use, a mid-portion (111) of the blade (110) extends radially outwardly with respect to the rotor axis (XI), from a blade root (112) to the blade tip (10); the mid-portion (111) defining an aerofoil (120) having a pressure side (130), a suction side (140), a leading edge (121) and a trailing edge (122),the aerofoil (120) being configured, in use, to generate lift in an airflow having an airflow direction (Da) from the leading edge (121) to the trailing edge 122; the blade tip (10) including a pressure side winglet (30) and a suction side winglet (40) joined at a winglet root (11); the pressure side winglet (30) extending in use along a length axis (X30), from the pressure side (130) of the aerofoil (120) and away from the suction side winglet (40); the suction side winglet (40) extending in use along a length axis (X40), from the suction side (140) of the aerofoil (120) and away from the pressure side winglet (30); each winglet (30, 40) having a leading edge (31, 41), a trailing edge (32, 42), an inward side (33, 44) joining the respective side (130, 140) of the aerofoil from which the winglet extends, and an outward side (35, 45) opposite the inward side (33, 44), the inward and outward sides (33, 35, 44, 45) extending in use in the airflow direction (Da) from the leading edge (31, 41) to the trailing edge (32, 42) of the respective winglet (30, 40); wherein each winglet (30, 40) includes a lightning receptor (230, 240), the lighting receptor (230, 240) being made from metal and arranged at a distal end of the winglet; and the blade tip further includes a blade tip connector (150) defining serrations (150') and extending from the winglet root (11) to form, in use, a serrated portion (122') of the trailing edge (122) of the mid-portion (111) of the blade (110); the blade tip connector (150) being made from metal and connected in electrically conductive relation to both of the lightning receptors (230, 240) and to a lightning conductor (113) of the blade (110), so that the lightning receptors (230, 240) are connected electrically to the lightning conductor (113) of the blade (110) via the blade tip connector (150).

19. A blade tip (10) according to claim 18, further including a lightning receptor connector (151); the lightning receptor connector (151) defining serrations (151') and forming a serrated trailing edge (32', 42') of each of the winglets (30, 40); the lightning receptor connector (151) being connected in electrically conductive relation to both of the lightning receptors (230, 240) and to the blade tip connector (150) at the winglet root (11), so that the lightning receptors (230, 240) are connected electrically to the blade tip connector (150) via the lightning receptor connector (151).

20. A blade tip according to claim 18 or claim 19, further including at least one chordwise connector (152) extending in the airflow direction (Da) at a respective one of the pressure and suction sides (130, 140) of the mid-portion (111) of the blade (110); the at least one chordwise connector (152) being connected in electrically conductive relation to the blade tip connector (150) and to the lightning conductor (113) of the blade (110), so that the blade tip connector (150) is connected electrically to the lightning conductor (113) of the blade (110) via the at least one chordwise connector (152).

Images