AEROFOIL TIP STRUCTURE, PARTICULARLY FOR A HAWT ROTOR BLADE
20210088026 ยท 2021-03-25
Inventors
Cpc classification
F03D1/0633
MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
Y02E10/74
GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
F03D1/0641
MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
F05B2240/30
MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
F03D1/0675
MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
F05B2240/307
MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
F05B2260/97
MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
International classification
Abstract
A tip structure may be arranged for example on a rotor blade (12) of a HAWT (10). The tip structure comprises a pressure side structure (50) arranged on a pressure side (43) of the blade, and a suction side structure (60) arranged on a suction side (44) of the blade (12). The pressure side and suction side structures (50, 60) have different pitch angles (P, S) so that the chord (CP2) of the pressure side structure (50) extends forwardly in the direction of motion (D) and relatively more radially outwardly away from the blade root, or less radially inwardly towards the blade root, than the chord (CS2) of the suction side structure (60), defining a relative twist angle (T) between the two structures (50, 60).
Claims
1. A tip structure (20, 21, 22, 23, 24, 25) for use in a use position at a free end region (13) of an aerofoil (12), the aerofoil extending along a length axis (L) from the free end region (13) to an opposite, supported end region (14), the aerofoil being mounted in use at the supported end region for motion in a direction of motion (D) in an airflow (W2) relative to the aerofoil, the airflow (W2) at the free end region being opposed to the direction of motion (D), wherein if the airflow (W2) is generated by rotation of the aerofoil about an axis of rotation (Y1), then the direction of motion (D) is defined as a tangent of a swept circle (40) at the tip structure, the swept circle defined by rotation of the tip structure about the axis of rotation; wherein, when considered in section in a section plane (Aref, Ai) normal to the length axis (L), the aerofoil defines a profile having a maximum length dimension defined by a chord (Cref, Ci), the chord being a straight line having a length between a leading edge point (41) on the profile and a trailing edge point (42) on the profile; the profile extending between the leading edge point and the trailing edge point on a first, pressure side (43) of the aerofoil and a second, suction side (44) of the aerofoil, the profile being configured to generate a reduced pressure on the suction side relative to the pressure side when the aerofoil is arranged in use in the airflow with the leading edge point upstream of the trailing edge point; the length axis (L) being defined as a line extending through an axis point (46, 47) on the chord in every position of the section plane (Aref, Ai) from the free end region to the supported end region of the aerofoil, the axis point being located at 25% of the length of the chord from the leading edge point; wherein a respective said section plane at the free end region of the aerofoil is defined as a first reference plane (Aref), and the chord of the aerofoil lying in the first reference plane is defined as a reference chord (Cref); a first pressure side plane (P1) containing the reference chord (Cref) is defined at an angle (P) from 75 to 90 relative to the first reference plane (Aref); and a first suction side plane (S1) containing the reference chord (Cref) is defined at an angle (S) from 75 to 90 relative to the first reference plane (Aref); the tip structure comprising a pressure side structure (50) extending from the pressure side (43) of the aerofoil, and a suction side structure (60) extending from the suction side (44) of the aerofoil; wherein, when considered in the use position: the pressure side structure (50), when considered in section in a second pressure side plane (P2) parallel with the first pressure side plane (P1), defines a profile (XP2) having a maximum length dimension defined by a chord (CP2), the chord being a straight line between a leading edge point (541) on the profile and a trailing edge point (542) on the profile, the leading edge point being upstream of the trailing edge point with respect to the airflow (W2), and the suction side structure (60), when considered in section in a second suction side plane (S2) parallel with the first suction side plane (S1), defines a profile (XS2) having a maximum length dimension defined by a chord (CS2), the chord being a straight line between a leading edge point (641) on the profile and a trailing edge point (642) on the profile, the leading edge point being upstream of the trailing edge point with respect to the airflow (W2); and wherein, when the direction of motion (D) is represented by a straight line respectively in the second pressure side plane (P2) and in the second suction side plane (S2): when considered in the second pressure side plane (P2), the chord (CP2) of the pressure side structure (50) extends at an angle P relative to the direction of motion (D); when considered in the second suction side plane (S2), the chord (CS2) of the suction side structure (60) extends at an angle S relative to the direction of motion (D); and the angle P is less positive or more negative than the angle S, wherein the angle P or S is defined as positive where the respective chord (CP2, CS2) extends forwardly and towards the supported end region (14) with respect to the direction of motion (D), and as negative where the respective chord (CP2, CS2) extends forwardly and away from the supported end region (14) with respect to the direction of motion (D).
2. A tip structure according to claim 1, wherein the angle P is at least 3 less positive or more negative than the angle S.
3. A tip structure according to claim 1, wherein the angle P is from 2 positive to 5 negative, and the angle S is from 2 positive to 9 positive.
4. A tip structure according to claim 1, wherein the angle S is positive and the angle P is negative, and the angle S is greater than the angle P.
5. A tip structure according to claim 1, wherein a suction side reference plane (Sref) containing the reference chord (Cref) is defined as normal to the first suction side plane (S1); and when the suction side structure (60) is projected in a direction normal to the suction side reference plane (Sref) to define a projected profile (61) in the suction side reference plane (Sref), the projected profile (61) of the suction side structure defining a total area Sa on one side of the first suction side plane and bounded by the suction side (44) of the aerofoil: the angle S is positive when the second suction side plane (S2) intersects the suction side reference plane (Sref) anywhere in a region comprising at least 50% of the area Sa.
6. A tip structure according to claim 1 wherein a pressure side reference plane (Pref) containing the reference chord (Cref) is defined as normal to the first pressure side plane (P1), and when the pressure side structure (50) is projected in a direction normal to the pressure side reference plane (Pref) to define a projected profile (51) in the pressure side reference plane (Pref), the projected profile (51) of the pressure side structure defining a total area Pa on one side of the first pressure side plane (P1) and bounded by the pressure side (43) of the aerofoil: the angle P is negative when the second pressure side plane (P2) intersects the pressure side reference plane (Pref) anywhere in a region comprising at least 50% of the area Pa.
7. A tip structure according to claim 1 wherein a pressure side reference plane (Pref) containing the reference chord (Cref) is defined as normal to the first pressure side plane (P1), and a suction side reference plane (Sref) containing the reference chord (Cref) is defined as normal to the first suction side plane (S1), and when the pressure side structure (50) is projected in a direction normal to the pressure side reference plane (Pref) to define a projected profile (51) in the pressure side reference plane (Pref), and the suction side structure (60) is projected in a direction normal to the suction side reference plane (Sref) to define a projected profile (61) in the suction side reference plane (Sref), the projected profile (51) of the pressure side structure defining a total area Pa on one side of the first pressure side plane (P1) and bounded by the pressure side (43) of the aerofoil, the projected profile (61) of the suction side structure defining a total area Sa on one side of the first suction side plane (S1) and bounded by the suction side (44) of the aerofoil: the angle P is less positive or more negative than the angle S when the second pressure side plane (P2) intersects the pressure side reference plane (Pref) anywhere in a region comprising at least 50% of the area Pa and the second suction side plane (S2) intersects the suction side reference plane (Sref) anywhere in a region comprising at least 50% of the area Sa.
8. A tip structure according to claim 1 wherein a pressure side reference plane (Pref) containing the reference chord (Cref) is defined as normal to the first pressure side plane (P1), and a suction side reference plane (Sref) containing the reference chord (Cref) is defined as normal to the first suction side plane (S1), and when the pressure side structure (50) is projected in a direction normal to the pressure side reference plane (Pref) to define a projected profile (51) in the pressure side reference plane (Pref), and the suction side structure (60) is projected in a direction normal to the suction side reference plane (Sref) to define a projected profile (61) in the suction side reference plane (Sref), the projected profile (51) of the pressure side structure defining a total area Pa on one side of the first pressure side plane (P1) and bounded by the pressure side (43) of the aerofoil, the projected profile (61) of the suction side structure defining a total area Sa on one side of the first suction side plane (S1) and bounded by the suction side (44) of the aerofoil: the angle P is from 2 positive to 5 negative and the angle S is from 2 positive to 9 positive when the second pressure side plane (P2) intersects the pressure side reference plane (Pref) anywhere in a region comprising at least 50% of the area Pa and the second suction side plane (S2) intersects the suction side reference plane (Sref) anywhere in a region comprising at least 50% of the area Sa.
9. A tip structure according to claim 1, wherein a pressure side reference plane (Pref) containing the reference chord (Cref) is defined as normal to the first pressure side plane (P1), and a suction side reference plane (Sref) containing the reference chord (Cref) is defined as normal to the first suction side plane (S1); and, when the pressure side structure (50) is projected in a direction normal to the pressure side reference plane (Pref) to define a projected profile (51) in the pressure side reference plane (Pref), and the suction side structure (60) is projected in a direction normal to the suction side reference plane (Sref) to define a projected profile (61) in the suction side reference plane (Sref), the projected profile (51) of the pressure side structure defining a total area Pa on one side of the first pressure side plane (P1) and bounded by the pressure side (43) of the aerofoil, the projected profile (61) of the suction side structure defining a total area Sa on one side of the first suction side plane (S1) and bounded by the suction side (44) of the aerofoil; and an intermediate axis point (47) is defined on the length axis (L) at a straight line distance (D1) within a range from 300 mm to 700 mm from a terminal axis point (46) defining a distal end of the aerofoil, the terminal axis point (46) being that axis point furthest from the supported end region (14), the chord (Ci) of the aerofoil passing through the intermediate axis point (47) having a length C1 between the respective leading edge point and trailing edge point of the aerofoil:
(Pa+Sa)(3.Math.C1).sup.2.
10. A tip structure according to claim 1, wherein each of the pressure side structure (50) and the suction side structure (60) is configured as a respective aerofoil having a pressure side (53, 63) and a suction side (54, 64) configured to generate a reduced pressure on the suction side relative to the pressure side when the tip structure is arranged in use in the airflow (W2); the pressure side (53) of the aerofoil of the pressure side structure (50) and the suction side (64) of the aerofoil of the suction side structure (60) facing generally towards the supported end region (14) when considered with respect to a direction of the length axis (L); the suction side (54) of the aerofoil of the pressure side structure (50) and the pressure side (63) of the aerofoil of the suction side structure (60) facing generally away from the supported end region (14) when considered with respect to a direction of the length axis (L).
11. A tip structure according to claim 1 wherein a pressure side reference plane (Pref) containing the reference chord (Cref) is defined as normal to the first pressure side plane (P1), and a suction side reference plane (Sref) containing the reference chord (Cref) is defined as normal to the first suction side plane (S1); and, when the pressure side structure (50) is projected in a direction normal to the pressure side reference plane (Pref) to define a projected profile (51) in the pressure side reference plane (Pref), and the suction side structure (60) is projected in a direction normal to the suction side reference plane (Sref) to define a projected profile (61) in the suction side reference plane (Sref), the projected profile (51) of the pressure side structure extending to a maximum distance Pd from the first pressure side plane (P1), the projected profile (61) of the suction side structure extending to a maximum distance Sd from the first suction side plane (51);
Sd>Pd.
12. A tip structure according to claim 1, wherein a third suction side plane (53) is defined in parallel with and between the first and second suction side planes (S1, S2), and the suction side structure (60), when considered in section in the third suction side plane (53), defines a profile having a maximum length dimension defined by a chord (CS3), the chord (CS3) being a straight line between a leading edge point (641) on the profile and a trailing edge point (642) on the profile, the leading edge point (641) being upstream of the trailing edge point (642) with respect to the airflow (W2); and wherein, when the direction of motion (D) is represented as a straight line respectively in each of the second and third suction side planes (S2, S3): the chord (CS2) of the suction side structure (60) extends in the second suction side plane (S2) at an angle S2 relative to the direction of motion (D), and (CS3) in the third suction side plane (53) at an angle S3 relative to the direction of motion (D), and the angle S3 is less positive or more negative, or less negative or more positive, than the angle S2, wherein the angle S2 or S3 is defined as positive where the respective chord (CS2, CS3) extends forwardly and towards the supported end region (14) with respect to the direction of motion (D), and as negative where the respective chord (CS2, CS3) extends forwardly and away from the supported end region (14) with respect to the direction of motion (0).
13. A tip structure according to claim 1, wherein a third pressure side plane (P3) is defined in parallel with and between the first and second pressure side planes (P1, P2), and the pressure side structure (50), when considered in section in the third pressure side plane (P3), defines a profile having a maximum length dimension defined by a chord (CP3), the chord (CP3) being a straight line between a leading edge point (541) on the profile and a trailing edge point (542) on the profile, the leading edge point (541) being upstream of the trailing edge point (542) with respect to the airflow (W2); and wherein, when the direction of motion (D) is represented as a straight line respectively in each of the second and third pressure side planes (P2, P3): the chord (CP2) of the pressure side structure (50) extends in the second pressure side plane (P2) at an angle P2 relative to the direction of motion (D), and in the third pressure side plane (P3) at an angle P3 relative to the direction of motion (D), and the angle P3 is more positive or less negative, or more negative or less positive, than the angle P2, wherein the angle P2 or P3 is defined as positive where the respective chord (CP2, CP3) extends forwardly and towards the supported end region (14) with respect to the direction of motion (D), and as negative where the respective chord (CP2, CP3) extends forwardly and away from the supported end region (14) with respect to the direction of motion (D).
14. A tip structure according to claim 1, wherein a pressure side reference plane (Pref) containing the reference chord (Cref) is defined as normal to the first pressure side plane (P1), and a suction side reference plane (Sref) containing the reference chord (Cref) is defined as normal to the first suction side plane (51); and, when the pressure side structure (50) is projected in a direction normal to the pressure side reference plane (Pref) to define a projected profile (51) in the pressure side reference plane (Pref), and the suction side structure (60) is projected in a direction normal to the suction side reference plane (Sref) to define a projected profile (61) in the suction side reference plane (Sref): the projected profile (51) of the pressure side structure and the projected profile (61) of the suction side structure converge forwardly with respect to the direction of motion (D) to define a projected profile (70) of a forwardly pointing leading edge (70) of the tip structure.
15. A tip structure according to any of claims 1-14 and including at least one attachment portion (30), the attachment portion being attachable to a free end region (13) of an aerofoil (12) to form an assembly.
16. A rotor blade comprising an aerofoil (12) and further comprising a tip structure according to any of claims 1-14.
17. A wind turbine comprising a rotor (11) having at least one blade (12) according to claim 16.
18. A wind farm (80) comprising a plurality of wind turbines (10) arranged to extract energy from a wind, the wind turbines being arranged respectively upstream and downstream of one another with respect to a direction (W1) of the wind, each wind turbine comprising a rotor (11) having at least one blade (12) according to claim 16.
19. A helicopter comprising a rotor having at least one blade (12) according to claim 16.
20. An aircraft comprising at least one aerofoil (12), the aerofoil having a tip structure according to any of claims 1-14.
21. A method of improving a mass efficiency P/I of a rotor (11); the rotor comprising at least one blade, the blade comprising an aerofoil (12); wherein P is an average output shaft power produced by the rotor over a target windspeed range, and/is a mass moment of inertia of the rotor; the method comprising: providing a tip structure (20, 21, 22, 23, 24, 25) according to claim 1; and arranging the tip structure at a free end region (13) of the aerofoil (12).
Description
[0050] Further features and advantages will be appreciated from the following illustrative embodiments of the invention which will now be described, purely by way of example and without limitation to the scope of the claims, and with reference to the accompanying drawings, in which:
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[0074] Reference numerals and characters appearing in more than one of the figures indicate the same or corresponding features in each of them.
[0075] Generally in this specification the terms suction side and pressure side are used to indicate on which side of the aerofoil 12 the respective feature is located.
[0076] Referring to
[0077] Each blade or aerofoil 12 extends along a length axis L from its free end region 13 to the opposite, supported end region 14.
[0078] Referring also to
[0079] In the example illustrated, the relative airflow W2 is generated by rotation of the aerofoil 12 about the axis of rotation Y1. In that case, the direction of motion D is defined as a tangent of a swept circle 40 at the tip structure 20, 21. The swept circle 40 is defined by rotation of the tip structure 20, 21 about the axis of rotation Y1, as shown in
[0080] It will be understood that the direction of the relative airflow W2, also referred to as the relative wind, is a consequence both of the direction of the wind W1 which impinges on the turbine rotor which extracts a part of its energy as shaft power, and also of the rotation of the blades 12, the vector sum being expressed as the angle of attack which depends on the pitch angle of the chord of the aerofoil 12 as well as the tip speed ratio. In this specification the pitch angle means the angle between the chord and the direction of motion D, often referred to in the field of wind turbine design as the twist angle. Thus it should be understood that in this specification, the relative airflow W2 is generated by rotation of the aerofoil 12 in the sense that the direction of the relative airflow W2 is affected by the rotation of the aerofoil 12.
[0081] The conventional HAWT is modified by a tip structure 21 which is mounted in a use position at the free end region of each aerofoil 12 as shown in
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[0083] The tip structure 20, 21 may be formed integrally with the aerofoil 12 or, as shown in
[0084] Referring to
[0085] The chord is a straight line having a length between a leading edge point 41 on the profile and a trailing edge point 42 on the profile. The length of the chord defines the maximum length dimension of the profile defining the chord. In
[0086] The profile of the aerofoil extends between the leading edge point 41 and the trailing edge point 42 on a first, pressure side 43 of the aerofoil and a second, suction side 44 of the aerofoil. The profile is configured to generate a reduced pressure on the suction side 44 relative to the pressure side 43 when the aerofoil is arranged in use in the airflow W2 with the leading edge point 41 upstream of the trailing edge point 42, as shown.
[0087] The illustrated embodiments show an asymmetric or cambered aerofoil 12. In the case of an asymmetric or cambered aerofoil, the leading edge point 41 and trailing edge point 42 are defined by the opposite ends of the camber line or mean centre line 45 of the profile as shown in
[0088] The tip structure 20, 21 comprises a pressure side structure 50 which extends from the pressure side 43 of the aerofoil 12, and a suction side structure 60 which extends from the suction side 44 of the aerofoil 12.
[0089] The length axis L of the aerofoil 12 is defined as a line extending through an axis point 46, 47 on the chord in every position of the section plane (Aref, Ai) from the free end region 13 to the supported end region 14 of the aerofoil 12. Each axis point 46, 47 is located at 25% of the length of the chord from the leading edge point 41.
[0090] The axis point 46 is referred to hereinafter as the terminal axis point, which is defined as that axis point furthest from the supported end region 14, thus defining a distal end of the aerofoil. The distal end may be identified at the last point along the length axis L where the profile contained in the section plane still defines an aerofoil; it may be in a section plane passing through the fillet 48 (
[0091] Referring to
[0092] The axis point 47 is defined on the length axis (L) at a straight line distance D1 (
[0093] The length axis L may be straight as illustrated, or alternatively may be curved, for example, where the free end region 13 of the blade 12 is swept so that it curves away from the supporting structure 15, 16 towards its tip, in which case the length axis L will follow the curve.
[0094] A first pressure side plane P1 containing the reference chord Cref is defined at an angle P from 75 to 90 relative to the first reference plane Aref, and a first suction side plane S1 containing the reference chord Cref is defined at an angle S from 75 to 90 relative to the first reference plane Aref. Preferably the first pressure side plane P1 and first suction side plane S1 are selected to be as near as possible normal to a mean central plane, respectively of the pressure side structure 50 and suction side structure 60, as illustrated in
[0095] In the examples illustrated in
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[0097] An angle P, S of less than 90 may be necessary for example where the tip structure is mounted on a rotor blade with a swept tip (i.e. a locally angled free end region 13), hence a curved length axis L, and the reference plane Aref is defined through the swept free end region 13 of the blade while the tip structure is arranged to be more nearly normal to the length axis of the longer, straight region of the blade which extends to its supported end region 14. This was the case for the geometry of the test tip structure when fitted to the test turbine, as further discussed below.
[0098] Referring again to
[0099] A suction side reference plane Sref containing the reference chord Cref is defined as normal to the first suction side plane S1. Thus it will be understood that the reference chord Cref also defines the intersection of Sref and S1.
[0100] A second pressure side plane P2 is defined in parallel with the first pressure side plane P1 to pass through the pressure side structure 50. The second pressure side plane P2 may be defined at any suitable position over the height hP of the pressure side structure 50. The pressure side pitch angle P, which is the angle between the chord of the pressure side structure 50 and the direction of motion D, is defined by reference to the chord CP2 of the pressure side structure 50 in the second pressure side plane P2.
[0101] In order to quantify the extent to which the pressure side pitch angle P varies (if at all) with the height hP of the pressure side structure 50, a third pressure side plane P3 is defined in parallel with and between the first and second pressure side planes P1, P2. The chord of the pressure side structure 50 in the third pressure side plane P3 is designated as CP3.
[0102] A second suction side plane S2 is defined in parallel with the first suction side plane S1 to pass through the suction side structure 60. The second suction side plane S2 may be defined at any suitable position over the height hS of the suction side structure 60. The suction side pitch angle S, which is the angle between the chord of the suction side structure 60 and the direction of motion D, is defined by reference to the chord CS2 of the suction side structure 60 in the second suction side plane S2.
[0103] In order to quantify the extent to which the suction side pitch angle S varies (if at all) with the height hS of the suction side structure 60, a third suction side plane 53 is defined in parallel with and between the first and second suction side planes S1, S2. The chord of the suction side structure 60 in the third suction side plane S3 is designated as CS3.
[0104] In the standard case as illustrated in
[0105] If alternatively one or both of the pressure and suction side structures is/are canted as shown in
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[0107] Similarly,
[0108] Thus, the chord of the pressure side structure 50 is shown in the second and third pressure side planes P2, P3 respectively as chord CP2, CP3, while the chord of the suction side structure 60 is shown in the second and third suction side planes S2, S3 respectively as chord CS2, CS3.
[0109] For ease of reference in
[0110] In each case the chord is a straight line between a leading edge point 541, 641 on the profile and a trailing edge point 542, 642 on the profile, the leading edge point being upstream of the trailing edge point with respect to the airflow W2 when considered in the use position of the tip structure 20, 21 as shown for example in
[0111] It will be understood of course that the section of the profile is the section of the outer surface contour of the respective pressure side or suction side structure in the respective plane; the internal structure is not shown, and may be solid or, more typically, hollow, depending on the method of fabrication (which may be for example in glass reinforced plastics, carbon fibre, or any other suitable materials) as well known in the art of aerofoil construction.
[0112] Preferably, in order to optimise the glide ratio, each of the pressure side structure 50 and the suction side structure 60 is configured as a respective aerofoil having a pressure side 53, 63 and a suction side 54, 64. The aerofoil is configured to generate a reduced pressure on the suction side 54, 64 relative to the pressure side 53, 63 when the tip structure is arranged in use in the airflow W2. When considered with respect to a direction of the length axis L, the pressure side 53 of the aerofoil of the pressure side structure 50 and the suction side 64 of the aerofoil of the suction side structure 60 may be arranged as shown to face generally towards the supported end region 14, while the suction side 54 of the aerofoil of the pressure side structure 50 and the pressure side 63 of the aerofoil of the suction side structure 60 are arranged to face generally away from the supported end region 14.
[0113] Although the tip structure 20, 21 is illustrated in an orientation which is generally normal (i.e. perpendicular) to the blade or aerofoil 12, as best seen in
[0114] The aerofoil in each respective pressure side plane or suction side plane need not have the shape illustrated, but may have any desired profile in accordance with well known aerofoil design principles to optimise the glide ratio and overall operating conditions when the tip structure is mounted in its intended use position. Irrespective of whether the pressure side and suction side structures are configured as aerofoils or as flat plates, the leading edge point and trailing edge point will be identified by the mean centre line of the profile as described above, with the chord defined as a straight line between those two points.
[0115] The direction of motion D is represented by a straight line (with the motion in the direction of the arrow as shown) in each of the second and third pressure side planes P2, P3 and the second and third suction side planes S2, S3.
[0116] It can be seen that when considered in the respective pressure side plane or suction side plane, the chord of the pressure side structure extends at an pressure side pitch angle P relative to the direction of motion D, while the chord of the suction side structure extends at a suction side pitch angle S relative to the direction of motion D.
[0117] It will be understood that the first, second and third pressure side planes and suction side planes may be angled somewhat obliquely relative to the true direction of motion D. For a rotating tip structure the direction of motion D will lie in the plane of rotation, i.e. a plane normal to the axis of rotation Y1 and containing the swept circle 40, the obliquity reflecting the pitch angle of the blade 12 at the tip structure which may be taken to be the angle of the reference chord Cref relative to the plane of rotation. In this case the direction of motion D may be represented by a straight line in each of the first, second and third pressure side planes and suction side planes, with the straight line in each case being the closest fit to the direction of motion in the respective plane. If the aerofoil 12 is a rotor blade, the direction of motion D might be considered as a straight line in the plane of rotation which is projected along the direction of the rotation axis Y1 onto the respective first, second or third pressure or suction side plane. If the aerofoil 12 is for example the wing of an aircraft, the direction of motion D may be considered as the direction in which a point is translated by the forward motion of the aircraft in straight and level flight. In each case, the angle P or S is defined relative to the straight line representing the direction of motion D in the respective pressure side plane or suction side plane.
[0118] The pressure side pitch angle P can be seen in the second and third pressure side planes P2, P3 respectively as pressure side pitch angles P2 and P3, as shown in
[0119] The suction side pitch angle S can be seen in the second and third suction side planes S2, S3 respectively as suction side pitch angles S2 and S3, as shown in
[0120] The difference between the respective pressure side and suction side pitch angles represents the relative twist angle T between the respective profiles of the pressure side and suction side structures, as shown in
[0121] Optionally, as exemplified by the illustrated embodiment and best seen in
[0122] Alternatively, either or both of the pressure side pitch angle P and the suction side pitch angle S may decrease smoothly and progressively with the height hP, hS of the respective pressure side or suction side structure; this can be arranged for example to reduce loading towards the respective extremity of the tip structure so as not to exceed the rated capacity of the turbine in its intended use situation.
[0123] In alternative embodiments the chord of the pressure side and/or suction side structure might lie in a flat plane for part or all of its height hP, hS so that the respective pressure side or suction side pitch angle P, S does not vary with height hP, hS in that region. In further alternative embodiments the twist of the pressure side and/or suction side structure may vary (increase or decrease) step-wise with height hP, hS. That is to say, the pressure side or suction side structure might be twisted in a local region (which is to say, the pitch angle P, S may vary rapidly with height hP, hS in that region) which marks a transition between two relatively less twisted regions (i.e. where the pitch angle P, S is constant or more constant with height hP, hS), the two regions having different pressure side or suction side pitch angle P, S.
[0124] In each case, and as exemplified by the illustrated embodiment, the angle P is less positive or more negative than the angle S.
[0125] That is to say, with respect to the length axis L of the aerofoil, the chord CP2 extends forwardly in the direction of motion D and relatively more radially outwardly away from the supported end region, or less radially inwardly towards the supported end region, than the chord CS2. A relative twist angle T greater than zero degrees is thus defined between the chords CP2, CS2 of the pressure and suction side structures.
[0126] In this specification, the angle P or S is defined as positive where the respective chord CP2, CP3, CS2, CS3 extends forwardly (i.e. in the direction of the arrow) and towards the supported end region with respect to the direction of motion D, and as negative where the respective chord CP2, CP3, CS2, CS3 extends forwardly (i.e. in the direction of the arrow) and away from the supported end region with respect to the direction of motion D.
[0127] The angular relationship between the pressure and suction side structures 50, 60 defined by the relative twist angle T generates an enhanced pressure differential across the respective wetted surfaces of the pressure and suction side structures which in tests is found to result in unexpectedly good performance, as further discussed below. Inter alia, the effective lateral diffusion and compression (perpendicular to the length axis L) generated by the angular arrangement of the respective wetted surfaces of the tip structure may be expected to reduce the radially outward (rolling) flow from the pressure side 43 to the suction side 44 of the blade 12, reducing the formation of a vortex at the tip and so reducing drag, as well as controlling aerodynamic losses by enhancing the pressure differential between the respective pressure and suction sides of the blade.
[0128] As exemplified by the illustrated embodiment, the tip structure may be arranged so that the angle S is positive and the angle P is negative, and the angle S is greater than the angle P. This arrangement is found to provide good performance when the tip structure is attached to the blade of a rotor, particularly a HAWT.
[0129]
[0130] For many aerofoils including those typically employed for a rotor blade, such as the rotor blade of a HAWT, good performance may be achieved where the angle P is from 2 positive to 5 negative, and the angle S is from 2 positive to 9 positive.
[0131] The pressure side structure 50 may be twisted in its height dimension so that the angle P.sub.3 is more positive or less negative, or more negative or less positive, than the angle P.sub.2. Alternatively or additionally, the suction side structure may be twisted in its height dimension so that the angle S.sub.3 is less positive or more negative, or less negative or more positive, than the angle S.sub.2, as illustrated.
[0132] In each case, the progressive or step-wise twist makes it possible to minimise drag at the fillet 48 at the root of the pressure side or suction side structure, and/or at the respective, upper or lower extremity or tip of the pressure side or suction side structure, by progressively increasing the pressure differential with respect to the height dimension of the blade.
[0133] Projected Surface Area
[0134] As shown in
[0135] The suction side structure 60 is projected in a direction normal to the suction side reference plane Sref to define a projected profile 61 in the suction side reference plane Sref.
[0136] For ease of reference, since the projected profile of the tip structure 20 including projected profiles 51, 61 as shown in
[0137] Referring to
[0138] The projected profile 61 of the suction side structure 60 defines a total area Sa on one side of the first suction side plane S1 and bounded by the suction side 44 of the aerofoil. The total area Sa as shown is the sum of the stippled area Sa (1) and the two areas Sa (2) with different, s shaped stippling. The areas Sa (2) contain those parts of the suction side structure occupied by the fillet 48 at its root 62 and the smooth curvature of its (upper or lower) end surface at its tip 65. The stippled region Sa (1) represents at least about 85% of the total projected surface area Sa.
[0139] In order to maximise the additional power generated by the pressure side structure 50 over at least most of its surface area, the angle P, taken as the angle P2, may be negative when the second pressure side plane P2 is defined in a position to intersect the pressure side reference plane Pref anywhere in a region comprising at least 50% of the area Pa.
[0140] In the example shown, this condition is satisfied where the pressure side reference plane Pref lies anywhere in the stippled region Pa (1).
[0141] Similarly, in order to maximise the additional power generated by the suction side structure 60 over at least most of its surface area, the angle S, taken as the angle S2, may be positive when the second suction side plane S2 is defined in a position to intersect the suction side reference plane Sref anywhere in a region comprising at least 50% of the area Sa.
[0142] In the example shown, this condition is satisfied where the suction side reference plane Sref lies anywhere in the stippled region Sa (1).
[0143] A favourable glide ratio may be obtained where the angle P, taken as the angle P2, is less positive or more negative than the angle S, taken as the angle S2, when the second pressure side plane P2 intersects the pressure side reference plane Pref anywhere in a region comprising at least 50% of the area Pa and the second suction side plane S2 intersects the suction side reference plane Sref anywhere in a region comprising at least 50% of the area Sa.
[0144] A favourable glide ratio may also be obtained where the angle P is from 2 positive to 5 negative and the angle S is from 2 positive to 9 positive when the second pressure side plane P2 (defining the angle P as P2) intersects the pressure side reference plane Pref anywhere in a region comprising at least 50% of the area Pa and the second suction side plane S2 (defining the angle S as S2) intersects the suction side reference plane Sref anywhere in a region comprising at least 50% of the area Sa.
[0145] For good performance the combined projected section area Pa, Sa of the pressure and suction side structures may be selected to be proportional to the length of the chord of the aerofoil as follows. Referring to
[0146] Referring to
[0147] It is found that good performance can be obtained where Sd>Pd, which is to say, the maximum distance Sd (i.e. maximum height hS) of the suction side structure 60 is greater than the maximum distance Pd (maximum height hP) of the pressure side structure 50, as illustrated by the alternative embodiment of
[0148] The profile of each of the pressure and suction side structure will generally comprise a radiused or conic leading edge and a rearwardly tapering tail region terminating in a point or a flat back as shown. Depending on the profile selected, the ratio of chord length to the maximum width or thickness dimension of the profile, i.e. its maximum dimension normal to the chord in the section containing the chord, may be generally less than 10:1 in each of the pressure and suction side structures.
[0149] Some or all of the preferred parameter values as discussed herein may be used in any desired combination to suit the individual use situation as guidelines when configuring the tip structure for use on the rotor blade of a HAWT or in other applications.
[0150] In practice, the geometry of the novel tip structure 20 incorporating its essential features as described above will be optimised for the aerofoil 12 to which the tip structure is to be attached, following the established principles of aerofoil design. Inter alia, the suction side pitch angle S and pressure side pitch angle P determine the loading on the aerofoil sections of the pressure and suction sides 50, 60 of the tip structure 20 and are selected to optimise lift relative to drag.
[0151] The pressure side and suction side structures 50, 60 may be configured to define any of a wide variety of different aerofoil profiles as known in the art, including combinations of different profiles over the respective height dimension hP or hS, and the suction side pitch angle S and pressure side pitch angle P may be selected within the range of values discussed below to define favourable operating conditions for the selected aerofoil profile or profiles. Those skilled in the art will appreciate for example that the suction side pitch angle S and pressure side pitch angle P may be varied inter alia to suit the camber of the aerofoil profile.
[0152] Preferably the suction side pitch angle S and pressure side pitch angle P are selected to obtain an angle of attack which optimises loading of the adjacent surfaces of the blade 12 relative to the drag from the tip structure, corresponding to the peak of the performance curve in
[0153] In the first tip structure 20 as shown, the pressure side pitch angle P2 is approximately 1.5 negative and the suction side pitch angle S2 is approximately 3 positive, giving an overall twist angle T between the sections in the respective planes P2, S2 of about 4.5.
[0154] A test tip structure was produced in three identical units which were attached as shown in
[0155] The four data sets are set out in tables 1-4 and presented in graphic form in
TABLE-US-00001 TABLE 1 Data set FB Field test data: Baseline turbine without tip structure Wind speed (m/s) Cp (Norm) 3.00 0.00 3.58 0.48 4.00 0.63 4.49 0.75 4.99 0.83 5.50 0.91 5.98 0.94 6.49 0.96 6.98 0.98 7.49 1.00 7.95 0.99 8.47 0.97 8.97 0.95 9.45 0.92 9.99 0.87 10.41 0.84 10.88 0.79 11.55 0.72 11.55 0.72 12.00 0.66
TABLE-US-00002 TABLE 2 Data set FT Field test data: Baseline turbine with Tip structure Wind speed (m/s) Cp (Norm) 3.17 0.24 3.56 0.48 4.00 0.64 4.49 0.78 5.00 0.87 5.50 0.93 6.00 0.97 6.50 1.00 6.99 1.02 7.46 1.02 7.98 1.02 8.49 1.01 8.97 0.98 9.54 0.95 9.94 0.93 10.50 0.87 10.97 0.82 11.56 0.76 12.00 0.72 12.00 0.72
TABLE-US-00003 TABLE 4 Data set CT CFD model data: Baseline turbine with Tip structure Wind speed (m/s) Cp (Norm) 5.00 0.81 6.00 0.93 7.00 0.96 8.00 0.97 9.00 0.93 10.00 0.85
TABLE-US-00004 TABLE 3 Data set CB CFD model data: Baseline turbine without tip structure Wind speed (m/s) Cp (Norm) 5.00 0.79 6.00 0.91 7.00 0.94 8.00 0.96 9.00 0.92 10.00 0.84
[0156] The turbine power coefficient Cp is a measure of the power extracted by the turbine as a proportion of the total available power in the wind, defined as Pm/(0.5.Math..Math.V3.Math.ar) wherein Pm is the measured power output of the rotor shaft, p is air density, V is the hub height wind speed (i.e. the speed of the wind W1 at the height of the hub), and ar is the area of the rotor.
[0157] The normalised turbine power coefficient Cp (Norm) in each data set as shown on the Y-axis of the graph was obtained by dividing the turbine power coefficient Cp at each data point by a reference value, which was selected to be the maximum measured value of Cp from the data underlying data set FB (Field test data: Baseline turbine without tip structure) before normalisation. For this reason Cp (Norm) for data set FB has a peak value of 1.00.
[0158] The features of the test tip structure were essentially the same as the features of the first tip structure 20 as discussed above, except that the geometry of the test tip structure was optimised for the blades of the test turbine. In the test tip structure the pressure side pitch angle P was within the range from 2 positive to 5 negative, and the suction side pitch angle S was within the range from 2 positive to 9 positive. The overall twist angle T was 6. As can be seen from
[0159] Field Test Parameters
[0160] The test was conducted to the IEC 61400-12 standard, Power performance measurements of electricity producing wind turbines on a 1 MW pitch-regulated variable speed turbine driving a generator connected to the power grid.
[0161] The rotor diameter was 61 m, each blade having an overall length of 30.5 m from the axis of rotation Y1, and the blades were pre-bent (swept) at the tip along their length axis L in the rest condition to compensate for deflection at the blade tip under load. The mass of each blade was approximately 3000 kg with the centre of mass at approximately 12 m from the hub, giving an mass moment (I) of 36000 kgm.
[0162] The test turbine was equipped with active pitch and yaw control systems. The pitch control system was arranged to adjust the pitch of each blade dependent on its angular position through the rotation cycle of the rotor. The cyclic pitch adjustment compensated for height dependent wind shear so as to reduce variable bending moments across the blades with each revolution of the rotor, as known in the art.
[0163] Power was measured at the grid connection by the power electronics system of the turbine generator before entering the grid. All wind conditions (including shear, turbulence, speed, direction) were measured by instruments on an on-site mast situated close to the turbine. All measurements were carried out to IEC standards with a statistically significant data set for each tabulated wind speed.
[0164] The turbine was run for a period of 2-3 months without the tip structure to generate the baseline data set FB. After fitting the tip structure to each of the turbine blades as shown in
[0165] The tip structure used in the field test was configured as shown in
[0166] The tip structure was attached as shown in
[0167] The total mass of the retrofit tip structure was 3.5 kg. The estimated mass of the short portion of the blade removed from the tip was about 0.7 kg so that the installation of the test tip structure added a net 2.8 kg to the mass of the blade with the centre of the added mass being positioned about 30.5 m from the axis of rotation, keeping the swept area the same.
[0168] CFD Test Parameters
[0169] The CFD model was based on a rotor with three static blades (i.e. blades which are fixed in relation to the hub) assuming a fully turbulent boundary layer with free airflow and no wind shear, with key parameters as follows:
[0170] StarCCM+ version 12 was used for the simulation with CATIA v5 used to generate the input CAD data.
[0171] The global domain represented a third of the swept area when viewed in the wind direction W1 and periodic boundary conditions were used to give rotation to the flow.
[0172] Cell sizes on the blade surface were defined in the range from 5 mm to 30 mm, with appropriate refinement to capture leading edge curvature in a smooth fashion.
[0173] A prism layer mesh was defined from the blade surface to capture boundary layer behaviour (wall y+<1.).
[0174] A volume mesh refinement area was defined around the blade tip to capture the trailing vortex with higher resolution.
[0175] The CFD model was a fully turbulent, steady simulation using the K-Epsilon turbulence model. The K-Omega SST turbulence model was also used with very similar results.
[0176] It can be seen that the data set CB generated by the CFD simulation for the baseline test turbine (without tip structure) was a fairly good fit to the corresponding real world data set FB, suggesting that the CFD simulation was a good model for the real world performance of the test turbine.
[0177] In addition to the CFD simulation, a wind tunnel scale model test was carried out. The wind tunnel used was of the closed section, closed return type with a nominal test flow speed of 40 m/s. The model used was a 50% scale model of the outer 3 m of the wind turbine blade constructed from an aluminium frame and 3D printed plastic components. The model was mounted to a high precision floor mounted force balance that measured six components, comprising three forces and three moments.
[0178] Models were tested with the same geometry as those simulated in CFD, and the trends observed in lift and drag results were in close agreement to the simulated results, giving confidence in the CFD approach.
[0179] It should be noted however that the CFD simulation assumed a fully turbulent boundary layer, whereas in real world conditions the surfaces of the blade and tip structure may be expected to exhibit significant amounts of laminar flow, which has a significantly lower skin friction drag than turbulent flow. This could be expected to give rise to at least about 2% greater Cp in the real world field test compared with the CFD simulation, which is reflected by the offset between the data sets CB and FB.
[0180] A tip structure or endplate comprising a pressure side structure in combination with a suction side structure when attached to a wingtip or blade tip as generally known in the art may be expected to provide enhanced separation between the pressure fields on the pressure and suction sides of the aerofoil, and so may be expected to deliver relatively lower drag, resulting in a net improvement in the power coefficient.
[0181] It may be noted that the expected improvement in Cp resulting from a conventional tip structure or endplate extending from both pressure and suction sides of the blade would be significantly greater than that provided by a conventional winglet or upturned blade tip which extends from only one (pressure or suction) side of the aerofoil as known in the art.
[0182] The predictable improvement in Cp resulting from the enhanced separation of the pressure fields by the pressure and suction side structures of the test tip structure is reflected by the difference between the two data sets CB and CT generated in the CFD simulation.
[0183] In the field test the turbine fitted with the novel tip structure performed surprisingly better than predicted by the CFD model, the data set FT indicating an increase in the power coefficient Cp of approximately 2.5%-4% when compared with the baseline turbine data set FB. It can be seen that the offset between data sets FB and FT is substantially greater than the offset between data sets CB and CT which reflects the predictable improvement in Cp resulting from separation of the pressure fields by the presence of the pressure and suction side structures.
[0184] The 2.5%-4% increase in Cp was calculated to correspond to an increase of approximately 2%-3% in annual energy production (AEP) from the turbine in view of the Weibull curve indicating the windspeed probability distribution for the field test site. Those skilled in the art will appreciate that the annual turbine energy production for each measured windspeed is calculated as the sum of the power output for that windspeed and the proportion of the year over which that windspeed is expected to occur, based on the Weibull curve The sum total annual power output over all measured windspeeds represents the total annual energy production (AEP) for the turbine.
[0185] As discussed above with reference to the performance of the blade, the tip structure may similarly perform better in the real world laminar flow condition of the boundary layer compared with the turbulent boundary layer assumed in the CFD model. This difference may be expected to make a contribution to the difference between the measured improvement in Cp between data sets FT and FB, and the relatively smaller improvement predicted by the CFD model in the difference between data sets CT and CB. However, the likely proportion of this contribution may be expected to correspond, at most, to the offset between data sets CB and FB (representing the influence of real world factors in tato over the CFD model), hence a few percentage points, as a proportion of the offset between data sets CB and CT which represent the overall predicted contribution of the tip structure. Thus, the laminar boundary layer condition at the real world tip structure as against the turbulent boundary layer condition of the CFD model may be inferred to contribute, at most, a few hundredths of a percentage point on the Cp(Norm) axis of the graph.
[0186] While the invention is not bound by theory, it is believed that the twist angle T between the pressure and suction side structures may generate an enhanced pressure differential across the radially outward surface of the tip structure resulting in an unexpectedly good binding of the airflow which in turn may be expected to dramatically alter the strength, size, and position of the vortices shed from the tip of the blade. This in turn will change how the turbine wake forms and evolves. Those skilled in the art will appreciate that changes in the size, shape, strength, or position of the wake will have an impact also on the upstream flow conditions, for example, resulting in a small increase in average velocity which contributes to an overall increase in power output.
[0187] It is also possible that by increasing the lift generated at the tip of the blade, the novel tip structure may have the effect of straightening the blades (in rotors either with or without blade pre-bend). The straighter blades may have the effect of slightly increase the swept area in use, which will increase the power output of the rotor.
[0188] The test site was unusual in that wind from one direction only was masked by a stand of tall trees, but flowed over flat ground from all other directions. The trees produced strong wind shear which would be expected to reduce power output to well below the level predicted by the CFD simulation which did not take account of wind shear.
[0189] However, the test results showed the power output during periods when the wind was from the direction of the treeline was equal to that predicted by the CFD study for free airflow. During periods when the wind was from any other direction, hence over flat ground, the performance was significantly better than predicted by the CFD study, as indicated by the field test data sets (FB, FT) which show aggregate data for all wind directions experienced during the test including from the direction of the treeline.
[0190] An analysis of the measured power output for different wind directions suggests that the novel tip structure not only provides surprisingly improved performance in free airflow but also performs surprisingly well in conditions of severe wind shear and turbulence.
[0191] Again, while the invention is not bound by theory, it is believed that by increasing the tip loading and changing the structure of the tip vortex, the turbine blades equipped with the novel tip structure may exhibit less dynamic twisting or deflection in the turbulent wind field, again increasing the power output because the aerofoils remain closer to the optimal angle of attack.
[0192] Further advantageously, the enhanced binding of the airflow and reduced vortex formation at the novel tip structure may provide a relatively more confined wake or stream tube downstream of the turbine. This in turn may be expected to increase the available power density and so improve the performance of further wind turbines arranged in the downstream region.
[0193] Thus, by equipping each of the wind turbines with the novel tip structure, the invention may improve the overall power output of a windfarm 80 relative to its total land area. Referring to
[0194] Advantageously, and as described and illustrated with reference to the first tip structure 20, the novel tip structure may have a relatively sharp angular transition between the blade (or the connection portion of the retrofit tip structure) and the pressure and suction side structures 50, 60. That is to say, the transition may be smoothly filleted as well known in the art, but the fillet 48 may be of relatively small radius so as to provide a clearly evident transition region between the wetted surfaces of the pressure and suction side structures 50, 60 and the surfaces of the blade 12.
[0195] The transition region is more distinct and localised, for example, than the corresponding region of a prior art upturned blade tip or winglet which has constant curvature so that it deviates progressively and smoothly from the length axis of the blade all the way to the tip.
[0196] The more localised and clearly evident transition region maximises the proportion of the overall surface area of the tip structure which is oriented in the direction required to generate the desired vector forces which give rise to the airflow pattern as explained above. Thus, relative to its size and compared with a more gradually and progressively curved blade tip, the novel tip structure may have a more pronounced binding effect on the airflow around the end of the blade.
[0197] The relatively small overall size of the tip structure means that it may be relatively light in weight when compared with the forces it generates it use.
[0198] A corollary of the relatively clearly marked transition region is that any angular discontinuity will introduce a potential source of drag, and so the drag penalty of the transition region must be balanced against the advantageously enhanced aerodynamic effect of the tip structure relative to its overall size and mass.
[0199]
[0200] The graph presents a data set derived from a CFD simulation with the same parameters used to produce data sets CB and CT (Tables 3 and 4), assuming a wind speed of 7 m/s and a rotational speed of 20 RPM for the rotor of the test turbine. The simulation modelled the same test tip structure attached to the test turbine as for data set CT, except that the twist angle T was varied, with the respective pressure side and suction side pitch angles P, S being optimised for each selected value of T.
[0201] The value T () on the X-axis of the graph is the value of the twist angle T in degrees.
[0202] The value Cp (%) on the Y-axis is the calculated increase in turbine power coefficient Cp generated by the tip structure as a percentage of the calculated increase in turbine power coefficient Cp obtained at zero twist angle (T=0).
[0203] The calculated increase in turbine power coefficient Cp at the optimal twist angle T=6 corresponds to the difference between data sets CT and CB (Tables 3 and 4) at the 7.00 m/s data point in the earlier described simulation.
[0204] The peak value Cp of approximately 128% indicates that the calculated increase in turbine power coefficient Cp at the optimal twist angle T=6 was about 128% of the calculated increase in turbine power coefficient Cp generated by the test tip structure with zero twist angle (T=0).
[0205] It can be seen that the twist angle T is an important determinant of the power output of the tip structure.
[0206]
[0207] The first data set (T=6) was based on the same test tip structure attached to the test turbine with the same geometry as for data set CT (including the twist angle T of 6). The data set was generated by varying the overall size of the test tip structure without changing its geometry.
[0208] The second data set (T=0) was based on the same parameters as the first data set, but with the test tip structure modified to align the pressure and suction side structures to provide zero twist angle (T=0). The respective pressure side and suction side pitch angles P, S were thus equal and were optimised for this condition. Again, the data set was generated by varying the overall size of the modified test tip structure with zero twist angle without otherwise changing its geometry.
[0209] The value Pa+Sa (%) on the X-axis of the graph is the combined projected area Pa and Sa of the pressure and suction side structures in the respective, pressure side and suction side reference planes Pref, Sref (
[0210] Thus, the minimum value of 100 indicates a combined projected area Pa+Sa equal to that of the test tip structure, and the value of 200 indicates a combined projected area Pa+Sa double that of the test tip structure.
[0211] The value Cp (%) on the Y-axis is the calculated increase in turbine power coefficient Cp generated by the tip structure as a percentage of the respective baseline value.
[0212] For the first data set (aT=6) the baseline value was the calculated increase in turbine power coefficient Cp obtained from the test tip structure, corresponding to the difference between data sets CT and CB (Tables 3 and 4) at the 7.00 m/s data point in the earlier described simulation.
[0213] Thus, for the first data set (aT=6) the value Cp=100% indicates an increase in turbine power coefficient Cp equal to that obtained from the test tip structure, hence equal to the difference between data sets CT and CB (Tables 3 and 4) at the 7.00 m/s data point.
[0214] For the second data set (aT=0) the baseline value was the calculated increase in turbine power coefficient Cp obtained from the modified test tip structure with zero twist angle (aT=0) and a combined projected area Pa+Sa equal to that of the test tip structure, hence the value of 100% on both X and Y axes.
[0215] The peak value Cp for the first data set (aT=6) indicates that the increase in turbine power coefficient Cp at the optimal twist angle T=6 can be optimised at about 132% of the increase obtained from the test tip structure used in the first simulation (data set CT, Table 4) by increasing the combined projected area Pa+Sa to about 142% of that of the test tip structure.
[0216] The second data set shows that a comparable value can be obtained from the tip structure with zero twist angle (aT=0) only by increasing its combined projected area Pa+Sa to about double that of the test tip structure.
[0217] The mass of the tip structure reflects its structural characteristics which must be adequate to transfer the applied loads to the blade. Mass will increase not only with the applied load but also as a non-linear function of overall size, since the same applied loads will generate a greater moment in a larger structure. Thus, an increase in area will result in a proportionately much larger increase in mass which reflects the additional materials together with the increased design loads on the tip structure which in turn require a thicker skin material.
[0218] In the earlier described field test (data sets FB and FT, Tables 1 and 2), the additional mass moment of each test tip structure when fitted to the test turbine was 2.8 kg-30.5 m=85.4 kgm, only 0.24% of the original mass moment of the blade without the tip structure.
[0219] In practice, it may be estimated that doubling the area of the tip structure with zero twist angle (aT=0) represented by the second data set of
[0220] Taking the example of the 1 MW test turbine, a normalised baseline figure for the mass efficiency P/I of each blade before fitting the novel tip structure can be calculated as 1.Math.10.sup.6/36000 kgm=27.78.
[0221] Based on an expected average value of 1.5% for the increase in the normalised turbine power coefficient Cp (Norm) as indicated by the difference between the CFD data sets CB and CT (Tables 3 and 4), the same blade when fitted with the test tip structure with optimal twist angle (aT=0) would be expected to provide a moderately higher mass efficiency P/I of 1.015.Math.10.sup.6/36085=28.13.
[0222] Taking the same average value of 1.5% for the increase in Cp (Norm), the same blade when fitted with the modified tip structure with zero twist angle (aT=0) but double the area would be expected to provide a reduced mass efficiency P/I of 1.015.Math.10.sup.6/36619=27.72.
[0223] The difference between data sets FB and FT is more difficult to average in view of the different measured windspeed data points. However, a data point occurs at 8.97 m/s in both data sets and can be seen from the graph (
[0224] Taking this value of 3% to reflect the surprisingly better real world performance of the test tip structure in the field test, the novel tip structure when fitted to the blade of the test turbine and with the twist angle optimised for the blade at aT=6 would be expected to provide a significantly higher mass efficiency P/I of 1.03.Math.10.sup.6/36085=28.54.
[0225] The novel tip structure can be used for example to increase the power output of a turbine without changing the blade length, either at the design stage or as a retrofit, for example, for an existing turbine which was rated for a higher class than is required for its actual or intended site location.
[0226] It can be inferred that similar benefits may be obtained from the novel tip structure in other applications, particularly when fitted to a blade which rotates about an axis with a similar form factor to that of a HAWT rotor blade.
[0227] Thus, the invention may be applied to obtain a rotor having at least one blade configured as an aerofoil, wherein the novel tip structure is fitted to the blade or each blade 12 of the rotor as shown in
[0228] Performance benefits may also be obtained when the novel tip structure is fitted to the tip of the wing or primary lifting surface of an aircraft, for example, as shown in
[0229] From the data presented above it will be understood that the invention may further be applied to provide a method of improving a mass efficiency P/I of a rotor comprising at least one blade, the blade comprising an aerofoil, wherein P is an average output shaft power produced by the rotor over a target windspeed range and I is a mass moment of inertia of the rotor, by arranging the novel tip structure as described above at a free end region of the or each of the blades.
[0230] It is found that good performance may be obtained where the projected profile 51 of the pressure side structure 50 and the projected profile 61 of the suction side structure 60 converge forwardly with respect to the direction of motion (D) to define a projected profile 70 of a forwardly pointing leading edge 70 of the tip structure. The trailing edge 71 of the tip structure may be bifurcated so that it tapers to a point 56, 66 at the tip (in the height dimension hP, hS) of each of the pressure and suction side structures, as shown. These features can be seen in first tip structure 20 and in the variant tip structure 21 (
[0231]
[0232] In the tip structure 22 (
[0233] Additional features such as vortex generators or a serrated trailing edge as known in the art may be applied to the novel tip structure to help maintain an attached airflow, allowing an increased angle of attack before the respective aerofoil stalls. This in turn may make it possible to provide a larger twist angle T between the pressure and suction side structures 50, 60 which further increases the pressure differential between the inwardly facing sides of the pressure side structure and suction side structure and so enhances the improvement in efficiency delivered by the novel aerofoil, particularly in mass efficiency P/I when used as a rotor blade in a HAWT.
[0234] By way of example,
[0235]
[0236]
[0237] The novel tip structure may incorporate an integrated lightning protection system comprising e.g. metallic inserts or foils to form a conductive pathway from the tower to the tip structure. Further features may be included as generally known in the art of aerofoil design and construction, for example, feathered edges and the like.
[0238] If desired, the attachment portion of a retrofit tip structure may be configured as an aerofoil to extend the overall rotor diameter of the rotor, alternatively, it may be configured to maintain the original diameter of the rotor after cutting the tip of the blade to attach the tip structure, as shown in the example of
[0239] In summary, preferred embodiments provide a tip structure which may be arranged for example on a rotor blade 12 of a HAWT 10. The tip structure comprises a pressure side structure 50 arranged on a pressure side 43 of the blade, and a suction side structure 60 arranged on a suction side 44 of the blade 12. The pressure side and suction side structures 50, 60 have different pitch angles P, S so that the chord CP2 of the pressure side structure 50 extends forwardly in the direction of motion D and relatively more radially outwardly away from the blade root, or less radially inwardly towards the blade root, than the chord CS2 of the suction side structure 60, defining a relative twist angle T between the two structures 50, 60.
[0240] Those skilled in the art will recognise that many further adaptations are possible within the scope of the claims.
[0241] In the claims, reference characters and numerals are provided in parentheses purely for ease of reference, and should not be construed as limiting features.