Abstract:
A blade for a partial pitch wind turbine is described, wherein the blade has a truncated aerodynamic profile provided at the pitch junction of the blade. The truncated trailing edge acts to prevent the formation of a trailing edge gap during pitching between the trailing edges of the inner and outer blade sections of a partial pitch blade, thereby reducing the negative impact of air leakage, etc., when pitching a partial pitch blade.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a wind turbine blade, in particular to a partial pitch wind turbine blade. 
     2. Description of Related Art 
     A wind turbine blade generally comprises an airfoil shaped profile having a relatively rounded leading edge and a relatively narrow trailing edge, the blade having a suction side (exposed to relatively low air pressure) and a pressure side (exposed to relatively high air pressure). 
     US patent application No. 2009/0148285 by Scholte-Wassink assigned to GE Energy discloses a wind turbine with a multi-section blade, where the outer section can be pitched relatively to the inner section. A pitch control assemble for an airplane wing is disclosed in U.S. Pat. No. 5,263,846. 
       FIG. 1  shows an enlarged pitch junction section of a partial pitch wind turbine blade. A partial pitch wind turbine blade  100  comprises an inner blade section  102  and an outer blade section  104  coupled together at a pitch junction, via a pitch system  106 . The outer blade section  104  is operable to be pitched relative to the inner blade section  102  by rotation of the pitch system  106 . The pitch angle of the outer blade section  104  can be controlled to regulate the operation of the wind turbine, by pitching the outer blade section  104  into or out of the wind direction at the turbine. 
     With reference to  FIG. 2 , one feature of partial pitch blades which contributes to efficiency losses in blade performance is the creation of a gap between the trailing edges of the adjacent blade sections, when projected along the longitudinal axis of the blade. In  FIG. 2 , the inner and outer blade section  102 , 104  have common leading edge  108 , and respective trailing edges  110   a , 110   b , with the pressure side of the overall blade  100  indicated at  112  and the suction side of the overall blade  100  indicated at  114 . When the outer blade section  104  is pitched relative to the inner blade section  102 , a gap (indicated at  116 ) is formed in the general blade profile between the two trailing edges  110   a ,  110   b , when seen along the length of the blade  100 . 
     The presence of this trailing edge gap  116  results in air leakage from the pressure side  112  of the blade  100  to the suction side  114 , as gap  116  means that there is no barrier between the pressure side of the outer blade section  104  and the suction side of the adjacent inner blade section  102 . 
     Sealing of a gap between adjacent parts of a wing of an airplane has been disclosed in U.S. Pat. No. 5,518,210 by Friberg assigned to McDonnel Douglas Corp. D 1  describes a wing where seal plates and attached to the airfoil so that the stream-wise gaps created by the deflected aileron are covered. Prevents airflow leakage through gaps between surface and aerofoil to reduce aerodynamic drag and improve flight efficiency. 
     SUMMARY OF THE INVENTION 
     It is an object of the invention to provide a partial pitch blade design which reduces the negative impact of such trailing edge gaps during wind turbine operation. 
     Accordingly, there is provided a partial pitch wind turbine blade of at least 35 metres length, the wind turbine blade having an aerodynamic airfoil profile with a leading edge and a trailing edge, the wind turbine blade comprising: 
     an inner blade section; and 
     an outer blade section, the blade sections coupled at a pitch junction such that the outer blade section is pitchable relative to the inner blade section, wherein the wind turbine blade comprises a truncated airfoil profile provided at said pitch junction, wherein the truncated profile is provided at the trailing edge. 
     A truncated profile at the pitch junction acts to reduce or prevent the formation of a gap between the trailing edges of the inner and outer blade sections when looking along the longitudinal direction of the blade profile, referred to as a trailing edge gap. Such a gap is caused when the surface of the pressure side of one of the blade sections intersects with the surface of the suction side of the other blade section, thereby forming a gap between the trailing edges of the adjacent blade sections, when seen along the longitudinal direction of the blade. Preventing the formation of such a trailing edge gap stops the creation of vortices at the pitch junction, and prevents air leakage from the pressure side of the blade to the suction side of the blade due to the presence of the gap. 
     For example, said truncated airfoil profile is arranged such that, when projected along the longitudinal axis of the blade, the surface of the pressure side of a first of said blade sections intersects with the surface of the suction side of a second of said blade sections for a pitch angle of the outer blade section greater than a pre-defined angle to prevent the formation of a trailing edge gap at the pitch junction between the trailing edges of the blade sections for pitch angles below said pre-defined angle. 
     The truncation at the pitch junction is dimensioned such that the trailing edge gap or profile gap is prevented from forming for an initial pre-determined range of pitch angles. If this pre-determined range of angles covers the most likely pitch angles for the predicted wind conditions for the turbine, then the inefficiencies created by tip losses, etc., present at the trailing edge gap do not occur for most of the total energy produced by the wind turbine. 
     Typically, said pre-defined pitch angle is between 4 and 12 degrees, for example between 6 and 10 degrees or 8 degrees or approximately 8 degrees. 
     Thus, for angles between 0 degrees pitch and the pre-defined pitch angle, for example 8 degrees pitch, there will be no crossover between the surface side of one section and the pressure side of the other, preventing the formation of a trailing edge gap and any associated vortices, etc. Preferably, the trailing edge gap is prevented from being formed for any pitch angle between a positive and negative value of the pre-defined pitch angle, for example between −8 degrees to +8 degrees. 
     For example, said inner blade section and said outer blade section comprise a truncated blade profile at said pitch junction, said airfoil profile truncated at the trailing edge of said profile, and wherein said truncated trailing edge is arranged such that when said outer blade section is pitched relative to said inner blade section, the truncated trailing edge surface of said outer blade section projected along the longitudinal axis of the blade intersects the truncated trailing edge surface of the inner blade section. 
     In this embodiment, both the inner and outer blade sections comprise truncated portions, preventing the formation of trailing edge gaps when pitched. The intersection or overlap between the edge surfaces ensures that a gap will not be formed when the blades are pitched. 
     For example, said truncated trailing edge is arranged such that said projected intersection of said truncated trailing edge surfaces occurs for when said outer blade section is pitched at a pitch angle of between 0 and 10 degrees, for example between 0 and 8 degrees. 
     This covers the majority of operating conditions of the wind turbine, and the greatest portion of annual energy consumption. Intersection may cover where the edge surfaces are completely in line, or where they cross at one point along the surface height as seen from an end of the turbine looking along the longitudinal direction of the turbine. 
     For example, 
     the inner blade section comprises a truncated blade profile at said pitch junction truncated at the trailing edge of said profile, and the outer blade section comprises an untruncated blade profile at said pitch junction; or 
     the inner blade section comprises an untruncated blade profile at said pitch junction, and the outer blade section comprises a truncated blade profile at said pitch junction truncated at the trailing edge of said profile; 
     wherein the truncated trailing edge surface is arranged such that when said outer blade section is pitched relative to said inner blade section, when projected along the longitudinal direction of the blade, the surface of said truncated trailing edge is intersected by the surface of the untruncated blade profile. 
     In this embodiment, only one of a blade&#39;s inner or outer blade sections is truncated. The truncation is arranged such that the trailing edge of the untruncated section will overlap with the surface of the truncated trailing edge, to prevent the formation of a trailing edge gap. Depending on which blade section is truncated, and on the direction of pitch, the surface of the truncated trailing edge may be intersected by the pressure side surface or the suction side surface of the untruncated section. For example, the overlap between surfaces at least occurs for outer blade section pitch angles of less than 8 degrees, i.e. to ensure that the overlap/intersection occurs for the majority of total energy production for the turbine. 
     In one embodiment, said inner blade section comprises a truncated airfoil profile at said pitch junction. 
     In such an embodiment, advantageously, said inner blade section comprises a truncated blade profile and an untruncated blade profile, wherein the chord length of said truncated blade profile is substantially equal to the chord length of the adjacent untruncated blade profile of said inner blade section. 
     If the inner blade section is truncated, advantageously, the truncation occurs adjacent the pitch junction of the blade, with the inner blade section relatively untruncated when moving away from the pitch junction. 
     Additionally or alternatively, said outer blade section comprises a truncated blade profile at said pitch junction. 
     In some embodiments, said outer blade section comprises a truncated blade profile and an untruncated blade profile and a transition zone between said truncated and untruncated blade profiles. 
     It will be understood that any truncation on the outer blade section may be dimensioned to approximate the chord length of the adjacent inner blade section. Accordingly, if the majority of the outer blade section has a relatively smaller chord that the inner blade section, a transition zone may be used to provide a smooth adjustment of blade profile along the longitudinal length of the blade. 
     The truncated section may be provided on only one, or both, of the blade sections. 
     For example, the chord length of the inner blade section at said pitch junction is substantially equivalent to the chord length of the outer blade section at said pitch junction 
     In order to improve the efficiency of the blade design, it is advantageous that there are no substantial discontinuities between the aerodynamic profiles of the adjacent blade sections. 
     For example, the blade comprises a shell member provided at said pitch junction, wherein said shell member forms an aerodynamic profile having said truncated trailing edge. 
     The truncated section may be retrofitted to existing blade designs in the form of a shell member or fairing fitted at or around the pitch junction. 
     For example, said shell member may be provided around at least one of the blade sections, or in a gap between said blade sections at said pitch junction. 
     In one embodiment, said truncated airfoil profile provided at said pitch junction comprises a chord length of between approximately 4-5 metres at said pitch junction, and wherein the height of the truncated end of said profile is at least 30 centimetres at said pitch junction. 
     Preferably, at least one of the blade sections comprises at the trailing edge a truncated blade profile at said pitch junction, wherein said truncated profile extends from said pitch junction a distance equivalent to between 2% and 50%, for example between 5% and 25% or between 5% and 15% or 10% of the total longitudinal length of said at least one blade section. 
     In embodiments where the inner blade section comprises a truncated profile, preferably the truncation longitudinally extends along said inner blade section from said pitch junction a distance substantially equivalent to between 2% and 50%, for example between 5% and 25% or between 5% and 15% or 10% of the total longitudinal length of said inner blade section. It will be understood that embodiments may be used wherein the truncation extends along even more than 50% or 70% or 90%, even up to approximately 100% of the longitudinal length of the inner blade section. 
     In embodiments where the outer blade section comprises a truncated profile, optionally, the truncation longitudinally extends along said outer blade section from said pitch junction a distance substantially equivalent to between 2% and 50%, for example between 5% and 25% or between 5% and 15% or 10% of the total longitudinal length of said outer blade section. 
     The truncation is optionally provided in the region of the pitch junction with the profiles of the blade sections away from this region being substantially unaltered. 
     For example, the wind turbine blade aerodynamic airfoil profile has a maximum thickness T and a chord length C from the leading edge of the profile to the trailing edge, and wherein truncation is performed such that T/C is at least 0.1 or at least 0.2 or at least 0.3. For example, in one embodiment, the profile is truncated such that the profile has a maximum thickness of 1.1 metres, and a chord length from the leading edge to the truncated trailing edge of 5 metres. 
     In one embodiment, said truncated airfoil profile provided at said pitch junction comprises a chord length of between approximately 4-5 metres at said pitch junction, and wherein the height of the truncated end of said profile is at least 30 centimetres at said pitch junction. 
     Further enhancements may include, but are not limited to, any combination of the following: the entire length of the inner and/or outer blade section may be truncated; the inner blade section profile comprises a stall-controlled blade profile and the outer blade section comprises a pitch-controlled blade profile; the surface area of said inner blade section is substantially equivalent to the surface area of said outer blade section; said inner blade section is approximately ⅓ of the length of the complete wind turbine blade; the wind turbine blade further comprises at least one stall fence provided at said pitch junction; the wind turbine blade further comprises at least one bridging member provided at said pitch junction to substantially cover the space between adjacent blade sections at said pitch junction. 
     There is also provided a wind turbine comprising a blade according to any of the embodiments as described above. 
     Such a wind turbine may comprise a turbine tower, a nacelle provided at the top of said tower, a rotor hub rotatably mounted on said nacelle, and at least two wind turbine blades provided on said rotor hub. 
     Embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which: 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an enlarged perspective view of a known pitch junction of a prior art partial pitch blade; 
         FIG. 2  is a cross-sectional view of the pitch junction of  FIG. 1  when pitched; 
         FIG. 3  is a perspective view of a two-bladed partial pitch wind turbine; 
         FIG. 4  is a perspective view of a wind turbine blade according to the invention for use in the wind turbine of  FIG. 3 ; and 
         FIG. 5  is a cross-sectional view of the pitch junction of the blade of  FIG. 4  when pitched; 
         FIG. 6  is a cross sectional profile of a basis blade surrounded by a shell member to provide a truncated trailing edge. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     With reference to  FIG. 3  a partial-pitch two-bladed wind turbine is indicated generally at  10 . The wind turbine  10  comprises a wind turbine tower  12 , a nacelle  14  provided at the top of said tower  12 , and a rotor hub  16  provided at said nacelle  14 . First and second partial pitch rotor blades  18 , 20  are provided on opposite sides of said rotor hub  16 . In  FIG. 2 , the tower  12  is shown provided on a wind turbine base  22 , which may comprise any suitable wind turbine foundation. It will be understood that while the illustrated embodiment describes the use of the invention for an on-shore wind turbine, it will be understood that the invention may equally apply to wind turbines for use in an off-shore environment. 
     The first and second partial pitch rotor blades  18 , 20  each comprise a blade body having a root end  18   a , 20   a  mounted to said rotor hub  16  and a distal tip end  18   b , 20   b . The rotor blades  18 , 20  comprise respective inner blade sections  24 , 26  provided at said root ends  18   a , 20   a , and respective outer blade sections  28 , 30  provided at said tip ends  18   b , 20   b . The rotor blades  18 , 20  further comprise a pitch system (not shown) provided in each blade at the junction between the inner blade sections  24 , 26  and the outer blade sections  28 , 30 . 
     The pitch system is operable to pitch the outer blade sections  28 , 30  relative to the inner blade sections  24 , 26 . In  FIG. 3 , the rotor blades  18  are shown unpitched (i.e. the outer blade sections  28 , 30  are pitched at a 0 degree pitch angle). It will be understood that the inner blade sections  24 , 26  and the outer blade sections  28 , 30  comprise a substantially circular end section for coupling to corresponding circular pitch systems. 
     While the embodiment shown is of a two-bladed wind turbine design, it will be understood that the invention may equally apply to wind turbines having any number of partial pitch blades, e.g. a three-bladed partial pitch wind turbine. 
       FIG. 4  shows a partial pitch wind turbine blade  18 , having root end  18   a  and tip end  18   b , and inner and outer blade sections  24 , 28 . As described in reference to  FIG. 3 , the blade sections  24 , 28  are coupled together at a pitch junction (indicated at  32 ), wherein the pitch junction end  24   a  of the inner blade section  24  and the pitch junction end  28   a  of the outer blade section  28  are coupled to a pitch system  34 , allowing the outer blade section  28  to be pitched relative to the inner blade section  24 . 
     The blade  18  comprises an aerodynamic blade profile, having a leading edge indicated at  36  (which generally faces into the wind) and a trailing edge indicated at  38  (which generally faces away from the wind). The upper surface of the blade  18  is the suction side of the blade  18  (indicated at  40 ), and the lower surface of the blade is the pressure side (indicated at  42 ). During rotation of the blade  18 , the pressure side  42  will be on the relatively high pressure side of the blade body, with the suction side  40  exposed to a lower air pressure than the pressure side  42 . 
     The blade  18  comprises a truncated blade profile in the region of the pitch junction  32 . The inner blade section  24  comprises a truncated trailing edge  44  adjacent the pitch junction  32 , and the outer blade section comprises a truncated trailing edge  46  adjacent the pitch junction  32 . The truncations  44 , 46  provided at the pitch junction  32  ensure that a trailing edge gap between the trailing edges of the adjacent blade section  24 , 28  is not formed when the outer blade section  28  is pitched, at least for an initial range of pitch angles. 
     With reference to  FIG. 5 , a cross-section of the blade  18  is shown at the pitch junction  34 , when the outer blade section  28  is pitched relative to the inner blade section  24 . The truncated trailing edges  44 , 46  are designed such that when the outer blade section  28  is pitched up to a pre-defined maximum angle (in a positive direction as indicated by arrow A), the trailing edges of the adjacent blade profiles will overlap in a longitudinal direction, preventing the formation of a trailing edge gap at the pitch junction  32 . This prevents air leakage from occurring at the pitch junction  32  from the pressure side  42  of the blade  18  to the suction side  40 . The pre-defined angle is typically between 4 and 12 degrees, for example between 6 and 10 degrees or 8 degrees. 
     The truncated surfaces  44 , 46  may be designed to be such a height that the blade profiles overlap to prevent trailing edge gaps for any typical pitch angle of the outer blade section  28  under normal operation. Optionally, the truncated surfaces  44 , 46  are dimensioned to ensure that no trailing edge gaps are formed for an initial pre-defined range of pitch angles, i.e. the most likely pitch angles to be used during turbine operation. Alternatively, the pitch angle may be limited by such maximum pre-defined angle under normal operation. The pre-defined angle is typically between 4 and 12 degrees, for example between 6 and 10 degrees or 8 degrees. The ranges of pitch angles are in positive as well as negative direction. 
     The wind turbine blade profile comprises a chord length C from the leading edge  36  to the trailing edge  38 , 44 , 46 , and a maximum thickness T (generally located towards the leading edge  36  of the profile). The truncation of the profile is generally performed on blades with a ratio of T/C is at least 20%, e.g. a profile having a thickness of approximately 1 metre and a chord length of approximately 5 metres. 
     In one example, a truncated airfoil profile provided at the pitch junction comprises a chord length of between approximately 4-5 metres at said pitch junction, wherein the height of the truncated trailing edge surface end of said profile is at least 30 centimetres at said pitch junction. Such an embodiment ensures that no trailing edge gaps are formed for pitch angles of between −8 to +8 degrees, which provides a range of pitch angles which covers the majority of total energy production for that particular wind turbine. 
     In the embodiment shown in  FIG. 4 , the outer blade section  28  comprises a tapered transition zone which transitions from the relatively long chord length of the truncated portion  46  of the inner blade section  28  adjacent the pitch junction  32 , and the relatively short chord length of the remainder of the outer blade section  28  extending towards the tip end  18   b  of the blade  18 . At the inner blade section  24 , the blade  18  has substantially the same chord length along most of the length of the inner blade section  24 , away from the circular root end  18   a . The chord length at the truncated portion  44  is substantially equivalent to the chord length on the adjacent untruncated portion. It will be understood that other embodiments of the invention may utilise alternate designs of truncation, and may or may not comprise transition zones between portions of the blade having different chord lengths, depending on the desired configuration of blade design. 
     In an alternate embodiment, the truncation of the trailing edge may be applied to only one of the blade section  24 , 28 , i.e. the inner blade section  24  is truncated and the outer blade section  28  is not, or vice versa. In such a case, the trailing edge of the untruncated blade section extends beyond the truncated trailing edge of the truncated blade section. The truncation is arranged such that a trailing edge gap will not be formed when the outer blade section is pitched, at least for an initial range of pitch angles. 
     As seen in  FIG. 4 , the truncated sections of the blade  18  extend along a portion of the blade trailing edge  38  a short distance from the pitch junction  32 . For example, the truncated trailing edges extend from the pitch junction  32  a distance substantially equivalent to at least approximately 10% of the length of the blade section in question. In a further alternative, the truncated trailing edge may extend along the entire length of the trailing edge of the inner blade section  24 . 
     As illustrated in  FIG. 6 , the blade may be provided as a basis blade  48  with a sharp trailing edge  50  and without a truncated trailing edge and a shell member  52  or fairing around one or both of said inner and outer blade sections at said pitch junction. The shell member  52  may be shaped to present an aerodynamic profile at the pitch junction with a truncated trailing edge ( 44 ,  46 ). The advantage of using such a shell member  52  is that the truncated trailing edge ( 44 ,  46 ) can be retrofitted to existing partial pitch basis blade designs. 
     The invention provides a new partial pitch blade design which improves wind turbine operational performance. As the truncation is dimensioned to prevent the formation of a trailing edge gap when the outer blade section is pitched, the effect of vortices and air leakage at the pitch junction is reduced, resulting in increased blade efficiency. 
     The invention is not limited to the embodiment described herein, and may be modified or adapted without departing from the scope of the present invention.