Abstract:
The present invention includes a set of airfoils, U rails and V rails taken together to describe a blade for use with a horizontal axis wind turbine. The blade&#39;s design includes a maximum thickness higher than conventional blades employed for the same use thereby providing better load bearing structural characteristics while at the same time maintaining the requisite aerodynamic qualities for similar blades. The blade has a maximum thickness of about 30% and a maximum lift coefficient of about 1.3.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application is a continuation of U.S. patent application Ser. No. 12/456,363, filed Jun. 16, 2009 now U.S. Pat. No. 8,241,000. 
    
    
     FIELD OF INVENTION 
     The present invention relates in general to rotor blades for wind turbines wherein the blade of the present invention comprises an airfoil that, in combination with designed airfoils at other stations, provides better load bearing structural characteristics than the generally acceptable blades. The blade is characterized by a family of airfoils distributed at spanwise stations at about 25%, 35%, 55%, 75% and 95% in the mid-span region and tip region. Specifically, the blade so described has a mid-span airfoil exhibiting a thickness of about 30%, a Reynolds number in the range of about 1.1×10 6  to 1.3×10 6  and a maximum lift coefficient of about 1.3. 
     BACKGROUND 
     Wind turbines operate at either a constant rotational speed or at variable rotational speeds that are proportional to the wind velocity. Peak power at high wind speeds is usually controlled through stall regulation or through the use of variable pitch turbine blades. A conventional horizontal axis wind turbine (HAWT) employed to generate electric power typically includes two or more turbine blades each associated with a central hub. The hub rotates about an axis and is connected to a shaft. Conversion of wind power into electrical power is accomplished in most wind powered systems by connecting the shaft to drive an electric generator. 
     The point of the turbine blade closest to the hub is called the root of the blade, while the point of the turbine blade farthest from the hub is called the tip of the blade. The portion therebetween is the mid-span. A line connecting root to tip is referred to as the span of the blade. A cross-section of a turbine blade taken perpendicular to the span is generally referred to as an airfoil. Theoretically, therefore, each turbine blade includes an infinite number of airfoils along that line and it is the collection of airfoils that fully describes the blade&#39;s contours and shape. Typically, however, a blade&#39;s shape is defined in reference to a finite number of the airfoil shapes for convenience. Further, once the airfoils are determined, it is the accepted practice that at least some portion of the blade is further designed by application of a computer program that interpolates between the fixed airfoils to create foils therebetween. 
     Blade design starts with airfoil shapes. Next, computer programs have been employed to complete the design of the blades. However, employing these programs has created problems with wind turbines. Allowing a CAD program to loft the blade surface for airfoil sections which have been arbitrarily placed for optimal aerodynamics may result in waves or ripples in the surface loft that can be deleterious to structural integrity. This occurs when the CAD Program forces a surface through the pre-defined airfoil section. In addition, the thickness of blades designed using CAD programs are often kept artificially low in order to minimize the computer program&#39;s negative effects on the curvatures between fixed airfoil stations. However, keeping the thickness low often results in blades that may not be able to bear the loads required; these blades may buckle. On the other hand, incorrectly assigned airfoil coordinates for thicker blades can result in less than desirable aerodynamic properties in some portions of the blade. Therefore, many blades used in wind turbines often sacrifice structural soundness and dependability in exchange for more aerodynamic attributes. Since the primary goal of a wind turbine is to convert the kinetic energy of the wind into electrical energy as inexpensively and efficiently as possible, operational efficiency of the wind turbine is negatively affected by a structure that can allow the blade to buckle under certain loads. The blade design of the present invention addresses these problems without the corresponding expected loss in aerodynamic character. 
     SUMMARY OF THE INVENTION 
     A wind turbine rotor blade and airfoil family is provided. The blade includes a root end, an opposite tip end, a leading edge and a trailing edge, each extending from root end to tip end. The airfoil also includes an upper surface and a lower surface. Chord perpendicular to the span (from root to tip) lies in the plane extending through the leading edge and trailing edge. Thickness is in the direction perpendicular to chord and to span and is typically expressed as the ratio of its measure to the measure of the chord at that spanwise station and termed “thickness”. Thickness is the distance between the upper surface and the lower surface. 
     The geometric shape of an airfoil is usually expressed in tabular form in which the x, y coordinates of both the upper and lower surfaces of the airfoil at a given cross-section of the blade are measured with respect to the chord line, which is the imaginary line connecting the leading edge of the airfoil and the trailing edge of the airfoil. Both x and y coordinates are expressed as fractions of the chord length. As previously alluded to, another important parameter of an airfoil is its thickness. The thickness of an airfoil refers to the maximum distance between the airfoil&#39;s upper surface and the airfoil&#39;s lower surface and is generally provided as a fraction of the airfoil&#39;s chord length. For example, a five percent thick airfoil has a maximum thickness (i.e., a maximum distance between the airfoil&#39;s upper surface and the airfoil&#39;s lower surface) that is five percent of the airfoil&#39;s chord length. 
     Similar to the description of the geometric shape of the airfoil, the exact placement of the airfoil relative to the pitch axis is defined by offsets x P  and y P  oriented with respect to fine pitch i.e. the pitch as oriented at below rated wind speeds. Where the suction surface is nominally downwind at fine pitch so x is downwind and y is orthogonal in the plane of rotation roughly towards the trailing edge. 
     The chord length of an airfoil or cross-section of a turbine blade will typically become larger if the length of the blade increases and will typically become smaller if the length of the blade becomes smaller. 
     Another important parameter for every airfoil or blade cross-section is its operating Reynolds number. Airfoil performance characteristics are expressed as a function of the airfoil&#39;s Reynolds number. As the length of a blade decreases, the blade&#39;s Reynolds number tends to decrease. For a particular airfoil along the blade&#39;s span, a small Reynolds number indicates that viscous forces predominate while a large Reynolds number indicates that inertial forces predominate. 
     In the present invention the cross-sections or airfoils are designed to work at relatively low Reynolds numbers (between about 1.15×10 6  and 1.3×10 6 ) and the maximum thickness has been preserved at about 30%. The cross-sections have been maintained aerodynamically sound therefore providing a blade that tolerates the high loads demanded at the maximum chord section of the blade while improving the overall performance of the blade without sacrificing nearly the performance that is sacrificed if less than adequate cross sections are employed. 
     Most blade designs are accomplished by first establishing parameters for several cross sections of the blade spaced out along the blade between its root and tip and then employing CAD to provide the parameters for cross sections therebetween. For turbines operating at Reynolds numbers such as those mentioned above, the thickness of the aerodynamically designed portion of a blade is typically no more than 25% because when the thickness exceeds 25% the cross sections are generally less than aerodynamically sound. It is widely accepted that aerodynamically sound cross sections are required to maximize the blade&#39;s utility and power performance. Therefore, a thickness above about 25% is not employed for wind turbine blades of the 10-16 meter variety. However, the resulting relatively thin blades do not adequately tolerate high loads and are less structurally sound. This disadvantage is typically accepted in the art as better than the alternative of cross sections that are not aerodynamically sound. 
     On the other hand, if a 30% maximum thickness blade including an airfoil for operation at Reynolds numbers between about 1.1 and 1.3×10 6  is created instead by careful design and paying careful attention to the effects of slight changes in chord length and twist on curvature, then the strength of the blade can be increased without the same level of damage to the aerodynamic properties of the blade. This is the advantage offered by the present invention. The present invention, therefore, stands counter to the industry standard, having a 30% maximum thickness yet still exhibiting aerodynamically acceptable cross sections for performance. 
     Other objects, features, and advantages of the present invention will be readily appreciated from the following description. The description makes reference to the accompanying drawings, which are provided for illustration of the preferred embodiment. However, such embodiment does not represent the full scope of the invention. The subject matter which the inventor does regard as his invention is particularly pointed out and distinctly claimed in the claims at the conclusion of this specification. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of a wind turbine embodying the present invention; 
         FIG. 2  is a perspective view of a blade of the wind turbine; 
         FIG. 2   a  is an end view of an airfoil of the present invention; 
         FIG. 3  is a cross section showing a family of airfoils belonging to the blade of the present invention; 
         FIG. 4  is an oblique view of the cross sections and v rails of the blade of the present invention; 
         FIG. 5  is an end view of  FIG. 4 ; 
         FIGS. 6 and 6   a  are tables showing design statistics applying to the four original airfoils; 
         FIG. 7  is a table showing certain design statistics of the five airfoils of the final design; 
         FIG. 8  is a table showing design statistics and pitch axis offset of the five airfoils of the final design; 
         FIG. 9  is a table showing design statistics and pitch axis offset of the five airfoils of the final design and other airfoils therebetween; 
         FIG. 10  is a table providing the coordinates of the Spanwise (V) rails of the present invention; and 
         FIG. 11  is a table providing coordinates of the Spanwise (V) rails and two additional V rails at the forward part line and the surfaces at the trailing edge. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     A horizontal axis wind turbine  10  having a tower  12 , a hub  14 , a horizontal axis  16 , and a plurality of blades  18 ,  19  and  20  each having a root  22 , a root region  24 , mid-span region  26 , a tip  28 , and a tip  30  region each comprising at least one airfoil  32 ,  34 ,  36 ,  38  and  40  of the present invention is presented in  FIG. 1 . As also shown in  FIGS. 2 and 2   a , each of said plurality of blades  18 ,  19  and  20  further comprises an upper surface  50 , a lower surface  52 , the airfoils  32 - 40 , a leading edge  54 , a trailing edge  56 , and a span  58  extending from root  22  to tip  28  and is further numerically characterized by a tip speed ratio. Each of said at least one airfoil  32 ,  34 ,  36 ,  38  and  40  comprises a chord  60  which is the distance between the leading edge  54  and the trailing edge  56 , a thickness  64  perpendicular to the chordline  60  extending between the upper surface  50  and the lower surface  52  and expressed as a percentage of the chord  60 , and a maximum lift coefficient and a design Reynolds number pertaining to each of said airfoils. These numeric values are shown in  FIG. 7 . Lift of the entire blade can be described by integrating the lift of all the airfoils in the blade and depends on the velocity of the air flow on the airfoil, the shape and contour of the airfoil, and the airfoil&#39;s angle of attach which is the angle between the chord and the vector resulting from all the combined air forces on the blade i.e. wind speed vector, airfoil&#39;s rotational velocity vector, and blade induced velocity vectors. 
     The root and transition region  24  extends outwardly from the horizontal axis  16  to a point approximately 20% of the distance between the axis  16  and the tip  28 ; the tip region  30  from the tip  28  back to the point approximately 90% of the distance from axis  16  to tip  28 ; and the mid-span region  26  covers the region between the tip region  30  and the root region  24 . 
     The blade design of the present invention started with defining four airfoils  32 ,  34 ,  36 ,  38  (See  FIG. 6 ) and later adding a fifth airfoil  40  at five stations along the blade (see  FIG. 7 ), and adding the barrel root  22 . The exact placement of each airfoil at its station relative to the pitch axis of the blade is defined by the offsets x P  and y P . (See  FIG. 8 ). The values of x P  and y P  for the stations are chosen to produce minimal curvature along spanwise cross-sections taken at Y=−200, −100, 0, 100 and 200 (Refer to  FIGS. 9 and 10 ). Traditionally, lofting includes using a CAD program to design the blade between the stations. Sometimes, the system designs a region of very high concave curvature which increases the tendency for the blade surface to buckle under a load. Buckling resistance can be improved by minimizing this curvature. 
     As shown in  FIGS. 5 and 10  spanwise cross-sections  70 - 80  called V rails were taken into account to help define the blade curvature in the spanwise direction. The relative positions of the spanwise cross-sections  70 - 80  are indicated as positions on a y coordinate, where y=0 is taken with the blade rotated 10 degrees nose up relative to the fine pitch orientation. The ability of the airfoils  32 ,  34 ,  36 ,  38  and  40  encompassed by each blade  18 ,  19  or  20  in the present invention to withstand buckling tendencies is attributed to the maximum thickness  64  embodied in the airfoil  40  encompassed by the blade  18 ,  19  or  20 . The ability of the blade  18 ,  19 , or  20  comprising this airfoil  40  to exhibit acceptable performance parameters can be attributed to the additional airfoils  32 ,  34 ,  36 , and  38  designed at selected stations. These additional airfoils function as U rails. These U rails and the V rails running from root  22  to tip  28  intersect. 
     In order to best maintain aerodynamic function of the blade while obtaining the structural advantage of the maximum thickness of 30%, the contours of the V rails were modified to reduce their curvature. These modifications took the form of slight alterations of twist and chord length of airfoils between the U rails all while maintaining the original intersections of the U and V rails. Differences before and after modifications are presented in  FIGS. 6   a  and  7 . 
     Slight shifting of one station by x P  or y P  (relative to fine pitch) may reduce the curvature in one spanwise cross-section and increase it in another. Therefore, a balance was struck by shifting slightly the placement of the stations. In the blade  18 ,  19  and  20  of the present invention, the spanwise rails placed at Y=−100, 0, and 100 are the most critical with regard to curvature. Therefore, the variables x P , y P , twist and chord length were taken into account and manipulated to create smooth sections between the established stations. For each of the original five airfoils and for several airfoils therebetween, x P , y P , chord length and twist were optimized to increase buckling resistance while retaining the intersections of the V rails with the original five airfoils. The optimized five stations are described by  FIGS. 8 and 9 . Upon correct manipulation, the resulting blade exhibited enhanced buckling resistance and load tolerance while maintaining aerodynamically acceptable cross-sections. While a 30% thickness blade will never perform as well as a 25% thickness at these Reynolds numbers, it will perform far better and exhibit load tolerances much improved over prior art blades at about and below 25% thickness. 
     In  FIG. 9 , stations and relevant airfoils located between the optimized five stations are also presented. These airfoils fully describe the shape of a wind turbine blade with tip speed ratio of 7.5 which exhibits higher buckling resistance than comparable prior art blades. 
     Thus, the present invention has been described in an illustrative manner. It is to be understood that the terminology that has been used is intended to be in the nature of words of description rather than of limitation. 
     Many modifications and variations of the present invention are possible in light of the above teachings. For example, the blade length can vary within the range, the maximum thickness may be larger than 30%, the number of airfoils selected as the base set may be higher or lower. Therefore, within the scope of the appended claims, the present invention may be practiced otherwise than as specifically described.